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Germanium and silicon ion beam deposition
Thin Solid Films, 92 (1982) 123-129
123
PREPARATION AND CHARACTERIZATION
GERMANIUM AND SILICON ION BEAM DEPOSITION* KIYOSHI MIYAKE AND TAKASHI TOKUYAMA
Central Research Laboratory, Hitachi Ltd., Kokubunji, Tokyo 185 (Japan) (Received December 22, 1981 ; accepted January 14, 1982)
Epitaxial growth of germanium and silicon on Si(100) substrates by low energy mass-separated ion beam deposition is demonstrated. Heteroepitaxial germanium films are obtained by irradiating Ge ÷ ions with a kinetic energy of 100 eV onto a silicon substrate at a substrate temperature of 300 °C. Homoepitaxial growth of silicon is possible at an ion energy of 200 eV and a substrate temperature of 740 °C. The crystalline structure of these films is discussed in detail.
1. INTRODUCTION Low temperature thin film formation techniques using plasma or ion beams have been attracting great interest in the field of electronic and optical device fabrication 1. These methods produce such film properties as strong adhesion, surface smoothness, controllability of crystalline orientations, the possibility of low temperature epitaxial growth of semiconductor materials 2 etc. Ion plating 3, ionized cluster beam deposition4 and plasma chemical vapour deposition s are examples of these approaches. Recently, there have been several reports of mass-separated ion beam deposition (IBD) of various materials 6-9. For example, Yagi e t al. 2 have reported the deposition of Ge ÷ and Si ÷. Low temperature epitaxial growth has been observed with many materials, although not with silicon. Silicon deposition in our previous work was not successful since the films obtained were amorphous because of poor vacuum conditions. In this paper, after briefly describing our improved IBD equipment, we discuss in detail the crystalline characterizations of the Ge ÷/Si heteroepitaxial and Si+/Si homoepitaxial films. 2. EXPERIMENTAL DETAILS The IBD system used in this study is shown schematically in Fig. 1. The system *Paper presented at the Fifth Symposium on Ion Sources and Ion-assisted Technology and International Workshop on Ion-based Techniques for Film Formation, Tokyo and Kyoto, Japan, June 1-5, 1981. 0040-6090/82/0000-0000/$02.75
© Elsevier Sequoia/Printed in The Netherlands
124
K. MIYAKE, T. TOKUYAMA
RASSSEPARATINGI~,GNET IONSOURCE ~
DEPOSITION CHAMBER
SUPPLY
Fig. 1. Schematic diagram of the ion beam deposition equipment.
consists of three principal parts: an ion source, a mass-separating magnet and an ultrahigh vacuum deposition chamber. Ge + and Si ÷ ion beams were separately generated from GeC14 and SiC14 gas discharges respectively in a Freeman-type source and were extracted at an energy of 10-30 keV. After mass separation using a 60 ° sector-type magnet, the ion beam was deflected at the horizontal focal point of the magnet by electrostatic deflectors to prevent target irradiation by chargeexchanged high energy neutral particles. The ion beam was then decelerated to an energy of 100-200 eV by an E × B lens system. The diameter of the deposited region was 7-10 ram. A typical mass spectrum of the decelerated ion beams for SIC14 gas discharges is shown in Fig. 2. An ion current range of 50-200 pA for Ge + and Si + ion beams was obtained. ?^^
{
Fig. 2. Typical mass spectrum of decelerated ion beams generated from an SiCI~ gas discharge.
Ge AND Si ION BEAM DEPOSITION
125
The silicon substrates for the depositions were (100)-oriented single crystals 20 mm x 2 0 m m x 0.3 mm which were mounted on heating blocks on a sample manipulator. Before deposition, the substrates were cleaned by conventional cleaning procedures. Neither flash desorption nor Ar ÷ ion sputter cleaning were carried out. The base pressure in the chamber was 5 x 10-lo Torr after 8 h of mild bake-out. During ion beam deposition the ambient pressure increased to 5 x 10-8 Torr because of the gas load from the ion source. The substrate temperature was monitored by a Pt-(Pt-Rh) thermocouple immersed in the heating block, and the ion current through the substrate was recorded by a current monitor during the deposition. The thicknesses of the deposited layers were a few hundred ~ngstrSms for Ge +/Si films and 2000-3000/~ for Si+/Si films. 3. RESULTS AND DISCUSSION
3.1. Ge +/Si heteroepitaxial growth Figure 3 shows a transmission electron microscopy (TEM) diffraction pattern for the Ge÷/Si heterostructure. Here, Ge ÷ ions were deposited at 100eV. The substrate temperature was 300°C and the vacuum pressure was 1 x 10 - 7 Torr. Perfect epitaxial relations between the deposited germanium lattice (a = 5.66/~) and the substrate silicon lattice (a = 5.43/~) are clearly indicated by the two sets of spots observed.
(a) (b) 1 lam Fig. 3. (a) Transmission electron diffraction pattern and (b) transmission electron micrograph of germanium deposited onto an Si(100) substrate (as~ = 5.43/~; ace = 5.66/~). The ion kinetic energy was 100 eV and the substrate temperature was 300 °C.
The crystalline structure for the germanium deposit was shown to be rather defective by T E M micrographs. Moir6 patterns were observed in the micrograph and the fringe spacing was in good agreement with that deduced from the difference in lattice constants. Many small dislocation loops were also found. As the substrate temperature is not high enough for solid phase epitaxial growth to occur, disorders in the ion-bombarded surface cannot fully recover and are thought to develop into these defects. In addition, large lattice mismatching between germanium and silicon is thought to be another reason for the defects.
126
K. MIYAKE, T. TOKUYAMA
The same structure was examined using Rutherford backscattering measurements. The results are shown in Fig. 4. The low yield for the aligned spectrum indicates a good epitaxial relation. However, the yield from the interface region shows some kind of interatomic mixing or alloy phase formation of germanium and silicon. The surface smoothness of the deposited germanium films was examined using an electron microscope. The surface replica micrograph shown in Fig. 5 indicates a very fiat surface structure.
siyle Cr,st
t
50
100 CHANNEL
150
200
NUMBER
Fig. 4. Rutherford backscattering (280keV He +) spectrum for deposited germanium films. The deposition conditions are the same as in Fig. 3.
Fig. 5. Surface replica micrograph ofa Ge+/Si film.
Ge
AND
Si
127
ION BEAM DEPOSITION
Heteroepitaxial growth of germanium on Si(111) substrates has been reported in one of our previous papers 2. The epitaxial temperature was as low as 300 °C at an ion energy of 100 eV. The present results are extensions of the previous ones to Si(100) substrates. Recently, Kuiper et aL to have reported that germanium epitaxial growth is possible at 230 °C by the ionized cluster beam deposition method, in which the average kinetic energy per atom is as small as 1 eV. To understand the effects of the ion bombarding energy on crystalline structures, further investigations are necessary.
3.2. 2sSi+/Si( lO0) homoepitaxial growth Si ÷ ion beam deposition was carried out using the same procedure. The ion acceleration energy was 200 eV and the ambient pressure during deposition was 5 x 10-a Torr. Secondary ion mass spectra of the deposited silicon films and the silicon substrates are shown in Figs. 6(a) and 6(b) respectively. It should be noted that the IBD films contained only the single isotope 28Si, i.e. the films were isotopically enriched. The carbon content included in the film was very small compared with our previous results on silicon IBD films where the pressure was 1 x 10-5 Torr and the ion current was 5 taA.
28Si+ (xlO) 2 ~
z
~
29Si +
~
30Si+
29SiH +
o (a)
28Si + (xl0)
, I
MAss NUMBER
44Si0 +
I
,I
°
(b)
o
II,
44Si0+
I
MASS NUMBER
Fig. 6. Secondary ion mass spectra for (a) an 2aSi + ion-beam-deposited film and (b) a silicon substrate.
The films deposited at room temperature were amorphous as shown in the reflection high energy electron diffraction (RHEED) pattern in Fig. 7. As the substrate temperature was increased to 600°C, (110) preferentially oriented polycrystalline films were obtained. At a substrate temperature of 740°C the deposited films became monocrystalline in structure as shown in the RHEED patterns in Fig. 8. The azimuthal directions of the probe electron beam were (a) <001> and (b) <011>. The three major spots in Fig. 8, which originate from the (220), (2]0) and (400) planes, indicate that the deposited layer is a film grown epitaxially from the Si(100) substrate. Weak spot patterns suggest the existence of twin structures. The same epitaxial structure was also examined by TEM. The transmission electron diffraction pattern shown in Fig. 9 indicates homoepitaxial growth. Sets of small spots indicate that twin structures are included in the deposited layer. The crystalline structure is rather defective and contains a lot of dislocation loops.
128
K. MIYAKE, T. TOKUYAMA
Fig. 7. R H E E D pattern of an Si+/Si IBD film deposited at room temperature.
(a)
(b)
Fig. 8. R H E E D patterns of Si +/Si IBD films deposited at 740 '~C. The ion kinetic energy was 200 eV. The azimuthal direction of the probe electron beam was (a) (001 ) and (b) ( 0 i I ).
1 pm Fig. 9. Transmission electron micrograph of an Si+/Si(100) IBD film deposited at 740 °C where the ion kinetic energy was 200 eV.
Ge AND Si ION BEAMDEPOSITION
129
In silicon deposition, it is well known that surface contamination from oxygen or carbon plays a great role in epitaxial growth TM. In this study the surface contamination on a silicon substrate was not completely removed before deposition. In our previous report 2, silicon ion beam deposition was carried out at a pressure of 1 × 10- 5 Torr, at which the impinging rate of residual gas atoms on a silicon surface was two orders of magnitude higher than the bombarding ion flux. Therefore the asdeposited films were always amorphous at an ion energy of 100 eV. In the present experiments, however, the impinging rate of Si ÷ ions was two orders or more higher than the residual gas atom flux. The epitaxial growth of silicon is thought to result from the improvements in vacuum pressure and ion current flux. 4. CONCLUSIONS Germanium and silicon films were epitaxially grown on Si(100) substrates by a low energy mass-separated ion beam deposition method. Germanium films deposited at 300 °C were epitaxially grown from the substrates, although a lot of dislocation loops were found in the deposited layer. Large lattice mismatching between germanium and silicon and the low substrate temperature for solid phase epitaxial growth are thought to have resulted in these defects. Silicon films, composed of 28Si+ ions deposited at an ion energy of 200eV and a substrate temperature of 740 °C, were isotopically enriched with the single species 28Si. The crystalline structure, observed by means of R H E E D and T E M measurements, indicated that the films were epitaxially grown on Si(100) substrates. However, the films were rather defective and contained twin structures. Further investigationsare necessary to obtain defect-free silicon epitaxial growth at low temperatures. ACKNOWLEDGMENTS The authors wish to thank Drs. M. Tamura and N. Natsuaki for their fruitful discussions and for TEM and Rutherford backscattering measurements. They are also grateful to H. Kakibayashi for his R H E E D measurements and to Dr. K. Yagi for his valuable discussions throughout the course of this work. REFERENCES 1 T. Tokuyama,K. Yagi, K. Miyake, M. Tamura, N. Natsuaki and S. Tachi, Nucl. Instrum. Methods, 182-183, Part I (1981)241. 2 K. Yagi, S. Tamura and T. Tokuyama,Jpn. J. Appl. Phys., 16 (1977) 245. 3 T. Itoh, T. Nakamura, M. Muromachi and T. Sugiyama,Jpn. J. Appl. Phys., 16 (1977) 553. 4 T. Takagi, I. Yamada and A. Sasaki, Thin Solid Films, 45 (1977) 569. 5 S. Suzuki, H. Takai, H. Okuda and T. Itoh, Proc. llth Conf. on Solid State Devices, Tokyo, 1979, in Jpn. J. Appl. Phys., Suppl. 1, 19(1980) 647. 6 J. Amano, P. BriceandR. P.W. Lawson,J. Vac. Sci. Technol.,13 (1976) 591. 7 J.H. Freeman, Nature (London), 275 (1978) 634. 8 G.E. Thomas and E. E. de Kluizenaar, Proc. 3rd Int. Congr. on Surface Physics and Chemistry, Grenoble, 1977, Socirt6 Fran~aise du Vide, Paris, 1977,p. 136. 9 J.S. Colligon,W. A. Grant, J. S. Williams and R. P. W. Lawson, Proc. Int. Conf. on Applications of Ion Beams to Metals, University of Warwick, 1975, in Inst. Phys. Conf. Ser. 28 (1976) 357. 10 A.E.T. Kuiper, G. E. Thomas and W. J. Schouten,J. Cryst. Growth, 51 (1981) 17. 11 A.G. CullisandG. R. Booker, J. Cryst. Growth, 9(1971) l12. 12 K. Itoh and K. Takahashi, Jpn. J. Appl. Phys., 7 (1968) 821.
123
PREPARATION AND CHARACTERIZATION
GERMANIUM AND SILICON ION BEAM DEPOSITION* KIYOSHI MIYAKE AND TAKASHI TOKUYAMA
Central Research Laboratory, Hitachi Ltd., Kokubunji, Tokyo 185 (Japan) (Received December 22, 1981 ; accepted January 14, 1982)
Epitaxial growth of germanium and silicon on Si(100) substrates by low energy mass-separated ion beam deposition is demonstrated. Heteroepitaxial germanium films are obtained by irradiating Ge ÷ ions with a kinetic energy of 100 eV onto a silicon substrate at a substrate temperature of 300 °C. Homoepitaxial growth of silicon is possible at an ion energy of 200 eV and a substrate temperature of 740 °C. The crystalline structure of these films is discussed in detail.
1. INTRODUCTION Low temperature thin film formation techniques using plasma or ion beams have been attracting great interest in the field of electronic and optical device fabrication 1. These methods produce such film properties as strong adhesion, surface smoothness, controllability of crystalline orientations, the possibility of low temperature epitaxial growth of semiconductor materials 2 etc. Ion plating 3, ionized cluster beam deposition4 and plasma chemical vapour deposition s are examples of these approaches. Recently, there have been several reports of mass-separated ion beam deposition (IBD) of various materials 6-9. For example, Yagi e t al. 2 have reported the deposition of Ge ÷ and Si ÷. Low temperature epitaxial growth has been observed with many materials, although not with silicon. Silicon deposition in our previous work was not successful since the films obtained were amorphous because of poor vacuum conditions. In this paper, after briefly describing our improved IBD equipment, we discuss in detail the crystalline characterizations of the Ge ÷/Si heteroepitaxial and Si+/Si homoepitaxial films. 2. EXPERIMENTAL DETAILS The IBD system used in this study is shown schematically in Fig. 1. The system *Paper presented at the Fifth Symposium on Ion Sources and Ion-assisted Technology and International Workshop on Ion-based Techniques for Film Formation, Tokyo and Kyoto, Japan, June 1-5, 1981. 0040-6090/82/0000-0000/$02.75
© Elsevier Sequoia/Printed in The Netherlands
124
K. MIYAKE, T. TOKUYAMA
RASSSEPARATINGI~,GNET IONSOURCE ~
DEPOSITION CHAMBER
SUPPLY
Fig. 1. Schematic diagram of the ion beam deposition equipment.
consists of three principal parts: an ion source, a mass-separating magnet and an ultrahigh vacuum deposition chamber. Ge + and Si ÷ ion beams were separately generated from GeC14 and SiC14 gas discharges respectively in a Freeman-type source and were extracted at an energy of 10-30 keV. After mass separation using a 60 ° sector-type magnet, the ion beam was deflected at the horizontal focal point of the magnet by electrostatic deflectors to prevent target irradiation by chargeexchanged high energy neutral particles. The ion beam was then decelerated to an energy of 100-200 eV by an E × B lens system. The diameter of the deposited region was 7-10 ram. A typical mass spectrum of the decelerated ion beams for SIC14 gas discharges is shown in Fig. 2. An ion current range of 50-200 pA for Ge + and Si + ion beams was obtained. ?^^
{
Fig. 2. Typical mass spectrum of decelerated ion beams generated from an SiCI~ gas discharge.
Ge AND Si ION BEAM DEPOSITION
125
The silicon substrates for the depositions were (100)-oriented single crystals 20 mm x 2 0 m m x 0.3 mm which were mounted on heating blocks on a sample manipulator. Before deposition, the substrates were cleaned by conventional cleaning procedures. Neither flash desorption nor Ar ÷ ion sputter cleaning were carried out. The base pressure in the chamber was 5 x 10-lo Torr after 8 h of mild bake-out. During ion beam deposition the ambient pressure increased to 5 x 10-8 Torr because of the gas load from the ion source. The substrate temperature was monitored by a Pt-(Pt-Rh) thermocouple immersed in the heating block, and the ion current through the substrate was recorded by a current monitor during the deposition. The thicknesses of the deposited layers were a few hundred ~ngstrSms for Ge +/Si films and 2000-3000/~ for Si+/Si films. 3. RESULTS AND DISCUSSION
3.1. Ge +/Si heteroepitaxial growth Figure 3 shows a transmission electron microscopy (TEM) diffraction pattern for the Ge÷/Si heterostructure. Here, Ge ÷ ions were deposited at 100eV. The substrate temperature was 300°C and the vacuum pressure was 1 x 10 - 7 Torr. Perfect epitaxial relations between the deposited germanium lattice (a = 5.66/~) and the substrate silicon lattice (a = 5.43/~) are clearly indicated by the two sets of spots observed.
(a) (b) 1 lam Fig. 3. (a) Transmission electron diffraction pattern and (b) transmission electron micrograph of germanium deposited onto an Si(100) substrate (as~ = 5.43/~; ace = 5.66/~). The ion kinetic energy was 100 eV and the substrate temperature was 300 °C.
The crystalline structure for the germanium deposit was shown to be rather defective by T E M micrographs. Moir6 patterns were observed in the micrograph and the fringe spacing was in good agreement with that deduced from the difference in lattice constants. Many small dislocation loops were also found. As the substrate temperature is not high enough for solid phase epitaxial growth to occur, disorders in the ion-bombarded surface cannot fully recover and are thought to develop into these defects. In addition, large lattice mismatching between germanium and silicon is thought to be another reason for the defects.
126
K. MIYAKE, T. TOKUYAMA
The same structure was examined using Rutherford backscattering measurements. The results are shown in Fig. 4. The low yield for the aligned spectrum indicates a good epitaxial relation. However, the yield from the interface region shows some kind of interatomic mixing or alloy phase formation of germanium and silicon. The surface smoothness of the deposited germanium films was examined using an electron microscope. The surface replica micrograph shown in Fig. 5 indicates a very fiat surface structure.
siyle Cr,st
t
50
100 CHANNEL
150
200
NUMBER
Fig. 4. Rutherford backscattering (280keV He +) spectrum for deposited germanium films. The deposition conditions are the same as in Fig. 3.
Fig. 5. Surface replica micrograph ofa Ge+/Si film.
Ge
AND
Si
127
ION BEAM DEPOSITION
Heteroepitaxial growth of germanium on Si(111) substrates has been reported in one of our previous papers 2. The epitaxial temperature was as low as 300 °C at an ion energy of 100 eV. The present results are extensions of the previous ones to Si(100) substrates. Recently, Kuiper et aL to have reported that germanium epitaxial growth is possible at 230 °C by the ionized cluster beam deposition method, in which the average kinetic energy per atom is as small as 1 eV. To understand the effects of the ion bombarding energy on crystalline structures, further investigations are necessary.
3.2. 2sSi+/Si( lO0) homoepitaxial growth Si ÷ ion beam deposition was carried out using the same procedure. The ion acceleration energy was 200 eV and the ambient pressure during deposition was 5 x 10-a Torr. Secondary ion mass spectra of the deposited silicon films and the silicon substrates are shown in Figs. 6(a) and 6(b) respectively. It should be noted that the IBD films contained only the single isotope 28Si, i.e. the films were isotopically enriched. The carbon content included in the film was very small compared with our previous results on silicon IBD films where the pressure was 1 x 10-5 Torr and the ion current was 5 taA.
28Si+ (xlO) 2 ~
z
~
29Si +
~
30Si+
29SiH +
o (a)
28Si + (xl0)
, I
MAss NUMBER
44Si0 +
I
,I
°
(b)
o
II,
44Si0+
I
MASS NUMBER
Fig. 6. Secondary ion mass spectra for (a) an 2aSi + ion-beam-deposited film and (b) a silicon substrate.
The films deposited at room temperature were amorphous as shown in the reflection high energy electron diffraction (RHEED) pattern in Fig. 7. As the substrate temperature was increased to 600°C, (110) preferentially oriented polycrystalline films were obtained. At a substrate temperature of 740°C the deposited films became monocrystalline in structure as shown in the RHEED patterns in Fig. 8. The azimuthal directions of the probe electron beam were (a) <001> and (b) <011>. The three major spots in Fig. 8, which originate from the (220), (2]0) and (400) planes, indicate that the deposited layer is a film grown epitaxially from the Si(100) substrate. Weak spot patterns suggest the existence of twin structures. The same epitaxial structure was also examined by TEM. The transmission electron diffraction pattern shown in Fig. 9 indicates homoepitaxial growth. Sets of small spots indicate that twin structures are included in the deposited layer. The crystalline structure is rather defective and contains a lot of dislocation loops.
128
K. MIYAKE, T. TOKUYAMA
Fig. 7. R H E E D pattern of an Si+/Si IBD film deposited at room temperature.
(a)
(b)
Fig. 8. R H E E D patterns of Si +/Si IBD films deposited at 740 '~C. The ion kinetic energy was 200 eV. The azimuthal direction of the probe electron beam was (a) (001 ) and (b) ( 0 i I ).
1 pm Fig. 9. Transmission electron micrograph of an Si+/Si(100) IBD film deposited at 740 °C where the ion kinetic energy was 200 eV.
Ge AND Si ION BEAMDEPOSITION
129
In silicon deposition, it is well known that surface contamination from oxygen or carbon plays a great role in epitaxial growth TM. In this study the surface contamination on a silicon substrate was not completely removed before deposition. In our previous report 2, silicon ion beam deposition was carried out at a pressure of 1 × 10- 5 Torr, at which the impinging rate of residual gas atoms on a silicon surface was two orders of magnitude higher than the bombarding ion flux. Therefore the asdeposited films were always amorphous at an ion energy of 100 eV. In the present experiments, however, the impinging rate of Si ÷ ions was two orders or more higher than the residual gas atom flux. The epitaxial growth of silicon is thought to result from the improvements in vacuum pressure and ion current flux. 4. CONCLUSIONS Germanium and silicon films were epitaxially grown on Si(100) substrates by a low energy mass-separated ion beam deposition method. Germanium films deposited at 300 °C were epitaxially grown from the substrates, although a lot of dislocation loops were found in the deposited layer. Large lattice mismatching between germanium and silicon and the low substrate temperature for solid phase epitaxial growth are thought to have resulted in these defects. Silicon films, composed of 28Si+ ions deposited at an ion energy of 200eV and a substrate temperature of 740 °C, were isotopically enriched with the single species 28Si. The crystalline structure, observed by means of R H E E D and T E M measurements, indicated that the films were epitaxially grown on Si(100) substrates. However, the films were rather defective and contained twin structures. Further investigationsare necessary to obtain defect-free silicon epitaxial growth at low temperatures. ACKNOWLEDGMENTS The authors wish to thank Drs. M. Tamura and N. Natsuaki for their fruitful discussions and for TEM and Rutherford backscattering measurements. They are also grateful to H. Kakibayashi for his R H E E D measurements and to Dr. K. Yagi for his valuable discussions throughout the course of this work. REFERENCES 1 T. Tokuyama,K. Yagi, K. Miyake, M. Tamura, N. Natsuaki and S. Tachi, Nucl. Instrum. Methods, 182-183, Part I (1981)241. 2 K. Yagi, S. Tamura and T. Tokuyama,Jpn. J. Appl. Phys., 16 (1977) 245. 3 T. Itoh, T. Nakamura, M. Muromachi and T. Sugiyama,Jpn. J. Appl. Phys., 16 (1977) 553. 4 T. Takagi, I. Yamada and A. Sasaki, Thin Solid Films, 45 (1977) 569. 5 S. Suzuki, H. Takai, H. Okuda and T. Itoh, Proc. llth Conf. on Solid State Devices, Tokyo, 1979, in Jpn. J. Appl. Phys., Suppl. 1, 19(1980) 647. 6 J. Amano, P. BriceandR. P.W. Lawson,J. Vac. Sci. Technol.,13 (1976) 591. 7 J.H. Freeman, Nature (London), 275 (1978) 634. 8 G.E. Thomas and E. E. de Kluizenaar, Proc. 3rd Int. Congr. on Surface Physics and Chemistry, Grenoble, 1977, Socirt6 Fran~aise du Vide, Paris, 1977,p. 136. 9 J.S. Colligon,W. A. Grant, J. S. Williams and R. P. W. Lawson, Proc. Int. Conf. on Applications of Ion Beams to Metals, University of Warwick, 1975, in Inst. Phys. Conf. Ser. 28 (1976) 357. 10 A.E.T. Kuiper, G. E. Thomas and W. J. Schouten,J. Cryst. Growth, 51 (1981) 17. 11 A.G. CullisandG. R. Booker, J. Cryst. Growth, 9(1971) l12. 12 K. Itoh and K. Takahashi, Jpn. J. Appl. Phys., 7 (1968) 821.