Accepted Manuscript

Title: Investigation of Structural and Optical Parameters ofCu-Ag–In–Se Thin Films Deposited by Thermal EvaporationMethod

Author: H.H. Gullu I. Candan E. Coskun M. Parlak

PII: S0030-4026(15)00326-5DOI: http://dx.doi.org/doi:10.1016/j.ijleo.2015.05.026Reference: IJLEO 55515

To appear in:

Received date: 21-3-2014Accepted date: 5-5-2015

Please cite this article as: H.H. Gullu, I. Candan, E. Coskun, M. Parlak, Investigationof Structural and Optical Parameters of Cu-AgndashInndashSe Thin Films Depositedby Thermal Evaporation Method, Optik - International Journal for Light and ElectronOptics (2015), http://dx.doi.org/10.1016/j.ijleo.2015.05.026

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Investigation of Structural and Optical Parameters of Cu-Ag–In–Se Thin

Films Deposited by Thermal Evaporation Method

H.H. Güllü1,3, İ. Candan1,3, E. Coşkun1,2, M. Parlak1,3,*

1Department of Physics, Middle East Technical University (METU), 06800 Ankara, Turkey

2Department of Physics, Çanakkale Onsekiz Mart University, 17100 Çanakkale, Turkey

3Center for Solar Energy Research and Applications (GÜNAM), METU, Ankara 06800, Turkey

*Corresponding author: Tel: +90 312 2107646; Fax: +90 312 2105099

Email: parlak@metu.edu.tr

Abstract

Annealing effect on the structural and optical properties of the quaternary Cu-Ag-In-Se thin film deposited by the thermal evaporation has been investigated. The evaporation source was prepared by using vertical Bridgman-Stockbarger crystal growth system. Structural analysis indicated that annealing the films following to the deposition resulted in the changes from amorphous to polycrystalline phase with the preferred orientation along (112) direction. In order to determine the optical properties of the thin films as a function of annealing temperature, the transmission measurements were carried out in between 300-2000 nm. The optical band gap values were lying in between 1.29 and 1.50 eV upon annealing the thin films in the temperature range of 300-500°C. The refractive indices of the samples were in the range of 2.7-3.8 depending on the wavelength region and annealing temperature by applying the envelope method. The other optical constants of the samples were also calculated using Cauchy Method and Single Oscillator Model.

Keywords: Thin film, chalcogenide, vapor deposition, X-ray diffraction, optical properties

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1. Introduction

One of the major research areas on thin film technology is the photovoltaic device

applications. It is the most promising one as a future energy technology. Since the thin film

technology provides fewer material uses, and simpler processing steps as compared to a

traditional wafer based crystalline solar cells. Thin films of a-Si, CdTe, Cu(In,Ga)Se2 and

related I-III-VI2 chalcopyrite compounds, have been of great interest for both the research and

commercial solar cell applications because of their many advantageous properties over the

bulk crystalline technologies. Mainly, their high absorption coefficients (> 104 cm-1) [1], only

a few microns of the material are needed to absorb the incident light, whereas in crystalline

silicon structure several microns are needed to obtain similar results. This leads to a decrease

in the constituent materials for the thin film devices, which results in lower costs. For this

purpose, recently, there has been a great deal of interest in the study of thin film growth

methods and characterization [2].

Polycrystalline thin film solar cells based on CuInSe2 (CIS) and AgInSe2 (AIS) ternary

chalcopyrite semiconductor compounds belong to a group of I-III-VI2 compounds are popular

in solar cell applications [3] and non-linear optical devices due to being stable and efficient

absorber material [4]. Family of these ternary chalcogenides is isoelectronic with the zinc-

blende II-VI compound semiconductors. By substituting two cations instead of one in a

regular zinc-blende type lattice, alternative series of bonds can be obtained which results in

differences with respect to the structural, electronic and optical properties. These chalcopyrite

semiconductors within the same family have direct band gap, and high absorption

coefficients, which make them popular in the device applications [5].

Studies on the quaternary compound, CuxAg1-xInSe2 (similarly Cu1-xAgxInSe2) (CAIS) films

as an absorber material for hetero-junction solar cells are a subject of interest since by

changing the composition of the constituent elements in the quaternary alloys, it enables to

the tuning of the material properties. Therefore, this quaternary alloy, which is the

combination of the ternary chalcopyrite compounds CIS and AIS, can give opportunity to

change the structural, optical and electrical characteristics of the material for the requirements

on usage. The optical band gap of these compounds for x = 0.5 is lying in the range of 1.05

eV - 1.24 eV [6, 7].

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In this work, we have studied on the structural and optical characteristics of thin films

deposited by using CAIS single crystal as an evaporation source. The characterization of the

thermally evaporated CAIS thin films were carried by means of X-ray diffraction and spectral

transmittance measurements. The post-annealing process was applied to the films under

nitrogen atmosphere, in order to see the effect of annealing process on the film properties. The

optical constants of the films were determined using Envelope and Cauchy Methods and

Single Oscillator Model.

2. Experimental Details

As an evaporation source for the thermal evaporation of the CAIS thin films, the CAIS crystal

was synthesized by mixing stoichiometric amount of high purity Cu, Ag, In and Se elements

(99.999% pure elements from Alfa-Aesar, US) which were in 0.5:0.5:1:2 molar ratio. These

elements were loaded into a chemically cleaned quartz crucible. Before sealing the quartz

ampoule, it was evacuated for 2 hours with Varian turbo pump to reach 10-5 Torr vacuum

level. Following to this process, the quartz ampule was placed in a specially designed vertical

Lindberg furnace. In order to satisfy homogeneity and also to start the initial chemical

reaction in the prepared mixture, the sealed quartz tube was heated slowly with the 100°C/hr

ramp rate up to 1050°C and kept at this temperature for 4 days, and by shaking frequently at

this temperature before starting the cooling. Then, the CAIS polycrystalline structure was

grown by using the Crystalox MSD-4000 model three-zone vertical Bridgman–Stockbarger.

The furnace was arranged so that the upper temperature zone was maintained about 1150 °C

which is over the melting point of the material to be grown, while the middle zone was kept

about 950°C and the lower zone was about 850°C. The temperature gradient was 10 K/cm and

the translation of the crucible was performed with 1.0 mm/h. Then, the cooling process was

done by the same rate as in heating treatment. Therefore, the crystal growth process in the

Bridgman-Stockbarger system was taken approximately 6 days. As a result of these processes,

the crystal extracted from the grown ingot was used as an evaporation source.

0ptically flat soda lime glasses were used as a substrate material for the thin film deposition.

Before starting to thermal deposition process, approximately 2 gr. crystal powder was

prepared as an evaporation source, and then placed into an alumina coated tungsten boat and

the substrate holder was placed about 15 cm above the source. During the deposition under

vacuum about 10-5 Torr, the substrate temperature was kept about 200°C and the deposition

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rate was around 4-5 Å/sec measured by Inficon XTM/2 deposition monitor connected to the

quartz crystal inside the vacuum chamber.

The annealing process was applied using annealing furnace under the nitrogen atmosphere in

the temperature range of 300-500°C for 30 minutes. Besides, the thickness of the as-grown

and annealed films was measured electromechanically by a Dektak 6M profilometer.

X-ray diffraction (XRD) measurements by using a Rigaku Miniflex XRD system equipped

with Cu Kα radiation source were used to obtain the structural properties of the deposited

CAIS thin films. Also, the peak matching analysis was made by using the computer software

and the database of international crystal diffraction (ICDD) cards. In order to complete the

structural analysis, the elemental atomic compositions of the samples were measured with an

energy dispersive X-ray analysis (EDXA) facility attached to the JSM-6400 scanning electron

microscope (SEM). Finally, the spectral transmittance measurements were carried out by

using Perkin-Elmer Lambda 950 UV/Vis/NIR spectrophotometer in the wavelength range of

300-2000 nm.

3. Results and Discussion

The compositions of the films obtained from EDX analysis showed that the films deposited

by thermal evaporation of CAIS single crystal powder are composed of Cu, Ag, In and Se

atoms. The atomic percentage of the constituent elements in the structure as a function of the

annealing temperatures was tabulated in Table 1. As seen from the table, the compositons of

both as-grown and annealed samples show Cu- and Ag- deficient; and In- and Se-rich

behavior. The different vapor pressures of the constituent elements result in the difficulty to

deposit the stoichometric thin film structure by thermal evaporation and may be responsible

for obtaining the off-stoichiometric CAIS thin films [6, 8]. Furthermore, it was found that

there was a remarkable change in atomic percentage of the constituent elements following to

the post-depositional annealing. As the annealing temperature increases, the decrease in the

atomic percentage of Se indicates the possible segregation and/or re-evaporation of Se atoms

from the film surface due to the high vapor pressure of selenium before making the bond with

the other elements [9].

In order to analyze the diffraction patterns of the deposited thin films, XRD measurements

were carried out. As seen from Fig.1, the XRD measurements indicate that the CAIS thin

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films deposited at the substrate temperature of 200 oC have amorphous structure in the as-

grown form. Therefore, post-thermal heat treatment under different temperatures were applied

to provide the thermal energy to the constituent elements to complete thermal kinetics for the

nucleation stage in the thin film structure. Although, the phases included in powder source

were not transformed to the structure of thermally deposited CAIS films, the transformation

from the amorphous to the polycrystalline phase took place at the annealing temperature of

300 oC. The reasons of having an amorphous for as-grown film and the transformation to

polycrystalline phase in this temperature may be due to deposition of films at low substrate

temperature, 200 oC when compared to the previous studies [6, 11, 12], since the impinging

atoms can prefer placing at fixed positions due to the insufficient thermal energy to complete

the migration and nucleation stages during deposition [6]. On the other hand, the annealed

films showed the same structural behavior in the quaternary phase with Cu0.5Ag0.5InSe2 thin

films [6]. Despite CAIS diffraction peaks of the samples, there were extra diffraction peaks in

the XRD pattern of each thin film. XRD data of the samples were compared with the most

appropriate ICDD database to identify the structures of these diffraction peaks.

In the multi-phase CAIS films, the main orientation direction was obtained at about 26.3°

along (112) direction with tetragonal crystalline structure [6]. Actually, the growth CIS

usually leads to the films with the preferred (112) faceting [14]. The same behavior has been

observed for AIS films [15]. The reported structural parameters calculated from this

diffraction phase are a = 5.937 Å and c = 11.633 Å [6,7]. The tetragonal distortion (c/2a) for

these chalcopyrite materials was found approximately as 0.051. In the case of CAIS material,

replacing Cu atom with Ag, it was evaluated that there is a distortion to form the tetragonal

structure using these ternary compounds. The previous works about AgInSe2 indicated that

significant deviation from the stoichiometry of the film structure resulted in secondary phase

formation (Ag2Se and In2Se3) [18]. Under this study, these additional binary phases were also

observed in the annealed films. It is understood that the thin films consist of a mixture of

crystalline phases (quaternary, ternary and binary) of the constituent atoms (Cu, Ag, In, Se) of

powder source. Therefore, annealing at high temperatures modifies the film structure.

The average microcrystalline grain size D was estimated from the XRD pattern using the

Scherrer’s formula [19] expressed as;

cos0K

D (1)

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where K is structure factor equal to 0.94 [20], 0 is the wavelength of X-rays, is defined

as full width half maximum (FWHM) value, and is the diffraction angle. The grain sizes of

the thin films deposited by thermal evaporation were found 32, 40 and 26 nm with the

increasing annealing temperature from 300 to 500 °C. The increase in grain size can be taken

as the indication of improvement in crystallinity following to the annealing process.

The optical characterization of the films was carried out by measuring the transmission of the

samples at room temperature in the wavelength range of 300-2000 nm. The wavelength

dependence of the transmission for all samples is shown in Fig.2. As seen from Fig.2, the

transmittance spectrum of the films shows interference fringes with a sharp fall at the band

edge in the region 800–2000 nm; whereas the interference effects disappear below this region.

This wavelength interval can be named as a strong absorption region for the films since the

transmission of the films decreases sharply with decreasing wavelength. Having uniform

sequential maxima and minima of the interference fringes at the transparent wavelength

region can be accepted as an indication of the optical homogeneity of the deposited films [21].

The optical transmission values of the as-grown films increased to about 90% in the

transparent region; and this increase depending on the increase in the wavelength was seen for

all the films. Although, the CAIS film annealed at 300 °C showed the same behavior with the

as-grown film, the film annealed at 400 and 500 °C had the different optical characteristics in

terms of their transmission spectra. Therefore, the systematic change between the films was

destroyed with the annealing at 400 °C. There was a wavelength shift in the transmission

spectrum of these samples and a remarkable decrease in the transmission of the 500 °C

annealed film. As observed from the XRD results, this decrease can be explained by the

structural modifications. In general, decrease in the transmittance of thin films is mainly

related with the surface morphology, the grain structure and the defect density [22]. In fact,

the composition of constituent elements, Cu, Ag and In in the film structure became more

effective with increasing annealing temperature, the segregation and/or re-evaporation of Se

in the film structure resulted in a Se deficiency as comparing with the as-deposited one;

therefore the surface may increase the construction of a pseudo-metallic surface forming a

more reflective surface [23].

The absorption coefficient ( ) of the samples was calculated using the transmission values by

means of the relation;

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))(

1ln(

1)(

Td (2)

where )(T is the normalized transmittance and d thickness of the thin films. The absorption

spectrum of the samples can be divided into four principle regions named as strong, medium,

weak and transparent regions depending on the variation of absorption coefficient with

wavelength [24]. As seen from Fig.3, the absorption coefficient decreases with increasing

annealing temperature in the medium and weak absorption regions. The absorption

coefficients of these CAIS samples vary in between 5.0x103 and 5.0x104 (cm-1) depending on

the annealing temperature.

In order to determine optical constants, such as refractive index n(λ), Envelope Method (EM)

developed by Swanepoel [24] was used. In this method, maximum and minimum values of

the transmittance data in the interference patterns must be defined as a function of the

wavelength . However, other method based on the approach of Manifacier et al. [25] needs

creating an upper and lower envelope of the transmission spectrum beyond the absorption

edge. For all of the CAIS films, this method was used, except the film annealed at 500 °C

because of non-systematic fringe distribution.

According to EM, the complex refractive index can be written as,

)()()( inn (3)

where )(n is the real and )( is the imaginary part of the complex refractive index )(n

.

For the calculation of )(n in the weak and medium absorption regions, the transmittance

maximum, MT and minimum, mT values for each wavelength ( ) must be determined.

Therefore, the refractive index, )(n , can be expressed as,

2/12/120

22 ])([ nnNNn s (4)

where is N equal to;

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mM

mMs

s

TT

TTnn

nnN

0

220 2

2(5)

.

In Eq.4 and 5, 0n is the refractive index of the surrounding environment (air) which is equal

to 10 n , MT and mT are the maximum and minimum values of the envelope curves obtained

from transmittance, respectively; sn is the refractive index of the glass substrate which is

approximately equal to 1.51 over the wavelength range of 300 - 1100 nm; and it was

calculated from its transmission spectrum using the relation [26];

1

112

SSs

TTn (6)

here Ts is the maximum value of transmittance of )(T .

As a result of the EM analysis, the refractive index values were found to be in the range of

2.5-3.8 in the wavelength region 800-2000 nm. On the other hand, annealing temperature

higher than 400 °C affected the film structures adversely and their optical measurement

values were not appropriate to use EM, there was a huge difference observed in the refractive

index as compared with the samples annealed at 300 and 400 °C. However, there was an

increase in n(λ) values for the films annealed at these temperatures because of the increases in

the reflectivity due to the structural and compositional changes with annealing.

In order to make further analysis and to compare the refractive index of the samples, the

dispersion relation of the film in the visible region can be described by the Cauchy's relation

[27];

42)(

CB

An (7)

where A, B and C are Cauchy constants that depend on the optical characteristics of the film.

Fig.4 shows the theoretical fit of Eq.7 to the experimental values. The calculated values of the

Cauchy parameters were given in Table 2 for all samples.

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Considering the basic equation for interference fringes;

mnd 2 (8)

where m is an integer defined for maxima or minima, the refractive indices at two adjacent

)(T maxima (or minima) at wavelengths 1 and 2 , 1n and 2n can be used to calculate the

thickness of the sample by the equation as;

)nn(2d

1221

21

(9)

This equation is very sensitive to errors in n values [19]. The results of the thickness

calculations by using Eq.9 and measured by Dektak profilometer were given in Table 3. The

homogeneity in the thickness of each sample was checked by measuring it from the different

parts of the surface area with the profilometer. The calculated and measured thicknesses of the

films were found to be in a good agreement with each other in the corresponding about 10 nm

measurement error interval. According to these results, the thickness of the films decreased

with the increase in the annealing temperature. That can be taken as the indication of the

decrease in the amount of some constituent elements in the structure of the films. Moreover,

the sharp decrease in the thickness of 500 °C annealing may support the idea that there was a

segregation and re-evaporation of constituent elements by resulting in the structural

modification.

Furthermore, the imaginary part of the complex refractive index, )( extinction coefficient

of the films was evaluated by using the expression [28];

)(4

)( (10)

and the variations of the extinction coefficients with the wavelength for each annealing

temperature were plotted in Fig.5. As seen from the figure, the calculated values )( of the

as-grown and annealed CAIS films show the similar variations in the wavelength range of 800

- 2000 nm.

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By the way, the dispersion of refractive index plays an important role in the optical properties

of thin film materials [29], and it was analyzed with the Single Oscillator Model (SOM) by

using the Wemple–Di Domenico relation [24, 25] as;

222 1)(

EE

EEEn

p

dp

(11)

where dE is related to the strength of the inter-band optical transitions, pE gives the average

energy gap also called oscillatory energy, and E is the photon energy in terms of h . The

SOM parameters, dE and pE were calculated from the [ 1)( 2 hn ] versus 2)( h plots for

as-grown and annealed films (see Fig.6), and the corresponding results were tabulated in

Table 4. By the analysis of dE values, it was defined that the strongest optical transition

probability is dominant for the samples annealed at 300 °C (see Table 4). In other words, the

optical properties of CAIS thin films strongly depend on the structural formations.

The obtained )(n values can be expressed in terms of the dielectric constant )( as [28];

2/12/1 ))()(())(()( imre in (12)

and by using Eq.3, the real and the imaginary part of the )( can be written as;

2)()( nre (13)

)(2)( im (14)

by the approximations of the calculated )( values which are very small as compared with

)(n values. The real part, )( re , was calculated by using Eq.13, and the result of this

calculation with respect to wavelength is given in Fig.7.

By using SOM model, the refractive index )0(n at zero photon energy (static refractive

index) is also known as the infinite wavelength dielectric constant that can be deduced from

the dispersion relationship by extrapolation of the linear part of the plots given in Fig.6 [28].

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Moreover, the static dielectric constant )0( can be determined from the corresponding

intercept value. The high-frequency dielectric constant can be obtained from the )( re

values of the films [28]. This optical dielectric constant related to the square of the refractive

index observed for the smallest band gap value corresponds to the long-wavelength refractive

index of the material. Therefore, these values were deduced at the wavelength region below to

the minimum absorption edge. The calculated optical constants by using these approaches and

Fig.7, were summarized in Table 4. According to the Lyddane-Sache-Teller relation [30], it

was observed that the CAIS films have hetero-polar crystal structure.

On the other hand, the plot of the optical absorption coefficient α versus photon energy

)( h showed principle regions as in the work of Wood and Tauc [31]. In the strong absorption

region, the absorption coefficient of a direct transition is related to the band gap energy and

can be expressed as;

2/1)()( gEhAh (15)

where A is an energy-independent constant and gE is the forbidden band gap energy.

Therefore, from the plot of 2)( h versus )( h (see Fig.8), the band gap values can be

defined by extrapolation of the straight line on the energy axis. Three distinct local maxima

were investigated in the high absorption region of the corresponding CAIS films. In other

words, these CAIS films have three intrinsic band-to-band transitions showing the splitting of

the valance band into three bands due to hybridization d-like valance band states and p-like

conduction band states [33]. This has been attributed [6, 15, 34] to the crystal-field splitting of

the valence bands and spin-orbit splitting. In compound semiconductors, the conduction band

minimum is mostly related with the cation (s-orbital), while anion p-orbital is corresponding

to the valence band maximum [35]. In I-III-VI2 compounds, usually s-orbital of III-group of

elements, In or Ga, contributes to the conduction band minimum while the Se 4p-orbitals

contribute to the valence band maximum [16]. However, the transition metals, Cu and Ag,

introduce d-orbital, which have an effect on the position of the valence band maximum. These

d-levels affect both the optical band gap and the separation of the spin-orbit splitting in the

valence bands. Calculated 2gE values can be accepted as the main band-to-band transition

because of having the longest linearity for the fundamental absorption edge. In literature, the

studies on the optical properties of CAIS films have shown that the compound has a direct

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optical band gap that ranges from 1.21 to 1.25 eV [7], and also due to being quaternary alloy

of CIS and AIS, it was also expected in between 1.05 and 1.24 eV. However, the contribution

of the elements, especially excess In, in the composition of the films depending on the

deposition method and annealing parameters may be the reason of high band gap values than

reported CAIS thin film studies.

From the quasi-cubic model for the splitting of p-like levels in a nearly cubic crystals,

including both the effects of a non-cubic crystalline field and spin-orbit interaction, by using

the adapted form to chalcopyrite compounds [16, 36], we can calculate the spin orbit splitting

of the valence bands in a cubic field, SO and crystal-field splitting of the valence bands in

the absence of spin-orbit interaction, CF from the three-fold optical band gap by using the

following expression;

2/1

22,1 )(

3

8)(

2

1)(

2

1

SOCFSOCFSOCFE (16)

where 2,1E are the energy differences in the form, 121 gg EEE and 232 gg EEE . The

corresponding splitting energies are related to the energies of the Г7 relative to the Г6 level in

the valence band of a chalcopyrite crystal [16]. The calculated values using Eq.16 were listed

in Table 5. It was observed that there was an increase in both SO and CF values with

increasing annealing temperature up to 400 °C. This can be related with structural changes

and modifications by post-thermal heat treatments as observed from the structural analysis. In

this quaternary compound, CAIS films with the chalcopyrite structure, the crystal field-

splitting can be related to the tetragonal distortion along the c-axis [16];

)2(2

3a

cbCF (17)

where b is the deformation potential. From the tetragonal distortion value calculated for the

main orientation of the CAIS structure, CF was calculated as in between -0.26 and -0.40 eV.

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4. Conclusion

In this work, detail structural and optical characteristics of the thermally evaporated and

annealed CAIS thin films were studied. According to the structural and the compositional

analyses carried out on the as-grown and annealed films, there was In-rich, Cu and Ag-

deficient structure as compared with the evaporation powder. These results imply that the

different vapor pressures of the constituent elements result in the difficulty to deposit the

stoichometric thin film structure by thermal evaporation of the sintered ternary powder and

may be responsible for obtaining the off-stoichiometric CAIS thin films. The XRD results

showed that the deposited CAIS samples are amorphous in nature and it transforms to the

polycrystalline phase as a result of annealing processes. Also, the resulting polycrystalline

phase is composed of the mixture of different crystalline phases. The room temperature

optical measurements were performed on the as-grown and annealed samples, and their

analyses by using the EM, Cauchy fitting and SOM allowed us to calculate and compare the

optical constants, n , and . The evaluated values were consistent and found to be in good

agreement with each other. The band gap energies obtained from the absorption spectrums of

the CAIS films were 1.38, 1.46, 1.50, 1.29 eV for the as-grown and annealed films at 300,

400 and 500 °C, respectively. The changes in these values imply the improvement in the

crystallinity upto the annealing at 400 oC, whereas further annealing above this value resulted

in the deformation in the structure by increasing the defect levels in the band edge region. The

distinct intrinsic transitions obtained from the variation of the absorption values with photon

energies are indicating the existence of the valence band splitting. As explained and

discussed, the variations in the structural and optical properties of the films were related with

the compositional changes initiated with the deposition and annealing processes.

Acknowledgement

This work was financed by Middle East Technical University (METU-BAP) under Grant No.

BAP-01-05-2013-005. Also, one of the authors would like to thank to TUBITAK-BIDEB for

the financial supports during this study.

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References

[1] N. Kavcar, Sol. Energ Mat. Sol. C., 52 (1998) 183-195

[2] K.L. Chopra, S.R. Das, Thin Film Solar Cells, Plenum Press, New York, 1983

[3] K.L. Chopra, K.L. Paulson, V. Dutta, Prog. Photovolt. 12 (2004) 69–92

[4] R. Asokamani, R.M. Amirthakumari, R. Rita, C. Ravi, Phys. Stat. Sol. (b) 213 (1999) 349

[5] J.L. Shay, B. Tell, H.M. Kasper, M.M. Schiavone, Phys. Rev. B 7 (1973) 10

[6] G.V. Rao, G.H. Chandra, O.M. Hussain, S. Uthana, B.S. Nadiu, J. Alloys Compd. 325

(2001) 12-17

[7] G.V. Rao, G.H. Chandra, P.S. Reddy, O.M. Hussain, K.T.R. Reddy, S. Uthana, Surf.

Instrum. and Vac. Technol. 67 (2002) 293-298

[8] C.M. Joseph, C.S. Menon, Semicond. Sci. Technol. 11 (1996) 1668

[9] A. Roth, Vacuum Technology, North Holland, Amsterdam, 1980

[10] R. Bacewicz, J.R. Durrant, T.F. Ciszek, S.K. Deb, Ternary and Multinary Compounds:

Proceedings of the 7th International Conference (1987) 155-160

[11] G.V. Rao, G.H. Chandra, P.S. Reddy, O.M. Hussain, K.T.R. Reddy, S. Uthana, J.

Optoelect. and Adv. Mat. 4 (2002) 387-392

[12] G.V. Rao, G.H. Chandra, O.M. Hussain, S. Uthana, B.S. Naidu, Cryst. Res. Technol. 36

(2000) 571–576

[13] H.V. Campe, Thin Solid Films 111 (1984) 17

[14] D. Liao, A. Rockett, J. Appl. Phys. 91 (2002) 1978

[15] P.P. Ramesh, S. Uthanna, B.S. Naidu, P.J. Reddy, J. Mater. Sci. Lett. 15 (1996) 116-119

[16] J.L. Shay, J.H. Wernick, Ternary Chalcopyrite Semiconductors: Growth, Electronic

Properties and Applications, Pergamon, Oxford, 1975

[17] L.I. Berger, Semiconductor Materials, CRC Press, New York, 1997

[18] I. Olekseyuk, O. Krykhovets, J. Alloys Compd. 316 (2001) 193-202

[19] E.F. Kaelble, Handbook of X-Rays, McGraw-Hill Inc., USA, 1967

[20] J.I. Langford, A.J.C. Willson, J. Appl. Cryst. 11 (1978) 102-113

[21] N. Khemiri, A. Sinaoui, M. Kanzari, Physica B 406 (2011) 1778-1783

[22] N. Revathi, P. Prathap, Y.P.V. Subbaiah, K. T. Ramakrishna Reddy, J. Phys. D 41 (2008)

155404

[23] H. Karaağaç, M. Kaleli, M. Parlak, J. Phys. D: Appl. Phys. 42 (2009) 165413

[24] R. Swanepoel, J. Phys. E 16 (1983) 1214

[25] J.C. Manifacier, J. Gasiot, J.P. Fillard, J. Phys. E 9 (1976) 1002

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[26] G.K.L. Goh, C.P.K. Liew, J. Kim, T.J. White, J. Cryst. Growth 291 (2006) 94

[27] M. Fox, Optical Properties of Solids, Oxford University Press, London, 2001

[28] S. Adachi, Optical Properties of Crystalline and Amorphous Semiconductors, Kluwer

Academic Publishers, Boston/Dordrecht/London, 1999

[29] A.F. Qasrawi, Opt. Mater. 29 (2007) 1751-1755

[30] A.J. Sievers, J.B. Page, Phys. Rev. B 41 (1990) 3455

[31] D.J. Wood, J. Tauc, Phys. Rev. B 5 (1972) 3144-3151

[32] E.A. Davis, N.F. Mott, Philos. Mag. 22 (1970) 903

[33] F. Habashi, Chalcopyrite: Its Chemistry and Metallurgy, McGraw-Hill, USA, 1978

[34] J. Shay, B. Tell, H. Kasper, L. Schiavone, Phys. Rev. B 7 (1973) 4485–4490

[35] M.S. Al-Kotb, W. Puff, S.A. Hassan, M. Mohsen, Mater. Sci. Forum 363-365 (2001)

150-152

[36] J.E. Rowe, J.L. Shay, Phys. Rev. B 3 (1971) 451

[37] L.L. Kazmerski, Polycrystalline and Amorphous Thin Films and Devices, Academic

Press, London, 1980

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List of Tables

Table 1 EDXA results of CAIS thin films at atomic percentage

Table 2 Three term Cauchy Parameters, and SOM parameters for as-grown and annealed

CAIS films

Table 3 Calculated and measured thickness values for the CAIS films

Table 4 Band gap values and quasicubic model parameters of the CAIS thin films

Table 5 Splitting energy values for as-grown and annealed CAIS films

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Figure Captions

Fig.1 XRD patterns for thermally evaporated CAIS thin films at different annealing

temperatures

Fig.2 The transmittance spectra for CAIS thin films

Fig.3 The variation of absorption coefficient with the photon energy for CAIS thin films

Fig.4 The refractive index of the films calculated by envelope method (EM)

Fig.5 Variation of the extinction coefficient for the as-grown and annealed CAIS thin films at

different temperature

Fig.6 Plots of )1( 2 n vs 2)( h for as-grown and annealed CAIS films

Fig.7 Real part of the dielectric constants for the CAIS films

Fig.8 The variation of 2)( h as a function of )( h for (a) as-grown and annealed at (b) 300

°C, (c) 400 °C, (d) 500 °C CAIS films

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Table 1 EDXA results of CAIS thin films at atomic percentage

Sample Cu (at %) Ag (at %) In (at %) Se (at %)

As-grown 0.86 1.86 42.20 55.08

Anl-300 1.14 1.76 43.39 53.71

Anl-400 1.76 1.58 43.49 53.17

Anl-500 1.77 2.47 43.77 52.04

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Table 2 Three term Cauchy Parameters, and SOM parameters for as-grown and annealed CAIS films

Sample A B (nm2) C (nm4) )0(n )0(

As-grown 2.50 7.91 x 105 5.53 x 109 2.30 5.29 10.56

Anl-300 2.60 8.15 x 104 1.59 x 1011 2.08 4.33 7.40

Anl-400 2.68 1.42 x 105 1.57 x 1011 2.21 4.88 8.94

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Table 3 Calculated and measured thickness values for the CAIS films

Thickness (nm)

Measured Calculated

As-grown 610 600

Anl-300 590 590

Anl-400 560 570

Anl-500 540 -

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Table 4 Band gap values and quasicubic model parameters of the CAIS thin films

Sample Eg1 (eV) Eg2 (eV) Eg3 (eV) Ed (meV) Ep (eV)

As-grown 1.20 1.38 1.45 12.9 2.06

Anl-300 1.23 1.46 1.63 13.5 2.46

Anl-400 1.26 1.50 1.75 14.4 2.45

Anl-500 1.06 1.29 1.41 - -

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Table 5 Splitting energy values for as-grown and annealed CAIS films

Sample - CF (eV) SO (eV)

As-grown 0.195 0.096

Anl-300 0.297 0.210

Anl-400 0.304 0.301

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Figure1

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Figure2

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Figure3

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Figure4

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Figure5

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Figure6

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Figure7

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Figure8