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Surfactant-mediated MBE growth of β-SiC on Si substrates
Materials Science and Engineering B61 – 62 (1999) 559 – 562
Surfactant-mediated MBE growth of b-SiC on Si substrates K. Zekentes *, K. Tsagaraki Foundation for Research and Technology-Hellas (FO.R.T.H.), P.O. Box 1527, Heraklion, Crete, 71110, Greece
Abstract A new approach for overcoming the problems of the heteroepitaxial b-SiC growth on Si substrates is hereby proposed. Namely, the surfactant effect is investigated by using Ge as adsorbate. The better control of the Si-surface conversion to SiC is targeted (conversion process). A structure is proposed taking into account eventual incorporation of the Ge in the SiC lattice. The growth experiments towards the proposed structure were performed by solid source MBE and the grown samples were ex-situ characterized by atomic force microscopy (AFM). In all cases, a clear difference between samples grown with and without the Ge adsorbate layer is observed. © 1999 Elsevier Science S.A. All rights reserved. Keywords: b-SiC; Heteroepitaxy; Molecular beam epitaxy; Surfactant
1. Introduction Heteroepitaxial growth of cubic (b) SiC on Si substrates has been widely investigated in the past. The large mismatches in the lattice parameter (about 22%) and thermal expansion coefficient (about 8%) precludes any conventional solution and leads to the growth of poor quality b-SiC layers. Crack-free films were produced by the two step growth approach i.e. conversion, at low temperature, of Si surface to b-SiC by exposing it to a carbon-containing precursor (conversion process) followed by a normal deposition step at higher substrate temperature. The driving idea was the formation of an initial continous and epitaxial layer of b-SiC in which the resulting strain is released through misfit dislocations. In this case, an extra b-SiC plane is introduced in every four silicon lattice planes thus creating a misfit dislocation. The localization of the misfit dislocations is considered to occur at the interface. The second step could be considered as an homoepitaxial growth step resulting in improved material quality. However, a simple calculation of the misfit shows that it is not completely removed by the above procedure and residual strain is present during the initial stages of growth. The local variations of the strain at the SiC/Si interface result in a higher number of stacking faults and twins * Corresponding author. Fax: +30-81-394106. E-mail address: [email protected] (K. Zekentes)
and in most cases in a small-angle misorientation of the initial b-SiC island nuclei [1,2]. Moreover, it has been shown theoretically [3] and experimentally [1] that the conversion process is not a single epitaxial process but rather a strong chemical reaction where interdiffusion plays an important role. Deceleration of this chemical reaction during the carbonization step by chemical passivation of the Si surface would resolve many of the above problems. For this purpose, it is proposed hereby to apply the surfactant-mediated growth approach [4]. Hereby, the term of surfactant do not refer only to materials wetting the substrate’s surface and thus reducing the surface tension but more generally to materials which lower the surface energy and change the kinetics of growth while they float on the top of the growing material. More precisely, it is proposed to perform Ge monolayer coverage of the Si (100) surface. 2. Proposed approach There are a few requirements for a material to act as a good surfactant (E. Kaxiras, private communication): (a) the binding of the surfactant atoms to the substrate must not be as strong as to create difficulty in segregation to the surface during growth; (b) the surfactant must produce a chemically passive surface so that the surface energy is lowered; and (c) the surfactant results in enhanced surface diffusion of the newly deposited atoms.
0921-5107/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 8 ) 0 0 4 7 3 - 5
560
K. Zekentes, K. Tsagaraki / Materials Science and Engineering B61–62 (1999) 559–562
It is well known that Ge forms dimers on the Si (100) surface and (2×1) is the more stable reconstruction. Thus, the Ge atoms have stronger bonds with the other Ge atoms than with the Si atoms of the substrate. This implies that the Ge atoms will not have in principle any difficulty in segregating to the surface during growth on top of the newly deposited carbon atoms. Furthermore, C atoms are smaller than Ge ones and it has been theoretically shown [5] that there is a repulsive Ge–C interaction and a preferential C – C interaction in a Si –Ge–C lattice. Moreover, there are some more advantages for using Ge as explained below. The Si–Ge system produces a lattice constant larger than that of Si and results in a smaller bandgap than that of Si. When grown on Si, these layers are under compressive strain. Therefore, use of SiGe or Si1 − x − y Gex Cy layers could accommodate elastically the above mentioned residual strain following the conversion process. Eventual partial incorporation of Ge atoms would not affect the electronic properties of the grown films as it is isovalent alloying element with C and Si. Therefore, no extra-doping is expected in this case. The structure that it is proposed to grow by solid source MBE is the following: Monolayer deposition of Ge atoms on Si(100) surface for the reasons explained above. Deposition of carbon atoms for forming five to six monolayers (ML) of b-SiC by the conversion process. The thickness of the carbonization layer should not exceed this value in order to limit the interdiffusion and avoid the formation of pits [1]. The growth temperature should be a trade-off between the minimum temperature for obtaining carbidic phase and the maximum one for effective surfactant effect. A temperature of 750 – 800°C would satisfy the above requirements. An optional step of Si1 − x − y Gex Cy layer formation at 500°C would accommodate the residual strain. A typical value of the residual-strain induced misfit is 1% and the minimum value of Ge composition x: 0.25 for y= 0 would be necessary. The final step of b-SiC deposition by simultaneous supply of silicon and carbon should start at 900°C as the alloys of the type Si1 − x − y Gex Cy exhibit thermal stability up to this temperature only [6]. However, use of higher temperatures will not introduce dislocations on this level because it is well known that strain relaxation proceeds in this case by the formation of SiC precipitates [6].
Carbon and silicon were supplied by e-gun evaporation. Ge was supplied from a Knudsen cell which was heated at 1200°C for obtaining a flux of 0.14 nm min − 1. The substrates were 2-in. Si (100) misoriented by 2°–4° towards [110] direction. Pyrolitic graphite was used as the carbon source. The substrates were cleaned chemically using a standard method and the Si oxide desorption procedure has been performed [7]. The growth of a silicon buffer layer has been performed at 725°C followed by a silicon layer where the substrate temperature was ramped to the Ge monolayer deposition temperature. Once the substrate temperature was stabilized, the Si supply was stopped and the Ge deposition (1 ML) step was performed by supplying only the Ge molecules. Following Ge deposition, the substrate temperature was increased to the conversion temperature and the carbon supply was launched. The same carbon sublimation rate and two substrate temperatures (725 and 800°C) at different process duration were used. The carbon sublimation rate was kept very low at levels where the initial stages of growth could be observed whereas the normal carbonization process is not optimized due to the low number of nucleation sites [1,8]. However, the initial stages can be observed by this way and the eventual increase of surface diffusion due to the surfactant effect (E. Kaxiras, private communication) could compensate this drawback. The samples were characterized by atomic force microscopy (AFM). A sample grown at 800°C and short (1 min) conversion time permitted to observe the really first stages of growth (Fig. 1). It is obvious that SiC growth starts at
3. Experimental results All experiments were performed in a solid source MBE chamber with a base pressure of 9×10 − 10 mbar.
Fig. 1. AFM showing the beginning of the conversion to b-SiC, of the stepped surface of the Si buffer layer on which 1 ML of Ge has been previously deposited.
K. Zekentes, K. Tsagaraki / Materials Science and Engineering B61–62 (1999) 559–562
561
Fig. 2. AFM showing the surface morphology of b-SiC grown on (a) bare Si surface and (b) Si-surface with 1 ML of Ge on the top.
the steps where a well oriented hillock along the steps is formed. The nature of the surface depressions at the steps (black squares) is not clear until now but it is highly probable that they are pits. However, pits are normally formed in a later stage of the conversion process when SiC covers more than 50% of the Si surface [1]. In this case, Ge do not effectively passivates the steps and defected SiC is formed.
A later stage of growth can be observed in Figs. 2 and 3. Here the comparison of the conversion process between Si surfaces without any Ge deposition and with 1 ML of Ge is straightforward. The conversion was performed in both cases at 730°C, under the same carbon flux and longer time (3 min) in comparison with the sample of Fig. 1. It is clear that in both cases growth proceeds by formation of 3D islands at the surface steps. However, there is a obvious difference
Fig. 3. Bigger magnification AFM of the surfaces of Fig. 2.
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K. Zekentes, K. Tsagaraki / Materials Science and Engineering B61–62 (1999) 559–562
between the two surfaces. The SiC islands formed on the Si surface containing Ge cover a larger part of the surface and they are formed in a lesser extent of the triangular-like stacks of material.
Acknowledgements K. Zekentes would like to thank Professor P. Kelires and E. Kaxiras for helpful discussions and to acknowledge NATO CRG program (Grant CGR 972187) for giving him the occasion to have numerous stimulating discussions with Professor R.F. Davis.
4. Conclusions The above experimental results show clearly that the deposition of the Ge monolayer influences the conversion process and a better morphology of the SiC 3D islands is obtained in this case. Moreover, it is clear that the Ge do not passivates the surface steps where the growth mainly proceeds. However, further experimental investigation like varying the Ge coverage is required to validate the advantages of the hereby proposed approach. Nevertheless, the self-organization/ alignement of the SiC islands along the surface steps has been observed for the first time. In conclusion a new growth approach has been proposed for growing heteroepitaxial b-SiC on Si substrates and for the first time the surfactant-mediated growth approach has been studied.
.
References [1] K. Zekentes, V. Papaioannou, B. Pecz, J. Stoemenos, J. Cryst. Growth 157 (1995) 392. [2] J. Stoemenos, C. Dezauzier, G. Arnaud, S. Contreras, J. Camassel, J. Pascual, J.L. Robert, Mater. Sci. Eng. B 29 (1995) 285. [3] M. Kitabatake, M. Deguch, T. Hirao, J. Appl. Phys. 74 (1993) 4438. [4] M. Copel, M.C. Reuter, E. Kaxiras, R.M. Tromp, Phys. Rev. Lett. 63 (1989) 632. [5] P.C. Kelires, Int. J. Mod. Phys. C 9 (1998) 357. [6] M.S. Goorsky, S.S. Iyer, K. Eberl, F. Legoues, J. Angilello, F. Cardone, Appl. Phys, Lett. 60 (1992) 2758. [7] A. Georgakilas, P. Panayotatos, J. Stoemenos, J.L. Mourrain, A. Christou, J. Appl. Phys. 71 (1992) 2679. [8] K. Zekentes, R. Callec, K. Tsagaraki, B. Sagnes, G. Arnaud, J. Pascual, J. Camassel, Mater. Sci. and Eng. B29 (1995) 138–141.
Surfactant-mediated MBE growth of b-SiC on Si substrates K. Zekentes *, K. Tsagaraki Foundation for Research and Technology-Hellas (FO.R.T.H.), P.O. Box 1527, Heraklion, Crete, 71110, Greece
Abstract A new approach for overcoming the problems of the heteroepitaxial b-SiC growth on Si substrates is hereby proposed. Namely, the surfactant effect is investigated by using Ge as adsorbate. The better control of the Si-surface conversion to SiC is targeted (conversion process). A structure is proposed taking into account eventual incorporation of the Ge in the SiC lattice. The growth experiments towards the proposed structure were performed by solid source MBE and the grown samples were ex-situ characterized by atomic force microscopy (AFM). In all cases, a clear difference between samples grown with and without the Ge adsorbate layer is observed. © 1999 Elsevier Science S.A. All rights reserved. Keywords: b-SiC; Heteroepitaxy; Molecular beam epitaxy; Surfactant
1. Introduction Heteroepitaxial growth of cubic (b) SiC on Si substrates has been widely investigated in the past. The large mismatches in the lattice parameter (about 22%) and thermal expansion coefficient (about 8%) precludes any conventional solution and leads to the growth of poor quality b-SiC layers. Crack-free films were produced by the two step growth approach i.e. conversion, at low temperature, of Si surface to b-SiC by exposing it to a carbon-containing precursor (conversion process) followed by a normal deposition step at higher substrate temperature. The driving idea was the formation of an initial continous and epitaxial layer of b-SiC in which the resulting strain is released through misfit dislocations. In this case, an extra b-SiC plane is introduced in every four silicon lattice planes thus creating a misfit dislocation. The localization of the misfit dislocations is considered to occur at the interface. The second step could be considered as an homoepitaxial growth step resulting in improved material quality. However, a simple calculation of the misfit shows that it is not completely removed by the above procedure and residual strain is present during the initial stages of growth. The local variations of the strain at the SiC/Si interface result in a higher number of stacking faults and twins * Corresponding author. Fax: +30-81-394106. E-mail address: [email protected] (K. Zekentes)
and in most cases in a small-angle misorientation of the initial b-SiC island nuclei [1,2]. Moreover, it has been shown theoretically [3] and experimentally [1] that the conversion process is not a single epitaxial process but rather a strong chemical reaction where interdiffusion plays an important role. Deceleration of this chemical reaction during the carbonization step by chemical passivation of the Si surface would resolve many of the above problems. For this purpose, it is proposed hereby to apply the surfactant-mediated growth approach [4]. Hereby, the term of surfactant do not refer only to materials wetting the substrate’s surface and thus reducing the surface tension but more generally to materials which lower the surface energy and change the kinetics of growth while they float on the top of the growing material. More precisely, it is proposed to perform Ge monolayer coverage of the Si (100) surface. 2. Proposed approach There are a few requirements for a material to act as a good surfactant (E. Kaxiras, private communication): (a) the binding of the surfactant atoms to the substrate must not be as strong as to create difficulty in segregation to the surface during growth; (b) the surfactant must produce a chemically passive surface so that the surface energy is lowered; and (c) the surfactant results in enhanced surface diffusion of the newly deposited atoms.
0921-5107/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 8 ) 0 0 4 7 3 - 5
560
K. Zekentes, K. Tsagaraki / Materials Science and Engineering B61–62 (1999) 559–562
It is well known that Ge forms dimers on the Si (100) surface and (2×1) is the more stable reconstruction. Thus, the Ge atoms have stronger bonds with the other Ge atoms than with the Si atoms of the substrate. This implies that the Ge atoms will not have in principle any difficulty in segregating to the surface during growth on top of the newly deposited carbon atoms. Furthermore, C atoms are smaller than Ge ones and it has been theoretically shown [5] that there is a repulsive Ge–C interaction and a preferential C – C interaction in a Si –Ge–C lattice. Moreover, there are some more advantages for using Ge as explained below. The Si–Ge system produces a lattice constant larger than that of Si and results in a smaller bandgap than that of Si. When grown on Si, these layers are under compressive strain. Therefore, use of SiGe or Si1 − x − y Gex Cy layers could accommodate elastically the above mentioned residual strain following the conversion process. Eventual partial incorporation of Ge atoms would not affect the electronic properties of the grown films as it is isovalent alloying element with C and Si. Therefore, no extra-doping is expected in this case. The structure that it is proposed to grow by solid source MBE is the following: Monolayer deposition of Ge atoms on Si(100) surface for the reasons explained above. Deposition of carbon atoms for forming five to six monolayers (ML) of b-SiC by the conversion process. The thickness of the carbonization layer should not exceed this value in order to limit the interdiffusion and avoid the formation of pits [1]. The growth temperature should be a trade-off between the minimum temperature for obtaining carbidic phase and the maximum one for effective surfactant effect. A temperature of 750 – 800°C would satisfy the above requirements. An optional step of Si1 − x − y Gex Cy layer formation at 500°C would accommodate the residual strain. A typical value of the residual-strain induced misfit is 1% and the minimum value of Ge composition x: 0.25 for y= 0 would be necessary. The final step of b-SiC deposition by simultaneous supply of silicon and carbon should start at 900°C as the alloys of the type Si1 − x − y Gex Cy exhibit thermal stability up to this temperature only [6]. However, use of higher temperatures will not introduce dislocations on this level because it is well known that strain relaxation proceeds in this case by the formation of SiC precipitates [6].
Carbon and silicon were supplied by e-gun evaporation. Ge was supplied from a Knudsen cell which was heated at 1200°C for obtaining a flux of 0.14 nm min − 1. The substrates were 2-in. Si (100) misoriented by 2°–4° towards [110] direction. Pyrolitic graphite was used as the carbon source. The substrates were cleaned chemically using a standard method and the Si oxide desorption procedure has been performed [7]. The growth of a silicon buffer layer has been performed at 725°C followed by a silicon layer where the substrate temperature was ramped to the Ge monolayer deposition temperature. Once the substrate temperature was stabilized, the Si supply was stopped and the Ge deposition (1 ML) step was performed by supplying only the Ge molecules. Following Ge deposition, the substrate temperature was increased to the conversion temperature and the carbon supply was launched. The same carbon sublimation rate and two substrate temperatures (725 and 800°C) at different process duration were used. The carbon sublimation rate was kept very low at levels where the initial stages of growth could be observed whereas the normal carbonization process is not optimized due to the low number of nucleation sites [1,8]. However, the initial stages can be observed by this way and the eventual increase of surface diffusion due to the surfactant effect (E. Kaxiras, private communication) could compensate this drawback. The samples were characterized by atomic force microscopy (AFM). A sample grown at 800°C and short (1 min) conversion time permitted to observe the really first stages of growth (Fig. 1). It is obvious that SiC growth starts at
3. Experimental results All experiments were performed in a solid source MBE chamber with a base pressure of 9×10 − 10 mbar.
Fig. 1. AFM showing the beginning of the conversion to b-SiC, of the stepped surface of the Si buffer layer on which 1 ML of Ge has been previously deposited.
K. Zekentes, K. Tsagaraki / Materials Science and Engineering B61–62 (1999) 559–562
561
Fig. 2. AFM showing the surface morphology of b-SiC grown on (a) bare Si surface and (b) Si-surface with 1 ML of Ge on the top.
the steps where a well oriented hillock along the steps is formed. The nature of the surface depressions at the steps (black squares) is not clear until now but it is highly probable that they are pits. However, pits are normally formed in a later stage of the conversion process when SiC covers more than 50% of the Si surface [1]. In this case, Ge do not effectively passivates the steps and defected SiC is formed.
A later stage of growth can be observed in Figs. 2 and 3. Here the comparison of the conversion process between Si surfaces without any Ge deposition and with 1 ML of Ge is straightforward. The conversion was performed in both cases at 730°C, under the same carbon flux and longer time (3 min) in comparison with the sample of Fig. 1. It is clear that in both cases growth proceeds by formation of 3D islands at the surface steps. However, there is a obvious difference
Fig. 3. Bigger magnification AFM of the surfaces of Fig. 2.
562
K. Zekentes, K. Tsagaraki / Materials Science and Engineering B61–62 (1999) 559–562
between the two surfaces. The SiC islands formed on the Si surface containing Ge cover a larger part of the surface and they are formed in a lesser extent of the triangular-like stacks of material.
Acknowledgements K. Zekentes would like to thank Professor P. Kelires and E. Kaxiras for helpful discussions and to acknowledge NATO CRG program (Grant CGR 972187) for giving him the occasion to have numerous stimulating discussions with Professor R.F. Davis.
4. Conclusions The above experimental results show clearly that the deposition of the Ge monolayer influences the conversion process and a better morphology of the SiC 3D islands is obtained in this case. Moreover, it is clear that the Ge do not passivates the surface steps where the growth mainly proceeds. However, further experimental investigation like varying the Ge coverage is required to validate the advantages of the hereby proposed approach. Nevertheless, the self-organization/ alignement of the SiC islands along the surface steps has been observed for the first time. In conclusion a new growth approach has been proposed for growing heteroepitaxial b-SiC on Si substrates and for the first time the surfactant-mediated growth approach has been studied.
.
References [1] K. Zekentes, V. Papaioannou, B. Pecz, J. Stoemenos, J. Cryst. Growth 157 (1995) 392. [2] J. Stoemenos, C. Dezauzier, G. Arnaud, S. Contreras, J. Camassel, J. Pascual, J.L. Robert, Mater. Sci. Eng. B 29 (1995) 285. [3] M. Kitabatake, M. Deguch, T. Hirao, J. Appl. Phys. 74 (1993) 4438. [4] M. Copel, M.C. Reuter, E. Kaxiras, R.M. Tromp, Phys. Rev. Lett. 63 (1989) 632. [5] P.C. Kelires, Int. J. Mod. Phys. C 9 (1998) 357. [6] M.S. Goorsky, S.S. Iyer, K. Eberl, F. Legoues, J. Angilello, F. Cardone, Appl. Phys, Lett. 60 (1992) 2758. [7] A. Georgakilas, P. Panayotatos, J. Stoemenos, J.L. Mourrain, A. Christou, J. Appl. Phys. 71 (1992) 2679. [8] K. Zekentes, R. Callec, K. Tsagaraki, B. Sagnes, G. Arnaud, J. Pascual, J. Camassel, Mater. Sci. and Eng. B29 (1995) 138–141.