Chapter-IV
Optostructural,Morphological,Compositional
and Electrical Conduction Studies on
MoBi2Se5 and MoBiInSe5 Mixed Metal
Chalcogenide Thin Films
CHAPTER – IV
Optostructural, Morphological, Compositional and Electrical Conduction Studies on MoBi2Se5 and MoBiInSe5 Mixed Metal
Chalcogenide Thin Films
Sr. No.
Title
Page No.
4. 1. Introduction
98
4. 2. Experimental Details 99
4. 2.1. Optostructural Studies on MoBi2Se5
Chalcogenide Thin Films 99
4. 2.2. Morphological, Compositional Studies on
MoBi2Se5 Chalcogenide Thin Films 102
4.2.3. Electrical Conduction Studies on MoBi2Se5
Chalcogenide Thin Films 103
4.3. Results and discussion
107
4.4. Conclusion
123
References
125
98
CHAPTER – IV
Optostructural, Morphological, Compositional and Electrical
Conduction Studies on MoBi2Se5 and MoBiInSe5 Mixed Metal
Chalcogenide Thin Films
4.1. Introduction
In this chapter Optostructural, morphological, Compositional and
Electrical Conduction studies carried out as deposited MoBi2Se5 and
MoBiInSe5 thin films, these films are characterized using Optical, Raman
spectral study, X-ray diffraction (XRD), Scanning Electron Microscopy
(SEM),Energy Dispersive X-ray Microanalysis (EDS) and Atomic Force
Microscopy (AFM) to investigate Optostructural, morphological and Electrical
Conduction properties. Optostructural, morphological and Electrical
Conduction studies are important to know the possible use of these ternary
and quaternary mixed metal chalcogenide thin films as conduction properties.
These properties are also highly sensitive to optical band gap, crystallite type
and size, surface morphology and grain size so our next study is to shed light
on optical, structural, morphological and Electrical properties of these
materials. In this chapter IV, Optostructural, morphological, Compositional and
Electrical Conduction properties of chemically deposited MoBi2Se5 and
MoBiInSe5 thin films are reported. Thin films of MoBi2Se5 and its alloys find
many applications such as in small scale thermoelectric power generator,
thermopile, thermoelectric refrigerators, thermoelectric- cooler, thermoelectric
and optical recording materials [1]. Malhotra et al. [2] have observed that
MoBi2Se5 films are good contenders for phase change optical media and that
the amorphous phase of these films is stable, their optical properties remain
unaffected by normal environmental conditions. Doping has been one of the
effective means of tuning the alloys phase change properties. Although a
variety of dopants including gallium [3], indium [4-7], arsenic [8] and selenium
[9] be used.
99
4. 2. Experimental Details
4.2.1. Optostructural Studies on MoBi2Se5 and MoBiInSe5 Chalcogenide
thin films
b) a) Optical absorption Study
Light incident on material is absorbed if it can cause an electronic
transition. In semiconductors, this process can occur by means of several
mechanisms, including the Direct interband (band-to-band) transition,Indirect
interbandtransitions,Impurity-to-bandandimputity-impurity transitions,Excitonic
transitions,Interband transitions, Phonon transitions
Semiconductors absorb light with energy larger than their band gap.
Absorption measurement can therefore be used to estimate their band gap
energy. Measurements at low temperature are more meaningful due to the
reduced amount of band tailing. To find dependence of the absorption
coefficient on frequency for direct/indirect gap semiconductors, the material is
made thin and its surfaces are polished to reduce scattering.
In present study, UV-Visible spectrophotometer (Hitachi model 330,
Japan) was used to determine absorption spectra of MoBi2Se5 and MoBiInSe5
in the wavelength range 350-850 nm. A glass slide of same thickness and
size was used as reference throughout all the measurements. One side of the
film was removed with the help of cotton swab moist in dil. HCl. The layer
thickness of the as deposited samples was measured by surfaceprofilometer
technique using highly sensitive. Absorption spectra was analysed to
determine absorption coefficient, optical band gap ‘Eg’ and mode of optical
transition for all the compositions.
c) Raman Spectral Study
Raman spectroscopy being used for the study of mixed metal
chalcogenide thin film semiconductors. It allows identification of the material
and information about frequencies, energies of electron states and electron
intraction, carrier concentration, crystal structure, impurity content, crystal
orientation, mechanical strain and temperature. Raman mapping, microscopy
100
and spectroscopy can provide varied and important information on many
different sample types; from-
� semiconductors to pharmaceuticals
� Polymers to minerals.
� Chemical composition
� Bonding, structure
� Phase, localization, size,
� Induced stress and reaction mechanisms can all be studied with the
modern Raman instrument.
Raman spectroscopy has been employed to provide high quality
sample information and characterization. Raman spectra of the crystal and
amorphous phases are quite easy to distinguish. This substrate (the darker
material in the video image, top spectral trace), from the amorphous feature
(the bright 15square and L-shaped).Raman spectroscopy is often the
technique of understanding the electronic properties of the materials used in
the manufacture of the transistors, diodes, and capacitors there has to be
patterning on the silicon which is achieved by doping or by depositing an
amorphous layer and by coating with dielectric and metal films properties of
the materials, there are important materials compatibility issues which can be
addressed with Raman spectroscopy. Raman spectroscopy is often the
technique of choice for studying the materials in an integrated circuit. It
provides information on interatomic bonding, crystallographic phase spatial
resolution for mixed metal chalcogenide thin films.
c) X-ray diffraction (XRD)
If the substance is crystalline, identification is usually carried out by X-
ray diffraction. Each crystalline solid has its own characteristics X-ray powder
pattern which may be used as a ‘fingerprint’ for its identification. The powder
patterns of most known inorganic solids are includes in an updated version of
the Powder Diffraction File; by using an appropriate search procedure,
unknowns can usually identified rapidly and unambiguously. Once the
substance has been identified, the next stage is to determine its structure and
its crystallite size.
101
An X-ray powder diffraction pattern is a set of lines or peakes, each of
different intensity and position (d-spacing or Bragg angle, θ), on either a strip
of photographic film or on a chart paper. For a given substance the line
positions are essentially fixed and are characteristics of that substance. The
intensities may vary somewhat from sample to sample, depending on the
method of preparation and the instrumental conditions. For identification
purposes, principle note is taken of line positions together with a
semiquantitative consideration of intensities. X-ray powder diffraction may be
used to measure the average crystallite size in a powdered sample, provided
the average diameter is less than about 2000A0. The broadening of lines
increases with decreasing particle size. The limit is reached with particle
diameters in the range roughly 20 to 100 A0.
In our study, X- ray diffraction (XRD) analysis was carried out using a
Philips PW-1710 X-ray diffractometer for the 2θ ranging from 00 to 1000 with
Cr Kα line used as a beam ( λ=2.89A0).
d) Thickness Measurement
The thickness of film is the most significant parameter that affects the
properties of the thin films. It may be measured either by in-situ monitoring of
the rate of the deposition or after the film is taken out form deposition
chamber. Technique of the first type often referred to as monitor methods
generally allow both monitoring and controlling of deposition rate of film
thickness. Any known physical quantity related to film thickness can be used
to measure the thickness. The method chosen should be convenient, reliable
and simple. One of the most convenient surfaceprofilometer and reliable
method for determining film thickness. In order to get more accurate results,
one should measure thickness using the films with maximum area so that,
weight difference is accurately measurable.
102
4.2.2. Morphological, Compositional Studies on MoBi2Se5 and MoBiInSe5
Chalcogenide thin films
a) Atomic Force Microscopy (AFM)
The two dimensional (2D) and three dimensional (3D) morphology of
films can be observed by AFM. A quantitative method to examine the surface
morphology and structure is obtained by analyzing the surface roughness
using AFM.The surface roughness can be given with a statistical parameter-
root mean square (rms or Rq) that is the standard deviation of the height (z)
values within a given area.
N2
i=1
(Zi - Zave)
R q = N
∑
4.1
Where, Zave is the average of z values within given area, Zi is current
Z value and N is the number of points within given area. Atomic Force
Microscopy (AFM) was performed on a JEOL-JSM- microscope.
b) Scanning Electron Microscopy (SEM)
As a first step in examining a solid, it is usually well worthwhile to have
a look at it under magnification. Electron microscopy is an extremely versatile
technique capable of providing structural information over a wide range of
magnification. With scanning electron microscopy (SEM) features up to tens
of micrometers can be seen and, because of the depth of focus of SEM
instruments, the resulting pictures have a definite three dimensional quality.
Some SEM instruments have the very valuable additional features of
providing an elemental analysis of sample composition.
In the scanning electron microscope, electrons from the electron gun
are focused to a small spot, 50 to 100 A0 in diameter, on the surface of the
sample. The electron beam is scanned systematically over the sample. Both
X-rays and secondary electrons are emitted by the sample; the former are
used for chemical analysis and the latter are used to build up an image of the
sample surface which is displayed on screen. A limitation with SEM
103
instrument is that the lower limit of resolution is 100 A0. Thus SEM is
invaluable in for surveying material under high magnification and providing
information on particle sizes and shapes.
In our investigation, scanning electron microscopy (SEM) and energy
dispersive X-ray analysis (EDS) was performed on a JEOL-JSM- 6360A
scanning microscope.
c) Energy Dispersive X-ray Microanalysis (EDS)
The combinatorial as deposited thin films of MoBi2Se5 and MoBiInSe5
were analyzed using EDS technique. The quantitative analysis for energy
dispersive X-ray analysis was performed for Mo, Bi, In and Se in the sample
at different points. Energy dispersive X-ray analysis (EDS) was performed on
a JEOL-JSM- 6360A scanning microscope. The chemically deposited
MoBi2Se5 and MoBiInSe5 thin film samples were cut into 1cm2 pieces and
mounted on the sample holder with conducting paste. The samples were
coated with a thin layer of gold to prevent charging of the samples. For
comparative studies, the electron beam was kept constant while analyzing the
samples. EDS spectrum obtained with an accelerating voltage of 10 kV,
acquisition time of 1 minute on a film within the precision of the energy
dispersive X-ray analysis, i.e. ± 2%.
4. 2.3. Electrical Conduction Studies on MoBi2Se5 and MoBiInSe5
Chalcogenide Thin Films
A) Setup for Electrical Resistivity Measurements
The electrical conductivity of the films was studied by using two point
D. C. probe method. As the contact resistance of the films is vary low (10-3
ohm) compared to film resistance, the two probe method is accurate and
hence used for electrical conductance measurements. Fig. 4.1b and 4.1b
shows photograph and schematic diagram of the electrical conductivity
measurement unit. The two brass plates of the size 10 x 5 x 0.5 cm are
grooved at the centre to fix the heating elements. Two strip heaters (65 Watts)
were kept parallel in between these two brass plates to achieve uniform
temperature. The two brass plates were then screwed to each other. The
104
sample was mounted on the upper brass plate at the centre. To avoid the
contact between the film and the brass plate, a mica sheet was placed
between the film and brass plate. The area of the film was defined and silver
emulsion (paste) was applied to ensure good electrical contact to the films.
The working temperature was recorded using a Chromel-Alumel
thermocouple (24 gauge) fixed at the centre of the brass plates. Testronix
model - 34C (power supply unit ) was used to pass the current through the
sample. The potential drop across the film was measured with the help of
Meco 801 digital multimeter and the current passed through the sample was
noted with a sensitive 4 digit picoammeter (Scientific equipment, Roorkee
DPM 111).The measurements were carried out by keeping the film system in
a light tight box, which was kept at room temperature.
a) Electrical Resistivity Measurement :
This is attributed to the fact that, intrinsically MoBi2Se5 and MoBiInSe5 is n-
type semiconductor. When trivalent impurity is added into MoBi2Se5 and
MoBiInSe5 lattice the MoBi2Se5 and MoBiInSe5 ions are substituted by indium
without distorting the structure, which is evident from XRD analysis. This
eventually enhances the carrier concentration which is evident from
conductivity measurements. The doping concentration does not change the
carrier type, provided that it enhances the carrier concentration. The retention
of carrier type is supported by TEP measurement.
105
Fig.4.1.a. Photograph showing the electrical conductivity
measurement assembly.
Fig.4.1b. Cross sectional view of electrical conductivity
measurement unit.
106
b) Thermoelectric Power Measurement (TEP):
Electrical measurement study revealed that there is remarkable change in
the activation energy of MoBi2Se5, and MoBiInSe5 Activation energy for
MoBiInSe5, at low temperature regions and in high temperature regions. This
is due to when trivalent impurity indium added into MoBi2Se5 lattice carrier
concentration goes on increasing. The negative sign indicates the material is
n-type semiconductor.
Fig. 4.2.a. Photograph showing the thermoelectric power measurement
assembly.
107
Fig.4.2b. Cross sectional view of thermoelectric power measurement
unit.
4. 3. Results and Discussion
4.3.1. Optical Analysis
Optical absorbance measurement of the film was used to estimate the band
gap energy from the position of absorption edge. Optical absorption
coefficient (α) of the material is of the order of 104 cm-1. Near the absorption
edge α is given by [10, 11]
α = A (hυ – Eg ) n / 2 / hυ
Where A is an energy dependant constant, hυ is photon energy and n=1/2
for direct band gap materials. The optical absorption data was used to plot a
graph of (αhυ) 2 vs. hυ. The plot of (αhυ) 2 vs. hυ yielded straight line at higher
energies indicating direct type of transition. Extrapolation of the plot to the x-
108
axis gives the energy band gap of the deposited film as shown in figure 4.3.
The band gap determined from (αhυ) 2 vs. hυ plots is found to be 1.78 eV to
1.47 eV. It was found that the optical energy gap decreases gradually with
increasing indium mole content [0.0ml to 1.0ml] as shown in figure. The
decreasing band gap is related with partical size from XRD data and SEM [12,
13]. Another reason is indium atoms are larger than bismuth atoms and
possesses higher energy atomic orbital which can lead to smaller energy gap
by raising top of the valence band and more importantly lowering the bottom of
the conduction band.
1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3
-20
0
20
40
60
80
100
120
140
160
180
( αα ααh
νν νν)2
(eV
/cm
)2
Photon Energy (hνννν), eV
-------- 1.0
------- 0.8
-------- 0.6
-------- 0.4
-------- 0.2
-------- 0.0
Fig. 4.3. The (αhν)2 vs. hν plots for the MoBi2Se5 and In doped MoBi2Se5
thin films having different composition.
109
Table 4.1.Composition parameter dependent properties of MoBi2Se5 and
In doped MoBi2Se5
4.3.2 Raman Spectral Study
Figure 4.4 (a-f) shows Raman spectra of the MoBi2Se5 and In doped
MoBi2Se5 thin films giving the identifications of the samples. In all the spectra
peaks are obtained at specific frequencies it reveals that all the samples
containing Mo, Bi, Se and in some cases In, the prominent peaks are at
frequency 2126 cm-1 i.e. from 2124 cm-1 to 2127 cm-1 and the peaks at
frequency 3140 cm-1 to 3244 cm-1 are also specific of these group of
compounds. Some times in the application of Raman spectroscopy it gives
different spectra i.e. when spectra of same compounds look different, this may
be due to the spectra sensitivity of the spectrometer or to the (pre-) resonance
Raman effect, which amplifies some lines in contrast to others. Therefore,
most collections of Raman spectra may be employed generally for work with
Raman spectra, independent of the wave length used for excitations [14].
MoBi(2-x)InxSe5
Composition (x)
Band gap
(Eg)
X1= 0.0 1.78
X2= 0.2 1.66
X3 = 0.4 1.60
X4= 0.6 1.57
X5= 0.8 1.52
X6= 1.0 1.47
110
500 1000 1500 2000 2500 3000 35000.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014
724
1743
2127
2197
2265
2470
2607
2686
2796
2894
3240
3324Ram
an
In
ten
sit
y
Wavenumber cm-1
b
Figure 4.4. Raman spectra for the In doped MoBi2Se5 thin films having
different composition. (a) x1 =0 (b) x2 = 0.2 (c) x3 = 0.4
(d) x4 = 0.6 (e) x5 = 0.8 (f) x6 = 1.0
500 1000 1500 2000 2500 3000 3500 40000.00
0.05
0.10
0.15
0.20
2126
2190
2435
2667
3245
3311
3495
Ram
an
In
ten
sit
y
Wavenumber cm-1
e
500 1000 1500 2000 2500 3000 3500 40000.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014
785
2126
2192
2310
2898
3035
3222
3338
3509
Ram
an
In
ten
sit
y
Wavenumber cm-1
f
500 1000 1500 2000 2500 3000 3500 40000.000
0.002
0.004
0.006
0.008
912
1334
2126
2497
2665
2827
3140
3208
3509
3323
Ram
an
In
ten
sit
y
Wavenumber cm-1
a
500 1000 1500 2000 2500 3000 35000.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040
1110
21242195
2669
3244
Ram
an
In
ten
sit
y
Wavenum ber cm-1
c
500 1000 1500 2000 2500 3000 35000.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
988
1736
2128
2193
2318
2560
2683
2821
3239
3329
3455
Ram
an
In
ten
sit
y
Wavenumber cm-1
d
111
4.3.3 XRD Analysis
Figure 4.5 (a-f) shows the XRD pattern of MoBi2Se5 and In doped
MoBi2Se5 thin films thin films having different composition (x =0.0, 0.2, 0.4,
0.6, 0.8, 1.0) deposited at 45 oC. The peaks become broader and some peaks
appeared at high θ values with increasing In doping. The deposited layers
exhibited polycrystalline nature which is explained by the presence of (221)
and (130) peaks. The plane indices are obtained by comparing the intensities
and positions of the peaks with JCPDS data. There are no JCPDS standard
data available for different composition of In doped MoBi2Se5. Hence the
plane indices are obtained by comparing the intensities and positions of the
peaks with those of MoSe2, Bi2Se3 and In2Se3 which are given by JCPDS file
no.77-1715, 81-0834, 71-0521, 72-2123, 77-2016, 72-2182, 81-0834, 40-
0908 and 24-0772. The formation of solid solution is expected because both
materials crystallises in orthorhombic structure. This small degree of
broadening occurs as a result of increase in strain in the film due to In
incorporation in the Bi lattice site. This indicates that the crystal quality
decreases with an increase of In content in the films.
The crystallite size of the film is calculated using by Scherrer formula [15]
Grain size= 0.9 λ / β cosθ
Where,
λ is the wavelength of the X-ray radiation used
β is the full width at half maximum
θ is Bragg’s angle
The crystallite size calculated for the In doped MoBi2Se5 thin films
having different composition. (a) x1 =0 (b) x2 = 0.2 (c) x3 = 0.4 (d) x4 = 0.6 (e)
x5 = 0.8 (f) x6 = 1.0 reflection is 47.7, 44.4, 40.1, 37.3, 33.5, 35.8 nm with
increasing In doping respectively.
112
20 40 60 80
0
10
20
30
40
(225
)
(104
)
Inetn
sity (
A.U
.)
2θθθθ (Degree)20 40 60 80
0
10
20
30
40
(12
1)
(02
1)
(00
9)
(20
0)
2θθθθ (Degree)
Inetn
sit
y (A
.U.)
20 40 60 80 100
0
10
20
30
40
50
(107
)
(134
)
(20
6)
(002
)
Inet
nsi
ty (
A.U
.)
2θθθθ (Degree)10 20 30 40 50 60 70 80
0
10
20
30
40
50
(224
)
(02
1)
(00
2)
2θθθθ (Degree)
Inetn
sit
y (
A.U
.)
20 40 60 80
0
10
20
30
40
50
(10
5)
(00
3)
(002
)
2θθθθ (Degree)
Ine
tnsity
(A.U
.)
20 40 60 800
10
20
30
40
(201
2)
(025)
(311)
Inetn
sity
(A.U
.)
2θθθθ (Degree)
a b
c d
e f
Figure 4.5. XRD for the In doped MoBi2Se5 thin films having different
composition. (a) x1 =0 (b) x2 = 0.2 (c) x3 = 0.4 (d) x4 = 0.6
(e) x5 = 0.8 (f) x6 = 1.0
113
Table 4.2 XRD results for the MoBi(2-x)InxSe thin films having different
composition.
Film Composition
MoBi(2-x)InxSe5
Standard ‘d’ value
(Ao)
Observed ‘d’ value
(Ao)
(hkl) planes
3.5070 3.5371 (311)
2.1080 2.1137 (025)
X1= 0.0
1.7053 1.6984 (20 12)
12.170 12.5895 (002)
9.8033 10.2031 (003)
X2= 0.2
35353 3.5205 (105)
3.0558 3.0664 (104)
X3 = 0.4 2.0234 2.0236 (225)
7.6485 7.8070 (200)
4.4888 4.5093 (009)
3.3641 3.3602 (021)
X4= 0.6
3.2314 3.2310 (121)
12.170 12.4350 (002)
2.5529 2.5680 (206)
1.7700 1.7768 (134)
X5= 0.8
1.5495 1.5472 (107)
12.170 12.6987 (002)
3.880 3.9102 (021)
X6= 1.0
2.0408 2.0477 (224)
114
4.3.4. AFM Analysis:-
A1 A2
B1 B2
C1 C2
Fig. 4.6. 2D and 3D (A1, A2, A3 and B1, B2, B3) AFM images for MoBi2Se5 and In doped MoBi2Se5 thin films samples respectively.
115
Figure 4.6 (A1, A2, A3) and (B1, B2, B3) shows 2D and 3D AFM images of
sample MoBi2Se5, In doped MoBi2Se5 thin films respectively. A quantitative
method to examine the surface morphology and structure is obtained by
analyzing the surface roughness using AFM. Figure shows and images
recorded for MoBi2Se5 and In doped MoBi2Se5 sample respectively. Figure
shows the Spherical and fibrous shaped grains uniformly grown over the
surface of the substrate. Figure shows the image of MoBi2Se5, In doped
MoBi2Se5 samples with the grains of about 200 to 350 nm.The surface
roughness can be given with a statistical parameter- root mean square (rms
or Rq) that is the standard deviation of the height (z) values within a given
area.
N2
i=1
(Zi - Zave)
R q = N
∑
4.1
Where,
Zave is the average of z values within given area
Zi is current Z value
N is the number of points within given area
All the results obtained from AFM data are in consistent with scanning
electron microscopy (SEM) results.
4.3.5. SEM Analysis
Figure 4.7(a-f) shows scanning electron micrograph of MoBi2Se5 and In
doped MoBi2Se5 thin films in the as-grown condition. The microstructure of the
films observed by SEM shows that the films are uniform, crack free and
covered all over the surface area [16]. The SEM micrograph of the sample
shows spherical and elongated fibrous structure. Some regions of overgrowth
were also observed. The scanning electron microphotographs of these films
were recorded on JEOL - 6360 scanning electron microscope (SEM).Grain
sizes were determined using the linear intercept technique [17]. The average
grain size (Ga) was calculated using the relation,
116
Ga = MN
L5.1
Where,
1.5 is geometry dependent proportionality constant,
L, the total test line length,
M, magnification,
N, the total number of intercepts,
The grain size of MoBi(2-x)InxSe thin films with x = 0.0 ml, 0.2 ml, 0.4 ml,
0.6 ml, 0.8 ml, 1.0 ml are 999, 994, 989, 984, 977, 970 nm respectively. All
grains having average size (985.5 nm) are composed of single type small
densely packed crystals. The grains show elongated fibrous structure
morphology and appear very homogeneous.
a b
c d
117
e f
Fig. 4.7. SEM micrographs for MoBi(2-x)InxSe5 thin films having different
composition (a) x1 =0 (b) x2 = 0.2 (c) x3 = 0.4 (d) x4 = 0.6 (e) x5 = 0.8
(f) x6 = 1.0
118
4.3.6. EDS Analysis
The stoichiometry, atomic and elemental wt% of MoBi2Se5 and In doped
MoBi2Se5 thin films was found by EDS. Figure 4.8 shows EDS spectrum of the
as deposited MoBi2Se5 and In doped MoBi2Se5 thin films giving the elemental
compositions of the samples. The samples were coated with a thin layer of
gold to be prevent charging of the samples. For comparative studies, the
electron beam was kept constant while analyzing samples. EDS spectrum
obtained with an accelerating voltage 10 Kv, acquisition time of 1 minute on a
film within the precision of energy dispersive X-ray analysis, i.e. ± 2%.
The percentage of Bi in the film is higher than expected, this is attributed
to the fact that, bismuth is more metallic and its reactivity towards Se2- is
higher. Moreover bismuth forms antisite defects [18] which is responsible for
slightly non-stoichiometry of MoBiInSe5.
a
119
b
c
Fig. 4.8 EDAX for the In doped MoBi2Se5 thin films having different
composition.
120
Table 4.3 EDS analysis of the MoBi2Se5 and In doped MoBi2Se5 thin films
Elements (Expected at %) Elements (Actual at %) Film Composition
MoBi(2-x)InxSe5 Mo Bi In Se Mo Bi In Se
X=0.0 12.5 25.0 0.0 62.5 10.62 28.94 0.0 60.44
X=0.6 12.5 20.83 8.928 62.5 10.56 29.78 4.23 55.43
X=1.0 12.5 12.5 12.5 62.5 11.07 22.92 9.07 56.94
4.3.7. Electrical Analysis
To examine the temperature dependence of the electrical conductivity in
more detail, electrical conductivity measurement was made in the temperature
range 300 K to 500 K under constant voltage (5 volt). The temperature
dependence of electrical conductivity of the semiconducting thin films is given
by [19],
σ = σο e(-Ea/κT)
Where Ea is conductivity activation energy, κ is Boltzmann constant and σ0 is
the temperature independent part of the conductivity. The variation of log σ
with 1 / T in the temperature range 300 K to 500 K is shown in figure 4.9.The
linear variation of the plot confirms the semiconducting nature of the film. The
plot reveals that the conductivity varies slowly with 1 / T and other above 445
K, where the conductivity varies abruptly with temperature. The activation
energy for conduction in low temperature region is the energy required to take
place between the defect level and valence bond or conduction band. At
sufficiently high temperature intrinsic conductivity starts and electron
conduction from valence bond to conduction band take place. From the
slopes of linear plots, activation energy for conduction was calculated for two
temperature region. The activation energy for low temperature region and in
high temperature region for MoBi2Se5, and MoBiInSe5 thin films is shown in
table 4.4.
121
We have found that the conductivity of the chemically deposited Indium
doped MoBi2Se5 thin films decreases as composition (x) increases from 0.0 to
1.0, as shown in Fig. 4.9. Doping of Indium in MoBi2Se5 results in smaller
crystallite size due to increase in nucleation centres [20], which ultimately
increases the intercrystalline barrier size. The charge carriers therefore have
to cross wide intercrystalline barriers and this may be responsible for the
decrease in conductivity.
1.6 1.7 1.8 1.9 2.0 2.1
-9
-8
-7
-6
-5
-4
-3
x= 0.0
x= 0.2
x= 0.4
x= 0.6
x= 0.8
x= 1.0
ln σσ σσ
(O
hm
-1 c
m-1)
1000/T (K-1)
Fig. 4.9 ln σ vs. 1000 / T plots for for the In doped MoBi2Se5 thin films having
different composition.
122
Table 4.4 Observed variation of activation energy (∆E) for MoBi2Se5, and
MoBiInSe5 films.
High temperature region Low temperature region Composition
MoBi(2-x)InxSe5 ∆E (eV) ∆E (eV)
X1= 0.0
X2=0.2
X3=0.4
X4=0.6
X5=0.8
X6=1.0
0.172 0.
0.102
0.083
0.041
0.0091
0.00378
0.150
0.081
0.047
0.018
0.0074
0.00189
4.3.8. TEP Analysis
During TEP measurement in a semiconductor, temperature gradient
yields the thermo electric effect. In which phonon travel from the hot end to
cold end because of electron phonon interactions. Thermo electric power (S)
Vs temperature curve for MoBi2Se5 and In doped MoBi2Se5thin films is shown
in figure 4.10. It is clear from the figure 4.10 that S goes on increasing with
temperature. The Seebeck coefficient of MoBi2Se5 and In doped MoBi2Se5 thin
films is quite high, -ve and increases with increasing temperature. The -ve sign
stems from a dominance of n-type charge carriers [21]. We measured the
Seebeck coefficients (S) of the series In doped MoBi2Se5 thin films with x= 0.0,
0.2, 0.4, 0.6, 0.8 and 1.0 in the range 300 K to 500 K is shown in Fig.4.10.
Fig. 4.10 shows that temperature dependence of thermoelectric power is
approximately linear in the low temperature region, whereas it deviated from
the linear behaviour at higher temperatures indicates nondegeneracy of the
material whose Seebeck coefficient is a weak function of the temperature.
Increase in Seebeck coefficient with increase in temperature can be attributed
123
to the increase in concentration and mobility of the charge carrier with rise in
temperature.
300 350 400 450 500600
700
800
900
1000
1100
x=0.0
x=0.2
x=0.4
x=0.6
x=0.8
x=1.0
S (
µµ µµ V
/K
)
Temperature (K)
Fig.4.10. Temperature dependence of the Seebeck coefficient for the
MoBi2Se5 , In doped MoBi2Se5 thin films having different composition.
4.4 Conclusion:
Arrested precipitation technique is applied successfully to deposit
stoichiometric, adherent and uniform deposition of MoBi2Se5 and In doped
MoBi2Se5 material in thin film form. Optostructural and SEM results obtained
shows material can be useful for device application such as best candidate for
broadband photo convertor and as a photo electrode in solar cells. These
features confirm the high quality of the chemically deposited MoBi2Se5 and In
doped MoBi2Se5 thin film is applicable to the deposition of quaternary
semiconductor compounds. Good quality films of thickness 1.2 to 1.5 µm
containing Mo, Bi, In and Se in an approximately 1:2:1:5 atomic ratios have
been deposited successfully by arrested precipitation technique. The
technique is simple and requires less monitoring. AFM images recorded for
MoBi2Se5 and In doped MoBi2Se5 sample respectively. Figure shows the
124
Spherical and fibrous shaped grains uniformly grown over the surface of the
substrate. X-ray diffraction patterns confirmed the proper phase formation of
the material. The films are mechanically stable since no cracks are observed in
the low magnification SEM image. Activation energy is different for low and
high temperature. MoBi2Se5 and In doped MoBi2Se5 exhibits an n-type
semiconducting behavior with a low electrical conductivity and very high
thermo power, which is strongly suitable for fabricating a thin film solar cell.
125
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