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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