Preparation of undoped and some doped ZnO
thin films by SILAR and their characterization
Dissertation submitted to The University of Burdwan in partial
fulfillment of the requirements for the degree of Doctor of
Philosophy in Science (Physics)
SHAMPA MONDAL
Department of Physics
The University of Burdwan
Burdwan
West Bengal, INDIA
2013
Dedicated to my parents & my son
THE UNIVERSITY OF BURDWAN GOLAPBGA, BURDWAN: 713104
WEST BENGAL, INDIA
Dr. Partha Mitra Phone: 0342 2657800 (O)
Associate Professor Fax : +91 342 2657800
Materials Science Laboratory e-mail: [email protected]
Department of Physics
Date:
Certificate from the Supervisor
This is to certify that the research work incorporated in the dissertation entitled
“Preparation of undoped and some doped ZnO thin films by SILAR and their
characterization” has been carried out at The University of Burdwan, Burdwan by
Shampa Mondal, under my supervision. Mrs. Mondal has followed the rules and
regulations as laid down by The University of Burdwan for the fulfillment of
requirements for the degree of Doctor of Philosophy in Science. Any other worker
anywhere has not published the results included in this dissertation.
(Dr. Partha Mitra)
ACKNOWLEDGEMENT
It is a distinct pleasure to express my deepest sense of gratitude to my research
supervisor, Dr. P. Mitra, Department of Physics, The University of Burdwan for kindly
suggesting me this challenging problem. His efficient guidance and constructive criticism
encourage me all along and helping through the course of the work. It was a privilege to
carry out the present research work under him.
Similar gratitude also goes to the Head of the Dept. Prof. S. Das for his kind co-
operation. The friendly and stimulating discussion with Dr. S. K. Pradhan, Dr. M. Pal,
Dr. S. Mukherjee and Dr. P. K. Chakraborty contributed greatly to the progress of my
work. I greatly acknowledge the help and guidance of all other faculty members of the
Department of Physics right from the beginning of my research work.
My thanks also goes to Mr. S. Patra, Mr. S. Bandyopadhyay, Mr. S. Lala, Mr. A.
Nandy, Smt. A. Sen, Mr. S. Sain, Mr. U. K. Bhaskar, Smt B. Ghosh and Mr. A. Banerjee
for their help during the experiment. My regards also goes to Dr. A. Dutta and Mr. K. P.
Kanta for their encouragement and suggestions during the progress of the work. With a
sense of gratitude, I am thankful to all the office and library staff of the Department of
Physics and the technical staff for all the help and cooperation.
My graceful thanks goes to my husband Mr. B. Roy Choudhury, my brothers Mr.
S. Mondal, Mr. A. Samanta and Mr. A. Roy Choudhury and my parent in-laws Mr. M.
K. Roy Choudhury and Smt. C. Roy Choudhury for their continuous inspiration
throughout the work.
I have no sufficient word to express my sincere gratitude to my parents Late R. N.
Mondal and Late K. Mondal for their blessings. I am grateful to my family members,
close relatives, all my colleagues of A.K.P.C Mahavidyalaya, well wishers and friends,
for their constant encouragement.
January 2013 (Shampa Mondal)
PREFACE
Preparation and characterization of ZnO and doped ZnO polycrystalline thin film
via different techniques have attracted considerable attention due to their wide
application prospects in various electronic and optoelectronic devices. Consideration of
simplicity, economy and input energy suggest that thin films of these materials be
deposited by a low temperature and simpler chemical route. The present work was taken
up to prepare ZnO and doped ZnO thin films following a relatively new and less
investigated wet chemical technique called Successive ion layer adsorption and reaction
(SILAR). The structural and morphological property of ZnO thin films synthesized by
SILAR has been studied and influence exerted by some metal doping on structural
properties, optical band gap and electrical resistance has been investigated.
The thesis contains nine chapters. Chapter 1 contains an introduction to ZnO and
its properties and importance of thin films of ZnO and doped ZnO. Different physical and
chemical techniques to prepare thin films have been discussed. The chapter ends with a
discussion on general characteristics of SILAR technique. Chapter 2 presents a brief
review of literature on preparation of ZnO and doped ZnO by different techniques and
their properties. Chapter 3 discusses the instrumental techniques used for characterization
of prepared materials in the present work. Theoretical consideration for evaluation of
preferred orientation and particle size has been discussed here. Characterization of ZnO
thin films prepared by SILAR has been discussed in chapter 4. Chapter 5 presents the
preparation of Cd doped ZnO thin films, their structural characterization and evaluation
of band gap energy. The preparation of Mn doped ZnO thin films and the influence of
Mn incorporation on structural properties and optical band gap of ZnO is discussed in
chapter 6. Chapter 7 deals with the preparation and characterization of Al doped ZnO
(AZO) thin films. Structural, morphological and electrical characterization of AZO thin
films has been discussed here. The influence of Ni incorporation in ZnO has been
discussed in chapter 8. Chapter 9 presents the concluding remarks and scope of future
work.
i
CONTENTS
Page No.
CHAPTER 1: Introduction
1.1 Zinc oxide and its properties 1
1.2 Thin films of ZnO and doped ZnO 6
1.3 Thin films and their deposition techniques 10
1.4 The technique of SILAR 16
References 20
CHAPTER 2: Literature review and Aim of the work
2.1 Review of Literature 24
2.2 Aims and objectives of the present work 39
References 40
CHAPTER 3: Instrumental techniques and Theoretical considerations
3.1 Instrumental techniques 48
3.1.1 X-ray Diffraction (XRD) Analysis 48
3.1.2 Electron Microscopes: SEM and TEM 51
3.1.3 Ultraviolet – Visible (UV-VIS) spectroscopy 52
3.1.4 Energy dispersive X-ray spectroscopy (EDS or EDX) 53
3.2 Theoretical considerations
3.2.1 Preferred orientation 53
3.2.2 Particle size estimation 54
References 56
CHAPTER 4: Preparation of ZnO thin films by SILAR and their
characterization
4.1 Introduction 57
ii
4.2 Preparation of ZnO thin films 58
4.2.1 Preparation of bath solutions 60
4.2.2 Optimization of pH and Concentration of the zincate baths 62
4.2.3 Deposition of ZnO films 63
4.2.4 Film thickness and its measurement 66
4.3 Structural characterization by XRD: Evaluation of particle size 69
4.4 Electron microscope studies 76
4.5 EDX and FTIR studies 80
4.6 Discussion of results on ZnO thin films 83
References 84
CHAPTER 5: Preparation of Cd doped ZnO thin films by SILAR and
their characterization
5.1 Preparation of Cd doped ZnO (Cd:ZnO) films 86
5.2 Structural characterization: Evaluation of particle size 87
5.3 SEM and EDX studies 89
5.4 Optical band gap evaluation of Cd:ZnO films 92
5.5 Discussion of results on Cd:ZnO thin films 94
References 95
CHAPTER 6: Preparation of Mn doped ZnO thin films by SILAR and
their characterization
6.1 Preparation of films and thickness measurements 96
6.2 Structural characterization: Evaluation of particle size and
strain 100
6.3 SEM and EDX studies 104
6.4 Evaluation of band gap from optical absorption 108
6.5 Discussion of results on Mn:ZnO thin films 111
References 112
iii
CHAPTER 7: Preparation of Al doped ZnO (AZO) thin films by SILAR
and their characterization
7.1 Preparation of AZO films 113
7.2 Structural characterization by XRD: Evaluation of TC(002) 114
7.3 Band gap evaluation from optical absorption 120
7.4 Electrical resistance measurements 121
7.5 Electrical resistance measurements in presence of LPG 125
7.6 Discussion of results on AZO thin films 130
References 132
CHAPTER 8: Preparation of Ni doped ZnO thin films by SILAR and
their characterization
8.1 Preparation of Ni doped ZnO (NZO) films 134
8.2 Structural characterization by XRD: Evaluation of particle size 135
8.3 SEM & EDX studies 138
8.4 Band gap evaluation from Optical absorption 140
8.5 Electrical characterization 141
8.6 Discussion of results on Ni:ZnO thin films 143
References 144
CHAPTER 9: Summary, Conclusions and Scope of future work
9.1 Summary and Conclusions 145
9.2 Scope of future Work 150
List of Publications 151
1
CHAPTER 1
Introduction
1.1 Zinc oxide and its properties
The earliest commercially produced semiconducting materials belong to II-VI
compounds. These are cadmium sulphide (CdS), cadmium selenide (CdSe), zinc oxide
(ZnO), tin dioxide (SnO2), zinc selenide (ZnSe), zinc telluride (ZnTe) etc. The ionicity of
such compounds is very high compared to elemental semiconductors such as silicon ( )Si
or germanium (Ge) and also III-V compounds. Polycrystallinity with crystallite
dimensions of the order of minority carrier diffusion length and good optical quality are
some of the important features which make the II-VI materials potential candidates for
their applications in optical and electronic devices. ZnO is one of the most versatile II-VI
materials and have long been subjects of investigation. The material ZnO is known since
the Bronze Age [1] and is an important topic of research in the 21st century. This is also
one of the most important materials that we come across in our day-to-day lives. Zinc
white is used as a pigment in paints and in coatings for paper. Some of the favorable
aspects of ZnO include its radiation hardness, abundance in nature and nontoxicity,
biocompatibility, excellent piezoelectric and semiconducting properties among many
others. Such multi-functional properties of ZnO make it suitable for applications in
electronic and optoelectronic devices.
2
The distinctive features of the material includes non-stoichiometric defect
structure, wide band gap (WBG) with high optical transparency in the visible region,
large variability of conductivity and high surface sensitive catalytic activity under
different atmospheres and high voltage-current nonlinearity etc. [2-4]. Besides being a
wide band gap semiconductor with a bandgap of around 3.2-3.37eV at room temperature
300K [5-6] and exciton binding energy of 60 meV (almost three times greater than that of
GaN, another most widely used WBG compound), it has several other aspects. ZnO
shows anisotropy in crystal structure and strong absorption in the ultraviolet range.
Accordingly it has got several potential applications in windows for photovoltaic solar
cell and heterojunction solar cells, surface acoustic wave (SAW) devices, IR reflective
coatings, piezo-electric and guided optical wave devices, blue and UV light emitting
diodes, phosphors, solid state gas sensors and transducers [7-14]. In transparent
conducting oxide form, the material have got applications in solid state display devices,
resistors, selective absorber components in solar collectors and in a number of electronic
and opto-electronic devices [15]. Being a large direct band gap material, it can transmit
most of the terrestrial sunlight (85%-95%) over the complete solar spectrum.
At ambient pressure and temperature, ZnO crystallizes in the hexagonal wurtzite
structure [7] having a 6-mm symmetry as shown in figure 1.1. It has a hexagonal lattice
belonging to the space group P633mc and is characterized by two interconnecting
sublattices of 2Zn
+ and 2O
− such that each Zn ion is surrounded by oxygen tetrahedra and
vice-versa. The bulk unit cell contains two Zn cations and two O anions. The crystal can
be viewed as a sequence of O-Zn double layers, which are stacked along c-axis. In fact,
the layers occupied by zinc atoms alternate in the lattice with layers occupied by oxygen
atoms. The effective ionic charges are about 1 to 1.2, which results in a polar c-axis
(Figure 1.1). The mean lattice constants are 3.25a = Å and 5.206c = Å [7]; the values
depend slightly on the stoichiometry of the oxide composition. The Zn-O distance is
1.992 Å parallel to the c-axis and nearly similar (~1.973 Å) in the other three directions
of the tetrahedral arrangement of nearest neighbours. Extensive literature review of the
3
different properties of ZnO (primarily single crystal ZnO) has been repoted by Hirschald
and his co-workers [16].
Figure 1.1: Crystal structure of ZnO (Hexagonal structure with 6-mm symmetry) [7]
The room temperature band gap value of ZnO corresponds to a strong absorption
in the ultraviolet range (λ≤387 nm). The nature of the absorption shows that the band gap
is of direct type. In 1969, Rossler [17] first reported the bulk band structure of ZnO by
Korringa-kohn-Rostoker calculations. In 1973, Bloom and Ortenburger [18] reported an
empirical pseudopotential calculation. In 1977, Chelikowsky [19] published the first self
consistently determined bulk band structure using a non-local pseudo-potential approach.
Using the Empirical Tight Binding Model (EBTM), Ivanov and Pollman [20] evaluated
the surface electronic structure.
Stoichiometric zinc oxide has the band structure typical of an insulator material.
When excess of zinc atoms are present, which often happens, ZnO becomes an n-type
semiconductor. Accordingly, irrespective of the preparation technique used, ZnO has a
c
a
4
characteristic n-type conductivity which results from stoichiometric deviation. The n-type
character can be deduced from the sign of the Hall coefficient and the thermoelectric
power. One of the problems in the quantitative determination of non-stoichiometric factor
δ in 1ZnO δ− is to achieve uniformity of defect concentrations in the sample. The non-
stoichiometric composition arises due to the presence of excess zinc in the form of zinc
interstitial (the excess zinc atom goes to interstitial space) and/or oxygen vacancy (the
excess zinc atom go into a lattice site resulting in the formation of an oxygen vacancy) as
point defects [21-22]. Both zinc interstitial ( )iZn and oxygen vacancy ( )o
V occupied by
electron pairs can serve as donor levels giving rise to conduction electrons resulting in n-
type conductivity in ZnO following the ionization schemes:
(1.1)i i
Zn Zn e+ −→ + →
(1.2)o o
V V e+ −→ + →
In equations (1.1) and (1.2), i
Zn+ and
oV
+ are singly ionised zinc interstitial and
oxygen vacancy respectively and e− is the electron released in the above ionization
processes. Both i
Zn+ and
oV
+can act as donor states through double ionization processes
at enhanced temperatures.
The electrical behaviour of ZnO is sufficiently modified by adsorption and
desorption of oxygen species from ambient air on the surface and this is particularly
important for thin film form of the material. The adsorption process may be either
physical (physisorption) or chemical (chemisorption) in nature. Physically adsorbed
oxygen forms a surface acceptor site for a conduction electron. The process does not
involve any transfer of electron between the adsorbate and the oxide material. The
chemisorption process involves capture or trapping of free electrons (conduction
electrons) by the adsorbed oxygen species. This leads to a surface double layer that is
5
actually formed between the charge transferred to the adsorbed gas and the opposing
charge, remaining in the semiconductor. The process is therefore an electronic transfer
process and the adsorbtion process thus directly controls the carrier density.
It has been observed that different adsorbed species become activated at different
temperature ranges. For instance, the following conversion scheme for oxygen species as
the function of increasing temperature, has been suggested [23]
2O (room temperature) → 2O
− (upto 200
oC) → 2O
− (upto 400oC) → 22O
− (above 400oC)
The trapping of conduction electrons to form negatively charged oxygen species
may be represented as 2 2O e O−
+ → , 2 2O e O− −+ → etc. The chemisorbed oxygen on the
surface and at the grain boundaries thus acts as an acceptor-like trap state causing a large
reduction in the electrical conductivity of the oxide material. A surface barrier is
produced by the chemisorbed oxygen species through electron exchange with the oxide
material and the grain boundary barrier height is modulated. The overall resistance is thus
governed by the non-stoichiometric defect states (acting as donor states for conduction
electrons) and chemisorbed oxygen species (acting as acceptor states for conduction
electrons). From a practical viewpoint, the most important consequence of chemisorption
is the ability of the oxide semiconductors to catalyse gas phase chemical reactions on
their surfaces. The resistance of the material is sensitive to the coverage of adsorbed
oxygen and any factor that changes this coverage will change the resistance. For a
semiconducting metal oxide, this can result in a measurable change in the electrical
conductivity, a phenomenon that is the basis of gas sensing sensor. The sensors in
resistive mode make use of this change in resistivity occuring on interaction with the gas
molecules. On exposure to a reducing gas, the trapped electrons are returned to the
conduction band due to the interaction of the reducing gas molecules and the
chemisorbed oxygen [24]. So the resistance of the oxide material changes. This effect has
been interpreted as the mechanism of gas sensing. The reducing gas molecule itself gets
oxidized in this process.
6
1.2 Thin films of ZnO and doped ZnO
Due to its versatility, ZnO has drawn considerable attention and has been
prepared and investigated in various physical forms such as single crystals,
sintered/ceramic pellets, thick films, thin films and nanostructures etc. [6, 9-10, 14, 25-
28]. Among different physical forms, the thin films of ZnO find a multitude of
immensely important applications in electronic and optoelectronic devices such as
photothermal conversion systems, transparent conductors, gas sensors for toxic and
combustible gases and heat mirrors among many others [11-13, 24]. It is also being
considered as a potential candidate in the new frontiers of research like spintronics [27].
Thin films form the basic for many electronic components and are of particular interest
for fabrication of large area arrays. Thus most of the device applications require ZnO in
polycrystalline thin film form. Simultaneous occurrence of high optical transparency
(≥80%) in the visible region and high conductivity may be conveniently obtained by
controlling the non-stoichiometry and/or dopants. Thus the thin films have been widely
studied during the last few decades because of their technological applications,
particularly in the field of semiconductor electronics.
A thin film can be visualised as a near surface region of a material whose
properties are different from those of the bulk. In general any solid or liquid system
possesing at most two-dimensional order of periodicity may be called a thin film. Thus a
thin film is a microscopically thin layer of material that is deposited onto a substrate [29].
The substrate may be glass, mica, metal or ceramic etc. Though a thin film, in general,
has a thickness of 1.0 µm or less, in practice, the thickness of a thin film may range from
a few hundred angstroms to several microns (0.01 - 10 µm). Films typically used in thin-
film applications range from a few angstroms to 100 µm thick (the width of a human
hair). Films of the order of few nanometer thicknesses are possible to fabricate in which
case they are called ‘Ultra thin films’. Thin materials may also be formed from a liquid or
a paste, in which case it is called a ‘thick film’.
7
The physical properties of polycrystalline thin films are different from those of
bulk single crystals in the sense that they are modified by their thickness as well as the
crystallite size. The physical properties include structural properties, electrical properties,
optical properties, mechanical properties etc. The change in structural properties includes
change in lattice parameters, partuicle size or grain size, stress, strain, etc. The change in
optical properties occurs in terms of band gap and other optical constants. The change in
electrical properties refers to change in carrier density, mobility etc. Also the way the
film is prepared affects its microstructure and properties. These perturbations affect the
electrical properties much more than optical properties since the band structure is
unaltered inside the bulk. In addition to thickness and crystallite size, the lattice impurity
and other structural defects also affect the electrical properties (e.g. conductivity) of the
films [30].
For polycrystalline thin films, the electrical properties are modified by the grain
boundaries as well. For polycrystalline oxide materials, such as zinc oxide, the electrical
properties are further modified by the adsorption of oxygen at the grain boundaries and
also on the surface. These grain boundaries generally contain fairly high density of
interface states which trap carriers from the bulk of the grain and scatter free carriers by
virtue of their inherent disorders and the presence of trapped charges. The interface states
result in a space charge limited region at the grain boundaries [31] which results in
potential barriers to charge transport.
Pure zinc oxide thin films have certain limitations in their application. They are
not stable against corrosive environments and in humid ambient and lack stability in
terms of thermal edging in air [32-33]. Adsorption of oxygen in the films modifies its
electrical conductivity and also modifies the surface morphology. To stabilize the ZnO
system against such changes and also to widen the potential areas where ZnO thin films
can be applied, dopant ions have to be incorporated into them to obtain certain desired
properties like wider or narrower band gap, higher optical absorbance, lower or higher
melting point, ferromagnetism, etc. Therefore polycrystalline ZnO films have been doped
8
with metals of group I. group II, group III and group V. Accordingly doped ZnO thin
films with improved stability and suitable structural, electrical and optical properties are
in constant demand for their potential application prospects.
Polycrystalline films of ZnO have been doped to enhance their properties with
Lithium ( )Li [34], tin ( )Sn [35], cadmium (Cd) [36], manganese (Mn) [37], silver (Ag)
[38], copper (Cu) and iron (Fe) [39], gallium (Ga) [40], indium (In) [41], aluminium (Al)
[15, 42], nickel (Ni) [43], phosphorous (P) [44-45], nitrogen [46] etc. Doping with IB
acceptors (Cu, Ag, Au) reduces the emission in the UV region and intensifies in the
visible region [47]. Doping with silver, phosphorous, nitrogen etc. has been primarily
done with the objective to get stable p-type conductivity [38, 44,46]. Doping with Gr. III
metal ions (such as Al , In ) is particularly done to get high transparency, stability and
high conductivity. On the other hand doping with Fe, Cu, Co etc. has been primarily
carried out to study their magnetic properties. Copper doping also shows interesting
thermoluminescence properties [48]. Another group III metal nickel (Ni) has been doped
for ferromagnetic study.
Mn doped ZnO (Mn:ZnO) is an extremely important material for its coexisting
magnetic, semi-conducting and optical properties [37]. Mn:ZnO is regarded as promising
material for spintronic applications as it shows room temperature ferromagnetism [49]. It
has also been utilized as a material for the manufacture of solar cells, transparent
electrodes, gas sensors, varistors, piezoelectric transducers, etc, due to its behaviour as a
dilute magnetic semiconductor (DMS) [50]. However the microstructural effects of Mn
doping in ZnO thin film are not well established [37].
Cd doping has been found to impart stability and also Cd doped ZnO films are
useful humidity sensors [51]. Cadmium oxide possesses cubic structure and a narrow
direct band gap of 2.3 eV, whereas ZnO possesses is a wide band gap of 3.2 eV [52-53].
Hence, it is possible to modify the physical properties of ZnO upon mixing with CdO.
Due to variation of the bandgap with doping percentage of Cd, they can be used as an
9
excellent candidate for the preparation of quantum wells, superlattices and other
configurations that involved bandgap engineering [51]. Cd doped ZnO nanowires show a
positive temperature coefficient of resistance effect, which is quite abnormal to pure ZnO
nanowires [54]. Although cadmium doped ZnO is one of the promising candidates in the
field of optoelectronics and also for the fabrication of ZnO based devices, the knowledge
of the physical properties of Cd doped ZnO has been very limited until recent times [55].
Doping with Al is primarily done to achieve high transparency, stability, high
conductivity and also, because it enhances the gas-sensing properties of the ZnO thin
films, which have immensely important industrial and domestic applications for detecting
hazardous gases, such as LPG [42, 56]. The enhancement of bandgap energy due to Al
incorporation offers the possibility to tailor its optical property. The bandgap
enhancement is particularly significant for nanocrystalline thin films. For transparent
conducting oxide (TCO) thin films like ZnO, it is always desired to improve the electrical
conduction without affecting its excellent optical properties. As such, it is very important
to optimize the process parameters of film growth and doping levels to have a desired
enhanced device performance. Al is chosen as dopant material because of its easy and
abundant availability. AZO films have got potential applications in solar cells, solid-state
display devices, optical coatings, heaters, defrosters, chemical sensors etc. [57-58].
Accordingly, synthesis of polycrystalline AZO thin films has been widely carried out
using different techniques.
Ni doped ZnO is considered as an important II-VI diluted magnetic
semiconductor (DMS) material due to its unique magneto-electrical and magneto-
transport properties [59]. Room temperature photoluminescence has been observed for
Ni-doped ZnO films. However the mechanism of conductivity change due to Ni
incoporation is still inconclusive [60]. The microstructural and optical properties of ZnO
are very much sensitive to the method of preparation, the type and amount of dopants.
Thus synthesis and characterization of doped ZnO thin films via different techniques
have attracted considerable attention.
10
1.3 Thin films and their deposition techniques
Thin films can be synthesized by many different processes. A thin film deposition
technique involves three steps: (i) Creation of atomic/molecular/ionic species, (ii)
transport of these species through a medium, and (iii) condensation of the species on a
substrate. The growth of a thin film can take place by different modes. One such is layer-
by-layer mode. In this case thin film is formed layer by layer on the substrate. This is
followed by formation of three-dimensional nuclei. Another possible mode is direct
three-dimensional growths of discrete nuclei. Depending on whether the species has
been created by a physical or a chemical process, thin film deposition techniques can be
broadly divided into two categories: physical and chemical [29]. In physical methods the
film material is moved from a target source with some form of energy to the substrate.
Chemical film fabrication methods involve chemical reactions and the precursors are
mostly components undergoing reaction at the substrate surface or in the vicinity of the
substrate.
In physical deposition technique, film is formed by atoms directly transported
from source to the substrate through gas phase [29]. The physical routes include different
forms of sputtering and evaporation. Commercial physical deposition systems require a
low-pressure environment and are classified as Physical vapor deposition or PVD. The
different physical vapor deposition systems includes evaporation, thermal evaporation,
electron beam evaporation, sputtering, reactive PVD etc.The material to be deposited is
placed in such a way that particles of material escape from the surface. They are then
allowed to arrive on a substrate to form a solid layer. The whole system is kept in a
vacuum deposition chamber. Since particles tend to follow a straight path, films
deposited by physical means are commonly directional, rather than conformal.
In evaporation technique, the material to be deposited is evaporated and the
evaporant vapor is allowed to impinge on the surface of the substrate. The evaporant
condenses on the substrate and is absorbed on it. This is done in a high vacuum. The
11
temperature of a material for evaporation may be raised by direct or indirect heating. In
thermal evaporation technique, an electric resistance heater is used for this purpose.
Electron beam evaporation fires a high-energy electron is used to boil a small spot of
material. Molecular beam epitaxy is a particular sophisticated form of thermal
evaporation. In this technique, slow streams of an element are directed at the substrate, so
that material deposits one atomic layer at a time [29]. The beam of material can be
generated by either physical means or by a chemical reaction (chemical beam epitaxy).
Sputtering is a popular method and one of the most flexible deposition techniques
for adhering thin films onto a substrate. In this technique, energetic ions in plasma
(usually a noble gas, such as Argon) are used to knock out or eject a few atoms at a time
from a "target". The ejection process, known as sputtering, takes place as a result of
momentum transfer between the impinging ions and the atoms of the target surface. The
sputtered atoms are condensed on a substrate to form a film. Since the process is not one
of evaporation, it is particularly suitable for compound or mixtures compared to
evaporation techniques where different components would tend to evaporate at different
rates. Different versions of sputtering are used by researchers. These are direct current
(dc) sputtering, where a dc current is used, radio frequency (rf) sputtering, where a rf
current is used and dc magnetron sputtering where a magnetic field is also applied. The
magnetic field is applied to confine the path of the ions.
In dc sputtering, a dc voltage is applied between the cathode (target) and anode
(on which the substrate is placed). The sputtered atoms reach the substrate with
randomized direction and energies due to collisions with gas atoms. If the cathode is an
insulator material, dc sputtering is not possible owing to building up of positive surface
charges. A high frequency rf field is applied in this case. Arrangements in which the
applied field is perpendicular to each of the electric and magnetic field is termed as
magnetron sputtering [29]. In reactive sputtering, a small amount of some non-noble gas
such as oxygen or nitrogen is mixed with the plasma-forming gas. After the material is
sputtered from the target, it reacts with this gas, so that the deposited film is a different
12
material, i.e. an oxide or nitride of the target material. Pulsed Laser deposition systems
work by an ablation process. Pulses of focused laser light vaporize the surface of the
target material and convert it to plasma.
Some methods fall outside these two categories i.e. physical and chemical
techniques. These are Chemical vapor deposition (CVD), Oxidation, and Plating etc. In
fact, reactive evaporation and sputtering is also referred to as hybrid techniques where
PVD and CVD are combined. In CVD, the film is formed through chemical reaction on
the surface of substrate followed by surface absorption. The technique generally uses a
gas-phase precursor, often a halide or hydride of the element to be deposited. The
reactive gas is introduced into the chamber and is allowed to decompose by heat or
plasma. The decomposition requires 800-1300oC [29]. The different CVD techniques are
Low-Pressure CVD (LPCVD), Plasma-Enhanced CVD (PECVD), Atmosphere-Pressure
CVD (APCVD) and Metal-Organic CVD (MOCVD)
In the case of MOCVD, an organometallic gas is used. PECVD uses an ionized
vapor (or plasma) as a precursor. The ionized plasma is used to transfer energy to the
reacting gases resulting in decomposition. Commercial PECVD relies on electromagnetic
means (electric current, microwave excitation), rather than a chemical reaction, to
produce plasma [29].0 Plating relies on liquid precursors, often a solution of water with a
salt of the metal to be deposited. Some plating processes are driven entirely by reagents
in the solution (usually for noble metals), but by far the most commercially important
process is electroplating.
Conventional physical rotes (vacuum techniques such as sputtering and
evaporation) renders better control over stoichiometry produces uniform and compact
films and generally produces good quality transparent films. They are generally safe (no
toxic gas emissions) and performs high deposition rate at room temperature. However
they require expensive capital instruments. Accordingly they are difficult to expand to
large scale. Chemical techniques of thin film deposition involving aqueous solution on
13
the other hand are cost-effective compared with vapor-phase techniques and simple. Thus
they offer the desirable cheapness and possibility of scaling up to industrial level.
Accordingly, chemical techniques have come out to be a good alternative for material
preparation during the past few decades. In chemical deposition techniques, a liquid
precursor undergoes a chemical change at a solid surface, leaving a solid layer. Thin
films from chemical deposition techniques tend to be conformal, rather than directional.
The different category of chemical deposition techniques include: Spray pyrolysis, Sol
gel, Chemical Bath deposition (CBD), Electroless deposition, Electrodeposition,
Anodization, Electrophoresis and SILAR etc.
Spray pyrolysis is a method of depositing films having thicknesses in the region
between thin film and thick film. Film deposition is carried out by spraying a solution
containing soluble salts of the constituent atoms of the desired compounds onto a
substrate. The substrate is maintained at elevated temperatures (typically 300-700°C).
The sprayed droplet reaches the hot substrate and undergoes pyrolytic (endothermic)
decomposition and forms a single crystallite or a cluster of crystallites of the product. The
thermal energy required for decomposition is provided by the hot substrate. The other
volatile by-products and the excess solvent escape in the vapor phase. Post deposition
sintering helps to the recombination of the constituent species and clustering. Finally a
coherent film is obtained. Several parameters affecting the deposition mechanism and
film properties are solution concentration, solution flow rate (spray rate), substrate
temperature, nature of the substrate, sprayer tip to substrate distance etc. Spray deposited
films generally have a rough microstructure. Oxides, sulphides and selenides are prepared
by this technique.
Electroless is a process typically used to obtain thick (micrometers) metal
structures on metallic or nonmetallic substrates. The process offers simplicity and
cheapness. In this method, the substrate on which the film to be deposited is chemically
activated and introduced in a solution containing a reducible form of the ion of the
desired metal. The ions are reduced at the substrate surface and the insoluble metal atoms
14
are incorporated into the surface. The process is often referred to as autocatalytic coating
technique. The occurrence of chemical changes owing to the passage of electric current
through an electrolyte is termed electrolysis. The deposition of any substance on an
electrode as a consequence of electrolysis is called electrodeposition. The phenomenon
of electrolysis is governed by Faraday’s laws [29].
In anodization technique, the metal to be anodized is made an anode and
immersed in an oxygen-containing electrolyte. The electrolyte may be aqueous,
nonaqueous or fused salt. The pH of the electrolyte plays an important role in obtaining a
coherent film. Growth may take place at constant voltage or at constant current. In
electrophoresis technique, electrically charged particles suspended in a liquid medium
are deposited on an electrode. The as-deposited films are loosely adherent coatings of
powder. Further post deposition treatment leads to adherent, compact and mechanically
strong surface coating. The electrophoresis technique is used to the deposition of both
conductors and nonconductors, including metals, alloys, salts, oxides, polymers etc. Sol-
gel method gained much interest because of its simplicity, low processing temperature,
stoichiometry control and its ability to produce uniform, chemically homogenous films
over large areas that can provide integration with other circuit elements. The substrate is
dipped in sol and gel to get the film [29].
In chemical bath deposition (CBD) method, deposition of thin films occurs due
to substrate maintained in contact with dilute chemical bath containing cationic and
anionic solutions [29, 61]. The film formation on substrate takes place when ionic
product (IP) exceeds solubility product (SP). However this results into precipitate
formation in the bulk of the solution which cannot be eliminated.
According to the solubility product principle, in a saturated solution of a weakly
soluble compound, the product of the molar concentrations of its ions (each concentration
term being raised to a power equal to the number of ions of that kind) is called the ionic
product. This is a constant at a given temperature and this constant is called the solubility
15
product. When the solution is saturated i.e. at equilibrium, IP SP= . But when the ionic
product exceeds the solubility product (in a supersaturated solution) i.e. SPIP⟩ ,
precipitation occurs [29, 62]. Under this situation ions combine on the substrate to form
nuclei. The kinetics of growth of a thin film in this process is determined by the ion-by-
ion deposition on the immersed surfaces. Initially, the film growth rate is negligible
because an incubation period is required for the formation of critical nuclei from a
homogeneous system onto a clean surface. Once nucleation occurs, the rate rises rapidly
until the rate of deposition equals rate of dissolution i.e. IP SP= . Consequently, the film
attains a terminal thickness. It seems that precipitate formation and wastage of material is
a common problem in CBD since ions combine to form nuclei in the solution also.
It is seen that different techniques ranging from simple to sophisticate ones has
been used to dope ZnO. Among the various chemical techniques, Spray pyrolysis is a
high temperature process and choice of suitable precursor solution is often not
convenient. Sol-gel is considered to be superior to most other conventional chemical thin
film deposition techniques for fabricating stoichiometric polycrystalline and uniformly
doped semiconducting thin films. However it requires costly precursors or sophisticated
reaction setups and the choice of the solvent is also often not convenient. Electroless
deposition is characterized with poor coverage whereas precipitate formation and wastage
of material is a common problem in chemical bath deposition (CBD). A relatively less
utilized and less investigate chemical technique is SILAR (Successive ionic layer
adsorption and reaction or Successive Ion layer adsorption and recation) [62-63]. In this
technique, thin films are obtained by immersing the substrate into separately placed
cationic and anionic precursors. Thus precipitate formation and wastage of material is
avoided in this technique. Accordingly, SILAR is often termed as modified version of
chemical bath deposition (modified CBD). Film formation takes place when SPIP⟩ .
Temperature, solvent, and particle size affect the solubility product [64]. The rate at
which nuclei forms on the substrate surface depends on the degree of supersaturation (S)
which is the ratio of IP and SP.
16
1.4 The technique of SILAR
The most used solution technique and also one of the oldest methods for thin film
growth is chemical bath deposition (CBD), sometimes called chemical deposition (CD),
or chemical solution deposition (CSD). CBD has been widely used for the deposition of
metal sulphides for various applications [65]. In CBD all the precursor ions (cations and
anions) are present at the same time in the reaction vessel. Typically CBD has a so-called
terminal thickness indicating a point where the growth of thin film is stopped due to
depletion of precursors in the solution. The film formation on substrate takes place when
ionic product exceeds solubility product. This results into precipitate formation in the
bulk of solution, which can not be eliminated. This causes the unnecessary formation of
precipitation and loss of material.
In SILAR, on the other hand, thin films are obtained by immersing the substrate
into separately placed cationic and anionic precursors for reaction at chosen temperatures.
Between every immersion the substrate is rinsed in distilled water or ion exchanged water
to avoid homogeneous precipitation in the solution. Sequential reaction on the substrate
surface under optimized conditions of concentration and pH of the reacting solutions
results in the formation of the film. Thus, precipitation formation i.e wastage of material
is avoided in SILAR method.
The SILAR method is a relatively less used and less investigated method. The
method was initially reported by Call et. al. [66] as chemical deposition method. It was
then used for copper oxide film deposition by Ristov et. al. [67]. The name SILAR was
ascribed to this method by Nicolau [68] since it involves ion-by-ion deposition and
discussed in subsequent papers of Nicolau and co-workers [69-70] and Ristov et. al. [71],
which deals with ZnS, CdZnS, CdS and ZnS thin films. Later on the technique has been
extended by many workers primarily to deposit sulphide thin films (ZnS, CdS, PbS, CuS,
MnS etc. and their doped versions) [62, 72-76] and not much effort has been made to
deposit oxide thin films and their doped version by this technique.
17
The basic building blocks in SILAR are ions instead of atoms. The substrate can
be introduced into various reactants for a specific length of time depending on the nature
and kinetics of the reaction. The immersion-reaction cycle can be repeated for any
number of times, limited only by the inherent problems associated with the deposition
technique and the substrate-thin film interface. The technique is called SILAR since it
involves adsorption of a layer of complex ion on the substrate followed by reaction of the
adsorbed ion layer. The different parameters that can affect the film growth process are
the nature of the bath solution, concentration of the bath solution and its pH value, nature
of the substrate and temperature of deposition. By proper optimisation of the deposition
parameters, good quality film can be achieved. Apart from being a relatively less studied
and less used process, it is an extremely simple to carry out. The thickness can be easily
controlled easily by varying the number of deposition cycles and thus both thin and thick
films can be prepared by this method.
In spite of its simplicity SILAR has number of advantages [62]: (i) unlike vapour
deposition method, SILAR does not require vacuum at any stage; (ii) The deposition can
be carried out at or close to room temperature; (iii) unlike high power methods such as
radio frequency magnetron sputtering, it does not cause local over heating that can be
detrimental for materials to be deposited and (iv) there are virtually no restrictions on
substrate material, dimensions or its surface profile. Thus, any insoluble surface to which
the solution has free access will be a suitable substrate for the deposition making the
technique convenient for large area deposition.
Adsorption of a substance on the surface of another substance is the basis of
SILAR [62]. Adsorption may be expected when two heterogeneous phases are brought
into contact with each other. In SILAR method, one is only concerned with adsorption in
liquid-solid sytem. The adsorption is a surface phenomenon between ions and surface of
the substrate and is possible due to attraction force between ions in the solution and
surface of the substrate. These forces may be cohesive or van-der-waals or chemical
attractive forces. Atoms or molecules of substrate surface are not surrounded by atoms or
18
molecules of their kind on all sides. So, they posses unbalanced or residual force and thus
can hold ions on them. The factors like concentration nd temperature of solution,
pressure, type of the substrate, area of the substrate etc. affect the adsorption process. The
reaction between pre-adsorbed cations and newly adsorbed anions forms the thin films of
desired material [62]. Figure 1.2 schematically presents SILAR growth.
Figure 1.2: Schematic of SILAR method [62]
In the first step of SILAR process, the cations present in the precursor solution are
adsorbed on the surface of the substrate (step a: adsorption of cations). In the next step,
excess adsorbed ions are rinsed away from the diffusion layer (step b: water rinsing of
loosely bound cations in deionized or distilled water). In the third step, the anions from
anionic precursor solution are introduced to the system (step c: adsorption of anions and
reaction of pre-absorbed cation with newly absorbed anion). In the last step of a SILAR
deposition cycle, the excess and unreacted species and the reaction byproduct from the
diffusion layer are removed (step d). This gives a material composed of two layers: the
inner (positively charged cations) and outer (negatively charged anions) layers. These
two layers form the Helmholtz electric double layer [62]. The substrate can be introduced
19
into various beakers containing the ionic precursors for a specific interval, withdrawn and
reintroduced into another beaker for reaction. By repeating these cycles a thin layer of a
material can be grown.
The maximum increase in the film thickness for one complete reaction cycle
(dipping cycle) is theoretically one monolayer. Dividing the measured overall film
thickness, by number of reaction cycles a numerical value of growth rate can be
determined [62]. A homogenous precipitation in the solution can result if the measured
growth rate exceeds the lattice constant of the material. In practice, however, the
thickness increase is typically less than or greater than a monolayer. The factors affecting
the growth phenomena are the quality of the precursor solutions, their pH and
concentrations, counter ions, individual rinsing and dipping times. In addition,
complexing agent and pretreatment of the substrate have been shown to affect the SILAR
growth.
It appears from the above discussion that SILAR is based on sequential reaction
of cations and anions at the substrate surface. Rinsing follows each reaction, which
enables heterogeneous reaction between the solid phase and the solvated ions in the
solution. If the anionic bath is water, SILAR reduce to a two-step process.
20
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24
CHAPTER 2
Literature review and Aim of the work
2.1 Review of Literature
Zinc oxide, being a very old material, has been prepared and characterized
thoroughly for their different properties both in undoped and doped form. Consequently
an extensive literature is available on this material. To date, a large number of
conventional as well as novel techniques of thin film deposition have been employed to
deposit thin films of ZnO and doped ZnO. The techniques used are several variants of the
evaporation and sputtering processes, LASER assisted techniques, different types of
chemical vapour deposition, spray pyrolysis, sol-gel, spin coating etc. This chapter
contains a brief survey of literature on different preparation techniques that has been
applied to deposit ZnO and doped ZnO thin films and their properties with particular
reference to structural properties such as preferred orientation, paticle size etc. The
morphological, electrical and optical properties of the synthesized films have also been
addressed in brief.
Almost all the usual preparation techniques used for thin film preparation
produces polycrystalline ZnO films. Earlier, vapour phase and reactive sputtering method
were the most applied techniques for the preparation of thin films of ZnO [1]. Good
25
conducting transparent ZnO was first fabricated in 1979 [2] using an enhanced reactive
evaporation technique for solar cell applications. Since then the technique has been
utilised [3-5] to prepare ZnO films. The films have been prepared by direct evaporation
of zinc oxide source or from a source of metallic zinc by reactive evaporation. Electron
beam evaporation technique has been utilized to deposit conducting and transparent films
of ZnO [6-7]. The resistivity and the rate of deposition have been found to depend on
oxygen partial pressure, input power and substrate temperature.
Sputtering is the most commonly used technique usd to obtain uniform films with
good orientation. Different sputtering techniques have been commonly used for the
deposition of ZnO thin film with substrate temperature ranging from room temperature to
500oC [8-27]. Apart from glass or corning glass substrate, sapphire, silicon (100),
alumina, quartz and metal substrates has been used in these works. Non-stoichiometric
ZnO films can be prepared by sputtering either a metallic zinc target in the presence of an
oxygen-argon atmosphere, or from an oxide target, usually in a gas mixture of hydrogen
and argon. For sputtered films, the structural properties and growth rates are strongly
influenced by various processing conditions, such as the gas phase composition, sputter
gas pressure (high pressure yields more porous layer and low pressure induces columnar
growth), plasma condition, sputter power (which influences actual deposition temperature
and diffusion at the surface during deposition), substrate type (which affects initial
growth and thus determines to a great extent the structure of the film) and finally, the
deposition geometry. Other than the deposition parameters, postdeposition treatments
also greatly affect the properties of the films. In general, crystallinity improves with
increase in substrate temperature. Further, if the distance between the substrate and the
target is less than the mean free path of the zinc atoms, films of better texture are
observed.
ZnO films have been grown using several variants of chemical vapour deposition
technique for piezoelectric, electro-optic and guided wave device applications [28-42]. In
conventional CVD systems hydrogen is normally used as the reducing gas for the ZnO
26
source, but these workers used NH3 in order to slow down the growth rate and improve
the film quality. The parameters governing the film growth rate in CVD process are
source temperature, substrate temperature, flow rate of the carrier gas and the distance
between source and the substrate. Natsume et. al. [34] deposited ZnO film by normal
CVD method with substrate temperature between 500-600oC. To obtain highly oriented
film growth at low substrate temperature, a variety of deposition methods have been
used. Shimizu et. al. [28] grew the film in the temperature range 150-350oC by plasma
enhanced MOCVD (Metal Oxide Chemical Vapour Deposition) using diethylzinc as the
source material. The adhesion of these films on sapphire substrate was stronger than that
on glass substrate. The same group [32] also reported photo-induced MOCVD growth of
ZnO film using a high pressure mercury lamp or xenon-mercury lamp as the light source.
Solanki and Collins [29] adopted a laser assisted MOCVD growth process using excimer
laser source. This method has got certain significant features e. g., lowering of substrate
temperature, selective deposition, high deposition rate and better crystal surface quality.
The properties of these films compared well with those of the films grown by the
MOCVD process. Khan and O’brian [31] used anhydrous zinc acetate for the growth of
ZnO in a low-pressure MOCVD method in the temperature range 350-420oC. The use
of zinc acetate as precursor for ZnO film deposition was first reported by them [31]. The
CVD technique has also been exploited to grow transparent conducting ZnO films. The
growth rate in this process was controlled by a complex multistep oxidation process that
was dominated by chemical reactions among free radicals.
Atomic layer epitaxy (ALE) and molecular beam epitaxy techniques have been
used to deposite the ZnO film [43-44]. Laser assisted techniques [45-50] has been
successfully utilised to deposit ZnO layer at atmospheric pressure. Lanno et. al. [45]
deposited the film by using pulsed laser e.g., a Q-switched Nd: YAG laser and a KrF
excimer laser. PLD (Pulsed Laser Deposition) method does not demand any further post-
deposition annealing treatment of the grown films and have been utilized to grow ZnO
thin film [49-50].
27
A number of cost effective chemical techniques also have been utilized to deposit
ZnO thin film. Transparent conducting films of zinc oxide have been successfully
prepared using spray pyrolysis technique [51 - 65]. This is a very useful and simple
technique for the preparation of different semiconducting thin films. It has been found to
be suitable for the preparation of ZnO films particularly for solar cell and gas sensor
applications. In this technique, an aqueous solution of zinc acetate is usually used as the
spray solution. This precursor is selected due to its high vapour pressure at relatively low
temperatures. The addition of a few drops of acetic acid prohibits the precipitation of zinc
hydroxide thereby making the solution suitable for spraying. This helps in producing
better quality optically transparent films. In this technique, chemicals in the form of
atomized droplets are brought in contact with the pre-heated substrate whose temperature
normally varies between 150-550oC. The deposition parameters that control the quality of
the film in this technique are substrate temperature, solution flow rate, air (carrier gas)
flow rate etc. Rol of substrate temperature on strucrural and morphological properties has
been reported by Gumus et. al. [62]. Wu et. al. [52] applied spray pyrolysis technique
using ultrasonic nebulization of zinc acetate solution. They obtained ZnO film on silicon
and silica substrates at 380oC. The spray pyrolysis process allows the coating of large
surface and it is compatible with mass production systems [61].
Dipping of the substrate in a liquid containing the metal ions followed by heat
treatment is one of the simplest methods of film preparation. The liquid can be either a
simple solution or sol or gel. Sol-gel has been widely used to synthesize ZnO thin films
[66-73]. Later, the technique has been used by many others [69-73] due to its simplicity
and non-requirement of any special apparatus. Ghodsi et. al. [73] used zinc acetate to
prepare ZnO films on glass substrate while Gupta et. al. [71] synthesized films on
conducting glass support (SnO2:F overlayer).
The chemical bath deposition (CBD) technique is an open-bath wet-chemical
method that has been widely employed for the synthesis of metal oxide thin films [74-
78]. A two-step modified CBD technique has been reported by Vijayan et. al. [77] to get
28
good quality adherent thin films of ZnO. On the other hand, a much simpler process is
chemical dipping in which the film is prepared from a simple aqueous solution instead of
a non-aqueous medium used in the sol-gel method. ZnO films by chemical deposition
were first obtained as a byproduct in an attempt to prepare the composite CdxZn1-xS film
by Call et. al. [79]. A precipitate of zinc hydroxide was formed by adding sodium
hydroxide to zinc sulphate. Heating the solution to 80-90oC initiated ZnO deposition.
The name SILAR was ascribed to this method by Nicolau et. al. [80] with the name
SILAR since it involves adsorption of a layer of complex ion on the substrate followed
by reaction of the adsorbed ion layer. Lupan et. al. [81] synthesized ZnO films using
sodium zincate bath as cationic precursor and named the technique succesisive chemical
solution deposition (SCSD). Recently Raidou et.al. [82] reported synthesis of ZnO thin
films by SILAR using ammonium zincate as cationic precursor. Different parameters
governing the film growth by SILAR includes choice of suitable metal salt complex as
precursor, temeprature of deposition, and concentration and pH of the reactant solution
among many others.
Another simple process, electroless deposition technique has also been utilised to
grow ZnO film [83-84]. In this technique, controlled homogeneous precipitate of metal
hydroxide is obtained by slow reaction on the surface of the substrate and the
corresponding oxide is obtained by post deposition heat treatment. Synthesis of ZnO
films at as low as 65oC has been reported using electrodeposition on ITO (In-coated tin
oxide) substrate [85]. Lupan et. al. reported preparation of ZnO thin films by
electrochemical deposition [86].
To conclude the discussion on the film deposition techniques, it may be noted that
various methods, ranging from a simple to a sophisticated one, have been utilised by
researchers to obtain ZnO film. The properties of the resulting films depend markedly on
the deposition parameters of each technique. As ZnO films have a large number of
commercial applications, the requirements of each application guide one to choose a
particular method over others.
29
To characterise the thin films of ZnO with respect to different growth and
processing conditions, X-ray diffraction analysis is normally performed. The most
important planes of ZnO with hexagonal wurtzite structures are (100), (101), (002) and
(110) [79]. It has been observed that with the increase of substrate or growth temperature,
the intensity of (002) peak perpendicular to the substrate increases at the expense of (100)
peak together with the increase of crystallite size i.e. crystallinity of the films. The effect
is almost universal irrespective of the preparation technique used. Such preferred
orientation is one of the most important properties of thin films. Together with the film
thickness and the microstructure, it is very important to control the orientation of the
crystallites in the films particularly for some of the device applications.
Pronounced (002) orientation of the crystallites was observed for ZnO films
prepared by activated reactive evaporation [5]. Other peaks of (101) and (102) were
present with small intensities. For sputtered films, the development of c-axis preferred
orientation was found to depend on the sputtering conditions. Films deposited from pure
metallic Zn and ceramic ZnO targets using dc magnetron sputtering also shows (002)
texture [21]. Films synthesized by rf sputtering on both glass and silicon
[ (001)]Si substrate [22, 25] shows preferred growth along c-axis. Strongly c-axis oriented
ZnO films on either Si/SiO2 or Si substrate has been reported using rf magnetron
sputtering [19]. The films exhibited a sharp fundamental absorption edge with a band gap
width of 3.31 eV [19]. Lee et. al. [8] also reported strong c-axis orientation in rf
magnetron sputtered film. They further reported that the strong c-axis orientation is a
function of film thickness. Atom beam sputtered films has been reported to have
preferred (002) growth [24]. Effect of surface roughness on the c-axis preferred
orientation for ZnO films deposited by rf magnetron sputtering has been discussed by Lee
et. al. [18]. Although Major et. al. [9] observed that orientation is independent of film
thickness, a decrease in preferred orientation was observed with increasing film thickness
for sputtered films [8]. Sputtered films are polycrystalline with grain size depending on
the substrate temperature. Higher substrate temperature results in larger grain sizes. Grain
size distribution in sputtered films is nearly monomodal. Jeong et. al. [20] reported
30
excellent transmittance (~95%) in the visible region for sputtered films. Hezam et. al.
[26] also reported high transmittance of ~85% in the visible region with an optical band
gap of 3.28 eV. The films are highly c-axis oriented with nearly spherical grains [26].
Suchea et. al. reported grain dimension in the range 10-50 nm for dc magnetron sputtered
ZnO film [21].
Natsume et. al. [34] obtained highly c-axis (002) oriented film produced by CVD
at a temperature range of 500-600oC. Maruyama and Shionoya [36] deposited c-axis
oriented ZnO films by a CVD process at 180oC. Shimizu et. al. [32] utilised plasma
enhanced MOCVD (PEMOCVD) technique to produce c-axis oriented ZnO films in the
temperature range 150-300oC on glass substrate and epitaxial film on sapphire
substrate.Different variants of CVD has been reported to grow preferred c-axis oriented
films [39-42]. Films synthesized by PLD [45] demonstrated how the preferential c-axis
oriented growth could be achieved by optimising the laser wavelength, fluence and
substrate temperature. Using PLD, Kotlyarchuk et. al. [49] repoted strongly oriented
grains in the basal plane direction and were grown along c-axis. Films had good
adherence with an optical transparency of ~85%. Average particle size of 55nm was
obtained from measured FWHM (Full width at half maximum intensity) value of 0.16o
[49].
XRD studies of spray pyrolytically deposited films [87] indicates that at
deposition temperatures less than 300oC, (101) and (100) are the most dominant
orientations. The (002) plane is present at these temperatures with significantly less
intensity. However, at an increased temperature (300oC), the (002) orientation becomes
progressively more important and the intensity of the (101) and (100) peaks start
decreasing. Film morphology is very sensitive to substrate temperature and thickness for
spray deposited films [87]. Uniform high quality ZnO thin films with high c-axis
orientation have been reported using spray pyrolysis [53, 58-60]. Highly transparent
polycrystalline ZnO thin film with a direct band gap of 3.27 eV has been reported using
spray pyrolysis [61]. The FWHM value for (002) peak was as low as ~0.23o and the
31
average particle size was 40 nm in this work [61]. Requirement of specific preparation
conditions and working temperature to obtain films having (100) or (002) orientation
using ultrasonic spray pyrolysis (USP) technique has been reported [62]. Ayouchi et. al.
[58] reported a dercease in bandgap from 3.33 eV to 3.31 eV as the substrate temperature
is increased. The resistivity of the films also increases and reaches maximum for 553K
substrate temperature. Selim et. al. [59] reported preferred c-axis orientation and the
polycrystalline film contained with needle like particles.
Although Okamura et. al. [88] reported non-oriented polycrystalline films
obtained using sol-gel, the technique has been successfully utilised to obtain highly c-axis
oriented ZnO films [67-68, 72]. Particle size of 40 nm has been repoted for sol-gel films
by Nirmala et. al. [72]. Using solgel technique, Khan et. al. reported particle size of ~20
nm [89] and Ilican et. al. [70] reported round grains with average particle size of 50 nm.
Crystallite size in the range 20-33 nm has been reported for spray pyrolysed film
[90] while particle size of 19.06 nm has been reported for spin-coated ZnO films [91]. In
all these reports evaluation of particle size was made using Scherrer relation according to
which the grain size is directly related to the full width at half maximum (FWHM)
intensity of X-ray diffraction peaky [92].
Polycrystalline hexagonal nano film with (002) preferred orientation has been
reported with a band gap of 3.24-3.27 eV and refractive index 2.29-2.34 using CBD
technique [77]. Electrodeposited films [85] has been found to give (101) as highest
orientation instead of (002) possibly due to low synthesis temperature. Call et. al. [79]
also obtained of ZnO with a slight preferred orientation toward (100). The film was
synthesized at 80-90oC. It has been observed that the direct band gap energy was
increased from 3.23 to 3.37 eV after annealing at 300oC. Electroless deposition also has
been reporte to give a band gap of 3 eV and transmittance of 85% [84].
32
It appears from the above discussion that c-axis orientation is a common
phenomenon in the ZnO film deposition by both physical and chemical processes. Such
preferred basal orientation is typically observed ZnO film since the surface energy
density of the (002) orientation is the lowest in hexagonal wurtzite ZnO structure [93].
Quantitative information concerning the preferential crystal orientation can be obtained
from the texture coefficient (TC) [94-95]. If TC(hkl)≈1 for all the (hkl) planes considered,
then the films are with a randomly oriented crystallite similar to the JCPDS reference. As
texture coefficient for a particular plane ( )hkl increases, this indicates that the preferential
growth of the crystallites in the direction of ( )hkl plane increases.
Reports on electrical measurements show wide scatter in resistivity value. Highly
pure (Stoichiometric) ZnO have very high resistance as expected. Deviation from
stoichiomtery produces zinc interstial and/or oxygen vacancy as donor states [77].
Electrons formed by the ionization of zinc atoms and/or oxygen vacancies directly
controls the number of charge carriers (electrons) available for conduction [77]. The film
stoichiometry depends on the deposition method used, deposition parameters, post
deposition treatments etc. The amount of nonstoichiometry controls the resistivity of ZnO
[17, 21]. Thus low resistive ZnO can be prepared either by adjusting the film
stoichiomtery or by doping [12]. Doping with appropriate metal atoms, such as, Al , Sn,
Ga, In, etc., the resistivity can be changed from values as high as 1010 Ω-cm to values as
low as 410− Ω-cm [21]. Jeong et. al. [20] reported highest resistivity of the order of 1410
Ω-cm for undoepd ZnO polycrystalline thin film while Al doping reduces it to the order
of 410− Ω-cm. The wide range of conductivities and conductivity changes make ZnO
films suitable for resistive mode gas sensors [21].
The morphology of the deposited films are influenced by many factors such as
deposition technique and deposition parameters, nature and temperature of substrate, post
depsotion treatments etc. Sputtered films have been reported to exhibit columnar
microstructure [8, 19] as well as spherical/nanogranular microstructure [21, 26]
33
depending on the dposition parameters. Morphology of films obtained by chemical routes
generally shows round or spherical shped grains, off spherical grains or bean like grains
[57-58, 72, 121, 128].
The first successful attempt to prepare Cd doped ZnO film by chemical technique,
to the best of our knowledge was made by Yogeeswaran et. al. [96] through combustion
synthesis of cadmium chloride ( )2CdCl in presence of zinc nitrate and urea. Cd-doping
resulted in band gap shrinkage [96] compared to pure ZnO and improved
photoelectrochemical response over the wavelength range ~300 to ~450 nm. Diffuse
reflectance spectroscopy showed the optical band gap of ZnO to shrink from 3.14 to 3.07
eV on Cd doping. Cd-doped ZnO nanowires exhibit a positive temperature coefficient of
resistance effect [97], which is quite abnormal as compared to pure ZnO. Further this can
be an excellent sensor material at room temperature. Although cadmium doped ZnO is
one of the promising candidates in the field of optoelectronics and also for the fabrication
of ZnO based devices [63], the knowledge of the physical properties of Cd doped ZnO
has been very limited until recent times. Cadmium oxide possesses cubic structure and a
narrow direct band gap of 2.3 eV, whereas ZnO possesses is a wide band gap of 3.2 eV
[98-99]. Hence, it is possible to modify the physical properties of ZnO upon mixing with
CdO. In recent times there are reports of Cd doped ZnO either in thin film or
nanostructured form with varying amount of Cd incorporation [63, 96, 100-105].
Substitution of zinc ion by isoelectronic element cadmium has been reported by
complicated physical processes such as pulsed laser deposition (PLD) [101], metal-
organic vapor phase epitaxy (MOVPE) [102], vapor-liquid-solid (VLS) [103]. Reports of
cadmium doped zinc oxide synthesized through chemical routes are relatively rare.
Among the chemical techniques, sol-gel [100, 104-105] and spray pyrolysis [63] has been
employed to deposit Cd doped ZnO films. The only effort to synthesize Cd doped ZnO
films by SILAR turned out to be unsuccessful [106]. The change in fundmental
absorption edge with Cd incorporation and cecrease of band gap with Cd doping has been
reported in these works [63, 105]. The optical constants of the films such as refractive
index, extinction coefficient and dielectric constants also changed with Cd doping and the
34
electrical conductivity of the films was improved by incorporation of Cd in the ZnO film
[105]. Scanning electron microscopy (SEM) images indicated that the films have a
wrinkle network with uniform size distributions [105]. Enhancement of polycrystallinity,
decrease of grain orientation in the (002) axis with an increase in the FWHM value
(indication of smaller grains) has been reported for spray pyrolysed films [63]. Alongwith
grain size, surface roughness value also decreased. The decrease in surface roughness
upon increasing the Cd concentration can be attributed to the polycrystallization of the
films which was confirmed by XRD analysis [63]. Undoped ZnO possesses larger grains
with offspherical shape. A substantial red shift of the band gap value was reported by PL
and optical transmittance measurements which can be interpreted in terms of band gap
modulation due to Cd doping. The electrical resistivity measurements show that the sheet
resistance of the films decreases for higher Cd concentrations, which is attributed to the
low resistance value of CdO [63]. Lowering of particle size due to Cd doping has been
attributed to strain developed in the material [104]. Decrease in band gap was also
reported in this work. The microstructure was found to compose of high density closely
packed nano/submicro rods over a large area [104].
The transition metal Manganese (Mn) has been doped primarily for its coexisting
magnetic, semi-conducting and optical properties [107]. Thus Mn doped ZnO has been
synthesized primarily to study its ferromagnetic behavior [108-109]. Due to its unique
magneto-optical, magneto-electrical, and magneto-transport properties, it is also
considered as a dilute magnetic semiconductor (DMS) material. DMS materials are
essential for future-generation spintronic device applications [110]. Various physical and
chemical techniques that has been used to deposit Mn doped ZnO thin films and
nanofilms includes atomic layer deposition (ALD) [111], sol-gel [72, 112-113], metal
organic chemical vapour deposition (MOCVD) [36], ion implantation [114], pulsed laser
deposition (PLD) [115] and solid state sintering [116] etc. While attempt to synthesize Cd
doped ZnO thin film by SILAR has remained an unsuccessful attempt [106], those on Mn
doped ZnO is nonexisting. Also the microstructural effects of Mn doping in ZnO thin
film are not well established [107].
35
Although Karamat et. al. [117] reported an increase in band gap energy with
increasing Mn incorporation (the reason was attributed to Burstein–Moss shift), most of
the workers have reported a decrease in band gap energy with increasing Mn content [72,
118-120]. The decrease in band gap value with increased Mn doping concentration has
been accounted due to the sp-d exchange interactions and has been theoretically
explained using the second–order perturbation theory [72, 119-120]. A decrease in band
gap energy from 3.27 eV for undoped ZnO to 2.78 eV for 3% Mn doped ZnO has been
reported by Senthilkumar et al [119] and has been attributed to s-d and p-d interactions
giving rise to band gap bowing. A decrease in transmittance in the visible due to Mn
doping has been reported for sol-gel films [72]. The crystalline nature (high c-axis
orientation) was found to be effectd by doping, by which more impurities were included
in the ZnO crystal [72]. The SEM images of ZnO resemble a granular surface. The
incorporation of Mn ions changed surface morphology to a wrinkle network [72]. Mn
doping was found to reduce the particle size from 40 nm to ~20 nm [72]. Similar effect
has been reported in Mn doped ZnO nanoparticles [121]. Microstructure consisting of
spherical nanoparticles and nanorods with wrinkle structure has also been reported by
Srinivasan et al [122]. Iintroduction of impurity level in the bandgap due to Mn
incorporation was reported by Wang et. al. [123] for Mn: ZnO nanocrystals. Hindrance of
grain growth due to Mn incorporation and decrease in band gap has been reported [119].
For transparent conducting oxide (TCO) thin films like ZnO, it is always desired
to improve the electrical conduction without affecting its excellent optical properties. The
interest in Al doped ZnO (AZO) films is primarily to explore the possibility of tailoring
its electrical and optical properties. Al is chosen as dopant material because of its easy
and abundant availability. Accordingly, synthesis of polycrystalline and nanocrystalline
AZO thin films has been widely carried out using different techniques such as ultrasonic
chemical vapor deposition [35], spray pyrolysis [60, 64], pulsed laser deposition [124 -
125], RF magnetron sputtering [8, 21, 23, 126], helicon-wave excited plasma (HWP)
deposition [127], electroless deposition [83], sol-gel [69, 73], pulsed laser ablation [46],
chemical beam deposition [74] and SILAR [128-129] among many others. For SILAR
36
deposited films, sodium zincate bath has been used as cationic precursor which always
introduces the possibility of highly mobile sodium ions in the deposited films that can be
detrimental for their practical applications.
The enhancement of band gap [60, 83, 73, 126], lowering of resistivity [60, 73,
126-130] and increase of optical transparency [60, 127] for Aluminum incorporation has
been reported in these works. The bandgap enhancement is particularly significant for
nanocrystalline thin films. The enhancement of band gap due to Al incorporation has
been explained by Burstein-Moss effect. Normally the optimum incorporation of
aluminium has been reported to be around 1-3 at. % [73, 124, 126]. Reports on the effect
of Al doping on microstructual properties such as particle size, crystallinity, c-axis
orientation, surface roughness etc. are relatively less [73, 126]. While Kim et. al. [126]
reported improved crystallinity; Zhou et al [73] reported enhanced preferred c-axis
orientation for AZO films. Kim et. al. [126] also reported stronger c-axis orientation with
Al incorporation. The orientation gets stronger with temperature. Similar observation of
enhanced c-axis prefree orientation due to Al incorporation has been reported by Jayaraj
et. al. [132]. Shan et. al. [130] reported that at low temperatures, (101) orientation
dominates. However the (002) orientation becomes predominant at high temperatures. At
low temperatures however (101) peak is predominant [130]. They also reported that Al
doping can decrease the refractive index of ZnO. For PLD films [131], samples grown at
low temperature shows amorphous nature, but sample grown at higher than 400oC show
preferred (002) orientation. An enhancement of band gap was also reported in PLD films.
Our initial results also suggested that aluminum incorporation increases grain size and
increases preferred c-axis orientation [129]. Film's resistivity and sheet resistance
significantly decreased as film thickness increased [126]. Higher surface roughness and
irregular surface structure occurred at 200 °C substrate temperature [126]. Films possess
high optical transmittance of approximately 90% and demonstrated an optical band gap
of 3.35 eV [126]. It is well known that the microstructural and optical properties of ZnO
films are very sensitive to the method of preparation, the type and amount of dopant.
Apart form this, the microstructural features of the materials depends on the fraction of
37
Al incorporated into the lattice (by replacing zinc substitutionally) and the fraction going
to the non-crystalline region (grain boundaries). So far, no conclusive effort has been
made in this direction. A detailed microstructural analysis coupled with optical and
electrical measurements is necessary to resolve this issue.
The lowering of particle size due to Al incorporation has been attributed to the
replacement of relatively bigger 2Zn
+ ion (0.074 nm) by the relatively smaller 3Al
+ ion
(0.054 nm) during the formation of AZO [64]. This leads to a decrease in the lattice
constants, which in turn is responsible for the change in the crystallite size. However, it
has been observed that the crystallite size in the doped films does not vary in any regular
pattern with Al -dopant concentration [64], which is attributed to the lattice disorder
produced in the films at higher dopant concentration due to difference in the ionic radii of
2Zn
+ and 3Al
+ . The electrical investigation revealed that with Al -doping the conductivity
of the ZnO film improves upto a certain percentage of Al incorporation, but beyond that
it decreased with further doping [59, 64]. The optical observations on the films indicate a
blue-shift in the absorption edge, improved emission in the UV region and a widening of
the bandgap with increasing Al -dopant concentration [64].
Ni doped ZnO (NZO) is considered as an important II-VI diluted magnetic
semiconductor (DMS) material due to its unique magneto-electrical and magneto-
transport properties [91, 133]. Studies on DMSs have been spurred on by the urge to
develop storage device and spin electronics [134-136]. Certainly, there are reported
results regarding the properties of Ni-doped ZnO thin films and nanofilms obtained by
various deposition techniques such as pulsed laser deposition [137-138], spin coating
[133, 139-140], atom beam sputtering [141], fast atom beam sputtering [24], pulsed
electrodeposition-assisted chemical bath deposition method [78], auto-combustion
method [142], reactive electron beam evaporation [143], etc. They have been prepared
primarily for study of their ferromagnetic properties. However reports describing the
38
synthesis and properties of NZO deposited by aqueous solution techniques at low
temperatures are rare.
A decrease in band gap has been reported for sol-gel NZO [144]. Spin coated
NZO also has been repoted with high preferred orientation [140]. The nanogranular
nature of the films has examined by transmission electron microscopy (TEM) [124].
Marginal increase in particle size from 24nm for undoped ZnO to 26.5 nm for Ni doped
ZnO has been reported for films prepared by SILAR technique [145] For solgel
synthesized NZO films [146], smooth surface with roughness limited to 4nm has been
reported.
High quality Ni doped ZnO with preferred c-axis orientation has ben reported by
PLD technique [138]. Ni doping also has been reported to decrease resistivity by almost
two orders of magnitude [141]. Ni has been reported to be present in divalent state [91,
141]. Transmission in NZO (~83%) was found to be less than that of pure ZnO (~90%).
Two important mechanisms reported in the literature viz. influence of d–d transition
bands and electron scattering from crystallites/grains are discussed as the possible causes
for the increase in conductivity on Ni doping in ZnO. However the mechanism of such
increase in conductivity is still inconclusive [141].
Yildiz et. al. [91] however reported enhanced resistivity for NZO due to reduction
in charge carrier concentration. They also reported that Ni doping reduces particle size.
Decrease in optical transparency and increase in resisitivity has been reported for fast
atom beam sputtered NZO [24]. High electrical conductivity of undoped film is explained
on the basis of presence of oxygen vacancies. Decrease of electrical conductivity due to
nickel doping is explained on the basis of compensation of oxygen vacancies [24].
Decrease in carrier density and mobility has also been reported for NZO films
synthesized by pulsed laser deposition [137]. It appears that a detailed understanding of
microstructure and its correlation with electrical property is needed for further
understanding.
39
2.2 Aims and objectives of the present work
Preparation and characterization of polycrystalline thin films ZnO and doped ZnO
via different techniques have attracted considerable attention due to their wide
application prospects. Consideration of simplicity, economy and input energy suggest
that thin films of these materials be deposited by a low temperature and simpler chemical
route. Therefore, it is necessary to develop a low temperature deposition methodology for
the growth of ZnO and doped ZnO films. The primary aim of the present work was to use
a relatively new and less utilized SILAR technique to prepare ZnO thin films from
different zinc complex solutions and their characterization. Compared to other chemical
techniques, SILAR has remained a relatively less investigated method for ZnO and doped
ZnO and the potential of this technique is yet to be explored in full.
Further since the preparation of thin films by SILAR can be carried out under
mild conditions and at lower processing temperatures, doping of metal atoms at low
temperatures may be particularly suitable by this method. The technique offers a wide
spectrum of deposition parameters to control such as choice of suitable precursors,
concentration and pH of the reacting precursors, temperature of deposition etc. Optimum
synthesis condition that provides regular growth for every particular dopant needs be
determined. The objective of the work therefore includes exploring the possibility of
utilizing SILAR to impurify ZnO thin film with some metals (Cd, Mn, Al and Ni).
Reports on synthesis of Cd and Mn doped ZnO by SILAR is nonexisting to the best of
our knowledge. Only a very few reports are available for Al and Ni doping in ZnO thin
films by SILAR. In almost all these reports, sodium containing cationic precursor was
used which always introduces the possibility of highly mobile sodium ions in the films
which can be detrimental for their practical applications. The aim of the work was to
prepare such doped films from different cationic precursors using the flexibility of
SILAR technique and the influence exterted by the metal dopants on the physical
properties (structural and morphological properties, optical band gap, electrical properties
etc.) of ZnO films.
40
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48
CHAPTER 3
Instrumental techniques and
Theoretical considerations
3.1 Instrumental techniques
This section deals with different experimental techniques which have been used to
characterize the ZnO and doped ZnO thin films. A brief discussion about X-ray
diffraction (XRD), Scanning electron microscope (SEM), Transmission electron
microscope (TEM), Energy dispersive X-rays (EDX) and Ultraviolet-visble (UV-VIS)
spectrophotometer has been included here.
3.1.1 X-ray Diffraction (XRD) Analysis
X-ray diffraction is a versatile and non-destructive analytical method to uniquely
identify the crystalline phases present and to study the structural properties. A given
substance always produces a characteristic diffraction pattern, whether the substance is
present in the pure state or as one constituent of a mixture of substances. This fact is the
basis for the diffraction method of analysis. Qualitative analysis for a particular substance
is accomplished by identification of the pattern of that substance. Quantitative analysis is
49
also possible, because the intensities of diffraction lines due to one constituent of a
mixture depend on the proportion of that constituent in the mixture.
As the spacing between the atomic arrays in a material is in the atomic range, to
look inside these arrays, one needs a light of wavelength in the order of atomic distance
i.e. angstrom (Å). When X-rays of wavelength λ is incident on a crystalline material
(single crystal or polycrystalline), which is mounted in the center of a diffractometer, at
an angle of incidence, they interact with the parallel atomic planes and produce a
diffraction pattern. The schematic of X-ray diffraction by lattice is shown in Figure 3.1.
Figure 3.1: Schematic of X-ray diffraction by lattice
The angle between the diffracted beam and the transmitted beam is always 2θ .
This is known as the diffraction angle and it is this angle, rather thanθ , which is usually
measured experimentally [1]. The condition for diffraction at any observable angle is
given by Bragg law [1]
2 sin (3.1)n dλ θ= →
50
where n is the order of diffraction and d is the interplaner spacing. Bragg law gives the
condition for strong reflection for rays reflected from adjacent atomic planes and it
occurs when the path difference between them is equal to an integral multiple )(n of
wavelength. The angle θ for which equation 3.1 is satisfied is called the Bragg angle.
The diffracted beam is detected in a counter (gas counter or solid state counter)
which moves through the angular range of reflections and the intensity of the diffracted
beam is recorded on a synchronously advancing chart. Comparing this data with the
standard JCPDS (Joint Committee of Powder Diffraction Standards) data file, one can
identify the structure of the substance, the lattice parameters and the planes present. From
X-ray diffraction patterns, the crystallite size or particle size ( )D can be found using the
Scherrer’s formula [1]
(3.2)cos
D
kD
λ
β θ= →
where k is a constant determined by the geometry of the crystallites and it is
approximately 0.9 for spherical particles, D
β is the full width at half maximum (FWHM)
intensity of the observed diffraction peak. The broadening considered in Scherrer
equation is due to particle size alone. The angular width at a point where the intensity has
fallen to half of its maximum value (Full width at half maximum intensity or FWHM) is a
measure of broadening of x-ray peak [1].
The crystal structure and orientation of the ZnO and doped ZnO films were
investigated from the X-ray diffraction (XRD) patterns. The x-ray diffraction (XRD)
profiles of the samples were recorded using −Ni filtered αCuK radiation (λ=1.5418 Å)
from a highly stabilized and automated Philips X-ray generator (PW 1830) operated at 40
kV and 20 mA. The experimental peak positions were compared with the standard
JCPDS files and the Miller indices were indexed to the peaks.
51
3.1.2 Electron Microscopes: SEM and TEM
Scanning Electron Microscope (SEM) is one of the most versatile instruments
available for the characterization of heterogeneous materials and surfaces on a
micrometer and sub-micrometer scale. SEM generates images by scanning the specimen
with a beam of electrons [2-3]. The electron beam from an electron gun is allowed to pass
through electromagnetic lenses before falling on the sample. The electromagnetic lenses
are used to focus the electrons into a very thin beam. The finely focused electron beam is
allowed to sweep rapidly over the surface of the specimen. The molecules in the
specimen are excited to high energy levels in this process and emit secondary electrons.
Apart from secondary electrons, back scattered electrons, characteristic X-rays and
photons of various energies are also produced. Primarily, the electrons so produced are
used to form an image of the specimen surface. The image signal is collected by a
detector [2-3]. The display devices provides for both visual observation and photographic
recording. Thus SEM can extract structural information of a thin film material. Thin films
are usually coated with a conductive material prior to imaging to render the surface
conductive.
Electron microscopes in which electrons are allowed to transmit through the
objects are known as transmission electron microscope (TEM). TEM consists of an
electron gun, central column, electromagnetic lenses and a fluorescent screen. The
electron gun is the source of electrons. The microscope column is an evacuated metal
tube through which the electron travels. The image of a sample is formed by illuminating
the sample with an electron beam and detecting the electrons that are transmitted through
sample [4]. The electromagnetic lenses are used to focus the electrons into a very thin
beam. Depending on the density of the material present, some of the electrons are
scattered and disappear from the beam. At the bottom of the microscope the unscattered
electrons hit a fluorescent screen, which gives rise to a “shadow image” of the specimen
with its different parts displayed in varied darkness according to the density. The image
can be directly studied by the operator or photographed with a camera.
52
3.1.3 Ultraviolet –Visible (UV-VIS) spectroscopy
UV-VIS spectroscopy refers to absorption spectroscopy in the UV-visible spectral
region. Studies by optical absorption explain the band structure of material. There are two
types of optical transitions, which can occur at the fundamental absorption edge of
crystalline as well as non-crystalline semiconducting materials. They are direct and
indirect transitions. When a light beam falls on a thin film semiconducting material, a
part of the beam will be reflected, another part will be transmitted through the film, and
the rest of the beam will be absorbed. Absorption of photons causes transition of the
electrons from valance band to conduction band. The absorption ability is measured by its
absorption coefficient ( )α which is a function of frequency [5-7] and is defined as [7]
(3.3)t
oI I e
α−= →
where I is the intensity of the transmitted beam, o
I is the intensity of the incident light
and t is the thickness of the film. The nature of transition is determined by using the
relation [5]
( ) ( ) (3.4)n
gh A h Eα ν ν= − →
where hν is the photon energy, g
E is the band gap energy, A is a constant and it is
function of index of refraction and hole/electron effective masses [5]. The constant n is
equal to two (2) for direct transition and equal to one (1) for indirect transition [5-6].
Several models of spectrophotometers of varying degree of sophistication are
available. These include single beam, double beam, relecting and multibeam instruments.
Deuterium or hydrogen lamp (for UV light) and tungsten lamp (for visible light) are
generally used as the light sources. In dual beam spectrophotometers, the incident light is
53
split into two beams of equal intensity, one of which passes through the reference cuvette
and the other through the sample cuvette. The instrument records the change in
absorbance in the sample with respect to the reference. The detector records the change in
absorbance in the sample. For our experiments, a dual beam UV-VIS spectrophotometer
(Shimadzu, Model No. UV-1800; Spectral resolution 1 nm) was used.
3.1.4 Energy dispersive X-ray spectroscopy (EDS or EDX)
EDX is an analytical technique used for the elemental analysis or chemical
characterization of a sample. It is one of the variants of X-ray fluorescence spectroscopy
which is based on the investigation of a sample through interactions between
electromagnetic radiation and matter. Quantitative estimation is made by analyzing the
X-rays emitted by the matter when it is bombarded with charged particles. Its
characterization capabilities lie on the fundamental principle that each element has a
unique atomic structure allowing X-rays that are characteristic of an element's atomic
structure to be identified uniquely from one another.
3.2 Theoretical considerations
3.2.1 Preferred orientation
Quantitative information concerning the preferred crystal orientation can be
obtained from the texture coefficient ( )TC , defined as [5, 8-9]
( )( ) / ( )
( ) (3.5)1 ( ) / ( )
o
o
n
I hkl I hklTC hkl
I hkl I hkln
= →
∑
where ( )TC hkl is the texture coefficient for the ( )hkl plane, n is the number of
diffraction peaks considered, ( )I hkl is the measured x-ray intensity of the ( )hkl plane
54
[converted to relative intensity when ( )TC hkl is evaluated, by taking the observed highest
intensity as hundred (100)] and ( )o
I hkl is the corresponding relative intensity according
to JCPDS card for ZnO [10]. ( )o
I hkl represents the x-ray intensities (relative intensities)
from standard ZnO powder with randomly oriented grains and with no preferred
orientation.
If ( ) 1TC hkl ≈ for all the (hkl) planes considered, then the films are with a
randomly oriented crystallite similar to the JCPDS reference, while values higher than 1
indicate the abundance of grains in a given (hkl) direction. Values of ( )TC hkl in the range
0 ( ) 1TC hkl⟨ ⟨ indicate lack of grains oriented in that direction. As ( )TC hkl increases, the
preferential growth of the crystallites along the plane ( )hkl enhances [5, 9].
3.2.2 Particle size estimation
Although Scherrer equation (Eqn. 3.2) is normally utilized to evaluate particle
size, it does not take account of microstrain present in polycrystalline thin films. The
broadening D
β in scherrer equation represents the broadening due to particle size alone
[1, 11]. However, the total contribution to the observed broadening in x-ray diffraction
peaks (called x-ray peak broadening or x-ray line broadening) is due to particle size
broadening ( )Dβ , broadening due to microstrain ( )s
β and instrumental broadening ( )iβ .
The instrumental broadening arises from various factors such as non-parallelism of the
incident x-ray beam; presence of other wavelengths apart from CuKα etc. [1] and it is a
constant for a particular experimental setup. Strain arises in polycrystalline materials due
to various defects (point defects, line defects, planar defects and volume defects) and this
gives rise to a broadening in x-ray diffraction peaks [1]. Thus the experimentally
observed broadening in x-ray diffraction pattern ( )o
β can be written as [1]:
(3.6)o D S i
β β β β= + + →
55
The broadening due to particle size and microstrain is obtained by subtracting
instrumental broadening from the observed or measured broadening and can be written as
(3.6)o i D S
β β β β β= − = + →
The broadening due to microstrain can be written as [1]:
4 tan (3.7)S
β ε θ= →
Equation 3.7 coupled with equations 3.6 and 3.2 gives the Williamson-Hall
equation [1, 11]
cos 4 sin (3.8)k
D
λβ θ ε θ= + →
where ε is the microstrain in the film.
Thus a plot of cosβ θ against 4 sinθ is a straight line and is called the
Williamspn-Hall (or W-H) plot. The slope of the plot represents average strain in the film
whereas the inverse of intercept of the straigt line on cosβ θ axis gives the crystallite
size ( )D according to equation 3.8.
The separation of crystallite size and microstrain can be done if the shapes of their
individual profiles are known. Normally a Gaussian function represents strain broadening
and a Lorentzian function represents crystallite size broadening [1].
56
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6. C. F. Klingshirn, in “Semiconductor Optics” (Springer-Verlag, Berlin, Heridelberg,
1997).
7. K. H. Kim, R. A. Wibowo and B. Munir, Materials Letters 60 (2006) 1931.
8. S. Ilican, Y. Caglar and M. Caglar, J. of Optoelectronics and Advanced Materials 10
(2008) 2578.
9. C. S. Prajapati and P. P. Sahay, Crystal Research Technology 46 (2011) 1086.
10. B. Post, S. Weissmann and H. F. McMurdie (eds.), Joint Committee on Powder
Diffraction standards, Inorganic Vol., Card No. 36-1451, International Centre for
Diffraction Data, Swarthmore, PA (1990).
11. H. P. Klug and L. E. Alexander, in “X-ray diffraction procedures for polycrystalline
and amorphous materials” (2nd
edition, Wiley-Interscience, 1974).
57
CHAPTER 4
Preparation of ZnO thin films by SILAR
and their characterization
4.1 Introduction
ZnO films by chemical deposition technique were first reported in 1980 by Call
et. al. [1] who obtained it as an accidental by-product in an attempt to deposit the
composite 1x x
Cd Zn S− films. Addition of zinc salt to a CdS depositing solution produced
CdS films containing large amount of ZnO impurities. Further experiments led to
refinement of the method for aqueous deposition of ZnO. The chemical deposition of
ZnO involving multiple dipping of the substrate in an aqueous solution of a zinc complex
was further developed in 1987 by Ristov et. al. [2]. The process was named as chemical
deposition process or chemical dipping process in these works [1-2]. The technique was
given the name SILAR (Successive ion layer adsorption and reaction) by Nicolau et. al.
[3]. Normally sodium zincate or ammonium zincate bath is used as the cationic precursor.
Jimenez-Gonzailez et. al. [4] and Raidou et. al. [5] deposited ZnO film using this
technique from ammonium zincate bath prepared from zinc sulphate as the starting
precursor. Preparation and characterization of ZnO films deposited from sodium zincate
and ammonium zincate complex with zinc sulphate as starting precursor has also been
reported by Chatterjee et. al. [6] and Mitra et. al. [7].
58
In the present investigation ZnO thin films were deposited from ammonium
zincate complex as cationic precursor with zinc acetate ( )3 222Zn CH COO H O as the
starting reagent for the first time. Zinc acetate has a number of distinctive properties. It is
known to be a ‘mono-precursor’ [8]. Also the ammonium acetate formed during its
reaction with ammonia is highly soluble in water which reduces the possibility of
impurity incorporation in the deposited films. Ammonium acetate is also a relatively
unusual example of a salt that melts at low temperatures. For comparison of physical
properties of the films synthesized from ammonium zincate bath, films were also
deposited from sodium zincate and zinc chloride as cationic precursors. This was also
essential since certain metals could not be doped from ammonium zincate bath (discussed
in subsequent chapters) while they could be successfully doped from other zinc complex
baths. Different experimental parameters such as chemical nature of the bath solution
including concentration and pH, temperature of deposition etc. governing the film growth
process have been studied.
4.2 Preparation of ZnO thin films
Figure 4.1 shows the simple process flow sheet for deposition of ZnO film from
cationic and anionic baths. The cationic bath is of ammonium zincate solution
( )4 22NH ZnO kept at room temperature (RT) and the anionic bath is of distilled water
maintained near boiling point (96–98oC). Sodium zincate [ ]2 2Na ZnO and zinc chloride
bath ( )2ZnCl as cationic precursor was also used in separate set of experiments. The
substrate (normally a glass slide) was alternatively dipped in the baths containing the zinc
complex solution and hot water. Although microscope glass substrates cannot withstand
very high temperatures, it is widely used since the electrical and optical measurements
are not disturbed by an underlying layer and are thus easier to interpret. Substrate
cleaning prior to deposition is an important step in thin film prepartion in order to remove
the contaminants that would otherwise affect the properties of the film. The sequence of
59
substrate cleaning steps was: overnight (24 hours) cleaning in chromic acid, rinsing in
double distilled water followed by ultrasonic cleaning in equivolume mixture of acetone
and ethyl alcohol. The cleaned substrate was tightly held in a holder so that only a
requisite area for film deposition is exposed. Thus the film deposition area could be
easily varied by adjusting the holder arrangement. For some specific experiments (Mn
doping in ZnO), quartz substrate was used and the cleaning of such substrate has been
discussed in chapter 6.
Pre-cleaned Substrate
Alternate Dipping
Figure 4.1: Process flow sheet for SILAR deposition of ZnO
( )4 22NH ZnO bath
(Cationic precursor)
Hot water bath
(Anionic precursor)
ZnO Film
Post-Deposition
Air Anneal
Single Phase
ZnO
60
4.2.1 Preparation of bath solutions
The ammonium zincate bath used for deposition of ZnO was prepared by slow
addition of ammonium hydroxide (~25% pure ammonia solution, Merck, Mol. Wt. 17.03
g/mol) to an aqueous solution of analytical grade zinc acetate
dihydrate ( )3 22.2Zn CH COO H O supplied by Merck. Addition of ammonia solution in
zinc acetate solution initially gives rise to a white precipitate of zinc hydroxide
[ ]2( )Zn OH according to the reaction
( )3 2 4 3 42( ) 2 2 (4.1)Zn CH COO NH OH Zn OH CH COONH+ = + →
However, on further addition of ammonia, the precipitate dissolved forming the
( )4 22NH ZnO bath following the reaction
( )2 4 4 2 22( ) 2 2 (4.2)Zn OH NH OH NH ZnO H O+ = + →
Ammonia solution was introduced slowly under continuous stirring until the
solution becomes clear and homogeneous. An excess of alkali is always required to have
a stable ammonium zincate bath [7, 9]. Thus the overall reaction leading to the formation
of ammonium zincate is obtained by adding equations (4.1) and (4.2):
( ) ( )3 4 4 2 3 4 22 24 2 2 (4.3)Zn CH COO NH OH NH ZnO CH COONH H O+ = + + →
The sodium zincate bath was prepared by addition of a solution sodium hydroxide
( NaOH pellets, Merck, mol. wt. 40) to an aqueous solution of zinc sulphate
( )4 2.7ZnSO H O [Merck, Mol. Wt. 287.54]. Similar to the case of ammonium bath,
initially a white precipitate of zinc hydroxide appeared according to the reaction
61
( )4 2 2 42 ( ) (4.4)ZnSO NaOH Zn OH Na SO+ = + →
However, on further addition of NaOH , the precipitate dissolved forming the 2 2Na ZnO
bath following the reaction
2 2 2 2( ) 2 2 (4.5)Zn OH NaOH Na ZnO H O+ = + →
Thus the overall reaction leading to the formation of sodium zincate is obtained
by adding equations (4.4) and (4.5):
4 2 2 2 4 24 2 (4.6)ZnSO NaOH Na ZnO Na SO H O+ → + + →
Similar to the case of ammonium zincate complex, an excess of NaOH is always
required to obtain a stable aqueous solution of sodium zincate [10]. Thus, to get a stable
solution of zinc complex ion ( )2Zn
+ , strong alkalinity is necessary.
In case of sodium zincate bath, only one stable zincate namely 2 2 2.4Na ZnO H O is
possible at room temperature [10]. The compound is highly soluble in water. However, a
measurable amount of ammonium zincate is not present in ammoniacal solution of zinc
oxide and instead, a series of complex cations can result in case of ammonium zincate
solution [9]. These are ( )3 2Zn NH
++ , ( )3 4
Zn NH++
and ( )3 6Zn NH
++ . Jimenez-
Gonzailez et. al. [4] also inferred the presence of tetraamminezinc (II), ( )3 4Zn NH
++ in
the solution. A stock solution of 0.5 molar (0.5 M) concentration was prepared in each
case and it was then diluted with double distilled water to get bath solutions of requisite
concentrations during the experiments. One of the problems encountered with the
ammonium zincate bath was that, due to evaporation of ammonia, the pH of the stock
solution slowly reduced with consequent precipitation of zinc hydroxide [9-10]. In such a
62
situation the pH was restored by adding appropriate amount of ammonia. Sodium zincate
bath was, however, stable for a reasonably long time as there was no possibility of
evaporation of sodium hydroxide.
4.2.2 Optimization of pH and Concentration of the zincate baths
The deposition rate process was studied by monitoring the variation of film
thickness with pH and concentration ( )C of the zincate bath and the number of dipping in
the reacting baths (cationic and anionic). The growth rate and quality of the film was
found to depend on the concentration of the zincate baths and the adherence of the film
on the substarte surface was found to be a stringent function of zincate bath pH. It was
found that above 0.125M concentration of the ammonium zincate bath, the growth
process was erratic and nonuniform. Particle absorption took place on the film surface
making the growth process nonuniform and resulting in poor quality films. For still
higher concentrations (in excess of 0.15M), film detachment from substrate surface took
place and no layer could be deposited. The ammonium zincate bath concentration was
therefore optimized at 0.1M to get good quality film with a reasonable growth rate.
Lower concentrations (less than 0.1 M) reduce the growth rate of the film in a linear
proportion. In case of sodium zincate bath, the growth process was found to be erratic
and nonuniform above 0.15M concentration. The bath concentration in this could be
optimized in the range 0.1-0.125M. For all further experiments the concentration of both
the zinacte bath was kept at 0.1M. For 0.1M solution of ammonium zincate complex, films produced with pH<10.7
was found to consist of non-adherent powder like precipitates indicating poor quality of
deposition. The bath solution also tends to lose stability for pH<10.7 and ammonium
zincate bath was found to be stable for pH values ≥ 10.7. However, above pH value of
10.9, the growth rate decreases abruptly. Also for more basic solutions (pH value in
excess of 10.9), it appears that dissolution of already deposited ZnO film occurs when
reintroduced into the solution. Thus the pH value was optimized in the range 10.75-10.85
63
for all further experiments from ammonium zincate bath with a minimum of ammonia
addition. For 0.1M sodium zincate complex, the pH value was similarly optimized in the
range 13.15-13.25 in order to get adherent films. The pH measurements were carried out
in a systronics pH meter (Model 335). Table 4.1 shows the optimized deposition
parameters for different zinc complex cationic baths used in the present investigation.
Table – 4.1: Optimized deposition parameters for different zinc complexes
Cationic
bath
Anionic
bath
( )C M
(Cationic
bath)
( )C M
(Anionic
bath)
pH
(Cationic
bath)
pH
(Anionic
bath)
T (oC)
(Cationic
bath)
T (oC)
(Anionic
bath)
( )4 22NH ZnO
2H O 0.1 M - 10.80 0.05±
- R. T. 97 1± oC
2 2Na ZnO 2H O 0.1 –
0.125 M
- 13.20 0.05±
- R. T. 97 1± o
C
2ZnCl* NaOH 0.1 M 0.075 M 4.70 0.05± 11.1 0.05±
R. T. 70
oC
* The details of ZnO film deposition from 2ZnCl solution as cationic bath and
NaOH solution as anionic bath has been discussed in section 6.1 of chapter 6.
4.2.3 Deposition of ZnO films
Pure zinc oxide films were deposited on microscope glass slide substrates by
alternate dipping of the substrate into 0.1M ammonium zincate ( )4 2NH ZnO bath kept
at room temperature and hot water maintained at 97 ± 1oC. The pH of the zincate bath
was maintained in the range 10.75-10.85 with appropriate ammonia addition during film
depsotion process.
64
The deposition process consisted of the following steps: (i) immersion of the pre-
cleaned substrate in the zincate solution kept at room temperature; (ii) withdrawal of the
substrate (which carries a thin layer of the complex zinc solution adhered to the surface
of the substrate) and (iii) introduction of the substrate with the zincate solution layer into
hot water bath (which lead to the chemical reaction between adsorbed zinc complex and
hot water on the substrate surface). Thus one complete cycle of dipping involves dipping
into the zincate bath, its withdrawal from the bath followed by dipping into hot water
bath. The dipping time in each bath was two (2) seconds (s). This cycle of dipping was
repeated several times in order to increase the overall film thickness.
The reaction occurring on the substrate surface leading to the formation of ZnO
may be represented as [7]:
( )4 2 2 422 (4.7)NH ZnO H O ZnO NH OH+ → + →
However, the detailed chemical reaction involving the presence of
tetraamminezinc (II) ( )2
3 4Zn NH
+ complex [4] in presence of an excess ammonical
solution may be quite complicated. The reaction involves release of zinc (II) ion in water
bath followed by reaction of this cation with hydroxyl ion ( )OH− present in the water
bath. Thus the possible reaction scheme may be represented as:
( )2
2
3 2 444 4 (4.8)Zn NH H O Zn NH OH
++ + − + → + →
2
22 ( ) (4.9)Zn OH Zn OH+ −+ → →
Since the reaction temperature is close to the boiling point of water, zinc
hydroxide breaks to give zinc oxide:
2 2( ) (4.10)Zn OH ZnO H O→ + →
65
Thus some amount of zinc hydroxide is always expected in the as-deposited film
and this has been experimentally observed [2, 6, 11]. A part of the ZnO so formed was
deposited onto the substrate as a strongly adherent film and another part of it formed a
precipitate in the hot water bath. Thus, only the strongly adherent microcrystals remained
on the surface. These crystals then serve as nuclei for further growth during subsequent
dipping [5, 7]. As a part of the precipitate during each dipping remained dispersed in the
hot water bath, its concentration increased with the number of dipping and to maintain
the uniformity of the film deposition process it is preferable to change the concentrated
hot water bath at regular intervals with a fresh water bath. The zincate solution was also
changed at definite intervals so that the concentration of the bath remained effectively
constant during the entire deposition process. This was particularly required when the
number of deposition cycle was significantly high. Both the hot water bath and zincate
bath was changed after each 25 dipping cycles. The deposited film was subsequently heat
treated in air at 200ºC for 2 hours to get milky white colored ZnO thin film.
In case of sodium zincate bath, the reaction occurring on the substrate surface
leading to the formation of ZnO may be represented as [6]:
2 2 2 2 (4.11)Na ZnO H O ZnO NaOH+ → + →
It can be seen that sodium ( )Na goes as NaOH by hydrolysis and excess Na , if
any, was removed when the film was thoroughly washed with distilled water.
It is to mention that the ammonium zincate bath used for deposition contains
ammonium acetate as well (eqn. 4.3). It appears that while the zincate precipitates as ZnO
in presence of high concentration of water when dipped in the hot water bath, the acetate
goes into the solution due to its high solubility in water. The much higher solubility of
ammonium acetate compared to sodium sulphate (contained alongwith sodium zincate in
eqn. 4.6) [12-13] reduces the possibility of impurity incorporation in the deposited films.
66
Thus use of sodium zincate bath [5] instead of ammonium zincate bath always introduces
the possibility of incorporation of highly mobile sodium ions in the film, which can be
detrimental for their practical applications. The use of sodium hydroxide as anionic
precursor for films deposited from 0.1 M 2ZnCl bath as cationic precursor and 0.075 M
NaOH bath as anionic precursor (discussed in section 6.1, chapter 6) also introduces the
possibility of incorporation of highly mobile sodium ions in the film. Also the higher
solubility of ammonium acetate compared to ammonium sulphate (obtained if zinc
sulphate is used [4-5, 7] instead of zinc acetate as starting reagent) reduces the possibility
of impurity incorporation (sulpher ion as impurity in case of ammonium sulphate
compared to carbon as impurity in case of ammonium acetate).
4.2.4 Film thickness and its measurement
Figure 4.2 shows the variation film thickness ( )t with number of dipping (25-100
dipping cycles) for ammonium zincate bath under optimized conditions. The result
implies a linear increase of ZnO film thickness with number of dippings. There is an
overall variation of ± 5% in the film thickness data (shown as error bars against each data
point of figure 4.2) which reflects a small variability in the deposition process arising
probably from small experimental scatter in the zincate bath concentration values as well
as due to the nonuniformity of the substrate handling procedure as the deposition is
carried out manually. The growth rate was ~0.0162 µm per dipping for 0.1M ammonium
zincate bath. The corresponding value 0.1 M sodium zincate bath was ~0.02 µm per
dipping. In other words the growth rate was ~0.162 µm per dipping per mole for
ammonium zincate bath and it was ~0.20 µm per dipping per mole for sodium zincate
bath. Table 4.2 shows the thickness ( )t values against number of dipping ( )N for
ammonium and sodium zincate bath. Average of three measurements is shown in the
table. The growth rate was ~0.021 µm per dipping for films deposited from 2ZnCl as
cationic precursor (Section 6.1, chapter 6).
67
Figure 4.2: Dependence of film thickness on number of dipping cycle
Table –4.2: Thickness of ZnO films from ammonium and sodium zincate baths
Number of
dipping
( )N
Thickness ( )mµ
[0.1M ( )4 22NH ZnO ]
Thickness ( )mµ
[0.1M 2 2Na ZnO ]
25 0.355 0.49
50 0.81 1.02
75 1.24 1.52
100 1.63 2.03
The film thickness was determined by weight difference-density consideration
method or gravimetry method [7, 14-15] using an electronic high-precision balance. The
gravimetry method measures the change in weight of the substrate due to film deposition
and using the known area of film deposition and utilizing the data of the theoretical
0 25 50 75 100 125
0.0
0.5
1.0
1.5
2.0
Thic
kn
ess (
µm
)
No. of dipping (N)
68
density of ZnO (5.6 gm/cm3) [11]. Thus, if
1' 'W and 2' 'W be the weights of the substrate
before and after film deposition in gm., ' 'A be the area of the deposited film in cm2 and
' 'ρ be the theoretical density of ZnO, then the film thickness can be evaluated using the
equation:
( )2 1 410W W
tAρ
−−= × mµ (4.12)→
A check of film thickness was made by measuring the thickness using cross-
sectional SEM. Figure 4.3 shows the cross-sectional SEM micrograph of ZnO film of
thickness 2.0 µm measured gravimetrically (obtained by 125 dipping from ammonium
zincate bath). An average thickness of 2.64 µm was obtained from SEM micrograph. The
value was an average of several measurements on different portions. This indicates a
porosity of ~ 32% in the deposited films. Similar experiments on films deposited from
sodium zincate bath shows a porosity of ~ 30% (not shown here for brevity). Those from
zinc chloride bath show a porosity of ~ 22% (Section 6.1, chapter 6).
Figure 4.3: Cross-sectional SEM of ZnO film prepared from ammonium zincate bath
Substrate
Film b
b
Thickness
69
The gravimetry method of film thickness determination has some limitations
because of non-uniformity, porosity and edge tapering effects in the chemically deposited
films with porous microstructure. The actual density of the film is always lower than the
theoretical density used in gravimetry technique which does not takes account of
porosity. Thus the actual thickness is always greater than the measure one using
theoretical density. However, this error does not affect the comparative data of measured
film thickness.
4.3 Structural characterization by XRD: Evaluation of particle size
Figure 4.4 shows the x-ray diffraction patterns of ZnO thin film prepared on glass
substrate from ammonium zincate bath. The diffraction patterns were recorded at room
temperature. Figure 4.4 (a) to 4.4 (d) shows the spectra of the samples heat treated at
200oC, 300
oC, 350
oC and 400
oC redpectively. The heat treatment was done in air for 2
hours. The materials were scanned in the range 20-60o. The 2θ variation was employed
with a 0.05 degrees step and a time step of 1 second. Intensity in arbitrary units is plotted
against 2θ in the figure.
It is seen from figure 4.4 (a) that diffraction peaks appear at 31.714o, 34.389o,
36.205o, 47.434
oand 56.576
o. The peak positions do not change significantly due to heat
treatment at different temperatures. The diffraction patterns reveal the formation of
phase-pure polycrystalline ZnO film with hexagonal wurtzite structure and good
crystalline quality without any appreciable changes from pure ZnO. All the peaks are in
good agreement with the Joint committee on powder diffraction standard (JCPDS) data
belonging to hexagonal ZnO structure [16]. The corresponding reflecting planes are
(100), (002), (101), (102) and (110) respectively. The XRD patterns of all the samples
exhibit enhanced intensities for the peaks corresponding to (002) plane, indicating
preferred orientation along c-axis.
70
Figure 4.4: X-ray diffraction pattern of ZnO thin films heat treated at (a) 200oC,
(b) 300oC, (c) 350
oC and (d) 400
oC
Table 4.3 shows the relative intensities for ZnO powder (with no preferred
orientation) [16] and ZnO thin film heat treated at 300oC (observed in this work). The
relative intensities for ZnO film was evaluated by taking the highest intensity of (002)
peak as hundred (100). The actual values of intensitie obtained are shown in the third
column of table 4.3 and the relative intensities are shown in the last column. The (101)
peak appears with maximum intensity in ZnO powder with no preferred orientation [16].
However, the (002) peak appears with maximum intensity in the SILAR deposited ZnO
films. Table shows the relative intensities for the first three peaks since they appeared
20 30 40 50 60
0
500
1000
1500
2000
(110)
(102)
(101)
(002)
(a)
2θ (degree)
20 30 40 50 60
0
500
1000
1500
2000
(b)
Inte
nsity (
a.u
.)
20 30 40 50 60
0
500
1000
1500
2000
(c)
20 30 40 50 60
0
500
1000
1500
2000
(100)
(d)
71
with high intensity in the XRD pattern of the films. Figure 4.5 shows the variations of
( )TC hkl for (002) peak evaluated using eqn. 3.5 (Chapter 3, section 3.2.1).
Table –4.3: Relative intensities of diffraction peaks for ZnO powder and ZnO thin film
Diffraction plane Relative intensities
for ZnO powder
( )o
I hkl [17]
Observed intensities
for ZnO film heat
treated at 3000C
Relative intensities
for ZnO film heat
treated at 3000C
( )I hkl
(100) 57 365 ~23.13
(002) 44 1578 100
(101) 100 445 ~28.20
Figure 4.5: Variation of (002)TC with temperature for ZnO films prepared from
ammonium zincate bath
Since three diffraction peaks were used ((100), (002), (101)), the maximum value
TC(hkl) possible is 3. The texture coefficient for the (002) orientation has been found to
increase from ~2.247 to ~2.291 as the annealing temperature is increased from 200oC to
300oC. No significant change is found with further heat treatment. All subsequent
200 250 300 350 400
2.20
2.25
2.30
2.35
2.40
TC
(0
02
)
T(oC)
72
measurements were made on films heat treated at 350oC. The value of the texture
coefficient indicates the maximum preferred orientation of the films along the diffraction
plane under consideration, meaning that the increase in preferred orientation is associated
with increase in the number of grains along that plane.
Thus with increase in annealing temperature, the crystallinity along (002) plane
improves upto 300oC and finally saturates. Table 4.4 shows the comparison of
(002)TC of ZnO films obtained from three different zinc complexes under optimized
deposition conditions of table 4.1. All the films were heat treated at 350oC for 2 hours
prior to XRD measurements. The x-ray patterns for films deposited from sodium zincate
bath and zinc chloride bath has been discussed in subsequent chapters (section 5.2 of
chapter 5 and section 6.2 of chapter 6). It is quite evident that films deposited from
ammonium zincate bath have highest preferred c-axis orientation.
Table – 4.4: (002)TC values for ZnO films prepared from different zinc complexes
Cationic precursor ( )TC hkl vale for (002) plane
( )4 22NH ZnO solution 2.29 0.01±
2 2Na ZnO solution 1.95 0.01±
2ZnCl solution 1.82 0.01±
Utilizing the X-ray diffraction data, the average particle size was estimated from
Williamson-Hall equation (Eqn. 3.8, section 3.2.2, chapter 3). The broadening due to
particle size and strain taken together i.e. β was obtained from the experimentally
observed broadening ( )oβ using the equation [17-18]:
(4.13)o i
β β β= − →
Diffraction data from standard silicon ( )Si powder was used to measure the
instrumental broadeningi
β . Normally silicon powder with very high particle size is used
73
as a standard [19-20]. Large particle size in the standard ensures that the broadening due
to particle size is negligible in the standard according to Scherre equation (Eqn. 3.2,
Section 3.1.1, chapter 3). Thus broadening observed in the x-ray diffraction pattern of the
standard is due to instrument only. Figure 4.6 shows the XRD pattern of standard silicon
powder. The peaks at 28.48o, 47.3
o, 56.08
o, 69.12
o, 76.22
o, 87.86
o, 94.8
o and 106.56
o
correspond to those from standard silicon.
Figure 4.6: XRD pattern of standard silicon powder
The broadening (in FWHM) against 2θ obtained for standard silicon sample was
plotted in a graph and was used as a reference. Figure 4.7 shows the broadening against
2θ obtained for standard silicon sample. A polynomial fitting of the experimentally
observed values is shown in figure 4.7. The corresponding fitting equation (polynomial
regression) is
2 (4.14)y a bx cx= + + →
40 60 80 100
0
1000
2000
3000
4000
5000
6000
7000
Inte
nsity (
a.
u.)
2θ (degree)
74
where y stands for the broadening in FWHM and x is the angle in 2o θ . a , b and c are
the constants with values 0.109, 47.742 10−− × and 51.263 10−× respectively. The
instrumental broadening at the observed peak positions for ZnO could be evaluated from
the graph as well as from equation 4.14 using the experimental value of 2θ .
Figure 4.7: Instrumental broadening against 2θ for standard Silicon powder.
X-ray line broadening analysis to estimate the observed ( )oβ was carried out
using computer software (MARQ2) [21-22]. The software utilizes Marquardt least-
squares procedure for minimizing the difference between the observed and simulated
diffraction patterns. The peak-shape and intensity of reflection is modeled with a pseudo-
Voigt (pV) analytical function, which is a combination of a Gaussian and a Lorentzian
functions representing lattice strain broadening and crystallite size broadening
respectively. The background intensity is subtracted by fitting the background with a
suitable linear function. A typical plot obtained from MARQ2 analysis for ZnO thin film
20 30 40 50 60 70 80 90 100
0.09
0.10
0.11
0.12
0.13
0.14
0.15
0.16
FW
HM
(d
egre
e)
2θ (degree)
75
synthesized from ammonium zincate bath is shown in Figure 4.8. The dotted curve
represents the experimental intensity data ( )oI and the continuous curve represents the
calculated (simulated) intensity data ( )cI . The difference plot ( )c o
I I− is shown at the
bottom.
Figure 4.8: Observed (dotted) and simulated (continuous) XRD patterns of ZnO film
heated at 350oC
From the values ofo
β obtained using MARQ2 fitting and the corresponding
values of instrumental broadeningi
β , β was calculated using equation 4.13. Figure 4.9
shows the plot of cosβ θ against θsin4 (W-H plots) for 350oC heat treated film. The
slope of the plot represents average strain in the films whereas the inverse of intercept on
cosβ θ axis gives the crystallite size ( )D according to the Williamson-Hall equation.
The particle size was evaluated using 0.9k = , which corresponds to spherical crystallites
and 1.5418λ = Å, the wavelength of αCuK radiation. The average value of particle size
for pure ZnO was ~22.75 nm and the strain in the film was ~ 32.04 10−× .
76
Figure 4.9: Williamson-Hall plots of ZnO film prepared from ammonium zincate bath
The lattice strain in polycrystalline films may arise from various factors. Several
lattice disorders such as point defects (vacancies in ZnO), line defects (dislocations),
planar defects (grain boundaries) and volume defects contribute to the strain. During
transfer and condensation of a liquid layer (zinc complex layer in this case) on a solid
support (glass substrate in this case), stretchning may occur which also may be a possible
source of stress in the films giving rise to lattice strain.
4.4 Electron microscope studies
Figure 4.10 shows the SEM image of ZnO films obtained from ammonium
zincate bath and heat treated at 350oC. Prior to imaging, deposition of thin gold layer was
made in an ion coater [GIKO Engineering ion coater IB-2] to enhance the emission of
secondary electrons for better imaging. The SEM unit (Model S530, Hitachi, Japan) was
operated at 20 kV. SEM investigation at normal incidence revealed polycrystalline
structure with smooth surface for the deposited films. The overall surface morphology
shows grains of nearly spherical shape and more or less uniformly covering the surface
without any cracks.
0.0 0.5 1.0 1.5 2.0 2.5
0.000
0.005
0.010
0.015
βcos
θ
4sinθ
77
Figure 4.10: SEM image of ZnO obtained from ammonium zincate bath
Figure 4.11 shows the TEM micrograph of ZnO powder scratched out from the
substrate. Particle sizes ranging between 23 and 28 nm was observed in the TEM image
with an average value of ~25.8 nm which matches well with x-ray value of 22.75 nm.
Figure 4.11: TEM image of ZnO film obtained from ammonium zincate bath
78
Figure 4.12, on the other hand, shows the TEM image of ZnO film obtained from
sodium zincate bath under optimized condition (Table 4.1). The film was heat treated at
350oC for 2 hours. Particle sizes ranging between 26 to 60 nm was observed in the TEM
image with an average value of ~41 nm. Figure 4.13, on the other hand, shows the
HRSEM (High resolution SEM) image of ZnO film obtained from zinc chloride bath as
cationic precursor under optimized conditions (Table 4.1).
Figure 4.12: TEM image of ZnO obtained from sodium zincate bath
Figure 4.13: HRSEM image of ZnO obtained from zinc chloride bath
79
HRSEM study at normal incidence was undertaken in a FEI FEG Nova 600
Nanolab at 10 kV. The image with magnification 10000× reveals structure consisting of
many spheroid-like nano particles with an average size of ~31.2 nm. The histogram of
particle size distribution is shown in figure 4.14.
Figure 4.14: Histogram of particle size distribution
Table 4.4 shows the particle size values onbtained form different zinc complexes
under optimized conditions of Table 4.1.
Table – 4.4: Particl size of ZnO from different zinc complexes
(Under optimized conditions)
Cationic precursor Particle size [ D (nm)]
( )4 22NH ZnO solution ~25.8 nm (TEM value)
2 2Na ZnO solution ~41 nm (TEM value)
2ZnCl solution ~31.2 nm (HRSEM value)
1 5 2 0 2 5 3 0 3 5 4 0 4 50
1 0
2 0
Particle diameter (nm)
Fre
qu
ency
(p
arti
cles
by
cou
nt)
80
The SEM image of ZnO film obtained from sodium zincate bath under optimized
deposition condition is shown in figure 4.15. The polycrystalline and porous nature is
revealed from the micrograph. The SEM photograph clearly illustrates the formation of
sub-micrometer crystallites distributed more or less uniformly over the surface. Although
no cracks could be detected, some holes indicating porosity is present. Agglomeration of
small crystallites also seems to be present in certain regions on the film surface. The
shape of the particles seems to be off spherical compared to nearly spherical particle for
those prepared from ammonium zincate and zinc chloride bath.
Figure 4.15: SEM image of ZnO film from sodium zincate bath
4.5 EDX and FTIR studies
Figure 4.16 shows EDX spectrum of ZnO film obtained from ammonium zincate
bath. EDX indicates that the products consist of zinc and oxygen elements. The silicon
signal appears from the substrate and the level of silicon contamination detected in the
films deposited is ~0.5 atomic %. No other impurity was detected in the films. Figure
4.17 on the other hand shows the EDX spectrum of ZnO film obtained from sodium
zincate bath. Apart from silicon, sodium was found to be present in approximately 0.7
atomic %. Trace amount of Calcium and Chlorine was also detected in the film.
81
Figure 4.16: EDX spectrum of ZnO obtained from ammonium zincate bath
Figure 4.17: EDX spectrum of ZnO obtained from sodium zincate bath
Fourier Transform Infra Red (FTIR) spectroscopy is a very useful tool to obtain
information about the chemical bonding and for investigating the vibrational properties of
0 5 10 15
0
5000
10000
15000
Zn
Zn
Ca
CaCl
Si
Na
Zn
Zn
O
Ca
Counts
Energy (KeV)
82
synthesized materials. This technique is based on the absorption of infrared radiation by
the material. When a material is irradiated with infrared radiation, absorbed IR radiation
usually excites molecules into a higher vibrational state. The wavelengths that are
absorbed by the sample are characteristic of its molecular structure. The band positions
and absorption peak not only depend on the chemical composition and structure of the
thin films but on the morphology of thin films also. FTIR analysis was performed using
Perkin-Elmer FTIR [FTIR spectrum RX1]. The FTIR spectrum of ZnO prepared from
ammonium zincate bath is shown in Figure 4.18. The FTIR spectra are usually presented
as plots of percent transmission (transmitted intensity) versus wavenumber (in cm-1). The
absorption band observed at 483.4 cm-1
is attributed to the ZnO stretching vibrations [23-
25].
Figure 4.18: FTIR spectrum of ZnO
The band at 1424.4 cm-1
may be attributed to C-O stretching frequencies [26] and
the band at 3443 may be attributed to O-H species in the film [26-27]. The band at 1122.5
could not be exactly assigned. It may be due to weakly bound acetic acid molecule [27].
4800 4200 3600 3000 2400 1800 1200 600 0
4
8
12
16
1122.5
1424.4
3443
483.4
%T
(a. u
.)
Wave number (cm)-1
83
4.6 Discussion of Results on ZnO thin films
An analysis of the results presented here indicates that strongly c-axis oriented
phase pure polycrystalline ZnO films can be prepared by SILAR technique from
ammonium zincate bath with zinc acetate as the staring precursor. The film growth rate is
a very sensitive function of zincate bath pH. Proper optimization of deposition
parameters such as bath concentrations and pH, temperarture of deposition resulted in
fairly uniform, mechanically hard and reproducible films. Clearly, if the deposition
conditions are not optimum, one can get non-adherent films.The pH has been optimized
in the range 10.80 0.05± for ammonium zincate bath and it is optimized the range
13.20 0.05± for sodium zincate bath to get adherent films on glass substrates. The growth
process follows an empirical linear behavior with number of dippoing cycle for both
sodium zincate and ammonium zincate bath. The growth rate is higher for sodium zincate
(~0.2 µm/diiping/mole) compared to ammonium zincate bath (~0.162 µm/diiping/mole).
The average particle size estimated by x-ray line broadening method was found to
be 22.75 nm (~25.8 nm from TEM) for films deposited from ammonium zincate bath. It
is evident from the present investigation that lowest particle size with highest preferred c-
axis orientation [Texture coeffcient value of ~2.29 for (002) plane] is obtained from
ammonium zincate complex. Films produced from sodium zincate bath exhibits highest
particle size (~41 nm from TEM) indicating possibly that sodium promotes grain growth.
SEM investigation shows round shape grains for films deposited from ammonium zincate
complex whereas it is off spherical for those deposited form sodium zincate bath. Grains
are uniformly distributed throughout the surface for films deposited from ammonium
zincate bath exhibiting the superiority of the films obtained from sodium zincate bath.
The porosity in the films deposited from zincate baths is quite high and it ranges between
30 to 32% as estimated from from cross sectional SEM observations. Films prepared
from ammonium zincat bath are phase pure containing no other impurities as revealed
from EDX. FTIR spectrum reveals the presence of ZnO stretching vibration.
84
References
1. R. L. Call, N. K. Jaber, K. Seshan and Jr. J. R. Whyte, Solar Energy Materials 2
(1980) 373.
2. M. Ristov, G. J. Sinadinovski, I. Grozdanov and M. Mitreski, Thin Solid Films 149
(1987) 65.
3. Y. F. Nicolau, M. Dupuy and M. Brunel, J. Electrochem. Soc. 137 (1990) 2915.
4. A. E. Jimenez-Gonzailez and P. K. Nair, Semicond. Sci. Technol. 10 (1995) 1277.
5. A. Raidou, M. Aggoer, A. Qachasu, L. Lanab and M. Fahoume, M. J. Cond. Mat. 12
(2012) 125.
6. A. P. Chatterjee, P. Mitra and A. K. Mukhopadhyay, J. Mat. Sc. 34 (1999) 4225.
7. P. Mitra and J. Khan, Mater. Chem. Phys. 98 (2006) 279.
8. A. Wojcik, M. Godlewski, E. Guziewicz, R. Minikayev and W. Paszkowicz, J.
Crystal Growth 310 (2008) 284.
9. M. C. Sneed and R. C. Brasted, in “Comprehensive Inorganic Chemistry”, Vol. 4
(Princeton, New York), 1955.
10. J. W. Mellor, in “A Comprehensive Treatise on Inorganic and Theoretical
Chemistry”, Vol. 4, (Longman-NY, USA, 1946) p. 521.
11. A. E. Rakhshani, Appl. Phys. A92 (2008) 413.
12. M. H. Lietzke, H. Marshall and L.William, Journal of Solution Chemistry 15 (1986)
903.
13. Flinn Scientific Inc, Material Safety Data sheet (MSDS, 2002).
14. S. S. Kale, R. S.Mane, H. M. Pathan, A. V. Shaikh, O. S. Joo and S. H. Han, Applied
Surface Science 253 (2007) 4335.
15. S. Ilican, Y. Caglar and M. Caglar, J. of Optoelectronics and Advanced Materials 10
(2008) 2578.
16. B. Post, S. Weissmann and H. F. McMurdie (eds.), Joint Committee on Powder
Diffraction standards, Inorganic Vol., Card No. 36-1451, International Centre for
Diffraction Data, Swarthmore, PA (1990).
17. B. E. Warren, in “X-ray diffraction” 2nd
Edition Courier Dover publications 1969.
85
18. H. P. Klug and L. E. Alexander, in “X-ray diffraction procedures for polycrystallime
and amorphous materials” (Wiley, New York, 1974).
19. P. Sharma, A. Gupta, K. V. Rao, F. J. Owens, R. Sharma, R. Ahuja, J. M. O. Gullen,
B. Johansson and G. A. Gehring, Nature Material 2 (2003) 673.
20. S. Patra, P. Mitra and S. K. Pradhan, Mat. Res. 14 (2011) 17.
21. X. Jumin and J. Wang, Mater. Lett. 49 (2001) 318.
22. B. Ghosh, H. Dutta and S. K. Pradhan, Journal of Alloys and Compounds 479 (2009)
193.
23. Z. Yang, Z. Ye, Z. Xu, B. H. Zhao, Physica E 42 (2009)116.
24. T. Ivanova, A. Harizanova, T. Koutzarova, B. Vertruyen, Materials Letters 64 (2010)
1147.
25. Z. R. Khan, M. S. Khan, Zulfequar, M. S. Khan, Materials Sciences and Applications
2 (2011) 340.
26. M. N. Kamalasanan and S. Chandra, Thin Solid Films 288 (1996) 112.
27. A. Pakdel and F. E. Ghodsi, Pramana - J. Phys. 76 (2011) 973.
86
CHAPTER 5
Preparation of Cd doped ZnO thin films by
SILAR and their characterization
5.1 Preparation of Cd doped ZnO (Cd:ZnO) films
Preparation of Cd doped ZnO was carried out from 0.1M sodium zincate
( )2 2Na ZnO bath kept at room temperature and hot water bath. Cadmium doping was
done by adding cadmium chloride (CdCl2.H2O, GR grade, Mol. Wt. 201.32) in sodium
zincate bath. Efforts to prepare Cd:ZnO films from ammonium zincate bath resulted in
instability of the bath as precipitates appear within the bath. The stability of ammonium
zincate bath is very sensitive with respect to pH due to evaporation of ammonia (Scetion
4.2.1, Chapter 4). Cadmium incorporation reduces the bath pH and results in unstable
ammonium zincate bath. The details of preparation of sodium zincate bath have already
been discussed in Chapter 4 section 4.2. The precleaned glass substrate was alternately
dipped in the cationic precursor (sodium zincate solution containing cadmium chloride)
for 2 s and for 2 s in hot water bath. The cadmium concentration was varied upto 10%
(atomic %) in the bath solution. More than 10% cadmium chloride could not be dissolved
in sodium zicate bath and precipitates appear. Fifty (50) dipping cycles were performed
for the present experiment. The thickness for pure ZnO film measured gravimetrically
was ~ 1.0 µm. The growth rate was found to decrease with increasing Cd doping. The
film thickness was ~0.94 µm for 5% Cd:ZnO and 0.85 µm for 10% Cd:ZnO.
87
5.2 Structural characterization: Evaluation of particle size
The X-ray diffraction patterns of undoped and Cd doped ZnO thin films are shown
in figure 5.1. The diffraction pattern for undoped ZnO is shown in figure 5.1 (a), while
figures 5.1(b) and 5.1(c) shows the diffractograms for 5% and 10% Cd:ZnO films
respectively. All the films were heat treated 350oC for 2 hr. prior to structural
characterization. Peaks for undoped ZnO appears at 31.60o, 34.35o, 36.20o, 47.55o and
56.55o corresponding to the reflecting planes (100), (002), (101), (102) and (110) which
are characteristics of hexagonal ZnO (Section 4.3, Chapter 4). The diffraction peaks are
shifted only marginally towards the low angle side due to slightly higher ionic radius of
cadmium (II) ion compared to that of zinc (II) ion [1-2].
Figure 5.1: XRD patterns of (a) ZnO, (b) 5% Cd doped ZnO and (c) 10% Cd doped ZnO
It is evident from figure 5.1(a) that undoped ZnO film prepared from sodium
zincate bath have a polycrystalline structure with strong preferred orientation in the (002)
20 30 40 50 60
0
500
1000
1500
2000
(110)
(102
)
(10
1)
(002)
(100)
(a)
2θ (degree)
0
500
1000
1500
2000
(b)
Inte
nsity (
a.u
.)
0
500
1000
1500
2000
(c)
88
direction. Compared with pure ZnO film, the intensity of (002) peak i.e. preferred c-axis
orientation decreases for Cd:ZnO films. The TC value for (002) plane of undoped ZnO
was ~1.95 and it decreases to ~0.6 for 10% Cd:ZnO film. [Evaluated using eqn. 3.5
(Section 3.2.1 of chapter 3) and following the method discussed in section 4.3 of chapter
4]. MARQ2 analysis (as discussed in section 4.3 of chapter 4) was carried out for
undoped ZnO sample and the fitting curve is shown in Figure 5.2. Intensity in arbitrary
units along y − axis is not shown in the figure.
Figure 5.2: Observed (dotted) and simulated (continuous) x-ray diffraction patterns of
ZnO. The difference plot is shown at the bottom.
Table 5.1 shows the values of peak positions, observed broadeningo
β obtained
using MARQ2 fitting and corresponding values of instrumental broadeningi
β . Using
these values, β was calculated from equation 4.13 (Section 4.3, Chapter 4). These values
of β were converted to radians for particle size estimation using Scherrer equation (Eqn.
3.2, Section 3.1.1, Chapter 3). The average value of particle size for undoped ZnO was
~36.73 nm. The actual particle size will be little higher than this since broadening due to
31.0 32.0 33.0 34.0 35.0 36.0 37.0
89
strain was not taken into account while particle size estimation. The x-ray value is less
than the TEM value of ~ 41 nm (Figure 4.12, Chapter 4).
Table – 5.1: Particle size in undoped ZnO film prepared from sodium zincate bath
Peak position
(2θ) o
β
(in degrees)
iβ
(in degrees)
β
(in degrees)
Particle
size (nm)
Average
particle
size
31.6 0.331 0.0979 0.2331 37.74
34.35 0.305 0.0981 0.2069 41.93 36.73 nm
36.2 0.397 0.0983 0.2988 30.51
The FWHM ( )β values for Cd:ZnO films was found to increase with Cd
incorporation. The average value of β in degrees for 5% Cd:ZnO and 10% Cd:ZnO are
0.2735 and 0.2899 respectively as opposed to 0.2427 for pure ZnO. The average particles
size comes out to be ~32 nm and ~29.9 nm respectively for 5% and 10% Cd:ZnO
respectively using the corresponding values of β . Such broadening of x-ray diffraction
peaks and decrease in particle size for Cd doped films has been reported by Maiti et. al.
[1] and Vijayalakshmi et. al. [3]. These observations along with decrease in relative
intensity of (002) peak confirms that Cd incorporation increases the degree of
polycrystallinity of the films. The decrease in particle size with increasing Cd
incorporation is possibly due to strain developed in the films due to replacement of
2Zn
+ ion by 2Cd
+ ion in ZnO lattice [1]. As explained in some literature, such increase of
strain energy may lead to a loss of preferred orientation and enhancement of random
orientation in polycrystalline ZnO [4-5].
5.3 SEM and EDX studies
Figures 5.3, 5.4 and 5.5 shows the SEM images of pure, 5% and 10% Cd: ZnO
films respectively. The images show a general view of the morphology of pure and Cd
doped ZnO films synthesized on glass substrate. The polycrystalline structure is revealed
90
from the SEM micrographs. The films are porous as evident from absence of close
packed morphology. The formation of sub-micrometer crystallites of varying sizes
indicates agglomeration and such agglomeration in certain regions of the films is evident
from the figures. Such agglomeration makes it difficult to evaluate the grain size form
SEM images. Some difference in surface morphology is observed for Cd:ZnO films
[Figures 5.4 and 5.5] compared to pure ZnO [Figure 5.3]. It appears that the morphology
gets less rougher for Cd:ZnO films. Similar observation of reduced surface roughness has
been reported for spray pyrolysed films [3].
Figure 5.3: SEM image of ZnO Figure 5.4: SEM image of 5% Cd:ZnO
,
Figure 5.5: SEM image of 10% Cd:ZnO
91
The compositional analysis of Cd doped ZnO films carried out by energy
dispersive X-ray (EDX) analysis is shown in figure 5.6 Figure 5.6 (a) shows the EDX
spectrum of 5% Cd:ZnO and 5.6 (b) shows the spectrum of 10% Cd:ZnO. The films were
repeatedly washed in hot water before EDX analysis.
Figure 5.6. EDX pattern of (a) 5% Cd:ZnO and (b) 10% Cd:ZnO
(a)
(b)
92
The EDX spectrum confirmed the presence of Zn, O and Cd elements in the
deposited films. The silicon signal appears from the substrate. Trace amount of carbon
was also detected in the film. Dopant concentration in these two cases was 5% and 10%
in the starting solution. Accordingly the expected Cd/Zn ratio was 0.05 and 0.1 in the
films. We actually obtained the Cd/Zn ratio in the films as 0.0075 and 0.0185
respectively indicating that the amount of Cd incorporation in the film is much less than
the amount of Cd in the starting solution.
5.4 Optical band gap evaluation of Cd:ZnO films
The optical absorption spectrum of the undoped ZnO and Cd doped ZnO thin
films were determined at room temperature in a UV-VIS spectrophotometer (Shimadzu,
Model No. UV-1800) in the wavelength range 200-500nm. The spectra were recorded by
taking a similar glass as a reference on which film deposition was carried out and hence
the absorption spectra obtained was from the films only (Section 3.1.3, Chapter 3). The
band gap of the films has been calculated from the absorption edge of the spectrum. Both
ZnO and CdO are considered as direct band gap materials [6]. The energy gap ( )gE can
thus be estimated by assuming direct transition between conduction band and valance
bands. Thus the direct band gap can be evaluated by putting 2n = in eqn. 3.4 (section
3.1.3, chapter 3) i.e. from the equation
( ) ( )2(5.1)
gh A h Eα ν ν= − →
The direct band gap is determined using this equation when linerar portion of
( )2
hα ν against hν plot is extrapolated to intersect the energy axis at α =0. Plot of
( )2
hα ν against hν for undoped and cadmium doped ZnO films are shown in figure 5.7.
The presence of a single slope in the plot suggests that the films have direct and allowed
93
transition. Figure 5.7 (a) shows the spectrum for pure ZnO while figures 5.7 (b) and 5.7
(c) shows the spectrum for 5% Cd:ZnO and 10% Cd:ZnO respectively.
Figure 5.7: Photon energy (eV) dependence of (a) ZnO, (b) 5% Cd:ZnO and
(c) 10% Cd:ZnO
It is seen that with the increase of cadmium doping level, the fundamental
absorption edge decreases. The value of g
E for undoped ZnO is 3.18 eV. It decreases to
3.14 eV for 5% Cd:ZnO and to 3.11 eV for 10% Cd:ZnO. The measured values are an
average of at least three measurements and are within the error limit of 0.01± eV. This
decrease can be accounted for the large difference in g
E values of ZnO and CdO [3, 7-9].
While Maiti et. al. [1] reported a decrease in band gap value from 3.29 eV for undoped
ZnO to 3.15 ev for 6% Cd doped ZnO, Vijayalakshmi et. al. [3] reported a decrease
from 3.12 eV for undoped to 2.96 eV for 25% Cd doped ZnO.
2.4 2.6 2.8 3.0 3.2 3.4
0
20
40
60
80
100
120
140
160
180
(c)
(a)(b)
(αh
ν)2
hν
94
5.5 Discussion of Results on Cd:ZnO thin films
Cd doped ZnO films could be successfully synthesized from sodium zincate bath
with cadmium chloride as source of Cd by SILAR. The films had good adherence to the
substrate. XRD spectra revealed that the films are polycrystalline with hexagonal ZnO
structure. Particle size evaluated using x-ray line broadening analysis shows a constantly
decreasing trend with increasing Cd incorporation. The average particle size of undoped
ZnO from sodium zincate bath is ~ 36.73 nm evaluated by x-ray line broadening method
neglecting strain broadening. The corresponding value from TEM is ~ 41 nm. The
average particle size reduces to ~32 nm for 5% Cd:ZnO and ~29.9 nm for 10% Cd:ZnO
evaluated by x-ray method. The undoped ZnO film is polycrystalline with strong
preferred c-axis orientation. The preferred orientation is lost and the degree of
polycrystallinity increases with increasing Cd incorporation. SEM shows polycrystalline
and porous nature of the films with surface morphology getting smoother due to Cd
incorporation. With increase of Cd doping, the fundamental absorption edge changes.
The value of fundamental absorption edge ( )gE is 3.18 eV for pure ZnO and it decreases
to 3.11 eV for 10% Cd:ZnO. Upon increasing the Cd concentration in the starting
solution, the amount of Cd in the solid films increases. The low incorporation of Cd into
the films (0.75% in the film against 5% in the starting solution for 5% Cd:ZnO film and
1.85% in the film against 10% in the starting solution for 10% Cd:ZnO film obtained
from EDX measurements) may be due to mild working conditions of SILAR technique.
More than 10% dopant addition in the starting solution was not possible since the starting
solution loses stability and precipitates appear within the bath. These observations along
with EDX observation confirm the replacement of zinc ion by cadmium ions in the ZnO
lattice. Although it is difficult to pedict the exact amount of Cd incorporation from EDX
analysis (which measures the atomic % by measuring the area under the curve in the
spectrum), marginal shift in diffraction peak positions and moderate reduction of optical
band gap energy apart from EDX estimation indicates that Cd incorporation in the films
is less than that in the starting solution.
95
References
1. U. N. Maiti, P. K. Ghosh, S. F. Ahmed, M. K. Mitra and K. K. Chattopadhyay, J.
Sol–Gel Sci. Technol. 41 (2007) 87.
2. F. Z. Wang, Z. Z. Ye, D. W. Ma, L. P. Zhu, F. Zhuge and H. P.He, Appl Phys Lett.
87 (2005) 143101.
3. S. Vijayalakshmi, S. Venkataraj and R. Jayavel, J. Phys. D: Appl. Phys. 41 (2008)
245403.
4. S. C. Seel, R. Carel and C. V. Thompson, in “Polycrystalline thin films: Structures,
textures, properties and applications II” (Mater. Res. Symp. Proc., Pittsburg, PA,
1996).
5. Y. E. Lee, Y. J. Kim and H. J. Kim, J. Mater. Res. 13 (1998) 1260.
6. H. Tabet-Derraz, N. Benramdane, D. Nacer, A. Bouzidi and M. Medles, Sol. Energy
Mater. Solar Cells 73 (2002) 249.
7. L. F. Dong, Z. Cui and Z. K. Zhang, Nanostruct. Mater. 8 (1997) 815.
8. H. Cao, J. Y. Xu, D. Z. Zhang, S. H. Chang, S. T. Ho, E. W. Seeling, X. Liu and R.
P. H. Chang, Phys. Rev. Lett. 84 (2000) 558410.
9. O. Vigil, L. Vaillant, F. Cruz, G. Santana, A. M. Acevedo and G. C. Puente, 2000
Thin Solid Films 361 (2000) 53.
96
CHAPTER 6
Preparation of Mn doped ZnO thin films
by SILAR and their characterization
6.1 Preparation of films and thickness measurements
Preparation of Mn doped ZnO film was carrid out from zinc chloride
( )2ZnCl solution as cationic precursor containing manganese (II) chloride as source of
Mn ion and sodium hydroxide ( )NaOH solution as anionic precursor. Efforts to
synthesize Mn doped ZnO from sodium zincate as well as ammonium zincate bath failed
possibly due to lowering of pH upon dopant addition. Although manganese chloride
could be dissolve in low concentrations in these baths, no film formation took place.
The concentration of the zinc chloride (ZnCl2.2H2O, Merck, Mol. Wt. 136.28)
bath and sodium hydroxide bath was optimized at 0.1 M and 0.075 M respectively for
synthesis of good quality adherent film. For concentrations more than 0.125 M for zinc
chloride bath and for concentrations more than 0.1 M for sodium hydroxide bath, the
growth process nonuniform resulting in poor quality and nonadherent films. The cationic
precursor was at room temperature and the temperature of anionic precursor was
optimized at 70oC.
97
One of the problems with zinc chloride solution is that complete dissolution of the
solute does not occur and precipitate appears on standing. Addition of three drops of
acetic acid (~ 0.3 cc by volume) gives a clear transparent solution. The pH of the
transparent solution was 4.70 ± 0.05. Sodium hydroxide solution (0.075 M) was prepared
by dissolving NaOH pellets (Merck, Mol. Wt. 40) in deionized water. The optimized pH
of the sodium hydroxide bath was 11.10± 0.05. Alongwith bath concentrations, the pH
and temperature of the baths were found to be optimum for getting adherent films on
substrate.
Although adherent films could be obtained on glass substrate, it was found during
the course of the experiment that adhesion of the films on quartz substrate was stronger
compared to glass. The adherence of the ZnO films on glass substrate was found to be
somewhat lesser compared to those deposited from sodium or ammonium zincate baths.
Both microscope glass slides and commercially available quartz substrates were used for
film deposition. For Mn doping, Manganese (II) chloride (MnCl2.4H2O, Merck, Mol.
Wt. 197.9) was dissolved in zinc chloride solution. Addition of manganese chloride
tetrahydrate gave the solution a slightly ash colouration. The resulting mixture was stirred
using a magnetic stirrer for about 10 minutes. After stirring the manganese chloride salt
gets completely dissolved in the solution. The manganese concentration was varied upto
5% in the zinc chloride solution for the preparation of doped films. For 10%≥ dopant
addition, the bath pH of the cationic precursor reduces reulting in slow growth rate and
poor quality of the coated films. Accordingly the dopant addition was restricted to 5
atomic %.
The quartz substrate was cleaned, before deposition, by etching in 1%
hydrofluoric acid )(HF for 24 hours followed by ultrasonic cleaning in equivolume
acetone and alcohol and thorough rinsing in deionized water. The cleaned substrate was
alternatively dipped in zinc chloride solution impurified with Mn (II) chloride and hot
NaOH solution. Dipping for 2 s in each bath constitutes one complete dipping cycle.
98
The film thickness )(t was built up by increasing the number of dipping cycle.
Fifty (50) dipping cycles were performed in the present experimens. The deposited films
were subsequently annealed in air at 350oC for 2 hr. Figure 6.1 shows the dependence of
film thickness (measured gravimetrically) on the number of dipping cycle )(N for
undoped ZnO films on quartz substrate.
It is seen from figure 6.1 that the film thickness follows a linear growth law with
number of dipping cycle and the growth rate was found to be 0.021 µm/dipping. There is
an overall variation of ± 5% in the film thickness data (shown as error bars against each
data point of Fig. 6.1). The growth rate was found to be uniformly low for glass substrate.
Figure 6.1: Dependence of film thickness on number of dipping cycle
The film thickness was verified against cross sectional SEM. Some portion of the
quartz substrate was acid etched to remove film from that area in order to create a step for
thickness measurement. Figure 6.2 shows the cross-sectional SEM micrograph of
undoped ZnO film of thickness 2.1 µm measured gravimetrically (obtained by 100
dipping). An average thickness of 2.56 µm was obtained from SEM micrograph.
0 25 50 75 100 125
0.0
0.5
1.0
1.5
2.0
2.5
Th
ickne
ss (
µm
)
No. of dipping (N)
99
The actual thickness determined from cross-sectional SEM is 22% higher than the
gravimetric value (2.56 µm measured by SEM as opposed to 2.1 µm measured
gravimetrically). The value was an average of several measurements on different
portions. This indicates an average porosity of ~ 22% in the deposited films. The films
are less porous compared to those from ammonium and sodium zincate baths (Section
4.2.4, Chapter 4).
Figure 6.2: Cross-sectional SEM of ZnO film
The growth rate was found to decrease with Mn incorporation. For 50 dipping, the
film thickness for pure ZnO was 1.05 µm. The corresponding thickness for 2% Mn:ZnO
film was 0.94 µm and for 5% Mn:ZnO, the thickness was 0.82 µm. Thus the growth rate
decreases with increasing Mn incorporation. The ZnO film was white in appearance and
Mn doped films were slightly brownish with a good adherence to the substrate.
Table 6.1 shows the thickness and growth rate values for undoped and Mn doped
films for 50 dipping on quartz substrate. The thickness of pure ZnO film was 0.80
mµ and Mn doped film it was 0.71 mµ on glass substrate.
Quartz substrate
Film
100
Table 6.1: Thickness and growth rate for ZnO and Mn:ZnO films
Film Thickness (µm) Growth rate
(µm/dipping)
ZnO 1.05 0.021
2% Mn:ZnO 0.94 0.0188
5% Mn:ZnO 0.82 0.0164
6.2 Structural characterization: Evalaution of particle size and strain
The X-ray diffraction patterns of undoped ZnO and Mn doped ZnO films
deposited on quartz substrate are shown in figure 6.3. The diffraction pattern for undoped
ZnO is shown in figure 6.3 (a). Figure 6.3 (b) and 6.3 (c) shows the diffractograms for
2% and 5% ZnOMn : films respectively. The films were heat treated at 350oC for 2 hr.
prior to structural characterization. The materials were scanned in the range 25-65o. It is
seen from figure 6.3 (a) that peaks appear at 31.708o, 34.397
o, 36.183
o, 47.516
o, 56.551
o
and 62.88o. The corresponding values for 5% Mn:ZnO are 31.69
o, 34.372
o, 36.168
o,
47.5o, 56.542
o and 62.87
o. The diffractogram of the sample reveals that all the peaks are
in good agreement with the JCPDS data belonging to hexagonal ZnO structure. The
corresponding reflecting planes are (100), (002), (101), (102), (110) and (103)
respectively. The (002) peak appears with maximum intensity at 34.397o. Apart from
ZnO characteristic peaks, no extra peaks due to manganese clusters, zinc or their complex
oxides could be detected within the detection limit of XRD. This observation is an
indication of the fact that the films do not have any phase segregation or secondary phase
formation as well as Mn incorporation into ZnO lattice.
It is evident from figure 6.3 (a) that undoped ZnO film have a polycrystalline
structure with preferred orientation along the (002) diffraction plane. Compared to
undoped ZnO film, the intensity of (002) peak decreases for Mn:ZnO films. This results
101
in an increase of relative intensity of (101) peak with respect to (002) peak. The (101)
peak appears with maximum intensity in ZnO powders with no preferred orientation
(JCPDS Card No. 36-1451). Thus crystalline nature of films was affected due to
enhancement of dopant concentration. Such loss of preferred c-axis orientation and
enhancement of polycrystalline nature with Mn incorporation has been reported by
Nirmala et. al [1]. The value of (002)TC for undoped ZnO is ~1.82 and it decreases to
~0.41 for 5% Mn:ZnO film [Evaluated using eqn. 3.5 (Section 3.2.1 of chapter 3) and
following the method discussed in section 4.3 of chapter 4].
Figure 6.3: X-ray diffraction pattern of (a) ZnO, (b) 2% Mn:ZnO and (c) 5% Mn:ZnO.
30 40 50 60
0
500
1000
1500
2000
2500
(103)
(110)
(102)
(101)
(100) (0
02)
(a)
2θ (degree)
30 40 50 60
0
500
1000
1500
2000
2500
(b)
Inte
nsity (
a.u
.)
30 40 50 60
0
500
1000
1500
2000
2500
(c)
102
The peaks of the diffraction pattern of the doped sample are slightly shifted to left
compared to undoped ZnO. This is possibly because the ionic radius of 2Mn
+ (0.83Å) is
larger than that of 2Zn
+ (0.74 Å) [1]. A typical plot of MARQ2 analysis for 5% Mn:ZnO
sample is shown in Figure 6.4.
Figure 6.4: Observed (dotted) and simulated (continuous) x-ray diffraction patterns of
5% Mn:ZnO on quartz substrate
The average value of particle size for undoped ZnO evaluated by x-ray line
broadening method is 29.71 0.01± nm. It decreases to 26.79 0.01± nm for 2% Mn:ZnO
and 23.76 0.01± nm for 5% Mn:ZnO. Figure 6.5 shows the W-H plots of ZnO, 2%
Mn:ZnO and 5% Mn:ZnO. The average microstrain in the films as determined from W-H
plots is 0.0013, 0.00137 and 0.00146 respectively for pure, 2% Mn:ZnO and 5% Mn:ZnO
films respectively. Thus the particle size decreases with increasing Mn incorporation and
the strain increases. The decrease in average particle size with increasing Mn doping i.e.
hindrance of grain growth upon Mn incorporation has been reported by other workers [2-
30.0 31.0 32.0 33.0 34.0 35.0 36.0 37.0 38.0
Inte
nsi
ty (
a. u
.)
2θ
103
4]. The decrease in crystal quality with Mn doping has been reported by Lee et al [5]. The
decrease in average particle size might be due to development of strain because of Mn
incorporation. Such enhancement of average microstrain with Mn incorporation has been
observed in the present work. The enhancement of strain due to Mn incorporation might
be due to larger ionic radius of Mn ion than Zn ion.
Figure 6.5: W-H plots of (a) pure ZnO, (b) 2% Mn:ZnO and (c) 5% Mn:ZnO films The X-ray diffraction patterns of undoped ZnO and 5% Mn doped ZnO films
deposited on glass substrates is shown in figure 6.6. The diffraction pattern for undoped
ZnO is shown in figure 6.6 (a). Figure 6.6 (b) shows the diffractograms for 5%
ZnOMn : film. The films were heat treated at 350oC for 2 hr. prior to structural
characterization. The step-scan data were recorded for the angular range 20o to 70
o.
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
0.000
0.005
0.010
0.015
0.020
(a)
4sinθ
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
0.000
0.005
0.010
0.015
0.020(b)
βco
sθ
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
0.000
0.005
0.010
0.015
0.020
(c)
104
30 40 50 60
0
400
800
1200
1600
2000
2400
2800
(a)
2θ (degree)
30 40 50 60
0
400
800
1200
1600
2000
2400
2800
(b)
Inte
nsity (
a.u
.)
Figure 6.6: X-ray diffraction pattern of (a) ZnO and (b) 5% Mn:ZnO deposited on glass
It is seen from figure 6.6 (a) that peaks appears at 31.7o, 34.42o, 36.2o, 47.46o,
56.56o and 62.84
o. Similar observation of loss of preferred orientation along c-axis is
evident from the figure. The preferred orientation of the films is governed by the total
system energy, which is the summation of the strain and surface energies [6]. Thus
increase in strain energy effects the preferred growth along c-axis since it is known that
the driving force towards preferred orientation arises out of total energy minimization of
the system.
6.3 SEM and EDX studies
Figure 6.7 shows the HRSEM micrograph of pure ZnO film prepared on quartz
substrate. HRSEM study was undertaken in a FEI FEG Nova 600 Nanolab at 5 kV. The
SEM image shows structure consisting of many spherical shaped nano particles with an
105
averge size of ~ 31 nm. This is similar to the result obtained on glass substrate (chapter 4,
Figure 4.18). The average particle size of ~31 nm matches well with that obtained using
x-ray line broadening analysis of ~29.71 nm. Figure 6.8 shows the SEM image of 5%
Mn:ZnO film on quartz substrate. Surface morphology of 5% Mn:ZnO film shows
wrinkle structure with formation of nanorods in certain regions.
Figure 6.7: HRSEM image of ZnO on quartz substrate
Figure 6.8: SEM image of 5% Mn:ZnO thin film
106
Similar observation of appearance of wrinkle structure due to Mn incorporation
has been reported by Nirmal et al [1]. Srinivasan et. al. [7] also reported microstructure
consisting of nanorods with wrinkle structure for Mn doped films. Formation of such
nanorods in Mn doped ZnO films have also been reported by Karamat et. al. [8]. Figure
6.9 shows the HRSEM image of 5% Mn:ZnO film on glass substrate with magnification
25000× [10].
Figure 6.9: HRSEM image of 5% Mn:ZnO
Figure 6.10 shows the energy dispersive X-ray spectrum of Mn:ZnO films
prepared on quartz substrate. Figure 6.10 (a) shows the EDX spectrum of 2% Mn:ZnO
and 6.10 (b) shows the spectrum of 5% Mn:ZnO. The EDX spectrum confirmed the
presence of Zn, O and Mn elements in the deposited films i.e. incorporation of Mn in
ZnO lattice. The silicon signal appears from the quartz substrate. Dopant concentration in
these two cases was 2% and 5% in the starting solution. Accordingly the expected Mn/Zn
ratio was 0.02 and 0.05 in the films. We actually obtained the Mn/Zn ratio in the films as
0.0131 and 0.0284 respectively indicating that the amount of Mn incorporation in the
film is less than the amount of Mn in the starting solution. The real Mn content in the
deposited films was 1.31% and 2.84% as obtained from EDX spectrum.
107
Figure 6.10. EDX pattern of (a) 2% Mn:ZnO and (b) 5% Mn:ZnO
(b)
(a)
108
Figure 6.11 reveals the EDX spectrum of 5% Mn:ZnO film on glass substrate.
Trace amount of calcium (Ca) impurity was also detected in the film.
Figure 6.11: EDX pattern of 5% Mn:ZnO
6.4 Evaluation of band gap from Optical absorption
The optical absorption spectra were recorded by using a similar quartz substrate
as a reference and hence the absorption due to the film only was obtained. Figure 6.12
shows the dependence of optical absorbance ( )α on wavelength ( )λ . While figure 6.12
(a) shows the dependence of α on λ for pure ZnO, figures 6.12 (b) and 6.12 (c) shows
dependence of α on λ for 2% Mn:ZnO and 5% Mn:ZnO respectively. Plot of
( )2
hα ν against hν for undoped and Mn doped ZnO films was derived from figure 6.12
and is shown in figure 6.13. Figure 6.13 (a) shows the spectrum of undoped ZnO while
figures 6.13 (b) and 6.13 (c) shows the spectrum of 2% Mn:ZnO and 5% Mn:ZnO
respectively. The direct band gap is determined using this equation when linerar portion
of ( )2
hα ν against hν plot is extrapolated to intersect the energy axis at α =0.
109
Figure 6.12. Plots of absorbance vs wavelength for (a) undoped ZnO; (b) 2% Mn:ZnO
and (c) 5% Mn:ZnO
Figure 6.13: Plots of ( )2ναh vs νh for (a) pure ZnO; (b) 2% Mn:ZnO and (c) 5% Mn:ZnO
360 380 400 420 440 460 480 500
1.0
1.5
2.0
2.5
3.0
3.5
4.0
(c)(b)
(a)
absorb
an
ce
(α
)
λ(nm)
2.4 2.6 2.8 3.0 3.2 3.4 3.6
0
20
40
60
80
100
120
140
160
180
200
(b)
(c)
(a)
(αh
ν)2
hν (eV)
110
It is seen that with the increase of manganese doping level, the fundamental
absorption edge decreases. The value of g
E for undoped ZnO is 3.22 0.01± eV. It
decreases to 3.13 0.01± eV for 2% Mn:ZnO and to 3.06 0.01± eV for 5% Mn:ZnO.
The decrease in band gap value with increased Mn doping concentration has been
accounted due to the sp-d exchange interactions and has been theoretically explained
using the second–order perturbation theory [1, 9-10]. A decrease in band gap energy from
3.27 eV for undoped ZnO to 2.78 eV for 3% Mn doped ZnO has been reported by
Senthilkumar et. al. [4] and has been attributed to s-d and p-d interactions giving rise to
band gap bowing.
Similar results on glass substrate for undoped 5% Mn:ZnO film is shown in figure
6.14. The value of g
E for undoped ZnO is 3.20 eV and it decreases to 3.04 eV for 5%
Mn:ZnO. The data for drawing figure 6.14 was extracted from the data of α versus λ .
Figure 6.14: Plots of ( )2ναh vs νh (in eV) for (a) pure ZnO and (b) 5% Mn:ZnO
2.4 2.6 2.8 3.0 3.2 3.4
0
20
40
60
80
100
120
140
160
(b)
(a)
(αh
ν)2
hν
111
6.5 Discussion of results on Mn:ZnO thin films
The primary aim of this investigation was to explore the possibility of doping or
impurifying ZnO with manganese by SILAR method. Mn doped ZnO films with different
percentage of Mn content (upto 5%) could be successfully synthesized by suitable choice
of cationic and anionic precursors under optimized deposition conditions. Zinc chloride
bath with manganese chloride as source of Mn ion was used as cationic precursor and
sodium hydroxide was used as anionic precursor. The film growth rate was found to
increase linearly with number of dipping cycle. Better adherence of Mn:ZnO films were
obtained on quartz substrate compared to glass substrate. More than 5% dopant addition
was difficult to obtain due to lowering of stability of the cationic bath. Particle size
evaluated using x-ray line broadening analysis shows a constantly decreasing trend with
increasing manganese incorporation. The average particle size of ~29.71 nm for undoped
ZnO evaluated by x-ray line broadening method matches well with HRSEM observation
(~31nm). The average particle size reduces to ~26.69 nm for 2% Mn:ZnO and ~23.76 nm
for 5% Mn:ZnO. The films are polycrystalline with an average porosity of ~22%. The
polycrystallinity of the films as well as the average microstrain (evaluated using
Williamson-Hall equation) increases with increasing Mn incorporation. Mn doping also
influences the morphology of the films. The undoped films contained nearly spherical
grains. On the other hand microstructure consisting of wrinkle structure was observed
due to Mn incorporation. The observation was similar for both quartz and glass
substrates. These observations along with EDX observation confirms the replacement of
zinc ion by manganese ions in the ZnO lattice. The real Mn content in the deposited film
was less than that in the starting solution as evident from EDX measurements. The
oxidation state of Mn in ZnO is controversial and no experiment was taken up in this
direction. This is important for magnetic properties on Mn:ZnO. Mn doping reduces the
value of fundamental absorption edge from ~3.22 eV for pure ZnO to ~3.06 eV for 5%
Mn:ZnO for films deposited on quartz substrate. Corresponding values on glass were
3.20 eV and 3.04 eV respectively. Incorporation of Mn has a strong effect on the
structural, morphological and optical properties of ZnO.
112
References
1. M. Nirmala and A. Anukaliani, Photonics Letters of Poland 2 (2010) 189.
2. J. Luo, J. K. Liang, Q. L. Liu, F. S. Liu, Y. Zhang, B. J. Sun and G. H. Rao, J. Appl.
Phys 97 (2005) 086106.
3. S. Deka and P. A. Roy, Solid State Communications 142 (2007) 190.
4. S. Senthilkumar, K. Rajendran, S. Banerjee, T. K. Chini and V. Sengodan, Materials
Science in Semiconductor Processing 11 (2008) 6.
5. J. H. Lee and B. O. Park, Thin Solid Films 426 (2003) 94.
6. U. C. Oh and J. H. Je, J. Appl. Phys. 74 (1993) 1692.
7. G. Srinivasan and J. Kumar, Applied Surface Science 254 (2008) 7285.
8. S. Karamat, S. Mahmood, J. J. Lin, Z. Y. Pan, P. Lee, T. L. Tan, S. V. Springham, R.
V. Ramanujan and R. S. Rawat, Applied Surface Science 254 (2008) 7285.
9. R. B. Bylsma, W. M. Becker, J. Kossut, U. Debska and D. Y. Short, Phys Rev B 33
(1986) 8207.
10. P. Singh, A. Kaushal and D. Kaur, J. Alloys and Compounds 471 (2009) 11.
113
CHAPTER 7
Preparation of Al doped ZnO (AZO) thin
films by SILAR and their characterization
7.1. Preparation of AZO films
Al doped ZnO films could be deposited from both sodium zincate and ammonium
zincate complex on glass substrate. The results presented here are for ammonium zincate
bath. Aluminium doping was carried out by adding hexahydrate aluminium chloride
(3 2.6AlCl H O , Merck) as the source of dopant and was added in requisite amount in the
ammonim zincate bath. Fifty (50) dipping cycles were performed for ZnO and AZO films
in the present experiment.
The aluminium concentration was varied upto 2% since optimum incorporation of
Al in ZnO has been reported to be around 1-3 atomic% [1-3]. All the deposited films
were white in color and homogeneous. Coated films were well adherent on glass
substrate. The thickness for pure ZnO film measured gravimetrically was ~ 0.8 µm (0.162
µm per dipping). Growth rate of the films was found to increase due to Al incorporation.
The thickness for 1% Al doped film was found to be ~0.96 µm (0.192 µm per dipping)
indicating higher growth rate due to Al incorporation.
114
7.2 Structural characterization by XRD: Evaluation of TC (002)
The X-ray diffraction patterns of undoped and Al doped ZnO films with different
Al content prepared from ammonium zincate bath are presented in figure 7.1. The films
were annealed at 350oC for 2 hr. in air prior to structural characterization. The diffraction
angle 2θ was scanned in the range 20o to 70
o.
.
Figure 7.1: X-ray diffraction pattern of (a) ZnO, (b) 0.5% AZO, (c) 0.75% AZO,
(d) 1% AZO, (e) 1.5% AZO and (f) 2% AZO
20 30 40 50 60 70
0
1150
2300
(103)
(110)
(102)
(101)
(100) (a)
2θ (degree)
0
1150
2300
(b)
0
1150
2300
(002)
(c)
0
1150
2300
(d)
Inte
nsity
(a. u.)
0
1150
2300
(e)
0
1150
2300
(f)
115
Figure 7.1 (a) shows the diffraction pattern for undoped ZnO while figures 7.1(b),
7.1 (c), 7.1 (d), 7.1(e) and 7.1 (f) shows the diffraction patterns for 0.5% AZO, 0.75%
AZO, 1% AZO, 1.5% AZO and 2% AZO respectively. It is seen from figure 7.1 (a) that
peaks appear at 31.75o, 34.389
o, 36.205
o, 47.434
o, 56.576
o and 62.855
o. The
corresponding reflecting planes are (100), (002), (101), (102), (110) and (103)
respectively. The (002) peak appears with maximum intensity in pure and all Al doped
films indicating all the samples have strong preferred c-axis orientation i.e. preferred
orientation of the crystals with c-axis perpendicular to the substrate. The other peaks
corresponding to (100), (101), (102), (110) and (103) are present with low relative
intensities. No measurable change in diffraction angles were found due to Al doping.
For films with high Al content (more than 1% AZO films), the relative intensity
of (100) and (101) peak increases indicating some loss of preferred c-axis orientation for
heavily doped films.
Table 7.1 shows the values of TC (002) for undoped and Al doped films. The
value increases initially and does not change much for films upto 1% doping. It then
starts decreasing sharply.
Table 7.1: TC(002) values for undoped and Al doped ZnO films
Film TC (002)
ZnO ~2.29
0.5% AZO ~2.32
0.75% AZO ~2.32
1% AZO ~2.33
1.5% AZO ~1.98
2% AZO ~1.83
116
The MARQ2 plot for 0.5% AZO sample is shown in Figure 7.2.
Figure 7.2: Observed (dotted) and simulated (continuous) XRD patterns of 0.5% AZO
Figure 7.3 shows the Williamson-Hall plots of pure ZnO, 1% AZO and 2% AZO
films. The average value of particle size estimated using 0.9k = for undoped ZnO is
~22.75 nm. It increases marginally to ~24.26 nm for 1% Al :ZnO and to ~25.13 nm for
2% Al :ZnO.
Thus with increasing doping concentration the particle size shows a slightly
increasing trend. While majority of the researchers have reported a marginal decrease in
grain size due to Al incorporation [4-6], Rakhshani [7] have reported that Al -doping
does not modify the size of the grains. Tewari et al. [4] however concluded that the
crystallite size does not vary in any regular pattern with Al incorporation. In all these
works, Scherrer equation (Eqn 3.2, Chapter 3) was applied to evaluate the grain size
which only takes account of particle size broadening. In our present work we have
117
utilized the Williamson-Hall equation which is more accurate since it takes account of
both instrumental broadening and strain broadening to estimate the particle size and
accordingly gives much more reliable results compared to Scherrer equation. Such
increase in particle size may be due to enhanced thickness of AZO films observed in our
present work. It seems that the film tends to lower its surface energy as it becomes
thicker during deposition. During the process the lower-surface-energy grains may
become larger as film thickness increases [8-9]. This is achieved by diffusion within a
thin surface layer of atoms from a particular crystallite to one having a lower surface
energy. The strain in the films was found to reduce from ~0.00204 for ZnO (section 4.3,
Chapter 4) to ~0.002 for 1% AZO film and ~0.00195 for 2% AZO film.
Figure 7.3: Williamson-Hall plots of (a) pure ZnO, (b) 1% Al :ZnO and (c) 2% Al :ZnO.
0.0 0.5 1.0 1.5 2.0 2.5
0.000
0.005
0.010
0.015
(a)
4sinθ
0.0 0.5 1.0 1.5 2.0 2.5
0.000
0.005
0.010
0.015
(b)
βco
sθ
0.0 0.5 1.0 1.5 2.0 2.5
0.000
0.005
0.010
0.015
(c)
118
Figure 7.4 shows the SEM micrograph of undoped ZnO film while figure 7.5
shows the same for 1% AZO film. The incorporation of Al into the lattice affects the
morphology as can be seen from the micrographs. Figure 7.4 (a) shows the SEM image at
normal incidence with magnification ×12000. It is evident that the microstructure
consists of many round shaped clearly defined grains (crystalline particles) covering the
substrate surface more or less uniformly. However there is agglomeration in certain
regions of the film which is clearly visible in the SEM image with magnification ×35000
[figure 7.4 (b)] of the same film].
The AZO film on the other hand shows particles with off spherical shape. Thus
Al doping seems to have modified the shape of the grains. Figure 7.5 (a) shows the SEM
image of 1% AZO film with magnification ×12000 while 7.5 (b) shows the SEM image
of the same film with magnification ×40000. The microstructure is composed of uniform
and compact interconnected grains. Also the film appears to have less porosity and more
rough than ZnO film indicating that the microstructure became denser with
Al incorporation. Similar observation of higher surface roughness due to Al doping has
been reported by Kim et. al. [3]. Thus incorporation of Al leads to a more continuous
film having higher density and less smooth surface compared to undoped ZnO.
Fig. 7.4 (a) Fig. 7.4 (b)
Figure 7.4: SEM image of ZnO (a) with magnification ×12000 and (b) with magnification ×35000
119
Fig. 7.5 (a) Fig. 7.5 (b)
Figure 7.5: SEM image of 1% AZO film (a) with magnification ×12000
and (b) with magnification ×40000
Figure 7.6 shows the energy dispersive X-ray spectrum of 1% Al :ZnO film.
Figure 7.6: EDX pattern of 1% Al :ZnO
120
Although no compositional analysis was attempted in the present study, the
incorporation of Al in the films was verified by the EDX result. The spectrum reveals
the presence of Zn, O and Al elements in the deposited films. The silicon signal appears
from the substrate. Trace amount of C and S impurities was also detected in the film.
7.3 Band gap evaluation from optical absorption
The optical absorbance spectrum was measured within the wavelength range of
200–500 nm. Plots of ( )2
hα ν against hν for undoped and Al doped ZnO films are
shown in figures 7.7. Figure 7.7 (a) shows the spectrum of pure ZnO while figures 7.7
(b) and 7.7 (c) shows the spectrum of 1% Al :ZnO and 2% Al :ZnO respectively.
Figure 7.7: Plots of ( )2
hα ν vs photon energy (in eV) of (a) pure ZnO, (b) 1% Al :ZnO
and (c) 2% Al :ZnO
2.4 2.6 2.8 3.0 3.2 3.4
0
20
40
60
80
100
120
140
160
180
(c)
(b)
(a)
(αh
ν)2
hν
121
The optical band gap shows an increase for 1% AZO compared to pure ZnO. For
pure ZnO the band gap is 3.23 0.01± eV and for 1% AZO, it increases to 3.29 0.01± eV.
Such widening of optical band gap with Al doping is well described by Burstein-Moss
effect [10-14]. For AZO films, compared with undoped ZnO films, the contribution from
3Al
+ ions on substitutional sites of 2Zn
+ ions and Al interstitial atoms determines the
widening of the band gap caused by increase in carrier concentration. This is the well
known Burstein–Moss effect and is due to the Fermi level moving into the conduction
band. Since Al -doping increases the carrier concentration in the conduction band, the
optical bandgap energy increases. Enhancement of band gap thus also ensures that Al
was successfully doped in the ZnO thin films. It is further observed in our present work
that a decrease in band gap occurs for 2% AZO film. The value of band gap for 2% AZO
is 3.18 0.01± . Such unusual red shift of fundamental absorption edge has been reported
by Mohanty et. al. [15] and has been explained in terms of stress relaxation mechanism.
The reduction in slope of the linear portion of the plot [figure 7.7 (c)] observed in our
present work suggest introduction of defect states within the band gap. Thus we interpret
this shift due to merging of an impurity band into the conduction band, thereby shrinking
the band gap. Formation of such impurity band giving rise to new donor electronic states
just below the conduction band is possible and this arises due to hybridization between
states of the ZnO matrix and of the Al dopant [16]. It seems that such formation of donor
levels compensates the Burstein–Moss effect and results in narrowing of the effective
band gap of AZO. The reduction of stress due to enhanced thickness [15] of AZO films
compared to pure ZnO may also have some contribution to the observed red shift. Our
present observation also suggests an enhanced growth rate giving rise to increased
thickness and lowering of strain due to Al incorporation.
7.4 Electrical resistance measurements
.
The electrical resistance measurement of pure and Al doped ZnO films was
carried out in the surface mode using the conventional DC two-point probe technique.
122
The resistance measurement was made in a Keithley 6514 system electrometer. The film
was kept in the dark inside a quartz tube furnace. Ohmic contacts using high conducting
silver paste (curing temperature 200oC) was made onto the surface of the film.
Approximately 20 mm long silver (Ag) contacts, separated by 5 mm, were made on the
films (30 mm× 25 mm) for electrical measurements. The width of the electrodes was
approximately 1 mm. The electrical resistance was measured at 100oC. The films were
heat treated at 200oC for 2 hr. prior to resistance measurements.
Electrical resistance measurement at 100oC is shown in figure 7.8. The figure
shows a marked decrease in resistance due to Al incorporation initially. While undoped
ZnO shows a resistance of ~2.28 MΩ, 1% AZO shows a resistance value of ~0.189 MΩ.
Such decrease in resistance confirms the substitutional replacement of 2Zn
+ ions by 3Al
+
and subsequent enhancement of carrier density. With further increase in Al concentration
the resistance value started to increase. The initial decrease of resistance with increase in
Al concentration has been attributed to increase in carrier concentration and also due to
increase in mobility [17]. Distribution of Al in the grains leading to interconnected
grains and a more continuous film as has been observed in our present work also might
contribute to the lowering of resistance by enhancing the mobility of charge carriers.
Beyond a certain doping concentration, a decrease in mobility but a small change
in carrier concentration has been reported [4, 18]. Our present observation suggests that
beyond a certain doping concentration, the doping atoms do not occupy the lattice sites
but possibly result in some kind of defects. Such limited incorporation of Al into ZnO
lattice is consistent with the results reported by researchers [1-3, 17, 19]. The defects
produced beyond this optimum level of doping gives rise to states within the band gap
reducing the effective band gap, observed in the present work. However their
contribution to enhanced carrier density is neutralized by drastic decrease in mobility and
thereby effectively reducing the resistance. Such drastic reduction of mobility due to
segregation of dopants at grain boundaries has been reported by Shrestha et. al. [5]. It
seems that the micro-mechanism of the influence of dopants is quite complicated. It
123
appears that beyond certain doping concentration there is segregation of dopant atoms at
the noncrystalline regions which produces disorder in the lattice. These defects act as
scattering centers giving rise to various scattering mechanisms and resulting in a sharp
decrease in the mobility.
Figure 7.8: Variation of electrical resistance at 100oC with Al concentration.
Figure 7.9 shows the data of the variation of the electrical conductance with
reciprocal temperature (1000/T) in the temperature range 300-400K with a control
accuracy of 1± K. The electrical conductance ( )Σ was directly evaluated from the
measured value of electrical resistance ( )R . The decrease in resistance with increasing
temperature following semiconducting behaviour of ZnO is observed.
The conduction process may be described by the following equation:
( )exp (7.1)o
EkT
∑ = ∑ − →
0.0 0.5 1.0 1.5 2.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
R (
M-o
hm
)
Al concentration (at %)
124
Figure 7.9: Temperature dependence of electrical conductance in the 300-400K range for
(a) undoped ZnO and (b) 1% AZO film
In equation 7.1, 1
R
∑ =
is the conductance at temperatureT , o
∑ is the pre-
exponential factor for the temperature range 300-400K, k is the Boltzmann’s constant,
T is the absolute temperature and E is the activation energy barrier value. The
experimentally obtained value of E is ~0.26 eV for pure ZnO and it seems to have
remain unaffected due to Al doping. Thus there is no change in activation barrier value
due to doping. An activation barrier value of 0.24-0.28 eV is normally associated with
neutral oxygen vacancy acting as donor state [20-21].
The decrease of resistance by approximately one order in the entire temperature
range studied shows that Al atoms are incorporated into the ZnO lattice and contributes
conduction electrons according to the equation
3 2 (7.2)Al Al e+ +→ + →
2Al
+ replaces 2Zn
+ and the electron released is free to contribute to electrical conduction.
2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.310
-8
10-7
10-6
10-5
(b)
(a)
Co
nd
ucta
nce
(o
hm
)-1
103/T (
oK)
-1
125
The value of effective density of conduction electrons (eff
n ) can be calculated
from the equation [22]:
(7.3)eff
n eσ µ= →
where 1
σρ
= is the electrical conductivity, e is the electronic charge and µ is the
mobility. The resistivity ρ was determined from the relationRL
Aρ = , where L is the
distance between the two silver electrodes and A is the area, which is the product of
length of the electrodes and thickness of the film. Here R is the measured resistance,
L ≅ 5 mm. The area A is the product of length of the electrodes ( ≅ 20 mm) and
thickness of the film ( ≅ 5 µ m). From our present results, the value of eff
n comes out to
be of the order of 1013
/cm3 at around 380K for ZnO film. For AZO film it is of the order
of 1014
/cm3 at 380K. While calculating
effn , the mobility value was assumed to be
constant and equal to 20 cm2 Vs
-1 i e. temperature dependence of mobility was not taken
into account [23]. This will introduce only a small error in the calculation and will not
affect the order of carrier density.
7.5 Electrical resistance measurements in presence of LPG
Some additional electrical measurements for AZO films were carried out in
presence of LPG (Liquefied Petroleum gas) since Al doped ZnO is a promising gas
sensor material [24-25]. However, only a few publications describing the sensing
behavior of Al doped ZnO thin films deposited by aqueous solution techniques at low
temperatures are available [26-27].
The electrical resistance of the films was measured before and after exposure to
LPG. The sensitivity of the film was determined at different operating temperatures in the
126
range 250-375oC in presence of LPG in air. Commercially available calibrated mixtures
of LPG were used for this purpose. Before exposing to LPG, the film was allowed to
equilibrate at each operating temperature for 30 minutes. The percent sensitivity was
estimated by measuring the percent reduction of resistance in presence of the target gas.
Thus if air
R and gas
R represents the equilibrium sample resistance in ambient air and under
test gas respectively, the percent sensitivity ( %)S can be expressed as [28]
% 100 (7.4)air gas
air
R RS
R
−= × →
Figure 7.10 shows the percent sensitivity ( %)S as a function of operating
temperature in presence of 1.6 vol% LPG in air. This value corresponds to 80% LEL
(Lower Explosive Limit) of LPG in air. Butane ( )4 10C H has a LEL of 1.6 vol% and
propane ( )3 8C H has a LEL of 2.1 vol%. Thus the LEL of LPG is generally taken to be 2
vol%. The exposure time to the target gas was 15 minutes.
Figure 7.10: Sensitivity vs. operating temperature for (a) ZnO and (b) 1% AZO film
200 250 300 350 400
0
20
40
60
80
100
(b)
(a)
S (
%)
T (oC)
127
It is observed that compared to undoped ZnO film, Al -doping enhances the
sensitivity of the films at all temperatures. The sensitivity increased with increasing
temperature of the sensor element, reaches a peak value and then decreases again. The
peak sensitivity for AZO film was observed at 325oC and the value of maximum
sensitivity was 87%. Similar characteristic has been reported by Sahay et. al. [29] with
peak sensitivity of ~89% at 325oC in presence of 1 vol% LPG.
Figure 7.11 shows sensing characteristics of the undoped and Al -doped film in
presence of 1.6 vol% LPG in air at 325oC. The plot of resistance ratio gas
air
R
Ragainst
time is shown in the figure 7.11. Faster response (short response time) is given by Al -
doped ZnO compared to undoped ZnO film.
Figure 7.11: Sensing characteristics in presence of 1.6 vol% LPG in air at 325oC for
(a) undoped ZnO and (b) 1% AZO film
0 2 4 6 8 10 12 14 16 18 20
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
Gas on
(b)
(a)
Rgas/R
air
Time (Minutes)
128
The gas sensing mechanisms normally accepted for semiconductor sensors
assume that the oxygen adsorbed on the surface of the oxide traps some of the conduction
electrons and thus decreases the material’s conductivity [30]. The surface adsorbed
oxygen species thus becomes negatively charged chemisorbed species ( 2O− , O
− or
)2O
− and acts as reaction centers for gas molecules. When reduction gas molecules come
into contact with this surface, they may interact with this chemisorbed oxygen species,
leading to an inverse charge transference [31]. Upon the return of the electrons to the
conduction band, conductivity increases. The reaction mechanism for LPG (containing
the hydrocarbons propane and butane) with surface adsorbed species leading to the final
products CO2 and H2O is quite complicated and proceeds through several intermediate
steps [29, 32].
Enhancement of sensitivity with operating temperature has been attributed to
increased speed of chemical reaction between the gas molecules and chemisorbed oxygen
species [29]. At high temperatures, gas molecules have enough thermal energy to react
with the chemisorbed species. However the appearance of peak sensitivity at 325oC has
not been explained. It has been established that the nature of the chemisorbed species is a
function of temperature [23]. While 2O
− is considered to be the prominent chemisorbed
species upto 500K, for temperatures higher than 500K, O− is the predominant species. As
is evident from experimental observation, reasonable sensitivity appears for temperatures
higher than 500K where the predominant species isO− . With increase in temperature, the
reaction 2 2O e O− −+ → , leading to the formation of O
− species proceeds at a faster rate
and thereby increasing the density of O− species. Since these are the active sites for
reaction with gas molecules, the sensitivity increases with temperature. This process must
accompany the process of enhancement of thermal energy of the gas molecules with
temperature in order to explain the reduction of sensitivity above a particular
temperature. With further increase in operating temperature a gradual change of the
adsorbed species from O− to 2
O− takes place according to the reaction 2
O e O− −+ → . The
129
latter species, although have high reactivity, is unstable and can go into the lattice as
lattice oxygen [23, 33]. Thus we can presume that at temperatures above 325oC, the
number of chemisorbed species available for surface activity is lowered. This lack in
number of chemisorbed species can slow down the catalytic oxidation reaction and leads
to a decrease in net yield of conduction electrons and hence sensitivity.
The enhancement of sensitivity due to Al incorporation might be due to increase
in electron concentration (carrier density) as has been observed in resistance
measurement as well as less porosity in the film as has been observed in SEM
measurements. Such increase in conduction electron concentration leads to an increased
density of surface active chemisorbed species which are the reaction centers for gas-
surface reaction. Also more porous film allows gas molecules to penetrate inside the film
and the resistance reduction process continues for a longer time. This delay the
attainment of equilibrium resistance value in presence of target gas and increase of
response time as has been observed for undoped film in the present work.
130
7.6 Discussion of Results on AZO thin films
Al -doped ZnO thin film could be successfully synthesized from ammonium
zincate complex with zinc acetate as the starting precursor by SILAR technique. Al
incorporation increases the growth rate of the film. Films had exceelllent adhesion on
glass slides. XRD spectra showed that the films have hexagonal structure with strong
preferred c-axis orientation. Texture coefficient of (002) plane increases due to Al
incorporation indicating improved crystallinity along c-axis. Particle size evaluated using
x-ray line broadening analysis and Williamson-Hall method shows a slightly increasing
trend with increasing Al incorporation alongwith a reduction in strain. The average
particle size for pure ZnO is ~22.75 nm. It increases to ~24.26 nm for 1% AZO film and
~25.13 nm for 2% AZO film. SEM micrograph shows round shaped particles for pure
ZnO. Al doping also seems to influence shape of the grains. AZO films show particles
with off spherical shape with compact interconnected grains. The morphology slaso
exhibits rougher surface compared to undoped ZnO. AZO film also appears to have
higher density (i. e. less porosity) compared to undoped ZnO and consist of compact
interconnected grains leading to a more continuous film. This fact along with
substituional replacement of divalent 2Zn
+ by trivalent 3Al
+ decreases the film resistance.
EDX analysis was carried out to check the incorporation of Al in the doped film.
The band gap of the film increases upto a certain level of doping due to increase
of carrier density. Beyond this limit, there is a narrowing of band gap possibly indicating
merging of an impurity band into the conduction band. The value of band gap for pure
ZnO is ~3.23 eV and it increases to ~3.29 eV for 1% AZO indicating a blue shift for 1%
AZO film. However for 2% AZO film, a decrease in band gap compared to pure ZnO is
observed indicating a red shift of fundamental absorption edge. Relaxation of strain due
to enhance growth and subsequent thickness of AZO films may also contribute to the
observed red shift.
131
The electrical resistance shows an initial decrease with increasing Al content.
With further enhancement of Al incorporation, the resistance increases. The electrical
resistance also decreases initially due to replacement of 2Zn
+ ions by 3Al
+ ions. Beyond a
certain level of doping, the electrical resistance increases due to drastic fall in mobility
arising out of segregation of dopants at grain boundaries. Al incorporation increases the
effective carrier density by approximately one order of magnitude in the temperature
range studied. However, it does not affect the value of activation barrier. The activation
barrier value of ~0.26 eVcan be attributed to oxygen vacancies acting as donor defect.
The sensitivity of the films in presence of LPG increases with temperature,
reaches a maximum and then decreases. The phenomena can be explained by the change
in nature of chemisorbed species with temperature as well as their number density.
Significantly high sensitivity of 87% with a reasonably fast response is observed for AZO
film in presence of 1.6 vol% LPG in air. The desired characteristics of a sensor material
need to be balanced with the processing costs for practical applications. Accordingly flms
synthesized by such a low temperature technique can be useful for sensor applications.
132
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134
CHAPTER 8
Preparation of Ni doped ZnO thin films by
SILAR and their characterization
8.1 Preparation of Ni doped ZnO (NZO) films
Ni doped ZnO thin films were deposited on glass substrates from ammonium
zincate bath. Nickel doping in ammonium zincate bath was carried out by adding nickel
chloride (2 2, 2NiCl H O , Merck) in ammonium zincate bath. The pH of the ammonium
zincate bath was adjusted to ~10.80. Fifty (50) dipping was performed for undoped ZnO
films. Addition of nickel chloride was found to reduce the pH of the zincate bath and
reduces the growth rate. The nickel concentration could be varied upto 10% in the bath
solution and more than 10% dopant addition, the pH reduces to less than 10.70 (the lower
limit for getting stable ammonium zincate bath). This makes the bath unstable and
unsuitable for film deposition as the growth process is a stringent function of pH. The
thickness for pure ZnO film measured gravimetrically was ~ 0.8 µm and the growth rate
of the films reduces with increasing Ni doping. The number of dipping in case of doped
films was adjusted to give more or less identical thickness as that of pure ZnO. Almost
100 dipping was required to get ~ 0.8 µm thick 10% Ni doped ZnO film for which the
bath pH was ~10.72.
135
8.2 Structural Characterization by XRD: Evaluation of particle size
Figure 8.1 shows the X-ray diffraction patterns of undoped and Ni doped ZnO
films. The films were annealed at 350oC for 2 hr. in air prior to structural
characterization. The diffraction pattern for undoepd ZnO is shown in figure 8.1 (a).
Figures 8.1 (b), 8.1 (c) and 8.1 (d) shows the diffractograms for 3% Ni:ZnO, 5% Ni:ZnO
and 10% Ni:ZnO respectively.
Figure 8.1: X-ray diffraction pattern of (a) ZnO, (b) 3% Ni:ZnO, (c) 5% Ni:ZnO and
(d) 10% Ni:ZnO
20 30 40 50 60 70
0
1000
2000
(103)
(110)
(102)
(101)
(002)
(100)
(a)
2θ (degree)
0
1000
2000
(b)
Inte
nsity
(a. u.)
0
1000
2000
(c)
0
1000
2000(d)
136
The appearance of peaks at 31.714o, 34.389
o, 36.205
o, 47.434
o, 56.576
o and
62.855o and the corresponding reflecting planes are shown in the figue. The (002) peak
appears with maximum intensity in pure and Ni doped films indicating all the samples
have high preferred c-axis orientation. Apart from ZnO characteristic peaks, no peaks
that correspond to either nickel, zinc or their complex oxides could be detected within the
detection limit of XRD suggesting the films do not have any phase segregation or
secondary phase formation and also indicating possible incorporation of Ni in ZnO
lattice. The diffraction angles are marginally shifted towards the right side. The ionic
radius of 2Zn
+ is 0.074 nm and of 2Ni
+ is 0.069 nm [1]. Ni has been reported to be present
in a divalent state in ZnO lattice [1-2]. The plot of MARQ2 analysis for 3% Ni:ZnO
sample is shown in Figure 8.2.
Figure 8.2: Observed (dotted) and simulated (continuous) x-ray diffraction patterns of
3% Ni: ZnO
137
From the values ofo
β obtained using MARQ2 fitting and the corresponding
values of instrumental broadeningi
β , β (FWHM) was calculated using equation 4.13
(Section 4.3 of chapter 4). Figure 8.3 shows the Williamson-Hall plots of
cosβ θ against θsin4 . The particle size for pure ZnO was ~22.75 nm (Section 4.3.1,
Chapter 4; TEM value ~25.8 nm). With increasing doping concentration the particle size
shows a slightly decreasing trend. It decreases to ~21.5 nm for 5% Ni:ZnO and further
decreases to ~20.5 nm for 10% Ni:ZnO.
Lupan et. al. [3] however reported a marginal increase in particle size from~ 24
nm for undoped ZnO to 26.5 nm for Ni doped ZnO for films synthesized by SILAR.
However they applied Scherrer equation to evaluate particle size. Instrumental and strain
broadening was not taken into account.
Figure 8.3: Williamson-Hall plots of (a) pure ZnO, (b) 3% Ni:ZnO, (c) 5% Ni:ZnO and (d) 10%
Ni:ZnO
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
0.000
0.005
0.010
0.015
(a)
βco
sθ
4sinθ
0.000
0.005
0.010
0.015
(b)
0.000
0.005
0.010
0.015
(c)
0.000
0.005
0.010
0.015
(d)
138
8.3 SEM and EDX studies
Figure 8.4 shows the SEM image of undoped and Ni doped ZnO films.
Figure 8.4: SEM image of (a) ZnO and (b) 10% Ni:ZnO thin film
(a)
(b)
139
Figure 8.4 (a) shows the SEM image of undoped ZnO film (Figure 7.4 (a),
Chapter 7; reproduced here for convenience) while figure 8.4 (b) shows the SEM image
of 10% Ni:ZnO films. SEM investigation at normal incidence revealed polycrystalline
structure with smooth surface for the deposited films. The overall surface structure shows
grains of nearly spherical shape and more or less uniformly covering the surface. The
grain size and grain shape appears not to be effected due to Ni incorporation; however Ni
doping seems to produce smoother and denser surface. Juan et. al. reported [4] smooth
surface with roughness limited to 4 nm for Ni doped ZnO films.
The compositional analysis of Ni doped ZnO film carried out by EDX analysis is
shown in figure 8.5. Figure 8.5 (a) shows the EDX spectrum of 5% Ni:ZnO and 8.5 (b)
shows the spectrum of 10% Mn:ZnO.
Figure 8.5: EDX pattern of (a) 5% Ni:ZnO and (b) 10% Ni:ZnO
200 400 600 800 1000
0
500
1000
1500
2000
2500
3000
3500
NiLa
O Ka
Zn La
SiKa
NiKbNiKa ZnKb
ZnKa
(a)
keV
200 400 600 800 1000
0
500
1000
1500
2000
2500
3000
3500
NiLa
O Ka
Zn La
SiKa
NiKbNiKa ZnKb
ZnKa
(b)
keV
140
The EDX spectrum confirmed the presence of Zn, O and Ni elements in the
deposited films. The silicon signal appears from glass substrate. Dopant concentration in
these two cases was 5% and 10% in the starting solution. Accordingly the expected Ni/Zn
ratio was 0.05 and 0.1 in the films. The values of Ni/Zn ratio actually obtained in the
films was 0.0303 and 0.0593 respectively indicating that the amount of Ni incorporation
in the film is less than the amount of Ni in the starting solution. The real Ni content in the
deposited films was 3.03% and 5.93% as obtained from EDX spectrum.
8.4 Band gap evaluation from Optical absorption
Plots of ( )2
hα ν against photon energy are shown in figure 8.6. The data for
plotting these graphs was obtained from optical absorbance measurement of α versus λ .
Figure 8.6 (a) shows the spectrum of pure ZnO while figures 8.6 (b) and 8.6 (c) shows the
spectrum of 5% Ni:ZnO and 10% Ni:ZnO respectively.
Figure 8.6: Plots of ( )2
hα ν vs photon energy (in eV) of (a) ZnO, (b) 5% Ni:ZnO and
(c) 10% Ni:ZnO
2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3
0
20
40
60
80
100
120
140
160
180
200
(c)(b)
(a)
(αh
ν)2
hν
141
It is seen that with the increase of nickel doping level, the fundamental absorption
edge decreases. The value of g
E for undoped ZnO is ~3.23eV. It decreases to ~3.21 eV
for 5% Ni:ZnO and to ~3.19eV for 10% Ni:ZnO. A decrease in band gap from 3.28 eV
for pure ZnO to 3.26 eV for Ni doped ZnO has been reported by Xiaolu et. al. [5]. The
decrease in band gap might be due to introduction of defect states within the band gap by
the Ni dopant ions. The decrease in the slope of the linear portion of the curve with
increasing Ni content observed in the present work supports the fact.
8.5 Electrical characterization
Figure 8.7 shows the data on the variation of the electrical conductance with
reciprocal temperature (1000/T) in the temperature range 300-400K for ZnO and Ni:ZnO.
The electrical conductance ( )Σ was directly evaluated from the measured value of
electrical resistance ( )R .
Figure 8.7: Temperature dependence of electrical conductance for (a) ZnO & (b) Ni:ZnO
2.4 2.6 2.8 3.0 3.2 3.410
-8
10-7
10-6
(b)
(a)
Conducta
nce (
ohm
−1 )
1000/T(oK
-1)
142
Figure 8.7(a) shows the variation of conductance for pure ZnO while figure 8.7(b)
shows the same for 10% Ni:ZnO. The conduction process may be described by the
following equation:
( )exp (8.1)o
EkT
∑ = ∑ − →
In this equation, o
∑ is the pre-exponential factor for the temperature range 300-
400K, k is the Boltzmann’s constant, T is the absolute temperature and E is the thermal
activation barrier value. The experimentally obtained value of E is ~0.261 eV for pure
ZnO and ~0.293 eV for Ni:ZnO. An activation barrier value of 0.24-0.28 eV is normally
associated with oxygen vacancy acting as donor state (Section 7.4, Chapter 7). The films
have high resistivity ( )ρ of the order of 104 Ω-cm and low effective donor density of the
order of 1013
/cm3 at room temperature. The effective donor density is governed by donor
defect states (oxygen vacancies in this case) as well as density of chemisorbed species on
the surface which acts as trap state for conduction electrons. It is evident from figure 8.7
that the electrical conductivity decreases with Ni doping. Such decrease in electrical
conductivity due to Ni incorporation has been explained on the basis of compensation of
oxygen vacancies and such compensation may also increase the activation energy value.
[2, 6]. The activation energy barrier value may also increase due to enhanced grain
boundary scattering [2]. Reduced particle size resulting in enhance grain boundary
scattering effect may also have some contribution in increasing the activation energy
barrier value.
143
8.6 Discussion of results on Ni:ZnO thin films The primary aim of the present investigation was to explore the possibility of
doping ZnO with nickel by SILAR technique. Ni doped ZnO films with different
percentage (3%, 5% and 10%) of Ni content could be successfully synthesized through
this technique. The films had good adherence to the substrate. Ni doping reduces the
growth rate. Characterization techniques of XRD, SEM and EDX were utilized to
investigate the effect of Ni doping on the microstructure of Ni:ZnO thin films. XRD
spectra showed that the films are of hexagonal structure with preferred c-axis orientation.
Structural characterization by x-ray diffraction reveals the polycrystalline nature of the
films. Particle size evaluated using x-ray line broadening analysis and Williamson-Hall
method shows a marginally decreasing trend with increasing nickel incorporation. The
average particle size of ~22.75 nm for undoped ZnO reduces to ~20.51 nm for 10%
Ni:ZnO. Surface morphology using SEM shows polycrystalline and porous structure with
grains distributed more or less uniformly over the substrate surface. These observations
along with EDX observation confirm the incorporation of Ni in ZnO. The real Ni content
as obtained from EDX spectrum in the deposited film was 3.03% and 5.93% respectively
as opposed to 5% and 10% in the starting solution indicating that the amount of dopant
incorporation in the films is less than the amount in the starting solution. With increase
of Ni doping, the fundamental absorption edge reduces moderately. It decreases to ~3.19
eV for 10% Ni:ZnO from ~3.23 eV for pure ZnO. The electrical conductance decreases
and the activation barrier value for electrical conduction increases for Ni doping. For pure
ZnO film, the value of activation barrier is ~0.261 eV and for 10% Ni:ZnO it is ~0.293
eV. The values are reproducible within an error limit of 0.005± eV. Compensation of
oxygen vacancies as well as grain boundary scattering effects accounts for this.
144
References
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(2008) 765.
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Sci: Mater Electron 22 (2011) 1473.
3. O. Lupan, S. Shishiyanu, L. Chow and T. Shishiyanu, Thin Solid Films 516 (2008)
3338.
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Tong and D. Y. Wei, Scinence China: Technological Sciences 53 (2010) 293.
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Condensed Matter 406 (2011) 3956.
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(2009) 1164.
145
CHAPTER 9
Summary, Conclusions and Scope of
future work
9.1 Summary and Conclusions
The research work embodied in this dissertation was undertaken primarily to
explore the possibility of using a relatively new and less utilized yet economic technique
to prepare ZnO and doped ZnO thin films and their characterization. Compared to other
chemical techniques, successive ionic layer adsorption and reaction (SILAR) technique
involving multiple dipping of a substrate in cationic and anionic precursors has remained
a relatively less investigated method for preparation and characterization ZnO and doped
ZnO thin films. Although a few researchers have earlier employed this technique for the
preparation of ZnO films, the potential of this technique is yet to be explored in full
particularly for doped ZnO films and their characteriztion.
By proper optimization of deposition parameters such as concentration and pH of
cationic and anionic precursors and temperature of deposition, it was possible to get
reproducible, good quality and strongly adherent ZnO films. Ammonium zincate, sodium
zincate and zinc chloride has been used as cationic precursors. For the zincate baths, hot
water was the anionic precursor. The concentration values of all the zincate baths have
been optimized to 0.1 M and the pH values have been optimized in the range
146
10.80 0.05± for ammonium zincate bath and in the range 13.20 0.05± for sodium zincate
bath to get adherent films. In both cases the anionic precursor was hot water maintained
near boiling point. For zinc chloride solution the concentratio and pH values have been
optimized at 0.1M and 4.70 0.05± respectively. The optimized concentration and pH
values of the corresponding anionic precursor ( NaOH solution) were 0.075 M and
11.10 0.05± respectively. The growth process follows an empirical linear behavior with
number of dipping cycle for both the zincate baths. The growth rates in terms of µm per
dipping per mole were ~0.2 and ~0.162 for sodium and ammonium zincate baths
respectively.
X-ray diffraction studies reveal the films are polycrystalline with a preferred c-
axis orientation. The temperature of heat treatment of the deposted films was optimized at
350oC as the texture coefficient (TC) value for (002) preferred orientation almost
saturates after 300oC. Polycrystalline thin films with lowest particle size of ~22.75 nm
estimated using x-ray line broadening analysis (~25.8 nm from TEM observation) and
with highest TC value for (002) plane (~2.29) was obtained from ammonium zincate
bath. Both instrumental broadening and strain broadening was taken into account while
particle size evaluation. All subsequent experiments were carried out for films heat
treated at 350oC. Films produced from sodium zincate bath exhibits highest particle size
(~41 nm from TEM measurements). Films deposited from ammonium zincate bath were
round shaped compared to off spherical shape obtained from sodium zincate bath. Films
from zinc chloride complex were also nearly spherical in shape. Morphology of films
prepared from ammonium zincate bath exhibits superiority over films obtained from
other zinc complexes. The use of zinc acetate as the staring reagent to prepare ammonium
zincate complex was attempted for the first time which reduces possibility of impurity
incorporation compared to other zinc complexes used so far by other researchers. The
porosity in the films deposited from zincate baths is quite high and it ranges between 30
to 32% as observed from cross sectional SEM. FTIR spectrum reveals the presence of
ZnO stretching vibration. The high resistivity of films alongwith high porosity may be
useful for applications in resistive mode gas sensors.
147
Taking into account the optimized parameters of film deposition and with proper
selection of cationic precursor, Cd doped ZnO thin films were successfully deposited
from sodium zincate bath by SILAR for the first time to the best of our knowledge. The
films had good adherence to the substrate. Particle size evaluated using x-ray line
broadening analysis shows a constantly decreasing trend with increasing cadmium
incorporation. The average particle size of undoped ZnO from sodium zincate bath is
~36.73 nm evaluated by x-ray line broadening method neglecting strain broadening. The
corresponding value evaluated from TEM is ~41 nm. The average particle size reduces to
~32 nm for 5% Cd:ZnO and ~29.9 nm for 10% Cd:ZnO evaluated by x-ray method. The
undoped ZnO film is polycrystalline with a preferred c-axis orientation. The preferred
orientation is lost and the degree of polycrystallinity increases with increasing Cd
incorporation. These observations along with EDX observation confirm the incorporation
of Cd in ZnO lattice. SEM shows polycrystalline and porous nature of the films with
surface morphology getting less rough due to Cd incorporation. With increase of Cd
doping, the fundamental absorption edge changes decreases. The value of fundamental
absorption edge is ~3.18 eV for pure ZnO and it decreases to ~3.11 eV for 10% Cd:ZnO.
The small shift in diffraction peak positions, moderate reduction of optical band gap as
well as EDX investigations indicates that Cd incorporation in the films is much less than
that in the starting solution possible due to low deposition temperature characteristics of
SILAR process. Our primary aim was however to explore the possibility of Cd
incoporporation in ZnO by SILAR. Cd doped ZnO films could be successfully
synthesized through this technique and from our experiments we have demonstrated that
the physical properties of ZnO can be well modified by cadmium doping.
Mn doped ZnO films with different percentage of Mn content (upto 5%) could be
successfully synthesized by suitable choice of cationic and anionic precursors under
optimized deposition conditions. Zinc chloride bath with manganese chloride as source of
Mn was used as cationic precursor and sodium hydroxide was used as anionic precursor.
Better adherence on quartz substrate was observed compared to glass. More than 5%
dopant addition is difficult since the cationic bath loses stability possibly due to lowering
148
of pH. Mn incorporation strongly affects the structural, morphological and optical
properties of ZnO. Enhancement of polycrystallinity, decrease of preferred c-axis
orientation, enhancement of microstrain and lowering of particle size was observed for
Mn doping. The average particle size of ~29.71 nm for undoped ZnO evaluated by x-ray
line broadening method (~ 31 nm from HRSEM measurement) reduces to ~23.76 nm for
5% Mn doping. The undoped films contained nearly spherical grains while Mn
incorporation gives wrinkle structure. These observations along with EDX observation
confirms the replacement of zinc ion by manganese ions in the ZnO lattice. The real Mn
content in the deposited film was less than that in the starting solution as obtained from
EDX measurements. Mn doping reduces the value of fundamental absorption edge from
~3.22 eV for pure ZnO to ~3.06 eV for 5% Mn:ZnO for films deposited on quartz
substrate.
Al -doped ZnO thin film could be successfully synthesized from ammonium
zincate complex with hexahydrate aluminium chloride as the source of dopant. Al
incorporation increases the growth rate of the film. The texture coefficient for (002) plane
increases upto a cerain doping percent indicating improved crystallinity along c-axis.
Average particle size increases marginally due to Al incorporation. AZO films show off
spherical and compact interconnected grains leading to a more continuous film. This fact
along with substituional replacement of divalent 2Zn
+ by trivalent 3Al
+ decreases the film
resistance upto a certain dopin level (~1 atomic %). The band gap of the film increases
upto a certain level of doping (~ 1 at.%) due to increase of carrier density. Beyond this
limit, there is a narrowing of band gap possibly indicating merging of an impurity band
into the conduction band. The value of band gap for pure ZnO is ~3.23 eV and it
increases to ~3.29 eV for 1% AZO indicating a blue shift for 1% AZO film. However for
2% AZO film, a decrease in band gap compared to undoped ZnO is observed indicating a
red shift of fundamental absorption edge. This may be due to enhancement of strain
(observed in the present experiment) as a consequence of increased growth rate of AZO
films and/or narrowing of band gap indicating merging of an impurity band into the
conduction band.
149
The electrical resistance shows a decrease with increasing Al content upto a
certain doping (~ 1 at.% doping level) due to replacement of 2Zn
+ ion by 3Al
+ ion. Al
incorporation decreases the resistance by approximately one order of magnitude in the
temperature range 300-400K. However, it does not affect the value of activation barrier
of ~0.26 eV which arises due to oxygen vacancies acting donor state for conduction
electrons. With further enhancement of Al incorporation, the resistance increases
possibly due to drastic fall in mobility
.
As AZO is an important gas sensing material, LPG sensing characteristics of the
films were studied as an immediate conceivable application of the AZO films prepared
by SILAR. Significantly high sensitivity of ~87% with a reasonably fast response is
observed for AZO film in presence of 1.6 vol% LPG in air at 325oC operating
temperature.
Ni doped ZnO films with different percentage (3%, 5% and 10%) of Ni content
could be successfully synthesized by SILAR. XRD studies revealed polycrystalline
structure with preferred c-axis orientation for Ni:ZnO films. Particle size shows a
marginally decreasing trend with increasing nickel incorporation. Surface morphology
using SEM shows polycrystalline and porous structure with round shaped grains
distributed more or less uniformly over the substrate surface. Ni incorporation does not
modify the shape of the particles. However a smother and denser surface is observed due
to Ni incorporation. These observations along with EDX observation confirm the
incorporation of Ni in ZnO. The real Ni content as obtained from EDX spectrum in the
deposited film was 3.03% and 5.93% respectively as opposed to 5% and 10% in the
starting solution indicating that the amount of dopant incorporation in the films is less
than the amount in the starting solution. With increase of Ni doping, the fundamental
absorption edge changes. It decreases to ~3.19 eV for 10% Ni:ZnO from ~3.23 eV for
pure ZnO. The electrical conductance decreases and the activation barrier value for
electrical conduction increases for Ni doping. For pure ZnO film, the value of activation
barrier is ~0.261 eV and for 10% Ni:ZnO it is ~0.293 eV.
150
9.2 Scope of future work
i) The technique of SILAR has been optimized for ZnO and doped ZnO films of
Cd , Mn , Al and Ni . The technique can be extended to prepare and characterize other
metal doped ZnO films. Apart from being a cost-effective and simple technique, the
method uses milder reaction conditions than those employed by most chemical methods
proposed in the literature. Doping of different metals in ZnO may be particularly suitable
by this method.
ii) Microstructural characterization for Cd doped ZnO films prepared successfully
by SILAR has been made. Cd:ZnO (in nanowire form) is an excellet humidity sensor
material as well as a promising PTC material. The technique may be suitable modify to
prepare materials in other physical forms suitable for application purpose. Electrical and
gas sensing characterization of Cd:ZnO films may be studied in future.
iii) Synthesis and characterization of Mn and Ni doped ZnO thin films has been made
in this work. Thes materials are important for their ferromagnetic properties. Magnetic
properties of these materials may be studied in future. With Cd and Mn doping, the
fundamental absorption edge changes. The materials are therefore also useful for
configurations that involved bandgap engineering.
iv) Preparation of Al doped ZnO and their structural, optical and electrical properties
has been studied. The AZO films also show significantly high sensitivity to LPG. AZO is
a promising gas sensor material apart from its application in many other areas such as
transparent conducting oxide (TCO). Since films synthesized by SILAR are highly
resistive and porous, they can be useful for resistive mode gas sensor applications. From
a practical point of view, the desired characteristics of a sensor material (or any other
material for industrial application) need to be balanced with the processing costs. The
SILAR technique offers the desirable cheapness and can be easlity scalable to industrial
level. Thus further investigation may be made on the gas sensing properties of AZO
films.
151
List of Publications:
1. “Preparation of Al-doped ZnO (AZO) Thin Film by SILAR” - S. Mondal, K. P.
Kanta and P. Mitra, Journal of Physical Sciences 12 (2008) 221.
2. “Hydrogen and LPG sensing properties of SnO2 films obtained by direct
oxidation of SILAR deposited SnS” - P. Mitra and S. Mondal, Bulletin of the
Polish Academy of Sciences: Technical Sciences 56 (2008) 295.
3. “Preparation of ZnS and SnS Nanopowders by Modified SILAR Technique”- S.
Patra, S. Mondal, and P. Mitra, Journal of Physical Sciences 13 (2009) 229.
4. “Effect of Manganese Incorporation in ZnO Thin Films Prepared by SILAR”- S.
Mondal and P. Mitra, Science and Society 10 (2012) 139.
5. “Preparation of Cadmium - doped ZnO thin films by SILAR and their
characterization” – S. Mondal and P. Mitra, Bull. Mater. Sci. 35 (2012) 751.
6. “Preparation of Ni doped ZnO thin films by SILAR and their Characterization”-S
Mondal and P Mitra, Indian Journal of Physics, “Published online” doi
10.1007/s12648-012-0198-8.
7. “Preparation of ZnO film on p-Si and I-V Characteristics of p-Si/n-ZnO” - S.
Mondal, K. P. Kanta and P. Mitra, Materials Research, “Published online”
doi.org/10.1590/S1516-14392012005000149.
8. “Preparation of manganese doped ZnO thin films by SILAR and their
characterization”- S. Mondal, S. R. Bhattacharyya and P. Mitra, Bull. Mater.
Sci., “Article in press” [Manuscript ID D-12-00036].
9. “Effect of Al doping on microstructure and optical band gap of ZnO thin film
synthesized by SILAR” - S. Mondal, S. R. Bhattacharyya and P. Mitra, Pramana
– Journal of Physics, “Article in press”.
10. “Structural and morphological characterization of ZnO film synthesized from
different zinc complexes as cationic precursor” - P. Mitra and S. Mondal,
Progress in Theoretical and Applied Physics, “Article in press”.