21

CHAPTER 2

GROWTH AND CHARACTERIZATION OF VERTICALLY

ALIGNED ZnO NANORODS

2.1 INTRODUCTION

In the past few years, wide band gap semiconductor nanostructures

such as nanorods, nanowires, and nanobelts have been of growing interest due

to their importance in both scientific research and potential applications,

including nano&electronics, nano&optoelectronics, nano&piezotronics,

gas/chemical sensors, surface acoustic wave devices (SAW) and field

emission device applications. Considerable effort has been devoted to the

synthesis and study of ZnO nanostructures of various morphologies.

ZnO nanomaterials have been synthesized by various techniques such as

chemical vapor deposition (Wu and Liu 2002), physical vapor deposition

(Kong et al 2001), molecular beam epitaxy (Heo et al 2002) and a simple

method that just involves heating a Zn powder containing catalyst

nanoparticles (Wang et al 2004). All these approaches apply the VLS

mechanism for nanowire growth, in which a metal or oxide catalyst is

necessary to dissolve feeding source atoms in the molten state to initiate the

growth of nanostructures. The UV stimulated emission from optically pumped

nanowires at room temperature has also been reported (Yang et al 2002). ZnO

nanobelts have been successfully synthesized by simply evaporating ZnO

powders without the presence of catalyst (Pan et al 2001). Nevertheless, a

high temperature, as high as the melting point of bulk ZnO, is needed in such

method. Among the various techniques, pulsed laser ablation is a simple

22

method compared with well&established techniques like metal organic

chemical vapor deposition and molecular beam Epitaxy. The PLD has been

widely used for the synthesis of nanostructures and thin films.

2.2 PULSED LASER DEPOSITION (PLD)

In the pulsed laser deposition technique, also called laser ablation,

high power laser pulses are used to evaporate material from a target surface

such that the stoichiometry of the material is preserved in the interaction. As a

result, intense “plume of particles” is generated from the target surface. The

plume expands away from the target with a strong forward directed velocity

and condenses on the substrate placed opposite to the target. A schematic

diagram of the typical PLD system is shown in Figure 2.1.

Figure 2.1 Schematic diagram of pulsed laser deposition setup

When the laser radiation is absorbed by a solid surface,

electromagnetic energy is first converted into electronic excitation and then

into thermal, chemical and even mechanical energy to cause evaporation,

substrate holder

Vacuum chamber ∼∼∼∼10-6

mbar

23

ablation, excitation, plasma formation and exfoliation. The ablated plume

contains a mixture of energetic species including atoms, molecules, ions,

clusters and micron&sized solid particulates. The collisional mean free path

inside the dense plume is very short. As a result, immediately after the laser

ablation, the plume rapidly expands into the vacuum from the target to form a

nozzle jet with hydrodynamic flow characteristics. PLD has several attractive

features, including stoichiometric transfer of material from the target,

generation of energetic species, hyperthermal reaction between the ablated

cation and molecular oxygen in the ablation plasma, and compatibility with

background gas pressure ranging from UHV to 100 Torr. This process

attributes to many advantages as well as disadvantages. The advantages are

flexibility, fast response, energetic evaporants and congruent evaporation.

It can be used for the synthesis of various materials including organic crystals,

semiconductors, dielectric materials and refractory materials. PLD is simpler,

easier and suitable technique for some complex and high melting point

compounds. It has been successfully used in carbon nanotube synthesis, with

high yield (Eason 2007). The disadvantages are the presence of micro&size

particulates and the narrow forward angular distribution that makes

large&area&scale&up a very difficult task.

One of the important advantages of PLD technique is that it can be

operated from ultra high vacuum to 100 Torr of ambient pressure.

The growth at low oxygen pressure leads to either the formation of thin film

or randomly aligned nanostructures (Yan et al 2003). The low pressure PLD

can be defined as if the oxygen pressure is in the range of 1 × 10&5 to 0.5 Torr.

The kinetic energy of ablated species is shown to have approximately several

100 eV. These energies are sufficiently high to create defects in the growing

thin films in vacuum or at low background pressure. The kinetic energy of the

ablated species can be reduced by increasing the ambient gas pressures.

Hence, the control and reproducibility over the nanostructure growth of oxide

24

and nitride materials can be enhanced in high&pressure PLD compared with

the low&pressure PLD (Eason 2007). Similarly, the high&pressure PLD can be

defined as if the oxygen pressure is in the range of 0.5 Torr to 100 Torr.

The number of collisions between ablated species and gas molecules is

strongly depends on the ambient gas pressure. The high gas pressure leads to

large number of collisions, which in turn results the formation of

nanoparticles in the gas phase (Okada et al 2005).

The nanoparticles act as a nucleation site for the growth of

nanostructures. This approach is called high&pressure PLD or nanoparticle

assisted PLD. Table 2.1 compares the typical growth parameters of high and

low pressure PLD. Both inert and reactive gases e.g, oxygen and argon can be

used as a gas source in PLD technique. In this study, efforts have been made

to optimize the aligned growth of ZnO nanorods by varying the PLD growth

parameters. The effect of laser wavelength, oxygen partial pressure and

substrate temperature on the growth of ZnO nanorods have been analyzed

without using any metal catalyst and templates.

Table 2.1 Comparison of the growth parameters of high and low#pressure

PLD

Growth Parameters High#pressure PLD Low#pressure PLD

Background pressure 0.5 & 100 Torr 10&5 & 0.5 Torr

Morphology Self&assembled ZnO nanorods

Mostly thin films

Growth temperature RT & 800 °C RT & 800 °C

Target & Substrate distance 20 & 40 mm 30 & 60 mm

Substrates Any substrates Any substrates

Material types Oxides and nitrides Oxides and nitrides

25

2.3 LOW#PRESSURE PULSED LASER DEPOSITION

2.3.1 Experimental Details

ZnO thin films were grown on GaN (0001), Al2O3 (0001) and

Si (100) substrates at different laser wavelengths using a pulsed laser

deposition technique. ZnO targets were prepared by grounding commercial

(Alfa Aesar) ZnO powder for 1 hour. Then the powders were mechanically

pressed and sintered at 900 °C for the duration of 8 hours. The Q&switched

Nd:YAG laser was used for ablating ZnO targets in the infrared (IR) and

visible wavelengths. The film deposition was carried out in a stainless steel

vacuum chamber evacuated by a diffusion pump to a base pressure of

1×10&5 mbar. The Q&switched Nd:YAG laser (λ = 1064 nm and 532 nm,

repetition rate of 10 Hz and the pulse duration of 8 ns) was focused by a lens

on the ZnO target at an angle of incidence of 45º. During the deposition, the

laser energy was maintained at above 30 mJ/pulse. The substrate to target

(T&S) distance was varied from 40 to 50 mm. A KrF excimer laser

(λ = 248 nm, laser energy with 200 mJ/pulse and repetition rate of 7 Hz) was

used to ablate ZnO targets in the UV laser wavelength. The stainless steel

chamber evacuated by a turbo molecular pump to a base pressure of

1×10&5 mbar was used for the KrF laser experiments. The film deposition time

was varied from 5&60 minutes. The oxygen background pressure (Po2) was in

the range of 5×10&5 Torr to 100 mTorr. The substrate temperature (Tsub) was

varied from 450 to 700 °C.

2.3.2 Scanning Electron Microscopy (SEM)

The surface morphology of the ZnO films was examined using

scanning electron microscopy (SEM, Stereoscan, LEO&440). ZnO thin films

deposited at different Po2, Tsub and T&S resulted in the randomly aligned ZnO

nanostructures. The diameter/width of the ZnO nanostructures were in the

26

range of 300 & 2000 nm. The SEM images of ZnO films grown on GaN and

Al2O3 substrates in the Po2 of 7.5×10&5 Torr are shown in Figure 2.2 (a) and

(b), respectively.

Figure 2.2 SEM micrographs of a, b) ZnO nanorods and microrods

grown on GaN and Al2O3 substrates at 7.5××××10#5

Torr c, d)

ZnO nanorods and microrods grown on GaN substrate at

7.5××××10#4

Torr and 10 mTorr e) ZnO wire#like structures

grown at 50 mTorr f) ZnO belt#like structures grown at

100 mTorr

27

As evident from the SEM image, irrespective of substrate it has

been observed a random distribution of ZnO nanorods and microrods.

The diameter of the ZnO nanorods grown on GaN substrate varies from

250&300 nm and length varies from 10&20 �m, whereas diameter of the

microrods varies from 1.2&1.6 �m and the length varies from 9&22 �m.

The average diameter and length of the ZnO nanorods grown on Al2O3

substrate were 300 nm and 8 �m, respectively. The diameter of the microrods

was about 0.9 �m and the length varies from 7&11 �m. The SEM images in

Figure 2.2 (c) and (d) show the ZnO nanorods and microrods grown on GaN

substrate in the Po2 of 7.5×10&4 and 10 mTorr.

The diameter and length of the nanorods and microrods were

smaller than that observed in relatively low oxygen pressure. ZnO wire&like

structure was obtained for the growth at the Po2 of 50 mTorr as shown in

Figure 2.2 (e). Figure 2.2 (f) shows the SEM micrograph of ZnO belt&like

structure grown at the Po2 of 100 mTorr. In all the above conditions,

randomly aligned ZnO nanostructures were observed. The alignment and

reproducibility of nanostructures were found to be major issues in

low&pressure PLD. From the SEM images, it is evident that features of frozen

droplets are absent at tip of the rods. This implies that the VLS mechanism is

not applicable for the ZnO rods in the present growth. In this work no metal

catalyst was used. This suggests the growth of ZnO rods via VS mechanism

(Roy et al 2003, Conley et al 2005).

2.3.3 Growth Mechanism of ZnO Nanorods and Microrods

Both ZnO nanorods and microrods were obtained on GaN and

Al2O3 substrate at the similar deposition conditions. The possible mechanism

for the growth of nanorods and microrods is depicted as shown in

Figure 2.3.

28

Figure 2.3 Schematic diagram of growth mechanism of ZnO nanorods

and microrods

The absorption coefficient “α” of the ZnO decreases with

increasing wavelength from UV (266 nm) to IR (1064 nm). The penetration

depth of the laser appears to be larger in the near IR region than in the UV

region. Koren et al (1989) reported that the density and size of particulates are

much higher for the YBCO films prepared by Nd:YAG laser at IR wavelength

as compared to one prepared with an excimer laser at UV wavelength.

Bilkova et al (2004) also demonstrated the formation of micrometric and

sub&micrometric sized particulates in the ZnO films grown using Nd:YAG

(λ=532 nm) laser. It has been shown that the penetration depth of the laser at

532 nm determines the size and density of particulates in the ZnO thin films.

When the experiment is conducted at low oxygen pressure, the number of

collisions between the ejected species and gaseous atoms is less before they

reach the substrate. Consequently, particulates are predominantly formed

from solidified liquid droplets that are expelled from the target (Chrisey and

29

Hubler 1994). These micro and nano size particulates may acts as seed

crystals for the growth of ZnO rods. Wang (2004) reported that the catalyst

droplet directs the growth direction and defines the diameter of the nanowires.

The nucleation can occur at any possible sites on the substrate. This reveals

that the particulates may induce the growth of ZnO nanostructure. Therefore,

it is likely that the diameter of the ZnO rods varies from nanometer to

micrometer size due to the different diameter of the particulates.

Figure 2.4 SEM micrographs of a, b) ZnO films grown on GaN and

Al2O3 substrates at 7.5××××10#5

Torr (λ = 1064 nm) c) ZnO film

grown on GaN substrate at 7××××10#4

Torr (λ = 248 nm)

The ZnO films were deposited at IR and UV wavelengths to further

confirm the particulates effect on the formation of nanorods and microrods.

Figure 2.4 (a) and (b) shows the SEM images of ZnO films grown on GaN

and Al2O3 substrate at the laser wavelength of λ = 1064 nm. The image

30

clearly shows the typical micron size particulates on the film surface. Here the

number of particulates ejected from the target is very high due to the large

penetration depth of the laser. On the other hand, the ejection of large number

of particulates may possibly hinder the growth of the rods. This could be the

reason for the absence of nanorods and microrods at higher laser wavelength.

When the experiment was carried out using KrF excimer laser (λ = 248 nm) a

relatively smooth surface of ZnO was observed, as shown in Figure 2.4 (c).

The probability for the formation of particulates in KrF excimer laser is very

less. Hence, no rods could be observed in the ZnO films. All the ZnO films

grown in IR and UV laser wavelengths do not reveal any rod like

morphology. This implies that the rod like growth may depend on the

penetration depth of the laser.

2.3.4 X#Ray Photoelectron Spectroscopy (XPS)

The composition of the ZnO nanorods and microrods were

examined using X&ray photoelectron spectroscopy. The XPS measurements

were performed with a VG scientific ESCALAB MK200X Electron

spectroscope for chemical analysis (ESCA) machine using Al&Kα X&ray as a

source (1486.6 eV, width 0.85 eV). The hemispherical analyzer was operated

with pass energy of 20 eV to give analyzer resolution of 0.4 eV. The base

pressure of the chamber was maintained at 1×10&10 mbar. The XPS spectrum

of the ZnO thin films grown on Al2O3 is shown in the Figure 2.5. The peaks

centered at 7 eV, 86.9 eV and 136.6 eV are due to the binding energy of Zn3d,

Zn3p and Zn3s, respectively. Peak centered at 527.8 eV is due to O1s peak

(Yuen et al 2007). The zinc and oxygen were only detected species in the

XPS spectrum. This shows that no other external metal catalyst was involved

in the ZnO rod growth.

31

Figure 2.5 XPS spectrum of ZnO nanorods grown on Al2O3 substrate

at 7.5××××10#5

Torr

2.3.5 X#Ray Diffraction (XRD)

The structural properties of the samples were studied using Philips

X&pert X&Ray Diffraction system using CuKα as a source (λ = 1.54 Å). Figure

2.6 shows XRD spectra of ZnO nanorods and microrods grown on Al2O3 and

GaN substrates at λ = 532 nm. The strong (0002) peak can be attributed to the

hexagonal wurtzite structure of ZnO. The lattice constants of ZnO and GaN

are very close to each other. Hence, the peaks due to (0002) plane of GaN and

ZnO overlap (Wang et al 2005). The FWHM of ZnO films grown in oxygen

ambient are 666 arcsec and 578 arcsec on GaN and Al2O3 substrate,

respectively. The higher FWHM of (0002) peak may be due to the overlap of

(0002) planes of both ZnO and GaN. The biaxial stress was calculated using,

σ = & 453.6 x 109 [(c&co)/ co] (2.1)

32

Figure 2.6 XRD patterns of ZnO nanorods and microrods grown on

GaN and Al2O3 substrates at 7.5××××10#5

Torr

where co = 0.5206 nm is the strain free c&axis lattice constant of ZnO (Oh et al

2005). The “c” is the lattice constant along c&axis of ZnO grown by PLD. The

estimated tensile stress is decreased from 3.48 GPa (on Al2O3) to 2.43 GPa

(on GaN). This shows the relaxation of the tensile stress in the ZnO films

grown on GaN substrate, which may be due to close lattice match.

2.3.6 Photoluminescence (PL)

The photoluminescence spectra were recorded at room temperature

using He&Cd (λ = 325 nm) laser as an excitation source with the excitation

power of 38 mW. Comparison of the PL spectra of ZnO films grown on GaN

and Al2O3 substrates is shown in Figure 2.7.

33

Figure 2.7 PL spectra of ZnO nanorods and microrods grown on GaN

and Al2O3 substrates

The ZnO films show near band edge (NBE) emission at 3.282 eV

and 3.277 eV on GaN and Al2O3 substrates in oxygen ambient, respectively.

The blue shift in the NBE emission of ZnO/GaN film is due to the low tensile

stress; this is in good correlation with XRD results. The PL intensity of ZnO

film grown on GaN in oxygen ambient is twice stronger than that of ZnO

grown on Al2O3. The NBE emission is due to the free exciton&exciton

recombination of ZnO. In addition to this, FWHM of ZnO/GaN film is

smaller (92 meV) than that of the ZnO/Al2O3 (101 meV) film. All the ZnO

films show weak defect related emission in the visible region. The emission

intensity is determined by the radiative and non&radiative transitions.

The non&radiative transition is generally induced by crystal imperfections

such as oxygen vacancies and zinc interstitials (Shan et al 2007) in ZnO thin

films.

34

2.4 HIGH # PRESSURE PULSED LASER DEPOSITION

2.4.1 Experimental Details

In the high&pressure PLD experiments the Po2 was maintained

between 6 and 8 Torr. Before loading into the chamber, the GaN (0001),

Al2O3 (0001) and Si (100) substrates were cleaned sequentially in acetone and

methanol for 3 min. The Si surfaces were etched in HF:H2O in the ratio of

1:10 for 1 min to remove the native oxide layer of the Si (100) substrate. Then

the substrates were rinsed with deionized (DI) water and dried in atmosphere.

The undoped ZnO targets were prepared by mechanically pressing the

commercial ZnO (Alfa Aesar & purity 99.99%) powder. The ZnO targets were

sintered at 900 °C for 8 hours in the atmosphere. The chamber was evacuated

to the base pressure of 8×10&6 Torr after loading the ZnO target and the

substrates. KrF excimer laser (λ = 248 nm, repetition rate of 10 Hz, pulse

duration of 30 ns and laser energy with 230 mJ/pulse) was used to irradiate

the ZnO target. The laser fluence was 3 J/cm2. The ablated species was then

deposited on a Si substrate that was mounted on a substrate heater. During

growth, the substrate temperature was varied from 550 to 700 °C in steps of

50 °C. The target to substrate distance was varied from 25 mm to 30 mm.

2.4.2 Field Emission Scanning Electron Microscopy (FESEM)

The surface morphology of ZnO films was analyzed using FESEM

(Hitachi S4700 system, with a resolution of 1.2 nm at 25 kV).

Figure 2.8 (a – e) shows top view of ZnO films grown on Si at Tsub of

500 °C, 550 °C, 600 °C, 650 °C and 700 °C, respectively. The laser fluence

was 3 J/cm2. Figure 2.9 (a) and (b) shows cross&sectional view of ZnO films

grown at Tsub of 650 and 600 °C, respectively.

35

Figure 2.8 FESEM micrographs (top view) of ZnO films grown on

Si substrates at a) 500 °C b) 550 °C c) 600 °C d) 650 °C

e) 700 °C

The film deposited at 500 °C shows sheet&like structure with a

rough surface. The SEM morphology turned into grain like structures with the

average diameter of ~ 1 `m at 550 °C. As the substrate temperature further

increased to 600 °C, the films were covered by near hexagonal crystals and

randomly aligned on the substrate, with the average diameter of ZnO grains as

36

~ 400 nm. ZnO films deposited at 650 °C show the well separated vertically

aligned hexagonal nanorods with tapered tip at the top. The average diameter

of ZnO nanorods was around 300 nm. When the substrate temperature was

increased to 700 °C the diameter of the nanorods further increased and starts

to contact with the neighboring nanorods. ZnO nanorods lost the ordered

alignment and lead to the formation of continuous ZnO film at higher

temperatures. The vertically aligned growth of ZnO nanorods is obvious from

the cross&sectional view of the ZnO films deposited at 650 and 600 °C,

respectively as shown in Figure 2.9 (a) & (b). The nanorods grown at 650 °C

show hardly any coalescence between the neighboring nanorods than that

grown at 600 °C.

ZnO films were deposited at the laser fluence of 2 J/cm2 to study its

effect on the growth of ZnO nanorods. Figure 2.9 (c) shows the FESEM

micrographs of ZnO films grown at 2 J/cm2. ZnO rod&like structures with

hexagonal facets are observed with continuous ZnO film in the background.

The density of the ZnO nanorods was low compared to ZnO nanorods grown

at 3 J/cm2. The background film consists of grains with diameter of ~ 300 nm.

This can be attributed to the low ablation rate and growth rate at reduced laser

fluence. The composition of the ZnO nanorods grown at 650 °C (3 J/cm2) was

analyzed using energy dispersive X&ray analysis (EDX). The EDX (Figure 2.9

(d)) confirmed the presence of zinc and oxygen species, the other impurities

were not found. This shows that the as grown nanorods followed the

vapor&solid growth mechanism rather than an impurity assisted vapor&liquid&

solid growth mechanism.

37

Figure 2.9 FESEM micrographs (cross sectional view) of ZnO films

grown on Si substrates with a laser fluence of 3 J/cm2 at

a) 650 °C b) 600 °C c) ZnO films grown at 650 °C with the

laser fluence of 2 J/cm2 and d) EDX spectra of ZnO

nanorods grown at 650 °C

Energy (keV)

Inte

nsi

ty (

cou

nts

)

(d)

38

2.4.3 Growth Mechanism of Aligned ZnO Nanorods

Figure 2.10 shows the schematic diagram for the growth

mechanism of ZnO nanorods. During the laser ablation, a short wavelength

and high power laser beam interact with sintered ZnO target (Step&I).

The strong laser&material interaction leads to the evaporation of Zn, O and

neutrals from the ZnO target. The background pressure determines the

number of collision between the ablated species and gaseous atoms. At a high

oxygen gas pressure above 1 Torr, the increased collisions lead to the

condensation of ablated species resulting in the formation of nanoparticles

(Step&II). The nanoparticles then reach the substrate surface and may act as a

nucleation site or seed crystal for the growth of nanostructures (Hartanto et al

2004, Kawakami et al 2003) (Step&III). The nucleation can occur at any

possible site on the substrate. Since ZnO nanoparticles are thermodynamically

favorable site for the growth of ZnO nanostructures, the nanorods growth can

be attributed to the formation of nanoparticles in the gas phase (Step&VI).

Figure 2.10 Schematic diagram represents the growth mechanism of

ZnO nanorods

This implies that nanorods could grow only at such a high gas

pressures whereas thin film growth was obtained when deposition was

39

conducted at low oxygen pressure. The nanoparticles formed in the gas phase

play an important role in the growth of vertically aligned nanorods.

2.4.4 X#Ray Diffraction

Figure 2.11 (a) shows the XRD patterns of ZnO films grown at

different substrate temperatures. All the samples show a pronounced (002)

peak of wurtzite ZnO, indicating the preferentially oriented growth along the

c&axis which is perpendicular to the substrate surface. The XRD peaks match

with the JCPDS card No: 00&003&0888. ZnO sheet&like structures grown at

500 °C exhibits (100), (002), (101), (102), (103), (112) and (004) peaks. This

indicates that the sheet&like structures are randomly aligned on the Si

substrate. The full width at half maximum (FWHM) of (002) peak is 0.19°.

The c&axis lattice constant ‘c’ was obtained using the formula: 2d sinθ = nλ

and the biaxial stress was calculated using,

σ = & 453.6 x 109 [(c&co)/ co] (2.2)

where c0 = 0.5206 is the strain&free lattice constant (Oh et al 2005). These

detailed data points were connected in lines in Figure 2.11 (b). As the Tsub

increases to 550 °C, high temperature provides enough energy for the adatoms

to gain high surface mobility, which promotes the formation of the closely

packed columnar grains oriented along the [002] direction and the (002) peak

was observed at 34.55°. The vertically aligned nanorods were obtained as the

temperature was increased to 600 and 650 °C. The (002) peak was observed at

34.46°, with a shift towards lower angle. The (002) peak position of nanorods

closely matches with (002) peak of bulk ZnO. The peak shifts towards higher

angle at 700 °C and the (002) peak was observed at 34.61°. The position of

(002) peak shifts towards higher angle with increase of Tsub. This may be

attributed to the unstable oxygen adatoms on the ZnO growth surface which

may be easier to desorbs at higher temperature. The FWHM of ZnO films

40

grown at 500, 550, 600, 650 and 700 °C are 0.19, 0.26, 0.26, 0.31 and 0.38°,

respectively. ZnO films grown at 500, 550, 600, 650 and 700 °C have

compressive stress of 1.82, 1.82, 0.6, 0.6 and 2.43 GPa, respectively. This

implies that due to the different thermal expansion coefficients of ZnO and Si,

the compressive stress would be induced parallel to the c&axis. ZnO grown at

higher temperature has high FWHM and large compressive stress compared

with ZnO grown at lower temperatures. The temperature dependence of the

film quality can be interpreted mainly by the mobility of the adatoms on the

substrate surface at different temperatures.

(a) (b)

Figure 2.11 a) XRD patterns of ZnO film deposited at different

temperatures and b) Plot between Tsub versus peak position,

FWHM and stress

During PLD, the kinetics of atomic arrangement is mainly

determined by substrate temperature and the energy of deposition atoms.

Therefore, at a high temperature, adatoms on the surface have high mobility.

41

There should be enough time for the adatoms to move on the surface to look

for the lowest energy sites before these adatoms are covered by the next layer

of atoms (Ye et al 2003). Otherwise, low substrate temperature results in poor

adatom mobility, which results in the degradation of crystallinity. On the

other hand, the decomposition of ZnO was promoted at high Tsub. In the

present work, it has been confirmed that the stress between ZnO film and Si

substrate increases due to the increase of the Tsub and the crystallinity of ZnO

film degrades at higher deposition temperature which is in agreement with

Liu et al (2004).

2.4.5 Micro # Raman spectroscopy

In the wurtzite ZnO, the number of atoms per unit cell is s = 4 and

there are a total of 12 phonon modes, namely, one longitudinal&acoustic (LA),

two transverse&acoustic (TA), three longitudinal&optical (LO), and six

transverse&optical (TO) branches. Raman spectroscopy has been commonly

employed to derive zone&center and some zone&boundary phonon modes in

ZnO. Because the space group ‘C’ describes the crystalline structure of the

wurtzite ZnO with 2 formula units in the primitive cell, the optical phonons at

the point of the Brillouin zone belong to the following irreducible

representation. The A1and E1 branches are both Raman and infrared active,

the two non&polar E2 branches are Raman active only, and the B1 branches are

inactive silent modes. The each A1 and E1 modes are split into LO and TO

components with different frequencies due to the macroscopic electric fields

associated with the LO phonons. Because the electrostatic forces dominate the

anisotropy in the short&range forces, the TO&LO splitting is larger than the

A1 & E1 splitting. For the lattice vibrations with A1 and E1 symmetries, the

atoms move parallel and perpendicular to the c axis, respectively.

The low&frequency E2 mode is associated with the vibration of the

heavy Zn sublattice, while the high&frequency E2 mode involves only the

42

oxygen atoms. In the case of highly oriented ZnO films, if the incident light is

exactly normal to the surface, only A1 (LO) and E2 modes are observed, and

the other modes are forbidden according to the Raman selection rules (Ozgur

et al 2005, Singamaneni et al 2003, Zhu et al 2009). The vibrational modes of

the as grown ZnO nanostructures were recorded using micro&Raman

spectroscopy. Figure 2.12 shows the micro&Raman spectra of ZnO films

grown on Si substrate at different Tsub of 500, 550, 600, 650 and 700 °C. The

spectra show Raman modes at 331, 665 (multi&phonon), 380 (A1 (TO)) and

439 cm&1 (E2H) (Ozgur et al 2005, Singamaneni et al 2003).

Figure 2.12 a) Micro # Raman spectra of ZnO film deposited at different

Tsub, at fluence of 3 J/cm2 and b) ZnO deposited at the Tsub

of 650 °C with the fluence of 2 J/cm

2

The Raman shifts observed at 304 and 521 cm&1 are correspond to

Si substrate. The E2H mode of ZnO films were observed at 439 cm&1. No shift

in E2H mode was observed for ZnO films grown at different temperatures. The

strong E2H mode can be attributed to low intrinsic defects associated with O

e.g. O vacancies (VO), since E2H mode is only associated with the vibration of

43

O atoms (Singamaneni et al 2003). The low VO of ZnO nanostructures can be

attributed to the high oxygen pressure during PLD growth. In addition to the

E2H mode, ZnO sheet&like structures grown at 500 °C shows additional mode

(AM) at 477 cm&1. The FWHM of ZnO films grown on Si substrates at Tsub of

500, 550, 600, 650 and 700 °C were 5.8, 6.4, 6.4, 6.9 and 7.1 cm&1,

respectively. These results are in good correlation with XRD and FESEM.

ZnO films deposited at 650 °C with the laser fluence of 2 J/cm2 possesses AM

mode at 477 cm&1 in addition to the well defined E2H at 439 cm&1. The decrease

in the laser fluence influences the growth of ZnO nanorods.

2.5 SUMMARY

Vertically aligned ZnO nanorods were grown on Si substrate using

high&pressure pulsed laser deposition. The randomly aligned ZnO nanorods,

microrods, nanowires and nanobelts were obtained in the Po2 of 4×10&4 Torr –

100 mTorr. The controllability and reproducibility on the growth of ZnO

nanostructures were found to be poor in low&pressure PLD.

The vertically aligned ZnO nanorods were realized in the high Po2 of

6 – 8 Torr. The (002) peak in XRD pattern implies the preferred orientation of

ZnO nanostructures along c&axis. The strong E2H mode shows that as grown

ZnO nanorods possesses good crystalline properties. The high pressure PLD

is suitable method to obtain aligned ZnO nanorods.