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Epitaxial growth of single-crystal Si1−xGex on Si(100) by ion beam sputter deposition
Thin Solid Films, 184 (1990) 117 123
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E P I T A X I A L G R O W T H OF S I N G L E - C R Y S T A L S i l _ x G e x O N Si(100) BY ION BEAM S P U T T E R D E P O S I T I O N F. MEYER, C. SCHWEBEL, C. PELLET AND G. GAUTHERIN
I.E.F., Bdt. 220, Universitk Paris XI, U.R.A. D0022, CNRS, 91405 Orsay (France) A. BUXBAUM AND M. EIZENBERG
Department ¢~fMaterials Engineering, The Solid Sate Institute, Technion Israel Institute c?f Technology, Ha~fa, 32000 ( L~rael) A. RAIZMAN Soreq Nuclear Research Center, Yavne, 70600 (Israel) (Received May 30, 1989)
The first results are reported on Si I -xGex epitaxial layers grown on Si(100) by ion beam sputter deposition in ultrahigh vacuum. Growth temperatures were varied from 300 to 700 °C, for compositions in the range 0.05 < x < 0.5. The properties of the grown films, such as morphology and structure, were studied by scanning electron microscopy and reflection high-energy electron diffraction respectively. Results indicate good monocrystallinity for the entire range of deposition temperature used. For deposition temperatures over 300 °C, the films have smooth surfaces for all compositions and thicknesses. The distorted lattice parameters of epitaxial Sio.vGe0. 3 layers were measured by double-crystal diffractometry and the tetragonal strain was calculated. Increasing deposition temperature results in strain relaxation. Layers 3000 A thick grown at 400 °C still retain a larger strain than that measured in a similar layer grown by molecular beam epitaxy. All these results may show the effects of energetic bombardment on the growing film.
1. INTRODUCTION Advances in Sil xGex technology depend largely upon improved growth methods and crystalline quality of the epitaxial layer. To date, Si-Ge layers have been grown by molecular beam epitaxy (MBE) 1'2, chemical vapor deposition (CVD) 3 and liquid phase epitaxy (LPE) 4. In these techniques, most of the species arriving at the substrate have a kinetic energy of less than about 0.2 eV. Although the mechanisms are not yet well elucidated, it is well known that bombardment of a growing film by energetic particles may result in modifications of the film properties 5 7. The additional energy may create nucleation sites and increase adatom mobilities, and hence allow a lowering of the temperature for epitaxial growth. Energetic bombardment may also result in modifications of the film microstructure in terms of grain size, lattice parameter, density, and surface topography, for instance, and therefore may alter the state of the stress of the films. Some researchers 6 have also shown the influence of low energy ion irradiation in 0040-6090/90/$3.50
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controlling the growth of metastable alloys. Unfortunately, energetic b o m b a r d m e n t may also induce defects during deposition. Taking into account all these considerations, the growth of high quality films requires an optimum value of kinetic energy from a few to a few hundred electronvolts 8. Low temperature silicon epitaxy has been reported using ion beam technologies such as ion beam sputter deposition (IBSD) 9'1°, ionized cluster beam deposition (ICBD) 11 and partially ionized vapour deposition (PIVD) lz. Recently, Zhur et al. x 3 have achieved good silicon epitaxy at temperatures as low as 400 °C by direct ion beam deposition (IBD) with ion energy of 10eV. They observed that germanium epitaxy was even easier to achieve, possibly because germanium is much less sensitive to vacuum conditions, surface cleaning and/or irradiation damage than silicon. They demonstrated that IBD, where ion energy and flux on the deposit are independent, is a very convenient tool to investigate the interactions between fast particles and a surface. In this paper we report our first results on the growth and the properties of Sil xGex films deposited on silicon by IBSD in an ultrahigh vacuum system. Growth temperatures were varied from 300 to 700°C for compositions in the 0.05 < x <0.50 range. Properties such as morphology, structure, composition and strain have been studied. Of course, IBSD provides a weaker control of deposition parameters than IBD, but has a wider applicability. For instance, we have shown that this technique leads to the formation of high quality silicon homoepitaxy TM and yttria-stabilized zirconia heteroepitaxy on silicon ~4. In our ultrahigh vacuum IBSD system, the only species that b o m b a r d the film emanate from the target. Such species include sputtered atoms and noble gas atoms either reflected from the target or trapped and sputtered later. Under ultrahigh vacuum conditions, these species do not collide with gas molecules and cannot lose any energy before impinging upon the deposit. The sputtered atoms arrive at the substrate with a rather broad energy distribution and an average energy about a few tens of electonvolts, weakly dependent on the incident energy (approximately El/3) 15 and relatively independent of the mass ratio M z / M 1 (where M1 is the atomic mass of the incident ion and M 2 is the mass of the target atom). Meanwhile, the reflected particle characteristics depend on this atomic ratio. The reflection coefficient and reflected energy strongly decrease with decreasing mass ratio ~s. 2. E X P E R I M E N T A L DETAILS
The Si-Ge alloys were grown in an ultrahigh vacuum IBSD apparatus described previously 1°. This system consists of three main parts: the ion source, the intermediate chamber to focus the beam and the deposition chamber, which contains a target-holder and a substrate-holder in which up to ten samples may be inserted. Additional equipment attached to the deposition chamber includes a residual gas analyser, a reflection high energy electron diffraction (RHEED) spectrometer and an Auger electron spectrometer. The base pressure is about 2 x 10 v Pa. The working pressure, mainly noble gas pressure, is about 2 z 10 - s Pa and the partial pressure of gaseous impurities ( H 2 0 , CO, C 0 2 ) remains below 2 x 10 v Pa during the deposition.
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Before their introduction into the sputtering system, 1 5 ~ c m p-type (100)oriented silicon wafers are degreased and chemically cleaned with the purpose of growing a thin silicon oxide film to protect the silicon surface. Samples are kept in ethanol until their loading, and immediately before deposition, oxide is eliminated in situ by flash heating at 900 °C for 2 min. To reduce the influence of high-energy reflected ions on the target, all our experiments were performed with Xe + ions instead o f A r + ions. Si 1 xGex alloys are formed by cosputtering of silicon and germanium from two different elemental targets with 20 keV Xe +. Composition control was simply achieved by masking more or less of the silicon target with the germanium target between each run. Because silicon and germanium targets are respectively above and below the substrate, the alloy composition was not expected to be uniform. Measurements of Si Ge uniformity across a sample (1 cm × 1 cm) by EDS showed that the alloy composition varied along vertical directions from a value about 0.5~o below to a value 0.5~o above the composition value at the centre (x ~ 30~o). However, an even larger discrepancy in uniformity (about 15~o) has been observed in the horizontal direction. This surprising result may perhaps show a difference of angular distributions of silicon and germanium sputtered particles. Additional experiments must be performed to verify this hypothesis. To improve composition uniformity, we shall have to use a more complex target, or to alternate germanium and silicon thin layers, or to rotate the substrate during deposition as is usual in conventional growth, by MBE. The morphology and structure of the as-grown layers were studied by scanning electron microscopy (SEM) and R H E E D respectively. The average composition of layers was obtained by energy dispersion spectrometry (EDS) and Rutherford backscattering spectrometry (RBS). Composition homogeneity was studied by Auger electron spectroscopy (AES) depth profiling. Finally, some samples were examined by double-crystal X-ray diffraction (DCD) to obtain the distorted lattice parameters of the epitaxial layers and their strain. 3.
RESULTS A N D DISCUSSION
3.1. Chemical composition When the xenon beam was placed at 45 ° relative to the target normal, we measured by RBS xenon contents of 0.14~o and 0.015~o in silicon films deposited at room temperature and 700 °C respectively 10. To reduce the noble gas incorporation still further, the incident angle of the beam was increased to 60 °, leading to a more specular reflection and a decrease in the number of xenon atoms reflected towards the sample (/~ = 15 ° relative to the target normal). After this modification, we observed no xenon contamination of our Si-Ge layers, even with proton induced Xray emission (PIXE). Composition homogeneity was examined by AES depth profiling and results show excellent uniformity with depth and a clean sharp interface between the Si-Ge and the silicon (Fig. 1). However, for samples grown at higher temperatures (about 700 °C), a weak surface segregation of germanium was observed, and the germanium concentration near the interface is about 25~o lower than near the surface.
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Fig. l. Auger depth profile in a Si0.TGeo.3 layer grown at 400 C. 3.2. Structure and morphology F i g u r e s 2(a) a n d 2(c) show r e p r e s e n t a t i v e R H E E D p a t t e r n s of S i - G e layers d e p o s i t e d at 300 a n d 700"~C on a silicon s u b s t r a t e (x = 30%). The p a t t e r n is c o m p o s e d of s t r e a k y spots, which indicates r e a s o n a b l y g o o d e p i t a x y even at the lower t e m p e r a t u r e (300 °C), a n d s o m e K i k u c h i lines are present in the p a t t e r n for the higher t e m p e r a t u r e . The crystalline q u a l i t y of the films a p p e a r s to be i n d e p e n d e n t of thickness a n d of c o m p o s i t i o n . It is to be n o t e d that, for low d e p o s i t i o n t e m p e r a t u r e (Ts = 300 °C), the crystalline quality is better for S i - G e alloys t h a n for p u r e silicon 1°. Such b e h a v i o u r has previously been o b s e r v e d by Bean et al. 2 F o r M B E epitaxial layers. RBS in channelling o r i e n t a t i o n (Ge[100]) of 0.9 M e V He + was also used to
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(c) (d) Fig. 2. (a) RHEED patterns ((110) azimuthal direction), (b) SEM micrograph for an Sio.6Ge0.4 film of 7000 ~ thickness grown at 300 °C; (c) RH EED patterns ((110) azimuthal direction), (d) SEM micrograph for an Sio.6Geo.,, film of 7000/~ thickness grown at 700 'C.
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121
EPITAXY BY ION BEAM SPUTTER DEPOSITION
examine the crystallinity of the Si-Ge films. At x = 30~, the Xmi. values range from 8To to 11~o when the deposition temperature decreases from 700 to 400 °C. We have noted some dechannelling with depth; this indicates a higher defect density near the interface in the layers of 3000~ thickness. These relatively high values of Xmi,, although we have obtained a Zml, about 2~o for silicon homoepitaxy 1°, are not surprising. First, the value of the lattice mismatch f = ( b - as)/as (b and a s are the lattice parameters of the alloy and of the substrate respectively) is high; a value of f = 1.26~o may be estimated assuming Vegard's law (f = 0.042x). For comparison, such a lattice mismatch leads to a ;~m~, = 15~ for an MBE epitaxial layer of 1000 thickness 4 grown at 550 °C. Regardless of film composition (0.05 ~< x ~< 0.5) and thickness (200/~ ~< t~< 1 p.m), the surface of the films remained continuous and smooth (Fig. 2(d)) for deposition temperatures greater than 300 °C. At 300 °C, the films had rough but still continuous surfaces, with regular hemispherical patterns as shown in Fig. 2(b). This behaviour is u n c o m m o n z'4. For instance, Bean et al. 2 noticed that the coalescence may still be incomplete in layers of 2500/~ thickness when deposition temperature is greater than 550 °C and x is larger than about 30~o. 3.3. Strain
The distorted lattice parameters of the epitaxial layers: b± and bll, normal and in-plane lattice constants respectively, were performed by DCD. X-Ray measurements were made with indium antimony as the first crystal and Cu K , radiation following a method recently described by Bhat et al. 16 Briefly, two different rocking curves with respect to the (511) plane are made and the alloy lattice parameters parallel and perpendicular to the interface can be calculated from these data. The results, for Sio.7Geo.3 alloy, as a function of deposition temperature, are plotted in Fig. 3. The thickness of the layers was about 3000/~ and thus greater than the critical thickness hc : 600 ~ 1 7 and it was ascertained that no deformation in the substrate occurred. The lattice constant perpendicular to the film is always larger than that 0.03 b tt
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along the surface, so the unit cell of the layer is not cubic but tetragonally distorted, corresponding to a compressive stress. The difference between perpendicular and inplane constants decreases as deposition temperature increases. The tetragonal strain eT = ( b ± - b I0/as is relatively large at low temperature (eT ~ 1.5~o at 400 °C) and is very weak at high temperature (ev ~ 0.03~ at 700 °C). For comparison, the vertical scale of Fig. 3 gives the values of the mismatch f = 1.26~o corresponding to an unstrained epitaxy (assuming Vegard's law and a cubic cell for Sio.TGe0.3) and to a strained epitaxy (commensurate growth). In the case of a commensurate growth, the in-plane lattice parameter is determined by the substrate (btl -= as) and the tetragonal strain is given by ev -
b , - bll
.l+v
(1)
where v is Poisson's ratio. This ratio is calculated by an interpolation between germanium (v = 0.273) and silicon (v = 0.280) is. Two observations can be made. First, as the temperature increases the two lattice constants (bll and b±) converge towards the cubic lattice constant of unstrained Sio.TGeo.3. This strain relaxation is probably a result of generation of misfit dislocations19; germanium diffusion may play only a reduced role for the higher temperature in our case 2°. Second, the layer of 3000 ~ thickness grown at 400 °C still retains a high strain (ev = 1.5~o) which is much larger than that measured in an MBE grown epitaxial layer of 2500,~ thickness (eT = 0.5~o for x = 30~o) 19. It is speculated that this additional intrinsic strain that appears at low epitaxial temperatures is related to the deposition method and in particular to the energetic b o m b a r d m e n t of the growing film, and may be explained by the "ion peening model" first suggested by d'Heurle 21. 4. CONCLUSION
We have prepared a variety ofSil xGex films by IBSD in an ultrahigh vacuum system. Epitaxial layers have been grown at low temperature (Ts = 300 °C). When the deposition temperature is greater than 300 °C, the surface of the films is smooth regardless of the thickness (200 ~ ~< t ~< 1 g m ) a n d the composition (0.05 ~< x ~< 0.5) of the films. All the Sio.TGeo. 3 films retain a compressive strain. This strain remains large at low deposition temperature (Ts = 400°C), even for a layer of 3000A thickness and is greater than that in a MBE layer with similar deposition parameters. These results may be related to the deposition method and in particular to energetic bombardment. Further investigations by transmission electron microscopy are under way to study the formation of misfit dislocations at the interface, and electrical characterizations (Hall effect measurements and Schottky diode characterizations) are to be carried out. ACKNOWLEDGMENTS
The authors from Orsay acknowledge that the pioneer works and contributions by Professor Christian Weissmantel have been essential for their own progress in the fields of ion beam sputter deposition. They will miss him.
Si~-x Gex EPITAXY
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The authors thank R. Laval and F. Pillier for EDS measurements and RHEED analyses respectively. They are also grateful to F. Fort and A. Chabrier for their technical assistance and to A. Charrier for the preparation of the figures. The kind help ofY. Gertner and Y. Saban in carrying out RBS analyses is greatly appreciated. The work at the Technion was supported by the Coleman Cohen Research Fund and by the Fund for the Promotion of Research at the Technion. REFERENCES I 2
3 4 5
6 7 8 9 10 I1 12 13 14 15 16 17 18 19 20 2l
E. Kasper and H. J. Herzog, Thin Solid Films, 44 (1977) 357. J . C . Bean, T. T. Sheng, L. C. Feldman, A. T. Fiory and R. T. Lynch, Appl. Phys. Lett., 44 (1984) 102. B.S. Meyerson, K.J. UramandF. K. LeGoues, Appl. Phys. Lett.,53(1988) 2555. J . R . Abelson, T. W. Sigmon, K. B. Kim and K. H. Weiner, Appl. Phys. Lett., 52 (1988) 230. J . M . E . Harper, J. J. Cuomo, R. J. G a m b i n o and H. R. K a u f m a n , in O. Aucielleo and R. Kelly (eds.), Ion Bombardment Modifications of Surfaces: Fundamentals and Applications, Elsevier, Amsterdam, 1984, p. 127. J.E. Green, S. A. Barnett, J. E. Sundgren and A. Rocket, in T. Itoh (ed.), Ion Beam Assisted Film Growth, Elsevier, Amsterdam, 1989, p. 101. S.M. Rossnagel and J. J. Cuomo, Thin Solid Films, 171 (1989) 143. T. Tagagi, Thin Solid Films, 92 (1982) 1. C. Weissmantel, G. Hecht and H. J. Hinneberg, J. Vac. Sci. Technol., 17 (1980) 812. C. Schwebel, F. Meyer, G. Gautherin and C. Pellet, J. Vac. Sci. Technol. B, 4 (1986) 1153. I. Y a m a d a and T. Takagi, Thin Solid Films, 80 (1981) 105. T. Narusawa, S. Shimizu and S. Komiya, J. Vac. Sci. Technol., 16 (1979) 366. R. A. Zhur, B. R. Appleton, N. Herbots, B. C. Larson, T. S. Noggle and S. J. Pennycook, J. Vac. Sci. Technol. A, 5 (1987) 2135. P. Legagneux, G. Garry, D. Dieumegard, C. Schwebel, C. Pellet, G. Gautherin and J. Siejka, Appl. Phys. Lett., 53 (1988) 1506. W. Eckstein and J. P. Biersack, Z. Phys. B-Condensed Matter, 63 (1986) 471. 1. B. Bhat, K. Patel, N. R. Taskar, J. E. Ayers and S. K. Gandhy, J. Crvst. Growth, 88 (1988) 23. J.C. Bean, J. Vac. Sci. Technol. B, 4(1986) 1427. W . A . Brantley, J. Appl. Phys., 44 (1973) 1534. A . T . Fiory, J.C. Bean, L.C. FeldmanandI. K. Robinson, J. Appl. Phys.,56(1984) 1227. B. Hollander, S. Mantl, B. Stritzker, H. Jorke and E. Kasper, J. Mater. Res., 4 (1989) 163. F . M . d'Heurle, Metall. Trans., 1 (1970) 725.
117
E P I T A X I A L G R O W T H OF S I N G L E - C R Y S T A L S i l _ x G e x O N Si(100) BY ION BEAM S P U T T E R D E P O S I T I O N F. MEYER, C. SCHWEBEL, C. PELLET AND G. GAUTHERIN
I.E.F., Bdt. 220, Universitk Paris XI, U.R.A. D0022, CNRS, 91405 Orsay (France) A. BUXBAUM AND M. EIZENBERG
Department ¢~fMaterials Engineering, The Solid Sate Institute, Technion Israel Institute c?f Technology, Ha~fa, 32000 ( L~rael) A. RAIZMAN Soreq Nuclear Research Center, Yavne, 70600 (Israel) (Received May 30, 1989)
The first results are reported on Si I -xGex epitaxial layers grown on Si(100) by ion beam sputter deposition in ultrahigh vacuum. Growth temperatures were varied from 300 to 700 °C, for compositions in the range 0.05 < x < 0.5. The properties of the grown films, such as morphology and structure, were studied by scanning electron microscopy and reflection high-energy electron diffraction respectively. Results indicate good monocrystallinity for the entire range of deposition temperature used. For deposition temperatures over 300 °C, the films have smooth surfaces for all compositions and thicknesses. The distorted lattice parameters of epitaxial Sio.vGe0. 3 layers were measured by double-crystal diffractometry and the tetragonal strain was calculated. Increasing deposition temperature results in strain relaxation. Layers 3000 A thick grown at 400 °C still retain a larger strain than that measured in a similar layer grown by molecular beam epitaxy. All these results may show the effects of energetic bombardment on the growing film.
1. INTRODUCTION Advances in Sil xGex technology depend largely upon improved growth methods and crystalline quality of the epitaxial layer. To date, Si-Ge layers have been grown by molecular beam epitaxy (MBE) 1'2, chemical vapor deposition (CVD) 3 and liquid phase epitaxy (LPE) 4. In these techniques, most of the species arriving at the substrate have a kinetic energy of less than about 0.2 eV. Although the mechanisms are not yet well elucidated, it is well known that bombardment of a growing film by energetic particles may result in modifications of the film properties 5 7. The additional energy may create nucleation sites and increase adatom mobilities, and hence allow a lowering of the temperature for epitaxial growth. Energetic bombardment may also result in modifications of the film microstructure in terms of grain size, lattice parameter, density, and surface topography, for instance, and therefore may alter the state of the stress of the films. Some researchers 6 have also shown the influence of low energy ion irradiation in 0040-6090/90/$3.50
~, Elsevier Sequoia/Printed in The Netherlands
118
F. MEYERe t al.
controlling the growth of metastable alloys. Unfortunately, energetic b o m b a r d m e n t may also induce defects during deposition. Taking into account all these considerations, the growth of high quality films requires an optimum value of kinetic energy from a few to a few hundred electronvolts 8. Low temperature silicon epitaxy has been reported using ion beam technologies such as ion beam sputter deposition (IBSD) 9'1°, ionized cluster beam deposition (ICBD) 11 and partially ionized vapour deposition (PIVD) lz. Recently, Zhur et al. x 3 have achieved good silicon epitaxy at temperatures as low as 400 °C by direct ion beam deposition (IBD) with ion energy of 10eV. They observed that germanium epitaxy was even easier to achieve, possibly because germanium is much less sensitive to vacuum conditions, surface cleaning and/or irradiation damage than silicon. They demonstrated that IBD, where ion energy and flux on the deposit are independent, is a very convenient tool to investigate the interactions between fast particles and a surface. In this paper we report our first results on the growth and the properties of Sil xGex films deposited on silicon by IBSD in an ultrahigh vacuum system. Growth temperatures were varied from 300 to 700°C for compositions in the 0.05 < x <0.50 range. Properties such as morphology, structure, composition and strain have been studied. Of course, IBSD provides a weaker control of deposition parameters than IBD, but has a wider applicability. For instance, we have shown that this technique leads to the formation of high quality silicon homoepitaxy TM and yttria-stabilized zirconia heteroepitaxy on silicon ~4. In our ultrahigh vacuum IBSD system, the only species that b o m b a r d the film emanate from the target. Such species include sputtered atoms and noble gas atoms either reflected from the target or trapped and sputtered later. Under ultrahigh vacuum conditions, these species do not collide with gas molecules and cannot lose any energy before impinging upon the deposit. The sputtered atoms arrive at the substrate with a rather broad energy distribution and an average energy about a few tens of electonvolts, weakly dependent on the incident energy (approximately El/3) 15 and relatively independent of the mass ratio M z / M 1 (where M1 is the atomic mass of the incident ion and M 2 is the mass of the target atom). Meanwhile, the reflected particle characteristics depend on this atomic ratio. The reflection coefficient and reflected energy strongly decrease with decreasing mass ratio ~s. 2. E X P E R I M E N T A L DETAILS
The Si-Ge alloys were grown in an ultrahigh vacuum IBSD apparatus described previously 1°. This system consists of three main parts: the ion source, the intermediate chamber to focus the beam and the deposition chamber, which contains a target-holder and a substrate-holder in which up to ten samples may be inserted. Additional equipment attached to the deposition chamber includes a residual gas analyser, a reflection high energy electron diffraction (RHEED) spectrometer and an Auger electron spectrometer. The base pressure is about 2 x 10 v Pa. The working pressure, mainly noble gas pressure, is about 2 z 10 - s Pa and the partial pressure of gaseous impurities ( H 2 0 , CO, C 0 2 ) remains below 2 x 10 v Pa during the deposition.
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Before their introduction into the sputtering system, 1 5 ~ c m p-type (100)oriented silicon wafers are degreased and chemically cleaned with the purpose of growing a thin silicon oxide film to protect the silicon surface. Samples are kept in ethanol until their loading, and immediately before deposition, oxide is eliminated in situ by flash heating at 900 °C for 2 min. To reduce the influence of high-energy reflected ions on the target, all our experiments were performed with Xe + ions instead o f A r + ions. Si 1 xGex alloys are formed by cosputtering of silicon and germanium from two different elemental targets with 20 keV Xe +. Composition control was simply achieved by masking more or less of the silicon target with the germanium target between each run. Because silicon and germanium targets are respectively above and below the substrate, the alloy composition was not expected to be uniform. Measurements of Si Ge uniformity across a sample (1 cm × 1 cm) by EDS showed that the alloy composition varied along vertical directions from a value about 0.5~o below to a value 0.5~o above the composition value at the centre (x ~ 30~o). However, an even larger discrepancy in uniformity (about 15~o) has been observed in the horizontal direction. This surprising result may perhaps show a difference of angular distributions of silicon and germanium sputtered particles. Additional experiments must be performed to verify this hypothesis. To improve composition uniformity, we shall have to use a more complex target, or to alternate germanium and silicon thin layers, or to rotate the substrate during deposition as is usual in conventional growth, by MBE. The morphology and structure of the as-grown layers were studied by scanning electron microscopy (SEM) and R H E E D respectively. The average composition of layers was obtained by energy dispersion spectrometry (EDS) and Rutherford backscattering spectrometry (RBS). Composition homogeneity was studied by Auger electron spectroscopy (AES) depth profiling. Finally, some samples were examined by double-crystal X-ray diffraction (DCD) to obtain the distorted lattice parameters of the epitaxial layers and their strain. 3.
RESULTS A N D DISCUSSION
3.1. Chemical composition When the xenon beam was placed at 45 ° relative to the target normal, we measured by RBS xenon contents of 0.14~o and 0.015~o in silicon films deposited at room temperature and 700 °C respectively 10. To reduce the noble gas incorporation still further, the incident angle of the beam was increased to 60 °, leading to a more specular reflection and a decrease in the number of xenon atoms reflected towards the sample (/~ = 15 ° relative to the target normal). After this modification, we observed no xenon contamination of our Si-Ge layers, even with proton induced Xray emission (PIXE). Composition homogeneity was examined by AES depth profiling and results show excellent uniformity with depth and a clean sharp interface between the Si-Ge and the silicon (Fig. 1). However, for samples grown at higher temperatures (about 700 °C), a weak surface segregation of germanium was observed, and the germanium concentration near the interface is about 25~o lower than near the surface.
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Fig. l. Auger depth profile in a Si0.TGeo.3 layer grown at 400 C. 3.2. Structure and morphology F i g u r e s 2(a) a n d 2(c) show r e p r e s e n t a t i v e R H E E D p a t t e r n s of S i - G e layers d e p o s i t e d at 300 a n d 700"~C on a silicon s u b s t r a t e (x = 30%). The p a t t e r n is c o m p o s e d of s t r e a k y spots, which indicates r e a s o n a b l y g o o d e p i t a x y even at the lower t e m p e r a t u r e (300 °C), a n d s o m e K i k u c h i lines are present in the p a t t e r n for the higher t e m p e r a t u r e . The crystalline q u a l i t y of the films a p p e a r s to be i n d e p e n d e n t of thickness a n d of c o m p o s i t i o n . It is to be n o t e d that, for low d e p o s i t i o n t e m p e r a t u r e (Ts = 300 °C), the crystalline quality is better for S i - G e alloys t h a n for p u r e silicon 1°. Such b e h a v i o u r has previously been o b s e r v e d by Bean et al. 2 F o r M B E epitaxial layers. RBS in channelling o r i e n t a t i o n (Ge[100]) of 0.9 M e V He + was also used to
i
i
i i i (a)
(b) n
(c) (d) Fig. 2. (a) RHEED patterns ((110) azimuthal direction), (b) SEM micrograph for an Sio.6Ge0.4 film of 7000 ~ thickness grown at 300 °C; (c) RH EED patterns ((110) azimuthal direction), (d) SEM micrograph for an Sio.6Geo.,, film of 7000/~ thickness grown at 700 'C.
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EPITAXY BY ION BEAM SPUTTER DEPOSITION
examine the crystallinity of the Si-Ge films. At x = 30~, the Xmi. values range from 8To to 11~o when the deposition temperature decreases from 700 to 400 °C. We have noted some dechannelling with depth; this indicates a higher defect density near the interface in the layers of 3000~ thickness. These relatively high values of Xmi,, although we have obtained a Zml, about 2~o for silicon homoepitaxy 1°, are not surprising. First, the value of the lattice mismatch f = ( b - as)/as (b and a s are the lattice parameters of the alloy and of the substrate respectively) is high; a value of f = 1.26~o may be estimated assuming Vegard's law (f = 0.042x). For comparison, such a lattice mismatch leads to a ;~m~, = 15~ for an MBE epitaxial layer of 1000 thickness 4 grown at 550 °C. Regardless of film composition (0.05 ~< x ~< 0.5) and thickness (200/~ ~< t~< 1 p.m), the surface of the films remained continuous and smooth (Fig. 2(d)) for deposition temperatures greater than 300 °C. At 300 °C, the films had rough but still continuous surfaces, with regular hemispherical patterns as shown in Fig. 2(b). This behaviour is u n c o m m o n z'4. For instance, Bean et al. 2 noticed that the coalescence may still be incomplete in layers of 2500/~ thickness when deposition temperature is greater than 550 °C and x is larger than about 30~o. 3.3. Strain
The distorted lattice parameters of the epitaxial layers: b± and bll, normal and in-plane lattice constants respectively, were performed by DCD. X-Ray measurements were made with indium antimony as the first crystal and Cu K , radiation following a method recently described by Bhat et al. 16 Briefly, two different rocking curves with respect to the (511) plane are made and the alloy lattice parameters parallel and perpendicular to the interface can be calculated from these data. The results, for Sio.7Geo.3 alloy, as a function of deposition temperature, are plotted in Fig. 3. The thickness of the layers was about 3000/~ and thus greater than the critical thickness hc : 600 ~ 1 7 and it was ascertained that no deformation in the substrate occurred. The lattice constant perpendicular to the film is always larger than that 0.03 b tt
b
0.02
"~
~ "~"
&S
unstrained epita~
/
4
[] perpendicular
0.01 /"
b
tl
-
.
&8
in plane
a8
0,00
I
300
400
I
500
I
I
600
700
800
deposition tcmperatuT¢('C) Fig. 3. Results of DCD measurements of lattice relaxation in Si0.TGeo.3 films of 3000/~ thickness as a function of deposition temperature.
122
v. MEYERet
al.
along the surface, so the unit cell of the layer is not cubic but tetragonally distorted, corresponding to a compressive stress. The difference between perpendicular and inplane constants decreases as deposition temperature increases. The tetragonal strain eT = ( b ± - b I0/as is relatively large at low temperature (eT ~ 1.5~o at 400 °C) and is very weak at high temperature (ev ~ 0.03~ at 700 °C). For comparison, the vertical scale of Fig. 3 gives the values of the mismatch f = 1.26~o corresponding to an unstrained epitaxy (assuming Vegard's law and a cubic cell for Sio.TGe0.3) and to a strained epitaxy (commensurate growth). In the case of a commensurate growth, the in-plane lattice parameter is determined by the substrate (btl -= as) and the tetragonal strain is given by ev -
b , - bll
.l+v
(1)
where v is Poisson's ratio. This ratio is calculated by an interpolation between germanium (v = 0.273) and silicon (v = 0.280) is. Two observations can be made. First, as the temperature increases the two lattice constants (bll and b±) converge towards the cubic lattice constant of unstrained Sio.TGeo.3. This strain relaxation is probably a result of generation of misfit dislocations19; germanium diffusion may play only a reduced role for the higher temperature in our case 2°. Second, the layer of 3000 ~ thickness grown at 400 °C still retains a high strain (ev = 1.5~o) which is much larger than that measured in an MBE grown epitaxial layer of 2500,~ thickness (eT = 0.5~o for x = 30~o) 19. It is speculated that this additional intrinsic strain that appears at low epitaxial temperatures is related to the deposition method and in particular to the energetic b o m b a r d m e n t of the growing film, and may be explained by the "ion peening model" first suggested by d'Heurle 21. 4. CONCLUSION
We have prepared a variety ofSil xGex films by IBSD in an ultrahigh vacuum system. Epitaxial layers have been grown at low temperature (Ts = 300 °C). When the deposition temperature is greater than 300 °C, the surface of the films is smooth regardless of the thickness (200 ~ ~< t ~< 1 g m ) a n d the composition (0.05 ~< x ~< 0.5) of the films. All the Sio.TGeo. 3 films retain a compressive strain. This strain remains large at low deposition temperature (Ts = 400°C), even for a layer of 3000A thickness and is greater than that in a MBE layer with similar deposition parameters. These results may be related to the deposition method and in particular to energetic bombardment. Further investigations by transmission electron microscopy are under way to study the formation of misfit dislocations at the interface, and electrical characterizations (Hall effect measurements and Schottky diode characterizations) are to be carried out. ACKNOWLEDGMENTS
The authors from Orsay acknowledge that the pioneer works and contributions by Professor Christian Weissmantel have been essential for their own progress in the fields of ion beam sputter deposition. They will miss him.
Si~-x Gex EPITAXY
BY ION BEAM SPUTTER DEPOSITION
123
The authors thank R. Laval and F. Pillier for EDS measurements and RHEED analyses respectively. They are also grateful to F. Fort and A. Chabrier for their technical assistance and to A. Charrier for the preparation of the figures. The kind help ofY. Gertner and Y. Saban in carrying out RBS analyses is greatly appreciated. The work at the Technion was supported by the Coleman Cohen Research Fund and by the Fund for the Promotion of Research at the Technion. REFERENCES I 2
3 4 5
6 7 8 9 10 I1 12 13 14 15 16 17 18 19 20 2l
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