In2Se3 thin films with different thicknesses have been deposited by thermal evaporation method on glass substrate under vacuum pressure of 10-6 Torr. Structural Properties of these films were studied by different analytical techniques. X- ray diffraction revealed as deposited films have amorphous nature and annealing effect enhanced crystalline structure. Structural studies by XRD results showed the polycrystalline nature of the films. The Full Width at Half Maximum (FWHM) values were observed from the XRD pattern and used to evaluate the microstructural parameters like crystallite size, strain, dislocation density. The optical absorption spectra of In2Se3 films were studied in the wavelength region of 250–2500 nm. The optical properties show that the band gap (Eg) values vary from 2.5 to 3.34 eV as an-nealing temperature varies from 150 to 350C.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3513

Mahalingam, T.

Mohan, A.

Suthagar, J.

SocioEconomic Challenges Journal (Sumy State University)

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE  
NANOMATERIALS: APPLICATIONS AND PROPERTIES 
Vol. 2 No 1, 01NTF07(5pp) (2013) 
 
 
2304-1862/2013/2(1)01NTF07(5) 01NTF07-1  2013 Sumy State University 
Investigation on the Structural and Optical Properties of Thermally Evaporated Indium 
Selenide Compound Material for Solar Cell Application 
 
A. Mohan, J. Suthagar, T. Mahalingam  
 
Thin Film Laboratory, Department of Physics, Karunya University, Coimbatore-641114, India 
 
 (Received 17 Мау; published online 30 August 2013) 
 
In2Se3 thin films with different thicknesses have been deposited by thermal evaporation method on 
glass substrate under vacuum pressure of 10-6 Torr. Structural Properties of these films were studied by 
different analytical techniques. X- ray diffraction revealed as deposited films have amorphous nature and 
annealing effect enhanced crystalline structure.  Structural studies by XRD results showed the 
polycrystalline nature of the films. The Full Width at Half Maximum (FWHM) values were observed from 
the XRD pattern and used to evaluate the microstructural parameters like crystallite size, strain, 
dislocation density. The optical absorption spectra of In2Se3 films were studied in the wavelength region of 
250–2500 nm. The optical properties show that the band gap (Eg) values vary from 2.5 to 3.34 eV as an-
nealing temperature varies from 150 to 350C. 
 
Keywords: In2Se3, Thermal Evaporation, XRD, Amorphous, Polycrystalline. 
 
 PACS number(s): 81.15.Ef, 68.55 –a, 78.20 -e 
 
 
1. INTRODUCTION: 
 
Indium Selenide (In2Se3) is one of the promising 
materials of chalcogenide alloys from the III – VI group 
semiconductor. Because of the better heterojunction 
with the In2Se3/CIS layer than the CdS/CIS layer 
combination [1], In2Se3 thin films are used in Diode [2], 
photo electrochemical application [3], Phase change 
random access memory (PRAM) [4] and used as a 
buffer layer for CIS solar cells. Also indium selenide is 
used as window layer (-In2Se3) and absorber layer (β-
In2Se3) in solar cells depending on optical band gap of 
thin films. -In2Se3 (window layer) with wide band gap 
1.8 eV is greater than β-In2Se3 (absorber layer) and 
InSe having a band gap 1.55 eV [5].  So far many 
techniques are used to prepare Indium Selenide thin 
films such as chemical bath deposition [6], Mechanical 
Alloying Techniques [7], MOCVD [8], Spray pyrolysis 
[9], sol gel method [10] and LP – LAPW method [11]. 
Among these methods, thermal evaporation is a 
promising method to prepare high quality 
semiconductor thin films. Many researchers 
investigated by this technique, but most of them 
concentrating on bilayer of In and Se stack deposition.  
In this present work we have prepared thin films of 
thermally evaporated Indium Selenide compound 
material and then studied structural and optical 
properties of Indium Selenide thin films with various 
thicknesses and at different annealing temperatures. 
 
2. EXPERIMENTAL PROCEDURE 
 
In2Se3 thin films were prepared by evaporating 
In3Se compound. Indium Selenide compound was kept 
in Molybdenum boat and then deposited on 
ultrasonically cleaned unheated soda lime glass 
substrates under the vacuum pressure of 10-6 torr. The 
successive film coatings of different thicknesses are 
100 nm, 200 nm and 300 nm and film thickness was 
maintained by using thickness monitor. A constant rate 
of evaporation ranging 1-3 Å/sec was maintained 
throughout the deposition process. Then films were 
annealed under open air atmosphere at 150C, 250C 
and 350C for about 30 minutes. The X-ray diffraction 
studies of both as deposited and annealed In2Se3 films 
were carried out using Shimadzu XRD-6000 X-ray 
Diffractometer. Here the films were exposed to Cu Kα 
source and the scattering angles were varied from 0 -
 90. The optical studies of the prepared films were 
analysed using UV-VIS-NIR spectrophotometer (Jasco-
570 UV/VIS/NIR Spectrophotometer) in the range of 
200 to 2500 nm. 
 
3. RESULTS AND DISCUSSIONS  
 
3.1 X-ray diffraction 
 
The X-ray diffraction (XRD 6000) studies are 
carried out for these samples, to find effects of 
thickness and annealing temperature on structure of 
InSe thin films.  
Fig 1 shows that recorded XRD pattern of as 
deposited, and annealed films of thickness 100 nm ,  
which revealed that as deposited and annealed at 
150°C, 250°C films exhibits amorphous nature. Further 
annealing at 350°C films enhanced crystalline nature. 
The prominent peaks corresponding to (4 1 0) plane 
was observed in the film. This recorded XRD pattern 
were compared with the JCPDS data (card number: 83-
0039) which confirm the orthorhombic crystal structure 
of In4Se3.  
The XRD pattern of annealed at 350°C films with 
thicknesses of 200 nm and 300 nm, shown in Fig 2 and 
3, have well grown orientation along (1 1 0), (3 0 0) and 
(2 0 2). These were compared with JCPDS data (card 
number: 89-0658), which confirmed hexagonal 
structure and presence of -In2Se3 and also similar 
peaks are found previous result. From XRD pattern 3 
(250°C) films show peak at (3 0 0) orientation, this 
confirms formation of -In2Se3. Annealing effect 
enhanced crystalline quality and increases the grain 
size. 
 
 J. SUTHAGAR, A. MOHAN PROC. NAP 2, 01NTF07 (2013) 
 
 
01NTF07-2 
 
 
Fig. 1 – XRD pattern of InSe film of thickness 100 nm at 
different annealed temperatures 
 
 
Fig. 2 – XRD pattern of InSe film of thickness 200 nm at 
different annealed temperatures 
 
The crystalline size (D) was calculated using Scher-
er’s formula [12] 
 
 
0 94
 

.  
cos
D  (1) 
 
where  is the wavelength of the X-ray beam used, 2 is 
the angle between the incident and scattered X-ray 
beam and  is the full width at half maximum (FWHM) 
and the dislocation density is estimated from the 
relation 
 
1
2
 
D
 (2) 
 
The number of crystallites per unit area (N) of the 
film was determined by using formula [13] 
 
 
3

t
N
D
 (3) 
 
where t is thickness of the film. 
The micro strain () developed in the InSe film is 
calculated from the relation 
 
 4  cos /  (4) 
 
 
 
Fig. 3 – XRD pattern of InSe film of thickness 300nm at 
different annealed temperatures 
 
Structural parameter of Indium Selenide films 
annealed at 350C shown in table 1. Increasing average 
grain size shows peaks are growing very narrow and 
decrease dislocation density, micro strain has improving 
crystalline structure. From above result increase grain 
size and decrease dislocation density, micro strain, 
number of crystallites show annealing at 350°C films 
have good crystalline structure than other films. 
Broader full width half maximum has very small 
grain and smaller FWHM have larger grain size. From 
the above XRD pattern we confirmed that a larger grain 
has very small FWHM and also thickness is an 
important parameter to prepare good crystalline films. 
 
3.2 Optical Studies 
 
Optical absorption and transmission of Indium Sele-
nide films deposited onto soda lime glass was recorded 
using UV-VIS-NIR spectrophotometer (Jasco-570 UV/VIS/ 
NIR Spectrophotometer) in the range of 200 to 2500 nm. 
The transmission spectra of the films as deposited and  
annealed at different temperature (150 °C - 350 °C)  with 
various thickness is shown in fig. 4. 
 INVESTIGATION ON THE STRUCTURAL AND OPTICAL PROPERTIES … PROC. NAP 2, 01NTF07 (2013) 
 
 
01NTF07-3 
Table 1 – Structural parameters of InSe thin films annealed at 350°C 
 
Thickness, 
nm 
2θ d FWHM 
Average 
Grain size, D 
nm 
Dislocation 
density (1018 
nm-2) 
N, 1018 /m2 Strain  
100 
24.30  
22.02  
29.60 
3.6598 
4.0189 
3.0155 
4.0000 
4.5000 
3.6500 
2.1 0.226 10.7 0.0976 
200 
24.927 
44.001 
30.315 
3.5691 
2.0562 
2.9460 
0.3096 
0.3265 
0.4051 
4.4 0.053 2.34 0.083 
300 
24.937 
44.011 
29.278 
3.5677 
2.0557 
3.0478 
0.3043 
0.3093 
0.3242 
4.8 0.043 2.71 0.075 
 
As deposited films higher transmission than 
annealing films and also interference pattern was 
formed by increasing thickness shown in Fig 4. Film 
thickness and annealing temperature played important 
role in the transmission spectrum by reducing 
transmission. The extinction coefficient can be 
calculated from the formula [14]: 
 
 
1
4


 
 
 
ln
    
T
k
t
 (5) 
 
where t is the thickness of the deposited film.  
 
 
 
 
Fig 4. – Transmission spectra of InSe thin films (a) 100 nm, (b) 200nm and (300nm) 
 
Energy band gap of InSe thin films were calculated 
by using optical transmission spectra. To find energy 
band gap, we plotted graph (h) vs. (h) 2 where  is 
absorption coefficient of the film and h is photon 
energy. The nature of the transition can be investigated 
on the basis of the dependence of the absorption 
coefficient with the incident photon energy hυ. 
 
 2  ( ) ( )   ngh A hv E  (6) 
where h is the incident photon energy, A is a constant 
and the exponent n depends on the type of transition, 
n = 0.5 for direct transition. The optical band gap of 
these films was determined by extrapolating the linear 
portion of the curve to the energy axis at (αh) 2  0. 
Hence, the direct optical band gap Eg can be evaluated 
from the plot of (αh) 2 vs h. 
 J. SUTHAGAR, A. MOHAN PROC. NAP 2, 01NTF07 (2013) 
 
 
01NTF07-4 
 
 
 
Fig. 5 – Energy Band Gap of InSe (a) 100 nm, (b) 200nm and 
(300nm) (c) 
 
The absorption coefficient can be calculated from 
the following the relation: 
 
 
4


    
k
 (7) 
 
The absorption coefficient of different thickness of 
InSe thin films plotted against wavelength and the 
graph is shown in Fig.6.  
Fig.6 clearly shows that the absorption coefficient 
was maximum for high In content and minimum for 
the low In content [17]. Lower thickness InSe films 
have more indium content than higher thickness films, 
which is confirmed from XRD pattern.  
The direct band gap of the films deposited at 
different thicknesses was found to be in the range from 
3.34 to 2.50eV.  The values of the estimated energy gap 
for amorphous films are greater than that obtained for 
crystalline InSe films, because in the polycrystalline 
material the electron transition is from band to band. 
But in amorphous materials the electron transitions 
may be either from localized states at the valance band 
edge to extended states in the conduction band or from 
extended states in the valance band to the localized 
states at the conduction edge. This leads to higher en-
ergies than those for polycrystalline materials. The 
energy band gap value was found to decrease with an 
increase in film thickness and annealing temperature. 
Decrease in energy band may be formation of  - In2Se3 
phase, which has larger band gap than previous results 
[15]. Formation of  - In2Se3 phase confirmed from the 
XRD pattern.  
 
 
 
 
Fig. 6 – Absorption coefficient of InSe films (a) as deposited 
and (b) annealing at 350ºC 
 
 
 
Fig. 7 – Energy band gap of InSe films with annealing 
temperature 
 INVESTIGATION ON THE STRUCTURAL AND OPTICAL PROPERTIES … PROC. NAP 2, 01NTF07 (2013) 
 
 
01NTF07-5 
The variation of energy band gap of InSe thin films 
with annealing temperature is shown in Fig 7. Films 
annealed at higher temperature are established lower 
band gap. This may be due to Sharpe edges in 
crystalline films. The decrease of band gap with 
increase of annealing temperature is attributed to 
increase of grain size and decrease in micro strain [16].  
Fig. 8 shows the optical band gap value decreases 
with an increase in film thickness due to an increase of 
particle size and decrease in micro strain. This is 
further explained from the three dimensional quantum 
size effects leading to decrease band gap with increase 
in particle size, which is well known for colloidal 
semiconductor. 
 
 
 
Fig. 8 – Energy band gap of InSe films with different 
thicknesses 
4. CONCLUSION 
 
Indium Selenide thin films have been prepared by 
thermal evaporation techniques on ultrasonically 
cleaned glass substrate. The annealing effect on 
structural and optical band gap has been studied. As 
deposited and annealed at lower temperature shows 
amorphous nature and annealed at 350°C films have 
hexagonal structure.  This confirms that annealing 
effect enhances crystalline quality. It is observed that 
optical band gap decrease 3.34eV to 2.50eV and which 
arise grain growth upon annealing. This is because of 
the formation of - In2Se3 phase, which has wider band 
gap for annealed films.  
 
ACKNOWLEDGMENT 
 
The authors are grateful to the Central Research 
Facility, Karunya University for supporting of this 
work. 
 
 
 
 
 
 
 
 
 
 
 
 
 
REFERENCES 
 
1. G. Gordillo, C. Calderon, Solar Energy Material and Solar 
Cells,77, 163 (2003). 
2. Atsufumi Hirohata, Jagadeesh S. Moodera, Geetha P. 
Berera, Thin Solid Films 510, 247 (2006). 
3. K. C. Rathod , M. R. Asabe ,A. V. Jadhav , V. B. Helavi , S. 
S. Chavan ,K. M. Garadkar , I. S. Mulla , P. P. Hankare, J. 
Mater Sci: Mater Electron, 22, 359 (2011). 
4. Heon Lee, Dae  Hawan Kang, Lung Tran, Mater. Sci. and 
Eng. B, 119 196 (2005). 
5. R. Sreekumar, R. Jayakrishnan, C. Sudha kartha, K. P. 
Vijyakumar, Y. Kashibawa and T. Abe, , Solar Energy Ma-
terial and Solar Cell, 90, 298 (2006). 
6. K. Bindu, C. Sudha Kartha, K.P. Vijayakumar, T. Abe, Y. 
Kasiwaba, App. Surf. Sci. 191, 138 (2002). 
7. S.M. El-Sayed, Vacuum 72, 169 (2004). 
8. Mohammad Afzaal, David Crouch, Paul O’Brien, Mater. 
Sci. and Eng. B 116,  391 (2005). 
9. Teny Theresa Johna, C. Sudha Karthaa, K.P. Vijayakuma-
ra, T. Abeb, Y. Kashiwaba, Vacuum 80, 870 ((2006)). 
10. Ibrahim H. Mutlu , Maharram Z. Zarbaliyev , Ferhat 
Aslan, J. of Sol-Gel Sci. and Tech., 43, 233 (2007). 
11. T. BenNasr n, H.BenAbdallah, R.Bennaceur, Physica B 
405, 3427 (2010).  
12.  S. Venkatachalam, Y.L. Jeyachandrn, P. Suresh Kumar, 
A. Dhayalraj, D. Mangalaraj , Sa.K. Narayandass, S. Ve-
lumani, Mater. Charact. 58, 794 (2007). 
13. T. Mahalingam, V. Dhanasekaran,  R. Chandramohan, 
Jin-Koo Rhee, J. of Mater. Sci., 47, 1950 (2012). 
14. Viswanathan,  S. Gopal, D. Mangala Raj, Sa. K. Narya-
nadass, O.F. Caltun, G. Rusu and Junsin Yi, Ionic, 10 
(2004). 
15. Aytunc Ates, Muthu Kundac1, Aykut Astam, Physica E 40, 
2709 (2008). 
16. S.Agilan, D. Mangalaraj, Sa. K. Narayandass, G. Mohan 
Rao, Physica B, 93 (2005). 
 


English

Investigation on the Structural and Optical Properties of Thermally Evaporated Indium Selenide Compound Material for Solar Cell Application

Electronic Sumy State University Institutional Repository

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE  NANOMATERIALS: APPLICATIONS AND PROPERTIES Vol. 2 No 1, 01NTF07(5pp) (2013)   2304-1862/2013/2(1)01NTF07(5) 01NTF07-1  2013 Sumy State University Investigation on the Structural and Optical Properties of Thermally Evaporated Indium Selenide Compound Material for Solar Cell Application  A. Mohan, J. Suthagar, T. Mahalingam   Thin Film Laboratory, Department of Physics, Karunya University, Coimbatore-641114, India   (Received 17 Мау; published online 30 August 2013)  In2Se3 thin films with different thicknesses have been deposited by thermal evaporation method on glass substrate under vacuum pressure of 10-6 Torr. Structural Properties of these films were studied by different analytical techniques. X- ray diffraction revealed as deposited films have amorphous nature and annealing effect enhanced crystalline structure.  Structural studies by XRD results showed the polycrystalline nature of the films. The Full Width at Half Maximum (FWHM) values were observed from the XRD pattern and used to evaluate the microstructural parameters like crystallite size, strain, dislocation density. The optical absorption spectra of In2Se3 films were studied in the wavelength region of 250–2500 nm. The optical properties show that the band gap (Eg) values vary from 2.5 to 3.34 eV as an-nealing temperature varies from 150 to 350C.  Keywords: In2Se3, Thermal Evaporation, XRD, Amorphous, Polycrystalline.   PACS number(s): 81.15.Ef, 68.55 –a, 78.20 -e   1. INTRODUCTION:  Indium Selenide (In2Se3) is one of the promising materials of chalcogenide alloys from the III – VI group semiconductor. Because of the better heterojunction with the In2Se3/CIS layer than the CdS/CIS layer combination [1], In2Se3 thin films are used in Diode [2], photo electrochemical application [3], Phase change random access memory (PRAM) [4] and used as a buffer layer for CIS solar cells. Also indium selenide is used as window layer (-In2Se3) and absorber layer (β-In2Se3) in solar cells depending on optical band gap of thin films. -In2Se3 (window layer) with wide band gap 1.8 eV is greater than β-In2Se3 (absorber layer) and InSe having a band gap 1.55 eV [5].  So far many techniques are used to prepare Indium Selenide thin films such as chemical bath deposition [6], Mechanical Alloying Techniques [7], MOCVD [8], Spray pyrolysis [9], sol gel method [10] and LP – LAPW method [11]. Among these methods, thermal evaporation is a promising method to prepare high quality semiconductor thin films. Many researchers investigated by this technique, but most of them concentrating on bilayer of In and Se stack deposition.  In this present work we have prepared thin films of thermally evaporated Indium Selenide compound material and then studied structural and optical properties of Indium Selenide thin films with various thicknesses and at different annealing temperatures.  2. EXPERIMENTAL PROCEDURE  In2Se3 thin films were prepared by evaporating In3Se compound. Indium Selenide compound was kept in Molybdenum boat and then deposited on ultrasonically cleaned unheated soda lime glass substrates under the vacuum pressure of 10-6 torr. The successive film coatings of different thicknesses are 100 nm, 200 nm and 300 nm and film thickness was maintained by using thickness monitor. A constant rate of evaporation ranging 1-3 Å/sec was maintained throughout the deposition process. Then films were annealed under open air atmosphere at 150C, 250C and 350C for about 30 minutes. The X-ray diffraction studies of both as deposited and annealed In2Se3 films were carried out using Shimadzu XRD-6000 X-ray Diffractometer. Here the films were exposed to Cu Kα source and the scattering angles were varied from 0 - 90. The optical studies of the prepared films were analysed using UV-VIS-NIR spectrophotometer (Jasco-570 UV/VIS/NIR Spectrophotometer) in the range of 200 to 2500 nm.  3. RESULTS AND DISCUSSIONS   3.1 X-ray diffraction  The X-ray diffraction (XRD 6000) studies are carried out for these samples, to find effects of thickness and annealing temperature on structure of InSe thin films.  Fig 1 shows that recorded XRD pattern of as deposited, and annealed films of thickness 100 nm ,  which revealed that as deposited and annealed at 150°C, 250°C films exhibits amorphous nature. Further annealing at 350°C films enhanced crystalline nature. The prominent peaks corresponding to (4 1 0) plane was observed in the film. This recorded XRD pattern were compared with the JCPDS data (card number: 83-0039) which confirm the orthorhombic crystal structure of In4Se3.  The XRD pattern of annealed at 350°C films with thicknesses of 200 nm and 300 nm, shown in Fig 2 and 3, have well grown orientation along (1 1 0), (3 0 0) and (2 0 2). These were compared with JCPDS data (card number: 89-0658), which confirmed hexagonal structure and presence of -In2Se3 and also similar peaks are found previous result. From XRD pattern 3 (250°C) films show peak at (3 0 0) orientation, this confirms formation of -In2Se3. Annealing effect enhanced crystalline quality and increases the grain size.   J. SUTHAGAR, A. MOHAN PROC. NAP 2, 01NTF07 (2013)   01NTF07-2   Fig. 1 – XRD pattern of InSe film of thickness 100 nm at different annealed temperatures   Fig. 2 – XRD pattern of InSe film of thickness 200 nm at different annealed temperatures  The crystalline size (D) was calculated using Scher-er’s formula [12]   0 94 .  cosD  (1)  where  is the wavelength of the X-ray beam used, 2 is the angle between the incident and scattered X-ray beam and  is the full width at half maximum (FWHM) and the dislocation density is estimated from the relation  12 D (2)  The number of crystallites per unit area (N) of the film was determined by using formula [13]   3tND (3)  where t is thickness of the film. The micro strain () developed in the InSe film is calculated from the relation   4  cos /  (4)    Fig. 3 – XRD pattern of InSe film of thickness 300nm at different annealed temperatures  Structural parameter of Indium Selenide films annealed at 350C shown in table 1. Increasing average grain size shows peaks are growing very narrow and decrease dislocation density, micro strain has improving crystalline structure. From above result increase grain size and decrease dislocation density, micro strain, number of crystallites show annealing at 350°C films have good crystalline structure than other films. Broader full width half maximum has very small grain and smaller FWHM have larger grain size. From the above XRD pattern we confirmed that a larger grain has very small FWHM and also thickness is an important parameter to prepare good crystalline films.  3.2 Optical Studies  Optical absorption and transmission of Indium Sele-nide films deposited onto soda lime glass was recorded using UV-VIS-NIR spectrophotometer (Jasco-570 UV/VIS/ NIR Spectrophotometer) in the range of 200 to 2500 nm. The transmission spectra of the films as deposited and  annealed at different temperature (150 °C - 350 °C)  with various thickness is shown in fig. 4.  INVESTIGATION ON THE STRUCTURAL AND OPTICAL PROPERTIES … PROC. NAP 2, 01NTF07 (2013)   01NTF07-3 Table 1 – Structural parameters of InSe thin films annealed at 350°C  Thickness, nm 2θ d FWHM Average Grain size, D nm Dislocation density (1018 nm-2) N, 1018 /m2 Strain  100 24.30  22.02  29.60 3.6598 4.0189 3.0155 4.0000 4.5000 3.6500 2.1 0.226 10.7 0.0976 200 24.927 44.001 30.315 3.5691 2.0562 2.9460 0.3096 0.3265 0.4051 4.4 0.053 2.34 0.083 300 24.937 44.011 29.278 3.5677 2.0557 3.0478 0.3043 0.3093 0.3242 4.8 0.043 2.71 0.075  As deposited films higher transmission than annealing films and also interference pattern was formed by increasing thickness shown in Fig 4. Film thickness and annealing temperature played important role in the transmission spectrum by reducing transmission. The extinction coefficient can be calculated from the formula [14]:   14   ln    Tkt (5)  where t is the thickness of the deposited film.      Fig 4. – Transmission spectra of InSe thin films (a) 100 nm, (b) 200nm and (300nm)  Energy band gap of InSe thin films were calculated by using optical transmission spectra. To find energy band gap, we plotted graph (h) vs. (h) 2 where  is absorption coefficient of the film and h is photon energy. The nature of the transition can be investigated on the basis of the dependence of the absorption coefficient with the incident photon energy hυ.   2  ( ) ( )   ngh A hv E  (6) where h is the incident photon energy, A is a constant and the exponent n depends on the type of transition, n = 0.5 for direct transition. The optical band gap of these films was determined by extrapolating the linear portion of the curve to the energy axis at (αh) 2  0. Hence, the direct optical band gap Eg can be evaluated from the plot of (αh) 2 vs h.  J. SUTHAGAR, A. MOHAN PROC. NAP 2, 01NTF07 (2013)   01NTF07-4    Fig. 5 – Energy Band Gap of InSe (a) 100 nm, (b) 200nm and (300nm) (c)  The absorption coefficient can be calculated from the following the relation:   4    k (7)  The absorption coefficient of different thickness of InSe thin films plotted against wavelength and the graph is shown in Fig.6.  Fig.6 clearly shows that the absorption coefficient was maximum for high In content and minimum for the low In content [17]. Lower thickness InSe films have more indium content than higher thickness films, which is confirmed from XRD pattern.  The direct band gap of the films deposited at different thicknesses was found to be in the range from 3.34 to 2.50eV.  The values of the estimated energy gap for amorphous films are greater than that obtained for crystalline InSe films, because in the polycrystalline material the electron transition is from band to band. But in amorphous materials the electron transitions may be either from localized states at the valance band edge to extended states in the conduction band or from extended states in the valance band to the localized states at the conduction edge. This leads to higher en-ergies than those for polycrystalline materials. The energy band gap value was found to decrease with an increase in film thickness and annealing temperature. Decrease in energy band may be formation of  - In2Se3 phase, which has larger band gap than previous results [15]. Formation of  - In2Se3 phase confirmed from the XRD pattern.      Fig. 6 – Absorption coefficient of InSe films (a) as deposited and (b) annealing at 350ºC    Fig. 7 – Energy band gap of InSe films with annealing temperature  INVESTIGATION ON THE STRUCTURAL AND OPTICAL PROPERTIES … PROC. NAP 2, 01NTF07 (2013)   01NTF07-5 The variation of energy band gap of InSe thin films with annealing temperature is shown in Fig 7. Films annealed at higher temperature are established lower band gap. This may be due to Sharpe edges in crystalline films. The decrease of band gap with increase of annealing temperature is attributed to increase of grain size and decrease in micro strain [16].  Fig. 8 shows the optical band gap value decreases with an increase in film thickness due to an increase of particle size and decrease in micro strain. This is further explained from the three dimensional quantum size effects leading to decrease band gap with increase in particle size, which is well known for colloidal semiconductor.    Fig. 8 – Energy band gap of InSe films with different thicknesses 4. CONCLUSION  Indium Selenide thin films have been prepared by thermal evaporation techniques on ultrasonically cleaned glass substrate. The annealing effect on structural and optical band gap has been studied. As deposited and annealed at lower temperature shows amorphous nature and annealed at 350°C films have hexagonal structure.  This confirms that annealing effect enhances crystalline quality. It is observed that optical band gap decrease 3.34eV to 2.50eV and which arise grain growth upon annealing. This is because of the formation of - In2Se3 phase, which has wider band gap for annealed films.   ACKNOWLEDGMENT  The authors are grateful to the Central Research Facility, Karunya University for supporting of this work.              REFERENCES  1. G. Gordillo, C. Calderon, Solar Energy Material and Solar Cells,77, 163 (2003). 2. Atsufumi Hirohata, Jagadeesh S. Moodera, Geetha P. Berera, Thin Solid Films 510, 247 (2006). 3. K. C. Rathod , M. R. Asabe ,A. V. Jadhav , V. B. Helavi , S. S. Chavan ,K. M. Garadkar , I. S. Mulla , P. P. Hankare, J. Mater Sci: Mater Electron, 22, 359 (2011). 4. Heon Lee, Dae  Hawan Kang, Lung Tran, Mater. Sci. and Eng. B, 119 196 (2005). 5. R. Sreekumar, R. 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Investigation on the Structural and Optical Properties of Thermally Evaporated Indium Selenide Compound Material for Solar Cell Application

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