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Investigation of porous Si grains by optical spectroscopy
:
Thin Solid Films 255 (1995)
Investigation
119- 122
of porous Si grains by optical spectroscopy
Soumyendu Guha”, Peter Steinerb, F. Kozlowskib, W. Langb “Naval Research Laboratorv, Washington, DC 20375, USA bFraunhofer Instilute for Solid State Technology, Hansastrasse 27d, W-8000 Munich, German!
Abstract Transparent high-porosity porous Si (PS) thin films are extremely fragile and are prone to laser damage even at a low laser power. In this report we show a very simple but elegant technique that protects optically clear granular (micron-size) free-standing PS from laser damage. This technique involves encapsulating PS grains in a polymethyl methacrylate thin film. We measure the absorption coefficient of these films as a function of PS concentration. We also study the photoluminescence (PL) and Raman active phonon modes in these films by a micro-Raman/luminescence instrument. The PL peak is also monitored as a function of incident power. A blue shift of the PL peak is observed that is attributed to saturation of low-lying electronic levels associated with Si nanocrystallites. Keywords:
Luminescence;
Optical spectroscopy; Raman scattering; Silicon
1. Introduction Light-induced degradation [l] of porous silicon is a major hindrance to its potential use in optoelectronic devices. Free-standing PS (FSPS) membranes are extremely fragile, and shatter easily in moderate laser powers during PL and Raman measurements. After a laser experiment, a black mark on PS at the location of the laser spot is usually observed when it is viewed with UV light. The aim of this investigation is to demonstrate how to reduce the laser damage of porous Si layers (PSLs), and study various properties of PS by optical technique without damaging the sample. Recently [2] we demonstrated that degradation of PS could be prevented by embedding PS flakes in polymethyl methacrylate (PMMA) discs. Photoluminescence from well-dispersed PS in colloidal suspension in various liquids [3] and polystyrene thin films [4] has also been reported; however the PL was quenched in colloidal solutions, as compared to PSLs. For optical devices one requires an optically clear and highly-luminescent material, sturdy enough to sustain the laser power without any degradation of PL intensity. In this report we show a very simple but elegant technique that protects optically clear granular (micron-size) FSPS from laser damage. This technique involves encapsulating PS grains in a PMMA thin film (PS:PMMA film). One can increase the thickness of the PMMA film to 0040-6090/95/$9.50 s 1995 ~ Elsevier SSDlOO40-6090(94)05635-S
Science S.A. All rights
reserved
few microns without degrading the PL intensity, and the thicker the PMMA coating is, the less prone PS is to laser damage. The absorption spectrum of PS grains can also be measured in these PS:PMMA-films. To acquire information on excited electronic states of PS by optical spectroscopy, one needs to conduct transmission experiments such as differential absorption spectroscopy and degenerate four-wave mixing. We have attempted these experiments on PS:PMMA films. However, the films were damaged when the excitation power was increased beyond 1 MW cm-*. Our results from these experiments are inconclusive at this point and we will report these results elsewhere.
2. Experimental details PSLs were formed by anodizing n-type silicon wafers by a double-cell technique [5]. We will report here the PL data on samples containing FSPS grains of 1 to 50 pm diameter (80% porosity) dispersed on a glass substrate and coated with PMMA. A viscous methyl methacrylate (MMA) solution was prepared by mixing 400-500 mg of MMA with 5 mg of benzoyl peroxide in a polyethylene vial and exposing it for a few minutes under an UV lamp in air. Thin films of poly-MMA (PMMA) containing PS grains (PS:PMMA film) were prepared by casting a thin film
S. Guha et ~1. I Thin Solid Films 255 (I 995) 119- 122
120
of PMMA on a glass substrate, then scraping FSPS grains from PSLs (previously etched on a Si wafer by anodization) onto the wet glass substrate coated with PMMA, and finally casting a PMMA layer 1 to 10 urn thick on top of the PS grains. A clear MMA surface layer of varying thickness was thus obtained in these samples. A typical thickness of the MMA surface layer was around l-2 urn. Raman and PL spectra were recorded with a microprobe Raman/luminescence imaging system. A SPEX scanning double monochromator was also used to collect the Raman and luminescence spectra. A cylindrical lens was used to make a line image of the incident laser beam. The PL time decay measurements were done by modulating the 457.95 nm line of an argon-ion laser with an acousto-optic modulator or with the 355 nm line of a pulsed Nd:YAG laser. A 0.25 meter SPEX double monochromator, a cooled red-sensitive Hamamatsu tube (R928P) with nanosecond rise time, and a Tektronics digitizer (Model 7612D) were used to record the PL decay. A Hitachi (U3000) UVvisible absorption spectrometer was used to measure the absorption spectra.
450 Frequency
1
500 Shift
550 (cm-l
600
1
,....,....,....,....,....I....
(b)
‘$....,....,....1 I ....,,...,..,
400
300
F.
2 s ?.
.
I
.
,
.
700
600 500 wavelength
.
.
,
.
,’ / /
. ..__..
900
800
(nm)
.
,
.
-\
.
,
.
.
.
.
,
-Frwd. PL - - -Bck.scatt.
\
.
icj.
PL
\ \
/
\
3. Kesults and discussion Fig. 1 shows an image of a 50-urn FSPS grain buried in a PMMA film on a glass substrate. The brightness in the sample was caused by the intense PL due to a laser beam of 1 urn diameter and 0.1 mW power from a HeNe laser.
Fig. 1. Imaged photograph PS:PMMA film.
of a porous
Si grain
embedded
in a
650
700 750 wavelength (nm)
800
850
900
Fig. 2. (a) Backscattered Raman spectra using the 457 nm line of an argon-ion laser. (b) Absorption spectrum of the PS:PMMA film. (c) Forward and backscattered luminescence spectra of the PS:PMMA film.
Fig. 2 shows the luminescence, absorption and Raman spectra of a PS:PMMA film. The Raman active Si-Si stretch in the phonon spectrum is the same as we reported in an earlier paper [2] except that we do not see any phonon modes from the PMMA film. The luminescence spectra (Fig. 2(c)) in the forward and backscattering geometries exhibit a PL peak around 720 nm, which is significantly higher than the observed peak (680 nm) in the PSLs. This indicates the presence of larger particles that were inadvertently selected during the scraping of PS from PSLs in making the PS:PMMA film. The absorption spectrum of the PS:PMMA film is shown in Fig. 2(b). The glass substrate background was removed from the spectrum by using a blank in the reference beam. We show the absorption spectra to demonstrate that PS grains embedded in PMMA film retain their characteristic absorption properties and can be used for differential transmission spectroscopic experiments. Normalized PL spectra from the PS:PMMA film as a function of incident laser power are shown in Fig. 3 (a). The laser power density at 2.5 mW was approximately
S. Guha et 01. / Thin Solid Films 255 (1995) Ii9-
122
Nonradiative
I
600
I
700 Wavelength
I
I
800 (nm)
I
I
900
Fig. 3. (a) Normalized PL spectra as a function of laser power with a micro-probe single-grating Raman instrument. A micron-size laser beam from a He Ne laser was used in a backscattering geometry. (b) Difference band between 2.5 mW and 0.1 mW PL spectra. The PL was blocked between 638 and 638 nm to avoid laser damage to the detector.
300 kW cm -‘. Without the PMMA coating on PS, the sample would have disintegrated at this power level. The shift of the PL peak is approximately 10 nm (25 meV) at this excitation wavelength and at a power level of approximately 100 kW cm -*. We have observed a blue shift between 10 and 20 nm of the PL peak at all excitation wavelengths between 355 nm and 633 nm in all PS:PMMA films as well as in PS:PMMA and PSLs [2]. A saturation of low-lying electronic states in PS as a function of incident laser power was reported before [2]. Note that, to observe the blue shift and the saturation effect. it is important that the PL is measured from the area where the laser beam strikes the sample, i.e. at which the saturation of elecronic levels occurs owing to the high power density of the incident laser beam. If the PL data from the adjacent areas that are not saturated by the incident power are collected as well, the saturation effect in the PL spectra will be washed out. Another important point to emhasize is that the blue shift is not the temperature effect. With increasing temperature, the PL peak in PS shifts to red. In Fig. 3(b) we show a difference spectrum that is obtained by subtracting the normalized 0.1 mW PL spectrum from the 2.5 mW PL spectrum. This spectrum shows a loss in oscillator strength of the PL band between 1.46 and I .77 eV (700 and 850 nm), and a gain in oscillator strength between 1.77 and 2.5 eV (500 and 700 nm). This is explained in the ensuing paragraphs. Si nanocrystallites of different sizes, being excited by the pump beam, create electron-hole pairs (see Fig. 4). The free electrons in the conduction band of each nanocrystallite decay, at first, non-radiatively to their respective lowest electronic states or traps ( le, l’e, l”e, etc., or T in Fig. 4) and they eventually recombine by emitting photons in the visible wavelengths.
1 “‘h 2”‘h
1.4 eV (200 PC)
1.8 (75
eV pet)
2.4 (48
eV psec)
Fig. 4. Possible mechanism for the observed PL band between I.4 and 2.4 eV in PS. Here T indicates surface states or traps. Approximate PL decay times I at three emission wavelengths are also indicated.
It has been observed by various investigators [6-81 as well as by us that the PL decay times in PS exhibit an approximate exponential increase with decreasing PL detection energy. The PL decay rate is also found to depend on the excitation wavelength and its pulse width. We have measured the PL decay time at room temperature at different detection energies with long (a few microseconds) and short (5 ns) pulses. With long (short) pulses, the decay time T was found to be 80 (8) us < r < 200 (20) us between I .4 and 1.7 eV, and 20 ( 1) ps < T < 80 (8) 11sbetween 1.8 and 2.4 eV. The observation of varying PL decay times in PS may be interpreted as due to the presence of traps in Si crystallites. There is always a probability of finding a larger number of traps in the large crystallites than in the small ones that have restricted volumes. Thus non-radiative recombinations through traps are more likely to occur as the size of the nanocrystals become larger. The observation of the varying PL decay times in PS at different detection energy is, probably, related to the varying number of traps as the size of the nanocrystal changes. At an increased pump intensity. those nanocrystallites with longer radiative decay time (bandgaps between 1.4 and 1.7 eV) get saturated first. while the nanocrystallites with a shorter radiative decay time gain oscillator strength till they get saturated as well. This complete saturation of low-lying excited electronic states happens around 1 mW of laser power, and beyond this power level the PL peak does not shift any more with increased incident power. We therefore observe an erosion of the PL band between 1.4 and 1.7 eV (700 and 900 nm), and a gain in the PL band between 1.8 and 2.4 eV (500 and 700 nm).
122
S. Guha et al.
I Thin Solid Films 255 (1995) 119-122
Our PL experiment as a function of laser power indicates that the broad PL band in PS arises due to exciton recombinations at different sites containing Si nanocrystallites of varying size. We believe that the localized surface states or traps are pinned at the band edge of each nanocrystallite, forming shallow donoracceptor type states. Since there is a wide distribution of Si particle sizes in PS, the exciton recombination from different spatial regions of PS gives rise to the wide PL band.
Acknowledgments The authors appreciate helpful suggestions from Dr. Al. Efros and Dr. M. Rosen during the preparation of this manuscript. We also acknowledge Prof. N. Peygambarian for discussing his preliminary PS data of hole-burning experiments. The authors acknowledge Renishaw PLC, UK, for the use of their Raman-microprobe system during a demonstration at NRL.
References 4. Conclusion In conclusion, we show that a PS:PMMA thin film can be used to conduct Raman, PL and transmission experiments. Differential absorption/transmission spectrocopy can also be conducted on these films; however, the threshold for damage of these films with an intense laser source was found to be around 1 MW cm-‘. Currently, we are trying to achieve a better dissolution of PS in various organic solvents before mixing the solution in MMA. We hope to fabricate optically clear PS:PMMA thin films with varying PS concentrations in the future.
[I] L.T. Canham, M.R.W.Y. Lerng, C. Pickering and J.M. Keen, Appl. Phys. Lett., 70 (1993) 422. [2] S. Guha, G. Hendershot, D. Peebles, P. Steiner, F. Kozlowski and W. Lang, Appl. Phys. Lett., 64 ( 1994) 613. [3] J.L. Heinrich, CL. Curtis, G.M. Credo, K.L. Kavanagh and M.J. Sailor, Science, 255 ( 1992) 66. [4] J.M. Laurhaas, G.M. Credo, J.L. Heinrich and M.J. Sailor, J. Am. Chetn. Sot., 112( 1992) 1911; Mat. Res. Sot. Symp. Proc., 256( 1992) 137. [S] A. Richter, P. Steiner, F. Kozlowski and W. Lang, IEEE Electron Deu. Left., EDL-I2 (1991) 691. [6] C. Delerue, G. Allan and M. Lannoo, Phys. Rev. B, 48( 1993) 11024. [7] L. Pavesi, M. Ceschini and F. Rossi, J. Lumin., 57 (1993) 131. [8] S. Finkbeiner, J. Weber, M. Rosenbauer and M. Stutzman, J. Lumin., 57 (1993) 231.
Thin Solid Films 255 (1995)
Investigation
119- 122
of porous Si grains by optical spectroscopy
Soumyendu Guha”, Peter Steinerb, F. Kozlowskib, W. Langb “Naval Research Laboratorv, Washington, DC 20375, USA bFraunhofer Instilute for Solid State Technology, Hansastrasse 27d, W-8000 Munich, German!
Abstract Transparent high-porosity porous Si (PS) thin films are extremely fragile and are prone to laser damage even at a low laser power. In this report we show a very simple but elegant technique that protects optically clear granular (micron-size) free-standing PS from laser damage. This technique involves encapsulating PS grains in a polymethyl methacrylate thin film. We measure the absorption coefficient of these films as a function of PS concentration. We also study the photoluminescence (PL) and Raman active phonon modes in these films by a micro-Raman/luminescence instrument. The PL peak is also monitored as a function of incident power. A blue shift of the PL peak is observed that is attributed to saturation of low-lying electronic levels associated with Si nanocrystallites. Keywords:
Luminescence;
Optical spectroscopy; Raman scattering; Silicon
1. Introduction Light-induced degradation [l] of porous silicon is a major hindrance to its potential use in optoelectronic devices. Free-standing PS (FSPS) membranes are extremely fragile, and shatter easily in moderate laser powers during PL and Raman measurements. After a laser experiment, a black mark on PS at the location of the laser spot is usually observed when it is viewed with UV light. The aim of this investigation is to demonstrate how to reduce the laser damage of porous Si layers (PSLs), and study various properties of PS by optical technique without damaging the sample. Recently [2] we demonstrated that degradation of PS could be prevented by embedding PS flakes in polymethyl methacrylate (PMMA) discs. Photoluminescence from well-dispersed PS in colloidal suspension in various liquids [3] and polystyrene thin films [4] has also been reported; however the PL was quenched in colloidal solutions, as compared to PSLs. For optical devices one requires an optically clear and highly-luminescent material, sturdy enough to sustain the laser power without any degradation of PL intensity. In this report we show a very simple but elegant technique that protects optically clear granular (micron-size) FSPS from laser damage. This technique involves encapsulating PS grains in a PMMA thin film (PS:PMMA film). One can increase the thickness of the PMMA film to 0040-6090/95/$9.50 s 1995 ~ Elsevier SSDlOO40-6090(94)05635-S
Science S.A. All rights
reserved
few microns without degrading the PL intensity, and the thicker the PMMA coating is, the less prone PS is to laser damage. The absorption spectrum of PS grains can also be measured in these PS:PMMA-films. To acquire information on excited electronic states of PS by optical spectroscopy, one needs to conduct transmission experiments such as differential absorption spectroscopy and degenerate four-wave mixing. We have attempted these experiments on PS:PMMA films. However, the films were damaged when the excitation power was increased beyond 1 MW cm-*. Our results from these experiments are inconclusive at this point and we will report these results elsewhere.
2. Experimental details PSLs were formed by anodizing n-type silicon wafers by a double-cell technique [5]. We will report here the PL data on samples containing FSPS grains of 1 to 50 pm diameter (80% porosity) dispersed on a glass substrate and coated with PMMA. A viscous methyl methacrylate (MMA) solution was prepared by mixing 400-500 mg of MMA with 5 mg of benzoyl peroxide in a polyethylene vial and exposing it for a few minutes under an UV lamp in air. Thin films of poly-MMA (PMMA) containing PS grains (PS:PMMA film) were prepared by casting a thin film
S. Guha et ~1. I Thin Solid Films 255 (I 995) 119- 122
120
of PMMA on a glass substrate, then scraping FSPS grains from PSLs (previously etched on a Si wafer by anodization) onto the wet glass substrate coated with PMMA, and finally casting a PMMA layer 1 to 10 urn thick on top of the PS grains. A clear MMA surface layer of varying thickness was thus obtained in these samples. A typical thickness of the MMA surface layer was around l-2 urn. Raman and PL spectra were recorded with a microprobe Raman/luminescence imaging system. A SPEX scanning double monochromator was also used to collect the Raman and luminescence spectra. A cylindrical lens was used to make a line image of the incident laser beam. The PL time decay measurements were done by modulating the 457.95 nm line of an argon-ion laser with an acousto-optic modulator or with the 355 nm line of a pulsed Nd:YAG laser. A 0.25 meter SPEX double monochromator, a cooled red-sensitive Hamamatsu tube (R928P) with nanosecond rise time, and a Tektronics digitizer (Model 7612D) were used to record the PL decay. A Hitachi (U3000) UVvisible absorption spectrometer was used to measure the absorption spectra.
450 Frequency
1
500 Shift
550 (cm-l
600
1
,....,....,....,....,....I....
(b)
‘$....,....,....1 I ....,,...,..,
400
300
F.
2 s ?.
.
I
.
,
.
700
600 500 wavelength
.
.
,
.
,’ / /
. ..__..
900
800
(nm)
.
,
.
-\
.
,
.
.
.
.
,
-Frwd. PL - - -Bck.scatt.
\
.
icj.
PL
\ \
/
\
3. Kesults and discussion Fig. 1 shows an image of a 50-urn FSPS grain buried in a PMMA film on a glass substrate. The brightness in the sample was caused by the intense PL due to a laser beam of 1 urn diameter and 0.1 mW power from a HeNe laser.
Fig. 1. Imaged photograph PS:PMMA film.
of a porous
Si grain
embedded
in a
650
700 750 wavelength (nm)
800
850
900
Fig. 2. (a) Backscattered Raman spectra using the 457 nm line of an argon-ion laser. (b) Absorption spectrum of the PS:PMMA film. (c) Forward and backscattered luminescence spectra of the PS:PMMA film.
Fig. 2 shows the luminescence, absorption and Raman spectra of a PS:PMMA film. The Raman active Si-Si stretch in the phonon spectrum is the same as we reported in an earlier paper [2] except that we do not see any phonon modes from the PMMA film. The luminescence spectra (Fig. 2(c)) in the forward and backscattering geometries exhibit a PL peak around 720 nm, which is significantly higher than the observed peak (680 nm) in the PSLs. This indicates the presence of larger particles that were inadvertently selected during the scraping of PS from PSLs in making the PS:PMMA film. The absorption spectrum of the PS:PMMA film is shown in Fig. 2(b). The glass substrate background was removed from the spectrum by using a blank in the reference beam. We show the absorption spectra to demonstrate that PS grains embedded in PMMA film retain their characteristic absorption properties and can be used for differential transmission spectroscopic experiments. Normalized PL spectra from the PS:PMMA film as a function of incident laser power are shown in Fig. 3 (a). The laser power density at 2.5 mW was approximately
S. Guha et 01. / Thin Solid Films 255 (1995) Ii9-
122
Nonradiative
I
600
I
700 Wavelength
I
I
800 (nm)
I
I
900
Fig. 3. (a) Normalized PL spectra as a function of laser power with a micro-probe single-grating Raman instrument. A micron-size laser beam from a He Ne laser was used in a backscattering geometry. (b) Difference band between 2.5 mW and 0.1 mW PL spectra. The PL was blocked between 638 and 638 nm to avoid laser damage to the detector.
300 kW cm -‘. Without the PMMA coating on PS, the sample would have disintegrated at this power level. The shift of the PL peak is approximately 10 nm (25 meV) at this excitation wavelength and at a power level of approximately 100 kW cm -*. We have observed a blue shift between 10 and 20 nm of the PL peak at all excitation wavelengths between 355 nm and 633 nm in all PS:PMMA films as well as in PS:PMMA and PSLs [2]. A saturation of low-lying electronic states in PS as a function of incident laser power was reported before [2]. Note that, to observe the blue shift and the saturation effect. it is important that the PL is measured from the area where the laser beam strikes the sample, i.e. at which the saturation of elecronic levels occurs owing to the high power density of the incident laser beam. If the PL data from the adjacent areas that are not saturated by the incident power are collected as well, the saturation effect in the PL spectra will be washed out. Another important point to emhasize is that the blue shift is not the temperature effect. With increasing temperature, the PL peak in PS shifts to red. In Fig. 3(b) we show a difference spectrum that is obtained by subtracting the normalized 0.1 mW PL spectrum from the 2.5 mW PL spectrum. This spectrum shows a loss in oscillator strength of the PL band between 1.46 and I .77 eV (700 and 850 nm), and a gain in oscillator strength between 1.77 and 2.5 eV (500 and 700 nm). This is explained in the ensuing paragraphs. Si nanocrystallites of different sizes, being excited by the pump beam, create electron-hole pairs (see Fig. 4). The free electrons in the conduction band of each nanocrystallite decay, at first, non-radiatively to their respective lowest electronic states or traps ( le, l’e, l”e, etc., or T in Fig. 4) and they eventually recombine by emitting photons in the visible wavelengths.
1 “‘h 2”‘h
1.4 eV (200 PC)
1.8 (75
eV pet)
2.4 (48
eV psec)
Fig. 4. Possible mechanism for the observed PL band between I.4 and 2.4 eV in PS. Here T indicates surface states or traps. Approximate PL decay times I at three emission wavelengths are also indicated.
It has been observed by various investigators [6-81 as well as by us that the PL decay times in PS exhibit an approximate exponential increase with decreasing PL detection energy. The PL decay rate is also found to depend on the excitation wavelength and its pulse width. We have measured the PL decay time at room temperature at different detection energies with long (a few microseconds) and short (5 ns) pulses. With long (short) pulses, the decay time T was found to be 80 (8) us < r < 200 (20) us between I .4 and 1.7 eV, and 20 ( 1) ps < T < 80 (8) 11sbetween 1.8 and 2.4 eV. The observation of varying PL decay times in PS may be interpreted as due to the presence of traps in Si crystallites. There is always a probability of finding a larger number of traps in the large crystallites than in the small ones that have restricted volumes. Thus non-radiative recombinations through traps are more likely to occur as the size of the nanocrystals become larger. The observation of the varying PL decay times in PS at different detection energy is, probably, related to the varying number of traps as the size of the nanocrystal changes. At an increased pump intensity. those nanocrystallites with longer radiative decay time (bandgaps between 1.4 and 1.7 eV) get saturated first. while the nanocrystallites with a shorter radiative decay time gain oscillator strength till they get saturated as well. This complete saturation of low-lying excited electronic states happens around 1 mW of laser power, and beyond this power level the PL peak does not shift any more with increased incident power. We therefore observe an erosion of the PL band between 1.4 and 1.7 eV (700 and 900 nm), and a gain in the PL band between 1.8 and 2.4 eV (500 and 700 nm).
122
S. Guha et al.
I Thin Solid Films 255 (1995) 119-122
Our PL experiment as a function of laser power indicates that the broad PL band in PS arises due to exciton recombinations at different sites containing Si nanocrystallites of varying size. We believe that the localized surface states or traps are pinned at the band edge of each nanocrystallite, forming shallow donoracceptor type states. Since there is a wide distribution of Si particle sizes in PS, the exciton recombination from different spatial regions of PS gives rise to the wide PL band.
Acknowledgments The authors appreciate helpful suggestions from Dr. Al. Efros and Dr. M. Rosen during the preparation of this manuscript. We also acknowledge Prof. N. Peygambarian for discussing his preliminary PS data of hole-burning experiments. The authors acknowledge Renishaw PLC, UK, for the use of their Raman-microprobe system during a demonstration at NRL.
References 4. Conclusion In conclusion, we show that a PS:PMMA thin film can be used to conduct Raman, PL and transmission experiments. Differential absorption/transmission spectrocopy can also be conducted on these films; however, the threshold for damage of these films with an intense laser source was found to be around 1 MW cm-‘. Currently, we are trying to achieve a better dissolution of PS in various organic solvents before mixing the solution in MMA. We hope to fabricate optically clear PS:PMMA thin films with varying PS concentrations in the future.
[I] L.T. Canham, M.R.W.Y. Lerng, C. Pickering and J.M. Keen, Appl. Phys. Lett., 70 (1993) 422. [2] S. Guha, G. Hendershot, D. Peebles, P. Steiner, F. Kozlowski and W. Lang, Appl. Phys. Lett., 64 ( 1994) 613. [3] J.L. Heinrich, CL. Curtis, G.M. Credo, K.L. Kavanagh and M.J. Sailor, Science, 255 ( 1992) 66. [4] J.M. Laurhaas, G.M. Credo, J.L. Heinrich and M.J. Sailor, J. Am. Chetn. Sot., 112( 1992) 1911; Mat. Res. Sot. Symp. Proc., 256( 1992) 137. [S] A. Richter, P. Steiner, F. Kozlowski and W. Lang, IEEE Electron Deu. Left., EDL-I2 (1991) 691. [6] C. Delerue, G. Allan and M. Lannoo, Phys. Rev. B, 48( 1993) 11024. [7] L. Pavesi, M. Ceschini and F. Rossi, J. Lumin., 57 (1993) 131. [8] S. Finkbeiner, J. Weber, M. Rosenbauer and M. Stutzman, J. Lumin., 57 (1993) 231.