The investigation of structural, electrical, and optical properties of thermal evaporated AgGaS2 thin films

Thin Solid Films 519 (2011) 2055–2061

Contents lists available at ScienceDirect

Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f

The investigation of structural, electrical, and optical properties of thermal evaporated AgGaS2 thin films H. Karaagac, M. Parlak ⁎ Department of Physics, Middle East Technical University, 06531 Ankara, Turkey

a r t i c l e

i n f o

Article history: Received 2 January 2010 Received in revised form 7 October 2010 Accepted 12 October 2010 Available online 21 October 2010 Keywords: X-ray diffraction Photoconductivity Chalcopyrite Thermal evaporation

a b s t r a c t AgGaS2 (AGS) thin films were deposited onto glass substrates by sequential thermal evaporation of AgGaS2 single crystalline powder and excess silver (Ag) interlayer. Systematic optimization to obtain single phase AgGaS2 thin films was carried out by changing the thickness of the excess silver layer. The structure and composition of as-grown and annealed films were studied by means of X-ray diffraction and energy dispersive X-ray analysis, respectively. The optical properties of AGS thin films determined by transmittance and reflection measurements showed that they had quite high absorption coefficient with the values around 104 (cm−1). The calculated band gap values were found to be between 2.30 and 2.75 eV depending on annealing temperature. The refractive index (n) and extinction coefficient (k) of the films were determined by the envelope method. Finally, photo-electrical measurements under different illumination intensities were carried out, and different sensitizing and recombination centers were defined. © 2010 Elsevier B.V. All rights reserved.

1. Introduction AgGaS2 is a member of chalcopyrite I–III–VI2 ternary semiconductor compounds. Such types of materials have extensively been studied because of their interesting electrical and optical properties. Tuning the band gap energies (ranging from 3.5 eV (CuAlS2) to 1.2 eV (AgInSe2)) makes them attractive for the light emitting devices and detectors in a wide wavelength range from UV to infrared [1]. Additionally, accommodation with different dopants results in formation of various solid solutions and enables them to be used as promising materials for solar cell applications [2,3]. These ternary compounds are generally classified under two groups, namely, copper (Cu-III–VI2) and silver (Ag-III–VI2) based chalcopyrites. Copper based compounds; especially Cu(InGa)Se2 have already proven their compliance for solar energy applications due to their suitable band gap and large absorption coefficient (105 cm−1). Recently, a 19.5% efficiency was reported for the solar cell constructed by using these copper based compounds as absorbing layer [4]. On the other hand, the silver based chalcopyrites attract much attention because of their potentials in non-linear optical and photonic applications [5]. AgGaS2 has a direct band gap of about 2.7 eV and good transparency in the wavelength range of 500– 1200 nm [6]. That is also the reason of its usage as the frequency doubler and tripler for CO2 laser output [7]. Their high transparency at 550 nm is used in the construction of optic parametric oscillator pumped by Nd:YAG laser [8]. Moreover, they are uniaxial and therefore

birefringent. The degree of birefringence is so high that it permits phase matching over a wide wavelength range in linear and non-linear optical interactions [9]. The AgGaS2 could be phase matched for the generation of second harmonics for fundamental wavelength ranging from 1.8 to 11 μm. In addition, the three wave mixing process can be extended to this range [10]. In terms of electrical properties, it is known that AgGaS2 behaves as semi-insulating (N108 Ω cm) material. And, it was found to be quite difficult to achieve useful conductivity of either n- or p-type due to the shallow impurities [11]. Previous attempts to improve their conductivity through annealing or impurity doping have failed, which prevents the extensive use of this material for optoelectronic applications as compared with Cu-based chalcopyrite compounds [12]. Different techniques have been reported so far for the preparation of AgGaS2 in both thin film and single crystalline form [6,13–15]. In this study, the aim is to improve the electrical and optical properties of AgGaS2 by studying the Ag–Ga–S system produced by a sequential evaporation of AgGaS2 and Ag films in a specific proportion by a double source thermal evaporation method. This system includes several forms of compounds like AgGaS2, AgGa5S8, AgGa3S5, and Ag9GaS6. We haven't encountered such a study on Ag–Ga–S system deposited by using intra-stacking layer of silver (Ag). Thus, herein, the effect of annealing on the structural, electrical, and optical properties of Ag–Ga–S thin films deposited by sequential thermal evaporation of AgGaS2 and Ag has been investigated. 2. Experimental details

⁎ Corresponding author. Tel.: +90 312 210 76 46; fax: +90 312 210 50 99. E-mail address: [email protected] (M. Parlak). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.10.027

Thin films of Ag–Ga–S (AGS) system were prepared onto the soda– lime glass substrates by thermal evaporation method using double

H. Karaagac, M. Parlak / Thin Solid Films 519 (2011) 2055–2061

3. Results and discussion 3.1. X-ray diffraction and composition analysis XRD measurements have been performed to identify the structural properties of the source material (powder) and deposited Ag–Ga–S thin films. To determine the effect of annealing on the film structure, diffraction measurements were carried out for the samples annealed in N2 atmosphere at the temperature range of 300–550 °C for 5 min. Fig. 1 shows the diffraction patterns obtained for the single crystalline powder AgGaS2, which was used as an evaporation source. It is clear from the pattern that the sintered powder has single crystalline structure and there is no diffraction peaks corresponding to any

30

20

(312)

40

50

(332) (008) (404) (316)

(323) (400)

60

(402)

(224) (116)

(301)

(104) (211)

(103)

(200)

(220)

(204)

Intensity (in arb. units)

(112)

powder AgGaS2

70

80

2θ (in degree) Fig. 1. X-ray diffraction pattern for the powder used as the evaporation source.

precipitation of secondary phases. Compared with JCPD card [16], it was deduced that this phase is corresponding to AgGaS2 with chalcopyrite structure well oriented in (112) direction. The lattice constants a and c are found to be 5.754 and 10.301 Å, respectively. Fig. 2 shows XRD patterns of as-grown and annealed films in the temperature range of 350–550 °C. As seen from Fig. 2, the as-grown film is in amorphous structure. However, annealing the sample at 350 °C results in the formation of Ag crystallites with (111), (200), (220) and (311) reflections of Ag cubic structure [17]. Up to the annealing temperature of 400 °C, there is no remarkable variation in terms of improvement in the structure. However, above 400 °C, AGS develops with co-existence of the Ag phase. Compared with JCPD cards, it is revealed that AGS is a ternary compound with hexagonal structure (a = b = 3.587, and c = 23.200 Å) [18]. All reflection planes with corresponding Miller indices (hkl) for the AGS were illustrated in Fig. 2. As observed from the pattern, the predominant (101) peak of hexagonal system represents a preferred orientation along this plane. With further increase in annealing temperature to 550 °C, it is seen that the intensity of reflection lines of both AGS and Ag phases increase, which is the indication of a better crystalline quality. The increase in grain sizes with the annealing observed from SEM images recorded with operating voltage of 15 kV (in Fig. 3) confirms the improvement in crystallinity as well. Thermal energy supplied by annealing leads to high mobile ad-atoms having a large diffusion

550°C

α

(311)

(220)

(021)

(116)

α

(1010) (110) (0111)

(200)

(111) α

(107)

(104)

(101) (012)

(006)

AGS α Ag

(003)

sources under vacuum of about 10−5 Pa. Substrates were placed approximately 15 cm above the evaporation sources and their temperature was kept constant at around 200 °C during the deposition cycle. Two quartz crucibles wrapped separately by tungsten heater wires were used as the evaporation boats for silver (Ag) and AgGaS2 precursors. The powder of AgGaS2 obtained by crushing a sintered ingot into fine grains was used as evaporation source. The compound was prepared by reacting high purity of Ag, Ga, and S elements weighted in stoichiometric ratio in an evacuated quartz crucible in a special designed horizontal furnace maintained at a temperature above melting point of the constituent elements (1050 °C) for a period of 96 h. Prior to the sequential evaporation of Ag and AgGaS2 films, the optimization of individual layers was carried out independently by conducting several depositions under the same conditions. The energy dispersive X-ray analyses (EDXA) (taken with the acceleration voltage of 20 kV) carried out for the deposited AgGaS2 films have revealed that there was almost no silver (1%) in composition, and a Gallium (Ga) and Sulfur (S) rich composition with an atomic percentage ratio of 5 and 8, respectively. The deviation from the stoichiometry of compound might be related with the decomposition of the starting material during evaporation, which can be attributed to the different vapor pressures of constituent elements. To compensate deficiency of Ag in the film composition, Ag was incorporated independently from a second source. The optimization for the silver was also carried out until the reproducibility of the samples was achieved. The individual precursors of AgGaS2 and Ag were deposited sequentially onto substrates in the order of AgGaS2/ Ag/AgGaS2 layers with thickness of approximately 900/(50–60)/ 900 nm, respectively. The thickness and rate of the deposited individual layers were monitored and controlled simultaneously by a quartz crystal monitor (Inficon XTM/2). The deposition of the layers in the given sequence was repeated until the reproducibility of films was achieved. Then, to examine the effect of annealing on properties of films, the samples were annealed in the temperature range of 350– 550 °C for a period of 5 min in nitrogen (N2) flow in a special designed furnace. The chemical composition of films was determined by a FEI Quanta 400 FEG model scanning electron microscopy (SEM) equipped with EDXA system. The phase and structure of samples were studied with the help of Rigaku Miniflex X-ray diffraction (XRD) system equipped with CuKα X-ray source in the range of 2θ from 10 to 80° with a scan speed of 2°/min−1. For the wavelength dependent transmittance measurements, a Pharmicia LKB Ultraspec III UV-VIS spectrophotometer was used in the wavelength range of 325–900 nm. In order to investigate the electro-optical properties of films, indium metal contacts were deposited onto the samples by thermal evaporation using suitable copper mask in the form of van der Pauw geometry. Silver paste was used to attach the copper electrodes to contacts. The ohmic behavior of contacts was determined by forward and reverse current–voltage characteristics. The photo-electrical measurements were carried out by using a Janis liquid nitrogen cryostat equipped with a Lakeshore 331 temperature controller in the temperature range of 100–430 K under vacuum of about 1.3 × 10−2 Pa.

Intensity (in arb.units)

2056

α

450°C

350°C

as-grown

10

20

30

40

50

60

70

80

2θ (in degree) Fig. 2. X-ray diffraction patterns of as-grown and AGS thin film annealed in the temperature range of 350–550 °C.

H. Karaagac, M. Parlak / Thin Solid Films 519 (2011) 2055–2061

2057

Fig. 3. SEM micrographs for (a) as-grown and (b) AGS films annealed at 550 °C.

distance which promotes the probability of collision process and triggers the nucleation resulting in a growth of film with larger grains [19]. The annealing AGS at 550 °C dominates the structure of AGS with the preferred orientation along (101) direction corresponding to Bragg's angle 2θ = 28.97. The elemental atomic composition of films has been studied by EDXA measurements. A typical spectrum for the powder AgGaS2 is shown in Fig. 4. Results revealed that the ratio of the constituent elements Ag:Ga:S in the powder and as-grown film was nearly 1:1:2 (23.72:27.81:48.47) and 1:5:8 (7:35.86:57.14), respectively. This implies that the stoichiometry of unknown (Ag,Ga)S in JCPDS database is likely to be AgGa5S8. There has been no study reported for AgGa5S8 structure so far. It is understood from the XRD patterns that the intensities of peaks corresponding to Ag phase increase upon increasing annealing temperature. The SEM pictures (Fig. 5) obtained for the as-grown and films annealed at 550 °C also confirm the evolution of silver phase. They were recorded with the same parameters (the same spot size, working distance and acceleration voltage). As seen in these micrographs, the agglomerations are distributed irregularly and their densities over the surface have increased with post-annealing. The EDXA measurements taken over these regions have revealed that the atomic percent of constituent

Fig. 4. EDXA spectrum for the source-powder AgGaS2 conducted at 20 kV.

elements Ag, Ga, and S is 79.50, 13.19, and 07.30, respectively. High atomic percent of Ag implies that these features with different geometries and sizes are corresponding to Ag crystallites probably improved by the crystallization of mobile Ag atoms diffused out from the grain boundaries to the surface by the subsequent post-annealing at high temperatures. In addition, it is likely to be due to the diffusion of Ag atoms from the deposited Ag-layer to the surface through the voids formed during the segregation of high volatile S triggered at high annealing temperatures. The most probable cause of not developing single phase in the structure could be the insufficient thermal energy driven by annealing to complete the reaction between the sequentially deposited AgGaS2/Ag/AgGaS2 film layers. To increase the interaction probability of these layers for the construction of ordered AGS structure, the mobility of silver atoms should be increased by raising annealing temperature. However, for the films deposited on soda–lime glass substrates, it is not feasible to do this over 550 °C due to the melting point of glass substrate.

3.2. Optical analysis Fig. 6 shows the transmittance spectrum measured in the wavelength range of 325–900 nm for the as-grown and films annealed at 350 and 550 °C. As clearly seen from the figure, postannealing has a pronounced effect on determining optical properties of films such as the shifts of band edges to lower wavelength and the increase in the average transmission from 20% to 38%. The enhancement in transparency with the increase of annealing temperature may be due to the modifications in the structure and surface morphology of the films. It is known that a smoother surface means less scattering of light from surface. The observed Ag agglomerations on the surface of the films and precipitation or nucleation of silver atoms firstly in the amorphous and then in the polycrystalline structure may be treated as the scattering centers responsible for the dispersion of incoming photons, and they subsequently result in the variation of absolute value of the transparency with the annealing. The silver related formations on the surface may act as the surface light scattering centers since they produce a more roughened surface, whereas unreacted constituent elements-related formations and precipitations act as bulk scatterings. Thereby, the observed fluctuation in transmittance could be the

2058

H. Karaagac, M. Parlak / Thin Solid Films 519 (2011) 2055–2061

Fig. 5. The SEM pictures indicating the evolution of Ag agglomerations obtained for (a) as-grown and (b) AGS thin films annealed at 550 °C.

result of reduction in the surface and bulk scatterings at different annealing temperatures. As it was deduced from the XRD analysis, the crystallinity of the samples was improving with increasing annealing temperature. This can be the indication of decreasing bulk scatterings. However, from the SEM pictures, it was understood that the density of surface scatterings increases as the annealing temperature increases. Thus, the higher transmittance observed at high annealing temperature could be attributed to the decrease in density of these scattering centers as a result of the improvement in structural homogeneity and a better crystallinity. The absorption coefficient (α) of the films was obtained by means of transmission (T) measurement through the following relation, α = 1 = d lnð1 = TÞ

ð1Þ

where d is the thickness of the film. The reflection measurements were also carried out for the AGS films and the low reflection values (lower than 10%) were observed. Therefore, it was not taken into account during the calculation of absorption coefficient. Fig. 7 shows the plot of absorption coefficient as a function of photon energy (hv) for the as-grown, and the films annealed at 350 and 550 °C. For all

samples it was found that the absorption coefficient above the fundamental band edge is around 104 cm−1, which is a characteristic value for the I–III–VI semiconductors. Dependence of absorption coefficient on photon energy follows an allowed direct interband transition given by   2 ðαhvÞ = A hv−Eg

ð2Þ

where Eg and A are the band gap energy and a constant that depends on the probability of transitions and material's refractive index, respectively [20]. The optical band gap energies were calculated from the plot of (αhv)2 vs. (hv) by extrapolating the linear region of the plot to energy axis of (αhv)2 = 0. The direct band gaps increase and the absorption edges shift to smaller wavelength (blue-shift) with increasing annealing temperature. The calculated band gap values are found to be as 2.30, 2.48, and 2.75 eV for the as-grown, and films annealed at 350 and 550 °C, respectively. It is known that the presence of unsaturated bonds and deviation from the stoichiometry result in the formation of localized 2.0x109 1.8x109

as-grown 350°C 550°C

(αhν)2(cm-2eV2)

1.6x109 1.4x109 1.2x109 1.0x109 8.0x108 6.0x108 4.0x108 2.0x108 0.0

1.5

2.0

2.5

3.0

hν (eV) Fig. 6. Transmittance spectrum for the as-grown, and films annealed at 350 and 550 °C.

Fig. 7. Plot of (αhν)2 against (hν) for as-grown and AGS thin films annealed at 350 and 550 °C.

H. Karaagac, M. Parlak / Thin Solid Films 519 (2011) 2055–2061

2059

states in the forbidden band gap. Since the as-grown samples are amorphous in nature and become crystalline by subsequent heat treatment, the increase in band gap is likely to be stemming from the reduction of defects and the decrease of structural disorder. The band gap value 2.75 eV obtained for the sample annealed at 550 °C agrees well with the reported value of 2.76 eV [21]. The optical constants (refractive index (n) and extinction coefficient (k)) of the AGS films were determined by the envelope method [22]. For the weak absorption regions, the refractive index can be determined by means of the following expression,    1 = 2 2 2 2 1=2 n = N + N −n0 ns

ð3Þ

where ns is the refractive index of substrate (soda–lime glass), n0 is the refractive index of air, which is equal to 1 and N is defined by   2 2 N = n0 + ns = 2 + 2n0 ns ðTM −Tm Þ = ðTM Tm Þ

Fig. 9. Spectral distribution of refractive index (n) for the AGS thin films.

ð4Þ

where TM and Tm stand for the maxima and subsequent minima of the transmission spectra connected by the fitted envelopes illustrated in Fig. 8. Figs. 9 and 10 show the spectral distribution of n and k for the asgrown, and the films annealed at 350 and 550 °C, respectively. Constants are determined in the wavelength range of 500–685 nm and 350–860 nm for n and k, respectively. From its spectral distribution, the refractive index has a decrease with increasing wavelength up to a specific wavelength and then starts to increase

Fig. 8. The envelope fittings of Tmax and Tmin for the (a) as-grown and (b) AGS thin film annealed at 350 °C.

with further increase in the wavelength over this point for the asgrown and film annealed at 350 °C. The decrease in refractive index with increasing wavelength implies that the material exhibits usual dispersion behavior. For the film annealed at 550 °C, the refractive index is almost wavelength independent in the studied wavelength range. As a result of the spectral distribution of refractive index, it can be concluded that the annealing process has a pronounced effect on the optical properties of the deposited films. This variation is closely related to the modification taking place in the structure and in the optical properties upon post-annealing process. The obtained refractive index values for the as-grown and annealed films are given in Table 1. The reported refractive index values for the AgGaS2 in the 500– 700 nm range lie between 2.5 and 2.6 [23], which is larger than those obtained for as-grown and films annealed at 550 °C, and very close to that obtained for the film annealed at 350 °C for the whole scanning wavelength range. The different structure and stoichiometry of AgGaS2 (tetragonal) and AgGa5S8 (hexagonal) make this difference plausible. The extinction coefficient values were estimated from the absorption coefficient through k = αλ/4π relation. Fig. 10 shows the spectral variation of k for the as-grown, and the films annealed at 350 °C and 550 °C. It is clear that the extinction coefficient decreases as the annealing temperature increases, which shows that films become highly transparent at long wavelengths. For the annealed samples, the k values are smaller than that of as-grown film in the 350–550 nm wavelength range. However, these values remain above the value of as-grown film over this range. It is known that extinction coefficient is a measure of the fraction of light lost, stemming from

Fig. 10. Spectral distribution of extinction coefficient (k) for the AGS thin films.

2060

H. Karaagac, M. Parlak / Thin Solid Films 519 (2011) 2055–2061

Table 1 Calculated refractive index values for AGS thin films. Sample

Wavelength range (nm)

n

As-grown 350 °C 550 °C

550–680 500–680 500–680

2.0–2.2 2.4–2.6 ~ 1.8

scattering and absorption in a medium or material. As can be seen from Fig. 6, the transmittance enhances (absorption decreases) with increasing annealing temperature. This suggests that the increase in the transparency is likely to be originating from the decrease observed in k with increasing annealing temperature in the same wavelength range. Since the optical constants are closely related to the density of defects in the structure, the elimination or formation of them with annealing may have remarkable effects on the optical properties of the films. 3.3. Photo-electrical analysis The temperature dependence of dark and illuminated conductivities under the illumination intensity of 17, 34, 55, 81, 113 mW/cm2 is shown in Fig. 11. The photoconductivity (PC) was evaluated by subtracting the illuminated conductivity from the dark conductivity. The variation of photoconductivity as a function of inverse temperature for the illumination intensity of 55, 81 and 113 mW/cm2 is indicated as an inset in Fig. 11. It is seen that the conductivity under illumination for all intensities is larger than dark conductivity and there is an exponential increase in the conductivity with increasing ambient temperature and illumination intensity. The variation of conductivity under light could be divided into two regions: low and high temperature regions. In the low temperature region (150– 260 K), the conductivity values for all illumination intensities are almost temperature-insensitive. However, in the high temperature region, there is a sharp exponential increase with increasing temperature. As observed from the variation of the photoconductivity as a function of temperature in Fig. 11, there are three different temperature regions. In the first region (150–300 K), the PC slightly decreases with increasing temperature and increases with increasing illumination intensity. In the second region (310–370 K), the PC exponentially increases with increasing temperature for each illumination intensity of 55, 81, 113 mW/cm2 with activation energies of 264, 340, and 350 meV, respectively. Existence of these regions with different activation energies indicates the presence of different trap

Fig. 11. The temperature dependence of dark and illuminated conductivity under the illumination intensity of 17, 34, 55, 81, and 113 mW/cm2. The inset of the figure shows photoconductivity as function of inverse temperature.

levels. The increase in activation energy with increasing illumination intensity implies that the Fermi level in the as-grown film is changed due to the increasing density of the defect centers [24]. Finally, in the third region, over 380 K the photoconductivity decreases with increasing temperature for all intensities. This phenomenon is known as the thermal quenching of photoconductivity [25]. Such a behavior is generally attributed to the co-existence of the sensitizing and the recombination centers in the structure [24]. To investigate the nature of recombination centers, the relation between the photocurrent (Ipc) and illumination intensity (Ф) was studied for the temperature levels of 150, 250 and 300 K. The plot of ln(Ipc) vs. ln (Ф) for these temperatures is shown in Fig. 12. The relation between photocurrent and the illumination intensity follows the Ipc α Фn expression, where the power exponent (n) designates the behavior of the recombination mechanism [26]. From the slope of the curve, it was found that the (n) values varied between 1 and 2, which is the indication of supralinear photoconductivity related with the twocentre model (for the case of n N 1). This model suggests that life-time of free carriers increases with increasing illumination intensity. Thereby, it becomes more photosensitive with increasing illumination intensity [25]. The dark conductivity and photoconductivity for the annealed films were quite different. A similar behavior of temperaturedependent photoconductivity was also observed for AgGaS2 thin films [12]. The changes observed in photoconductivity probably occur due to the structural modification introduced by the deposition of the conducting silver layer. By illuminating the films, free charge carriers are generated depending on illumination intensity. The life-time of generated free carriers has a pronounced effect on the conductivity, which could be affected partially by defects and the space charge regions present on the surface or grain boundaries. Thus, the observed photosensivity only in as-grown film probably originates from the layer by layer deposition of the films. The easier injection of generated carriers in such a deposited structure is likely to be the result of the conductive Ag-layer. The photo-and thermally-generated carriers are easily gathered with the applied field in this conducting layer and transported to the probes without encountering so many defects (lifetime killing) and space charge regions responsible for building barrier potentials. 4. Conclusions In this study, to prevent silver (Ag) deficiency in the structure, AgGaS2 thin films were deposited by inserting excess interlayer of silver between sequentially evaporated AgGaS2 layers. Direct growth of AgGaS2 thin films from the stoichiometric sintered single crystalline powder has resulted in thin films deficient in Ag but rich in Ga and S.

Fig. 12. The photocurrent-intensity relation for the as-grown AGS thin film.

H. Karaagac, M. Parlak / Thin Solid Films 519 (2011) 2055–2061

In order to overcome the deficiency of Ag in the structure, double sources were used, namely, one for AgGaS2 single crystal and the other for Ag. During the growth process, excess stack layer was inserted by following AgGaS2/Ag/AgGaS2 deposition sequence. XRD analysis has shown that AGS grows with co-existence of Ag phase and the crystallinity of the structure increases by increasing annealing temperature. Optical band gap values calculated by transmission and reflection were found to have changed between 2.30 and 2.75 eV upon the increase in annealing temperature from 350 to 550 °C. The evaluated values of refractive indexes by envelope method have varied between 1.8 and 2.6 in the studied wavelength region. Finally, photoconductivity measurements taken under different illumination intensities have shown that the segregation of constituent elements resulted in generation of different recombination and sensitizing centers at different energy levels. They may have occurred as a result of the structural modification introduced during the deposition. Acknowledgement This work was supported by Turkish Scientific and Research Council (TUBITAK) under Grant no. 108T019. References [1] B.M. Basol, A. Halani, C. Leidhalm, G. Norsworthy, V.K. Kapur, A. Swatzlander, R. Matson, Prog. Photovolt. Res. Appl. 8 (2000) 227. [2] J.L. Shay, J.H. Wernick, Ternary Chalcopyrite Semiconductors: Growth, Electronic Properties and Applications, Pergamon Press, Oxford, 1975.

2061

[3] Y.S. Murthy, O.M. Hussain, B.S. Naidu, P.J. Reddy, Mater. Lett. 10 (1991) 12. [4] M.A. Contreas, K. Ramanathan, J. Abushama, F. Hasson, D.L. Young, B. Egaas, R. Noufi, J. Word, A. Duda, Prog. Photovolt. Res. Appl. 13 (2005) 209. [5] P.C. Ricci, A. Anedda, R. Corpino, C.M. Carbonaro, M. Marceddu, I.M. Tiginyanu, V.V. Ursaki, J. Phys. Chem. Solids 66 (2005) 1950. [6] H.C. Hsu, Thin Solid Films 419 (2002) 237. [7] F.K. Hopkious, Laser Focus World 31 (1995) 87. [8] F. Rotermund, V. Petrov, F. Noock, Opt. Commun. 185 (2000) 177. [9] A. Chahed, O. Benhelal, S. Laksari, B. Abbar, B. Bouhafs, N. Amrane, Phys. B 367 (2005) 142. [10] B.J. Zhao, S.F. Zhu, F.L. Yu, H.Y. Li, D.Y. Gao, Z.H. Li, Cryst. Res. Technol. 33 (1998) 943. [11] B. Tell, H.M. Kasper, Phys. Rev. B 4 (1971) 12. [12] H.V. Campe, Thin Solid Films 111 (1984) 17. [13] Y. Akaki, S. Kurihara, M. Shirahama, K. Tsurugida, S. Seto, T. Kakeno, K. Yoshino, J. Phys. Chem. Solids 66 (2005) 1858. [14] E. Treser, V. Kramer, J. Cryst. Growth 128 (1993) 664. [15] S.H. Eom, D.J. Kim, Y.-M. Yu, Y.D. Choi, J. Alloy. Compd. 388 (2005) 190. [16] Powder Diffraction File, Joint Committee on Powder Diffraction Standarts, Card 270615. [17] Powder Diffraction File, Joint Committee on Powder Diffraction Standarts, Card 040783. [18] Powder Diffraction File, Joint Committee on Powder Diffraction Standarts, Card 321008. [19] N.M. Shah, J.R. Ray, K.J. Patel, V.A. Kheraj, M.S. Desai, C.J. Panchal, Thin Solid Films 517 (2008) 3639. [20] S.M. Patel, V.G. Kapale, Thin Solid Films 148 (1987) 143. [21] C.C. Garry, MRS Bull. 7 (1998) 28. [22] H. Karaagac, M. Kaleli, M. Parlak, J. Phys. DAppl. Phys. 42 (2009) 165413. [23] L. Artus, Y. Bertrand, L. Soler, J. Phys. CSolid State Phys. 20 (1987) 111. [24] D. Redfield, R.H. Bube, Photoinduced Defects in Semiconductors, Cambridge University Press, New-York, 1996. [25] R.H. Bube, Photoconductivity of Solids, Willey, New-York, 1960. [26] R.H. Bube, Photoelectronic Properties of Semiconductors, Cambridge University Press, Cambridge, 1992.