- Email: [email protected]
Deep levels in ZnSe epitaxial layers examined by piezoelectric photoacoustic spectroscopy
ELSEVTER
Journal of Crystal Growth 184/185 (1998) 1151~1154
Deep levels in ZnSe epitaxial layers examined by piezoelectric photoacoustic spectroscopy Kenji Yoshinoa**, Yasuhiro Nakagawaa, Atsuhiko Fukuyamab, Kouji Maeda”, Minoru Yoneta”, Hiroshi Saito”, Masakazu Ohishi”, Tetsuo Ikari” aDepartment of Electrical and Electronic Engineering, Miyazaki University, I-I Gakuen, Kibanadai-nishi, Myazaki 889.21, Japan b Department of Materials Science, Miyazaki University. l-l Gakuen, Kibanadai-nishi, Miyazaki 889.21, Japan ‘Department qf Applied Physics, Okayama University of Science, I-l Ridai-cho, Okayama 700, Japan
Abstract Piezoelectric photoacoustic (PPA) measurements (MBE) are carried out at room and liquid nitrogen
of nondoped ZnSe epitaxial layers grown by molecular beam epitaxy temperatures. Distinct peaks due to the band-gaps of ZnSe and the
substrate GaAs are clearly observed. Five PPA peaks are observed in the nondoped ZnSe thin films with VI/II ratio of 12 at liquid nitrogen temperature for the first time. This indicates that five types of intrinsic defects exist in ZnSe epitaxial layers. The obtained energies of these defects are 1.20 + 0.01, 0.97 + 0.03, 0.65 & 0.03 and 0.06 f 0.01 eV, respectively. ;Q 1998 Elsevier Science B.V. All rights reserved. Zinc selenide (ZnSe); Molecular (PL); Nonradiative recombination
Keywords:
beam epitaxy (MBE); Piezoelectric
1. Introduction II-VI compounds, especially zinc selenide (ZnSe), have been widely investigated to develop blue light-emitting devices [l]. However, the device applications of ZnSe-based II-VI compounds are not yet realized. It is important to understand the nature of the crystalline defects in the materials for
*Corresponding author. Fax: [email protected].
+ 81 985 58 2876; e-mail:
0022-0248/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PI1 SOO22-0248(97)00622-2
photoacoustic
(PPA); Photoluminescence
their practical use. In order to know the crystalline defects, photoluminescence (PL) spectroscopy has usually been carried out. Although we know radiative recombinations by the PL measurements, nonradiative recombination processes cannot be characterized at all. Recently, many studies on the nonradiative recombination in ZnSe, such as deeplevel transient spectroscopy (DLTS) [2], have been paid great attention since these crystalline defects may prevent the CW laser oscillation at room temperature. However, it is known that DLTS measurements for high resistivity ZnSe are difficult
1152
K. Yoshino et al. /Journal
of‘Ctystal
and distinguishing between radiative and nonradiative centers is not easy. Photoacoustic (PA) measurements have recently been used as one of the new methods to study the physical properties of semiconductors [3-61. The great advantage of the PA measurement is that the nonradiative recombination processes are measured directly. Then, the PA techniques may complement the PL techniques. To detect the PA signal, a gas-microphone has been generally used in the PA measurements. We used a piezoelectric transducer (PZT) in this study. A PZT detector is more sensitive than a gas-microphone for detecting the PA signal and can be used even at low temperatures. Furthermore, piezoelectric PA (PPA) spectroscopy is much easier than the DLTS methods since no electrodes are needed in the PPA system. So far, only a few papers have reported the PPA technique on ZnSe epitaxial layers. We have previously carried out PPA measurements of ZnSe at room temperature [7]. In this paper, PPA measurements of ZnSe are carried out at room and liquid nitrogen temperatures. The usefulness of the PPA measurements to study the nonradiative centers in the ZnSe epitaxial layers grown by MBE is discussed.
2. Experimental
Growth 1841185
(1998)
1151-1154
sample was mounted on the cold finger of a cryostat (Oxford Co., Ltd.: optistat DN-V model). A xenon lamp was used as an excitation source (Ushio Spex Co., Ltd.: UXL-SOOD). The probe light was mechanically chopped and exposed on the surface of the ZnSe layers. The PPA signals were detected by a PZT. The disk of 5 mm 0, was attached directly on the rear surface of the samples (GaAs substrate side) with silver paste. We confirmed that the silver paste gives good thermal and mechanical contacts between the sample and the PZT. The output signals from the PZT were amplified by a digital lock-in amplifier and accumulated by a personal computer. The detailed experimental procedures have been reported in a previous paper
C91.
3. Results and discussion The PPA spectra of the ZnSe/GaAs films grown by MBE are shown in Fig. 1 under a modulation
1OOHz R. T.
procedure
The nondoped ZnSe epitaxial layers were grown on semi-insulating (SI) GaAs(0 0 1) by MBE methods at a substrate temperature of 250°C. Zn and Se beams were post-heated at 600 and 400°C respectively. The VI/II ratio of the two samples in this study were about 12 and 15. The thickness of the layers and SI-GaAs substrate were about 1.0 and 500 urn, respectively. The epitaxial layers were of high quality since streaky reflection high-energy electron diffraction (RHEED) patterns were observed and the PL spectra at low temperature show sharp free-exciton emissions. Details of the MBE growth techniques, the RHEED patterns and the PL spectra of the nondoped ZnSe thin films were described in a previous paper [S]. The PPA measurements were carried out at room and liquid nitrogen temperatures with a modulated frequency from 100 to 500 Hz. The
1.5
2
Photon
2.5
Energy
3
(eV)
Fig. 1. Piezoelectric photoacoustic spectra of ZnSe/GaAs(O 0 1) grown under VI/II ratio of 12 (upper) and 15 (lower) at room temperature using a modulation frequency of 100 Hz.
K. Yoshino et al. /Journal
qf Crystal Growth 184/185 (1998) 1151-1154
frequency of 100 Hz at room temperature. The intensity of the PPA signals are normalized by the wavelength dependence of incident light power and the vertical scale of the figure between 1.3 and 2.5 eV is multiplied by 10. In the ZnSe/GaAs film grown under the VI/II ratio of 15, a sharp peak is observed at 2.70 eV. This peak energy is in good agreement with the bandgap energy (E,) obtained by electroreflectance measurements [lo]. Two weak bands at 1.45 and 2.05 f 0.03 eV (labeled B in the figure) are observed. Since the E, of GaAs is about 1.45 eV at room temperature, we consider that the peak at 1.45 eV in Fig. 1 is due to the E, of GaAs [9,11] and the band B is due to an intrinsic defect in the ZnSe layer. From the energy difference between the B band and the E,, the activation energy is estimated to be 0.65 + 0.03 eV. This value corresponds to that obtained by DLTS measurements for N-doped ZnSe thin film [12]. Since the 0.65 + 0.05 eV band is not observed in the PL spectrum [S], it is considered that this is due to nonradiative recombination. The deep level with an activation energy of 0.65 ) 0.03 eV is detected optically for nondoped ZnSe epitaxial layers grown by MBE for the first time. For the sample grown under a VI/II ratio of 12, on the other hand, a weak and broad bands are observed around the 2.70 eV region. The PPA intensity of the peaks corresponding to the band gaps of ZnSe and GaAs with a VI/II ratio of 12 is very small in comparison with those of a VI/II ratio of 15. This indicates that the absorption coefficient of the ZnSe layer with a VI/II ratio of 12 becomes smaller than that of a layer with a VI/II ratio of 15. The peak due to the band gap of GaAs is weakly observed. This peak energy at 1.54 eV corresponds well to that reported by Ikari et al. [9,11]. Since the signals of the PPA spectra of the ZnSe/GaAs films with VI/II ratios of 12 and 15 under a modulation frequency of 100 Hz at 80 K are weak, the higher modulation frequency of 500 Hz is used. The low temperature PPA spectrum of the ZnSe/GaAs film with a VI/II ratio of 12 is shown in Fig. 2. The broad band around the 2.70 eV region seen at room temperature in Fig. 1 is now composed of two peaks, i.e., the peak at 2.77 eV (labelled C) is due to an intrinsic defect and the peak at 2.82 eV corresponds to the E, of ZnSe
1153
C 5ooHz SOK
ZnSc i 1
VI/II=12
B
D
In
1
11
1.5
I
I
I
I
2
I
I.
h
*
I
I
I
2.5
I
I
I
I
3
Photon Energy (eV) Fig. 2. Piezoelectric photoacoustic spectra of ZnSe/GaAs(O 0 1) grown under VI/II ratio of 12 at liquid nitrogen temperature using modulation frequency of 500 Hz.
[13]. Similarly, the broad band around 2.1 eV at room temperature is decomposed into three peaks (labelled B and D) at 80 K as shown in Fig. 2. Furthermore, the weak but sharp peak at 1.62 f 0.01 eV (labelled A) is clearly resolved at 80 K. We obtained the activation energy for this level as 1.20 f 0.01 eV. This value corresponds well to that obtained by the DLTS measurements for N-doped ZnSe thin film [a]. The peak C at 2.76 f 0.01 eV appears clearly in comparison with that at room temperature. The activation energy for peak C is estimated as 0.06 + 0.01 eV, which is not reported in nondoped ZnSe epitaxial layers so far. However, the peak C is not clearly observed in ZnSe/GaAs grown under VI/II= 15. Although the bound exciton line of the deep acceptor (Ifeep), accompanied by LO phonon replicas, is clearly observed in the PI, spectrum of ZnSe/GaAs grown under VI/II = 12, no such lines are seen in ZnSe/GaAs grown under a VI/II = 15. Then we consider that the peak C in PPA spectrum is related to Zn vacancy [14].
1154
K. Yoshino et al. /Journal
qf Crystal Growth 1841185 (1998) 1151-1154
Besides the band B, a new band D appears at 80 K in ZnSe grown under VI/II = 12. However, only the band D is observed in the ZnSe grown under VI/II = 15. By using its energy of 1.85 _+ 0.03, the activation energy for this level is calculated as 0.97 f 0.03 eV. This energy corresponds to those found in N-doped ZnSe epitaxial layers [2,15] but this is the first report on nondoped ZnSe epitaxial layers.
Acknowledgements The authors would like to thank Mr. H. Yokoyama, T. Shimizu, M. Kawahara and D. Maruoka of Miyazaki University for their experimental assistance and useful discussions.
References 4. Conclusions The nondoped ZnSe thin films with VI/II ratio of 12 and 15 grown on GaAs substrate by MBE have been characterized by means of PPA measurements at room and liquid nitrogen temperatures. Seven peaks in the PPA spectrum are confirmed in this study, and two of them are attributed to the band gap energies of ZnSe and GaAs. The other peaks activation energies of 1.20 + 0.01, having 0.97 f 0.03, 0.65 + 0.03 and 0.06 + 0.01 eV are considered to be due to intrinsic defects formed in nondoped ZnSe epitaxial layers. The origin of the intrinsic defect with activation energy of 0.06 F 0.01 eV is related to the Zn-vacancy. In order to make detailed assignment for observed peaks, further experiments are needed. The PPA signals of nondoped ZnSe/GaAs grown by MBE are successfully measured at liquid nitrogen temperatures for the first time. We found that PPA measurement using a piezoelectric transducer is a useful method for characterizing the nonradiative centers in ZnSe epitaxial layers.
Cl1 K. Yoshino, Jpn. J. Appl. Phys. 34 (1995) 6331. 121 A. Ohki, Y. Kawaguchi, K. Ando, A. Katsui, Appl. Phys. Lett. 59 (1991) 671. T. Ikari, K. Maeda, K. Futagami, Jpn. J. c31 A. Fukuyama, Appl. Phys. 32 (1993) 2567. M T. Ikari, K. Maeda, K. Futagami, Jpn. J. Appl. Phys. 33 (1994) L351. I51 Am. Zegadi, Ab. Zegadi, E. Ahmed, R.D. Pilkington, A.E. Hill, R.D. Tomlinson, Cryst. Res. Technol. 31 (1996) 247. B.K. Chaudhuri, Cd A.K. Ghosh, K.K. Som. S. Chatterjee, Phy. Rev. B 51 (1995) 4842. A. Fukuyama, K. Maeda, M. c71 K. Yoshino, Y. Nakagawa, Yoneta, H. Saito, M. Ohishi, T. Ikari, Mater. Sci. Eng. B, to be published. PI M. Ohisihi, M. Yoneta, H. Saito, in preparation. K. Maeda, K. Futagami, S. [91 T. Ikari, A. Fukuyama, Shigetomi, Y. Akashi, Phys. Rev. B 46 (1992) 10173. Jpn. J. Appl. Phys. 35 (1996) Cl01 T. Kita, T. Nishino, 5367. II111T. Ikari, H. Yokoyama, S. Shigetomi, K. Futagami, Jpn. J. Appl. Phys. 29 (1990) 887. T. Ido, Jpn. J. Appl. Cl21 H. Goto, T. Tanoi, M. Takemura, Phys. 34 (1995) L827. 1131D. Theis, Phys. Stat. Sol. (b) 79 (1977) 125. Cl41 M. Ohishi, Jpn. J. Appi. Phys. 25 (1986) 1546. Cl51 K. Tanaka, Z. Zhu, T. Yao, Appl. Phys. Lett. 66 (1995) 3349.
Journal of Crystal Growth 184/185 (1998) 1151~1154
Deep levels in ZnSe epitaxial layers examined by piezoelectric photoacoustic spectroscopy Kenji Yoshinoa**, Yasuhiro Nakagawaa, Atsuhiko Fukuyamab, Kouji Maeda”, Minoru Yoneta”, Hiroshi Saito”, Masakazu Ohishi”, Tetsuo Ikari” aDepartment of Electrical and Electronic Engineering, Miyazaki University, I-I Gakuen, Kibanadai-nishi, Myazaki 889.21, Japan b Department of Materials Science, Miyazaki University. l-l Gakuen, Kibanadai-nishi, Miyazaki 889.21, Japan ‘Department qf Applied Physics, Okayama University of Science, I-l Ridai-cho, Okayama 700, Japan
Abstract Piezoelectric photoacoustic (PPA) measurements (MBE) are carried out at room and liquid nitrogen
of nondoped ZnSe epitaxial layers grown by molecular beam epitaxy temperatures. Distinct peaks due to the band-gaps of ZnSe and the
substrate GaAs are clearly observed. Five PPA peaks are observed in the nondoped ZnSe thin films with VI/II ratio of 12 at liquid nitrogen temperature for the first time. This indicates that five types of intrinsic defects exist in ZnSe epitaxial layers. The obtained energies of these defects are 1.20 + 0.01, 0.97 + 0.03, 0.65 & 0.03 and 0.06 f 0.01 eV, respectively. ;Q 1998 Elsevier Science B.V. All rights reserved. Zinc selenide (ZnSe); Molecular (PL); Nonradiative recombination
Keywords:
beam epitaxy (MBE); Piezoelectric
1. Introduction II-VI compounds, especially zinc selenide (ZnSe), have been widely investigated to develop blue light-emitting devices [l]. However, the device applications of ZnSe-based II-VI compounds are not yet realized. It is important to understand the nature of the crystalline defects in the materials for
*Corresponding author. Fax: [email protected].
+ 81 985 58 2876; e-mail:
0022-0248/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PI1 SOO22-0248(97)00622-2
photoacoustic
(PPA); Photoluminescence
their practical use. In order to know the crystalline defects, photoluminescence (PL) spectroscopy has usually been carried out. Although we know radiative recombinations by the PL measurements, nonradiative recombination processes cannot be characterized at all. Recently, many studies on the nonradiative recombination in ZnSe, such as deeplevel transient spectroscopy (DLTS) [2], have been paid great attention since these crystalline defects may prevent the CW laser oscillation at room temperature. However, it is known that DLTS measurements for high resistivity ZnSe are difficult
1152
K. Yoshino et al. /Journal
of‘Ctystal
and distinguishing between radiative and nonradiative centers is not easy. Photoacoustic (PA) measurements have recently been used as one of the new methods to study the physical properties of semiconductors [3-61. The great advantage of the PA measurement is that the nonradiative recombination processes are measured directly. Then, the PA techniques may complement the PL techniques. To detect the PA signal, a gas-microphone has been generally used in the PA measurements. We used a piezoelectric transducer (PZT) in this study. A PZT detector is more sensitive than a gas-microphone for detecting the PA signal and can be used even at low temperatures. Furthermore, piezoelectric PA (PPA) spectroscopy is much easier than the DLTS methods since no electrodes are needed in the PPA system. So far, only a few papers have reported the PPA technique on ZnSe epitaxial layers. We have previously carried out PPA measurements of ZnSe at room temperature [7]. In this paper, PPA measurements of ZnSe are carried out at room and liquid nitrogen temperatures. The usefulness of the PPA measurements to study the nonradiative centers in the ZnSe epitaxial layers grown by MBE is discussed.
2. Experimental
Growth 1841185
(1998)
1151-1154
sample was mounted on the cold finger of a cryostat (Oxford Co., Ltd.: optistat DN-V model). A xenon lamp was used as an excitation source (Ushio Spex Co., Ltd.: UXL-SOOD). The probe light was mechanically chopped and exposed on the surface of the ZnSe layers. The PPA signals were detected by a PZT. The disk of 5 mm 0, was attached directly on the rear surface of the samples (GaAs substrate side) with silver paste. We confirmed that the silver paste gives good thermal and mechanical contacts between the sample and the PZT. The output signals from the PZT were amplified by a digital lock-in amplifier and accumulated by a personal computer. The detailed experimental procedures have been reported in a previous paper
C91.
3. Results and discussion The PPA spectra of the ZnSe/GaAs films grown by MBE are shown in Fig. 1 under a modulation
1OOHz R. T.
procedure
The nondoped ZnSe epitaxial layers were grown on semi-insulating (SI) GaAs(0 0 1) by MBE methods at a substrate temperature of 250°C. Zn and Se beams were post-heated at 600 and 400°C respectively. The VI/II ratio of the two samples in this study were about 12 and 15. The thickness of the layers and SI-GaAs substrate were about 1.0 and 500 urn, respectively. The epitaxial layers were of high quality since streaky reflection high-energy electron diffraction (RHEED) patterns were observed and the PL spectra at low temperature show sharp free-exciton emissions. Details of the MBE growth techniques, the RHEED patterns and the PL spectra of the nondoped ZnSe thin films were described in a previous paper [S]. The PPA measurements were carried out at room and liquid nitrogen temperatures with a modulated frequency from 100 to 500 Hz. The
1.5
2
Photon
2.5
Energy
3
(eV)
Fig. 1. Piezoelectric photoacoustic spectra of ZnSe/GaAs(O 0 1) grown under VI/II ratio of 12 (upper) and 15 (lower) at room temperature using a modulation frequency of 100 Hz.
K. Yoshino et al. /Journal
qf Crystal Growth 184/185 (1998) 1151-1154
frequency of 100 Hz at room temperature. The intensity of the PPA signals are normalized by the wavelength dependence of incident light power and the vertical scale of the figure between 1.3 and 2.5 eV is multiplied by 10. In the ZnSe/GaAs film grown under the VI/II ratio of 15, a sharp peak is observed at 2.70 eV. This peak energy is in good agreement with the bandgap energy (E,) obtained by electroreflectance measurements [lo]. Two weak bands at 1.45 and 2.05 f 0.03 eV (labeled B in the figure) are observed. Since the E, of GaAs is about 1.45 eV at room temperature, we consider that the peak at 1.45 eV in Fig. 1 is due to the E, of GaAs [9,11] and the band B is due to an intrinsic defect in the ZnSe layer. From the energy difference between the B band and the E,, the activation energy is estimated to be 0.65 + 0.03 eV. This value corresponds to that obtained by DLTS measurements for N-doped ZnSe thin film [12]. Since the 0.65 + 0.05 eV band is not observed in the PL spectrum [S], it is considered that this is due to nonradiative recombination. The deep level with an activation energy of 0.65 ) 0.03 eV is detected optically for nondoped ZnSe epitaxial layers grown by MBE for the first time. For the sample grown under a VI/II ratio of 12, on the other hand, a weak and broad bands are observed around the 2.70 eV region. The PPA intensity of the peaks corresponding to the band gaps of ZnSe and GaAs with a VI/II ratio of 12 is very small in comparison with those of a VI/II ratio of 15. This indicates that the absorption coefficient of the ZnSe layer with a VI/II ratio of 12 becomes smaller than that of a layer with a VI/II ratio of 15. The peak due to the band gap of GaAs is weakly observed. This peak energy at 1.54 eV corresponds well to that reported by Ikari et al. [9,11]. Since the signals of the PPA spectra of the ZnSe/GaAs films with VI/II ratios of 12 and 15 under a modulation frequency of 100 Hz at 80 K are weak, the higher modulation frequency of 500 Hz is used. The low temperature PPA spectrum of the ZnSe/GaAs film with a VI/II ratio of 12 is shown in Fig. 2. The broad band around the 2.70 eV region seen at room temperature in Fig. 1 is now composed of two peaks, i.e., the peak at 2.77 eV (labelled C) is due to an intrinsic defect and the peak at 2.82 eV corresponds to the E, of ZnSe
1153
C 5ooHz SOK
ZnSc i 1
VI/II=12
B
D
In
1
11
1.5
I
I
I
I
2
I
I.
h
*
I
I
I
2.5
I
I
I
I
3
Photon Energy (eV) Fig. 2. Piezoelectric photoacoustic spectra of ZnSe/GaAs(O 0 1) grown under VI/II ratio of 12 at liquid nitrogen temperature using modulation frequency of 500 Hz.
[13]. Similarly, the broad band around 2.1 eV at room temperature is decomposed into three peaks (labelled B and D) at 80 K as shown in Fig. 2. Furthermore, the weak but sharp peak at 1.62 f 0.01 eV (labelled A) is clearly resolved at 80 K. We obtained the activation energy for this level as 1.20 f 0.01 eV. This value corresponds well to that obtained by the DLTS measurements for N-doped ZnSe thin film [a]. The peak C at 2.76 f 0.01 eV appears clearly in comparison with that at room temperature. The activation energy for peak C is estimated as 0.06 + 0.01 eV, which is not reported in nondoped ZnSe epitaxial layers so far. However, the peak C is not clearly observed in ZnSe/GaAs grown under VI/II= 15. Although the bound exciton line of the deep acceptor (Ifeep), accompanied by LO phonon replicas, is clearly observed in the PI, spectrum of ZnSe/GaAs grown under VI/II = 12, no such lines are seen in ZnSe/GaAs grown under a VI/II = 15. Then we consider that the peak C in PPA spectrum is related to Zn vacancy [14].
1154
K. Yoshino et al. /Journal
qf Crystal Growth 1841185 (1998) 1151-1154
Besides the band B, a new band D appears at 80 K in ZnSe grown under VI/II = 12. However, only the band D is observed in the ZnSe grown under VI/II = 15. By using its energy of 1.85 _+ 0.03, the activation energy for this level is calculated as 0.97 f 0.03 eV. This energy corresponds to those found in N-doped ZnSe epitaxial layers [2,15] but this is the first report on nondoped ZnSe epitaxial layers.
Acknowledgements The authors would like to thank Mr. H. Yokoyama, T. Shimizu, M. Kawahara and D. Maruoka of Miyazaki University for their experimental assistance and useful discussions.
References 4. Conclusions The nondoped ZnSe thin films with VI/II ratio of 12 and 15 grown on GaAs substrate by MBE have been characterized by means of PPA measurements at room and liquid nitrogen temperatures. Seven peaks in the PPA spectrum are confirmed in this study, and two of them are attributed to the band gap energies of ZnSe and GaAs. The other peaks activation energies of 1.20 + 0.01, having 0.97 f 0.03, 0.65 + 0.03 and 0.06 + 0.01 eV are considered to be due to intrinsic defects formed in nondoped ZnSe epitaxial layers. The origin of the intrinsic defect with activation energy of 0.06 F 0.01 eV is related to the Zn-vacancy. In order to make detailed assignment for observed peaks, further experiments are needed. The PPA signals of nondoped ZnSe/GaAs grown by MBE are successfully measured at liquid nitrogen temperatures for the first time. We found that PPA measurement using a piezoelectric transducer is a useful method for characterizing the nonradiative centers in ZnSe epitaxial layers.
Cl1 K. Yoshino, Jpn. J. Appl. Phys. 34 (1995) 6331. 121 A. Ohki, Y. Kawaguchi, K. Ando, A. Katsui, Appl. Phys. Lett. 59 (1991) 671. T. Ikari, K. Maeda, K. Futagami, Jpn. J. c31 A. Fukuyama, Appl. Phys. 32 (1993) 2567. M T. Ikari, K. Maeda, K. Futagami, Jpn. J. Appl. Phys. 33 (1994) L351. I51 Am. Zegadi, Ab. Zegadi, E. Ahmed, R.D. Pilkington, A.E. Hill, R.D. Tomlinson, Cryst. Res. Technol. 31 (1996) 247. B.K. Chaudhuri, Cd A.K. Ghosh, K.K. Som. S. Chatterjee, Phy. Rev. B 51 (1995) 4842. A. Fukuyama, K. Maeda, M. c71 K. Yoshino, Y. Nakagawa, Yoneta, H. Saito, M. Ohishi, T. Ikari, Mater. Sci. Eng. B, to be published. PI M. Ohisihi, M. Yoneta, H. Saito, in preparation. K. Maeda, K. Futagami, S. [91 T. Ikari, A. Fukuyama, Shigetomi, Y. Akashi, Phys. Rev. B 46 (1992) 10173. Jpn. J. Appl. Phys. 35 (1996) Cl01 T. Kita, T. Nishino, 5367. II111T. Ikari, H. Yokoyama, S. Shigetomi, K. Futagami, Jpn. J. Appl. Phys. 29 (1990) 887. T. Ido, Jpn. J. Appl. Cl21 H. Goto, T. Tanoi, M. Takemura, Phys. 34 (1995) L827. 1131D. Theis, Phys. Stat. Sol. (b) 79 (1977) 125. Cl41 M. Ohishi, Jpn. J. Appi. Phys. 25 (1986) 1546. Cl51 K. Tanaka, Z. Zhu, T. Yao, Appl. Phys. Lett. 66 (1995) 3349.