- Email: [email protected]
Time-resolved photoluminescence of porous silicon under double-pulse excitation
J O U R N A L OF
NON-C NESOLI ELSEVIER
Journal of Non-CrystallineSolids 198-200 (1996) 965 968
Time-resolved photoluminescence of porous silicon under double-pulse excitation Shuji Komuro a,*, Takitaro Morikawa ", Patrick O'Keeffe b, Yoshinobu Aoyagi Faculo' of Engineering, Toyo Unil,ersit~, Kawagoe, Saitama 350, Japan b Semtconductors Laboratoo,, RIKEN, Wako, Saitama 351-01, Japan
Abstract In naturally oxidized porous Si (PS) it was found that the blue-green emission band centered at 440 nm appears and its photoluminescence (PL) intensity is enhanced by a double-pulse excitation. The PL enhancement of the blue-green band strongly depends on the pulse time interval between the first pulse and the second pulse. This means that the PL enhancement is due to the radiative recombination of carriers re-excited by the second pulse. The origin of the PL is considered to be radiative transitions via oxygen induced defect states. The time decay of PL is not single exponential but takes a double-exponential form. A simple carrier recombination model taking account of oxygen induced defect states is proposed on the basis of the time-resolved PL induced by double-pulse excitation.
1. Introduction As for the origin of strong visible photoluminescence (PL) observed from porous Si (PS), models based on quantum size effects [1], siloxene derivatives [2], and radiative surface states [3] have been proposed to date. The PL peak energy and intensity of PS dramatically change as a consequence of its preparation conditions a n d / o r subsequent treatments such as immersing in solution [4,5], thermal annealing [6], and oxidation [7-9]. However, application of PS to optical devices requires clarification of the preparation processes as well as the origin of this PL. Time-resolved PL (TRPL) measurements have been carried out to understand the carrier recombination process in PS [10,11]. Here, we have employed, for the first time, a double-pulse excitation method for
* Corresponding author. Tel: + 81-492 391 366; fax: + 81-492 331 855; e-mail: [email protected].
TRPL, which is expected to be more informative than a single-pulse method for understanding the PS luminescence mechanism. This will make the carrier recombination process clearer by varying the pulse time interval between the first excitation pulse and the second excitation pulse, because observation of the radiative recombination of carriers re-excited by the second pulse enables us to characterize the detailed PL mechanism.
2. Experimental The substrate used was a p-type (100)Si wafer with a resistivity of 10-20 [2 cm. The PS sample was preparcd by standard anodization [9]. This PS sample was allowed to naturally oxidize at room temperature in the dark for three months. All PL measurements were carried out at room temperature using a N 2 laser which has a pulse width of 0.3 ns, a
0022-3093/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0022-3093(96)00097-X
966
S. Komuro et al. / Journal of Non-Crystalline Solids 198 200 (1996) 965-968
wavelength of 337 nm and peak power of 250 kW. The excitation laser beam scattered from the PS surface was cut by a UV37 filter and PL spectra were recorded using a boxcar integrator with a gatewidth of 60 ns. Pulses for double-pulse excitation TRPL were obtained by splitting a single pulse of the N 2 laser. The PS sample was excited by the first pulse directly, and the second pulse after a time interval (At) provided by an optical delay line.
1.4
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In order to investigate the effect on PL of oxidation, PL spectra for both an as-prepared sample and a naturally oxidized one were measured first under single-pulse excitation. The typical PL spectrum for as-prepared PS is shown in Fig. l(a) with a single broad luminescence profile centered at 620 nm. After natural oxidation, the PL spectrum changes from Fig. l(a) to (b). Comparing the PL spectra of Fig. l(a) with (b), two dominant differences are observed in PL features after oxidation. Firstly, the PL peak positioned at 620 nm in Fig. l(a) shifts toward higher energy, 600 nm. It should also be noted that the PL intensity increases with oxidation. These spectra changes are consistent with previously reported PS oxidation effects [7-9]. Secondly, an additional emission band appears in the high energy side of the spectrum at about 440 nm.
.--.
i
. . . . . . . . .
i
. . . . . . . . .
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) .........
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500
600
700
800
Wavelength (nm)
Fig. I. PL spectra for (a) as-prepared and (b) naturally oxidized PS.
. . . . . . . . .
400
i
. . . . . . . . .
500
"~.
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. . . . . . . . .
i .........
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3. Results
1.2
i .........
Double pulse excitation
lstpulR
l 1 l"
"~ltA~
t . , . l l l l l t l l
600
700
800
Wavelength (rim) Fig. 2. The changes in PL for different pulse time intervals (At) under double-pulse excitation.
To understand the carrier recombination process in oxidized PS, TRPL under double-pulse excitation was then carried out. In particular, carrier dynamics associated with the high energy band in highly oxidized PS has been focused on. Double-pulse excitation is essential in providing sufficient excitation of carriers to the high energy PL band. Fig. 2 shows a series of double-pulse excited TRPL spectra measured at different pulse time intervals, A t = 0.2 ns to 2.2 ns as shown in the inset. The single-pulse excited TRPL spectrum is also shown as a reference. Two distinct bands, the high energy PL band (HEB) and the low energy PL band (LEB) centered at 440 nm and 600 nm, are observed, respectively. In the case of single-pulse excitation, the PL intensity of the LEB is about two times larger than that of the HEB. However, HEB PL intensities were dramatically changed by varying At under double-pulse excitation. The changes in PL peak intensity of HEB and LEB are shown in Fig. 3(a) and (b), respectively, as a function of At. By decreasing At from 3.1 ns to 0.2 ns, the HEB PL intensity increased by a factor of 3.6 whereas the LEB hardly changed. For At < 1.0 ns, the HEB is dominant. On the contrary, the LEB is the main PL feature for A t > 1.0 ns. Under single-pulse excitation, the intensity ratio H E B / L E B was independent of the excitation power level. Hence, the observed enhancement of the HEB can be attributed directly to double-pulse excitation.
S. Komuro et al. / Journal of Non-Crystalline Solids 198-200 (1996) 965-968 1.5
\ O
A~
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5
on the best fit of the double-exponential form to the changes in PL intensity of both HEB and LEB. This suggests that radiative carrier recombination via both the HEB and LEB consists of two lifetime components, a fast component (ZFASV) and a slow one (~'sLow). F r o m the analysis of both the HEB and the LEB, the PL intensity decays with lifetimes of %VAST=3 ns, ZHSLOw = 7 0 ns for the HEB and fLEASv = 10 ns, ZLSLOw = 800 ns for the LEB. The two fast components of %VAST = 3 ns in the HEB and ZLFASv = 10 ns in the LEB strongly depend on A t for the enhancement of PL intensity of HEB as shown in Figs. 2 and 3.
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Pulse Time Interval At (ns) Fig. 3. P L p e a k i n t e n s i t y d e p e n d e n c e on p u l s e t i m e i n t e r v a l ( A t ) for (a) the H E B at 4 4 0 n m a n d (b) the L E B at 6 0 0 nm. L i n e s are
4. D i s c u s s i o n
d r a w n as g u i d e s for the eye.
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. . . .
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250
Delay Time (ns) Fig. 4. P L peak i n t e n s i t y d e p e n d e n c e o n d e l a y t i m e for (a) the H E B at 4 4 0 n m a n d (b) the L E B at 6 0 0 nm.
Fig. 4 shows the time decay of the PL peak intensities of both the HEB and LEB under doublepulse excitation. The decay is highly non-exponential. The experimental data could not be fitted by either a single-exponential decay or a stretched-exponential decay of the form Icx exp{-(t/z)¢}, where I, ~- and /3 are the PL intensity, carrier lifetime and fitting parameter, respectively [13], and which arises from the presence of a broad distribution of relaxation lifetimes. The data, however, could be fitted to a double exponential of the form, I cx exp(--t/'rFAST) + e x p ( - - t / Z s L o w ) as shown by the solid lines in Fig. 4. These calculated lines are based
In order to understand the above described results, Fourier transform infrared ( F T - I R ) measurements have been performed. W e observed the appearance of H S i - O 3 absorption peaks at 2250 cm-~ and that of H 2 S i - O 2 at 2195 cm l confirming the oxidation of the Si back-bond. Furthermore, the absorption at 1100 c m - ~ is due to the oxidation of the outermost surface resulting in S i - O - S i bridge formation. The above F T - I R measurement suggests that the HEB strongly depends on the oxidation of the PS surface layer. Therefore, the origin of the HEB is considered to be radiative transitions via oxygen induced defect levels, whereas the LEB results from S i / S i O x interface states, as for typical as-prepared PS [3,12]. Taking into account the PL intensity dependence of the HEB on A t, it seems that the enhancement can be achieved by re-excitation of carriers to HEB before the beginning of carrier relaxation. The second pulse excites carriers not only from ground states to the HEB and to LEB but also from both the HEB and LEB to higher states. However, for A t > 3 ns no selective enhancement of PL intensity on HEB is achieved. Therefore, carriers re-excited to the HEB by the second pulse for A t < 3 ns are considered to be provided from the LEB. To visualize these radiative processes more clearly a simple model can be proposed to explain the carrier excitation and recombination in highly oxidized PS under double-pulse excitation. This schematic model is shown in Fig. 5. The existence of a double-exponential dependence indicates that there
968
S. Komuro et al. / Journal of Non-Crystalline Solids 198-200 (1996) 965-968
• •
HEB ~
•
V•
'II j
the inter-pulse time interval less than 1 ns. The origin o f this PL is considered to be radiative transitions via o x y g e n induced defect states. The time decay o f P L intensity for the H E B is non-exponential and takes a d o u b l e - e x p o n e n t i a l form. A simple carrier r e c o m b i n a t i o n m o d e l taking into account the o x y g e n induced defect states has been p r o p o s e d on the basis o f the T R P L caused by double-pulse excitation.
¢'4
~
•
O.
•
LEB
References
Fig. 5. A model proposed to explain carrier excitation and recombination in highly oxidized PS under double-pulse excitation.
exist c o n f i n e d e n e r g y levels f r o m w h i c h radiative r e c o m b i n a t i o n occurs with two different lifetimes. Carriers w h i c h are initially excited by the first pulse (solid lines) relax in a c o m p e t i n g process via both the H E B and the L E B . U n d e r the condition o f A t < 3 ns, the second pulse e n h a n c e s PL intensity o f the H E B through re-excitation o f carriers f r o m the L E B to the H E B before the b e g i n n i n g o f the carrier relaxation (broken lines).
5. C o n c l u s i o n s It has been o b s e r v e d in naturally o x i d i z e d PS that the b l u e - g r e e n e m i s s i o n band ( H E B ) centered at 440 n m appears and its PL intensity is e n h a n c e d by a second pulse f o l l o w i n g the first excitation pulse with
[1] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1096. [2] M.S. Brandt, H.D. Fuchs, M. Stutzmann, J. Weber and M. Cardona, Solid State Commun. 81 (1992) 302. [3] F. Koch, V. Petrova-Koch, T. Mnschik, A. Nikolov and V. Garilenko, Mater. Res. Soc. Proc. 238 (1993) 197. [4] X.Y. Hou, G. Shi, W. Wang, F.L. Zhang, P.H. Hao, D.M. Huang and X. Wang, Appl. Phys. Lett. 62 (1993) 1097. [5] A. Nakajima, T. Itakura, S. Watanabe and N. Nakayama, Appl. Phys. Lett. 61 (1992) 46. [6] K. Shiba, K. Sakamoto, S. Miyazaki and M. Hirose, Jpn. J. Appl. Phys. 32 (1993) 2722. [7] A. Takazawa, T. Tamura and M. Yamada, Appl. Phys. Lett. 63 (1993) 940. [8] Y. Xiao, M.J. Heben, J.M. McCullough, Y.S. Tsuo, J.I. Pankove and S.K. Deb, Appl. Phys. Lett. 62 (1993) 1152. [9] P. O'Keeffe, S. Komuro, T. Kato, T. Morikawa and Y. Aoyagi, Jpn. J. Appl. Phys. 33 (1994)7117. [10] D.I. Kovalev, I.D. Yaroshetzhii, T. Muschik, V. Petrova-Koch and F. Koch, Appl. Phys. Lett. 64 (1994) 214. [11] T. Matsumoto, M. Daimon, T. Futagi and H. Mimura, Jpn. J. Appl. Phys. 31 (1992) L619. [12] P. O'Keeffe, S. Komuro, T. Kato, T. Morikawa and Y. Aoyagi, Appl. Phys. Lett. 66 (1995) 836. [13] N. Ookubo, H. Ono, Y. Ochiai, Y. Mochizuki and S. Matsui, Appl. Phys. Lett. 61 (1992) 940.
NON-C NESOLI ELSEVIER
Journal of Non-CrystallineSolids 198-200 (1996) 965 968
Time-resolved photoluminescence of porous silicon under double-pulse excitation Shuji Komuro a,*, Takitaro Morikawa ", Patrick O'Keeffe b, Yoshinobu Aoyagi Faculo' of Engineering, Toyo Unil,ersit~, Kawagoe, Saitama 350, Japan b Semtconductors Laboratoo,, RIKEN, Wako, Saitama 351-01, Japan
Abstract In naturally oxidized porous Si (PS) it was found that the blue-green emission band centered at 440 nm appears and its photoluminescence (PL) intensity is enhanced by a double-pulse excitation. The PL enhancement of the blue-green band strongly depends on the pulse time interval between the first pulse and the second pulse. This means that the PL enhancement is due to the radiative recombination of carriers re-excited by the second pulse. The origin of the PL is considered to be radiative transitions via oxygen induced defect states. The time decay of PL is not single exponential but takes a double-exponential form. A simple carrier recombination model taking account of oxygen induced defect states is proposed on the basis of the time-resolved PL induced by double-pulse excitation.
1. Introduction As for the origin of strong visible photoluminescence (PL) observed from porous Si (PS), models based on quantum size effects [1], siloxene derivatives [2], and radiative surface states [3] have been proposed to date. The PL peak energy and intensity of PS dramatically change as a consequence of its preparation conditions a n d / o r subsequent treatments such as immersing in solution [4,5], thermal annealing [6], and oxidation [7-9]. However, application of PS to optical devices requires clarification of the preparation processes as well as the origin of this PL. Time-resolved PL (TRPL) measurements have been carried out to understand the carrier recombination process in PS [10,11]. Here, we have employed, for the first time, a double-pulse excitation method for
* Corresponding author. Tel: + 81-492 391 366; fax: + 81-492 331 855; e-mail: [email protected].
TRPL, which is expected to be more informative than a single-pulse method for understanding the PS luminescence mechanism. This will make the carrier recombination process clearer by varying the pulse time interval between the first excitation pulse and the second excitation pulse, because observation of the radiative recombination of carriers re-excited by the second pulse enables us to characterize the detailed PL mechanism.
2. Experimental The substrate used was a p-type (100)Si wafer with a resistivity of 10-20 [2 cm. The PS sample was preparcd by standard anodization [9]. This PS sample was allowed to naturally oxidize at room temperature in the dark for three months. All PL measurements were carried out at room temperature using a N 2 laser which has a pulse width of 0.3 ns, a
0022-3093/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0022-3093(96)00097-X
966
S. Komuro et al. / Journal of Non-Crystalline Solids 198 200 (1996) 965-968
wavelength of 337 nm and peak power of 250 kW. The excitation laser beam scattered from the PS surface was cut by a UV37 filter and PL spectra were recorded using a boxcar integrator with a gatewidth of 60 ns. Pulses for double-pulse excitation TRPL were obtained by splitting a single pulse of the N 2 laser. The PS sample was excited by the first pulse directly, and the second pulse after a time interval (At) provided by an optical delay line.
1.4
.........
J
"~
1
•~
0,6
~
0.4
~
0,2
In order to investigate the effect on PL of oxidation, PL spectra for both an as-prepared sample and a naturally oxidized one were measured first under single-pulse excitation. The typical PL spectrum for as-prepared PS is shown in Fig. l(a) with a single broad luminescence profile centered at 620 nm. After natural oxidation, the PL spectrum changes from Fig. l(a) to (b). Comparing the PL spectra of Fig. l(a) with (b), two dominant differences are observed in PL features after oxidation. Firstly, the PL peak positioned at 620 nm in Fig. l(a) shifts toward higher energy, 600 nm. It should also be noted that the PL intensity increases with oxidation. These spectra changes are consistent with previously reported PS oxidation effects [7-9]. Secondly, an additional emission band appears in the high energy side of the spectrum at about 440 nm.
.--.
i
. . . . . . . . .
i
. . . . . . . . .
i
. . . . . . . . .
1
0.8 @
0.6
C:I
0.4
~3
0.2
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400
.
\
[ ~ ']K '~~. /
~
At=0.4ns At=l.2ns At=2.2ns
) .........
f-I~iAt I-,q-
/, /
| ~ 1 ~
500
600
700
800
Wavelength (nm)
Fig. I. PL spectra for (a) as-prepared and (b) naturally oxidized PS.
. . . . . . . . .
400
i
. . . . . . . . .
500
"~.
A,..,- /-
1
Single pulseexcitation(At=0ns) 0
. . . . . . . . .
i .........
m __ --0Zns
3. Results
1.2
i .........
Double pulse excitation
lstpulR
l 1 l"
"~ltA~
t . , . l l l l l t l l
600
700
800
Wavelength (rim) Fig. 2. The changes in PL for different pulse time intervals (At) under double-pulse excitation.
To understand the carrier recombination process in oxidized PS, TRPL under double-pulse excitation was then carried out. In particular, carrier dynamics associated with the high energy band in highly oxidized PS has been focused on. Double-pulse excitation is essential in providing sufficient excitation of carriers to the high energy PL band. Fig. 2 shows a series of double-pulse excited TRPL spectra measured at different pulse time intervals, A t = 0.2 ns to 2.2 ns as shown in the inset. The single-pulse excited TRPL spectrum is also shown as a reference. Two distinct bands, the high energy PL band (HEB) and the low energy PL band (LEB) centered at 440 nm and 600 nm, are observed, respectively. In the case of single-pulse excitation, the PL intensity of the LEB is about two times larger than that of the HEB. However, HEB PL intensities were dramatically changed by varying At under double-pulse excitation. The changes in PL peak intensity of HEB and LEB are shown in Fig. 3(a) and (b), respectively, as a function of At. By decreasing At from 3.1 ns to 0.2 ns, the HEB PL intensity increased by a factor of 3.6 whereas the LEB hardly changed. For At < 1.0 ns, the HEB is dominant. On the contrary, the LEB is the main PL feature for A t > 1.0 ns. Under single-pulse excitation, the intensity ratio H E B / L E B was independent of the excitation power level. Hence, the observed enhancement of the HEB can be attributed directly to double-pulse excitation.
S. Komuro et al. / Journal of Non-Crystalline Solids 198-200 (1996) 965-968 1.5
\ O
A~
No o
5
on the best fit of the double-exponential form to the changes in PL intensity of both HEB and LEB. This suggests that radiative carrier recombination via both the HEB and LEB consists of two lifetime components, a fast component (ZFASV) and a slow one (~'sLow). F r o m the analysis of both the HEB and the LEB, the PL intensity decays with lifetimes of %VAST=3 ns, ZHSLOw = 7 0 ns for the HEB and fLEASv = 10 ns, ZLSLOw = 800 ns for the LEB. The two fast components of %VAST = 3 ns in the HEB and ZLFASv = 10 ns in the LEB strongly depend on A t for the enhancement of PL intensity of HEB as shown in Figs. 2 and 3.
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.
.
.
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.
.
.
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.
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.
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Pulse Time Interval At (ns) Fig. 3. P L p e a k i n t e n s i t y d e p e n d e n c e on p u l s e t i m e i n t e r v a l ( A t ) for (a) the H E B at 4 4 0 n m a n d (b) the L E B at 6 0 0 nm. L i n e s are
4. D i s c u s s i o n
d r a w n as g u i d e s for the eye.
1.2
tm
. . . .
. . . .
i
. . . .
i
. . . .
i
. . . .
i
. . . .
1
0.6
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~(b)
•'•"•&"'•
~.' .... 0 -50 0
~ .... 50
(a)
~ . . , ~ I~ ~" 7 " r - n - ~ . . , 100 150 200
250
Delay Time (ns) Fig. 4. P L peak i n t e n s i t y d e p e n d e n c e o n d e l a y t i m e for (a) the H E B at 4 4 0 n m a n d (b) the L E B at 6 0 0 nm.
Fig. 4 shows the time decay of the PL peak intensities of both the HEB and LEB under doublepulse excitation. The decay is highly non-exponential. The experimental data could not be fitted by either a single-exponential decay or a stretched-exponential decay of the form Icx exp{-(t/z)¢}, where I, ~- and /3 are the PL intensity, carrier lifetime and fitting parameter, respectively [13], and which arises from the presence of a broad distribution of relaxation lifetimes. The data, however, could be fitted to a double exponential of the form, I cx exp(--t/'rFAST) + e x p ( - - t / Z s L o w ) as shown by the solid lines in Fig. 4. These calculated lines are based
In order to understand the above described results, Fourier transform infrared ( F T - I R ) measurements have been performed. W e observed the appearance of H S i - O 3 absorption peaks at 2250 cm-~ and that of H 2 S i - O 2 at 2195 cm l confirming the oxidation of the Si back-bond. Furthermore, the absorption at 1100 c m - ~ is due to the oxidation of the outermost surface resulting in S i - O - S i bridge formation. The above F T - I R measurement suggests that the HEB strongly depends on the oxidation of the PS surface layer. Therefore, the origin of the HEB is considered to be radiative transitions via oxygen induced defect levels, whereas the LEB results from S i / S i O x interface states, as for typical as-prepared PS [3,12]. Taking into account the PL intensity dependence of the HEB on A t, it seems that the enhancement can be achieved by re-excitation of carriers to HEB before the beginning of carrier relaxation. The second pulse excites carriers not only from ground states to the HEB and to LEB but also from both the HEB and LEB to higher states. However, for A t > 3 ns no selective enhancement of PL intensity on HEB is achieved. Therefore, carriers re-excited to the HEB by the second pulse for A t < 3 ns are considered to be provided from the LEB. To visualize these radiative processes more clearly a simple model can be proposed to explain the carrier excitation and recombination in highly oxidized PS under double-pulse excitation. This schematic model is shown in Fig. 5. The existence of a double-exponential dependence indicates that there
968
S. Komuro et al. / Journal of Non-Crystalline Solids 198-200 (1996) 965-968
• •
HEB ~
•
V•
'II j
the inter-pulse time interval less than 1 ns. The origin o f this PL is considered to be radiative transitions via o x y g e n induced defect states. The time decay o f P L intensity for the H E B is non-exponential and takes a d o u b l e - e x p o n e n t i a l form. A simple carrier r e c o m b i n a t i o n m o d e l taking into account the o x y g e n induced defect states has been p r o p o s e d on the basis o f the T R P L caused by double-pulse excitation.
¢'4
~
•
O.
•
LEB
References
Fig. 5. A model proposed to explain carrier excitation and recombination in highly oxidized PS under double-pulse excitation.
exist c o n f i n e d e n e r g y levels f r o m w h i c h radiative r e c o m b i n a t i o n occurs with two different lifetimes. Carriers w h i c h are initially excited by the first pulse (solid lines) relax in a c o m p e t i n g process via both the H E B and the L E B . U n d e r the condition o f A t < 3 ns, the second pulse e n h a n c e s PL intensity o f the H E B through re-excitation o f carriers f r o m the L E B to the H E B before the b e g i n n i n g o f the carrier relaxation (broken lines).
5. C o n c l u s i o n s It has been o b s e r v e d in naturally o x i d i z e d PS that the b l u e - g r e e n e m i s s i o n band ( H E B ) centered at 440 n m appears and its PL intensity is e n h a n c e d by a second pulse f o l l o w i n g the first excitation pulse with
[1] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1096. [2] M.S. Brandt, H.D. Fuchs, M. Stutzmann, J. Weber and M. Cardona, Solid State Commun. 81 (1992) 302. [3] F. Koch, V. Petrova-Koch, T. Mnschik, A. Nikolov and V. Garilenko, Mater. Res. Soc. Proc. 238 (1993) 197. [4] X.Y. Hou, G. Shi, W. Wang, F.L. Zhang, P.H. Hao, D.M. Huang and X. Wang, Appl. Phys. Lett. 62 (1993) 1097. [5] A. Nakajima, T. Itakura, S. Watanabe and N. Nakayama, Appl. Phys. Lett. 61 (1992) 46. [6] K. Shiba, K. Sakamoto, S. Miyazaki and M. Hirose, Jpn. J. Appl. Phys. 32 (1993) 2722. [7] A. Takazawa, T. Tamura and M. Yamada, Appl. Phys. Lett. 63 (1993) 940. [8] Y. Xiao, M.J. Heben, J.M. McCullough, Y.S. Tsuo, J.I. Pankove and S.K. Deb, Appl. Phys. Lett. 62 (1993) 1152. [9] P. O'Keeffe, S. Komuro, T. Kato, T. Morikawa and Y. Aoyagi, Jpn. J. Appl. Phys. 33 (1994)7117. [10] D.I. Kovalev, I.D. Yaroshetzhii, T. Muschik, V. Petrova-Koch and F. Koch, Appl. Phys. Lett. 64 (1994) 214. [11] T. Matsumoto, M. Daimon, T. Futagi and H. Mimura, Jpn. J. Appl. Phys. 31 (1992) L619. [12] P. O'Keeffe, S. Komuro, T. Kato, T. Morikawa and Y. Aoyagi, Appl. Phys. Lett. 66 (1995) 836. [13] N. Ookubo, H. Ono, Y. Ochiai, Y. Mochizuki and S. Matsui, Appl. Phys. Lett. 61 (1992) 940.