- Email: [email protected]
Hydrogenating and annealing effects of GaAs
Solid State Communicacionr. Vol. 90, No. 9. pp. 585-588. 1994 Ekwer Scmur Ltd rnnredin Great Britain 0038- io98;94 57.00 + .oo
HYDROGENATING
AND ANNEALING
EFFECTS OF GaAs
Y.T. Oh, T.W. Kang’ and C.Y. Hong Department
of Physics, Dongguk University, 3-26 Pil-dong, Chung-ku. Seoul 100-715, Korea S.B. Park
Department
of Physics, Taegu University Kyungsan. Kyungpuk 713-800. Korea and T.W. Kim
Department
of Physics, Kwangwoon
University, 447-l Wolgye-dong, Nowon-ku, Seoul 139-701, Korea
(Received
13 October
1993 by C. N. R. Rue)
Photoluminescence (PL) and photoreflectance (PR) measurements on a nominally undoped GaAs layer grown by molecular beam epitaxy have been carried out to investigate the neutralization of the carbons and the variation of the charges in the GaAs. After the GaAs was hydrogenated and annealed at 400°C. PL measurements showed that the relative intensity ratio between the donor-bound exciton and the carbon acceptor increased by a factor of 2. PR measurements indicate that the variation of the broadening parameter is induced by a neutralization of the carbon acceptors. These results were caused by the neutralization of the carbon due to the combination of the hydrogenic ions and the carbon ions in the GaAs or by the separation of the hydrogen atoms resulting from the thermal treatment.
THE ROLE of the hydrogen atom is very attractive because of the significant improvement in the electrical and optical propertics of GaAs due to its injection into the semiconductor materials [I -101. Chevallicr et 01. [S] reported that the Si donor impurities in Si-doped GaAs were decreased by hydrogen passivation and that activation of the Si was observed after thermal treatment at 400°C. McCluskey et ul. [3] reported elimination of the donors and the deep levels in Si-doped GaAs by hydrogen passivation. Although Hall-elTect and capacitance-voltage measurements are usually used to investigate the charge distribution in such materials, metal contacts are required for those measurements. Modulation spectroscopy methods are also powerful tools for optically investigating the variation of the charge distribution and they do not need metal contacts [I l-211. Among these
l Author to whom all correspondence addressed.
should be 585
modulation spectroscopy techniques, photoreflectance (PR) is a method which measures the modulated quantity of the dielectric constant as a function of the wavelength by applying a monochromatic light as an alternating current signal [ 121. Modulated signals appear at all energy states with critical points such as transitions between bands and subbands in quantum wells. This communication reports the results of photoluminescence (PL) and PR measurements which were performed in order to investigate the effects of hydrogenation and thermal treatment on a nominally undoped GaAs layer grown by molecular beam epitaxy (MBE). The results of the PL measurements showed neutralization of the carbon ions by hydrogen ions. The variation of the charges in the materials was investigated by PR measurements. The nominally undoped-GaAs layer used in this study was grown by MBE. The SO-period GaAs/ A10,3Ga0,7As (I = 5nm) superlattice was grown on semi-insulating GaAs, and subsequently, the 1 pm GaAs layer was grown on the superlattice. As-grown
586
HYDROGENATING
1.36
1.4
1.44 Energy (ev)
1.46
AND ANNEALING
1.52
Fig. I. PR spectra of (a) the as-grown GaAs, (b) the hydrogenated GaAs. and the GaAs annealed at (c) 400°C. (d) 500°C. and (e) 600°C. samples were exposed to a hydrogen plasma at a pressure of 0.5Torr for 60min at a power density of 0.06 W cm-’ using a capacitively coupled r.f. (I 3.56 MHz) plasma system. The hydrogenated GaAs was annealed at 400, 500. and 600°C in a nitrogen atmosphcrc for approximately 5 min. The FL measurements were carried out using a 75cm monochromator equipped with an RCA 31034 photomultiplicr tube. The excitation source was the 5145 A line of’ an Ar-ion laser, and the sample temperature was controlled between 10 and 300K by using a He displex system. The PR measurements were performed using a 75cm monochromator equipped with a 750 watt tungsten lamp as a probe source and the 6328A line of a He-Ne laser as a modulation source. Figure I shows the results of the room-temperature PR spectra for the as-grown, the hydrogenated, and the annealed GaAs. The PR spectrum of the as-grown GaAs shows the typical shape in the low-field limit. The typical modulation spectroscopy signal in the low electrical-field region has a third-derivative line shape and obeys the following equation [I I]: AR/R
= R,[Ce”(E - ER + lT)-“1,
EFFECTS OF GaAs
Vol. 90, No. 9
spectrum and using equation (I) was 1.43 eV. The total luminescence intensity of the PR spectrum for the hydrogenated GaAs remarkably decreased in comparison to that of the as-grown GaAs and recovered for the hydrogenated and annealed GaAs. The broadening parameter of the as-grown GaAs was 13.2meV. The broadening parameters of the GaAs sample hydrogenated after rapid thermal annealing (RTA) at 400 and 500°C were 7.8 and 13.8meV. respectively. Since the variation of the broadening parameters is induced by the variance of the charge densities or the surface state densities, the charge densities or the surface state densities can be changed by hydrogenation and annealing. Figure 2 shows the results of the PL spectra at IOK for the as-grown, the hydrogenated, and the annealed GaAs. The peaks at 1.5171, 1.5137. and I .5120eV are related to a free exciton (X). a neutral donor-bound exciton (Do, X). and a neutral acceptorbound exciton (A”, X), respectively. The defect-related exciton (n, k’) appears in the range between 1.504 and I .5 I 1 eV, and the peak at 1.493 eV is considered to be an impurity due to the carbon acceptor (u, A”),. The peak at 1.5148 eV located between the (De, X) and (X) peaks is attributed to an antisite defect complex bound exciton 122).The (d, X) peak is related either to the Ga vacancy or its related acceptor defect [23] or to the As-stabilized (2 x 4) surface reconstruction [24]. The luminescence intensity of the PL spectrum after hydrogenation was lowered by as much as about six times in comparison to that of the as-grown GaAs. The decrease in the PR and PL intensities was caused by damage to the crystallinity due to hydrogenation,
‘I’= 16 K
(I)
where C and fI are the amplitude and the phase, respectively. ER and I’ show the energy gap and the broadening parameter, respectively. n is the dimension factor of the critical point. The values of n are 2.5, 3. and 2 for the three-, two-, and one-dimension cases. respectively [I I]. The value of the energy band can at room temoerature determined from the PR
(0)
&&
1
1.45
1.48 1.51 Energy (cV)
1 i4
Fig. 2. PL spectra of (a) the as-grown GaAs, (b) the hydrogenated GaAs, and the GaAs annealed at (c) 400°C. (d) 500°C. and (e) 600°C.
Vol. 90, No. 9
HYDROGENATING
AND ANNEALING
The intensity of the total luminescence for the hydrogenated and annealed GaAs increased in comparison with that for the hydrogenated GaAs. The (d, X) peak around 1.505 eV vanished after hydrogenation. This was caused by passivation, resulting from the hydrogenation of the acceptors related to the Ga vacancies, and this result was in good agreement with the results obtained by Pan er al. [6]. The (d, X) peak appeared after thermal treatment at 400’C. When the annealing temperature increased, the (d,X) peak increased rapidly. The appearance of the (d,X) peak originates from the separation of the passivation ions associated with either the Ga vacancies or the defect complexes. However, the (e,A”)c peak due to the carbon acceptor did not change, regardless of the deterioration of the GaAs crystallinity after hydrogenation. This behavior was induced by the formation of a hydrogen ion (H+) due to hydrogenation and a carbon ion in the GaAs. The increase of the neutral carbon acceptor compensated for the decrease of the PL intensity caused by hydrogenation. Although the increase of the PL spectrum intensity for the GaAs annealed at 400°C indicated the recovery of the crystallinity damage due to the hydrogenation. the complexes due to the Ht and C- still existed. The increase of the (A”, X) peak intensity shows augmcntation of the neutral carbon acceptors. When the thermal
m.zn
0.22
q *e $ 0 0
0.16
2
EFFECTS OF GaAs
587
treatment temperature became 500°C the decrease in the intensity of the (e, A o)c peak was due to the carbon acceptor separation from the hydrogen atoms. The variation of the broadening parameters in the PR spectra correlated with the intensity of the (4, A o)c peak. Figure 3 shows a comparison of the broadening parameter and the intensity ratio of the (e,A”)r to (Do, X) taken from the PR and PL spectra of Figs. 1 and 2 for (a) the as-grown GaAs and the GaAs annealed at (b) 4OO”C,(c) 500°C and (d) 600°C after hydrogenation. When the GaAs was annealed at 400’C after hydrogenation, the neutralization of the carbon acceptors increased while the PL intensity ratio of the (e, A’), to (Do, X) and the value of the broadening parameter decreased. Although the ionization of the neutral carbon acceptors resulting from separation of the hydrogen atoms decreased the intensity of the (e, A’), peak for the GaAs annealed at 500°C after hydrogenation. the value of the broadening parameter increased. These results indicate that the decrease of the surface potential caused by the decrease of the carriers reduces the value of the broadening parameter after hydrogenation and that the increase of the surface potential due to the ionized carbon acceptors resulting from the thermal treatment incrcascs the value of the broadening parameter. In summary, the PL measurements showed that the intensity due to the (e, A’), did not change and that the (d, X) peak disappeared after hydrogenation. The relative intensity ratios between (fI’,X) and (e, A’), for the as-grown sample and for the hydrogenated sample which was subsequently annealed at 400°C increased by a factor of 3. This result indicates the formation of neutral carbon acceptors resulting from the combination of Ht caused by hydrogenation and C- in GaAs. The separation of the hydrogen ions which neutralized the carbon acceptors was observed for hydrogenated GaAs which was subsequently annealed at 500 and 600°C. The variation of the broadening parameter in the PR spectra was caused by a neutralization or an ionization of the carbon acceptors. - This work was supported by the Korean Science and Engineering Foundation in 1993.
Acknowledgement
REFERENCES Fig. 3. Comparison of the broadening parameter with the ratio of the free carbon-acceptor (e, A), signal to the neutral donor-bound exciton (& X) signal taken from the PR and PL spectra of Figs I and 2 for (a) the as-grown GaAs sample and the hydrogenated GaAs samples which were subsequently annealed at (b) 400°C (c) 500°C. and (d) 600’C.
I. 2. 3.
A. Slaoui. K. Barhdadi, J.C. Muller & P. Siffer, Appl. Phys. A39, I59 (1986). A.R. Calawa, Appl. Phys. Left. 33, 12 (1987). F.P. McCluskey, L. PfeitTer, K.W. West, J. Lopata, M.L. Schnoes, T.D. Harres, S.J. Pearton & W.C. Dautremont-Smith, Appl. Phys. Letr. 54, I8 (1989).
588 4. 5. 6. 7. 8. 9.
10. Il. 12. 13.
HYDROGENATING
AND ANNEALING
B. Clejaud, F. Gendron, M. Krause & W. Ulrici, Phys. Rev. Lett. 65, 14 (1990). J. Chevallier. W.C. Dautremont-Smith. C.W. Tu & S.J. Peat-ton. Appl. Phys. Lett. 47,2 (1985). N. Pan, S.S. Bose, M.H. Kim, G.E. Stillman, G. Chambers, G. Devane, C.R. Ito & M. Feng. Appl. Phys. Lett. 51, 8 (1987). A.R. Calawa, Appl. Phys. L-err. 33, 12 (1978). Michihro Kobayashi, Teruo Yokoyama & Shin-i&ho Narita, Jpn J. Appl. Phys. 22, 10 (1983). W.C. Dautremont-Smith, J.C. Nabity, V. Swaminathan, M. Stavola, J. Chevallier, C.W. Tu & S.J. Pearton, Appl. Phys. Lett. 49, 17 (1986). A. Callegari, P.D. Hoh, D.A. Buchanan & D. Lacey, Appl. Phys. Lett. 54, 4 (1989). Modulation M. Cardona, Spectroscopy, Academic Press, New York, (1969). D.E. Aspnes, Handbook on Senticonductors, Vol. 2, North-Holland, New York (1980). C. Van Hof, K. Deneffe, J. De Boeck, D.J.
14. 5. 6. 17. 18. 19. 20. 21. 22.
24.
EFFECTS
OF GaAs
Vol. 90. No. 9
Arent & G. Borghs, Appl. Phys. Lett. 54, 608 ( 1989). U.K. Reddv. M. Longerbone & B.P. Gu. J. Appf. Phys.62, 4858 (1587). X. Yin, F.H. Pollak, L. Pawiowicz. T. O’Niel & M. Hafizi, Appl. Phys. Lett. 56, 1278 (1990). R.N. Bhattacharya, H. Shen, P. Parayanthal, F.H. Pollak, T. Coutts & H. Aharoni, Phys. Rev. B37, 4044 (1988). 0. Gerolo & J.C. Woolley, Canadian J. Phys. 49, 1335 (1971). R.C. Bowmann, Jr., R.L. Ah & K.W. Brown, Proc. SPIE 794,96 (1987). U.K. Reddy, G. Ji. T. Henderson, H. Morkoc & J.N. Schulman. J. Appl. Phys. 62, 145 (1987). N. Bottka. D.K. Gaskill. R.S. Sillmon, R. Henry & R. Glosser, J. Elec. Mat. 17, 161 ( 1988). D.E. Aspnes, Phys. Rev. BlO, 4228 (1982). F. Briones. R.F. Marks & L. Vina, J. Appl. Phys. 59, 3 (1986). H. Kiinzel & K. Ploog. Appl. Phys. Lett. 37, 4 ( 1980).
HYDROGENATING
AND ANNEALING
EFFECTS OF GaAs
Y.T. Oh, T.W. Kang’ and C.Y. Hong Department
of Physics, Dongguk University, 3-26 Pil-dong, Chung-ku. Seoul 100-715, Korea S.B. Park
Department
of Physics, Taegu University Kyungsan. Kyungpuk 713-800. Korea and T.W. Kim
Department
of Physics, Kwangwoon
University, 447-l Wolgye-dong, Nowon-ku, Seoul 139-701, Korea
(Received
13 October
1993 by C. N. R. Rue)
Photoluminescence (PL) and photoreflectance (PR) measurements on a nominally undoped GaAs layer grown by molecular beam epitaxy have been carried out to investigate the neutralization of the carbons and the variation of the charges in the GaAs. After the GaAs was hydrogenated and annealed at 400°C. PL measurements showed that the relative intensity ratio between the donor-bound exciton and the carbon acceptor increased by a factor of 2. PR measurements indicate that the variation of the broadening parameter is induced by a neutralization of the carbon acceptors. These results were caused by the neutralization of the carbon due to the combination of the hydrogenic ions and the carbon ions in the GaAs or by the separation of the hydrogen atoms resulting from the thermal treatment.
THE ROLE of the hydrogen atom is very attractive because of the significant improvement in the electrical and optical propertics of GaAs due to its injection into the semiconductor materials [I -101. Chevallicr et 01. [S] reported that the Si donor impurities in Si-doped GaAs were decreased by hydrogen passivation and that activation of the Si was observed after thermal treatment at 400°C. McCluskey et ul. [3] reported elimination of the donors and the deep levels in Si-doped GaAs by hydrogen passivation. Although Hall-elTect and capacitance-voltage measurements are usually used to investigate the charge distribution in such materials, metal contacts are required for those measurements. Modulation spectroscopy methods are also powerful tools for optically investigating the variation of the charge distribution and they do not need metal contacts [I l-211. Among these
l Author to whom all correspondence addressed.
should be 585
modulation spectroscopy techniques, photoreflectance (PR) is a method which measures the modulated quantity of the dielectric constant as a function of the wavelength by applying a monochromatic light as an alternating current signal [ 121. Modulated signals appear at all energy states with critical points such as transitions between bands and subbands in quantum wells. This communication reports the results of photoluminescence (PL) and PR measurements which were performed in order to investigate the effects of hydrogenation and thermal treatment on a nominally undoped GaAs layer grown by molecular beam epitaxy (MBE). The results of the PL measurements showed neutralization of the carbon ions by hydrogen ions. The variation of the charges in the materials was investigated by PR measurements. The nominally undoped-GaAs layer used in this study was grown by MBE. The SO-period GaAs/ A10,3Ga0,7As (I = 5nm) superlattice was grown on semi-insulating GaAs, and subsequently, the 1 pm GaAs layer was grown on the superlattice. As-grown
586
HYDROGENATING
1.36
1.4
1.44 Energy (ev)
1.46
AND ANNEALING
1.52
Fig. I. PR spectra of (a) the as-grown GaAs, (b) the hydrogenated GaAs. and the GaAs annealed at (c) 400°C. (d) 500°C. and (e) 600°C. samples were exposed to a hydrogen plasma at a pressure of 0.5Torr for 60min at a power density of 0.06 W cm-’ using a capacitively coupled r.f. (I 3.56 MHz) plasma system. The hydrogenated GaAs was annealed at 400, 500. and 600°C in a nitrogen atmosphcrc for approximately 5 min. The FL measurements were carried out using a 75cm monochromator equipped with an RCA 31034 photomultiplicr tube. The excitation source was the 5145 A line of’ an Ar-ion laser, and the sample temperature was controlled between 10 and 300K by using a He displex system. The PR measurements were performed using a 75cm monochromator equipped with a 750 watt tungsten lamp as a probe source and the 6328A line of a He-Ne laser as a modulation source. Figure I shows the results of the room-temperature PR spectra for the as-grown, the hydrogenated, and the annealed GaAs. The PR spectrum of the as-grown GaAs shows the typical shape in the low-field limit. The typical modulation spectroscopy signal in the low electrical-field region has a third-derivative line shape and obeys the following equation [I I]: AR/R
= R,[Ce”(E - ER + lT)-“1,
EFFECTS OF GaAs
Vol. 90, No. 9
spectrum and using equation (I) was 1.43 eV. The total luminescence intensity of the PR spectrum for the hydrogenated GaAs remarkably decreased in comparison to that of the as-grown GaAs and recovered for the hydrogenated and annealed GaAs. The broadening parameter of the as-grown GaAs was 13.2meV. The broadening parameters of the GaAs sample hydrogenated after rapid thermal annealing (RTA) at 400 and 500°C were 7.8 and 13.8meV. respectively. Since the variation of the broadening parameters is induced by the variance of the charge densities or the surface state densities, the charge densities or the surface state densities can be changed by hydrogenation and annealing. Figure 2 shows the results of the PL spectra at IOK for the as-grown, the hydrogenated, and the annealed GaAs. The peaks at 1.5171, 1.5137. and I .5120eV are related to a free exciton (X). a neutral donor-bound exciton (Do, X). and a neutral acceptorbound exciton (A”, X), respectively. The defect-related exciton (n, k’) appears in the range between 1.504 and I .5 I 1 eV, and the peak at 1.493 eV is considered to be an impurity due to the carbon acceptor (u, A”),. The peak at 1.5148 eV located between the (De, X) and (X) peaks is attributed to an antisite defect complex bound exciton 122).The (d, X) peak is related either to the Ga vacancy or its related acceptor defect [23] or to the As-stabilized (2 x 4) surface reconstruction [24]. The luminescence intensity of the PL spectrum after hydrogenation was lowered by as much as about six times in comparison to that of the as-grown GaAs. The decrease in the PR and PL intensities was caused by damage to the crystallinity due to hydrogenation,
‘I’= 16 K
(I)
where C and fI are the amplitude and the phase, respectively. ER and I’ show the energy gap and the broadening parameter, respectively. n is the dimension factor of the critical point. The values of n are 2.5, 3. and 2 for the three-, two-, and one-dimension cases. respectively [I I]. The value of the energy band can at room temoerature determined from the PR
(0)
&&
1
1.45
1.48 1.51 Energy (cV)
1 i4
Fig. 2. PL spectra of (a) the as-grown GaAs, (b) the hydrogenated GaAs, and the GaAs annealed at (c) 400°C. (d) 500°C. and (e) 600°C.
Vol. 90, No. 9
HYDROGENATING
AND ANNEALING
The intensity of the total luminescence for the hydrogenated and annealed GaAs increased in comparison with that for the hydrogenated GaAs. The (d, X) peak around 1.505 eV vanished after hydrogenation. This was caused by passivation, resulting from the hydrogenation of the acceptors related to the Ga vacancies, and this result was in good agreement with the results obtained by Pan er al. [6]. The (d, X) peak appeared after thermal treatment at 400’C. When the annealing temperature increased, the (d,X) peak increased rapidly. The appearance of the (d,X) peak originates from the separation of the passivation ions associated with either the Ga vacancies or the defect complexes. However, the (e,A”)c peak due to the carbon acceptor did not change, regardless of the deterioration of the GaAs crystallinity after hydrogenation. This behavior was induced by the formation of a hydrogen ion (H+) due to hydrogenation and a carbon ion in the GaAs. The increase of the neutral carbon acceptor compensated for the decrease of the PL intensity caused by hydrogenation. Although the increase of the PL spectrum intensity for the GaAs annealed at 400°C indicated the recovery of the crystallinity damage due to the hydrogenation. the complexes due to the Ht and C- still existed. The increase of the (A”, X) peak intensity shows augmcntation of the neutral carbon acceptors. When the thermal
m.zn
0.22
q *e $ 0 0
0.16
2
EFFECTS OF GaAs
587
treatment temperature became 500°C the decrease in the intensity of the (e, A o)c peak was due to the carbon acceptor separation from the hydrogen atoms. The variation of the broadening parameters in the PR spectra correlated with the intensity of the (4, A o)c peak. Figure 3 shows a comparison of the broadening parameter and the intensity ratio of the (e,A”)r to (Do, X) taken from the PR and PL spectra of Figs. 1 and 2 for (a) the as-grown GaAs and the GaAs annealed at (b) 4OO”C,(c) 500°C and (d) 600°C after hydrogenation. When the GaAs was annealed at 400’C after hydrogenation, the neutralization of the carbon acceptors increased while the PL intensity ratio of the (e, A’), to (Do, X) and the value of the broadening parameter decreased. Although the ionization of the neutral carbon acceptors resulting from separation of the hydrogen atoms decreased the intensity of the (e, A’), peak for the GaAs annealed at 500°C after hydrogenation. the value of the broadening parameter increased. These results indicate that the decrease of the surface potential caused by the decrease of the carriers reduces the value of the broadening parameter after hydrogenation and that the increase of the surface potential due to the ionized carbon acceptors resulting from the thermal treatment incrcascs the value of the broadening parameter. In summary, the PL measurements showed that the intensity due to the (e, A’), did not change and that the (d, X) peak disappeared after hydrogenation. The relative intensity ratios between (fI’,X) and (e, A’), for the as-grown sample and for the hydrogenated sample which was subsequently annealed at 400°C increased by a factor of 3. This result indicates the formation of neutral carbon acceptors resulting from the combination of Ht caused by hydrogenation and C- in GaAs. The separation of the hydrogen ions which neutralized the carbon acceptors was observed for hydrogenated GaAs which was subsequently annealed at 500 and 600°C. The variation of the broadening parameter in the PR spectra was caused by a neutralization or an ionization of the carbon acceptors. - This work was supported by the Korean Science and Engineering Foundation in 1993.
Acknowledgement
REFERENCES Fig. 3. Comparison of the broadening parameter with the ratio of the free carbon-acceptor (e, A), signal to the neutral donor-bound exciton (& X) signal taken from the PR and PL spectra of Figs I and 2 for (a) the as-grown GaAs sample and the hydrogenated GaAs samples which were subsequently annealed at (b) 400°C (c) 500°C. and (d) 600’C.
I. 2. 3.
A. Slaoui. K. Barhdadi, J.C. Muller & P. Siffer, Appl. Phys. A39, I59 (1986). A.R. Calawa, Appl. Phys. Left. 33, 12 (1987). F.P. McCluskey, L. PfeitTer, K.W. West, J. Lopata, M.L. Schnoes, T.D. Harres, S.J. Pearton & W.C. Dautremont-Smith, Appl. Phys. Letr. 54, I8 (1989).
588 4. 5. 6. 7. 8. 9.
10. Il. 12. 13.
HYDROGENATING
AND ANNEALING
B. Clejaud, F. Gendron, M. Krause & W. Ulrici, Phys. Rev. Lett. 65, 14 (1990). J. Chevallier. W.C. Dautremont-Smith. C.W. Tu & S.J. Peat-ton. Appl. Phys. Lett. 47,2 (1985). N. Pan, S.S. Bose, M.H. Kim, G.E. Stillman, G. Chambers, G. Devane, C.R. Ito & M. Feng. Appl. Phys. Lett. 51, 8 (1987). A.R. Calawa, Appl. Phys. L-err. 33, 12 (1978). Michihro Kobayashi, Teruo Yokoyama & Shin-i&ho Narita, Jpn J. Appl. Phys. 22, 10 (1983). W.C. Dautremont-Smith, J.C. Nabity, V. Swaminathan, M. Stavola, J. Chevallier, C.W. Tu & S.J. Pearton, Appl. Phys. Lett. 49, 17 (1986). A. Callegari, P.D. Hoh, D.A. Buchanan & D. Lacey, Appl. Phys. Lett. 54, 4 (1989). Modulation M. Cardona, Spectroscopy, Academic Press, New York, (1969). D.E. Aspnes, Handbook on Senticonductors, Vol. 2, North-Holland, New York (1980). C. Van Hof, K. Deneffe, J. De Boeck, D.J.
14. 5. 6. 17. 18. 19. 20. 21. 22.
24.
EFFECTS
OF GaAs
Vol. 90. No. 9
Arent & G. Borghs, Appl. Phys. Lett. 54, 608 ( 1989). U.K. Reddv. M. Longerbone & B.P. Gu. J. Appf. Phys.62, 4858 (1587). X. Yin, F.H. Pollak, L. Pawiowicz. T. O’Niel & M. Hafizi, Appl. Phys. Lett. 56, 1278 (1990). R.N. Bhattacharya, H. Shen, P. Parayanthal, F.H. Pollak, T. Coutts & H. Aharoni, Phys. Rev. B37, 4044 (1988). 0. Gerolo & J.C. Woolley, Canadian J. Phys. 49, 1335 (1971). R.C. Bowmann, Jr., R.L. Ah & K.W. Brown, Proc. SPIE 794,96 (1987). U.K. Reddy, G. Ji. T. Henderson, H. Morkoc & J.N. Schulman. J. Appl. Phys. 62, 145 (1987). N. Bottka. D.K. Gaskill. R.S. Sillmon, R. Henry & R. Glosser, J. Elec. Mat. 17, 161 ( 1988). D.E. Aspnes, Phys. Rev. BlO, 4228 (1982). F. Briones. R.F. Marks & L. Vina, J. Appl. Phys. 59, 3 (1986). H. Kiinzel & K. Ploog. Appl. Phys. Lett. 37, 4 ( 1980).