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Si heterodiodes
Materials Science and Engineering B65 (1999) 150 – 152 www.elsevier.com/locate/mseb
The influence of hydrogen passivation of silicon on the photocurrent of CdS/Si heterodiodes B. Ullrich a,*, T. Lo¨her b, Y. Segawa a, T. Kobayashi c a
Photodynamics Research Center, The Institute of Physical and Chemical Research (RIKEN), 19 -1399 Koeji, Nagamachi, Aoba-ku, Sendai 980 -0868, Japan b Department of Chemistry, Uni6ersity of Tokyo, 7 -3 -1 Hongo, Bunkyo-ku, Tokyo 113, Japan c Department of Physics, Uni6ersity of Tokyo, 7 -3 -1 Hongo, Bunkyo-ku, Tokyo 113, Japan Received 16 February 1999
Abstract A CdS/p-Si:H heterojunction is formed by the evaporation of CdS on a hydrogen passivated Si wafer. It is found that the substrate passivation procedure has a strong influence on the photocurrent properties of the CdS/p-Si interface. In fact, in addition to the photoresponse in the red and infrared spectral ranges, the CdS/p-Si:H heterodiode also reveals photocurrent in the blue and green. This is in contrast to CdS/p-Si and CdS/p-InP devices, which do not exhibit photocurrent in the absorption region of CdS. The positive influence of the passivation on the optoelectronic properties of the CdS/p-Si:H heterodiode is explained by the prevention of interface reactions during the formation of the device. The dependence of the photocurrent of the CdS/p-Si:H heterodiode on an applied bias gives evidence of a homojunction-like behavior. © 1999 Published by Elsevier Science S.A. All rights reserved.
Utilizing the well known intrinsic n-type conductivity of the II-VI compound semiconductor CdS, the CdS/pSi heterojunction (HJ) was intensively studied with regard to the applicability to solar cells [1 – 5]. In spite of the huge lattice mismatch of : 7%, solar power
Fig. 1. Current – voltage characteristic of the CdS/p-Si:H heterodiode in the dark. The lines are guides for the eyes. * Corresponding author. Fax: +81-22-2282010. E-mail address: [email protected] (B. Ullrich)
conversion efficiencies of up to 11% were achieved [5–7], moving the CdS/Si solar cell into a competitive range in comparison with other photovoltaic systems [8]. Surprisingly, the photodetecting features of the CdS/p-Si HJ have not been extensively studied. Since the absorption region of CdS starts at around 500 nm, the CdS/p-Si HJ ought to have the potential to produce photodiodes with high sensitivity to blue and green light. However, experimental evidence points to extreme difficulties in obtaining CdS/p-Si HJ’s with photocurrent in the absorption region of CdS [5,7,9]. In this paper we report on a CdS/p-Si photodiode with pronounced photocurrent in the blue and green range of the spectrum. The HJ is formed by the evaporation of CdS powder (99.999%) from a Langmuir cell heated to a temperature of 750°C. The powder is evaporated for 60 min at a pressure of 0.4 × 10 − 4 Pa while keeping the temperature of the Si substrate at 300 K. The deposited film thickness is : 1 mm. The substrate used is a commercial (111) Si(B) wafer with doping concentration and thickness of : 1017 cm − 3 and 0.42 mm, respectively. The wafer is hydrogen passivated by the RCA® etching procedure, providing a monomer hydrogen terminated surface. Hence, we call the sample CdS/p-Si:H HJ. In
0921-5107/99/$ - see front matter © 1999 Published by Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 9 ) 0 0 1 8 3 - X
B. Ullrich et al. / Materials Science and Engineering B65 (1999) 150–152
Fig. 2. Spectra of the short-circuit photocurrent of (a) CdS/p-Si; (b) CdS/p-InP and (c) CdS/p-InP heterodiodes formed by evaporation, spray pyrolysis and laser ablation, respectively. Curve (d) shows the absorption coefficient of thin film CdS on quartz. Curve (a) is taken from Ref. [9].
Fig. 3. Short-circuit photocurrent of the CdS/p-Si:H heterodiode.
order to ascertain the rectification of the heterodiode by current–voltage measurements electric contacts are realized by silver paste dots on the surface and the back of the sample. The rectifying current – voltage characteristic of the CdS/H-Si diode is shown in Fig. 1. By means of X-ray analysis it is established that the CdS film exhibits wurtzite structure with a perpendicularly oriented crystallographic axis with respect to the film surface. For the photocurrent measurements the same contacts are used and the CdS surface is illuminated by monochromatic light with an intensity of : 100 mW cm − 2. The photocurrent is detected using a lock-in amplifier by chopping the impinging light at 278 Hz. All data presented refer to room temperature. A typical photocurrent spectrum of a CdS/p-Si HJ formed by evaporation is shown by curve (a) in Fig. 2. For comparison with other heterojunctions formed by CdS on small bandgap materials, curves (b) and (c) reveal the photocurrent spectra of CdS/p-InP HJ’s
151
formed by spray pyrolysis and laser ablation, respectively. The CdS/p-InP HJ was expected to represent a ‘perfect HJ’ with appealing optoelectronic features due to the marginal lattice mismatch of 0.3% [10]. In addition, like CdS, InP has a direct fundamental transition. However, according to curves (b) and (c) in Fig. 1, the CdS/InP samples do not show a more expanded range of photosensitivity than curve (a). Apparently, irrespective of the preparation method, lattice match conditions and the transition nature of the substrate, the deposition of CdS on small bandgap materials results in samples without photocurrent in the absorption region of CdS. The same phenomenon was observed with CdSe/Si samples [9]. Curve (d) shows the absorption coefficient of CdS evaluated from the transmission of an evaporated film on quartz. A greatly contrasting spectral response is achieved with the CdS/p-Si:H HJ as demonstrated in Fig. 3. The response shows a pronounced peak at 500 nm, which corresponds to the fundamental transition of wurtzite crystalline CdS at 300 K [11]. We believe that the minimum in the photocurrent around 570 nm is caused by the crossover of the absorption process from CdS to Si. This clear evidence of the contribution of CdS absorption to the photocurrent of the CdS/p-Si:H HJ indicates a strong influence of hydrogen passivation on the optoelectronic features of the HJ. It is known that the hydrogen passivation blocks interfacial reactions by saturation of dangling bonds on the Si surface [12], whereas on the other hand, the growth of ZnS on unpassivated Si is characterized by an initial adsorption of a sulfur layer [13]. We should stress at this point that only electron-hole pairs generated in the depletion area or within a diffusion length of the depletion area contribute to the current flow of a diode. Therefore, in the range of CdS absorption, CdS/p-Si and CdS/p-InP junctions exhibit photocurrent only if the depletion width in the CdS film is comparable to that in the Si substrate [2], or, at least, the diffusion length of the minority carriers in the CdS film is long enough to reach the depletion region in the Si substrate [9]. Obviously, the above conditions are not satisfied by the samples in Fig. 2 and it is very likely that the lack of photocurrent below 500 nm is caused by an interfacial sulfur layer. However, besides interfacial reactions, no interfacial spike is expected in the conduction band of the CdS/p-Si HJ [2,3,7] and the photocurrent dependence on the depletion width (W) ought to behave according to the following expression [14] Iph 8 (1− exp(− aW)),
(1)
where a is the absorption coefficient. Assuming that the donor and acceptor concentrations in CdS and Si are approximately the same, i.e. ND, CdS : NA, Si =N, we write [15]
152
W=
'
B. Ullrich et al. / Materials Science and Engineering B65 (1999) 150–152
4oj (V − V +Vloss), qN bi
(2)
where oj is the junction permittivity, q is the elementary charge, Vbi is the built-in potential, V is the applied bias and Vloss is the dissipated voltage due to various reasons, such as the arrangement of the contacts, sample size and voltage drops at the contacts. The signs of V and Vloss are positive for forward bias and negative for reverse bias. Fig. 4 (a) and (b) show a comparison between Eqs. (1) and (2) and the measurements at 500 and 800 nm, for forward and reverse bias, respectively. The following values are used to fit the data: a(500 nm): a(800 nm)= 1×104 cm − 1, N =1017 cm − 3, oj =3.7 (which corresponds to the ‘reduced’ value of or, Si =11.9 and or, CdS = 5.4 [15]), Vbi =0.4 V and Vloss =0.5 V. We want to emphasize that the similarity of the voltage dependence of the photocurrent at 500 and 800 nm on the backward bias in Fig. 4 (b) is clear evidence that the extension of the CdS depletion width is comparable to that in the Si substrate. Hence, the CdS/p-Si:H heterodiode represents an abrupt HJ, i.e. a HJ with a homojunction-like depletion region, and the electric field is not wasted by the presence of interfacial imperfections highlighted in previous work on CdS/p-InP HJ’s [16]. Furthermore, Fig. 4 (b) shows the influence of the minority carriers. The signal rises more slow at 500 nm than at 800
nm due to the clearly smaller mobility of the holes (B50 cm2 V − 1 s − 1) in CdS than that of electrons in Si (1300 cm2 V − 1 s − 1), which determines the photocurrent at these two wavelengths, respectively. On the other hand, in forward direction (Fig. 4 (a)), the photocurrent is composed by majority and minority carriers of both joining materials and no difference is observed for differently polarized voltages. In conclusion, it is demonstrated that the CdS/p-Si:H HJ shows a blue enhanced photocurrent in contrast to CdS/p-Si and CdS/p-InP HJ’s. The sensitivity to blue light of the CdS/p-Si:H HJ is attributed to the avoidance of interfacial reactions, allowing the formation of a depletion region with similar widths in film and substrate. These results show that the CdS/p-Si:H HJ is a technologically attractive hetero-pairing for the creation of photodiodes applicable in the blue and green spectral range.
Acknowledgements B.U. would like to thank Dr H. Nguyen Cong, Professor P. Chartier and Dr H. Sakai for the samples formed by spray pyrolysis and laser ablation. The work is partly supported by the Research for the Future of the Japan Society for the Promotion of Science to B.U. and T.K.
References
Fig. 4. Dependence of the photocurrent of the CdS/p-Si:H heterodiode on the applied bias in (a) forward and (b) backward direction at 500 and 800 nm. The lines are calculated by Eqs. (1) and (2).
[1] H. Okimura, M. Kawakami, Y. Sakai, Jpn. J. Appl. Phys. 6 (1967) 908. [2] H. Okimura, R. Kondo, Jpn. J. Appl. Phys. 9 (1969) 274. [3] F.M. Livingstone, W.M. Tsang, A.J. Barlow, R.M. De La Ru, W. Duncan, J. Phys. D 10 (1959) 1977. [4] C. Coluzza, M. Garozzo, G. Maletta, D. Margadonna, R. Tomaciello, Appl. Phys. Lett. 37 (1980) 569. [5] E. Scafe´, G. Maletta, R. Tomaciello, P. Alessandrini, A. Camanzi, L. de Angelis, F. Galluzzi, Solar Cells 10 (1983) 17. [6] R.R. Arya, P.M. Sarro, J.J. Loferski, Appl. Phys. Lett. 41 (1982) 355. [7] T. Hayashi, T. Nishikura, K. Nishimura, Y. Ema, Jpn. J. Appl. Phys. 28 (1989) 1174. [8] A.L. Fahrenbruch, R.H. Bube, Fundamentals of Solar Cells, Chapter 1, Academic Press, New York, 1983, p. 17. [9] P.A. Kovalenko, V.A. Korotkov, L.M. Panasjuk, Proc. Int. Conf. on Physics and Chemistry of Semicond. Heterojunctions, 1971, Akademiai Kiado´, Budapest, 1971, p. 363. [10] S. Wagner, J.L. Shay, K.J. Bachmann, E. Buehler, Appl. Phys. Lett. 26 (1975) 229. [11] V.V. Sobolev, V.I. Donetskikh, E.F. Zagainov, Sov. Phys. Semicond. 12 (1978) 646. [12] D.L. Smith, Thin-Film Deposition, Chapter 6, McGraw-Hill, New York, 1995, p. 241. [13] C. Maierhofer, S. Kulkarni, M. Alonso, T. Reich, K. Horn, J. Vac. Sci. Technol. B 9 (1991) 2238. [14] J. Singh, Optoelectronics, Chapter 6, The McGraw-Hill, Singapore, 1996, p. 258. [15] S.M. Sze, Semiconductor Devices, Physics and Technology, Chapter 3, Wiley, New York, 1985, p. 80. [16] B. Ullrich, N.M. Dushkina, H. Ezumi, S. Keitoku, T. Kobayashi, Solid State Comm. 103 (1997) 635.
The influence of hydrogen passivation of silicon on the photocurrent of CdS/Si heterodiodes B. Ullrich a,*, T. Lo¨her b, Y. Segawa a, T. Kobayashi c a
Photodynamics Research Center, The Institute of Physical and Chemical Research (RIKEN), 19 -1399 Koeji, Nagamachi, Aoba-ku, Sendai 980 -0868, Japan b Department of Chemistry, Uni6ersity of Tokyo, 7 -3 -1 Hongo, Bunkyo-ku, Tokyo 113, Japan c Department of Physics, Uni6ersity of Tokyo, 7 -3 -1 Hongo, Bunkyo-ku, Tokyo 113, Japan Received 16 February 1999
Abstract A CdS/p-Si:H heterojunction is formed by the evaporation of CdS on a hydrogen passivated Si wafer. It is found that the substrate passivation procedure has a strong influence on the photocurrent properties of the CdS/p-Si interface. In fact, in addition to the photoresponse in the red and infrared spectral ranges, the CdS/p-Si:H heterodiode also reveals photocurrent in the blue and green. This is in contrast to CdS/p-Si and CdS/p-InP devices, which do not exhibit photocurrent in the absorption region of CdS. The positive influence of the passivation on the optoelectronic properties of the CdS/p-Si:H heterodiode is explained by the prevention of interface reactions during the formation of the device. The dependence of the photocurrent of the CdS/p-Si:H heterodiode on an applied bias gives evidence of a homojunction-like behavior. © 1999 Published by Elsevier Science S.A. All rights reserved.
Utilizing the well known intrinsic n-type conductivity of the II-VI compound semiconductor CdS, the CdS/pSi heterojunction (HJ) was intensively studied with regard to the applicability to solar cells [1 – 5]. In spite of the huge lattice mismatch of : 7%, solar power
Fig. 1. Current – voltage characteristic of the CdS/p-Si:H heterodiode in the dark. The lines are guides for the eyes. * Corresponding author. Fax: +81-22-2282010. E-mail address: [email protected] (B. Ullrich)
conversion efficiencies of up to 11% were achieved [5–7], moving the CdS/Si solar cell into a competitive range in comparison with other photovoltaic systems [8]. Surprisingly, the photodetecting features of the CdS/p-Si HJ have not been extensively studied. Since the absorption region of CdS starts at around 500 nm, the CdS/p-Si HJ ought to have the potential to produce photodiodes with high sensitivity to blue and green light. However, experimental evidence points to extreme difficulties in obtaining CdS/p-Si HJ’s with photocurrent in the absorption region of CdS [5,7,9]. In this paper we report on a CdS/p-Si photodiode with pronounced photocurrent in the blue and green range of the spectrum. The HJ is formed by the evaporation of CdS powder (99.999%) from a Langmuir cell heated to a temperature of 750°C. The powder is evaporated for 60 min at a pressure of 0.4 × 10 − 4 Pa while keeping the temperature of the Si substrate at 300 K. The deposited film thickness is : 1 mm. The substrate used is a commercial (111) Si(B) wafer with doping concentration and thickness of : 1017 cm − 3 and 0.42 mm, respectively. The wafer is hydrogen passivated by the RCA® etching procedure, providing a monomer hydrogen terminated surface. Hence, we call the sample CdS/p-Si:H HJ. In
0921-5107/99/$ - see front matter © 1999 Published by Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 9 ) 0 0 1 8 3 - X
B. Ullrich et al. / Materials Science and Engineering B65 (1999) 150–152
Fig. 2. Spectra of the short-circuit photocurrent of (a) CdS/p-Si; (b) CdS/p-InP and (c) CdS/p-InP heterodiodes formed by evaporation, spray pyrolysis and laser ablation, respectively. Curve (d) shows the absorption coefficient of thin film CdS on quartz. Curve (a) is taken from Ref. [9].
Fig. 3. Short-circuit photocurrent of the CdS/p-Si:H heterodiode.
order to ascertain the rectification of the heterodiode by current–voltage measurements electric contacts are realized by silver paste dots on the surface and the back of the sample. The rectifying current – voltage characteristic of the CdS/H-Si diode is shown in Fig. 1. By means of X-ray analysis it is established that the CdS film exhibits wurtzite structure with a perpendicularly oriented crystallographic axis with respect to the film surface. For the photocurrent measurements the same contacts are used and the CdS surface is illuminated by monochromatic light with an intensity of : 100 mW cm − 2. The photocurrent is detected using a lock-in amplifier by chopping the impinging light at 278 Hz. All data presented refer to room temperature. A typical photocurrent spectrum of a CdS/p-Si HJ formed by evaporation is shown by curve (a) in Fig. 2. For comparison with other heterojunctions formed by CdS on small bandgap materials, curves (b) and (c) reveal the photocurrent spectra of CdS/p-InP HJ’s
151
formed by spray pyrolysis and laser ablation, respectively. The CdS/p-InP HJ was expected to represent a ‘perfect HJ’ with appealing optoelectronic features due to the marginal lattice mismatch of 0.3% [10]. In addition, like CdS, InP has a direct fundamental transition. However, according to curves (b) and (c) in Fig. 1, the CdS/InP samples do not show a more expanded range of photosensitivity than curve (a). Apparently, irrespective of the preparation method, lattice match conditions and the transition nature of the substrate, the deposition of CdS on small bandgap materials results in samples without photocurrent in the absorption region of CdS. The same phenomenon was observed with CdSe/Si samples [9]. Curve (d) shows the absorption coefficient of CdS evaluated from the transmission of an evaporated film on quartz. A greatly contrasting spectral response is achieved with the CdS/p-Si:H HJ as demonstrated in Fig. 3. The response shows a pronounced peak at 500 nm, which corresponds to the fundamental transition of wurtzite crystalline CdS at 300 K [11]. We believe that the minimum in the photocurrent around 570 nm is caused by the crossover of the absorption process from CdS to Si. This clear evidence of the contribution of CdS absorption to the photocurrent of the CdS/p-Si:H HJ indicates a strong influence of hydrogen passivation on the optoelectronic features of the HJ. It is known that the hydrogen passivation blocks interfacial reactions by saturation of dangling bonds on the Si surface [12], whereas on the other hand, the growth of ZnS on unpassivated Si is characterized by an initial adsorption of a sulfur layer [13]. We should stress at this point that only electron-hole pairs generated in the depletion area or within a diffusion length of the depletion area contribute to the current flow of a diode. Therefore, in the range of CdS absorption, CdS/p-Si and CdS/p-InP junctions exhibit photocurrent only if the depletion width in the CdS film is comparable to that in the Si substrate [2], or, at least, the diffusion length of the minority carriers in the CdS film is long enough to reach the depletion region in the Si substrate [9]. Obviously, the above conditions are not satisfied by the samples in Fig. 2 and it is very likely that the lack of photocurrent below 500 nm is caused by an interfacial sulfur layer. However, besides interfacial reactions, no interfacial spike is expected in the conduction band of the CdS/p-Si HJ [2,3,7] and the photocurrent dependence on the depletion width (W) ought to behave according to the following expression [14] Iph 8 (1− exp(− aW)),
(1)
where a is the absorption coefficient. Assuming that the donor and acceptor concentrations in CdS and Si are approximately the same, i.e. ND, CdS : NA, Si =N, we write [15]
152
W=
'
B. Ullrich et al. / Materials Science and Engineering B65 (1999) 150–152
4oj (V − V +Vloss), qN bi
(2)
where oj is the junction permittivity, q is the elementary charge, Vbi is the built-in potential, V is the applied bias and Vloss is the dissipated voltage due to various reasons, such as the arrangement of the contacts, sample size and voltage drops at the contacts. The signs of V and Vloss are positive for forward bias and negative for reverse bias. Fig. 4 (a) and (b) show a comparison between Eqs. (1) and (2) and the measurements at 500 and 800 nm, for forward and reverse bias, respectively. The following values are used to fit the data: a(500 nm): a(800 nm)= 1×104 cm − 1, N =1017 cm − 3, oj =3.7 (which corresponds to the ‘reduced’ value of or, Si =11.9 and or, CdS = 5.4 [15]), Vbi =0.4 V and Vloss =0.5 V. We want to emphasize that the similarity of the voltage dependence of the photocurrent at 500 and 800 nm on the backward bias in Fig. 4 (b) is clear evidence that the extension of the CdS depletion width is comparable to that in the Si substrate. Hence, the CdS/p-Si:H heterodiode represents an abrupt HJ, i.e. a HJ with a homojunction-like depletion region, and the electric field is not wasted by the presence of interfacial imperfections highlighted in previous work on CdS/p-InP HJ’s [16]. Furthermore, Fig. 4 (b) shows the influence of the minority carriers. The signal rises more slow at 500 nm than at 800
nm due to the clearly smaller mobility of the holes (B50 cm2 V − 1 s − 1) in CdS than that of electrons in Si (1300 cm2 V − 1 s − 1), which determines the photocurrent at these two wavelengths, respectively. On the other hand, in forward direction (Fig. 4 (a)), the photocurrent is composed by majority and minority carriers of both joining materials and no difference is observed for differently polarized voltages. In conclusion, it is demonstrated that the CdS/p-Si:H HJ shows a blue enhanced photocurrent in contrast to CdS/p-Si and CdS/p-InP HJ’s. The sensitivity to blue light of the CdS/p-Si:H HJ is attributed to the avoidance of interfacial reactions, allowing the formation of a depletion region with similar widths in film and substrate. These results show that the CdS/p-Si:H HJ is a technologically attractive hetero-pairing for the creation of photodiodes applicable in the blue and green spectral range.
Acknowledgements B.U. would like to thank Dr H. Nguyen Cong, Professor P. Chartier and Dr H. Sakai for the samples formed by spray pyrolysis and laser ablation. The work is partly supported by the Research for the Future of the Japan Society for the Promotion of Science to B.U. and T.K.
References
Fig. 4. Dependence of the photocurrent of the CdS/p-Si:H heterodiode on the applied bias in (a) forward and (b) backward direction at 500 and 800 nm. The lines are calculated by Eqs. (1) and (2).
[1] H. Okimura, M. Kawakami, Y. Sakai, Jpn. J. Appl. Phys. 6 (1967) 908. [2] H. Okimura, R. Kondo, Jpn. J. Appl. Phys. 9 (1969) 274. [3] F.M. Livingstone, W.M. Tsang, A.J. Barlow, R.M. De La Ru, W. Duncan, J. Phys. D 10 (1959) 1977. [4] C. Coluzza, M. Garozzo, G. Maletta, D. Margadonna, R. Tomaciello, Appl. Phys. Lett. 37 (1980) 569. [5] E. Scafe´, G. Maletta, R. Tomaciello, P. Alessandrini, A. Camanzi, L. de Angelis, F. Galluzzi, Solar Cells 10 (1983) 17. [6] R.R. Arya, P.M. Sarro, J.J. Loferski, Appl. Phys. Lett. 41 (1982) 355. [7] T. Hayashi, T. Nishikura, K. Nishimura, Y. Ema, Jpn. J. Appl. Phys. 28 (1989) 1174. [8] A.L. Fahrenbruch, R.H. Bube, Fundamentals of Solar Cells, Chapter 1, Academic Press, New York, 1983, p. 17. [9] P.A. Kovalenko, V.A. Korotkov, L.M. Panasjuk, Proc. Int. Conf. on Physics and Chemistry of Semicond. Heterojunctions, 1971, Akademiai Kiado´, Budapest, 1971, p. 363. [10] S. Wagner, J.L. Shay, K.J. Bachmann, E. Buehler, Appl. Phys. Lett. 26 (1975) 229. [11] V.V. Sobolev, V.I. Donetskikh, E.F. Zagainov, Sov. Phys. Semicond. 12 (1978) 646. [12] D.L. Smith, Thin-Film Deposition, Chapter 6, McGraw-Hill, New York, 1995, p. 241. [13] C. Maierhofer, S. Kulkarni, M. Alonso, T. Reich, K. Horn, J. Vac. Sci. Technol. B 9 (1991) 2238. [14] J. Singh, Optoelectronics, Chapter 6, The McGraw-Hill, Singapore, 1996, p. 258. [15] S.M. Sze, Semiconductor Devices, Physics and Technology, Chapter 3, Wiley, New York, 1985, p. 80. [16] B. Ullrich, N.M. Dushkina, H. Ezumi, S. Keitoku, T. Kobayashi, Solid State Comm. 103 (1997) 635.