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Si layers on patterned Si substrate
PERGAMON
Solid State Communications 112 (1999) 255–259 www.elsevier.com/locate/ssc
Improvement of photoluminescence of strained SiGe/Si layers on patterned Si substrate Jun-Jie Si a,*, Li-Wei Guo b, Qin-Qing Yang a, Jun-Hua Gao a, Da Teng a, Jun-Ming Zhou b, Qi-Ming Wang a a
State Key Lab. on Integrated Optoelectronics, Institute of Semiconductor, Chinese Academy of Science, Beijing 100083, People’s Republic of China b MBE Lab. Institute of Physics, Chinese Academy of Science, Beijing 100083, People’s Republic of China Received 26 March 1999; accepted 14 July 1999 by S. Ushioda
Abstract A strained SiGe/Si superlattice structure has been grown on a patterned Si substrate and its photoluminescence has been studied. The patterned substrate is composed of pyramid-like structures. It is found that there are Ge-rich SiGe quantum wires (QWR) at the crossings of adjacent planes that form the pyramid-like structure. Photoluminescence of strained the SiGe layer grown on a planar substrate and a patterned substrate was compared. The total intensity of photoluminescence from the patterned substrate was 5.2 times larger than that from the planar substrates. The result is discussed and it is believed that this increase in photoluminescence is related to the observed QWRs. q 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Nanostructures; A. Semiconductors; A. Thin films; E. Luminescence
1. Introduction How to make light emitting devices from silicon-based materials, and to develop production techniques compatible with current VLSI (very-large-scale-integration) techniques, have been one of the most important topics in the opto-electronics area. People hope to realize silicon monolithic opto-electronic integration as well as transformation from microelectronics to optoelectronics [1]. Due to its indirect bandgap structure, the optical transition possibility of silicon is very low compared with direct bandgap structure materials. A strained SiGe/Si quantum structure could be a possible candidate for improving light emitting efficiency by the so-called “energy-band engineering”. Although the SiGe layer still retains its direct bandgap character in the strained SiGe/Si quantum structure, the confinement to hole carriers is enhanced due to larger valence band* Corresponding author. Morisaky-Nozaki Laboratory, Department of Communications and Systems, University of ElectroCommunications, 1-5-1 Chofugaoka, Chofu-shi, Tokyo 182-8585, Japan. E-mail address: [email protected] (J.-J. Si)
offset between Si and the strained SiGe. Randomly distributed Ge atoms in the strained SiGe layer acting as momentum scattering centers can strengthen the overlap of electron and hole wave functions and greatly increase the NP (nophonon) assisted optical transition probability [2]. The NP transition of quantum well (QW) structure has been observed and widely studied in order to improve the strained SiGe/Si QW luminescence property [3]. An important way to further increase Si-based material luminescence is by using low-dimensional quantum structures. Ohms [4] and Sanders [5] reported that the band structure of a Si quantum wire (QWR) can be changed from an indirect band into a quasi-direct band when its width is smaller than 0.8 nm due to mixing of the X and L bands. The radiative probability of quantum dots will be much increased due to the breakdown of momentum conservation in the dots. In this paper, in order to confirm the role of a low-dimensional structure, we made a patterned substrate with regularly distributed inverse pyramid-like structures. We hoped to get the low-dimensional structure at the crossings of crystal planes. We then grew a strained SiGe/Si quantum structure on it. A clear increase in photoluminescence was
0038-1098/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S0038-109 8(99)00349-X
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Fig. 1. Planar section AFM photograph of the patterned substrate. Inset shows the sketch of a single inverse-pyramid structure.
Fig. 2. The PL spectrum of the strained SiGe/Si SLs on the patterned area (a) and the planar area (b) on the same substrate. Dot lines are Gaussian fits of the peak.
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buffer layer and a 30 nm Si cap layer. Nominal Si0.7Ge0.3 well width was 2.5 nm, with a Si barrier width of 3 nm. The morphology of the epitaxial film was studied by a Shimadzu SPM 9500 atomic force microscope (AFM) and a JEM 200 CX transmission electron microscope (TEM). The photoluminescence of the strained structures grown on both patterned and planar substrates was measured, respectively, at low temperature (10 K). The sample was cooled by a Helium closed-cycle refrigerator and excited by a multiple-line argon laser. The luminescence signal was monochromatized by SPEX 1404 double grating monochromator and detected by a liquid-nitrogen cooled Ge detector. The standard lock-in technique was used.
3. Results and discussion
Fig. 3. The TEM photograph of SiGe/Si on the patterned substrate: (a) top view; and (b) cross-section view.
observed. It is shown that the improvement is related to the SiGe low-dimensional quantum structure.
2. Experiment A 2-in. diameter n-type CZ–Si (100) wafer with a resistivity of 3–5 V cm was used as the substrate. A 150 nm thick SiO2 film was formed on the surface by thermo-oxidation at 10508C. By reactive ion-beam etching using a Plasma Therm 520/540, we finished photolithography and got uniformly distributed windows, 2 × 2 mm2 in area. Then using an alkaline etchant, we made the inverse pyramid structures. Half of the wafer was patterned in this way, while the other half was kept flat for comparison. After removing the surface SiO2 layer, 11 periods of Si/Si0.7Ge0.3 were grown by a VG-80 SS-MBE system with a 200 nm Si
Fig. 1 shows the inverse-pyramid structure as measured by AFM. It is composed of four (111) planes and one (100) plane at the bottom. The height of the inverse-pyramid can be controlled by varying the etch time. Self-limitation will be reached if the etch time is long enough. Then the four (111) planes will meet and the bottom (100) plane will become a dot. We did not reach the self-limitation in our case. The enlarged width of the upper square relative to the mask is caused by cross-etch which is an inherent characteristic of the wet etch method. Fig. 2 shows the PL spectra of the strained SiGe layer on planar and patterned substrates. It can be clearly seen that the luminescence efficiency of the strained SiGe layer on the patterned substrate is much higher than that on the planar substrate when excited by the same laser power (both 20 mW/mm 2). By integrating the intensity curves, we find that the total intensity of luminescence is increased by a factor of 5.2. This increase in PL is partially attributed to the increment of surface area of the patterned substrate relative to the planar substrate. A simple calculation shows that p the increment of surface area is at most by a factor of 3: Thus, the extra increment must be due to other mechanisms. We believe that the increment is related to the SiGe/Si low-dimensional structures. Fig. 3 shows the top-view and the cross-sectional view of the SiGe/Si structure on the patterned substrate by TEM. We can recognize the QWR structures that are indicated by contrast pattern at the crossings of adjacent (111) planes from the top-view picture. This pattern may arise from two causes. One is that, there is a narrow and thick SiGe layer at the crossings, which means a geometrical QWR structure. The other could be, that there is a narrow SiGe layer at the crossings which has the same thickness but a larger Ge content compared with that on the plane. It means a content QWR structure. Though there are many reports on geometrical QWR grown at the bottom of V-grooves [6,7], here we believe our QWR is a content QWR structure similar to Hartman’s results [8]. It is formed by condensation of the Ge atoms, that is Ge in the planar area diffusing into the crossings. There are two proofs
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J.-J. Si et al. / Solid State Communications 112 (1999) 255–259
Fig. 4. Excitation and transportation of photon generated carriers in the strained SiGe layer on the patterned substrate: hn is the excitation light; hn 1 the light emitted from quantum wells; hn 2 the light emitted from quantum wires.
supporting our conclusion. First, if it were a geometrical QWR, the contrast pattern should change from layer to wire monotonically. The undulated contrast pattern should not appear. If it were a content QWR, there would be a Ge depletion region, where Ge content is smaller due to the Ge atoms diffusing into the wire. Thus the Ge content at the center of the planar area and at the crossings are relatively higher. Then the undulated contrast pattern of the TEM picture can be explained. Second, AFM measurement shows that the thickness difference of the epitaxial layer on the plane and at the crossings is no more than 2%. It means that the contrast pattern of the picture is not due to a thickness variation. The generation mechanism of QWR formed by Ge diffusion is determined by the strained SiGe epitaxial layer growth dynamics. For both incident Si and Ge atoms, they prefer to stay at the kink and step-edge on the surface. Under the same growing condition, the incident Ge atom on the surface has a larger mobility than the Si atom. At the crossings of the planes, there are a lot of kinks and steps where the incident atoms have more possibility to occupy those positions. Thus Ge atoms are more likely to be gathered at the crossings, which forms the Ge-rich QWR. In Fig. 2, we can distinguish the no-phonon assisted optical transition from the transverse-optical phonon assisted transition peak of SiGe layer grown on a planar substrate. From the cross-sectional picture in Fig. 3, we know the thickness of the strained SiGe layer on (100) is larger than that on (111). The former one is about 1.4 times thicker than the latter one. We can consider that the peak from the
strained layer on the patterned substrate is a sum of peaks from the SiGe layer on the (100) plane and (111) planes. Due to the quantum confinement effect, the peak position of the SiGe layer on (111) plane will be higher than that on the (100) plane. Christen and Bimberg studied the optical recombination line shape of excitons in QW in detail [9]. There are two causes that make PL peak broadening. One is the lifetime broadening, which is related to the phonons and carrier scattering. The line shape follows the Lorentzian shape. It is dominated when the measurement is carried under higher temperature. The other is the statistical broadening, which is related to the interface fluctuation and alloy content fluctuation. The line shape follows the Gaussian shape. It is dominated when the measurement is carried under lower temperature. Since our PL is measured at 10 K, we use a Gaussian curve to fit the peak of the SiGe layer on the patterned substrate. The four fitting peaks, which individually represent the peak from SiGe on the (100) and (111) plane, are shown in Fig. 2. One possible explanation for 5.2 times increase of integral luminescence intensity is that the QWR can enhance the capture and recombination of the carriers in QW area. Due to its small practical size, optical transition of the SiGe QWR is quite small compared with that of the QW. Fig. 4 gives a sketch of the photoexcited carriers generation, recombination and transportation in QWR and QW structures on a patterned substrate. The carriers, which are generated and collected by SiGe QWR, can be thermally excited or tunneled into QW area. This results in an increase in the recombination of carriers in SiGe QW.
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4. Conclusion
References
A strained SiGe/Si superlattice was fabricated on an inverse-pyramid substrate. The photoluminescence of the superlattice was found to be 5.2 times larger than that on the planar substrate. It is found that the structure of the strained SiGe/Si layer mainly consists of two quantum structures. One is the QW layer on the (111) plane and (100) plane, and the other is QWR at the crossings of the adjacent planes. The QWR is Ge-riched and is formed by Ge atoms in QW diffusing into the crossings. It is analyzed that the increase in photoluminescence intensity is related to QWR.
[1] K.D. Hirschman, L. Tsybeskov, S.P. Duttagupta, P.M. Fauchet, Nature 384 (1996) 338. [2] J. Weber, M.L. Alonso, Phys. Rev. B 40 (1989) 5683. [3] U. Menczigar, E. Friess, M. Gailand, G. Abstreiter, H. Kibbel, H. Presting, E. Kasper, Thin Solid Film 222 (1992) 227. [4] T. Ohno, K. Shiraishi, T. Ogawa, Phys. Rev. Lett. 69 (1992) 2400. [5] C.G. Sanders, Y.C. Chang, Appl. Phys. Lett. 60 (1992) 2525. [6] V. Higgs, E.C. Lightowlers, N. Usami, T. Mine, S. Fukatsu, Y. Shiraki, Appl. Phys. Lett. 67 (1995) 1709. [7] J. Zhang, X.M. Zhang, A. Matsumara, A. Marinopooulou, J. Hartury, N. Anwar, G. Parry, M.H. Xie, S.M. Mokler, J.M. Fernanderz, B.A. Joyce, J. Cryst. Growth 150 (1995) 950. [8] A. Hartmann, L. Vescan, C. Dieker, H. Luth, J. Appl. Phys. 77 (1995) 1959. [9] J. Christen, D. Bimberg, Phys. Rev. B 42 (1990) 7213.
Acknowledgements One of the authors wishes to thank Dr Wu Ju for his TEM measurement. This work was partially supported by China National Nature Science Foundation.
Solid State Communications 112 (1999) 255–259 www.elsevier.com/locate/ssc
Improvement of photoluminescence of strained SiGe/Si layers on patterned Si substrate Jun-Jie Si a,*, Li-Wei Guo b, Qin-Qing Yang a, Jun-Hua Gao a, Da Teng a, Jun-Ming Zhou b, Qi-Ming Wang a a
State Key Lab. on Integrated Optoelectronics, Institute of Semiconductor, Chinese Academy of Science, Beijing 100083, People’s Republic of China b MBE Lab. Institute of Physics, Chinese Academy of Science, Beijing 100083, People’s Republic of China Received 26 March 1999; accepted 14 July 1999 by S. Ushioda
Abstract A strained SiGe/Si superlattice structure has been grown on a patterned Si substrate and its photoluminescence has been studied. The patterned substrate is composed of pyramid-like structures. It is found that there are Ge-rich SiGe quantum wires (QWR) at the crossings of adjacent planes that form the pyramid-like structure. Photoluminescence of strained the SiGe layer grown on a planar substrate and a patterned substrate was compared. The total intensity of photoluminescence from the patterned substrate was 5.2 times larger than that from the planar substrates. The result is discussed and it is believed that this increase in photoluminescence is related to the observed QWRs. q 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Nanostructures; A. Semiconductors; A. Thin films; E. Luminescence
1. Introduction How to make light emitting devices from silicon-based materials, and to develop production techniques compatible with current VLSI (very-large-scale-integration) techniques, have been one of the most important topics in the opto-electronics area. People hope to realize silicon monolithic opto-electronic integration as well as transformation from microelectronics to optoelectronics [1]. Due to its indirect bandgap structure, the optical transition possibility of silicon is very low compared with direct bandgap structure materials. A strained SiGe/Si quantum structure could be a possible candidate for improving light emitting efficiency by the so-called “energy-band engineering”. Although the SiGe layer still retains its direct bandgap character in the strained SiGe/Si quantum structure, the confinement to hole carriers is enhanced due to larger valence band* Corresponding author. Morisaky-Nozaki Laboratory, Department of Communications and Systems, University of ElectroCommunications, 1-5-1 Chofugaoka, Chofu-shi, Tokyo 182-8585, Japan. E-mail address: [email protected] (J.-J. Si)
offset between Si and the strained SiGe. Randomly distributed Ge atoms in the strained SiGe layer acting as momentum scattering centers can strengthen the overlap of electron and hole wave functions and greatly increase the NP (nophonon) assisted optical transition probability [2]. The NP transition of quantum well (QW) structure has been observed and widely studied in order to improve the strained SiGe/Si QW luminescence property [3]. An important way to further increase Si-based material luminescence is by using low-dimensional quantum structures. Ohms [4] and Sanders [5] reported that the band structure of a Si quantum wire (QWR) can be changed from an indirect band into a quasi-direct band when its width is smaller than 0.8 nm due to mixing of the X and L bands. The radiative probability of quantum dots will be much increased due to the breakdown of momentum conservation in the dots. In this paper, in order to confirm the role of a low-dimensional structure, we made a patterned substrate with regularly distributed inverse pyramid-like structures. We hoped to get the low-dimensional structure at the crossings of crystal planes. We then grew a strained SiGe/Si quantum structure on it. A clear increase in photoluminescence was
0038-1098/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S0038-109 8(99)00349-X
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J.-J. Si et al. / Solid State Communications 112 (1999) 255–259
Fig. 1. Planar section AFM photograph of the patterned substrate. Inset shows the sketch of a single inverse-pyramid structure.
Fig. 2. The PL spectrum of the strained SiGe/Si SLs on the patterned area (a) and the planar area (b) on the same substrate. Dot lines are Gaussian fits of the peak.
J.-J. Si et al. / Solid State Communications 112 (1999) 255–259
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buffer layer and a 30 nm Si cap layer. Nominal Si0.7Ge0.3 well width was 2.5 nm, with a Si barrier width of 3 nm. The morphology of the epitaxial film was studied by a Shimadzu SPM 9500 atomic force microscope (AFM) and a JEM 200 CX transmission electron microscope (TEM). The photoluminescence of the strained structures grown on both patterned and planar substrates was measured, respectively, at low temperature (10 K). The sample was cooled by a Helium closed-cycle refrigerator and excited by a multiple-line argon laser. The luminescence signal was monochromatized by SPEX 1404 double grating monochromator and detected by a liquid-nitrogen cooled Ge detector. The standard lock-in technique was used.
3. Results and discussion
Fig. 3. The TEM photograph of SiGe/Si on the patterned substrate: (a) top view; and (b) cross-section view.
observed. It is shown that the improvement is related to the SiGe low-dimensional quantum structure.
2. Experiment A 2-in. diameter n-type CZ–Si (100) wafer with a resistivity of 3–5 V cm was used as the substrate. A 150 nm thick SiO2 film was formed on the surface by thermo-oxidation at 10508C. By reactive ion-beam etching using a Plasma Therm 520/540, we finished photolithography and got uniformly distributed windows, 2 × 2 mm2 in area. Then using an alkaline etchant, we made the inverse pyramid structures. Half of the wafer was patterned in this way, while the other half was kept flat for comparison. After removing the surface SiO2 layer, 11 periods of Si/Si0.7Ge0.3 were grown by a VG-80 SS-MBE system with a 200 nm Si
Fig. 1 shows the inverse-pyramid structure as measured by AFM. It is composed of four (111) planes and one (100) plane at the bottom. The height of the inverse-pyramid can be controlled by varying the etch time. Self-limitation will be reached if the etch time is long enough. Then the four (111) planes will meet and the bottom (100) plane will become a dot. We did not reach the self-limitation in our case. The enlarged width of the upper square relative to the mask is caused by cross-etch which is an inherent characteristic of the wet etch method. Fig. 2 shows the PL spectra of the strained SiGe layer on planar and patterned substrates. It can be clearly seen that the luminescence efficiency of the strained SiGe layer on the patterned substrate is much higher than that on the planar substrate when excited by the same laser power (both 20 mW/mm 2). By integrating the intensity curves, we find that the total intensity of luminescence is increased by a factor of 5.2. This increase in PL is partially attributed to the increment of surface area of the patterned substrate relative to the planar substrate. A simple calculation shows that p the increment of surface area is at most by a factor of 3: Thus, the extra increment must be due to other mechanisms. We believe that the increment is related to the SiGe/Si low-dimensional structures. Fig. 3 shows the top-view and the cross-sectional view of the SiGe/Si structure on the patterned substrate by TEM. We can recognize the QWR structures that are indicated by contrast pattern at the crossings of adjacent (111) planes from the top-view picture. This pattern may arise from two causes. One is that, there is a narrow and thick SiGe layer at the crossings, which means a geometrical QWR structure. The other could be, that there is a narrow SiGe layer at the crossings which has the same thickness but a larger Ge content compared with that on the plane. It means a content QWR structure. Though there are many reports on geometrical QWR grown at the bottom of V-grooves [6,7], here we believe our QWR is a content QWR structure similar to Hartman’s results [8]. It is formed by condensation of the Ge atoms, that is Ge in the planar area diffusing into the crossings. There are two proofs
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J.-J. Si et al. / Solid State Communications 112 (1999) 255–259
Fig. 4. Excitation and transportation of photon generated carriers in the strained SiGe layer on the patterned substrate: hn is the excitation light; hn 1 the light emitted from quantum wells; hn 2 the light emitted from quantum wires.
supporting our conclusion. First, if it were a geometrical QWR, the contrast pattern should change from layer to wire monotonically. The undulated contrast pattern should not appear. If it were a content QWR, there would be a Ge depletion region, where Ge content is smaller due to the Ge atoms diffusing into the wire. Thus the Ge content at the center of the planar area and at the crossings are relatively higher. Then the undulated contrast pattern of the TEM picture can be explained. Second, AFM measurement shows that the thickness difference of the epitaxial layer on the plane and at the crossings is no more than 2%. It means that the contrast pattern of the picture is not due to a thickness variation. The generation mechanism of QWR formed by Ge diffusion is determined by the strained SiGe epitaxial layer growth dynamics. For both incident Si and Ge atoms, they prefer to stay at the kink and step-edge on the surface. Under the same growing condition, the incident Ge atom on the surface has a larger mobility than the Si atom. At the crossings of the planes, there are a lot of kinks and steps where the incident atoms have more possibility to occupy those positions. Thus Ge atoms are more likely to be gathered at the crossings, which forms the Ge-rich QWR. In Fig. 2, we can distinguish the no-phonon assisted optical transition from the transverse-optical phonon assisted transition peak of SiGe layer grown on a planar substrate. From the cross-sectional picture in Fig. 3, we know the thickness of the strained SiGe layer on (100) is larger than that on (111). The former one is about 1.4 times thicker than the latter one. We can consider that the peak from the
strained layer on the patterned substrate is a sum of peaks from the SiGe layer on the (100) plane and (111) planes. Due to the quantum confinement effect, the peak position of the SiGe layer on (111) plane will be higher than that on the (100) plane. Christen and Bimberg studied the optical recombination line shape of excitons in QW in detail [9]. There are two causes that make PL peak broadening. One is the lifetime broadening, which is related to the phonons and carrier scattering. The line shape follows the Lorentzian shape. It is dominated when the measurement is carried under higher temperature. The other is the statistical broadening, which is related to the interface fluctuation and alloy content fluctuation. The line shape follows the Gaussian shape. It is dominated when the measurement is carried under lower temperature. Since our PL is measured at 10 K, we use a Gaussian curve to fit the peak of the SiGe layer on the patterned substrate. The four fitting peaks, which individually represent the peak from SiGe on the (100) and (111) plane, are shown in Fig. 2. One possible explanation for 5.2 times increase of integral luminescence intensity is that the QWR can enhance the capture and recombination of the carriers in QW area. Due to its small practical size, optical transition of the SiGe QWR is quite small compared with that of the QW. Fig. 4 gives a sketch of the photoexcited carriers generation, recombination and transportation in QWR and QW structures on a patterned substrate. The carriers, which are generated and collected by SiGe QWR, can be thermally excited or tunneled into QW area. This results in an increase in the recombination of carriers in SiGe QW.
J.-J. Si et al. / Solid State Communications 112 (1999) 255–259
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4. Conclusion
References
A strained SiGe/Si superlattice was fabricated on an inverse-pyramid substrate. The photoluminescence of the superlattice was found to be 5.2 times larger than that on the planar substrate. It is found that the structure of the strained SiGe/Si layer mainly consists of two quantum structures. One is the QW layer on the (111) plane and (100) plane, and the other is QWR at the crossings of the adjacent planes. The QWR is Ge-riched and is formed by Ge atoms in QW diffusing into the crossings. It is analyzed that the increase in photoluminescence intensity is related to QWR.
[1] K.D. Hirschman, L. Tsybeskov, S.P. Duttagupta, P.M. Fauchet, Nature 384 (1996) 338. [2] J. Weber, M.L. Alonso, Phys. Rev. B 40 (1989) 5683. [3] U. Menczigar, E. Friess, M. Gailand, G. Abstreiter, H. Kibbel, H. Presting, E. Kasper, Thin Solid Film 222 (1992) 227. [4] T. Ohno, K. Shiraishi, T. Ogawa, Phys. Rev. Lett. 69 (1992) 2400. [5] C.G. Sanders, Y.C. Chang, Appl. Phys. Lett. 60 (1992) 2525. [6] V. Higgs, E.C. Lightowlers, N. Usami, T. Mine, S. Fukatsu, Y. Shiraki, Appl. Phys. Lett. 67 (1995) 1709. [7] J. Zhang, X.M. Zhang, A. Matsumara, A. Marinopooulou, J. Hartury, N. Anwar, G. Parry, M.H. Xie, S.M. Mokler, J.M. Fernanderz, B.A. Joyce, J. Cryst. Growth 150 (1995) 950. [8] A. Hartmann, L. Vescan, C. Dieker, H. Luth, J. Appl. Phys. 77 (1995) 1959. [9] J. Christen, D. Bimberg, Phys. Rev. B 42 (1990) 7213.
Acknowledgements One of the authors wishes to thank Dr Wu Ju for his TEM measurement. This work was partially supported by China National Nature Science Foundation.