- Email: [email protected]
n-si structures by MBE and their electrical and optical properties
Journal of Luminescence 80 (1999) 473— 477
Fabrication of p-Si/b-FeSi balls/n-si structures by MBE and their electrical and optical properties T. Suemasu*, T. Fujii, M. Tanaka, K. Takakura, Y. Iikura, F. Hasegawa Institute of Materials Science, University of Tsukuba, 1-1-1 Tennohdai, Tsukuba, Ibaraki 305-8573, Japan
Abstract We report on the formation technique of single-crystalline b-FeSi balls ((100 nm) embedded in a Si p—n junction region by Si molecular beam epitaxy (MBE). b-FeSi films grown on Si (0 0 1) by reactive deposition epitaxy (RDE) aggregated into islands after annealing at 850°C in ultrahigh vacuum. The islands of b-FeSi aggregated further into a ball shape by following the Si MBE overgrowth at 750°C. It was found from X-ray diffraction (XRD) patterns that the epitaxial relationship between the two materials, and single-crystalline nature were preserved even after the annealing and the Si overgrowth. Capacitance—voltage (C—») characteristics and transmission electron microscope (TEM) images revealed that a lot of defects were introduced around the embedded b-FeSi balls with an increase of embedded b-FeSi quantity. 1999 Elsevier Science B.V. All rights reserved. PACS: 81.05.Hd; 81.10.Jt; 81.15.Hi Keywords: b-FeSi ; Reactive deposition epitaxy (RDE); Si-MBE; Aggregation; Light emitting diode
1. Introduction In the past few years, semiconducting iron disilicide b-FeSi has attracted considerable attention, because it has a direct band gap of about 0.85 eV (1.46 lm) at room temperature (RT) [1—4], which is near the absorption minimum of silica optical fibers. In addition to that, due to the large refractive index of b-FeSi (5.6) [5] compared to Si (3.5), p-Si/b-FeSi /n-Si double heterostructure (DH) is very interesting for light-emitting devices. Home-
* Corresponding author: Tel.:#81-298-53-5111; fax: #81298-55-7440; e-mail: [email protected].
wood et al. realized the first Si/b-FeSi light-emit ting diode (LED) operating at a wavelength of 1.5 lm [6]. This fact confirms that b-FeSi is a promising candidate for Si-based optoelectronic devices. However, it was fabricated by implantation of Fe> into the depletion region of a Si p—n diode. Consequently, a long-time annealing of high temperature is necessary in order to recover damaged Si crystals caused by the implantation. Among several techniques for epitaxial growth of b-FeSi , reactive deposition epitaxy (RDE) is supposed to have less damage on Si crystals, because it is just the deposition of Fe on a hot Si substrate [7—10]. We have studied the epitaxial growth of b-FeSi on Si(0 0 1) by RDE, and tried to fabricate Si/b-FeSi /Si structure [11,12].
0022-2313/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 8 ) 0 0 1 5 8 - 6
474
T. Suemasu et al. / Journal of Luminescence 80 (1999) 473—477
In this paper, we present the formation technique of single-crystalline b-FeSi balls ((100 nm) em bedded in a Si p—n junction region by Si molecular beam epitaxy (MBE). In the course of the fabrication, transformation of RDE growth b-FeSi layers into islands and balls are observed by a cross-sectional transmission electron microscope (TEM). Electron traps near the Si p—n junction will be discussed using the carrier concentration profiles obtained from capacitance—voltage (C—») measurements, and TEM images.
3. Results and Discussion 3.1. Fabrication of p-Si/b-FeSi balls/n-Si structures Fig. 1 shows cross-sectional images of the 200 A> -thick b-FeSi sample at each stage, i.e., sam ples after (a) the RDE growth, (b) the annealing, (c)
2. Experimental An ion-pumped MBE system equipped with a 30 keV RHEED and electron gun evaporation sources for Fe and Si was used. The base pressure of the MBE system was less than 1;10\ Torr and it was kept below 5;10\ Torr during the evaporation of Fe and Si. N-type epitaxial Si(0 0 1) substrates (10—20 ) cm 20;20;20;0.50 mm) were used. The substrate was cleaned and a thin protective oxide layer was formed by the RCA method. After loading the substrate into the growth chamber, the substrate was heated at 850°C for 20 min to remove the protective oxide, which yielded a well developed 2;1/1;2 RHEED pattern. After cleaning the surface, the substrate was cooled and Fe of 99.99% purity was deposited to grow b-FeSi layers at 470°C. Three kinds of samples with different b-FeSi thicknesses (100 A> , 200 A> and 400 A> ) were prepared. After the growth of b-FeSi , the substrates were annealed at 850°C for 30 min in UHV in order to smoothen the surface. The annealing temperature must be lower than the b- to a-FeSi transition temperature of about 940°C. Finally, a 1 lm-thick p-Si overlayer was grown epitaxially at 750°C by MBE with HBO irradiation [13]. The carrier concentration of the p-Si layer was determined to be 4;10 cm\ from Hall measurements. The deposition rate of Fe and Si was 0.1 A> /s and 0.4 A> /s, respectively, in this work. They were controlled by an electron impact emission spectroscopy (EIES) sensor. Ohmic contacts were made with Au on both sides, and then the samples were mesa-etched using a solution of HF : HNO : H O"3 : 10 : 6. TEM images were observed along the [1 1 0] azimuth of Si.
Fig. 1. TEM images of the 200 A> - thick b-FeSi sample after (a) the RDE growth, (b) the annealing, and (c) the 0.1 lm-thick p-Si growth. (d) SEM image after the 1 lm-thick p-Si growth.
T. Suemasu et al. / Journal of Luminescence 80 (1999) 473—477
the 0.1 lm-thick p-Si growth and (d) the 1 lm-thick p-Si growth. X-ray diffraction (XRD) patterns for these stages are presented in Fig. 2. In the epitaxial growth of b-FeSi on Si (0 0 1), the [0 1 0] and [0 0 1] axes of b-FeSi are expected to orient along the Si 11 1 02 direction under a compressive strain with the lattice mismatches of 1.4 and 2.1% at room temperature (RT), respectively. As can be seen in Fig. 2a, no peak other than those from (1 0 0)-oriented b-FeSi is observed for the as grown sample. This fact indicates that the b-FeSi layer is grown epitaxially on the Si (0 0 1) with single-crystalline nature. After the annealing, the b-FeSi layer aggregated into islands, dipping into the Si so that only one half of the height of these islands extends above the Si surface. The atomic force microscopy (AFM) shows us a clear appearance of the aggregation [12]. This is because the aggregation occurs so that the interface energy due to the lattice mismatch minimizes by decreasing the contact area between the two materials. The epitaxial relationship to the substrate is preserved even after annealing as shown in Fig. 2b. This phenomena is unique compared to the other latticemismatched epitaxial growth system, such as Ge
Fig. 2. XRD patterns of the 200 A> - thick b-FeSi sample after (a) the RDE growth, (b) the annealing and (c) the 1 lm-thick p-Si growth.
475
dots on Si [14] and InAs dots on GaAs [15], where the Stranski-Krastanow (S—K) growth mode was observed. To our knowledge, aggregation of these materials together with monocrystalline and epitaxial relationship to the substrates even after annealing has not been reported so far. Figs. 1c and 1d show that the Si MBE overgrowth starts at the bare parts of the Si on the b-FeSi /Si surface, and gradually covers the b-FeSi islands. In compari son to the annealed sample as in Fig. 1b, it was found that the b-FeSi islands aggregated further into a ball shape embedded in Si crystals. This is because a ball shape has a smaller contact area than a b-FeSi island sandwiched by Si. As can be seen in Fig. 2c, the epitaxial relationship was maintained even after the transformation of b-FeSi islands to a ball shape. As to the other two samples with different b-FeSi thicknesses, the b-FeSi were also embedded in single-crystalline nature. 3.2. Electrical properties of p>-Si/b-FeSi balls/n-Si structures Fig. 3 shows the current—voltage (I—») characteristics of the diodes measured at RT. Compared to the 100 A> -thick b-FeSi sample, the current was larger by one order of magnitude for the 200 A> thick b-FeSi sample. The ideality factors of both
Fig. 3. Forward-biased I—» characteristics of the p-Si/b-FeSi balls/n-Si(0 0 1) structures measured at RT.
476
T. Suemasu et al. / Journal of Luminescence 80 (1999) 473—477
samples were around 1.5. Depletion region of all the samples spreads towards the n-Si side because of the p>—n junction. It is well-known that boron accumulation occurs at the substrate [16], when the substrates are cleaned by the RCA method. Consequently, it can be said that the balls are embedded in the p-Si side. Therefore, the immediate influence of the b-FeSi balls on the ideality factor is small. Fig. 4 shows the carrier concentration profiles in the n-Si side from the p—n junction. It was clear that the carrier concentration close to the p—n junction decreased except for the 100 A> -thick b-FeSi sample. This tendency was clearly observed for the 400 A> -thick b-FeSi sample. These results indicate that electron traps caused by defects located near the p—n junction. There exists some papers about introduction of deep levels after iron silicidation. In those papers, origin of the deep levels was inferred to be Fe-related complexes. In our work, the 100 A> -thick b-FeSi sample showed good rectifying behavior and no deep levels were observed. Fig. 5 shows the TEM images of the p-Si/b-FeSi /n-Si structures for the (a) 100 A> - and (b) 400 A> -thick b-FeSi samples. For the 400 A> -thick b-FeSi sample, as in Fig. 5b, the b-FeSi balls do not have a neat spherical shape compared to the other sample. Furthermore, there are a lot of dislocations
propagating along the Si+1 1 1, planes. Thus, the balls of this sample were supposed to strain the Si substrate, and therefore, the origin of the electron traps was regarded as a strain between b-FeSi and Si rather than Fe-related complexes. We have observed high-resolution TEM images of the Si/bFeSi interface for the 100 A> -thick b-FeSi sample, and found out that the atomic structure close to the interface is highly distorted even for the 100 A> thick b-FeSi sample, where the edge-type disloca tion appears at the interface. We therefore speculate that this kind of interface plays a dominant role
Fig. 4. C—» characteristics of the p-Si/b-FeSi balls/n-Si(0 0 1) structures measured at RT.
Fig. 5. TEM images of p-Si/b-FeSi /n-Si(0 0 1) structures for the (a) 100 A> - and (b) 400 A> -thick b-FeSi samples.
T. Suemasu et al. / Journal of Luminescence 80 (1999) 473—477
in the appearance of the defects. Further investigations are needed to probe the origin of the deep levels. But the dependence of quantity of the embedded b-FeSi suggests that size reduction of the b-FeSi balls is one of the possible ways to decrease the density of deep levels.
4. Conclusions We have demonstrated the fabrication technique of single-crystalline b-FeSi balls embedded in an Si p—n junction region by Si molecular beam epitaxy. In spite of ball-shaped b-FeSi , (1 0 0) oriented relationship of b-FeSi to the Si (0 0 1) substrates was preserved even after the annealing and the Si overgrowth. The C—V characteristics and TEM observation indicated the existence of deep level near the p—n junction region for the sample with large quantity of b-FeSi . Acknowledgements The authors express their sincere thanks to Dr. Baumann (Texas Instruments Tsukuba R&D Center Ltd.) for fruitful discussions on the TEM results. This work was partially supported by a Grant-in-Aid for Scientific Research (B) No. 09450121 and the Priority Area ‘‘Spin Controlled Semiconductor Nanostructures’’ No. 10138204
477
from the Ministry of Education, Science, Sports and Culture, Japan.
References [1] M.C. Bost, J.E. Mahan, J. Appl. Phys. 58 (1985) 2696. [2] M.C. Bost, J.E. Mahan, J. Vac. Sci. Technol. B 4 (1986) 1336. [3] N. Cherief, C. D’ Anterroches, R.C. Cinti, T.A. Nguyen Tan, J. Derrien, Appl. Phys. Lett. 55 (1989) 1671. [4] N.E. Christensen, Phys. Rev. B 42 (1990) 7148. [5] V. Bellani, G. Guizzetti, F. Marabelli, M. Patrini, Solid State Commun. 96 (1995) 751. [6] D. Leong, M. Harry, K.J. Reeson, K.P. Homewood, Nature 387 (1997) 686. [7] J.E. Mahan, K.M. Geib, G.Y. Robinson, R.G. Long, X. Yan, G. Bai, M.A. Nicolet, M. Nathan, Appl. Phys. Lett. 56 (1990) 2126. [8] K.M. Geib, J.E. Mahan, R.G. Long, M. Nathan, G. Bai, J. Appl. Phys. 70 (1991) 1730. [9] A.H. Reader, J.P.W.B. Duchateaun, J. Timmers, F.J.G. Hakkens: Appl. Surf. Sci. 73 (1993) 3105. [10] L. Wang, L. Qin, Y. Zheng, Q. Shen, X. Chen, X. Lin, C. Lin, S. Zou, Appl. Phys. Lett. 65 (1994) 3105. [11] M. Tanaka, Y. Kumagai, T. Suemasu, F. Hasegawa, Jpn. J. Appl. Phys. 36 (1997) 3624. [12] T. Suemasu, M. Tanaka, T. Fujii, S. Hashimoto, Y. Kumagai, F. Hasegawa, Jpn. J. Appl. Phys. 36 (1997) L1225. [13] T. Tatsumi, H. Hirayama, N. Aizaki, Appl. Phys. Lett. 50 (1987) 1234. [14] D.J. Eaglemann, M. Cerullo, Phys. Rev. Lett. 64 (1990) 1943. [15] D. Leonard, M. Krishnamurthy, C.M. Reaves, S.P. Denbaars, P.M. Petroff, Appl. Phys. Lett. 63 (1993) 3203. [16] A. Casel, E. Kasper, H. Kibbel, E. Sasse, J. Vac. Sci. Technol. B 5 (1987) 1650.
Fabrication of p-Si/b-FeSi balls/n-si structures by MBE and their electrical and optical properties T. Suemasu*, T. Fujii, M. Tanaka, K. Takakura, Y. Iikura, F. Hasegawa Institute of Materials Science, University of Tsukuba, 1-1-1 Tennohdai, Tsukuba, Ibaraki 305-8573, Japan
Abstract We report on the formation technique of single-crystalline b-FeSi balls ((100 nm) embedded in a Si p—n junction region by Si molecular beam epitaxy (MBE). b-FeSi films grown on Si (0 0 1) by reactive deposition epitaxy (RDE) aggregated into islands after annealing at 850°C in ultrahigh vacuum. The islands of b-FeSi aggregated further into a ball shape by following the Si MBE overgrowth at 750°C. It was found from X-ray diffraction (XRD) patterns that the epitaxial relationship between the two materials, and single-crystalline nature were preserved even after the annealing and the Si overgrowth. Capacitance—voltage (C—») characteristics and transmission electron microscope (TEM) images revealed that a lot of defects were introduced around the embedded b-FeSi balls with an increase of embedded b-FeSi quantity. 1999 Elsevier Science B.V. All rights reserved. PACS: 81.05.Hd; 81.10.Jt; 81.15.Hi Keywords: b-FeSi ; Reactive deposition epitaxy (RDE); Si-MBE; Aggregation; Light emitting diode
1. Introduction In the past few years, semiconducting iron disilicide b-FeSi has attracted considerable attention, because it has a direct band gap of about 0.85 eV (1.46 lm) at room temperature (RT) [1—4], which is near the absorption minimum of silica optical fibers. In addition to that, due to the large refractive index of b-FeSi (5.6) [5] compared to Si (3.5), p-Si/b-FeSi /n-Si double heterostructure (DH) is very interesting for light-emitting devices. Home-
* Corresponding author: Tel.:#81-298-53-5111; fax: #81298-55-7440; e-mail: [email protected].
wood et al. realized the first Si/b-FeSi light-emit ting diode (LED) operating at a wavelength of 1.5 lm [6]. This fact confirms that b-FeSi is a promising candidate for Si-based optoelectronic devices. However, it was fabricated by implantation of Fe> into the depletion region of a Si p—n diode. Consequently, a long-time annealing of high temperature is necessary in order to recover damaged Si crystals caused by the implantation. Among several techniques for epitaxial growth of b-FeSi , reactive deposition epitaxy (RDE) is supposed to have less damage on Si crystals, because it is just the deposition of Fe on a hot Si substrate [7—10]. We have studied the epitaxial growth of b-FeSi on Si(0 0 1) by RDE, and tried to fabricate Si/b-FeSi /Si structure [11,12].
0022-2313/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 8 ) 0 0 1 5 8 - 6
474
T. Suemasu et al. / Journal of Luminescence 80 (1999) 473—477
In this paper, we present the formation technique of single-crystalline b-FeSi balls ((100 nm) em bedded in a Si p—n junction region by Si molecular beam epitaxy (MBE). In the course of the fabrication, transformation of RDE growth b-FeSi layers into islands and balls are observed by a cross-sectional transmission electron microscope (TEM). Electron traps near the Si p—n junction will be discussed using the carrier concentration profiles obtained from capacitance—voltage (C—») measurements, and TEM images.
3. Results and Discussion 3.1. Fabrication of p-Si/b-FeSi balls/n-Si structures Fig. 1 shows cross-sectional images of the 200 A> -thick b-FeSi sample at each stage, i.e., sam ples after (a) the RDE growth, (b) the annealing, (c)
2. Experimental An ion-pumped MBE system equipped with a 30 keV RHEED and electron gun evaporation sources for Fe and Si was used. The base pressure of the MBE system was less than 1;10\ Torr and it was kept below 5;10\ Torr during the evaporation of Fe and Si. N-type epitaxial Si(0 0 1) substrates (10—20 ) cm 20;20;20;0.50 mm) were used. The substrate was cleaned and a thin protective oxide layer was formed by the RCA method. After loading the substrate into the growth chamber, the substrate was heated at 850°C for 20 min to remove the protective oxide, which yielded a well developed 2;1/1;2 RHEED pattern. After cleaning the surface, the substrate was cooled and Fe of 99.99% purity was deposited to grow b-FeSi layers at 470°C. Three kinds of samples with different b-FeSi thicknesses (100 A> , 200 A> and 400 A> ) were prepared. After the growth of b-FeSi , the substrates were annealed at 850°C for 30 min in UHV in order to smoothen the surface. The annealing temperature must be lower than the b- to a-FeSi transition temperature of about 940°C. Finally, a 1 lm-thick p-Si overlayer was grown epitaxially at 750°C by MBE with HBO irradiation [13]. The carrier concentration of the p-Si layer was determined to be 4;10 cm\ from Hall measurements. The deposition rate of Fe and Si was 0.1 A> /s and 0.4 A> /s, respectively, in this work. They were controlled by an electron impact emission spectroscopy (EIES) sensor. Ohmic contacts were made with Au on both sides, and then the samples were mesa-etched using a solution of HF : HNO : H O"3 : 10 : 6. TEM images were observed along the [1 1 0] azimuth of Si.
Fig. 1. TEM images of the 200 A> - thick b-FeSi sample after (a) the RDE growth, (b) the annealing, and (c) the 0.1 lm-thick p-Si growth. (d) SEM image after the 1 lm-thick p-Si growth.
T. Suemasu et al. / Journal of Luminescence 80 (1999) 473—477
the 0.1 lm-thick p-Si growth and (d) the 1 lm-thick p-Si growth. X-ray diffraction (XRD) patterns for these stages are presented in Fig. 2. In the epitaxial growth of b-FeSi on Si (0 0 1), the [0 1 0] and [0 0 1] axes of b-FeSi are expected to orient along the Si 11 1 02 direction under a compressive strain with the lattice mismatches of 1.4 and 2.1% at room temperature (RT), respectively. As can be seen in Fig. 2a, no peak other than those from (1 0 0)-oriented b-FeSi is observed for the as grown sample. This fact indicates that the b-FeSi layer is grown epitaxially on the Si (0 0 1) with single-crystalline nature. After the annealing, the b-FeSi layer aggregated into islands, dipping into the Si so that only one half of the height of these islands extends above the Si surface. The atomic force microscopy (AFM) shows us a clear appearance of the aggregation [12]. This is because the aggregation occurs so that the interface energy due to the lattice mismatch minimizes by decreasing the contact area between the two materials. The epitaxial relationship to the substrate is preserved even after annealing as shown in Fig. 2b. This phenomena is unique compared to the other latticemismatched epitaxial growth system, such as Ge
Fig. 2. XRD patterns of the 200 A> - thick b-FeSi sample after (a) the RDE growth, (b) the annealing and (c) the 1 lm-thick p-Si growth.
475
dots on Si [14] and InAs dots on GaAs [15], where the Stranski-Krastanow (S—K) growth mode was observed. To our knowledge, aggregation of these materials together with monocrystalline and epitaxial relationship to the substrates even after annealing has not been reported so far. Figs. 1c and 1d show that the Si MBE overgrowth starts at the bare parts of the Si on the b-FeSi /Si surface, and gradually covers the b-FeSi islands. In compari son to the annealed sample as in Fig. 1b, it was found that the b-FeSi islands aggregated further into a ball shape embedded in Si crystals. This is because a ball shape has a smaller contact area than a b-FeSi island sandwiched by Si. As can be seen in Fig. 2c, the epitaxial relationship was maintained even after the transformation of b-FeSi islands to a ball shape. As to the other two samples with different b-FeSi thicknesses, the b-FeSi were also embedded in single-crystalline nature. 3.2. Electrical properties of p>-Si/b-FeSi balls/n-Si structures Fig. 3 shows the current—voltage (I—») characteristics of the diodes measured at RT. Compared to the 100 A> -thick b-FeSi sample, the current was larger by one order of magnitude for the 200 A> thick b-FeSi sample. The ideality factors of both
Fig. 3. Forward-biased I—» characteristics of the p-Si/b-FeSi balls/n-Si(0 0 1) structures measured at RT.
476
T. Suemasu et al. / Journal of Luminescence 80 (1999) 473—477
samples were around 1.5. Depletion region of all the samples spreads towards the n-Si side because of the p>—n junction. It is well-known that boron accumulation occurs at the substrate [16], when the substrates are cleaned by the RCA method. Consequently, it can be said that the balls are embedded in the p-Si side. Therefore, the immediate influence of the b-FeSi balls on the ideality factor is small. Fig. 4 shows the carrier concentration profiles in the n-Si side from the p—n junction. It was clear that the carrier concentration close to the p—n junction decreased except for the 100 A> -thick b-FeSi sample. This tendency was clearly observed for the 400 A> -thick b-FeSi sample. These results indicate that electron traps caused by defects located near the p—n junction. There exists some papers about introduction of deep levels after iron silicidation. In those papers, origin of the deep levels was inferred to be Fe-related complexes. In our work, the 100 A> -thick b-FeSi sample showed good rectifying behavior and no deep levels were observed. Fig. 5 shows the TEM images of the p-Si/b-FeSi /n-Si structures for the (a) 100 A> - and (b) 400 A> -thick b-FeSi samples. For the 400 A> -thick b-FeSi sample, as in Fig. 5b, the b-FeSi balls do not have a neat spherical shape compared to the other sample. Furthermore, there are a lot of dislocations
propagating along the Si+1 1 1, planes. Thus, the balls of this sample were supposed to strain the Si substrate, and therefore, the origin of the electron traps was regarded as a strain between b-FeSi and Si rather than Fe-related complexes. We have observed high-resolution TEM images of the Si/bFeSi interface for the 100 A> -thick b-FeSi sample, and found out that the atomic structure close to the interface is highly distorted even for the 100 A> thick b-FeSi sample, where the edge-type disloca tion appears at the interface. We therefore speculate that this kind of interface plays a dominant role
Fig. 4. C—» characteristics of the p-Si/b-FeSi balls/n-Si(0 0 1) structures measured at RT.
Fig. 5. TEM images of p-Si/b-FeSi /n-Si(0 0 1) structures for the (a) 100 A> - and (b) 400 A> -thick b-FeSi samples.
T. Suemasu et al. / Journal of Luminescence 80 (1999) 473—477
in the appearance of the defects. Further investigations are needed to probe the origin of the deep levels. But the dependence of quantity of the embedded b-FeSi suggests that size reduction of the b-FeSi balls is one of the possible ways to decrease the density of deep levels.
4. Conclusions We have demonstrated the fabrication technique of single-crystalline b-FeSi balls embedded in an Si p—n junction region by Si molecular beam epitaxy. In spite of ball-shaped b-FeSi , (1 0 0) oriented relationship of b-FeSi to the Si (0 0 1) substrates was preserved even after the annealing and the Si overgrowth. The C—V characteristics and TEM observation indicated the existence of deep level near the p—n junction region for the sample with large quantity of b-FeSi . Acknowledgements The authors express their sincere thanks to Dr. Baumann (Texas Instruments Tsukuba R&D Center Ltd.) for fruitful discussions on the TEM results. This work was partially supported by a Grant-in-Aid for Scientific Research (B) No. 09450121 and the Priority Area ‘‘Spin Controlled Semiconductor Nanostructures’’ No. 10138204
477
from the Ministry of Education, Science, Sports and Culture, Japan.
References [1] M.C. Bost, J.E. Mahan, J. Appl. Phys. 58 (1985) 2696. [2] M.C. Bost, J.E. Mahan, J. Vac. Sci. Technol. B 4 (1986) 1336. [3] N. Cherief, C. D’ Anterroches, R.C. Cinti, T.A. Nguyen Tan, J. Derrien, Appl. Phys. Lett. 55 (1989) 1671. [4] N.E. Christensen, Phys. Rev. B 42 (1990) 7148. [5] V. Bellani, G. Guizzetti, F. Marabelli, M. Patrini, Solid State Commun. 96 (1995) 751. [6] D. Leong, M. Harry, K.J. Reeson, K.P. Homewood, Nature 387 (1997) 686. [7] J.E. Mahan, K.M. Geib, G.Y. Robinson, R.G. Long, X. Yan, G. Bai, M.A. Nicolet, M. Nathan, Appl. Phys. Lett. 56 (1990) 2126. [8] K.M. Geib, J.E. Mahan, R.G. Long, M. Nathan, G. Bai, J. Appl. Phys. 70 (1991) 1730. [9] A.H. Reader, J.P.W.B. Duchateaun, J. Timmers, F.J.G. Hakkens: Appl. Surf. Sci. 73 (1993) 3105. [10] L. Wang, L. Qin, Y. Zheng, Q. Shen, X. Chen, X. Lin, C. Lin, S. Zou, Appl. Phys. Lett. 65 (1994) 3105. [11] M. Tanaka, Y. Kumagai, T. Suemasu, F. Hasegawa, Jpn. J. Appl. Phys. 36 (1997) 3624. [12] T. Suemasu, M. Tanaka, T. Fujii, S. Hashimoto, Y. Kumagai, F. Hasegawa, Jpn. J. Appl. Phys. 36 (1997) L1225. [13] T. Tatsumi, H. Hirayama, N. Aizaki, Appl. Phys. Lett. 50 (1987) 1234. [14] D.J. Eaglemann, M. Cerullo, Phys. Rev. Lett. 64 (1990) 1943. [15] D. Leonard, M. Krishnamurthy, C.M. Reaves, S.P. Denbaars, P.M. Petroff, Appl. Phys. Lett. 63 (1993) 3203. [16] A. Casel, E. Kasper, H. Kibbel, E. Sasse, J. Vac. Sci. Technol. B 5 (1987) 1650.