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ZnS multiple quantum wells
PHYSICA
Physica B 191 (1993) 136-139 North-Holland SDI: 0921-4526(93)E0096-Y
Ultraviolet laser and photodetector of CdZnS/ZnS multiple quantum wells T. T a g u c h i a, Y. Y a m a d a b, T. O h n o a, J . T . Mullins a a n d Y. M a s u m o t o b aDepartment of Electrical Engineering, Faculty of Engineering, Osaka University, Japan hlnstitute of Physics, University of Tsukuba, Japan Ultraviolet (UV) lasers have been constructed for the first time from multiple quantum well (MQW) heterostructures of Cd~Zn1 xS/ZnS (x =0.11-0.31) strained-layer superlattices. Stimulated emission can be observed either under optical pumping at RT or under pulsed injection at 30K in the spectral range of 357-390 nm. Structures of the laser were fabricated by the gaseous low-pressure MOCVD method. A spectral narrowing in the emission spectrum with increasing current in the UV injection diode was clearly observed in the vicinity of 375 nm at 30 K. An UV photodetector has been successfullyprepared from this MQW system, in which a spectral responsivityat 366 nm exhibits a high efficiencyof about 60 mA/W.
1. Introduction Since the first successful reports of II-VI g r e e n - b l u e laser [1] and blue laser [2] diodes, development of I I - V I quantum well lasers in the violet-ultraviolet wavelength region has become the next target in the fields of optical storage m e m o r y and biomedical testing devices [3]. We have already reported the great success in the fabrication of a C d Z n S / Z n S strained-layer superlattice (SLS) [4] for UV photodetectors [3] and optically pumped lasers [5,6] due to ZnS, having the largest band-gap energy among II-VI materials. CdxZnt_xS-based quantum well structures exhibit fundamental absorption edges which can be varied from green to UV [4]. This type of SLS was, for the first time, grown by E n d o h and Taguchi [7] using low-pressure metalorganic chemical vapour deposition ( M O C V D ) in 1989. Preliminary investigations of the stimulated laser emission have already been done, which may lead to the possibility of producing an ultraviolet laser diode from this
type of the SLS based on ZnS in the optically pumped C d Z n S / Z n S SLSs at 1 0 K or at room temperature (RT) with the use of a resonant excitation of the exciton absorption line [6]. Such experimental results have suggested that the origin of optical gain is different from that in a I I I - V multiple-quantum well ( M Q W ) system in which an electron-hole plasma plays a principal role in providing optical gain. Evidence of exciton lasing predominantly in optically pumped II-VI QWs [6,8] makes it possible to interpret the exciton-assisted stimulated emission process. In this paper, we will report the ultraviolet (UV) stimulated emission of C d Z n S / Z n S MQWs under optical pumping and pulsed injection conditions. It is noted that the injection-type diode exhibits spectral narrowing at 30 K under a pulsed current mode. Characteristics of the photodetector from a C d Z n S / Z n S M Q W have been examined by illuminating 366 nm light at RT.
2. CdZnS/ZnS growth and laser structures Correspondence to: T. Taguchi, Department of Electrical Engineering, Faculty of Engineering, Osaka University, Suita, Osaka 565, Japan.
Cubic-structured C d Z n S / Z n S SLSs were grown by low-pressure metalorganic chemical
0921-4526/93/$06.00 (~ 1993-Elsevier Science Publishers B.V. All rights reserved
137
T. Taguchi et al. I On CdZnS/ZnS multiple quantum wells
v a p o u r deposition at 0 . 3 T o r r using gaseous sources on ( 1 0 0 ) - o r i e n t e d G a A s substrates, following the deposition of a 1.5 Ixm thick ZnS buffer layer. The M Q W structures used in this study consisted of 10, 50 and 150 pairs of 22 ,~ or 41/~ CdxZnl_xS well layers (Lw) separated by 81/~ ZnS barrier layers (LB). The Cd composition ratio in the ternary alloy well layer varied f r o m 0.11 to 0.31 whose lattice mismatch between the well and the barrier layers is estimated to be approximately from 0.8 to 3.8%. As a result, the ternary alloy well layers are assumed to be thickness under a biaxial compressive strain. The individual well and barrier layer are believed to be within the critical layer thickness. T w o laser structures, shown in figs. 1(1) and (2), were fabricated as follows. (1) Optically p u m p e d laser structure. The 50 pairs M Q W sample grown on a ( 1 0 0 ) semiinsulating G a A s substrate was cleaved to approximately a 1 m m long resonator with uncoated facets. A cap layer of ZnS has a thickness of about 0.05 Ixm. For observation of the stimulated emission, the M Q W layer plane was excited perpendicularly and the polarisation was selected
t ZnS (0.05 pm) lCdZnS/Zn S j50 MQW ZnS
as the T E mode. A frequency-tripled Q-switched N d : Y A G laser linked to a tunable dye laser system was used as the excitation source. The pulse width was approximately 5 ns and the repetition rate was 50 Hz. (2) Injection laser diode structure. The laser diode ( L D ) structure with a 1 m m long resonator was fabricated on a layer of n-type ZnS d o p e d with iodine ( Z n S : I ) / ( 1 0 0 ) n ÷ G a A s substrate and was based on a window stripe structure with a length of 2000 Ixm and a stripe width of 15 ixm. The forward current flow layer was SiO 2. The active layer was a M W Q structure consisting of 10 pairs of 4 1 / k u n d o p e d CdZnS well layers and 81/k ZnS barrier layers. This M Q W structure was e m b e d d e d in a p - n junction made from ZnS layers doped with Na [9] acceptors and I [10] donors. The carrier concentrations of Na acceptors and I donors were estimated to be approximately 1016cm -3 and 8 × 1019 cm -3 at RT, respectively, both of which were confirmed by means of Hall effect measurement. Au and A u Ge electrodes were used as the ohmic contacts on the p-type Z n S : N a layer and n + - G a A s substrate, respectively.
3. Stimulated emission spectra under optical pumping
SI (100)GaAs Figure 2 shows the stimulated emission spectra observed at R T from the front face of the three
(I) ,~Au I~!
n÷- GaAs
I
SiO2 (0.5 um) ZnS: No (0.5 pm) CdZnS/ZnS 10MOW ZnS:I () 2pm)
f
RT
(a)
>., Ul C C I
Fig. 1. (1) Structure of an optically pumped laser prepared on SI (100) GaAs substrate. (2) Structure of an injection laser diode with a stripe window prepared on an n+-GaAs substrate.
(c)
x=0.22 x :0.31
0
r//////,~ A u G e
(2)
(b)
x =0.11
330
I
I ii I
I
i
I
350 370 390 W evelengt h (nm)
i
1-
410
Fig. 2. Stimulated emission spectra obtained at RT in the three CdxZn~ xS/ZnS MQWs (a:x=0.11, b:x=0.22 and c:x =0.31).
138
T. Taguchi et al. / On C d Z n S / Z n S multiple quantum wells
SLS (L w = 41 A and L B = 81 A) samples differing in the composition x ( a : x = 0.11, b : x = 0 . 2 2 and c : x = 0 . 3 1 ) . Each stimulated emission spectrum was obtained under resonant excitation of the n = 1 heavy-hole excitonic absorption peaks. The peak positions of each stimulated emission are located at (a) 357.2 nm, (b) 374.9 nm and (c) 390.2 nm, respectively. It should be particularly noted that each peak position of the stimulated emission is located at lower energies compared to the absorption and photoluminescence peaks (see the paper of Yamada et al. in this volume). The energy separation between the absorption and photoluminescence peaks may be related to the Stokes shift which becomes large with increasing composition x. This leads to the spectral diffusion of excitons towards the low-energy localised states in the inhomogeneous line [3]. CdxZnl_xS/ZnS
(Lw = 41 ]~ and L B = 81 ~ ) p - n junction diode under a forward bias condition. Typical characteristics of LD with both cleaved facets under pulsed operation at 30 K are shown in this figure. The pulse width and the repetition rate were 300 ns and 4 Hz, respectively. Spontaneous emission was obtained at 371.5nm at 8V and its linewidth was estimated to be about 20 nm. From the voltage-versus-current characteristics, the built-in voltage for causing stimulated emission was 30V. Above 30V, the emission intensity becomes strong and the peak position moves towards the longer-wavelength side (about 40meV) in comparison with that of spontaneous emission. The stimulated emission of linewidth less than 3 nm was clearly observed at the wavelength of 375.5nm above the threshold current of about 6 k A / c m 2 at 38V. The present energy position is close to that of the stimulated emission observed at 10 K under optical pumping [6].
4. Spectral narrowing of the injection diode
Figure 3 shows the spontaneous electroluminescence (a) and stimulated emission (b) spectra at 30 K from the MQW Cd0.3Zn0.7S/ZnS
6
CdxZnl-x S (41 ,~,)/ ZnS (61 A) x=0.3 30K
A375.5nm //38V 6kA/cm2
c
.c_c
l./J
•
360
./
8V
370 380 Wovelengt h (nm)
0-3
:90
Fig. 3. Spontaneous electroluminescence (a) and stimulated emission (b) associated with spectral narrowing at 30 K from a Cdo 3Zn0,TS/ZnS MQW diode under pulsed injection condition: (a) 8 V and (b) 38 V.
5. UV photodetector
An UV photodetector was fabricated from a MQW structure with 150 cycles and its photoresponse was measured by illuminating light of 366nm. We have previously reported that an application of the electric field induces a significant change in the valence band offset [11] which leads to an energy shift towards shorter wavelength in the manner of type-II optical absorption. This effect leads to an apparent shift of the excitonic absorption edge in the QW and has been tentatively interpreted in terms of the Wannier-Stark localisation of electrons and holes which are strongly localised in the extremely narrow well width under reverse-biased electric fields. Figure 4 shows the photocurrent response as a function of input power at 366 nm at RT of a Cd0.3Zn0.7S/ZnS MQW photodiode (L w = 22 ~ , L s = 81 ~ and 150 cycles) under an electric field of 1 V. With increasing input power the photocurrent increases linearly. The peak response of the MQW diode is estimated to be about 60 m A /
T. Taguchi et al. / On CdZnS/ZnS multiple quantum wells
lasing at UV wavelength has been demonstrated for the first time in wide-gap II-VI quantum structures. It was found that the characteristics of stimulated emission in the range of 350 to 390 nm under optical pumping at RT depend upon the composition x. The CdZnS/ZnS M Q W system is a promising candidate for fabricating an UV photodetector with high response.
CdZnS MQW RT 104 V/cm
o -,,1)<
139
E v
E 5 4 3 2
References
1
0
I
0
I
I
1 2 Power
i
I
3 4 (IJW)
I
5
Fig. 4. Photoresponse of a Cdo3Zn0.7S/ZnS MQW photodetector at 366 nm at RT.
W, which can provide an efficiency higher than that of an ordinary Si photodiode.
6. Conclusions
A laser diode structure of CdxZn~_xS/ZnS MQWs in the ultraviolet wavelength range (340400 nm) has b e e n fabricated for the first time using a conventional low-pressure MOCVD method. Spectral narrowing in the UV emission above the threshold current can be observed at 30 K under pulsed operation of 38 V and about 6 k A / c m 2. In spite of a low acceptor concentration of ZnS doped with Na, the possibility of
[1] M.A. Haase, J. Qiu, J.M. DePuydt and H. Cheng, Appl. Phys. Lett. 59 (1991) 1272. [2] H. Okuyama, T. Miyajima, Y. Morinaga, F. Hiei, M. Ozawa and K. Akimoto, Electron. Lett. 28 (1992) 1798. [3] T. Taguchi, Y. Endoh, T. Ohno and Y. Nozue, J. Lumin. 54 (1992) 123. [4] T. Taguchi, Y. Endoh and Y. Nozue, Appl. Phys. Lett. 56 (1991) 342. [5] Y. Yamada, J. Mullins, Y. Masumoto and T. Taguchi, Conference Digest of the 13th International Semiconductor Lasers Conference, Takamatsu (Business Center for Academic Societies, Japan, 1992) p. 162. [6] Y. Yamada, Y. Masumoto, J. Mullins and T. Taguchi, Appl. Phys. Lett. 61 (1992) 2190. [7] Y. Endoh and T. Taguchi, Mat. Res. Soc. Symp. 161 (1990) 211. [8] J. Ding, H. Jeon, T. Ishihara, A.V. Nurmikko, H. Luo, N. Samarth and J. Furdana, Surf. Sci. 287 (1992) 616. [9] T. Ohno and T. Taguchi, J. Cryst. Growth 107 (1991) 649. [10] Z. Kawazu, Y. Kawakami and T. Taguchi, Mater. Sci. Forum 38-41 (1988) 555. [11] T. Ohno and T. Taguchi, Japan J. Appl. Phys. 30 (1991) L512.
Physica B 191 (1993) 136-139 North-Holland SDI: 0921-4526(93)E0096-Y
Ultraviolet laser and photodetector of CdZnS/ZnS multiple quantum wells T. T a g u c h i a, Y. Y a m a d a b, T. O h n o a, J . T . Mullins a a n d Y. M a s u m o t o b aDepartment of Electrical Engineering, Faculty of Engineering, Osaka University, Japan hlnstitute of Physics, University of Tsukuba, Japan Ultraviolet (UV) lasers have been constructed for the first time from multiple quantum well (MQW) heterostructures of Cd~Zn1 xS/ZnS (x =0.11-0.31) strained-layer superlattices. Stimulated emission can be observed either under optical pumping at RT or under pulsed injection at 30K in the spectral range of 357-390 nm. Structures of the laser were fabricated by the gaseous low-pressure MOCVD method. A spectral narrowing in the emission spectrum with increasing current in the UV injection diode was clearly observed in the vicinity of 375 nm at 30 K. An UV photodetector has been successfullyprepared from this MQW system, in which a spectral responsivityat 366 nm exhibits a high efficiencyof about 60 mA/W.
1. Introduction Since the first successful reports of II-VI g r e e n - b l u e laser [1] and blue laser [2] diodes, development of I I - V I quantum well lasers in the violet-ultraviolet wavelength region has become the next target in the fields of optical storage m e m o r y and biomedical testing devices [3]. We have already reported the great success in the fabrication of a C d Z n S / Z n S strained-layer superlattice (SLS) [4] for UV photodetectors [3] and optically pumped lasers [5,6] due to ZnS, having the largest band-gap energy among II-VI materials. CdxZnt_xS-based quantum well structures exhibit fundamental absorption edges which can be varied from green to UV [4]. This type of SLS was, for the first time, grown by E n d o h and Taguchi [7] using low-pressure metalorganic chemical vapour deposition ( M O C V D ) in 1989. Preliminary investigations of the stimulated laser emission have already been done, which may lead to the possibility of producing an ultraviolet laser diode from this
type of the SLS based on ZnS in the optically pumped C d Z n S / Z n S SLSs at 1 0 K or at room temperature (RT) with the use of a resonant excitation of the exciton absorption line [6]. Such experimental results have suggested that the origin of optical gain is different from that in a I I I - V multiple-quantum well ( M Q W ) system in which an electron-hole plasma plays a principal role in providing optical gain. Evidence of exciton lasing predominantly in optically pumped II-VI QWs [6,8] makes it possible to interpret the exciton-assisted stimulated emission process. In this paper, we will report the ultraviolet (UV) stimulated emission of C d Z n S / Z n S MQWs under optical pumping and pulsed injection conditions. It is noted that the injection-type diode exhibits spectral narrowing at 30 K under a pulsed current mode. Characteristics of the photodetector from a C d Z n S / Z n S M Q W have been examined by illuminating 366 nm light at RT.
2. CdZnS/ZnS growth and laser structures Correspondence to: T. Taguchi, Department of Electrical Engineering, Faculty of Engineering, Osaka University, Suita, Osaka 565, Japan.
Cubic-structured C d Z n S / Z n S SLSs were grown by low-pressure metalorganic chemical
0921-4526/93/$06.00 (~ 1993-Elsevier Science Publishers B.V. All rights reserved
137
T. Taguchi et al. I On CdZnS/ZnS multiple quantum wells
v a p o u r deposition at 0 . 3 T o r r using gaseous sources on ( 1 0 0 ) - o r i e n t e d G a A s substrates, following the deposition of a 1.5 Ixm thick ZnS buffer layer. The M Q W structures used in this study consisted of 10, 50 and 150 pairs of 22 ,~ or 41/~ CdxZnl_xS well layers (Lw) separated by 81/~ ZnS barrier layers (LB). The Cd composition ratio in the ternary alloy well layer varied f r o m 0.11 to 0.31 whose lattice mismatch between the well and the barrier layers is estimated to be approximately from 0.8 to 3.8%. As a result, the ternary alloy well layers are assumed to be thickness under a biaxial compressive strain. The individual well and barrier layer are believed to be within the critical layer thickness. T w o laser structures, shown in figs. 1(1) and (2), were fabricated as follows. (1) Optically p u m p e d laser structure. The 50 pairs M Q W sample grown on a ( 1 0 0 ) semiinsulating G a A s substrate was cleaved to approximately a 1 m m long resonator with uncoated facets. A cap layer of ZnS has a thickness of about 0.05 Ixm. For observation of the stimulated emission, the M Q W layer plane was excited perpendicularly and the polarisation was selected
t ZnS (0.05 pm) lCdZnS/Zn S j50 MQW ZnS
as the T E mode. A frequency-tripled Q-switched N d : Y A G laser linked to a tunable dye laser system was used as the excitation source. The pulse width was approximately 5 ns and the repetition rate was 50 Hz. (2) Injection laser diode structure. The laser diode ( L D ) structure with a 1 m m long resonator was fabricated on a layer of n-type ZnS d o p e d with iodine ( Z n S : I ) / ( 1 0 0 ) n ÷ G a A s substrate and was based on a window stripe structure with a length of 2000 Ixm and a stripe width of 15 ixm. The forward current flow layer was SiO 2. The active layer was a M W Q structure consisting of 10 pairs of 4 1 / k u n d o p e d CdZnS well layers and 81/k ZnS barrier layers. This M Q W structure was e m b e d d e d in a p - n junction made from ZnS layers doped with Na [9] acceptors and I [10] donors. The carrier concentrations of Na acceptors and I donors were estimated to be approximately 1016cm -3 and 8 × 1019 cm -3 at RT, respectively, both of which were confirmed by means of Hall effect measurement. Au and A u Ge electrodes were used as the ohmic contacts on the p-type Z n S : N a layer and n + - G a A s substrate, respectively.
3. Stimulated emission spectra under optical pumping
SI (100)GaAs Figure 2 shows the stimulated emission spectra observed at R T from the front face of the three
(I) ,~Au I~!
n÷- GaAs
I
SiO2 (0.5 um) ZnS: No (0.5 pm) CdZnS/ZnS 10MOW ZnS:I () 2pm)
f
RT
(a)
>., Ul C C I
Fig. 1. (1) Structure of an optically pumped laser prepared on SI (100) GaAs substrate. (2) Structure of an injection laser diode with a stripe window prepared on an n+-GaAs substrate.
(c)
x=0.22 x :0.31
0
r//////,~ A u G e
(2)
(b)
x =0.11
330
I
I ii I
I
i
I
350 370 390 W evelengt h (nm)
i
1-
410
Fig. 2. Stimulated emission spectra obtained at RT in the three CdxZn~ xS/ZnS MQWs (a:x=0.11, b:x=0.22 and c:x =0.31).
138
T. Taguchi et al. / On C d Z n S / Z n S multiple quantum wells
SLS (L w = 41 A and L B = 81 A) samples differing in the composition x ( a : x = 0.11, b : x = 0 . 2 2 and c : x = 0 . 3 1 ) . Each stimulated emission spectrum was obtained under resonant excitation of the n = 1 heavy-hole excitonic absorption peaks. The peak positions of each stimulated emission are located at (a) 357.2 nm, (b) 374.9 nm and (c) 390.2 nm, respectively. It should be particularly noted that each peak position of the stimulated emission is located at lower energies compared to the absorption and photoluminescence peaks (see the paper of Yamada et al. in this volume). The energy separation between the absorption and photoluminescence peaks may be related to the Stokes shift which becomes large with increasing composition x. This leads to the spectral diffusion of excitons towards the low-energy localised states in the inhomogeneous line [3]. CdxZnl_xS/ZnS
(Lw = 41 ]~ and L B = 81 ~ ) p - n junction diode under a forward bias condition. Typical characteristics of LD with both cleaved facets under pulsed operation at 30 K are shown in this figure. The pulse width and the repetition rate were 300 ns and 4 Hz, respectively. Spontaneous emission was obtained at 371.5nm at 8V and its linewidth was estimated to be about 20 nm. From the voltage-versus-current characteristics, the built-in voltage for causing stimulated emission was 30V. Above 30V, the emission intensity becomes strong and the peak position moves towards the longer-wavelength side (about 40meV) in comparison with that of spontaneous emission. The stimulated emission of linewidth less than 3 nm was clearly observed at the wavelength of 375.5nm above the threshold current of about 6 k A / c m 2 at 38V. The present energy position is close to that of the stimulated emission observed at 10 K under optical pumping [6].
4. Spectral narrowing of the injection diode
Figure 3 shows the spontaneous electroluminescence (a) and stimulated emission (b) spectra at 30 K from the MQW Cd0.3Zn0.7S/ZnS
6
CdxZnl-x S (41 ,~,)/ ZnS (61 A) x=0.3 30K
A375.5nm //38V 6kA/cm2
c
.c_c
l./J
•
360
./
8V
370 380 Wovelengt h (nm)
0-3
:90
Fig. 3. Spontaneous electroluminescence (a) and stimulated emission (b) associated with spectral narrowing at 30 K from a Cdo 3Zn0,TS/ZnS MQW diode under pulsed injection condition: (a) 8 V and (b) 38 V.
5. UV photodetector
An UV photodetector was fabricated from a MQW structure with 150 cycles and its photoresponse was measured by illuminating light of 366nm. We have previously reported that an application of the electric field induces a significant change in the valence band offset [11] which leads to an energy shift towards shorter wavelength in the manner of type-II optical absorption. This effect leads to an apparent shift of the excitonic absorption edge in the QW and has been tentatively interpreted in terms of the Wannier-Stark localisation of electrons and holes which are strongly localised in the extremely narrow well width under reverse-biased electric fields. Figure 4 shows the photocurrent response as a function of input power at 366 nm at RT of a Cd0.3Zn0.7S/ZnS MQW photodiode (L w = 22 ~ , L s = 81 ~ and 150 cycles) under an electric field of 1 V. With increasing input power the photocurrent increases linearly. The peak response of the MQW diode is estimated to be about 60 m A /
T. Taguchi et al. / On CdZnS/ZnS multiple quantum wells
lasing at UV wavelength has been demonstrated for the first time in wide-gap II-VI quantum structures. It was found that the characteristics of stimulated emission in the range of 350 to 390 nm under optical pumping at RT depend upon the composition x. The CdZnS/ZnS M Q W system is a promising candidate for fabricating an UV photodetector with high response.
CdZnS MQW RT 104 V/cm
o -,,1)<
139
E v
E 5 4 3 2
References
1
0
I
0
I
I
1 2 Power
i
I
3 4 (IJW)
I
5
Fig. 4. Photoresponse of a Cdo3Zn0.7S/ZnS MQW photodetector at 366 nm at RT.
W, which can provide an efficiency higher than that of an ordinary Si photodiode.
6. Conclusions
A laser diode structure of CdxZn~_xS/ZnS MQWs in the ultraviolet wavelength range (340400 nm) has b e e n fabricated for the first time using a conventional low-pressure MOCVD method. Spectral narrowing in the UV emission above the threshold current can be observed at 30 K under pulsed operation of 38 V and about 6 k A / c m 2. In spite of a low acceptor concentration of ZnS doped with Na, the possibility of
[1] M.A. Haase, J. Qiu, J.M. DePuydt and H. Cheng, Appl. Phys. Lett. 59 (1991) 1272. [2] H. Okuyama, T. Miyajima, Y. Morinaga, F. Hiei, M. Ozawa and K. Akimoto, Electron. Lett. 28 (1992) 1798. [3] T. Taguchi, Y. Endoh, T. Ohno and Y. Nozue, J. Lumin. 54 (1992) 123. [4] T. Taguchi, Y. Endoh and Y. Nozue, Appl. Phys. Lett. 56 (1991) 342. [5] Y. Yamada, J. Mullins, Y. Masumoto and T. Taguchi, Conference Digest of the 13th International Semiconductor Lasers Conference, Takamatsu (Business Center for Academic Societies, Japan, 1992) p. 162. [6] Y. Yamada, Y. Masumoto, J. Mullins and T. Taguchi, Appl. Phys. Lett. 61 (1992) 2190. [7] Y. Endoh and T. Taguchi, Mat. Res. Soc. Symp. 161 (1990) 211. [8] J. Ding, H. Jeon, T. Ishihara, A.V. Nurmikko, H. Luo, N. Samarth and J. Furdana, Surf. Sci. 287 (1992) 616. [9] T. Ohno and T. Taguchi, J. Cryst. Growth 107 (1991) 649. [10] Z. Kawazu, Y. Kawakami and T. Taguchi, Mater. Sci. Forum 38-41 (1988) 555. [11] T. Ohno and T. Taguchi, Japan J. Appl. Phys. 30 (1991) L512.