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High deposition rates for microcrystalline silicon with low temperature plasma enhanced chemical vapor deposition processes
Journal of Non-Crystalline Solids 227–230 Ž1998. 861–865
High deposition rates for microcrystalline silicon with low temperature plasma enhanced chemical vapor deposition processes P. Hapke ) , F. Finger Forschungszentrum Julich, ISI-PV, D-52425-Julich, Germany ¨ ¨
Abstract Intrinsic microcrystalline silicon Ž m c-Si:H. was prepared with plasma enhanced chemical vapor deposition ŽPECVD. from silanerhydrogen mixtures at 2008C with the aim to increase the deposition rate. Using a plasma excitation frequency of 95 MHz we obtain an increase of the deposition rate by a factor of 25 from that of our standard PECVD process at 13.56 MHz. This increase is obtained by the combination of a higher plasma excitation frequency, an increased silane concentration ŽSC. and larger discharge powers. Material prepared under these conditions at a deposition rate of 0.46 nm sy1 maintains crystallinity and electronic properties with dark conductivities, s D f 10y7 S cmy1, and spin densities in the range of 10 16 cmy3. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Microcrystalline silicon; Deposition rate; Conductivity
1. Introduction Microcrystalline silicon Ž m c-Si:H. is a promising material for device applications such as solar cells w1x and color sensors w2x. Compared to amorphous silicon Ža-Si:H., the absorption coefficient of m c-Si:H is considerably larger in the infrared spectral region. However, we estimate that for a sufficient absorption, a film thickness of several microns is necessary which is a critical point because the deposition rates of m c-Si:H in standard plasma enhanced chemical vapor deposition ŽPECVD. are, in general, very low.
)
Corresponding author. Tel.: q49-2461 61 2594; fax: q492461 61 3735; e-mail: [email protected].
Using PECVD processes, high quality m c-Si:H Žhigh crystalline volume fraction. is obtained with a dilution of silane in hydrogen at deposition rates of about 0.02 nm sy1 . One possibility to increase the rate is to increase the plasma excitation frequency, nex Ž13.56 MHz F nex F 95 MHz., which has been shown to increase the deposition rate by a factor of 4 from 0.02 nm sy1 to 0.08 nm sy1 at a low plasma power level of 5 W and dilution of SiH 4 in H 2 Ž3% SiH 4 . for doped ²n:-type w3x and ²i:-material w4x. In addition, the structural properties such as grain size and crystalline volume fraction are increased. A further possibility to increase the deposition rate is to increase the silane concentration ŽSC. in the gas phase w5,6x, with the drawback that the structural properties deteriorate, i.e., the larger the SC, the larger the amorphous volume fraction. Another pos-
0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 Ž 9 8 . 0 0 3 4 3 - 3
862
P. Hapke, F. Fingerr Journal of Non-Crystalline Solids 227–230 (1998) 861–865
sibility that may affect the deposition rate is the plasma power, which has been found not only to increase the rate w7,8x, but also affects the structural properties w8,9x. It has been also reported, that Ar-dilution in a certain range increases the deposition rate w10,11x. With other deposition techniques such as PECVD using fluorinated gases w12x or hot wire CVD w13x, deposition rates of 2 and 1.5 nm sy1 have been achieved, respectively. However, using fluorinated gases experimental precautions might be needed, while for hot wire films, a hydrogen posttreatment might be necessary w14x. Deposition rates up to 0.8 nm sy1 have been achieved using ECWRplasmas w15x, but the deposition on large areas is not shown yet. The deposition of m c-Si:H using PECVD with SiH 4 and H 2 has the advantages of the inherent hydrogen passivation of defects and the compatibility with the well established a-Si:H technology. Furthermore, this technique is routinely used with large substrate size and the method would be the first choice, provided, similar high deposition rates as in the alternative techniques are achieved. This is the topic of the present paper. Here, we concentrate on SiH 4rH 2-plasmas, the excitation frequency, the SC and the plasma power to increase the deposition rate. The deposited films are characterized with respect to their structural and electronic properties.
smooth and rough borosilicate glass ŽCorning 7059., polished and rough crystalline silicon and on rough quartz glass. Electron spin resonance ŽESR. measurements were performed on powder samples deposited on Al foil w16x. The sample thickness was between 0.5 and 1.6 m m.
3. Results In Fig. 1, the deposition rates for ²i:-layers deposited at an excitation frequency of 95 MHz as a function of the SC and various plasma powers are shown. As expected, the deposition rate increases with the SC and the plasma power. At a plasma power of 5 W and SC s 2%, we have a low deposition rate of about 0.08 nm sy1 and material which is fully microcrystalline with a crystalline volume fraction of almost 100% w17x. Increasing the SC to 7% leads to an increase of the deposition rate to 0.15 nm sy1 , but the film has a larger amorphous phase fraction ŽFig. 2., and with further increase of the SC to 8%, the material becomes completely amorphous. Increasing the plasma power to 30 W results in an increase of the rate to 0.41 nm sy1 at SC s 7%. The largest investigated plasma power was 50 W. Start-
2. Experimental details Intrinsic m c-Si:H films are deposited with PECVD using an plasma excitation frequency of 95 MHz. As deposition system we used a conventional diode-type reactor with an electrode spacing of 1.2 cm. The substrate temperature was 2008C, the pressure of the reaction gases was 300 mTorr and the substrate size was 100 cm2 . As gases, we used silane which was highly diluted in hydrogen. The SC ŽSC s SiH 4rŽSiH 4 q H 2 .. in the gas phase was varied between 2 and 8%. Various plasma powers were applied: 5 to 50 W. The sample’s Raman spectra ŽAr-Laser, lex s 488 nm, 100 mW. were measured. The dark conductivity was measured at 300 K under high vacuum conditions using the standard coplanar contacts configuration. The samples are grown on
Fig. 1. Deposition rate as a function of the silane concentration for films deposited at various plasma powers. Lines are drawn as guides for the eye.
P. Hapke, F. Fingerr Journal of Non-Crystalline Solids 227–230 (1998) 861–865
863
Fig. 2. Raman spectra of intrinsic m c-Si:H films deposited at an excitation frequency of 95 MHz, a SC of 2 and 7% and various plasma powers: 5 W Ža., 50 W Žb.. Fig. 3. Ratio R c of the integrated Raman intensities as a function of SC for various plasma powers Žlines are guides to the eye..
ing at SC s 2% in the gas phase we have a rate of 0.14 nm sy1 and at SC s 7% a deposition rate of 0.46 nm sy1 was found. Compared to the deposition rate of m c-Si:H prepared under standard conditions Ž13.56 MHz, 2% SiH 4 , 5 W, rate: 0.02 nm sy1 ., we have an enlargement of about a factor of 25 for the deposition rate. In Fig. 2, the Raman spectra of intrinsic m c-Si:H films deposited with a SC of 2% and 7% at various plasma powers Ž5 W, 50 W. are shown. Starting at a plasma power of 5 W ŽFig. 2a. and SC s 2%, the film is crystalline. Increasing the SC to 7% results in the growth of amorphous Ž480 cmy1 . and crystalline phase Ž520 cmy1 ., i.e., at this deposition condition we are in the transition regime of microcrystalline to amorphous growth. At larger plasma power Ž50 W, Fig. 2b., the Raman spectrum of the film deposited with SC s 2% shows also a large intensity at 520 cmy1 , i.e., this film is also microcrystalline with a crystalline volume fraction of about 100%, as estimated from TEM investigations w17x. Increasing the SC to 7% results only in a small increase of the amorphous component in the Raman spectrum. Therefore, comparing the Raman spectra of the films deposited at SC s 7%, we can state that by increasing the plasma power the structure is improved with respect to the crystalline phase. In Fig. 3 the ratio, R c , of the integrated intensity of the crystalline phase Ž Ic . to the overall area of the Raman spectrum Ž Ic q Ia . is plotted as a function of
the SC. We keep in mind that the ratio of the integrated intensities is only a semiquantitative measure of the crystalline volume fraction w18x and is a lower boundary w17x. For smaller SCs, R c is large for low and high plasma powers. Note that the intensity ratio, R c , in this ²i:-type material is larger than in ²n:-type material with doping of 2% prepared under similar conditions w17x. With increasing SC, R c decreases and is zero at a plasma power of 5 W and SC s 8%. On the other hand, at plasma powers of 30 and 50 W, crystalline growth is observed at these SCs ŽSC s 7%., which means that for the investigated power range the transition zone of microcrystalline to amorphous growth is shifted to larger SCs if the plasma power is increased.
4. Discussion While the effect of shifting the transition zone is also observed at low excitation frequencies, the margin for variation is larger at higher frequencies. At low frequency one already starts with high discharge powers to obtain good crystallinity and the threshold for amorphous growth is already at low SC. At high frequencies, one can extend SC and simultaneously increase the power and thus obtain at the same time
864
P. Hapke, F. Fingerr Journal of Non-Crystalline Solids 227–230 (1998) 861–865
5. Conclusion It has been shown that the deposition rate of m c-Si:H increases by about a factor of 25 compared to standard conditions, combining high plasma excitation frequencies, high plasma power and high SCs. Using these conditions, the structure with respect to the crystalline volume fraction is improved compared with low rate material, i.e., the transition of microcrystalline to amorphous material is shifted to higher SCs. The high deposition rate material maintains good electronic properties with dark conductivities s D - 4)10y6 Ž V cm.y1 and spin densities N F 10 16 cmy3 .
Fig. 4. Dark conductivity as a function of the silane concentration for samples deposited at a plasma power of 50 W.
larger growth rates and good crystallinity. The latter could be also due to the much reduced maximum ion energies at higher frequency. Summarizing our results, we can state that by using higher excitation frequencies, higher plasma powers and higher SCs, it is possible to deposit m c-Si:H with high deposition rates and improved structural properties. It should be mentioned that the further increase of the plasma power Ž) 50 W. leads to a deterioration of the structural properties, presumably through high energy ion bombardment of the growing film. For the application of these intrinsic layers in microcrystalline solar cells, we consider the deposition rate and the electronic properties. In Fig. 4, the data of the room temperature conductivity as a function of SC are shown for the series deposited at a plasma power of 50 W. In the investigated range of the SC, the dark conductivity, s D , varies between 8)10y8 and 4)10y6 Ž V cm.y1 . It is interesting to note that our intrinsic m c-Si:H has such dark conductivities without additional treatment such as microdoping or gas purifying w1x. ESR measurements show that the spin density of this higher rate ²i:material is F 10 16 cmy3 . However, to know which electronic quality of the ²i:-layer is necessary for the cell can only be tested in the device itself, which is the task for the next work.
Acknowledgements This work is supported by the Bundesminister fur ¨ Bildung und Forschung ŽBMBF, Germany.. We thank D. Steinbacher and J. Wolff for technical support and J. Muller for ESR measurements. ¨
References w1x J. Meier, P. Torres, R. Platz, S. Dubail, U. Kroll, J.A. Anna Selvan, N. Pellaton Vaucher, C. Hof, D. Fischer, H. Keppner, A. Shah, K.-D. Ufert, P. Giannoules, J. Koehler, Mater. Res. Soc. Symp. Proc. 420 Ž1996. 3. w2x H. Stiebig, D. Knipp, P. Hapke, F. Finger, J. Non-Cryst. Solids 227–230 Ž1998. 1330. w3x F. Finger, P. Hapke, M. Luysberg, R. Carius, H. Wagner, M. Scheib, Appl. Phys. Lett. 65 Ž1994. 2588. w4x P. Hapke, Thesis, RWTH Aachen, Germany, 1995. w5x A. Matsuda, T. Goto, Mater. Res. Soc. Symp. Proc. 164 Ž1990. 3. w6x M. Nakata, T. Namikawa, H. Shirai, J. Hanna, I. Shimizu, Mater. Res. Soc. Symp. Proc. 192 Ž1990. 481. w7x C.C. Tsai, B. Anderson, R. Thompson, Mater. Res. Soc. Symp. Proc. 192 Ž1990. 475. w8x K. Prasad, Thesis, University of Neuchatel, Switzerland, 1991. w9x S. Hasegawa, S. Narikawa, Y. Kurata, Philos. Mag. B 48 Ž1983. 431. w10x H. Keppner, P. Torres, J. Meier, R. Platz, D. Fischer, U. Kroll, S. Dubail, J.A. Anna Selvan, N. Pellaton Vaucher, Y. Ziegler, R. Tscharner, C. Hof, N. Beck, M. Goetz, P. Pernet,
P. Hapke, F. Fingerr Journal of Non-Crystalline Solids 227–230 (1998) 861–865
w11x w12x w13x w14x
M. Goerlitzer, N. Wyrsch, J. Veuille, J. Cuperus, A. Shah, J. Pohl, Mater. Res. Soc. Symp. Proc. 452 Ž1997. 865. S. Usui, M. Kikuchi, J. Non-Cryst. Solids 34 Ž1979. 1. N. Shibata, K. Fukuda, H. Ohtoshi, J. Hanna, S. Oda, I. Shimizu, Jpn. J. Appl. Phys. 26 Ž1987. L10. A. Middya, J. Guillet, J. Perrin, J. Huc, J. Moncel, J. Parey, G. Rose, Mater. Res. Soc. Symp. Proc. 377 Ž1995. 119. A. Middya, J. Guillet, J. Perrin, J. Bourre, Mater. Res. Soc. Symp. Proc. 420 Ž1996. 289.
865
w15x M. Scheib, B. Schroder, H. Oechsner, Mater. Res. Soc. ¨ Symp. Proc. 420 Ž1996. 437. w16x F. Finger, C. Malten, P. Hapke, R. Carius, R. Fluckiger, H. ¨ Wagner, Philos. Mag. Lett. 70 Ž1994. 247. w17x M. Luysberg, P. Hapke, R. Carius, F. Finger, Philos. Mag. A 75 Ž1997. 31. w18x S. Veprek, F.-A. Sarrot, Z. Iqbal, Phys. Rev. B 36 Ž1987. 3344.
High deposition rates for microcrystalline silicon with low temperature plasma enhanced chemical vapor deposition processes P. Hapke ) , F. Finger Forschungszentrum Julich, ISI-PV, D-52425-Julich, Germany ¨ ¨
Abstract Intrinsic microcrystalline silicon Ž m c-Si:H. was prepared with plasma enhanced chemical vapor deposition ŽPECVD. from silanerhydrogen mixtures at 2008C with the aim to increase the deposition rate. Using a plasma excitation frequency of 95 MHz we obtain an increase of the deposition rate by a factor of 25 from that of our standard PECVD process at 13.56 MHz. This increase is obtained by the combination of a higher plasma excitation frequency, an increased silane concentration ŽSC. and larger discharge powers. Material prepared under these conditions at a deposition rate of 0.46 nm sy1 maintains crystallinity and electronic properties with dark conductivities, s D f 10y7 S cmy1, and spin densities in the range of 10 16 cmy3. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Microcrystalline silicon; Deposition rate; Conductivity
1. Introduction Microcrystalline silicon Ž m c-Si:H. is a promising material for device applications such as solar cells w1x and color sensors w2x. Compared to amorphous silicon Ža-Si:H., the absorption coefficient of m c-Si:H is considerably larger in the infrared spectral region. However, we estimate that for a sufficient absorption, a film thickness of several microns is necessary which is a critical point because the deposition rates of m c-Si:H in standard plasma enhanced chemical vapor deposition ŽPECVD. are, in general, very low.
)
Corresponding author. Tel.: q49-2461 61 2594; fax: q492461 61 3735; e-mail: [email protected].
Using PECVD processes, high quality m c-Si:H Žhigh crystalline volume fraction. is obtained with a dilution of silane in hydrogen at deposition rates of about 0.02 nm sy1 . One possibility to increase the rate is to increase the plasma excitation frequency, nex Ž13.56 MHz F nex F 95 MHz., which has been shown to increase the deposition rate by a factor of 4 from 0.02 nm sy1 to 0.08 nm sy1 at a low plasma power level of 5 W and dilution of SiH 4 in H 2 Ž3% SiH 4 . for doped ²n:-type w3x and ²i:-material w4x. In addition, the structural properties such as grain size and crystalline volume fraction are increased. A further possibility to increase the deposition rate is to increase the silane concentration ŽSC. in the gas phase w5,6x, with the drawback that the structural properties deteriorate, i.e., the larger the SC, the larger the amorphous volume fraction. Another pos-
0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 Ž 9 8 . 0 0 3 4 3 - 3
862
P. Hapke, F. Fingerr Journal of Non-Crystalline Solids 227–230 (1998) 861–865
sibility that may affect the deposition rate is the plasma power, which has been found not only to increase the rate w7,8x, but also affects the structural properties w8,9x. It has been also reported, that Ar-dilution in a certain range increases the deposition rate w10,11x. With other deposition techniques such as PECVD using fluorinated gases w12x or hot wire CVD w13x, deposition rates of 2 and 1.5 nm sy1 have been achieved, respectively. However, using fluorinated gases experimental precautions might be needed, while for hot wire films, a hydrogen posttreatment might be necessary w14x. Deposition rates up to 0.8 nm sy1 have been achieved using ECWRplasmas w15x, but the deposition on large areas is not shown yet. The deposition of m c-Si:H using PECVD with SiH 4 and H 2 has the advantages of the inherent hydrogen passivation of defects and the compatibility with the well established a-Si:H technology. Furthermore, this technique is routinely used with large substrate size and the method would be the first choice, provided, similar high deposition rates as in the alternative techniques are achieved. This is the topic of the present paper. Here, we concentrate on SiH 4rH 2-plasmas, the excitation frequency, the SC and the plasma power to increase the deposition rate. The deposited films are characterized with respect to their structural and electronic properties.
smooth and rough borosilicate glass ŽCorning 7059., polished and rough crystalline silicon and on rough quartz glass. Electron spin resonance ŽESR. measurements were performed on powder samples deposited on Al foil w16x. The sample thickness was between 0.5 and 1.6 m m.
3. Results In Fig. 1, the deposition rates for ²i:-layers deposited at an excitation frequency of 95 MHz as a function of the SC and various plasma powers are shown. As expected, the deposition rate increases with the SC and the plasma power. At a plasma power of 5 W and SC s 2%, we have a low deposition rate of about 0.08 nm sy1 and material which is fully microcrystalline with a crystalline volume fraction of almost 100% w17x. Increasing the SC to 7% leads to an increase of the deposition rate to 0.15 nm sy1 , but the film has a larger amorphous phase fraction ŽFig. 2., and with further increase of the SC to 8%, the material becomes completely amorphous. Increasing the plasma power to 30 W results in an increase of the rate to 0.41 nm sy1 at SC s 7%. The largest investigated plasma power was 50 W. Start-
2. Experimental details Intrinsic m c-Si:H films are deposited with PECVD using an plasma excitation frequency of 95 MHz. As deposition system we used a conventional diode-type reactor with an electrode spacing of 1.2 cm. The substrate temperature was 2008C, the pressure of the reaction gases was 300 mTorr and the substrate size was 100 cm2 . As gases, we used silane which was highly diluted in hydrogen. The SC ŽSC s SiH 4rŽSiH 4 q H 2 .. in the gas phase was varied between 2 and 8%. Various plasma powers were applied: 5 to 50 W. The sample’s Raman spectra ŽAr-Laser, lex s 488 nm, 100 mW. were measured. The dark conductivity was measured at 300 K under high vacuum conditions using the standard coplanar contacts configuration. The samples are grown on
Fig. 1. Deposition rate as a function of the silane concentration for films deposited at various plasma powers. Lines are drawn as guides for the eye.
P. Hapke, F. Fingerr Journal of Non-Crystalline Solids 227–230 (1998) 861–865
863
Fig. 2. Raman spectra of intrinsic m c-Si:H films deposited at an excitation frequency of 95 MHz, a SC of 2 and 7% and various plasma powers: 5 W Ža., 50 W Žb.. Fig. 3. Ratio R c of the integrated Raman intensities as a function of SC for various plasma powers Žlines are guides to the eye..
ing at SC s 2% in the gas phase we have a rate of 0.14 nm sy1 and at SC s 7% a deposition rate of 0.46 nm sy1 was found. Compared to the deposition rate of m c-Si:H prepared under standard conditions Ž13.56 MHz, 2% SiH 4 , 5 W, rate: 0.02 nm sy1 ., we have an enlargement of about a factor of 25 for the deposition rate. In Fig. 2, the Raman spectra of intrinsic m c-Si:H films deposited with a SC of 2% and 7% at various plasma powers Ž5 W, 50 W. are shown. Starting at a plasma power of 5 W ŽFig. 2a. and SC s 2%, the film is crystalline. Increasing the SC to 7% results in the growth of amorphous Ž480 cmy1 . and crystalline phase Ž520 cmy1 ., i.e., at this deposition condition we are in the transition regime of microcrystalline to amorphous growth. At larger plasma power Ž50 W, Fig. 2b., the Raman spectrum of the film deposited with SC s 2% shows also a large intensity at 520 cmy1 , i.e., this film is also microcrystalline with a crystalline volume fraction of about 100%, as estimated from TEM investigations w17x. Increasing the SC to 7% results only in a small increase of the amorphous component in the Raman spectrum. Therefore, comparing the Raman spectra of the films deposited at SC s 7%, we can state that by increasing the plasma power the structure is improved with respect to the crystalline phase. In Fig. 3 the ratio, R c , of the integrated intensity of the crystalline phase Ž Ic . to the overall area of the Raman spectrum Ž Ic q Ia . is plotted as a function of
the SC. We keep in mind that the ratio of the integrated intensities is only a semiquantitative measure of the crystalline volume fraction w18x and is a lower boundary w17x. For smaller SCs, R c is large for low and high plasma powers. Note that the intensity ratio, R c , in this ²i:-type material is larger than in ²n:-type material with doping of 2% prepared under similar conditions w17x. With increasing SC, R c decreases and is zero at a plasma power of 5 W and SC s 8%. On the other hand, at plasma powers of 30 and 50 W, crystalline growth is observed at these SCs ŽSC s 7%., which means that for the investigated power range the transition zone of microcrystalline to amorphous growth is shifted to larger SCs if the plasma power is increased.
4. Discussion While the effect of shifting the transition zone is also observed at low excitation frequencies, the margin for variation is larger at higher frequencies. At low frequency one already starts with high discharge powers to obtain good crystallinity and the threshold for amorphous growth is already at low SC. At high frequencies, one can extend SC and simultaneously increase the power and thus obtain at the same time
864
P. Hapke, F. Fingerr Journal of Non-Crystalline Solids 227–230 (1998) 861–865
5. Conclusion It has been shown that the deposition rate of m c-Si:H increases by about a factor of 25 compared to standard conditions, combining high plasma excitation frequencies, high plasma power and high SCs. Using these conditions, the structure with respect to the crystalline volume fraction is improved compared with low rate material, i.e., the transition of microcrystalline to amorphous material is shifted to higher SCs. The high deposition rate material maintains good electronic properties with dark conductivities s D - 4)10y6 Ž V cm.y1 and spin densities N F 10 16 cmy3 .
Fig. 4. Dark conductivity as a function of the silane concentration for samples deposited at a plasma power of 50 W.
larger growth rates and good crystallinity. The latter could be also due to the much reduced maximum ion energies at higher frequency. Summarizing our results, we can state that by using higher excitation frequencies, higher plasma powers and higher SCs, it is possible to deposit m c-Si:H with high deposition rates and improved structural properties. It should be mentioned that the further increase of the plasma power Ž) 50 W. leads to a deterioration of the structural properties, presumably through high energy ion bombardment of the growing film. For the application of these intrinsic layers in microcrystalline solar cells, we consider the deposition rate and the electronic properties. In Fig. 4, the data of the room temperature conductivity as a function of SC are shown for the series deposited at a plasma power of 50 W. In the investigated range of the SC, the dark conductivity, s D , varies between 8)10y8 and 4)10y6 Ž V cm.y1 . It is interesting to note that our intrinsic m c-Si:H has such dark conductivities without additional treatment such as microdoping or gas purifying w1x. ESR measurements show that the spin density of this higher rate ²i:material is F 10 16 cmy3 . However, to know which electronic quality of the ²i:-layer is necessary for the cell can only be tested in the device itself, which is the task for the next work.
Acknowledgements This work is supported by the Bundesminister fur ¨ Bildung und Forschung ŽBMBF, Germany.. We thank D. Steinbacher and J. Wolff for technical support and J. Muller for ESR measurements. ¨
References w1x J. Meier, P. Torres, R. Platz, S. Dubail, U. Kroll, J.A. Anna Selvan, N. Pellaton Vaucher, C. Hof, D. Fischer, H. Keppner, A. Shah, K.-D. Ufert, P. Giannoules, J. Koehler, Mater. Res. Soc. Symp. Proc. 420 Ž1996. 3. w2x H. Stiebig, D. Knipp, P. Hapke, F. Finger, J. Non-Cryst. Solids 227–230 Ž1998. 1330. w3x F. Finger, P. Hapke, M. Luysberg, R. Carius, H. Wagner, M. Scheib, Appl. Phys. Lett. 65 Ž1994. 2588. w4x P. Hapke, Thesis, RWTH Aachen, Germany, 1995. w5x A. Matsuda, T. Goto, Mater. Res. Soc. Symp. Proc. 164 Ž1990. 3. w6x M. Nakata, T. Namikawa, H. Shirai, J. Hanna, I. Shimizu, Mater. Res. Soc. Symp. Proc. 192 Ž1990. 481. w7x C.C. Tsai, B. Anderson, R. Thompson, Mater. Res. Soc. Symp. Proc. 192 Ž1990. 475. w8x K. Prasad, Thesis, University of Neuchatel, Switzerland, 1991. w9x S. Hasegawa, S. Narikawa, Y. Kurata, Philos. Mag. B 48 Ž1983. 431. w10x H. Keppner, P. Torres, J. Meier, R. Platz, D. Fischer, U. Kroll, S. Dubail, J.A. Anna Selvan, N. Pellaton Vaucher, Y. Ziegler, R. Tscharner, C. Hof, N. Beck, M. Goetz, P. Pernet,
P. Hapke, F. Fingerr Journal of Non-Crystalline Solids 227–230 (1998) 861–865
w11x w12x w13x w14x
M. Goerlitzer, N. Wyrsch, J. Veuille, J. Cuperus, A. Shah, J. Pohl, Mater. Res. Soc. Symp. Proc. 452 Ž1997. 865. S. Usui, M. Kikuchi, J. Non-Cryst. Solids 34 Ž1979. 1. N. Shibata, K. Fukuda, H. Ohtoshi, J. Hanna, S. Oda, I. Shimizu, Jpn. J. Appl. Phys. 26 Ž1987. L10. A. Middya, J. Guillet, J. Perrin, J. Huc, J. Moncel, J. Parey, G. Rose, Mater. Res. Soc. Symp. Proc. 377 Ž1995. 119. A. Middya, J. Guillet, J. Perrin, J. Bourre, Mater. Res. Soc. Symp. Proc. 420 Ž1996. 289.
865
w15x M. Scheib, B. Schroder, H. Oechsner, Mater. Res. Soc. ¨ Symp. Proc. 420 Ž1996. 437. w16x F. Finger, C. Malten, P. Hapke, R. Carius, R. Fluckiger, H. ¨ Wagner, Philos. Mag. Lett. 70 Ž1994. 247. w17x M. Luysberg, P. Hapke, R. Carius, F. Finger, Philos. Mag. A 75 Ž1997. 31. w18x S. Veprek, F.-A. Sarrot, Z. Iqbal, Phys. Rev. B 36 Ž1987. 3344.