Thin Solid Films 519 (2011) 5329–5334

Contents lists available at ScienceDirect

Thin Solid Films

j ourna l homepage: www.e lsev ie r.com/ locate / ts f

Formation of CuIn1− xAlxSe2 thin films studied by Raman scattering

J. Olejníček a,c,⁎, C.A. Kamler b, S.A. Darveau a, C.L. Exstrom a, L.E. Slaymaker a, A.R. Vandeventer a,N.J. Ianno b, R.J. Soukup b

a Department of Chemistry, University of Nebraska at Kearney, 905 W. 25th St. Kearney, NE 68849-1150, USAb Department of Electrical Engineering, University of Nebraska-Lincoln, 209N WSEC Lincoln, NE 68588-0511, USAc Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic

⁎ Corresponding author at: Department of ChemistKearney, 905 W. 25th St. Kearney, NE 68849-1150, Ufax: +1 308 865 8399.

E-mail address: olejn@fzu.cz (J. Olejníček).

0040-6090/$ – see front matter. Published by Elsevierdoi:10.1016/j.tsf.2011.02.030

a b s t r a c t

a r t i c l e i n f o

Article history:Received 17 February 2010Received in revised form 7 February 2011Accepted 9 February 2011Available online 24 February 2011

Keywords:Copper Aluminium Indium SelenideChalcopyritesRaman SpectroscopySolar cellsX-ray diffraction

CuIn1−xAlxSe2 (CIAS) thin films (x=0.06, 0.18, 0.39, 0.64, 0.80 and 1) with thicknesses of approximately1 μm were formed by the selenization of sputtered Cu―In―Al precursors and studied via X-ray diffraction,inductively coupled plasma mass spectrometry and micro-Raman spectroscopy at room temperature.Precursor films selenized at 300, 350, 400, 450, 500 and 550 °C were examined via Raman spectroscopy in therange 50–500 cm−1 with resolution of 0.3 cm−1. Sequential formation of InxSey, Cu2−xSe, CuInSe2 (CIS) andCIAS phases was observed as the selenization temperature was increased. Conversion of CIS to CIAS wasinitiated at 500 °C. For all CuIn1−xAlxSe2 products, the A1 phonon frequency varied nonlinearly with respectto the aluminum composition parameter x in the range 172 cm−1 to 186 cm−1.

ry, University of Nebraska atSA. Tel.: +1 308 865 8565;

B.V.

Published by Elsevier B.V.

1. Introduction

In the last 20 years, the chalcopyrite semiconductors of theCuInSe2 (CIS) family have been investigated as extremely promisingmaterials for high-efficiency photovoltaic devices [1–3]. Currently thegreatest energy conversion efficiency of 19.9% has been achieved witha CuIn1−xGaxSe2 (CIGS) (x~0.3) absorber layer that has a band gap of1.2 eV [4]. The band gap currently accepted as the ideal for terrestrialphotovoltaic applications is 1.37 eV [5]. However, with band gapsgreater than 1.3 eV the efficiency of CIGS devices is limited bydegradation of the electronic properties of the CIGS layer, leading tolosses in fill factor and open circuit voltage, and a decrease in thejunction quality factor [6,7]. Moreover, gallium is a scarce andexpensive material. Therefore, attention has been focused onidentifying an alternate solar cell absorber material. One of thesepotential materials is another chalcopyrite, CuIn1−xAlxSe2 (CIAS). TheCIAS band gap can be varied between 1.04 eV for CuInSe2 and 2.68 eVfor CuAlSe2 (CAS). Since the band gap energy of CAS is significantlyhigher than the similar chalcopyrite CuGaSe2 (CGS) with Eg only1.69 eV, CIAS can reach the theoretical ideal bandgap value forterrestrial single-junction solar cells with only 27% Al substitution [8].This was experimentally confirmed in Ref. [9], which shows that

alloying with Al allows the band gap to increase with less variation inlattice spacing than with Ga.

CuIn1−xAlxSe2 thin films or nanocrystals have been preparedusing a variety of methods such as sequential deposition of theconstituents followed by selenization [10–12], co-evaporation[8,9,13], flash evaporation [14], rf magnetron sputtering of mixedbinary selenides [15] and chemical bath deposition [16,17]. To date,the world record in energy conversion efficiency for CIAS thin filmssolar cells 16.9% was reported by Marscillac et al. in 2002 for a samplewith 13% Al substitution with a band gap of 1.16 eV [9]. In order tofurther increase efficiency and overcome this record efficiency value itis necessary to determine the structural and electro-optical propertiesof CIAS thin films. Spectroscopic characterization and correlation toelemental composition and phase behavior is necessary in order tounderstand and develop this material.

In this work CuIn1−xAlxSe2 thin films were studied as a function ofcomposition and selenization conditions in an attempt to investigatethe effect of the aluminum addition on the structural and opticalproperties measured by Raman spectroscopy and X-ray diffraction(XRD).

2. Experimental details

CuIn1−xAlxSe2 thin films were deposited onto soda lime glass (SLG)by a two-step process consisting of DC magnetron sputtering ofcomposite Cu―In―Al metallic precursors, followed by selenization inan Ar and Se atmosphere. All precursor layers were sputtered fromcomposite targets with fixed stoichiometric ratios of Cu0.45In0.55−yAly

Fig. 1. XRD patterns of CIAS samples selenized in an Argon atmosphere with Se vaporsat 550 °C for 60 min.

Fig. 3. Variation in lattice spacing d(112) of CIAS thin films with the Al/(In+Al) ratio.

5330 J. Olejníček et al. / Thin Solid Films 519 (2011) 5329–5334

(y=0.05, 0.15, 0.25, 0.35, 0.45, and 0.55). This limited copper deficiency,Cu/(In+Al)=0.82, was used in order to minimize the formation ofCu2−xSe phases during the selenization process. Prior to deposition thechamber was evacuated to a base pressure of 1×10−4 Pa with a turbomolecular pump. High purity (99.998%) Ar was used as the working gas.The argon flow rate was set to 40 sccm for all samples and the totaldeposition time was approximately 30 min. A deposition rate of 15 nm/min produced thin films with thicknesses near 450 nm before seleniza-tion. (After selenization the thickness approximately doubled.) A set of 6samples with initial ratio x=Al/(In+Al)=0.09, 0.27, 0.46, 0.64, 0.82and 1 was prepared.

All samples were selenized in a quartz halogen lamp heating systemusing a solid pure selenium source (Alfa Aesar, purity: 99.999+%). Theselenium was placed in a graphite container along with the substrate.The container was loaded into a quartz tube, which was evacuated by arotary mechanical pump to a base pressure of less than 1 Pa and thenfilled with 1 atm of pure Argon. (A similar selenization system isdescribed in Ref. [18] in detail.) A thermocouple was embedded in thecontainer and served as the process temperature monitor. A computerwas used to control the temperature profile during the experiment.Because formationofCIAS starts at a temperatureof490 °C [11], thefinalselenization temperature was set to 550 °C. The temperature profileduring selenizationwas the following: a one step ramp and soakprocesswith a 5 min rampdirectly to thefinal temperature 550 °C followed by a60 min soak and controlled cool down to return the container to roomtemperature. The total reaction time was 65 min.

Fig. 2. Detail of the XRD patterns from Fig. 1. The position of the (112) peak for differentvalue of parameter x is illustrated.

The final samples were studied by X-ray diffraction and Ramanspectroscopy. X-ray characterizationwas donewith the Rigaku D/Max-Bdiffractometer with Cu alpha line 1.5406 Å in the 2 thetamode. A Ramanspectroscope (Horiba/Jobin Yvon Lab RAM HR800) with a He―Ne laser(λ=632.81 nm) was used to measure the Raman scattering and theRaman spectra (50–500 cm−1 range, 0.3 cm−1 resolution) of thin filmswere recorded. Inductively coupled plasmamass spectroscopy (ICP-MS)was carried out by Evans Analytical Group LLC to determine theelemental composition of the final material. Surface morphology of thefilms was observed using scanning electron microscopy (SEM) and theJEOL JSM 840A SEM operating at 10 kV was used for this purpose.

3. Results and discussion

3.1. X-ray diffraction

XRD patterns of CIAS samples selenized under a one step processare shown in Fig. 1. From these patterns, it is observed that all thefilms are polycrystalline in nature. The presence of weak peaks such asthe (101), (103), and (211), and especially the well resolved doublet-peaks (220/204) and (312/116), which appear for high values of theparameter x, confirm that the structure is chalcopyrite. Someunwanted phases are presented in the spectra of samples withx=0.39 and 0.80. These phases were identified as hexagonal crystalsof CuSe (see also the hexagonal plates in SEM images — Fig. 5B) andpure Se crystallizing in space group P3_221. A significant shift of allpeaks to higher 2θ positions is observedwith increasing Al ratio due toa decrease in d-spacings and unit cell dimensions. Detailed examples

Table 1Full data measured on CIAS thin films: Initial x ratio (from sputtering targets), final xratio (after selenization), the (112) peak position, FWHM of (112) peak, crystallite size(S), lattice spacing d(112), lattice parameters a and c, tetragonal distortion, freeparameter (u), bond lengths AI―CVI and BIII―CVI in AIBIIIC2

VI chalcopyrite and the RamanA1 phonon frequency.

CIAS-1 CIAS-2 CIAS-3 CIAS-4 CIAS-5 CIAS-6

Initial x 0.09 0.27 0.46 0.64 0.82 1.00x (ICP-MS) 0.06 0.18 0.39 – 0.80 1.00(112) [2θ] 26.77 26.73 26.97 – 27.60 27.83FWHM [°] 0.30 0.29 0.31 – 0.35 0.29S [nm] 27 28 26 – 23 28d (112) [Å] 3.33 3.33 3.30 – 3.23 3.20a [Å] 5.77 5.78 5.72 – 5.63 5.59c [Å] 11.53 11.56 11.46 – 11.08 10.952-c/a 0.001 −0.001 −0.003 0.032 0.043u 0.250 0.250 0.249 0.258 0.261dI–VI [Å] 2.50 2.50 2.48 2.45 2.44dIII–VI [Å] 2.50 2.50 2.48 2.40 2.37A1 [cm−1] 172.4 171.9 173.3 178.8 182.8 185.9

Fig. 4. XRD patterns of CuIn1−xAlxSe2 with x=0.80 after selenization in Se atmosphere(without Ar) at 550 °C for 60 min. The pressure of Se vapors measured in quartz tubewas less than 100 Pa. Typical defects are phase separation and the lack of purecrystallinity.

5331J. Olejníček et al. / Thin Solid Films 519 (2011) 5329–5334

of this shift are presented in Fig. 2, where the normalized (112)reflections are shown. From the graph it can be seen that the shift inthe (112) peak is negligibly small until the composition parameter xreaches a value of at least 0.3. The dependence of lattice spacing d(112) on Al content is shown in Fig. 3. The lattice parameters a and c,which are presented in Table 1, were calculated from peak positionscorresponding to reflections (112), (204) and (312). Their valuesvaried in the range 5.59 Å to 5.77 Å for a and 10.95 Å to 11.56 Å for c,and it seems that their dependence on Al content is nonlinear. Thesimilar deviation from Vegard's law for CIAS thin films has beenreported also in some previous work [10,19,20]. Also, the data on

Fig. 5. SEM images of CuIn1− xAlxSe2 thin films with A) x=0.18 B) x=0.39 C) x=0.80 an

single crystal and bulk published in Refs. [21] and [22] showed thelinear variation. Reddy et al. who studied this nonlinearity for CIASthin films showed that variation from the expected linearity at lowervalues of x indicated that Al is not completely incorporatedsubstitutionally and the unincorporated Al is segregated as Al2O3 atthe surface of thin film [19].

The tetragonal distortion (2-c/a), which is an important parameterin chalcopyrite compounds, is almost zero for CuIn1−xAlxSe2 with alow value of the parameter x and significantly growswith the increasein Al concentration. This fact could be indicative of tension in a crystalstructure and therefore on the crystallite size (S). However thecrystallite size calculated from a full width at half maximum (FWHM)of XRD peak (112) by using the Scherrer formula [23] reveals theminimum for x close to 0.5 and maximum for x close to 0 or 1 (seeTable 1). This result can be explained by comparing Figs. 1 and 4, inwhich the latter presents XRD patterns of CuInAl precursors selenizedin Se vapor (without Ar). The pressure of Se vapors in the graphitecontainer was not measured but the total pressure in the quartz tubewas less than 100 Pa during all time. By comparing these two figures itis possible to see that during selenization both main phases, CIS andCAS, are formed separately and the final CIAS crystals grow by mixingof these two phases. It means that the broadening of (112) peak canbe caused not only by a decrease in crystallinity or by the increase inthe concentration of ordered defect compounds but also by thepresence of other CIAS phases with slightly lower or higherconcentrations of Al. This effect, of course, cannot be present duringthe formation of pure CuInSe2 or CuAlSe2.

3.2. Scanning electron microscopy

The SEM images of four selected samples with x=0.18, 0.39, 0.80and 1.00 are presented in Fig. 5A–D respectively. They reveal that morehomogeneous distribution is exhibited by the samples with low x or

d D) x=1.00. White circles in panel B represent unwanted CuSe hexagonal crystals.

Fig. 6. Raman spectra of CIAS samples selenized in an Argon atmosphere with Se vaporsat 550 °C for 60 min.

Fig. 7. Comparison of the position of normalized peak of the A1 phonon from Fig. 6.

5332 J. Olejníček et al. / Thin Solid Films 519 (2011) 5329–5334

with x close to 1. Samples with x around 0.5 revealed some of theunwanted phases and impurities such as Cu2−xSe and pure un-reactedSe. For example, in Fig. 5B, it is possible to see hexagonal plates of CuSecrystals (the encircled areas), whichwere also identified in the XRD andRaman spectra (Figs. 1 and 6). The easily visible white nanorods inFig. 5C (also confirmed by XRD) represent the rest of the Selenium,which did not react with precursors. An excess of selenium was alsoconfirmedby ICP-MS in all samples regardless of Al content (see Table2)and is probably a consequence of the temperature drop at the end of theselenization process. Unreacted selenium in Se vapor condenses in anAratmosphere on the surface of each sample immediately when thetemperature drops below 221 °C. This problem can probably be solvedby evacuating the tube before the temperature drops below this criticalvalue and will be a subject of further experiments. Another unwantedeffect thatwas noticed after the selenization procedurewas a significantloss of Al. Before selenization the Al/(Cu+In+Al) ratio was 0.05, 0.15,0.25, 0.35, 0.45 and 0.55 after selenization this ratio was almost 30%lower. A similar effectwithAl losswas reported in a previous paper [17].

3.3. Raman spectroscopy

The Raman spectra of CuIn1−xAlxSe2 thin films were recorded atroom temperature and the results are presented in Fig. 6. According tothe selection rules of Raman scattering, by excitation with a verticallypolarized laser beam, only the A1, B1, and B2 modes can be observed.The most intense line within 172–186 cm−1 could be assigned to theA1 mode which is the strongest mode generally observed in theRaman spectra of AIBIIIC2VI chalcopyrite compounds. This mode resultsfrom the motion of the CVI atom with the AI and BIII atoms remainingat rest and its frequency is

ω =

ffiffiffiffiffiffiffiffiffikMC

;

sð1Þ

Table 2Chemical composition of CuIn1− xAlxSe2 thin films measured by ICP-MS.

CIAS-1 CIAS-2 CIAS-3 CIAS-4 CIAS-5 CIAS-6

Cu [%] 20.3 19.8 23.0 – 22.6 26.3In [%] 24.6 16.9 11.3 – 3.8 –

Al [%] 1.5 3.8 7.3 – 14.6 20.7Se [%] 53.6 59.5 58.4 – 59.0 53.0Al/(In+Al) 0.06 0.18 0.39 – 0.80 1.00Cu/(In+Al) 0.78 0.96 1.24 1.23 1.27Al/(Cu+In+Al) 0.03 0.09 0.18 0.36 0.44

where k is the force constant and MC is the mass of the CVI atom [24].There are two more very weak peaks at 212 and 232 cm−1

corresponding to the B2(TO) and B2(LO) modes of CuInSe2 [25]. Asimilar B2 phonon corresponding to the CuAlSe2 chalcopyrite, with atheoretical position between 350 and 400 cm−1 [25], was not observed.In the case of the CuAlSe2 thin film, an additional weak signal around243 cm−1 was detected. Roa et al. [26] and Gebicky et al. [27]interpreted this signal as a phonon of E and F symmetry, respectively,while Eifler et al. [28] considers this signal as an indication of possiblecrystal imperfections or impurities from mixed crystal growth. Insamples with x=0.39 and 0.64 it is possible to find a signal withfrequency 258 cm−1. This signal is related to CuSe contamination and itwas reported in some earlier work that it is possible to remove thiscontamination by further thermal annealing techniques [29,30].Appearance of this signal in the Raman spectra is consistent withresults providedbyXRDand SEMand it is probably a consequence of thefact that formation of CIAS or CAS phases starts at a temperature of490 °C [11], while the formation of CIS and CuSe phases starts almostimmediately after reaching the melting point of selenium, 221 °C. Thesame signal 258 cm−1 did not appear in the first two samples becausethe initial ratio of Cu to In was less than or very close to one and almostall the copper was immediately employed to build the CuInSe2 phase. Adetailed study of influence of the initial ratio of Cu/In on the Ramansignal was presented in Ref. [31].

The normalized peak of A1 phonons is depicted in Fig. 7. The A1

phonon frequency reveals a significant shift with the increase ofAl/(In+Al) content from172 cm−1 (A1-mode frequency of CuInSe2) to186 cm−1 (A1-mode frequency of CuAlSe2). The final shift is in good

Fig. 8. A1 phonon frequency as a function of Al content.

Fig. 9. A1 phonon frequency as a function of bond lengths dIII–VI. Fig. 11. Deconvolution of the signal corresponding to 500 °C from Fig. 10.

5333J. Olejníček et al. / Thin Solid Films 519 (2011) 5329–5334

agreement with all previous published studies focused on Ramanspectroscopy of CuAlSe2 material [26–28,32,33]. From Fig. 8 it can beseen that for xb0.4 the shift of theA1 band is negligibly small and aswellas the lattice costants a and c, or plane spacing d, does not follow thelinear Vegard law. The similar nonlinear shift in A1mode frequencywasnoticed by Chuang-Ming et al. [34] on thin films of CuIn1−xGaxSe2. Theother authors who studied A1 phonon frequency as a function of Gacontent in CuIn1−xGaxSe2 thin films or nanocrystals noticed only alinear shift [30,35,36]. No similar study focused on CuIn1−xAlxSe2 hasyet beenpublished. Because the frequency of A1mode is given by Eq. (1)and because themass of CVI atom in all CuIn1−xAlxSe2 samples remainsthe same, the force constant k must be slightly changed during gradualsubstitution of Al for In. As shown in Table 1, the length of the BIII―CVI

bond obtained from Abrahams-Bernstein relations [37] is decreasingwith increasing Al ratio. This is due to the fact that the Al―Se bond isstronger than In―Se and it was observed that variation of this bondlength is also nonlinear, like the lattice constants and A1 phononfrequencies. The relationship between the bond length BIII―CVI and theRaman A1 mode is illustrated in Fig. 9. This relationship is almostperfectly linear.

The temperature evolution of the selenization process is demon-strated in Fig. 10 for a sample with x=0.64. All spectra were taken onthe same sample after repetitive selenization at 300, 350, 400, 450, 500and 550 °C. Selenization was done in Se atmosphere (without argon)and, after each step, the sample was cooled to room temperature,removed from graphite container, a Raman spectrum was taken, the

Fig. 10. Raman spectra of CuIn1− xAlxSe2 thin films with x=0.64 after repetitiveselenization in Se vapors for increasing temperature.

sample with a new portion of Se granules was inserted back into agraphite container and selenized at a new temperature Tn=Tn−1+50,where Tn−1 was the selenization temperature of the previous step. Thereaction time for each step was 30 min, which means that the totalselenization time for this measured sample, after reaching the finaltemperature of 550 °C, was 3 h. As can be seen in Fig. 10 the sequentialformation of InxSey, CuSe, CIS and CIAS phases with increasingtemperature was observed. Formation of InxSey, CuSe and CIS wasinitiated almost immediately when solid Se granules were evaporated.After the second step of selenization at a temperature of 350 °C, theInxSey phase completely disappears because it is partially converted intochalcopyrite CuInSe2 (by reactionwithCu2Se), and partially into ternaryselenide (Al,In)2Se3 which also serves as a reaction partner for Cu2Se, toform the quaternary chalcopyrite Cu(In,Al)Se2 according to 1/2Cu2Se+1/2(Al,In)2Se3→Cu(In,Al)Se2 [11]. All these results are in very goodagreement with real-time XRD measurements published by Jost et al.[11]. Conversion of CIS to CIAS was initiated at 500 °C and from thecorresponding graph in Fig. 11 it is possible to see that both phases formseparately. Dual phase formation of CIS and CAS under these conditionswas common in samples with an Al/(In+Al) ratio close to 0.5.Homogeneous CIAS films from these precursors could be formed at550 °C, but with a processing time of at least 60 min. However, forsamples close to CIS or CAS stoichiometry, the processing time forsingle-phase formation was closer to 30 min. Another positive effect oflong time annealing can be noticed by comparing Figs. 6 and 10. Forsamples with x close to 0.5, after 60 min of annealing the CuSe phase isstill present in significant quantity (see the two middle lines in Fig. 6)while for the sample with the same composition of initial precursors,after 3 h of selenization and annealing, this unwanted phase is absent(top line in Fig. 10).

4. Conclusion

Thin films of CuIn1−xAlxSe2 with a greatly varying compositionrange were deposited on soda-lime glass by magnetron sputtering ofmetallic precursors followed by selenization in an Ar+Se atmosphere.Analysis by XRD confirmed that all layers are crystalline with thechalcopyrite structure and lattice constants vary nonlinearly withcomposition parameter x. Raman scattering was performed at roomtemperature and it was discovered that the A1 mode frequencyincreases from 172 cm−1 (CIS) to 186 cm−1 (CAS) and its dependenceon Al/(In+Al) ratio is also nonlinear. Based on these two results it wasfound that it is possible to describe the A1 mode frequency as a linearfunction of lattice parameters. Sequential formation of InxSey, Cu2−xSe,CIS and CIAS phases was observed from temperature resolved Ramanspectra, and conversionof CIS to CIASwas initiated at 500 °C. Dual phase

5334 J. Olejníček et al. / Thin Solid Films 519 (2011) 5329–5334

formation was observed in samples with an Al/(In+Al) ratio close to0.5.

Acknowledgement

This work was supported by the U.S. Department of Energy, Officeof Science (Grant No. DE-FG02-06ER64235) and Office of Energy andRenewable Energy (Grant No. DE-FG3608GO88007) and by theNebraska Research Initiative Program (NRI).

References

[1] A. Rockett, R.W. Birkmire, J. Appl. Phys. 70 (1991) R81.[2] C. Guillen, J. Herrero, Sol. Energ. Mater. 23 (1991) 31.[3] U. Rau, H.W. Schock, Appl. Phys. A-Mater. 69 (1999) 131.[4] I. Repins, M.A. Contreras, B. Egaas, C. DeHart, J. Scharf, C.L. Perkins, B. To, R. Noufi,

Prog. Photovoltaics Res. Appl. 16 (2008) 235.[5] M.A. Contreras, K. Ramanathan, J. AbuShama, F. Hasoon, D.L. Young, B. Egass, R.

Noufi, Prog. Photovoltaics Res. Appl. 13 (2005) 209.[6] W.N. Shafarman, R. Klenk, B.E. McCandless, J. Appl. Phys. 79 (1996) 7324.[7] U. Rau, M. Schmidt, A. Jasenek, G. Hanna, H.W. Schock, Sol. Energy Mater. Sol. Cells

67 (2001) 137.[8] P.D. Paulson, M.W. Haimbodi, S. Marsillac, R.W. Birkmire, W.N. Shafarman, J. Appl.

Phys. 91 (2002) 10153.[9] S. Marsillac, P.D. Paulson, M.W. Haimbodi, R.W. Birkmire, W.N. Shafarman, Appl.

Phys. Lett. 81 (2002) 1350.[10] J. López-García, C. Guillén, Thin Solid Films 517 (2009) 2240.[11] S. Jost, F. Hergert, R. Hock, M. Purwins, R. Enderle, Phys. Status Solidi A 203 (2006)

2581.[12] J.H. Yuna, R.B.V. Chalapathy, J.C. Lee, J. Song, K.H. Yoon, Solid State Phenom. 124–

126 (2007) 975.[13] Y.B.K. Reddy, V.S. Raja, Phys. B 381 (2006) 76.

[14] K. Srinivas, J.N. Kumar, G.H. Chandra, S. Uthanna, J. Mater. Sci. Mater. Electron. 17(2006) 1035.

[15] B. Munir, R.A. Wibowo, E.S. Lee, K.H. Kim, J. Ceram. Process Res. 8 (2007) 252.[16] B. Kavitha, M. Dhanam, Mater. Sci. Eng. B 140 (2007) 59.[17] C.L. Exstrom, J. Olejnicek, S.A. Darveau, A. Mirasano, D.S. Paprocki, M.L. Schliefert,

M.A. Ingersoll, L.E. Slaymaker, R.J. Soukup, N.J. Ianno, C.A. Kamler, Mater. Res. Soc.Symp. Proc. 1165 (2009)8 1165-M05-03.

[18] F.O. Adurodijaa, J. Songa, S.D. Kimb, S.H. Kwonc, S.K. Kima, K.H. Yoona, B.T. Ahnc,Thin Solid Films 333 (1999) 13.

[19] Y.B.K. Reddy, V.S. Raja, B. Sreedhar, J. Phys. D Appl. Phys. 39 (2006) 5124.[20] F. Itoh, O. Saitoh, M. Kita, H. Nagarnori, H. Oike, Sol. Energ. Mater. Sol. Cell 50

(1998) 119.[21] W. Gebicky, M. Igalson, W. Zajac, R. Trykozko, J. Phys. D Appl. Phys. 23 (1990) 964.[22] I.V. Bodnar, I.N. Tsyrelchuk, I.A. Victorov, J. Mater. Sci. Lett. 13 (1994) 762.[23] A.L. Patterson, Phys. Rev. 56 (1939) 978.[24] I.V. Bodnar, L.V. Golubev, V.G. Plotnichenko, E.A. Smolyaninova, Phys. Status Solidi

B 105 (1981) K111.[25] F.W. Ohrendorf, H. Haeuseler, Cryst. Res. Technol. 34 (1999) 339.[26] L. Roa, J.C. Chervin, A. Chevy, M. Davila, P. Grima, J. Gonzáles, Phys. Status Solidi

198 (1996) 99.[27] W. Gebicki, J. Filipowicz, R. Bacewicz, J. Phys. Condens. Matter 8 (1996) 8695.[28] A. Eifler, E.A. Kudritskaya, I.V. Bodnar, V. Riede, J. Phys. Chem. Solids 64 (2003)

1983.[29] H. Miyazaki, R. Mikami, A. Yamada, M. Konagai, Jpn. J. Appl. Phys. 43 (2004) 4224.[30] J. Olejnicek, C.A. Kamler, A. Mirasano, A.L. Martinez-Skinner, M.A. Ingersoll, C.L.

Exstrom, S.A. Darveau, J.L. Huguenin-Love, M. Diaz, N.J. Ianno, R.J. Soukup, Sol.Energ. Mat. Sol. C. 94 (2010) 8.

[31] J.H. Park, I.S. Yang, H.Y. Cho, Appl. Phys. A 58 (1994) 125.[32] Yu.N. Azhnyuk, V.V. Artamonov, I.V. Bodnar, J. Appl. Spectrosc. 43 (1985) 1276.[33] S. Chichibu, A. Kamata, J. Appl. Phys. 77 (1995) 5470.[34] C.M.Xu.Y. Sun, L. Zhang, Y.M. Xue, Q. He, H.T. Liu, Chin. Phys. Lett. 23 (2006) 1002.[35] S. Theodoropoulou, D. Papadimitriou, N. Rega, S. Siebentritt, M.Ch. Lux-Steiner,

Thin Solid Films 511–512 (2006) 690.[36] S. Roy, P. Guha, S.N. Kundu, H. Hanzawb, S. Chaudhuri, A.K. Pal, Mater. Chem. Phys.

73 (2002) 24.[37] S.C. Abraham, J.L. Bernstein, J. Chem. Phys. 52 (1970) 5607.