- Email: [email protected]
High-quality and low-temperature epitaxial Si films deposited at very high deposition rate
Journal of Crystal Growth 225 (2001) 335–339
High-quality and low-temperature epitaxial Si films deposited at very high deposition rate Ralf B. Bergmann*, Lars Oberbeck, Thomas A. Wagner Institut fu¨r Physikalische Elektronik, Universita¨t Stuttgart, Pfaffenwaldring 47, Stuttgart D-70569, Germany
Abstract Low-temperature epitaxy at temperatures between 5508C and 6508C using ion-assisted deposition enables the formation of Si films with minority-carrier diffusion lengths deposited at rates previously only conceivable using hightemperature chemical vapor deposition at growth temperatures exceeding 10008C. Using quantum efficiency and photoluminescence measurements, we investigate charge carrier recombination in Si films formed by ion-assisted deposition at temperatures between 4608C and 6508C. Silicon films deposited at a temperature between 4608C and 5108C display relatively short minority-carrier diffusion lengths peaked at a deposition rate around 0.25 mm/min, while we find high diffusion lengths >20 mm in Si films deposited at a temperature 55508C with deposition rates between 0.2 and 0.8 mm/min. At a deposition temperature of 6508C we achieve a minority carrier diffusion length of 40 mm in a 21 mm thick epitaxial Si film deposited at a rate of 0.8 mm/min. # 2001 Elsevier Science B.V. All rights reserved. PACS: 73.50.Gr; 81.15.Jj; 81.15.Hi; 85.40.Sz Keywords: A1. Characterization; A3. Physical vapor deposition processes; B2. Semiconducting silicon; B3. Solar cells
1. Introduction Epitaxial deposition of Si at temperatures above 10008C is a well established process that yields high-quality, device grade Si films at a high deposition rate of around 1 mm/min. Many device processes, however, require epitaxial growth at low temperatures of less than 6508C. Restrictions of the deposition temperature may be imposed either by unwanted interdiffusion of dopants or multi*Corresponding author. Tel.: +49-711-685-7163, fax: +49711-685-7143. E-mail address: [email protected] (R.B. Bergmann).
layered structures [1,2] or by the use of foreign substrates [3,4]. Epitaxial growth of Si at deposition temperatures below 6508C is usually impeded by low deposition rates and comparatively low electronic quality of the resulting Si films. In this paper, we investigate low-temperature Si epitaxial growth at deposition temperatures Tdep 46508C. We use ion-assisted deposition (IAD) as a means of systematically studying the dependence of the film properties on practically relevant deposition parameters such as deposition temperature Tdep and deposition rate rdep . This deposition technique is similar to molecular beam epitaxy (MBE), however, ionized Si atoms which are accelerated towards the growing surface
0022-0248/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 0 9 0 6 - X
336
R.B. Bergmann et al. / Journal of Crystal Growth 225 (2001) 335–339
provide additional energy for the growth process. In addition, using a physical vapor deposition process such as IAD permits one, in contrast to chemical vapor deposition (CVD), to separate the chemical decomposition kinetics from the kinetics of the epitaxial growth process. For a detailed description of IAD see Ref. [5].
2. Low-temperature Si epitaxy A compilation of literature data on epitaxial growth rates at Tdep 46508C shows [4,6] that epitaxy based on CVD processes exhibits deposition rates limited to rdep 40:1 mm/min. These epitaxial rates approximately display an exponential increase with deposition temperature, an observation that most probably reflects the decomposition kinetics of the precursor gases rather than limitations fundamentally related to the growth process itself. In contrast to these results, epitaxial deposition rates obtained from MBE [7] or IAD (this work), which use Si evaporation from a solid Si source, are independent of deposition temperature. Recently, Thiesen et al. [8] demonstrated silicon epitaxy by means of hot-wire CVD (HW CVD) with deposition rates exceeding 0.1 mm/min at Tdep ¼ 4508C. This approach appears to be promising in terms of deposition rates; however, up to now it has not been proven that the electronic properties of Si epitaxial layers deposited by HW CVD are suitable for electronic devices. At deposition temperatures below 4508C, a critical thickness exists, above which epitaxial growth turns into polycrystalline or amorphous growth. The critical thickness as function of the deposition temperature follows an Arrhenius law with an activation energy of Eact ¼ 0:4520:5 eV [7,9].
3. Low-temperature epitaxy by IAD Our recent work demonstrated that epitaxial growth of several-mm-thick Si films is possible by IAD at deposition temperatures Tdep 54358C [6].
Furthermore, majority-carrier mobility as determined by room temperature Hall-effect measurements approaches values obtained from bulk-Si and is fairly insensitive of deposition conditions once Tdep 55408C. Minority-carrier diffusion length L, however, critically depends on Tdep [5]. The dependence of L on rdep as well as the nature of defects limiting the material quality is yet unresolved. In order to study the properties of lowtemperature epitaxial Si films, we first deposit a highly boron doped, 1.5-mm-thick buffer layer onto a (1 0 0)-oriented Si substrate at elevated temperatures onto which epitaxial Si films with a thickness d in the range of 104d421 mm and a boron doping of around 1 1017 cm3 are consecutively deposited at 4608C4Tdep 46508C. Finally, a phosphorus-doped Si film with a thickness of 0.5 mm serves to form a pn-junction that allows for electrical characterization of the films. The low-temperature epitaxial Si films are at no stage of their processing exposed to temperatures higher than their respective deposition temperature Tdep . Internal quantum efficiency (IQE) measurements on these samples serve to determine the minority-carrier diffusion length L within the films [10], while the density of extended structural defects is evaluated by Secco etching [11]. Details of the sample preparation and the evaluation procedure are described elsewhere [5]. In addition, we use photoluminescence (PL) spectroscopy in order to detect the presence of defect levels.
4. Characterization of epitaxial films Fig. 1 displays the dependence of the minoritycarrier diffusion length L on deposition temperature and rate. It reveals two distinct parameter regimes: (i) At Tdep 45108C, we observe a peak of L at rdep 0:25 mm/min. (ii) At Tdep 55508C, the diffusion length L is fairly insensitive to rdep even up to a very high deposition rate of 0.81 mm/min. Secco etching employed in this study permits identification of dislocations and stacking faults. The dislocation density according to etch pit counts on the surface of our epitaxial Si films is on the order of 105 cm2. Stacking faults with a
R.B. Bergmann et al. / Journal of Crystal Growth 225 (2001) 335–339
Fig. 1. Minority-carrier diffusion length L as a function of deposition rate rdep for deposition temperatures Tdep ¼ 4608C, 5108C, 5508C and 6508C, respectively. Epitaxy at Tdep 45108C results in maximum L for rdep 0:25 mm/min while epitaxy at Tdep 55508C produces high L even for a deposition rate of rdep ¼ 0:81 mm/min (dashed lines are visual guides only).
337
photon energies above 1 eV we see band edge (BE) luminescence with characteristic phonon-associated peaks [12]. Below a photon energy of 1 eV a broad distribution of PL intensity appears that indicates a broad defect distribution. Especially remarkable are the luminescence peaks at energies between 0.72 and 0.77 eV. These peaks correspond to deep levels with energies between 0.41 and 0.46 eV below the conduction band and have in literature been associated with copper decorated defects [13]. Copper atoms may be sputtered from components of the electron gun systems. We, therefore, conjecture that the PL peaks between 0.72 and 0.77 eV may be related to copper decorated dislocations. We did not, however, observe the so-called D-lines commonly associated with dislocations in Si [14] and we were not able to identify Cu by means of secondary ion mass spectroscopy (SIMS) with a resolution limit of about 1016 cm3.
5. Discussion
Fig. 2. Photoluminescence spectra of epitaxial Si films deposited at rdep ¼ 0:3 mm/min and Tdep ¼ 4608C, 5108C and 6508C, respectively. Peaks at photon energies above 1 eV stem from band edge luminescence, while peaks at lower energies are related to deep-level defects. The error in peak height is estimated to about 15%.
density of around 103–104 cm2 have only been found on films deposited at 6508C. Fig. 2 shows PL spectra excited by an Ar+ ion laser operating at a wavelength of 488 nm with a power density of 10 W/cm2. For comparability of the spectral intensity, all measurements are performed at 12 K using the same optical setup. A germanium detector serves to record the PL spectra at a sample temperature of 12 K. For
Charge carrier recombination in Si films may be dominated by dislocations or point defects in the bulk of the films or by surface or interface recombination. A comparison of data on minority-carrier diffusion length (see Fig. 1) and etch pit densities reveals that there is no correlation between minority-carrier diffusion length and dislocation density. Electrical simulations show [5] that minority-carrier diffusion lengths L > d=2 in films with thickness d represent effective diffusion lengths, since recombination is not limited any more by recombination centers within the bulk of the Si film alone but also by recombination at the surface and the interface between the substrate and the Si film. Our PL investigations demonstrate that samples prepared at Tdep ¼ 4608C have, independent of deposition rate, a high defect density at PL energies below 1 eV. As a result of the preceding discussion, we conclude that the diffusion length L in films deposited at Tdep ¼ 4608C must be limited by point defects of yet unresolved nature. At Tdep 55108C, this defect density is strongly reduced see Fig. 2 and our PL setup is presently not
338
R.B. Bergmann et al. / Journal of Crystal Growth 225 (2001) 335–339
sensitive enough to detect differences in defect luminescence intensity of films deposited at higher temperatures. Apart from the sample deposited at Tdep=5508C, the intensity of the BE luminescence rises with Tdep, indicating enhanced radiative recombination and hence improved material quality at higher deposition temperatures. This observation is consistent with the results presented in Fig. 1. As a consequence of the results described above, we distinguish two regimes for the growth of lowtemperature epitaxial Si films: (i) At Tdep 45108C, minority-carrier diffusion length L is limited by deep-level recombination centers. The low diffusion length observed at low deposition rates of rdep50.2 mm/min is most probably limited by impurities from the gas phase incorporated during growth. Assuming a constant partial pressure of impurities in the growth system and a partial pressure of Si proportional to the growth rate, the impurity concentration in a Si film originating from the gas phase should be / 1=rdep . Film quality thus increases with deposition rate. At high deposition rates, however, thermal annealing of local structural defects is probably too slow to compensate for the formation of recombination centers and therefore L decreases again. (ii) At Tdep 55508C, minority-carrier diffusion length is, at least to some extent, limited by interface recombination for the parameters used in this study. Point defects have, as observed by PL, a much lower concentration as compared to Si films formed at Tdep ¼ 4608C. In the temperature regime of 5508C4Tdep 46508C, material quality still increases with deposition temperature and appears to fully benefit from reduced incorporation of impurities at high growth rates. Films exhibit diffusion lengths L>20 mm at Tdep ¼ 5508C and up to 40 mm at Tdep ¼ 6508C:
6. Summary and conclusions The present investigation demonstrates that low-temperature epitaxy of Si using IAD enables
high quality epitaxial growth with deposition rates previously only conceivable by epitaxial growth at temperatures exceeding 10008C. Further work will concentrate on the identification of the nature of point defects in low-temperature epitaxial Si films and the improvement of surface preparation in order to reduce the dislocation density within the Si films. At Tdep 45108C minority-carrier diffusion length L is limited by the presence of deep level defects and diffusion lengths peak at deposition rates rdep 0:25 mm. At Tdep ¼ 5508C minoritycarrier diffusion lengths L > 20 mm are obtained at deposition rates of 0.55rdep 50:8 mm/min, with rdep limited by the electron gun evaporator employed in the growth system. We achieve a maximum diffusion length of L ¼ 40 mm in a 21 mm thick epitaxial layer deposited at 6508C with a deposition rate of 0.8 mm/min.
Acknowledgements The authors thank S. Amann, G. Bilger, K. Brenner and M. Gerlach for technical assistance, M. Schubert for carefully reading the manuscript, and J.H. Werner for continuous support and stimulating discussions. The authors gratefully acknowledge the support of the German Ministry for Education, Science, Research, and Technology (BMBF) under Contract No. 0329818.
References [1] B.S. Meyerson, in: M.L. Hitchman, K.F. Jensen (Eds.), Chemical Vapor Deposition, Academic Press, London, 1993 p. 219, (Chapter 5). [2] M.A. Herman H. Sitter, Molecular Beam Epitaxy, Springer, Berlin, 1996 p. 327 (Chapter 7.1) [3] R.B. Bergmann, J. Ko¨hler, R. Dassow, C. Zaczek, J.H. Werner, Phys. Stat. Sol. A 166 (1998) 587. [4] R.B. Bergmann, T.J. Rinke, L. Oberbeck, R. Dassow, in: P.L.F. Hemment, V.S. Lysenko, A.N. Nazarov (Eds.), Perspectives, Science and Technologies for Novel Silicon on Insulator Devices, NATO Science Series 3. High Technology, Vol. 73, Kluwer Academic Publishers, Dordrecht, 2000, p. 109. [5] L. Oberbeck, R.B. Bergmann, J. Appl. Phys. 88 (2000) 3015.
R.B. Bergmann et al. / Journal of Crystal Growth 225 (2001) 335–339 [6] L. Oberbeck, T.A. Wagner, R.B. Bergmann, Mater Res. Soc. Symp. Proc. 609 (2000) A7.1.1. [7] D.J. Eaglesham, J. Appl. Phys. 77 (1995) 3597. [8] J. Thiesen, E. Iwaniczko, K.M. Jones, A. Mahan, R. Crandall, Appl. Phys. Lett. 75 (1999) 992. [9] A.I. Nikiforov, B.Z. Kanter, O.P. Pchelyakov, Thin Solid Films 336 (1998) 179.
339
[10] R. Brendel, M. Hirsch, R. Plieninger, J.H. Werner, IEEE Trans. Electron Devices 43 (1996) 1104. [11] F. Secco d’Aragona, J. Electrochem. Soc. 119 (1972) 948. [12] P.J. Dean, Phys. Rev. 161 (1967) 711. [13] A.A. Istratov, E.R. Weber, Appl. Phys. A 66 (1998) 123. [14] E.A. Steinmann, H.G. Grimmeiss, Semicond. Sci. Technol. 13 (1998) 124.
High-quality and low-temperature epitaxial Si films deposited at very high deposition rate Ralf B. Bergmann*, Lars Oberbeck, Thomas A. Wagner Institut fu¨r Physikalische Elektronik, Universita¨t Stuttgart, Pfaffenwaldring 47, Stuttgart D-70569, Germany
Abstract Low-temperature epitaxy at temperatures between 5508C and 6508C using ion-assisted deposition enables the formation of Si films with minority-carrier diffusion lengths deposited at rates previously only conceivable using hightemperature chemical vapor deposition at growth temperatures exceeding 10008C. Using quantum efficiency and photoluminescence measurements, we investigate charge carrier recombination in Si films formed by ion-assisted deposition at temperatures between 4608C and 6508C. Silicon films deposited at a temperature between 4608C and 5108C display relatively short minority-carrier diffusion lengths peaked at a deposition rate around 0.25 mm/min, while we find high diffusion lengths >20 mm in Si films deposited at a temperature 55508C with deposition rates between 0.2 and 0.8 mm/min. At a deposition temperature of 6508C we achieve a minority carrier diffusion length of 40 mm in a 21 mm thick epitaxial Si film deposited at a rate of 0.8 mm/min. # 2001 Elsevier Science B.V. All rights reserved. PACS: 73.50.Gr; 81.15.Jj; 81.15.Hi; 85.40.Sz Keywords: A1. Characterization; A3. Physical vapor deposition processes; B2. Semiconducting silicon; B3. Solar cells
1. Introduction Epitaxial deposition of Si at temperatures above 10008C is a well established process that yields high-quality, device grade Si films at a high deposition rate of around 1 mm/min. Many device processes, however, require epitaxial growth at low temperatures of less than 6508C. Restrictions of the deposition temperature may be imposed either by unwanted interdiffusion of dopants or multi*Corresponding author. Tel.: +49-711-685-7163, fax: +49711-685-7143. E-mail address: [email protected] (R.B. Bergmann).
layered structures [1,2] or by the use of foreign substrates [3,4]. Epitaxial growth of Si at deposition temperatures below 6508C is usually impeded by low deposition rates and comparatively low electronic quality of the resulting Si films. In this paper, we investigate low-temperature Si epitaxial growth at deposition temperatures Tdep 46508C. We use ion-assisted deposition (IAD) as a means of systematically studying the dependence of the film properties on practically relevant deposition parameters such as deposition temperature Tdep and deposition rate rdep . This deposition technique is similar to molecular beam epitaxy (MBE), however, ionized Si atoms which are accelerated towards the growing surface
0022-0248/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 0 9 0 6 - X
336
R.B. Bergmann et al. / Journal of Crystal Growth 225 (2001) 335–339
provide additional energy for the growth process. In addition, using a physical vapor deposition process such as IAD permits one, in contrast to chemical vapor deposition (CVD), to separate the chemical decomposition kinetics from the kinetics of the epitaxial growth process. For a detailed description of IAD see Ref. [5].
2. Low-temperature Si epitaxy A compilation of literature data on epitaxial growth rates at Tdep 46508C shows [4,6] that epitaxy based on CVD processes exhibits deposition rates limited to rdep 40:1 mm/min. These epitaxial rates approximately display an exponential increase with deposition temperature, an observation that most probably reflects the decomposition kinetics of the precursor gases rather than limitations fundamentally related to the growth process itself. In contrast to these results, epitaxial deposition rates obtained from MBE [7] or IAD (this work), which use Si evaporation from a solid Si source, are independent of deposition temperature. Recently, Thiesen et al. [8] demonstrated silicon epitaxy by means of hot-wire CVD (HW CVD) with deposition rates exceeding 0.1 mm/min at Tdep ¼ 4508C. This approach appears to be promising in terms of deposition rates; however, up to now it has not been proven that the electronic properties of Si epitaxial layers deposited by HW CVD are suitable for electronic devices. At deposition temperatures below 4508C, a critical thickness exists, above which epitaxial growth turns into polycrystalline or amorphous growth. The critical thickness as function of the deposition temperature follows an Arrhenius law with an activation energy of Eact ¼ 0:4520:5 eV [7,9].
3. Low-temperature epitaxy by IAD Our recent work demonstrated that epitaxial growth of several-mm-thick Si films is possible by IAD at deposition temperatures Tdep 54358C [6].
Furthermore, majority-carrier mobility as determined by room temperature Hall-effect measurements approaches values obtained from bulk-Si and is fairly insensitive of deposition conditions once Tdep 55408C. Minority-carrier diffusion length L, however, critically depends on Tdep [5]. The dependence of L on rdep as well as the nature of defects limiting the material quality is yet unresolved. In order to study the properties of lowtemperature epitaxial Si films, we first deposit a highly boron doped, 1.5-mm-thick buffer layer onto a (1 0 0)-oriented Si substrate at elevated temperatures onto which epitaxial Si films with a thickness d in the range of 104d421 mm and a boron doping of around 1 1017 cm3 are consecutively deposited at 4608C4Tdep 46508C. Finally, a phosphorus-doped Si film with a thickness of 0.5 mm serves to form a pn-junction that allows for electrical characterization of the films. The low-temperature epitaxial Si films are at no stage of their processing exposed to temperatures higher than their respective deposition temperature Tdep . Internal quantum efficiency (IQE) measurements on these samples serve to determine the minority-carrier diffusion length L within the films [10], while the density of extended structural defects is evaluated by Secco etching [11]. Details of the sample preparation and the evaluation procedure are described elsewhere [5]. In addition, we use photoluminescence (PL) spectroscopy in order to detect the presence of defect levels.
4. Characterization of epitaxial films Fig. 1 displays the dependence of the minoritycarrier diffusion length L on deposition temperature and rate. It reveals two distinct parameter regimes: (i) At Tdep 45108C, we observe a peak of L at rdep 0:25 mm/min. (ii) At Tdep 55508C, the diffusion length L is fairly insensitive to rdep even up to a very high deposition rate of 0.81 mm/min. Secco etching employed in this study permits identification of dislocations and stacking faults. The dislocation density according to etch pit counts on the surface of our epitaxial Si films is on the order of 105 cm2. Stacking faults with a
R.B. Bergmann et al. / Journal of Crystal Growth 225 (2001) 335–339
Fig. 1. Minority-carrier diffusion length L as a function of deposition rate rdep for deposition temperatures Tdep ¼ 4608C, 5108C, 5508C and 6508C, respectively. Epitaxy at Tdep 45108C results in maximum L for rdep 0:25 mm/min while epitaxy at Tdep 55508C produces high L even for a deposition rate of rdep ¼ 0:81 mm/min (dashed lines are visual guides only).
337
photon energies above 1 eV we see band edge (BE) luminescence with characteristic phonon-associated peaks [12]. Below a photon energy of 1 eV a broad distribution of PL intensity appears that indicates a broad defect distribution. Especially remarkable are the luminescence peaks at energies between 0.72 and 0.77 eV. These peaks correspond to deep levels with energies between 0.41 and 0.46 eV below the conduction band and have in literature been associated with copper decorated defects [13]. Copper atoms may be sputtered from components of the electron gun systems. We, therefore, conjecture that the PL peaks between 0.72 and 0.77 eV may be related to copper decorated dislocations. We did not, however, observe the so-called D-lines commonly associated with dislocations in Si [14] and we were not able to identify Cu by means of secondary ion mass spectroscopy (SIMS) with a resolution limit of about 1016 cm3.
5. Discussion
Fig. 2. Photoluminescence spectra of epitaxial Si films deposited at rdep ¼ 0:3 mm/min and Tdep ¼ 4608C, 5108C and 6508C, respectively. Peaks at photon energies above 1 eV stem from band edge luminescence, while peaks at lower energies are related to deep-level defects. The error in peak height is estimated to about 15%.
density of around 103–104 cm2 have only been found on films deposited at 6508C. Fig. 2 shows PL spectra excited by an Ar+ ion laser operating at a wavelength of 488 nm with a power density of 10 W/cm2. For comparability of the spectral intensity, all measurements are performed at 12 K using the same optical setup. A germanium detector serves to record the PL spectra at a sample temperature of 12 K. For
Charge carrier recombination in Si films may be dominated by dislocations or point defects in the bulk of the films or by surface or interface recombination. A comparison of data on minority-carrier diffusion length (see Fig. 1) and etch pit densities reveals that there is no correlation between minority-carrier diffusion length and dislocation density. Electrical simulations show [5] that minority-carrier diffusion lengths L > d=2 in films with thickness d represent effective diffusion lengths, since recombination is not limited any more by recombination centers within the bulk of the Si film alone but also by recombination at the surface and the interface between the substrate and the Si film. Our PL investigations demonstrate that samples prepared at Tdep ¼ 4608C have, independent of deposition rate, a high defect density at PL energies below 1 eV. As a result of the preceding discussion, we conclude that the diffusion length L in films deposited at Tdep ¼ 4608C must be limited by point defects of yet unresolved nature. At Tdep 55108C, this defect density is strongly reduced see Fig. 2 and our PL setup is presently not
338
R.B. Bergmann et al. / Journal of Crystal Growth 225 (2001) 335–339
sensitive enough to detect differences in defect luminescence intensity of films deposited at higher temperatures. Apart from the sample deposited at Tdep=5508C, the intensity of the BE luminescence rises with Tdep, indicating enhanced radiative recombination and hence improved material quality at higher deposition temperatures. This observation is consistent with the results presented in Fig. 1. As a consequence of the results described above, we distinguish two regimes for the growth of lowtemperature epitaxial Si films: (i) At Tdep 45108C, minority-carrier diffusion length L is limited by deep-level recombination centers. The low diffusion length observed at low deposition rates of rdep50.2 mm/min is most probably limited by impurities from the gas phase incorporated during growth. Assuming a constant partial pressure of impurities in the growth system and a partial pressure of Si proportional to the growth rate, the impurity concentration in a Si film originating from the gas phase should be / 1=rdep . Film quality thus increases with deposition rate. At high deposition rates, however, thermal annealing of local structural defects is probably too slow to compensate for the formation of recombination centers and therefore L decreases again. (ii) At Tdep 55508C, minority-carrier diffusion length is, at least to some extent, limited by interface recombination for the parameters used in this study. Point defects have, as observed by PL, a much lower concentration as compared to Si films formed at Tdep ¼ 4608C. In the temperature regime of 5508C4Tdep 46508C, material quality still increases with deposition temperature and appears to fully benefit from reduced incorporation of impurities at high growth rates. Films exhibit diffusion lengths L>20 mm at Tdep ¼ 5508C and up to 40 mm at Tdep ¼ 6508C:
6. Summary and conclusions The present investigation demonstrates that low-temperature epitaxy of Si using IAD enables
high quality epitaxial growth with deposition rates previously only conceivable by epitaxial growth at temperatures exceeding 10008C. Further work will concentrate on the identification of the nature of point defects in low-temperature epitaxial Si films and the improvement of surface preparation in order to reduce the dislocation density within the Si films. At Tdep 45108C minority-carrier diffusion length L is limited by the presence of deep level defects and diffusion lengths peak at deposition rates rdep 0:25 mm. At Tdep ¼ 5508C minoritycarrier diffusion lengths L > 20 mm are obtained at deposition rates of 0.55rdep 50:8 mm/min, with rdep limited by the electron gun evaporator employed in the growth system. We achieve a maximum diffusion length of L ¼ 40 mm in a 21 mm thick epitaxial layer deposited at 6508C with a deposition rate of 0.8 mm/min.
Acknowledgements The authors thank S. Amann, G. Bilger, K. Brenner and M. Gerlach for technical assistance, M. Schubert for carefully reading the manuscript, and J.H. Werner for continuous support and stimulating discussions. The authors gratefully acknowledge the support of the German Ministry for Education, Science, Research, and Technology (BMBF) under Contract No. 0329818.
References [1] B.S. Meyerson, in: M.L. Hitchman, K.F. Jensen (Eds.), Chemical Vapor Deposition, Academic Press, London, 1993 p. 219, (Chapter 5). [2] M.A. Herman H. Sitter, Molecular Beam Epitaxy, Springer, Berlin, 1996 p. 327 (Chapter 7.1) [3] R.B. Bergmann, J. Ko¨hler, R. Dassow, C. Zaczek, J.H. Werner, Phys. Stat. Sol. A 166 (1998) 587. [4] R.B. Bergmann, T.J. Rinke, L. Oberbeck, R. Dassow, in: P.L.F. Hemment, V.S. Lysenko, A.N. Nazarov (Eds.), Perspectives, Science and Technologies for Novel Silicon on Insulator Devices, NATO Science Series 3. High Technology, Vol. 73, Kluwer Academic Publishers, Dordrecht, 2000, p. 109. [5] L. Oberbeck, R.B. Bergmann, J. Appl. Phys. 88 (2000) 3015.
R.B. Bergmann et al. / Journal of Crystal Growth 225 (2001) 335–339 [6] L. Oberbeck, T.A. Wagner, R.B. Bergmann, Mater Res. Soc. Symp. Proc. 609 (2000) A7.1.1. [7] D.J. Eaglesham, J. Appl. Phys. 77 (1995) 3597. [8] J. Thiesen, E. Iwaniczko, K.M. Jones, A. Mahan, R. Crandall, Appl. Phys. Lett. 75 (1999) 992. [9] A.I. Nikiforov, B.Z. Kanter, O.P. Pchelyakov, Thin Solid Films 336 (1998) 179.
339
[10] R. Brendel, M. Hirsch, R. Plieninger, J.H. Werner, IEEE Trans. Electron Devices 43 (1996) 1104. [11] F. Secco d’Aragona, J. Electrochem. Soc. 119 (1972) 948. [12] P.J. Dean, Phys. Rev. 161 (1967) 711. [13] A.A. Istratov, E.R. Weber, Appl. Phys. A 66 (1998) 123. [14] E.A. Steinmann, H.G. Grimmeiss, Semicond. Sci. Technol. 13 (1998) 124.