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Growth mechanisms of diamond crystals and films prepared by chemical vapor deposition
230
Diamond and Related Materials, 1 (1992) 230-234 Elsevier Science Publishers B.V., Amsterdam
Growth mechanisms of diamond crystals and films prepared by chemical vapor deposition A. M. Bonnot, B. S. Mathis, J. Mercier, J. Leroy Laboratoire d'Etudes des PropriOti's Eleetroniques des Solides, CNRS, BP 166X, 38042 Grenoble Cedex (France)
J. P. Vitton Lahoratoire Sciences des Mat~riaux du Centre de Recherche et de fechnologie de Kodak, Z.I. Nord, BP 103, 71102 ChOlon Sadne Cedex (France)
Abstract Diamond crystals and films were prepared either by hot-filament-assisted or microwave-plasma-assistedCVD techniques. Depending on the technique used and on the synthesis parameters, modifications of diamond crystal habit were observed and are analyzed by considering the growth mechanisms of(ll 1) and (100) faces. Consequently, film surfaces with either predominantly (111) faces or only (100) faces were obtained. Effects of surface reconstruction, defect inclusions and etching are considered in view of modifying the relative < 111 > and < 100 > growth rates and thus the crystal and film surface morphologies.
1. Introduction Although growth of diamond at low pressure where it is thermodynamically metastable is now well stated, there are still some crucial problems which preclude its use, especially in the fields of optics and electronics. Whatever the technique used, the continuity of the films results from the juxtaposition of cubo-octahedral diamond crystals, randomly oriented onto the substrate. Consequently, the film roughness makes them absorbing and the grain boundaries deteriorate their electrical properties. Actually, the challenge is to succeed in growing heteroepitaxial films. However, even without heteroepitaxial growth, it has been shown [1] that the film smoothness and thus the transparency can be improved by growing <100> textured films whose surface is formed with a mosaic of only (100) faces parallel to the substrate. Considering that the film texture is induced by the preferential growth direction, it is thus very important to know how to favor one growth direction over an other. In the same way, the dependence of the single crystal habit on the experimental technique, hotfilament-assisted CVD (HFCVD) or microwave-plasmaassisted CVD (MWPCVD), and on synthesis conditions has been investigated.
2. Experimental details Low pressure isolated diamond crystals and thin films have been synthetized by HFCVD and MWPCVD. Details of the HFCVD arrangement have been described
elsewhere [2]. Typical synthesis conditions were a 0.5-2 at.% methane proportion in hydrogen and a 8001000'~C substrate temperature. The samples prepared by'~ MWPCVD (2.45 GHz) were obtained by adjusting the methane concentration from 0.5 to 5at.% at a pressure of 90 Torr. The temperature could be varied from 800 to 1000 °C, measured by optical pyrometry. Microwave power was maintained around 1200 W. For both synthesis techniques, the substrates were either scratched or non-scratched silicon wafers.
3. Results and discussion Whatever the technique used, diamond crystals are cubo-octahedral. However, depending on the technique used and on the synthesis parameters, variations in the growth mechanism and thus in the development of the crystal faces were observed, leading to different crystal habit and film surface morphologies.
3.1. Diamond growth with hot-filament-assisted CVD Figure 1 (a) shows a diamond crystal prepared with a 2 vol. % methane proportion in hydrogen, a 800 'C substrate temperature and a 150 V substrate bias. Its cubo-octahedral habit is also typical of high pressure diamond crystals synthetized in metallic solvent, while natural diamond crystals generally have an octahedral habit [3]. It has been emphasized [-4] that the octahedral habit of natural diamond crystals is related to growth conditions in silicate solution which prevent diamond surface reconstruction, whereas the cubo-octahedral
0925 9635/92/$05.00 :~', Elsevier Science Publishers B.V. All rights reserved
4. M. Bonnot et al. ' Growth mechanisms ot diammld and lilm.s prepared by chemical ~'apor deposition
(a)
(b)
{el Fig. I. {al Habit. (bl surface morphology of a (l Ill face. and (c) surface morphology of a (1001 face of a diamond crystal prepared by H F C V D with a 2vo1.% CH~ proportion in H 2 and a 800 C substrate
temperature.
habit of high pressure synthetic diamond crystals has been related to surface reconstruction of the (100) face which then behaves as a fiat face with a slow growth rate. Effectively, for an unreconstructed surface, the (111) face, which has one dangling bond normal to the surface, is a fiat face [4] with a slow growth rate when compared
231
with those of the (100) and (110), which have two dangling bonds and for which growth sites are steps and kinks. Considering that the lesser developed is a face, the higher is its growth rate, diamond growth on unreconstructed surface leads to an octahedral habit. However, it is supposed that with high pressure and high temperature synthesis techniques, hydrogen which fills the dangling bonds is desorbed and the (111) and (100) faces reconstruct. In particular, reconstruction of the (100) surface reduces its number of dangling bonds to one and gives it a flat character and thus a slower growth rate. Scanning tunneling microscopy results [5] on low pressure epitaxial diamond growth on a (100) oriented diamond substrate confirm the reconstruction of the (100) diamond surface. Thus, lowering of the < 1 0 0 > growth rate with surface reconstruction leads to cubo-octahedral habits of diamond crystals synthetized either at high pressure or at low pressure. l-igure 1 {b) and (c) show that the (100) faces of HFCVD diamond crystals are smooth and slightly concave with low undulating steps, but that the (111) faces are rough with macrosteps parallel to their edges. The smoothness of the (100) faces is an indication of a stable layer growth mechanism for which growth spiral and low undulating steps are often observed [4] and tends to confirm its flat character. The incurved edges of the (100) face, with a straight line in the middle, are an indication of preferential growth sites on ledges, developing tilted planes at the corners of the (100) face. On the contrary, the roughness of the (l l l) faces indicates unstable growth conditions which have been attributed to alternative growth in the < 1 1 0 > and in the < 1 0 0 > directions [6]. As previously reported [2], the Raman diamond line of the (111) faces has a full width at half maximum (FWHM) larger than that of the (100) faces and shows evidence of more Raman graphite-like bands. This is an indication of a more defective growth in the (111) sectors than in the 1100) ones. It can be a consequence of the metastable growth conditions and of the matching between the projection of the chair atom configuration in successive (l 11) planes and the hexagonal atom configuration in the basal plane of graphite [7] for which growth of graphite on (1111 faces is facilitated. It can be also enhanced by surface reconstruction which contErs s o m e s p 2 bonding-like character to the (1 I1) surface. Figure 2 shows the cubo-octahedral habit of diamond crystals, varying from nearly octahedral [Fig. 2 (a)) with little development of (100) faces for 1 vol. % CH4 and 800 C . to nearly cubic (Fig. 2 (b/) with little development of(1111 faces for 2 vol. % CH., and 970 C. This variation goes along with an increase in the crystal growth rate from 0.7 ~,tmh to 2 tam/h. The well defined habit with sharp edges observed for both synthesis conditions is the indication of a higher growth rate in the < 1 1 0 >
232
A. M. Bonnot et al. / Growth mechanisms of diamond and films prepared by chemical vapor deposition
diamond film surface morphology shown in Fig, 3. It is composed of predominantly (111) faces oriented nearly parallel to the substrate. However, with synthesis conditions for nearly octahedral crystal habit numerous twins are also grown. This twin growth slows down the apparition of the <100> texture with film thickness. For a thickness of 10 ~tm it has not been possible to obtain predominantly (100) faces on the film surface. Figure 4 shows the crystal habit prepared with a 2 vol. % C H 4 concentration and a 1000 °C substrate temperature. A similar crystal growth habit has already been observed [6], but apparently only with hotfilament-assisted CVD. The roughness of the cubooctahedral < 110 > edges, the development of re-entrant (110) faces from the edges of (111) faces and the overgrowth in the center of (100) faces indicate that the growth does not proceed via edges, but rather by renucleation on the (100) faces. The development of (110) faces is the consequence of a lowering of the <110>
(a)
Fig. 3. Surface morphology of a diamond film prepared by HFCVD with a 2vol. % CH4 proportion in Ha and a 970 ~C substrate temperature.
(b) Fig. 2. Variation parameters: (a) temperature, 6 h 970 °C substrate
of the cubo-octahedral habit with HFCVD synthesis l vol. % CH 4 proportion in H2, 800 °C substrate deposition time; (b) 2 vol. % CH 4 proportion in HE, temperature, 3.5 h deposition time.
direction than in the <111> and <100> ones. The higher growth rate for nearly cubic crystals goes along with an increase in the < 111 > growth rate which could be the consequence of more stable (111) growth conditions as the methane concentration and the substrate temperature are increased. When the scratching of the substrate was more efficient, the nucleation rate was increased and diamond films whose thickness is of the order of 10 tam have been obtained. Synthesis conditions for nearly cubic habit crystal growth (2 vol. % C H 4 and 970 °C) lead to the
Fig. 4. Habit of diamond crystals prepared by HFCVD with a 2 vol. % CH 4 proportion in H2 and a 1000 °C substrate temperature.
4. M. 13onnot et al. , Growth mechanisms ol diamond and lihns prepared by chemical capor depo.siti+m
233
growth rate which could be the consequence of preferential (110)face graphitization and etching for high deposition temperature. When re-entrant (110) planes cover the ( 111 ) faces, growth is stopped and can only proceed on (100) faces. With hot-filament CVD, this results in re-nucleation on (100) faces. Despite the fact that it is not possible to directly compare the influence of the parameters in HFCVD and M P W C V D (mainly because they are bound together in each technique), we can mention that the evolution of the growth morphology described above for HFCVD looks very similar to the diamond growth behavior obtained with MPWCVD. Nevertheless, we have obserxed some differences and one of them is briefly described in the following section.
3.2. (I00) Pyramidal-like growth with microwaveassisted C VD Figure 5 shows the crystal habit for 5 vol. % methane and 1 vol. % oxygen proportions in hydrogen and for a 950 "(' substrate temperature. Although the (l l l) faces show similar (110) re-entrant planes to the H F C V D crystal tFig. 4), continuation of the growth has given rise to pyramidal growth on the (100) faces rather than to re-nucleation. Figure 6 shows that for a high nucleation rate, this type of pyramidal growth gives rise to very smooth film, the surface of which is composed of a mosaic of (100) faces with no graphitic inclusions, as is demonstrated by the corresponding Raman spectrum which evidences only the Raman diamond line with a 5 cm 1 full width at half maximum.
I
Fig. 5. ~100) Pyramidal-like growth of diamond crystals prepared by M W P ( ' V D with 5 vol. % CHa and vol. % O 2 proportions in H 2 arid a 950 (" substrate temperature.
4. Conclusion
With the hot-filament-assisted CVD technique, an increase of both the methane proportion in hydrogen from l vol. % to 2 vol. % and substrate temperature from 800 C to 9 7 0 C has modified the cubo-octahedral habit of well faceted diamond crystals from nearly octahedral to nearly cubic. This evolution corresponds to an increase in the ratio of the <111 > and < 1 0 0 > growth rates which has been attributed to more stable
I
I
I
I 1333.4
crfi ~
wavenumber (I/cm]
Ca)
(b)
Fig. 6. la) Surface lllorphology and (b) Raman spectrum of a diamond film prepared b~' M W P C V D for a 5 vol. '!0 CH+ and I x,ol. % 0 2 proportions in hydrogem a 050 C substrate temperature and a 4 h deposition time.
234
A. M. Bonnot et al.
Growth mechanisms of diamond and.films prepared by chemical t:apor deposition
(l 1 l) growth conditions as both the C H 4 and substrate temperature are increased. Films formed with nearly cubic crystals show predominantly (111) faces nearly parallel to the substrate. With microwave-plasmaassisted CVD, using hydrogen with 5 vol% methane and l vol% oxygen, films with a surface composed of only (100) faces have been prepared. These synthesis conditions lead to a stopping of the growth of (111) faces by slowing d o w n the < 1 1 0 > growth rate, and to a pyramidal type growth of (100) faces, as evidenced by isolated d i a m o n d crystals morphology.
References 1 C. Wild, W. Mfiller-Sebert, T. Eckermann and P. Koidl, Proc. Applied Diamomt ConJl, Auburn, Elsevier, 1991, p. 197. 2 A. M. Bonnot, Phys. Rev. B, 41 (1990) 6040. 3 [. Sunagawa, J. Cryst. Growth, 99 (1990) 1[56. 4 H. Kanda, T. Ohsawa, O. Fukunaga and I. Sunagawa, J. Cry.st. Growth, 94 (1989) 115. 5 T. Tsuno, T. Imai, Y. Nishibayashi, K. Hamada and N. Fujimori, Jap. J. Apl. Phys., 30 (1991) 1063. 6 J.S. Kim, M.H. Kim, S. S. Park and J. Y. Lee, J. Appl. Phys., 67 (1990) 3354. 7 A. R. Badzian, T. Badzian, R. Roy, R. Messier and K. E. Spear, Mat. Res. Bull., 23 (1988) 531.
Diamond and Related Materials, 1 (1992) 230-234 Elsevier Science Publishers B.V., Amsterdam
Growth mechanisms of diamond crystals and films prepared by chemical vapor deposition A. M. Bonnot, B. S. Mathis, J. Mercier, J. Leroy Laboratoire d'Etudes des PropriOti's Eleetroniques des Solides, CNRS, BP 166X, 38042 Grenoble Cedex (France)
J. P. Vitton Lahoratoire Sciences des Mat~riaux du Centre de Recherche et de fechnologie de Kodak, Z.I. Nord, BP 103, 71102 ChOlon Sadne Cedex (France)
Abstract Diamond crystals and films were prepared either by hot-filament-assisted or microwave-plasma-assistedCVD techniques. Depending on the technique used and on the synthesis parameters, modifications of diamond crystal habit were observed and are analyzed by considering the growth mechanisms of(ll 1) and (100) faces. Consequently, film surfaces with either predominantly (111) faces or only (100) faces were obtained. Effects of surface reconstruction, defect inclusions and etching are considered in view of modifying the relative < 111 > and < 100 > growth rates and thus the crystal and film surface morphologies.
1. Introduction Although growth of diamond at low pressure where it is thermodynamically metastable is now well stated, there are still some crucial problems which preclude its use, especially in the fields of optics and electronics. Whatever the technique used, the continuity of the films results from the juxtaposition of cubo-octahedral diamond crystals, randomly oriented onto the substrate. Consequently, the film roughness makes them absorbing and the grain boundaries deteriorate their electrical properties. Actually, the challenge is to succeed in growing heteroepitaxial films. However, even without heteroepitaxial growth, it has been shown [1] that the film smoothness and thus the transparency can be improved by growing <100> textured films whose surface is formed with a mosaic of only (100) faces parallel to the substrate. Considering that the film texture is induced by the preferential growth direction, it is thus very important to know how to favor one growth direction over an other. In the same way, the dependence of the single crystal habit on the experimental technique, hotfilament-assisted CVD (HFCVD) or microwave-plasmaassisted CVD (MWPCVD), and on synthesis conditions has been investigated.
2. Experimental details Low pressure isolated diamond crystals and thin films have been synthetized by HFCVD and MWPCVD. Details of the HFCVD arrangement have been described
elsewhere [2]. Typical synthesis conditions were a 0.5-2 at.% methane proportion in hydrogen and a 8001000'~C substrate temperature. The samples prepared by'~ MWPCVD (2.45 GHz) were obtained by adjusting the methane concentration from 0.5 to 5at.% at a pressure of 90 Torr. The temperature could be varied from 800 to 1000 °C, measured by optical pyrometry. Microwave power was maintained around 1200 W. For both synthesis techniques, the substrates were either scratched or non-scratched silicon wafers.
3. Results and discussion Whatever the technique used, diamond crystals are cubo-octahedral. However, depending on the technique used and on the synthesis parameters, variations in the growth mechanism and thus in the development of the crystal faces were observed, leading to different crystal habit and film surface morphologies.
3.1. Diamond growth with hot-filament-assisted CVD Figure 1 (a) shows a diamond crystal prepared with a 2 vol. % methane proportion in hydrogen, a 800 'C substrate temperature and a 150 V substrate bias. Its cubo-octahedral habit is also typical of high pressure diamond crystals synthetized in metallic solvent, while natural diamond crystals generally have an octahedral habit [3]. It has been emphasized [-4] that the octahedral habit of natural diamond crystals is related to growth conditions in silicate solution which prevent diamond surface reconstruction, whereas the cubo-octahedral
0925 9635/92/$05.00 :~', Elsevier Science Publishers B.V. All rights reserved
4. M. Bonnot et al. ' Growth mechanisms ot diammld and lilm.s prepared by chemical ~'apor deposition
(a)
(b)
{el Fig. I. {al Habit. (bl surface morphology of a (l Ill face. and (c) surface morphology of a (1001 face of a diamond crystal prepared by H F C V D with a 2vo1.% CH~ proportion in H 2 and a 800 C substrate
temperature.
habit of high pressure synthetic diamond crystals has been related to surface reconstruction of the (100) face which then behaves as a fiat face with a slow growth rate. Effectively, for an unreconstructed surface, the (111) face, which has one dangling bond normal to the surface, is a fiat face [4] with a slow growth rate when compared
231
with those of the (100) and (110), which have two dangling bonds and for which growth sites are steps and kinks. Considering that the lesser developed is a face, the higher is its growth rate, diamond growth on unreconstructed surface leads to an octahedral habit. However, it is supposed that with high pressure and high temperature synthesis techniques, hydrogen which fills the dangling bonds is desorbed and the (111) and (100) faces reconstruct. In particular, reconstruction of the (100) surface reduces its number of dangling bonds to one and gives it a flat character and thus a slower growth rate. Scanning tunneling microscopy results [5] on low pressure epitaxial diamond growth on a (100) oriented diamond substrate confirm the reconstruction of the (100) diamond surface. Thus, lowering of the < 1 0 0 > growth rate with surface reconstruction leads to cubo-octahedral habits of diamond crystals synthetized either at high pressure or at low pressure. l-igure 1 {b) and (c) show that the (100) faces of HFCVD diamond crystals are smooth and slightly concave with low undulating steps, but that the (111) faces are rough with macrosteps parallel to their edges. The smoothness of the (100) faces is an indication of a stable layer growth mechanism for which growth spiral and low undulating steps are often observed [4] and tends to confirm its flat character. The incurved edges of the (100) face, with a straight line in the middle, are an indication of preferential growth sites on ledges, developing tilted planes at the corners of the (100) face. On the contrary, the roughness of the (l l l) faces indicates unstable growth conditions which have been attributed to alternative growth in the < 1 1 0 > and in the < 1 0 0 > directions [6]. As previously reported [2], the Raman diamond line of the (111) faces has a full width at half maximum (FWHM) larger than that of the (100) faces and shows evidence of more Raman graphite-like bands. This is an indication of a more defective growth in the (111) sectors than in the 1100) ones. It can be a consequence of the metastable growth conditions and of the matching between the projection of the chair atom configuration in successive (l 11) planes and the hexagonal atom configuration in the basal plane of graphite [7] for which growth of graphite on (1111 faces is facilitated. It can be also enhanced by surface reconstruction which contErs s o m e s p 2 bonding-like character to the (1 I1) surface. Figure 2 shows the cubo-octahedral habit of diamond crystals, varying from nearly octahedral [Fig. 2 (a)) with little development of (100) faces for 1 vol. % CH4 and 800 C . to nearly cubic (Fig. 2 (b/) with little development of(1111 faces for 2 vol. % CH., and 970 C. This variation goes along with an increase in the crystal growth rate from 0.7 ~,tmh to 2 tam/h. The well defined habit with sharp edges observed for both synthesis conditions is the indication of a higher growth rate in the < 1 1 0 >
232
A. M. Bonnot et al. / Growth mechanisms of diamond and films prepared by chemical vapor deposition
diamond film surface morphology shown in Fig, 3. It is composed of predominantly (111) faces oriented nearly parallel to the substrate. However, with synthesis conditions for nearly octahedral crystal habit numerous twins are also grown. This twin growth slows down the apparition of the <100> texture with film thickness. For a thickness of 10 ~tm it has not been possible to obtain predominantly (100) faces on the film surface. Figure 4 shows the crystal habit prepared with a 2 vol. % C H 4 concentration and a 1000 °C substrate temperature. A similar crystal growth habit has already been observed [6], but apparently only with hotfilament-assisted CVD. The roughness of the cubooctahedral < 110 > edges, the development of re-entrant (110) faces from the edges of (111) faces and the overgrowth in the center of (100) faces indicate that the growth does not proceed via edges, but rather by renucleation on the (100) faces. The development of (110) faces is the consequence of a lowering of the <110>
(a)
Fig. 3. Surface morphology of a diamond film prepared by HFCVD with a 2vol. % CH4 proportion in Ha and a 970 ~C substrate temperature.
(b) Fig. 2. Variation parameters: (a) temperature, 6 h 970 °C substrate
of the cubo-octahedral habit with HFCVD synthesis l vol. % CH 4 proportion in H2, 800 °C substrate deposition time; (b) 2 vol. % CH 4 proportion in HE, temperature, 3.5 h deposition time.
direction than in the <111> and <100> ones. The higher growth rate for nearly cubic crystals goes along with an increase in the < 111 > growth rate which could be the consequence of more stable (111) growth conditions as the methane concentration and the substrate temperature are increased. When the scratching of the substrate was more efficient, the nucleation rate was increased and diamond films whose thickness is of the order of 10 tam have been obtained. Synthesis conditions for nearly cubic habit crystal growth (2 vol. % C H 4 and 970 °C) lead to the
Fig. 4. Habit of diamond crystals prepared by HFCVD with a 2 vol. % CH 4 proportion in H2 and a 1000 °C substrate temperature.
4. M. 13onnot et al. , Growth mechanisms ol diamond and lihns prepared by chemical capor depo.siti+m
233
growth rate which could be the consequence of preferential (110)face graphitization and etching for high deposition temperature. When re-entrant (110) planes cover the ( 111 ) faces, growth is stopped and can only proceed on (100) faces. With hot-filament CVD, this results in re-nucleation on (100) faces. Despite the fact that it is not possible to directly compare the influence of the parameters in HFCVD and M P W C V D (mainly because they are bound together in each technique), we can mention that the evolution of the growth morphology described above for HFCVD looks very similar to the diamond growth behavior obtained with MPWCVD. Nevertheless, we have obserxed some differences and one of them is briefly described in the following section.
3.2. (I00) Pyramidal-like growth with microwaveassisted C VD Figure 5 shows the crystal habit for 5 vol. % methane and 1 vol. % oxygen proportions in hydrogen and for a 950 "(' substrate temperature. Although the (l l l) faces show similar (110) re-entrant planes to the H F C V D crystal tFig. 4), continuation of the growth has given rise to pyramidal growth on the (100) faces rather than to re-nucleation. Figure 6 shows that for a high nucleation rate, this type of pyramidal growth gives rise to very smooth film, the surface of which is composed of a mosaic of (100) faces with no graphitic inclusions, as is demonstrated by the corresponding Raman spectrum which evidences only the Raman diamond line with a 5 cm 1 full width at half maximum.
I
Fig. 5. ~100) Pyramidal-like growth of diamond crystals prepared by M W P ( ' V D with 5 vol. % CHa and vol. % O 2 proportions in H 2 arid a 950 (" substrate temperature.
4. Conclusion
With the hot-filament-assisted CVD technique, an increase of both the methane proportion in hydrogen from l vol. % to 2 vol. % and substrate temperature from 800 C to 9 7 0 C has modified the cubo-octahedral habit of well faceted diamond crystals from nearly octahedral to nearly cubic. This evolution corresponds to an increase in the ratio of the <111 > and < 1 0 0 > growth rates which has been attributed to more stable
I
I
I
I 1333.4
crfi ~
wavenumber (I/cm]
Ca)
(b)
Fig. 6. la) Surface lllorphology and (b) Raman spectrum of a diamond film prepared b~' M W P C V D for a 5 vol. '!0 CH+ and I x,ol. % 0 2 proportions in hydrogem a 050 C substrate temperature and a 4 h deposition time.
234
A. M. Bonnot et al.
Growth mechanisms of diamond and.films prepared by chemical t:apor deposition
(l 1 l) growth conditions as both the C H 4 and substrate temperature are increased. Films formed with nearly cubic crystals show predominantly (111) faces nearly parallel to the substrate. With microwave-plasmaassisted CVD, using hydrogen with 5 vol% methane and l vol% oxygen, films with a surface composed of only (100) faces have been prepared. These synthesis conditions lead to a stopping of the growth of (111) faces by slowing d o w n the < 1 1 0 > growth rate, and to a pyramidal type growth of (100) faces, as evidenced by isolated d i a m o n d crystals morphology.
References 1 C. Wild, W. Mfiller-Sebert, T. Eckermann and P. Koidl, Proc. Applied Diamomt ConJl, Auburn, Elsevier, 1991, p. 197. 2 A. M. Bonnot, Phys. Rev. B, 41 (1990) 6040. 3 [. Sunagawa, J. Cryst. Growth, 99 (1990) 1[56. 4 H. Kanda, T. Ohsawa, O. Fukunaga and I. Sunagawa, J. Cry.st. Growth, 94 (1989) 115. 5 T. Tsuno, T. Imai, Y. Nishibayashi, K. Hamada and N. Fujimori, Jap. J. Apl. Phys., 30 (1991) 1063. 6 J.S. Kim, M.H. Kim, S. S. Park and J. Y. Lee, J. Appl. Phys., 67 (1990) 3354. 7 A. R. Badzian, T. Badzian, R. Roy, R. Messier and K. E. Spear, Mat. Res. Bull., 23 (1988) 531.