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Growth of single-phase Cu(In,Al)Se2 photoabsorbing films by selenization using diethylselenide
Thin Solid Films 517 (2009) 2175–2177
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
Growth of single-phase Cu(In,Al)Se2 photoabsorbing films by selenization using diethylselenide M. Sugiyama a,⁎, A. Umezawa a, T. Yasuniwa a, A. Miyama a, H. Nakanishi a, S.F. Chichibu b a b
Department of Electrical Engineering, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda 278−8510, Japan CANTech, Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, 2-1-1 Katahira, Aoba, Sendai 980-8577, Japan
a r t i c l e
i n f o
Available online 6 November 2008 Pacs: 73.20.At 73.30.+y 73.40.Sx 85.30.Hi
a b s t r a c t Selenization growth of purely single-phase, polycrystalline CuIn1 − xAlxSe2 (0 ≤ x ≤ 0.26) alloy films was demonstrated using a less-hazardous metal-organic selenide, diethylselenide [(C2H5)2Se: DESe], by simple thermal annealing of metal precursor. Approximately 2.0 µm thick alloy films exhibited X-ray diffraction peaks originating exclusively from the chalcopyrite structure. Transitions seen in the low temperature photoluminescence spectra were attributed to characteristic donor–acceptor pair emissions of the state-ofthe-art CuIn1 − xAlxSe2 photoabsorbing layers. © 2008 Elsevier B.V. All rights reserved.
Keywords: CIAS Selenization Solar cell materials Diethylselenide
1. Introduction Chalcopyrite Cu(In,Ga)(S,Se)2 alloys are used as the light-absorbing medium of high conversion efficiency (η), low cost, lightweight, and radiation-resistant solar cells. For example, a co-evaporation method provides full flexibility for device optimization, and a high η of 19.9% has been demonstrated using a small-area CuIn1 − xGaxSe2 (CIGS) absorber [1]. Selenization methods using H2Se gas [2,3] or elemental Se vapor [4,5] are the conventional techniques for obtaining highquality CIGS thin films. The bandgap energy of CIGS (x = 0.6) being 1.4 eV should be ideal for use in solar cells. However, growing single phase CIGS (x = 0.6) alloys or CIGS solid solutions of high CuGaSe2 (CGS) molar faction is difficult because of unwanted compositional separation into CuInSe2 (CIS) and CGS, or compositional gradation due to the difference in reaction rates of the two end-point compounds [6]. In addition, the open-circuit voltage and conversion efficiency of CIGS solar cells do not increase proportionally with the bandgap, because of insufficient grain size and crystal quality of CIGS films [7]. CuIn1 − xAlxSe2 (CIAS) alloy is also an alternative candidate for the chalcopyrite solar cells, since the CuAlSe2 (CAS) molar fraction x required for obtaining the ideal bandgap is equal to 0.2−0.35, which is lower than in case of CIGS. Polycrystalline CIAS films have been deposited by a variety of methods such as evaporation [8,9], selenization [10,11], and chemical bath deposition [12]. Moreover, several research groups have recently fabricated CIAS-based solar cells [13,14], and a high η of 16.9%
⁎ Corresponding author. E-mail address: [email protected] (M. Sugiyama). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.10.083
has been demonstrated [14]. Similar to the CIGS growth, selenization technique is an appropriate method for fabricating large-size CIAS solar cells. However, the growth of CIAS films has been difficult because the matrix contains chemically active aluminum [15]. The authors have proposed the use of a less hazardous metal-organic selenide, diethylselenide [(C2H5)2Se, DESe], for growing CIGS films by the selenization method [16,17]. Since DESe decomposes into atomic Se more easily than H2Se gas or Se vapor, reaction rates capable for forming CIS and CGS can be expected. Therefore, formation of segregation-free CIAS alloy films with high x might be possible. This article describes the advantages of using DESe for growing CIAS films by the selenization method. Single-phase polycrystalline films of CIAS (0 ≤x ≤ 0.26) alloys were obtained without additional annealing after selenization. 2. Experiments Sequentially stacked, approximately 670 nm thick metallic Cu/In/ Al layers were used as the starting material, generally called a precursor. They were evaporated on non-intentionally heated Mocoated soda-lime glass (Mo/SLG) substrates. The gross thickness and molar fraction of x, i.e. the Al content of the precursors, were controlled by in-situ monitoring of each layer thickness using a quartz crystal oscillator. The deposition pressure was 1 × 10− 3 Pa. The precursors were selenized using DESe at 515 °C for 90 min under atmospheric pressure. The flow rates of DESe and the N2 carrier gas were 35 μmol/min and 2 L/min, respectively. The details of the selenization equipment have been described in our previous reports [16,17].
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M. Sugiyama et al. / Thin Solid Films 517 (2009) 2175–2177
Fig. 1. Surface SEM images of CIAS films [x = (a) 0.03, (b) 0.19, and (c) 0.26], and (d) crosssectional SEM image of CIAS films (x = 0.26).
The surface morphology was observed with a scanning electron microscope (SEM). Lattice constants and the crystal phase were evaluated by X-ray diffraction (XRD) measurements. The value of x in the CIAS films was determined from the lattice parameter assuming Vegard's law [18]. Photoluminescence (PL) was excited by the 532 nm line of a frequency-doubled quasi-cw Nd:YAG laser (60 mW) and dispersed by a 50 cm focal length grating monochromator. Phasesensitive detection was carried out using a GaAs:Cs photomultiplier and a liquid-N2-cooled Ge photodetector. 3. Results and discussion Approximately 2.0 μm thick single-phase polycrystalline CIAS alloy films were obtained by the present selenization technique without postgrowth annealing [8–10] process. The CIAS films (x = 0.03, 0.19, and 0.26) consist of densely packed, 1–3 μm diameter grains, as shown in the surface SEM images in Fig. 1(a)–(c). In addition, a representative cross-sectional SEM image of CIAS films (x = 0.26) is also shown in Fig. 1(d). The films adhered well to the Mo/ SLG substrate. A representative XRD pattern of a CIAS (x = 0.26) film is shown in Fig. 2(a). Single-phase CIAS solid solution without any secondary phases, such as (In,Al)2Se3 or Cu2Se, was successfully obtained. Phase separation was also examined by XRD measurements. The XRD patterns of the CIAS films near the (312) and (116) diffraction peak angles are shown in Fig. 2(b). The expected diffraction angles for the strain-free CIS and CAS are represented by solid vertical lines. In addition, the symbols indicate the expected diffraction angles for the CIAS in each film [18]. As shown, single-phase CIAS alloys within the entire range of molar fractions were formed; i.e., distinct peaks particular to CIS, CAS were not observed. The diffraction peaks shifted to the higher angle with an increase in x. The lattice constants of a and c axis are also shown in Fig. 2(b). The result indicates the decrease in lattice parameter with the increase in x, according to Vegard's law [18]. The XRD pattern of the CIAS (x = 0.06 and 0.19) films exhibited an unwanted peak due to Al-rich CIAS, in addition to the primary CIAS peak. The inclusion of unwanted CIAS phase may be due to the preferential quick selenization of the Al layer of the precursors. In our method, DESe decomposed into atomic Se more easily than H2Se gas or Se vapor, because the bond strength of Se−C2H5 (243 kJ/mol) is weaker than that of H−Se (276 kJ/mol) or Se = Se (332 kJ/mol) [19,20]. This might result in a faster solid-phase growth of CIAS films. The formation of a single-phase CIAS solid solution without additional annealing clearly indicates the superiority of using DESe.
Fig. 2. XRD patterns of (a) a CIAS (x = 0.26) film and (b) CIAS films as a function of x. The expected angles of (312) and (116) diffraction peaks for CIS and CAS are indicated by vertical lines. The symbols indicate the expected diffraction angles for the CIAS in each film.
PL spectra of CIAS films at 77 K are shown as a function of x in Fig. 3. The symbols indicate the A-exciton transition energy of the CIAS in each film [21]. We note that the spectral discontinuity at 0.88 eV is due to the grating ghost called Wood's anomaly. All
Fig. 3. PL spectra of CIAS films at 77 K as a function of x.
M. Sugiyama et al. / Thin Solid Films 517 (2009) 2175–2177
samples exhibited a broad luminescence band consisting of two or three components. This is characteristic to single-phase CIAS films. Other PL peaks around 1.4−2.7 eV, which are generally emitted from CAS or CIAS alloys of high CAS molar fraction, were completely missing (data not shown). The PL spectra of the CIAS (x = 0.06 and 0.19) films exhibited a board tail higher than the A-exciton transition energy of the primary CIAS. This result may be due to the inclusion of Al-rich CIAS judging from the XRD measurement. The PL bands shown in Fig. 3 are regarded as a superposition of two PL bands originating from a defect-related donor–acceptor pair (DAP) and a free-to-bound recombination, because the higher peak energy showed a blue shift with an increase in the excitation power (data not shown) [22]. Since there are only a limited number of reports on the defect/impurity levels in CIAS alloys, the donor and the acceptor responsible for the emission have not been clearly identified. We note that deep-level emissions assignable to carbon contamination were not observed. The results are similar to the cases of CIGS thin films grown by selenization [16,17] and CIS epilayers grown by metalorganic vapor phase epitaxy [23] using DESe. The PL result may imply that the films are suitable photoabsorbing layer of CIAS-based solar cells. The superior data presented herein indicates that the selenization growth of CIAS alloys using DESe has both industrial (less hazardous, easy to handle, and reasonable cost) and physical advantages (film uniformity, less prone to phase separation, and an ability to grow high quality films). 4. Conclusions In summary, selenization growth of single-phase, polycrystalline CIAS alloy films was demonstrated using DESe without postgrowth thermal annealing. Low temperature PL spectra of the films were dominated by characteristic DAP emission, indicating that the films maybe suitable for the use as photoabsorbing layers in CIAS-based solar cells. These properties indicate that the selenization growth of CIAS alloys using DESe has both industrial (less hazardous, ease of handling, and reasonable cost) and physical (film uniformity, less prone to phase separation, and an ability to grow high-quality films) advantages.
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Acknowledgments The authors thank C. Fujiwara, T. Sato, and T. Abe for their help in the experiments. They would also like to thank Professor T. Iida, Dr. K. Tang, and Dr. B. Fan for stimulating discussions. This work was supported in part by Marubun Research Promotion Foundation, Japan.
References [1] I. Repins, M.A. Contreras, B. Egaas, C. DeHart, J. Scharf, C.L. Perkins, B. To, R. Noufi, Prog. Photovolt. Res. Appl. 16 (2008) 235. [2] T. Wada, T. Negami, M. Nishitani, Appl. Phys. Lett. 62 (1993) 1943. [3] B.M. Basol, V.K. Kapur, A. Halani, C.R. Leidholm, J. Sharp, J.R. Sites, A. Swartzlander, R. Matson, H. Ullal, J. Vac. Sci. Technol. A 14 (1996) 2251. [4] K. Reddy, I. Forbes, R. Miles, M. Carter, P. Dutta, Mater. Lett. 37 (1998) 57. [5] F.D. Dejene, V. Alberts, J. Mater. Sci. 14 (2003) 89. [6] M. Marudachalam, H. Hichri, R. Klenk, R.W. Birkmire, W.N. Shafrman, J.M. Schultz, Appl. Phys. Lett. 67 (1995) 3978. [7] R. Herberholz, R. Herberholz, V. Nadenau, U. Riihle, C. K6ble, H.W. Schock, B. Dimmler, Sol. Energy Mater. Sol. Cells 49 (1997) 227. [8] F. Itoh, O. Saitoh, M. Kita, H. Nagamori, H. Oike, Sol. Energy Mater. Sol. Cells 50 (1998) 119. [9] Dhananjay, J. Nagaraju, S.B. Krupanidhi, Solid State Commun. 127 (2003) 243. [10] E. Halgand, J.C. Bernede, S. Msrsillac, J. Kessler, Thin Solid Films 480 (2005) 443. [11] G. Zoppi, I. Forbes, P. Nasikkar, R.W. Miles, Mater. Res. Soc. Symp. Proc. 1012 (2007) Y12–02. [12] B. Kavitha, M. Dhanam, Mater. Sci. Eng. B 140 (2007) 59. [13] P.D. Paulson, M.W. Haimbodi, S. Marsillac, R.W. Birkmire, W.N. Shafarman, J. Appl. Phys. 91 (2002) 10153. [14] S. Marsillac, P.D. Paulson, M.W. Haimbodi, R.W. Birkmire, W.N. Shafarman, Appl. Phys. Lett. 81 (2002) 1350. [15] S. Chichibu, S. Shirakata, S. Isomura, H. Nakanishi, Jpn. J. Appl. Phys. 36 (1997) 1703. [16] S.F. Chichibu, M. Sugiyama, M. Obasami, A. Hayakawa, T. Mizutani, H. Nakanishi, T. Negami, T. Wada, J. Cryst. Growth 243 (2002) 404. [17] M. Sugiyama, A. Kinoshita, M. Fukaya, H. Nakanishi, S.F. Chichibu, Thin Solid Films 515 (2007) 5867. [18] I.V. Bodnar, I.N. Tsyrelchuk, I.A. Victorov, J. Mater. Sci. Lett. 13 (1994) 762. [19] T.L. Cottrell, The Strength of Chemical Bonds, Butterworths, London, 1954. [20] L. Pauling, The Nature of The Chemical Bond, 3rd ed., Cornell University Press, 1960, p. 562. [21] S. Shirakata, H. Miyake, Jpn. J. Appl. Phys. 41 (2002) 77. [22] E. Zacks, A. Halperin, Phys. Rev. B 6 (1972) 3072. [23] S. Chichibu, Appl. Phys. Lett. 70 (1997) 1840.
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
Growth of single-phase Cu(In,Al)Se2 photoabsorbing films by selenization using diethylselenide M. Sugiyama a,⁎, A. Umezawa a, T. Yasuniwa a, A. Miyama a, H. Nakanishi a, S.F. Chichibu b a b
Department of Electrical Engineering, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda 278−8510, Japan CANTech, Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, 2-1-1 Katahira, Aoba, Sendai 980-8577, Japan
a r t i c l e
i n f o
Available online 6 November 2008 Pacs: 73.20.At 73.30.+y 73.40.Sx 85.30.Hi
a b s t r a c t Selenization growth of purely single-phase, polycrystalline CuIn1 − xAlxSe2 (0 ≤ x ≤ 0.26) alloy films was demonstrated using a less-hazardous metal-organic selenide, diethylselenide [(C2H5)2Se: DESe], by simple thermal annealing of metal precursor. Approximately 2.0 µm thick alloy films exhibited X-ray diffraction peaks originating exclusively from the chalcopyrite structure. Transitions seen in the low temperature photoluminescence spectra were attributed to characteristic donor–acceptor pair emissions of the state-ofthe-art CuIn1 − xAlxSe2 photoabsorbing layers. © 2008 Elsevier B.V. All rights reserved.
Keywords: CIAS Selenization Solar cell materials Diethylselenide
1. Introduction Chalcopyrite Cu(In,Ga)(S,Se)2 alloys are used as the light-absorbing medium of high conversion efficiency (η), low cost, lightweight, and radiation-resistant solar cells. For example, a co-evaporation method provides full flexibility for device optimization, and a high η of 19.9% has been demonstrated using a small-area CuIn1 − xGaxSe2 (CIGS) absorber [1]. Selenization methods using H2Se gas [2,3] or elemental Se vapor [4,5] are the conventional techniques for obtaining highquality CIGS thin films. The bandgap energy of CIGS (x = 0.6) being 1.4 eV should be ideal for use in solar cells. However, growing single phase CIGS (x = 0.6) alloys or CIGS solid solutions of high CuGaSe2 (CGS) molar faction is difficult because of unwanted compositional separation into CuInSe2 (CIS) and CGS, or compositional gradation due to the difference in reaction rates of the two end-point compounds [6]. In addition, the open-circuit voltage and conversion efficiency of CIGS solar cells do not increase proportionally with the bandgap, because of insufficient grain size and crystal quality of CIGS films [7]. CuIn1 − xAlxSe2 (CIAS) alloy is also an alternative candidate for the chalcopyrite solar cells, since the CuAlSe2 (CAS) molar fraction x required for obtaining the ideal bandgap is equal to 0.2−0.35, which is lower than in case of CIGS. Polycrystalline CIAS films have been deposited by a variety of methods such as evaporation [8,9], selenization [10,11], and chemical bath deposition [12]. Moreover, several research groups have recently fabricated CIAS-based solar cells [13,14], and a high η of 16.9%
⁎ Corresponding author. E-mail address: [email protected] (M. Sugiyama). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.10.083
has been demonstrated [14]. Similar to the CIGS growth, selenization technique is an appropriate method for fabricating large-size CIAS solar cells. However, the growth of CIAS films has been difficult because the matrix contains chemically active aluminum [15]. The authors have proposed the use of a less hazardous metal-organic selenide, diethylselenide [(C2H5)2Se, DESe], for growing CIGS films by the selenization method [16,17]. Since DESe decomposes into atomic Se more easily than H2Se gas or Se vapor, reaction rates capable for forming CIS and CGS can be expected. Therefore, formation of segregation-free CIAS alloy films with high x might be possible. This article describes the advantages of using DESe for growing CIAS films by the selenization method. Single-phase polycrystalline films of CIAS (0 ≤x ≤ 0.26) alloys were obtained without additional annealing after selenization. 2. Experiments Sequentially stacked, approximately 670 nm thick metallic Cu/In/ Al layers were used as the starting material, generally called a precursor. They were evaporated on non-intentionally heated Mocoated soda-lime glass (Mo/SLG) substrates. The gross thickness and molar fraction of x, i.e. the Al content of the precursors, were controlled by in-situ monitoring of each layer thickness using a quartz crystal oscillator. The deposition pressure was 1 × 10− 3 Pa. The precursors were selenized using DESe at 515 °C for 90 min under atmospheric pressure. The flow rates of DESe and the N2 carrier gas were 35 μmol/min and 2 L/min, respectively. The details of the selenization equipment have been described in our previous reports [16,17].
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M. Sugiyama et al. / Thin Solid Films 517 (2009) 2175–2177
Fig. 1. Surface SEM images of CIAS films [x = (a) 0.03, (b) 0.19, and (c) 0.26], and (d) crosssectional SEM image of CIAS films (x = 0.26).
The surface morphology was observed with a scanning electron microscope (SEM). Lattice constants and the crystal phase were evaluated by X-ray diffraction (XRD) measurements. The value of x in the CIAS films was determined from the lattice parameter assuming Vegard's law [18]. Photoluminescence (PL) was excited by the 532 nm line of a frequency-doubled quasi-cw Nd:YAG laser (60 mW) and dispersed by a 50 cm focal length grating monochromator. Phasesensitive detection was carried out using a GaAs:Cs photomultiplier and a liquid-N2-cooled Ge photodetector. 3. Results and discussion Approximately 2.0 μm thick single-phase polycrystalline CIAS alloy films were obtained by the present selenization technique without postgrowth annealing [8–10] process. The CIAS films (x = 0.03, 0.19, and 0.26) consist of densely packed, 1–3 μm diameter grains, as shown in the surface SEM images in Fig. 1(a)–(c). In addition, a representative cross-sectional SEM image of CIAS films (x = 0.26) is also shown in Fig. 1(d). The films adhered well to the Mo/ SLG substrate. A representative XRD pattern of a CIAS (x = 0.26) film is shown in Fig. 2(a). Single-phase CIAS solid solution without any secondary phases, such as (In,Al)2Se3 or Cu2Se, was successfully obtained. Phase separation was also examined by XRD measurements. The XRD patterns of the CIAS films near the (312) and (116) diffraction peak angles are shown in Fig. 2(b). The expected diffraction angles for the strain-free CIS and CAS are represented by solid vertical lines. In addition, the symbols indicate the expected diffraction angles for the CIAS in each film [18]. As shown, single-phase CIAS alloys within the entire range of molar fractions were formed; i.e., distinct peaks particular to CIS, CAS were not observed. The diffraction peaks shifted to the higher angle with an increase in x. The lattice constants of a and c axis are also shown in Fig. 2(b). The result indicates the decrease in lattice parameter with the increase in x, according to Vegard's law [18]. The XRD pattern of the CIAS (x = 0.06 and 0.19) films exhibited an unwanted peak due to Al-rich CIAS, in addition to the primary CIAS peak. The inclusion of unwanted CIAS phase may be due to the preferential quick selenization of the Al layer of the precursors. In our method, DESe decomposed into atomic Se more easily than H2Se gas or Se vapor, because the bond strength of Se−C2H5 (243 kJ/mol) is weaker than that of H−Se (276 kJ/mol) or Se = Se (332 kJ/mol) [19,20]. This might result in a faster solid-phase growth of CIAS films. The formation of a single-phase CIAS solid solution without additional annealing clearly indicates the superiority of using DESe.
Fig. 2. XRD patterns of (a) a CIAS (x = 0.26) film and (b) CIAS films as a function of x. The expected angles of (312) and (116) diffraction peaks for CIS and CAS are indicated by vertical lines. The symbols indicate the expected diffraction angles for the CIAS in each film.
PL spectra of CIAS films at 77 K are shown as a function of x in Fig. 3. The symbols indicate the A-exciton transition energy of the CIAS in each film [21]. We note that the spectral discontinuity at 0.88 eV is due to the grating ghost called Wood's anomaly. All
Fig. 3. PL spectra of CIAS films at 77 K as a function of x.
M. Sugiyama et al. / Thin Solid Films 517 (2009) 2175–2177
samples exhibited a broad luminescence band consisting of two or three components. This is characteristic to single-phase CIAS films. Other PL peaks around 1.4−2.7 eV, which are generally emitted from CAS or CIAS alloys of high CAS molar fraction, were completely missing (data not shown). The PL spectra of the CIAS (x = 0.06 and 0.19) films exhibited a board tail higher than the A-exciton transition energy of the primary CIAS. This result may be due to the inclusion of Al-rich CIAS judging from the XRD measurement. The PL bands shown in Fig. 3 are regarded as a superposition of two PL bands originating from a defect-related donor–acceptor pair (DAP) and a free-to-bound recombination, because the higher peak energy showed a blue shift with an increase in the excitation power (data not shown) [22]. Since there are only a limited number of reports on the defect/impurity levels in CIAS alloys, the donor and the acceptor responsible for the emission have not been clearly identified. We note that deep-level emissions assignable to carbon contamination were not observed. The results are similar to the cases of CIGS thin films grown by selenization [16,17] and CIS epilayers grown by metalorganic vapor phase epitaxy [23] using DESe. The PL result may imply that the films are suitable photoabsorbing layer of CIAS-based solar cells. The superior data presented herein indicates that the selenization growth of CIAS alloys using DESe has both industrial (less hazardous, easy to handle, and reasonable cost) and physical advantages (film uniformity, less prone to phase separation, and an ability to grow high quality films). 4. Conclusions In summary, selenization growth of single-phase, polycrystalline CIAS alloy films was demonstrated using DESe without postgrowth thermal annealing. Low temperature PL spectra of the films were dominated by characteristic DAP emission, indicating that the films maybe suitable for the use as photoabsorbing layers in CIAS-based solar cells. These properties indicate that the selenization growth of CIAS alloys using DESe has both industrial (less hazardous, ease of handling, and reasonable cost) and physical (film uniformity, less prone to phase separation, and an ability to grow high-quality films) advantages.
2177
Acknowledgments The authors thank C. Fujiwara, T. Sato, and T. Abe for their help in the experiments. They would also like to thank Professor T. Iida, Dr. K. Tang, and Dr. B. Fan for stimulating discussions. This work was supported in part by Marubun Research Promotion Foundation, Japan.
References [1] I. Repins, M.A. Contreras, B. Egaas, C. DeHart, J. Scharf, C.L. Perkins, B. To, R. Noufi, Prog. Photovolt. Res. Appl. 16 (2008) 235. [2] T. Wada, T. Negami, M. Nishitani, Appl. Phys. Lett. 62 (1993) 1943. [3] B.M. Basol, V.K. Kapur, A. Halani, C.R. Leidholm, J. Sharp, J.R. Sites, A. Swartzlander, R. Matson, H. Ullal, J. Vac. Sci. Technol. A 14 (1996) 2251. [4] K. Reddy, I. Forbes, R. Miles, M. Carter, P. Dutta, Mater. Lett. 37 (1998) 57. [5] F.D. Dejene, V. Alberts, J. Mater. Sci. 14 (2003) 89. [6] M. Marudachalam, H. Hichri, R. Klenk, R.W. Birkmire, W.N. Shafrman, J.M. Schultz, Appl. Phys. Lett. 67 (1995) 3978. [7] R. Herberholz, R. Herberholz, V. Nadenau, U. Riihle, C. K6ble, H.W. Schock, B. Dimmler, Sol. Energy Mater. Sol. Cells 49 (1997) 227. [8] F. Itoh, O. Saitoh, M. Kita, H. Nagamori, H. Oike, Sol. Energy Mater. Sol. Cells 50 (1998) 119. [9] Dhananjay, J. Nagaraju, S.B. Krupanidhi, Solid State Commun. 127 (2003) 243. [10] E. Halgand, J.C. Bernede, S. Msrsillac, J. Kessler, Thin Solid Films 480 (2005) 443. [11] G. Zoppi, I. Forbes, P. Nasikkar, R.W. Miles, Mater. Res. Soc. Symp. Proc. 1012 (2007) Y12–02. [12] B. Kavitha, M. Dhanam, Mater. Sci. Eng. B 140 (2007) 59. [13] P.D. Paulson, M.W. Haimbodi, S. Marsillac, R.W. Birkmire, W.N. Shafarman, J. Appl. Phys. 91 (2002) 10153. [14] S. Marsillac, P.D. Paulson, M.W. Haimbodi, R.W. Birkmire, W.N. Shafarman, Appl. Phys. Lett. 81 (2002) 1350. [15] S. Chichibu, S. Shirakata, S. Isomura, H. Nakanishi, Jpn. J. Appl. Phys. 36 (1997) 1703. [16] S.F. Chichibu, M. Sugiyama, M. Obasami, A. Hayakawa, T. Mizutani, H. Nakanishi, T. Negami, T. Wada, J. Cryst. Growth 243 (2002) 404. [17] M. Sugiyama, A. Kinoshita, M. Fukaya, H. Nakanishi, S.F. Chichibu, Thin Solid Films 515 (2007) 5867. [18] I.V. Bodnar, I.N. Tsyrelchuk, I.A. Victorov, J. Mater. Sci. Lett. 13 (1994) 762. [19] T.L. Cottrell, The Strength of Chemical Bonds, Butterworths, London, 1954. [20] L. Pauling, The Nature of The Chemical Bond, 3rd ed., Cornell University Press, 1960, p. 562. [21] S. Shirakata, H. Miyake, Jpn. J. Appl. Phys. 41 (2002) 77. [22] E. Zacks, A. Halperin, Phys. Rev. B 6 (1972) 3072. [23] S. Chichibu, Appl. Phys. Lett. 70 (1997) 1840.