Thin Solid Films 535 (2013) 57–61
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A study of kesterite Cu2ZnSn(Se,S)4 formation from sputtered Cu–Zn–Sn metal precursors by rapid thermal processing sulfo-selenization of the metal thin films R.A. Wibowo a,⁎, H. Yoo a, A. Hölzing a, R. Lechner b, S. Jost b, J. Palm b, M. Gowtham c, B. Louis c, R. Hock a a b c
Chair for Crystallography and Structural Physics, Friedrich-Alexander-University Erlangen-Nürnberg, Staudtstraße 3, D-91058 Erlangen, Germany AVANCIS GmbH & Co. KG, Otto-Hahn-Ring 6, D-81739 München, Germany Saint-Gobain Recherche, 39 Quai Lucien Lefranc, 93303 Aubervilliers Cedex, France
a r t i c l e
i n f o
Available online 7 December 2012 Keywords: Kesterite Sulfo-selenization X-ray diffraction Intermetallic compounds
a b s t r a c t Cu–Zn–Sn intermetallic thin films were sputtered on Mo-coated soda-lime glass substrates from elemental targets. Samples representing a wide range of compositions around the 2:1:1 kesterite ratio of the Cu–Zn–Sn material system have been investigated. Crystalline phase content and chemical composition of the metal precursors were characterized by X-ray phase analysis and X-ray fluorescence. The metal precursor films were then processed into metal chalcogenides by rapid thermal processing in sulfur ambient with a maximum process temperature around 500 °C. Thin films were investigated by X-ray powder diffraction, X-ray fluorescence and Raman spectroscopy to identify their phase contents as a function of precursor composition and initial intermetallic crystalline phase content. Compositional regions of kesterite crystallization as well as remaining secondary chalcogenide phases were identified. Consequences of the obtained results for the thin film crystallization of Kesterite absorbers for solar cell fabrication by rapid thermal processing of metallic precursors will be discussed. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In the past few years, kesterite-based Cu2ZnSn(S,Se)4 (CZTSSe) thin film photovoltaic has been demonstrating a tremendous increase in its efficiency record which brings a vision on a low-cost and high efficiency solar cell closer to reality. Several kesterite thin film absorber preparations leading to highly efficient cell (>6% efficiency) have employed hydrazine process (10.1%), co-evaporation (9.15%), hot injection (7.2%) thermal evaporation (6.8%), ion beam sputtering (6.7%), and co-evaporation (6.1%) [1–6]. Nevertheless, a kesterite film preparation by two-stage process of metal precursor film deposition followed by chalcogenization is still preferred since it offers economically benign and adaptable approach for large scale production as it has been applied in chalcopyrite-based photovoltaic production line [7]. In conjunction with the efforts to comprehend a kesterite formation in the two-stage process, the influence of Cu–Zn–Sn intermetallic precursor film chemical and phase compositions on the crystalline phase formation of the chalcogenized films has attracted our particular interest. It was reported that Cu6Sn5 and Cu5Zn8 (or CuZn) intermetallic phases along with elemental Sn exist in the precursor films having near kesterite metallic compositional ratio of [Cu]:[Zn]:[Sn] = 2:1:1 regardless of the film preparation methods [8–10]. These precursors may lead to the kesterite and possible unwanted/secondary phase formations after chalcogenization process, for instances Cu2Sn(S,Se)3, ⁎ Corresponding author. Tel.: +49 9131 8525195; fax: +49 9131 8525182. E-mail address:
[email protected] (R.A. Wibowo). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.11.074
Sn(S,Se)2, Zn(S,Se) or Cux(S,Se)y, depending on how much their initial composition deviates from the kesterite metallic ratios [11]. We are not aware of any comprehensive investigations for identifying all phases that may co-exist in the sputtered intermetallic precursors and chalcogenized films under broad composition. In this contribution, a study of the phase formation as a result from chalcogenization of intermetallic precursors is carried out with a specific goal to collect diffractograms as phase analysis libraries over broad compositional range. It is expected that such phase libraries could also be informative to trace any possible secondary phases in the typical kesterite photovoltaic device compositional range of Cu-rich and Zn-poor which are difficult to resolve from kesterite phase e.g. Cu2Sn(S,Se)3 and Zn(S,Se). 2. Experimental details Cu–Zn–Sn multilayer precursor films consisted of 100 alternate elemental layer stacks were sputtered on Mo-coated soda-lime glass substrates at room temperature from three high purity elemental targets. Total precursor film thicknesses were controlled from 750 nm to 1000 nm depending on the desired metallic composition. The sputtering working pressure was optimized at 10 −2 Pa with deposition rates of about 2 Å/s. Visual inspection revealed that as-deposited precursor films showed a typical reflective and bright metallic-like surface. Crystalline phase contents of the precursors were characterized by spatially resolved X-ray diffraction (XRD, Phillips X-Pert Pro-MPD). The compositional range of the precursor films was characterized by X-ray fluorescence (XRF, Bruker AXS SRS
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3400S). The XRF was quantitatively calibrated on a routine basis via inductively coupled plasma optical emission spectroscopy. On the metallic precursors, selenium (Se) capping layers with a thickness of 1700 nm were thermally evaporated. The sulfo-selenization was performed by rapid thermal processing (RTP) of stacked precursor film/thermally evaporated Se in sulfur-containing ambient at a temperature around 550 °C. The as-sulfo-selenized thin films were subsequently investigated by XRD to identify their phase contents as a function of initial precursor composition and crystalline phase content. Phase identification of both precursors and sulfo-selenized films was carried out by Jade 9 software which came with bundled International Committee of Diffraction Database (ICDD) files [12]. All XRD patterns of films were refined using Rietveld refinement with the aid of Topas software for determining reference diffraction patterns of possible intermetallic and chalcogenide phases. Several structure models and space groups (s.g.) were employed in this refinement, including Cu2ZnSn(S,Se)4 (s.g. I-4), Cu2Sn(S,Se)3 (s.g. F-43m), Zn(S,Se) (s.g. F-43m), Cu2 − x(S,Se) (s.g. F-43m), and Sn(S,Se)2 (s.g. P-3 m1) [13]. The lattice parameters of chalcogenides structure models were determined using Vegard's law [14] and the occupational factors of anions (S and Se) for all structure models were varied to allow chalcogenide structure models having various [S]/([S] +[Se]) ratios from 0 to 1. The March Dollase model was used to determine the phase content correctly and a pseudo-Voigt profile function was used to fit the Bragg reflections. A micro-Raman spectroscopy (Horiba Jobin Yvon LabRam micro Raman system) at a wavelength of 633 nm with information depth is below 100 nm was complementary employed for confirming the phases in the sulfo-selenized films as well. 3. Results 3.1. Precursor films The metallic Cu–Zn–Sn precursor films cover a wide compositional range and can generally be mapped into five classifications as summarized in Table 1. Concerning the stoichiometric metal ratio of kesterite [Cu]:[Zn]:[Sn]= 2:1:1, the corresponding metallic composition in precursor films should be 50 at.% Cu, 25 at.% Zn and 25 at.% Sn. Therefore, an elemental excess from this value is termed as “metal-rich” whereas an elemental deficiency is termed as “metal-poor”. A visual description of the precursor compositional ranges is best presented using a Cu–Zn– Sn ternary alloy phase diagram at 180 °C (sketched in black) as shown in Fig. 1 [15]. The superimposed Cu2(S,Se)–Zn(S,Se)–Sn(S,Se)2 phase diagram (sketched in red) is a modification from Refs. [16] and [17] and is presented in metal atomic percent unit, thus considering only the amount of metals. The use of Fig. 1 facilitates some expectations regarding possible chalcogenide phases which form after sulfo-selenization with respect to initial precursor phases and composition. Representative X-ray diffraction patterns of as-deposited precursor films with different compositional range are depicted in Fig. 2. Generally speaking, all precursor films form mostly binary Cu–Zn and Cu–Sn intermetallic alloys. An additional elemental Sn is detected in precursors with certain compositional ranges. There are no any Zn–Sn intermetallic
Fig. 1. Compositional ranges of sputtered metallic precursor films represented on the superimposed ternary Cu–Zn–Sn (black) and the modified metal chalcogenide (red) phase diagrams. Nos. 1, 2, 3, 4 and 5 are Cu2ZnSn(SxSe1−x)4, Cu2ZnSn3S8, Cu4SnS9, Cu2Sn(SxSe1-x)3 and Cu2Sn4S9, respectively. See Table 1 for symbol details.
phases recognized in all precursor films which are in a good agreement with the Zn–Sn binary phase diagram [18]. It is qualitatively identified that the films with compositional range No. 1 exhibit typical apparent cubic Cu5Zn8 (ICDD # 01-071-0397) and weak tetragonal Sn (ICDD # 97-010-6072) reflections. The precursor film phase contents at this compositional range reflect closely the Cu–Zn–Sn equilibrium phase diagram in Fig. 1. However, the phase diagram suggests that another CuZn phase should exist instead of only Cu5Zn8 in this compositional range. It was found that the CuZn (s.g. Pm3m) phase could only be resolved by using Rietveld refinement since the Bragg reflections of CuZn overlap with Cu5Zn8 at 2θ ~ 43° and 62.5°. This finding reveals that the cubic Cu5Zn8 and CuZn naturally co-exist. Additionally, a discrepancy with the Cu–Zn–Sn phase diagram is found after identifying that a Cu6Sn5
Table 1 Summary of precursor film compositional classification along with the respective compositional range. No.
1 2 3 4 5
Compositional classification
Cu-poor, Zn-rich, Sn-poor Cu-rich, Zn-rich, Sn-poor Cu-rich, Zn-poor, Sn-poor Cu-rich, Zn-poor, Sn-rich Cu-poor, Zn-rich, Sn-rich
Symbol
(▲) (x) (○) (□) (●)
Metallic precursor film composition (at. %) Cu
Zn
Sn
40.2–48.3 51.2–69.0 61.4–75.1 54.1 48.3–49.2
27.5–48.0 23.7–38.0 17.7–20.9 20.8 23.2–27.5
10.8–24.1 7.3–16.1 8.4–20.9 25.1 24.1–27.5
Fig. 2. Representative X-ray diffraction patterns of metallic precursor films with different compositional ranges of No. 1 (Cu 40.2 at.%, Zn 48.0 at.% and Sn 11.9 at.%), No. 2 (Cu 62.2 at.%, Zn 27.1 at.% and Sn 10.7 at.%), No. 3 (Cu 75.1 at.%, Zn 12.1 at.% and Sn 12.8 at.%), No. 4 (Cu 54.1 at.%, Zn 20.8 at.% and Sn 25.1 at.%) and No. 5 (Cu 49.2 at.%, Zn 23.2 at.% and Sn 27.5 at.%).
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phase diffraction pattern (ICDD # 00-045-1488) is absent at this precursor compositional range. Reducing more Zn content in the precursor films up to precursor film compositional region No. 2 introduces clearly the monoclinic Cu6Sn5 phase reflections besides Cu5Zn8 (and CuZn) in the absence of elemental Sn reflections. Precursor films with compositional range No. 3 show a clearly recognized cubic Cu41Sn11 intermetallic phase (ICDD # 00-030-0510) on the left shoulder of overlapped Cu6Sn5, Cu5Zn8 and CuZn phase reflection at 2θ ~ 43°. The appearance of the Cu41Sn11 as detected in the precursor films compositional range No. 3 could be interpreted as a direct consequence from a highly excessive Cu amount. Precursor films with compositional range Nos. 4 and No. 5 contain identical phases of CuZn and Cu6Sn5 with an exception of a detected elemental Sn in compositional range No. 5. In these films, some characteristic reflections of Cu5Zn8 phase are completely missing, which deduces that it could totally be absent or be in an extremely weak intensity beyond the sensitivity of X-ray diffractometer. Any elemental Zn reflections are absent throughout precursor compositional ranges.
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Fig. 4. Raman spectra of the Cu-poor, Zn-rich, Sn-rich sulfo-selenized film. The film represents a A1 two-mode behavior of Cu2ZnSnSe4 and Cu2ZnSnS4.
3.2. Sulfo-selenized films Representative X-ray diffraction patterns of sulfo-selenized films with various precursor compositions are depicted in Fig. 3. All films possess identical precursor compositions with the ones presented in Fig. 2. The sulfo-selenized films are polycrystalline multi-phase in nature regardless their precursor compositions. Phase identification results suggest that these films exhibit a polycrystalline kesterite CZTSSe, Zn(S,Se) and Cu2(S,Se) phases. These muliphases of chalcogenides could qualitatively be recognized from the shifts of all Bragg reflections to higher diffraction angles with respect to Cu2ZnSnSe4 (CZTSe, ICDD # 01-070-8930), ZnSe (ICDD # 01-088-2345) and Cu2−xSe (ICDD # 00-006-0680). This shift is an obvious effect from sulfur incorporation into the CZTSe, ZnSe and Cu2−xSe crystal lattices after sulfoselenization, resulting sulfur and selenium-containing multiphases. The quantitative Rietveld refinement demonstrates that the best fitting of all multiphase reflections as depicted in Fig. 4 is satisfied by using chalcogenide structure models with a S occupancy factor of 0.3 yielding identified phases of Cu2ZnSn(S0.3Se0.7)4, Cu2−x(S0.3Se0.7) and Zn(S0.3Se0.7). It can also be seen that a weak hexagonal MoSe2 phase (ICDD # 00-029-0914) is present over the entire sulfo-selenized films as a consequence from the reaction between Mo back contact with selenium at the interface between Mo and kesterite absorber layer and at the end of the sulfo-selenization process. All observable phases in sulfo-selenized films
Fig. 3. Representative X-ray diffractograms of sulfo-selenized films with different metallic precursor film compositional ranges.
maintain their reflections at consistent diffraction angles which suggests that sulfo-selenized films contain an identical [S]/([S]+[Se]). At the Cu-poor, Zn-rich and Sn-poor film composition (compositional range No. 1), Zn(S0.3Se0.7) and Cu2 − x(S0.3Se0.7) are clearly seen apart from main kesterite reflections. As the precursor composition changes from Zn-rich to Zn-poor (compositional range No. 2 to 3), the Cu2 − x(S0.3Se0.7) reflection intensity in sulfo-selenized films increases significantly at the expense of Zn(S0.3Se0.7). Taking a look at the phase diagram of Fig. 1 at the precursor compositional range Nos. 2 and 3, one can see that it is apparent that Cu2 − x(S0.3Se0.7) and the kesterite CZTSSe phases are dominant. These results show that extreme precursor compositional deviations from the kesterite [Cu]:[Zn]:[Sn] metallic ratio of 2:1:1 such as in compositional range Nos. 1–3 lead to the enhancement of the clear and resolvable secondary phases from the main kesterite. The diffraction pattern of sulfo-selenized films with precursor film compositional range No. 4 shows a set of broad reflections of kesterite-like single phase without any noticeable Zn(S0.3Se0.7) and Cu2 − x(S0.3Se0.7) secondary phases. A possible contribution from a Cu2Sn(S,Se)3 phase to these broad reflections is also considered, however, to be very weak since the precursor film compositional range No. 4 is located at the far end of Cu2Sn(S,Se)3-containing region in the Cu2Sn(S,Se)3–Cu2ZnSn(S,Se)4–Sn(S,Se)2 phase triangle. On a basis of Rietveld refinement, most probable suggestion regarding the origin of these broad reflections comes from a contribution of dual kesterite CZTSSe phases as seen by the split of (112), (220/ 204) and (312/116) main reflections. Current sulfo-selenization process is likely to form two kesterite CZTSSe phases of Se-rich CZTSSe and S-rich CZTSSe at this compositional range with each having different [S]/([S] + [Se]) ratio and with identical diffraction pattern of dual-kesterite CZTSSe nanowires [19]. An increase of Sn amount into the precursor compositional range No. 5 tends to promote a formation of trigonal SnSe2 secondary phase (ICDD # 01-089-2939) at Bragg reflection 2θ ~ 14.56°. Phase identification only reveals an unexpectedly SnSe2 (001) preferred orientation. Fig. 4 represents a typical Raman spectrum of the sulfo-selenized film having precursor compositional range No. 5 (Cu 49.2 at.%, Zn 23.2 at.% and Sn 27.5 at.%). The film exhibits two-mode A1 behavior at ~197 cm−1 (CZTSe) and at ~323 cm−1 (Cu2ZnSnS4, CZTS) which can be assigned to Se–Se and S–S vibrations while the cations remain at rest [20]. The other CZTSe Raman peaks are obvious at 171, and 230 cm −1 while the other CZTS Raman peak is clearly detected at 355 cm −1. Compare with pure A1 Se–Se of CZTSe and A1 S-S CZTS [21], this Raman characteristic spectrum of sulfo-selenized film may originate from a shift of the A1 Se–Se mode towards higher frequencies
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as S incorporated into the films after sulfo-selenization process. This two-mode behavior is suggested to be the results of large mass difference between the Se and S as well as substantial difference between the frequencies of the respective phonons [20]. Moreover, the broader Raman peaks of sulfo-selenized films may assign a structural disorder related to random distribution of S and Se atoms in the lattice [21]. A comparison of the A1 peak position and S-content of these Raman measurement results with that of Ref. [21] deduces that the sulfo-selenized films may possess [S]/([S] + [Se]) ratio about 0.25, in a close value with our Rietveld refinement results. Taking into account that a SnSe2 and Zn(S0.3Se0.7) secondary phases are well recognized in the X-ray diffractogram (Fig. 3), a Raman spectrum of SnSe2 at ~191 cm −1 [20] must be overlapped with the A1 Se–Se mode of CZTSe whereas a ZnSe peak at ~250 cm −1 [21] could not be detected using current Raman excitation wavelength. It is seen that the possible existence of any Cu2Sn(S, Se)3 peaks [22] is still difficult to confirm in the sulfo-selenized films. 4. Discussion The detected intermetallic phases in the precursor films indicate that sputtering deposition of Cu–Zn–Sn multilayer has naturally triggered the alloying of Cu, Zn and Sn in a similar manner with that of Ref. [8]. In a circumstance where Cu, Zn and Sn co-exist, Zn is completely consumed by Cu, forming Cu5Zn8 and CuZn intermetallic phases. This is supported by the fact that the precursor phase identification result over entire compositional ranges shows that Cu5Zn8 or CuZn is always present without any remaining elemental Zn phase. The reaction of Sn with Cu to form Cu–Sn intermetallic phases may occur provided that the remaining Cu after reacted with Zn is still in significant amount. Precursors with compositional region Nos. 2, 3 and 4 are good examples to show the formation of Cu–Sn intermetallic phases (Cu6Sn5 and Cu41Sn11). Nevertheless, the as-deposit precursor films seem not to follow strictly the Cu–Zn–Sn equilibrium phase diagram. The existence of Cu5Zn8 (in precursor compositional range Nos. 2 and 3) and Cu41Sn11 (precursor compositional range No. 3) does not satisfy the equilibrium phase contents as described in the Cu–Zn–Sn phase diagram (see Fig. 1). The existence of Cu5Zn8 and Cu41Sn11 is therefore, thought as a result from an interfacial reaction between Cu–Zn and Cu–Sn layers in accordance with the effective heat formation theory [23] and in a good agreement with experimental observations [24,25]. Under our particular sulfo-selenization process, the observed sulfo-selenized film phases are most likely to form from the reaction between intermetallic and elemental phases with Se melts to form metal selenides and succeeded by the diffusion of S gas phase into the metal selenide lattices. A general sequential phase formation in the presence of Se melt is therefore proposed as a reaction between Cu5Zn8, CuZn, Cu6Sn5 and Cu41Sn11 intermetallic phases with Se melt to form Cu2 − xSe, SnSe2 and ZnSe. These metal chalcogenide phases are predicted to form after the decomposition of Cu6Sn5 (415 °C) [26] and melting of Zn (420 °C) [27] as also suggested by in-situ XRD experimental results of CZTS film formation [10]. Sulfurization substitutes partially Se in crystal lattices with S to form Cu2 − x(S0.3Se0.7), and Zn(S0.3Se0.7) as determined from Rietveld refinement result. The detected Cu2(S0.3Se0.7), and Zn(S0.3Se0.7) secondary phases in the as-sulfo-selenized films with extremely Cu and Zn-rich composition (compositional range Nos. 1–3) are strong evidences for confirming this reaction since these phases shall remain unreacted as a normal consequence of excessive elemental Cu and Zn amount from their stoichiometry in kesterite. Surprisingly, the sulfurization stage does not seem to modify SnSe2 into Sn(S,Se)2 or Sn(S0.3Se0.7)2 as expected. Neither qualitative phase identification nor Rietveld refinement recognizes any indications of S incorporation to the SnSe2 phase. At present, the Cu2Sn(S,Se)3 phase formation from Cu2 − x (S0.3Se0.7) and SnSe2 that is expected to precede CZTSSe formation [11] cannot be traced with current ex-situ characterization
technique. At the end of sulfo-selenization process, the formation of kesterite Cu2ZnSn(S0.3Se0.7)4 is most likely accomplished by reaction of Cu2 − x(S0.3Se0.7), Zn(S0.3Se0.7) and SnSe2. Moreover, the presence of SnSe2 secondary phase in the sulfo-selenized film also implies that our current sulfo-selenization process provides a sufficient chalcogen vapor pressure that stabilizes kesterite CZTSSe and hence minimizes Sn loss as normally occurs during kesterite formation [28]. A dual kesterite phase in the precursor compositions 4 and 5 arises an assumption that S diffuses effectively only at the surface of films that produces a S-rich kesterite CZTSSe while the bulk sulfoselenized film is Se-rich one. More investigations on the chalcogen compositional gradient in the sulfo-selenized films are needed to clarify this assumption. Similar studies have been performed in order to optimize kesterite compositional range by narrowing down the precursor film compositional range around the 2:1:1 kesterite metallic ratio. This effort has yielded a kesterite CZTSSe thin film photovoltaic device with a current progress of 6.6% efficiency on 1.34 cm 2 active area as presented elsewhere [29]. 5. Conclusion A study of the sulfo-selenized film phase contents and kesterite formation as a function of precursor initial crystalline phase and composition was performed. The detected intermetallic phases in the precursor films indicate that sputtering deposition of Cu–Zn–Sn multilayer has triggered the alloying of Cu, Zn and Sn mostly as CuxZny and CuxSny. The favorable precursor film composition of Cu-poor, Zn-rich and Sn-rich for fabricating highly efficient kesterite photovoltaic exhibits intermetallic CuZn, Cu6Sn5 and elemental Sn phases. These typical composition and phase content lead to a predominant kesterite Cu2ZnSn(S0.3Se0.7)4 and secondary SnSe2 phases after sulfo-selenization. All phases are formed from sequential reactions of Se melt with intermetallic phases and elemental Sn succeeded by partial substitution of Se by S. The Zn(S0.3Se0.7) and Cu2 − x(S0.3Se0.7) phases could clearly be resolved by extremely deviating the precursor film composition from the near stoichiometric kesterite metallic ratio and this is found to be very informative for tracing any unwanted/secondary phases which may co-exist in the sulfo-selenized films. A possible future direction of this study may employ precursor films which are prepared directly from intermetallic CuZn (brass), Cu6Sn5 (bronze) and elemental Sn sputtering targets or from some metallic layering sequences which are optimized to meet the typical precursor films having CuZn, Cu6Sn5 and Sn phases. References [1] D. Aaron, R. Barkhouse, O. Gunawan, T. Gokmen, T.K. Todorov, D.B. Mitzi, Prog. Photovolt. Res. Appl. 20 (2011) 6. [2] I. Repins, C. Beall, N. Vora, C. DeHart, D. Kuciauskas, P. Dippo, B. To, J. Mann, W.C. Hsu, A. Goodrich, R. Noufi, Sol. Energy Mater. Sol. Cells (2012), http: //dx.doi.org/10.1016/j.solmat.2012.01.008. [3] Q. Guo, G.M. Ford, W.C. Yang, B.C. Walker, E.A. Stach, H.W. Hillhouse, R. Agrawal, J. Am. Chem. Soc. 132 (2010) 17384. [4] K. Wang, O. Gunawan, T. Todorov, B. Shin, S.J. Chey, N.A. Bojarczuk, D. Mitzi, S. Guha, Appl. Phys. Lett. 97 (2010) 143508. [5] H. Katagiri, K. Jimbo, S. Yamada, T. Kamimura, W.S. Maw, T. Fukano, T. Ito, T. Motohiro, Appl. Phys. Exp. 1 (2008) 041201. [6] A. Redinger, D.M. Berg, P.J. Dale, S. Siebentritt, J. Am. Chem. Soc. 133 (2011) 3320. [7] A.A. Rockett, Curr. Opin. Solid State Mater. Sci. 14 (2010) 143. [8] G. Zoppi, I. Forbes, R.W. Miles, P.J. Dale, J.J. Scragg, L.M. Peter, Prog. Photovolt. Res. Appl. 17 (2009) 315. [9] K.H. Kim, I. Amal, Electron. Mater. Lett. 7 (2011) 225. [10] R. Schurr, A. Hölzing, S. Jost, R. Hock, T. Voβ, J. Schulze, A. Kirbs, A. Ennaoui, M. Lux-Steiner, A. Weber, I. Kötschau, H.-W. Schock, Thin Solid Films 517 (2009) 2465. [11] F. Hergert, R. Hock, Thin Solid Films 515 (2007) 5953. [12] International Centre for Diffraction Data (ICDD), 12 Campus Blvd., Newtown Square, PA 19073–3273 U.S.A., 2011. [13] Inorganic Crystal Structure Database (ICSD), Fachinformationzentrum Karlsruhe, Germany and the U.S. Department of Commerce, 2011. [14] L. Vegard, Z. Phys. 5 (1921) 17. [15] C.Y. Chou, S.W. Chen, Acta Mater. 54 (2006) 2393.
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