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Near band-edge optical absorption in pure GaAs
Solid State Communications,
Vol. 11, pp. 1187—1191, 1972. Pergamon Press.
Printed in Great Britain
NEAR BAND-EDGE OPTICAL ABSORPTION IN PURE GaAs Dale E. Hill Monsanto Company, St. Louis, Missouri 63166 (Received 28 July 1972 by V.B. 1-lannay)
The optical absorption spectrum of pure epitaxial GaAs at 1.38 K and 4.2°K shows strong lines related to the n = 1 and 2 exciton, and the neutral donor-exciton. Also reported here are siz weaker absorption lines, unexpectedly strong n — 2 exciton absorption, and a neutral donor-exciton oscillator strength of 170. Transmission samples with Burstein shifted transparent GaAs substrates are used and yield data equivalent to that from self-supporting samples.
THE OPTICAL absorption of GaAs at E
~.
E
0
transmission measurements according to the
Has been measured by Sturge’ on material for which the ionized impurity concentration was in the 3 x 10~_10~ range. It is now possible to grow epitaxial layers which are purer by more
usual equation with ax replaced by aLxL + asxs. The substrate absorption was measured for each sample and was found to be small and nearly flat over the region of interest. The
than two orders of magnitude. Recently optical absorption data for such material havepaper beendata 2 In this reported by Sell and Dingle. are presented which corroborate these results
thickness of the epitaxial layers down to about 7~m was determined by measuring the interfererice fringes observed in the infrared reflectivity. Dimensions for each thinner sample were obtained by matching its absorption curve to those of thicker samples, for overlapping ranges.
and show structure not observed by those authors. The samples used in this work were of two types. One is the usual self-supporting kind. The second type of sample consists of an undoped layer grown on < 110> n-type substrates doped to a level of 1 to 2 x 10~ electrons/cm.3 For such samples the absorption of the epitaxial layer can be measured without removing the substrate, since the absorption of the substrate is shifted to higher energies by the Burstein effect. The epitaxial layers should be quite strain-free since there is negligible difference in lattice constant between undoped GaAs and GaAs so doped. The measurements contained in this paper indicate that this is essentially true.
The samples were immersed in liquid helium which was pumped to 1.5—2.OmmHg during measurement. The transmitted light was analyzed with a Jarrell Ash 0.5m Ebert Grating Monochrometer and a Bausch and Lomb 0.25m Grating Moriochrometer was used as the light source. Scattered light in the system was estimated from the apparent transmission at the exciton peak and in the continuum region for samples which were thick enough to insure that no real transmission should occur in these regions. This value was then used as the zero transmission value.
The absorption coefficient for the epitaxial layers on doped substrates was obtained from
Figure 1 shows the absorption coefficient as a function of energy for several thicknesses of a 1187
1188
NEAR BAND-EDGE OPTICAL ABSORPTION IN PURE GaAs
8185
~
8180
WAVELENGTH IN ANGSTROMS 8175 8170
//~
~
Vol. 11, No.9
8165
8160
~1
102 1.514
1.515
I
I
I
1.516
1.518
1.519
1.517 PHOTON ENERGY IN eV
FIG. 1. Absorption coefficient vs. energy for GaAs at 1.38°K. Resolution — 0.04 meV. Sample 971 with substrate;., — 53 .2 ~im; x, = 9.9~m;u, t 6.6J2m. Sample 971 self-supporting; A, t 22/~Lm.Sample 629829—4; o, t = 6.2~m. The number of data points shown on the curves is limited to minimize confu-
sion. The experimental data is continuous. high purity sample and one sample of larger impurity concentration. The high purity sample (No. 971) was grown in the same run with specimens on high resistivity substrates which exhibited exhaustion concentrations of about 3 x 10~cm~and 77°K mobilities of 150,000cm2 V sec. This corresponds to an ionized impurity 3 of about 3 x 10 4cm~. The other concentration sample shown has an estimated ionized impurity concentration in the mid- 10~cm3 range. It is apparent that much more structure is seen for purer material. For an ionized impurity concentration of 3 x 106 _1017 the absorption peak has a full width at half maximum of about 5 meV which is roughly the full energy span shown in Fig. 1. The strong absorption line at about 1.5151eV is identified with the n 1 exciton, the line at 1.5182eV with the n = 2 exciton, and the sharper line at 1.5141 with the exciton bound to a neutral donor, in agreement with Sell and Dingle. The consistency in the position of these major features to better than 0.1meV is good evidence that the samples are strain-free for both sets of data. \dditional absorption related to the n ~ 3 exciton is identifiable in the high energy region of the spectrum. The structure which Sell and
Dingle report at 1.5135eV is not seen in this work, even for a specimen five times thicker than the one they used. This could be related to an impurity not present in our samples. The identification of the n = 1 exciton and the exciton-neutral donor line absorption seem firm, since they occur at positions where photoluminescence lines have been associated with these entities by various authors.46 It is observed in the present work that the exciton line broadens with increased temperature, as as would be expected, to a full width at half maximum of nearly 1 meV at 4.2°K. The line 1.5141eV, however, does not change width up to 4.2°K. This is the behaviour expected for a bound exciton, in agreement with the identification above. Further, the variation of absorption coefficient at 1.5141eV between the 53,~imsample and the thinner ones indicates that this is an impurity-related line, and that the concentration was not the same for the two samples. If one takes an average of the integrated neutral donor-exciton absorption for these samples, an oscillator strength7 of about 170 is obtained using the neutral donor concentration
Vol. 11, No. 9
NEAR BAND-EDGE OPTICAL ABSORPTION IN PURE GaAs
as 3 x 1013 cm~.The accuracy of this figure is probably limited to ±50 per cent by the lack of better knowledge of the neutral donor density. Such ‘giant’ oscillator strengths have been 8 predicted for excitons bound to neutral donors and observed for CdS.7 Further, a radiative lifetime of 0.05 nsec can be inferred9 on the basis of this oscillator strength. For CdS the lifetime predicted on this basis have been confirmed by more direct measurements.9 The lifetime for the exciton-neutral donor state at these temperatures has been measured as 1.07 nsec by Hwang and Dawson. 10 The reason for disagreement of this magnitude is not understood. In order to obtain reliable data for high absorption coefficients, thinner samples are necessary. Thus, samples in the thickness range of 1—6~imhave been prepared on conducting substrates and measured. However progressively thinner samples show the high-energy end of the spectrum depressed with respect to the low energy portion. This depression is such that a 5~Lm sample shows the n — 2 peak reduced to about 1/2 the value observed for the n = 1 line, while a sample = 1 micron thick exhibits no n = 2 absorption peak, even though the n = 1 peak is reasonably strong and the neutral donor related line is still clearly visible. Thus the data for thin samples will not consistently fit that for the thicker ones over the range of energy under consideration. These thinner samples also show that both the continuous portion of the absorption and the n = 2 line decrease in strength with respect to the n = 1 peak. Sell and Dingle did not report on any specimens thicker than l0~m and did not calculate absorption coefficient from their data. From their figures it is clear that they observe the same effect. The absorption not related to discrete exciton effects can be estimated by comparison with the broken line of Fig. 1 depicting the discrete exciton portion of the theory of Glioski et a!. ~ which includes a broadening parameter EB. The non-exciton portion of the continuous absorption could (hi.i/E be an Urbach-type band edge, a 12exp In 0), asE is found in less pure GaAs. this case 0 would be less than 1meV, in contrast to values of 2 meV and greater 12 found for less pure samples.
1189
These thin-sample effects are very pronounced, but the precise reasons for them are not clear. The effect of finite sample thickness relative to the various exciton radii has been estimated theoretically13 and is not sufficient to account for these effects. Therefore, surface effects are very likely play an important role. These could be caused either by effects near the substrate interface or near the air interface. Since similar effects seem to occur on samples with or without substrates2 it is likely that at least some problem is related to the air interface surface. Substrate interface effects could reduce the continuous portion of the spectrum by a Burstein shift of the absorption edge, if the carrier concentration is somewhat higher in this region. Regardless of the exact nature of these effects, it is reasonable that the n = 2 lines should be affected before the other lines, since this line is related to the most loosely bound particle observed. The fit of the data with the above theory represented by the broken line in Fig. 1 requires Eg 1.51917eV, R = 4.1meV and EB = 0.17meV. The less pure samples shown would require changing EB to about 1 meV, while the data of Sturge can be fitted with an EB of almost 3 meV, indicating the strong dependence on impurity content. Mthough the polariton nature of the excitonj4 is not accounted for by this theory, this correction2 would still leave the value of E 0 = 1.5 192 ±0.0002 in agreement with Dingle and Sell. The value of the exchange energy seems not definitely established 2, 14 and could change E~ if the larger values are correct. The theory also gives us a means of examining the relative magnitude of the absorption for the n = 1 and 2 exciton for the case of thick samples, where surface effects are not large. If one subtracts an estimate of the observed continuous absorption from that observed for the n = 2 excitori one gets a peak absorption coefficient of about 9 x lOBcm1. The calculated curve, which was fitted to3cm the n for the 1 line, n = gives 2 a value of about 1.6 x lO line, as one would expect from the 1/na dependence of simple theory. Thus the observed absorption strength for the n 2 exciton is nearly six times greater than expected relative to the
1190
NEAR BAND-EDGE OPRICAL ABSORPTION IN PURE GaAs
Vol. 11, No.9
n - 1 line. This difference is not understood at the present.
The fourth line is seen only as a shoulder in absorption and may have been undetectable in
The line at 1.5174eV has been tentatively 2 The two associated with impurity effects. smaller lines at about 0.2 and 0.4 meV higher energies are observed for the first time in this work. If these lines are donor related, one might expect the absorption for the 53.2~m sampie to differ from that for the thinner samples as observed for the neutral donor-excitori line. However, this is not the case. A pair of photoluminescence lines in this region have been associated with the n 2 and 3 excitons. ~ This identification is very likely not correct.
In summary, this paper presents data on the absorption coefficient of pure GaAs which shows the absorption lines of the n 1 and 2 exciton, the exciton bound to a neutral donor and several other features not reported previously. The inferred exciton binding energy is 4.1 ±0.2meV, and the band gap is 1.5192eV ±0.0002. An absorption strength for the n - 2 excitori of about 6 times that predicted by theory is reported. A ‘giant’ oscillator strength of 170 for the excitori bound to a neutral donor is found and a corresponding radiative lifetime of 0.05 nsec is calculated. Finally a supported absorption sample is used which is much simpler to prepare and easier to handle, and yet yields data which are as good as those for the unsupported sample.
The remaining features of Fig. 1 have not been previously reported. They are the four small lines on the low energy side of the exciton peak whose widths are slit-width limited. These are almost certainly related to the neutral donor since the strongest three occur at the same position, within experimental error, as photoluminescence lines identified with the neutr al donor by Rossi, Wolf, Stillman and Dimmock. ~
Acknowledgement — The author would like to express his appreciation to Dr. R. Walline and J. Holtz for providing the excellent samples, to Dr P.T. Bailey for helpful discussions, and to Dr. D.D. Sell for access to his manuscript prior to publication.
REFERENCES 1.
STURGE M.D., Phys. Rev. 127, 768 (1962.
2. 3.
SELL D.D. and DINGLE R., Bull. Am. Phys. Soc. Series II, 17, 325 (1972), paper EE2, and SELL D.D., to be published. WOLFE C.M., STILLMAN G.E. and DIMMOCK J.O., J. appi. Phys. 41, 504 (1970).
4.
SELL D.D., DINGLE R., STOKOWSKI S.E. and DILORENZO J.V., Phys. Rev. Lett. 27, 1644 (1971).
5.
ROSSI J.A., WOLFE C.M., STILLMAN G.E. and DIMMOCK J.O., Solid State Commun. 8, 2021 (1970).
6.
GILLEO MA., BAILEY PT. and HILL D.E., Phys. Rev. 174, 898 (1968); BOGARDUS E.H. and BEBB H.B., Phys. Rev. 176, 993 (1968); SHAH J.,LIETE R.C.C. and NAHORY E., Phys. Rev. 184, 811 (1969).
7.
THOMAS D.G. and HOPFIELD
8.
RASHBA E.I. and GURGENISHVILI G.E., Soviet Phys. Solid State, 4, 759 (1962).
9.
HENRY C.H. and NASSAU K., Phys. Rev. Bi, 1628 (1970).
J.J.,
Phys. Rev. 175, 1021 (1968).
10.
HWANG C.J. and DAWSON L.R., Solid State Commun. 10, 443 (1972).
11.
GLINSKY R.G., LONG KS. and WOOLLEY J.C., Phys. Status Solidi (b) 48, 815 (1971).
12.
PANKOVE J.I., Phys. Rev. 140, A2059 (1965).
13.
BENDOE B., Phys. Rev. B3, 1999 (1971).
14.
GILLEO, M.A., BAILEY P.T. and HILL D.E., J. Luminescence 1, 2, 562 (1970).
15.
BIMBERG D. and SCHAIRER W., Phys. Rev. Lett. 28, 442 (1972).
Vol. 11, No.9
NEAR BAND-EDGE OPTICAL ABSORPTION IN PURE GaAs
Das optische Absorptionsspektrum eines reinen epitaxialen GaAs zeigt bei 1.38°K und bei 4.2°K starke, an n = 1 und 2 exciton- an das neutrale donatorexciton-verwandte Banden. Es werden auch sechs schwachere Absorptionslinienmeine unerwartet n = 2 Excitonabsorption und eine neutrales Donartorexciton mit der Oscillatorstarke von 170 berichtet. Durchlassigkeitsproben mit Burstein-verschobenen, durchsichtigen GaAs-Substraten werden verwendet und sie geben den selbsttragenden Proben equivalente Werte.
1191
Vol. 11, pp. 1187—1191, 1972. Pergamon Press.
Printed in Great Britain
NEAR BAND-EDGE OPTICAL ABSORPTION IN PURE GaAs Dale E. Hill Monsanto Company, St. Louis, Missouri 63166 (Received 28 July 1972 by V.B. 1-lannay)
The optical absorption spectrum of pure epitaxial GaAs at 1.38 K and 4.2°K shows strong lines related to the n = 1 and 2 exciton, and the neutral donor-exciton. Also reported here are siz weaker absorption lines, unexpectedly strong n — 2 exciton absorption, and a neutral donor-exciton oscillator strength of 170. Transmission samples with Burstein shifted transparent GaAs substrates are used and yield data equivalent to that from self-supporting samples.
THE OPTICAL absorption of GaAs at E
~.
E
0
transmission measurements according to the
Has been measured by Sturge’ on material for which the ionized impurity concentration was in the 3 x 10~_10~ range. It is now possible to grow epitaxial layers which are purer by more
usual equation with ax replaced by aLxL + asxs. The substrate absorption was measured for each sample and was found to be small and nearly flat over the region of interest. The
than two orders of magnitude. Recently optical absorption data for such material havepaper beendata 2 In this reported by Sell and Dingle. are presented which corroborate these results
thickness of the epitaxial layers down to about 7~m was determined by measuring the interfererice fringes observed in the infrared reflectivity. Dimensions for each thinner sample were obtained by matching its absorption curve to those of thicker samples, for overlapping ranges.
and show structure not observed by those authors. The samples used in this work were of two types. One is the usual self-supporting kind. The second type of sample consists of an undoped layer grown on < 110> n-type substrates doped to a level of 1 to 2 x 10~ electrons/cm.3 For such samples the absorption of the epitaxial layer can be measured without removing the substrate, since the absorption of the substrate is shifted to higher energies by the Burstein effect. The epitaxial layers should be quite strain-free since there is negligible difference in lattice constant between undoped GaAs and GaAs so doped. The measurements contained in this paper indicate that this is essentially true.
The samples were immersed in liquid helium which was pumped to 1.5—2.OmmHg during measurement. The transmitted light was analyzed with a Jarrell Ash 0.5m Ebert Grating Monochrometer and a Bausch and Lomb 0.25m Grating Moriochrometer was used as the light source. Scattered light in the system was estimated from the apparent transmission at the exciton peak and in the continuum region for samples which were thick enough to insure that no real transmission should occur in these regions. This value was then used as the zero transmission value.
The absorption coefficient for the epitaxial layers on doped substrates was obtained from
Figure 1 shows the absorption coefficient as a function of energy for several thicknesses of a 1187
1188
NEAR BAND-EDGE OPTICAL ABSORPTION IN PURE GaAs
8185
~
8180
WAVELENGTH IN ANGSTROMS 8175 8170
//~
~
Vol. 11, No.9
8165
8160
~1
102 1.514
1.515
I
I
I
1.516
1.518
1.519
1.517 PHOTON ENERGY IN eV
FIG. 1. Absorption coefficient vs. energy for GaAs at 1.38°K. Resolution — 0.04 meV. Sample 971 with substrate;., — 53 .2 ~im; x, = 9.9~m;u, t 6.6J2m. Sample 971 self-supporting; A, t 22/~Lm.Sample 629829—4; o, t = 6.2~m. The number of data points shown on the curves is limited to minimize confu-
sion. The experimental data is continuous. high purity sample and one sample of larger impurity concentration. The high purity sample (No. 971) was grown in the same run with specimens on high resistivity substrates which exhibited exhaustion concentrations of about 3 x 10~cm~and 77°K mobilities of 150,000cm2 V sec. This corresponds to an ionized impurity 3 of about 3 x 10 4cm~. The other concentration sample shown has an estimated ionized impurity concentration in the mid- 10~cm3 range. It is apparent that much more structure is seen for purer material. For an ionized impurity concentration of 3 x 106 _1017 the absorption peak has a full width at half maximum of about 5 meV which is roughly the full energy span shown in Fig. 1. The strong absorption line at about 1.5151eV is identified with the n 1 exciton, the line at 1.5182eV with the n = 2 exciton, and the sharper line at 1.5141 with the exciton bound to a neutral donor, in agreement with Sell and Dingle. The consistency in the position of these major features to better than 0.1meV is good evidence that the samples are strain-free for both sets of data. \dditional absorption related to the n ~ 3 exciton is identifiable in the high energy region of the spectrum. The structure which Sell and
Dingle report at 1.5135eV is not seen in this work, even for a specimen five times thicker than the one they used. This could be related to an impurity not present in our samples. The identification of the n = 1 exciton and the exciton-neutral donor line absorption seem firm, since they occur at positions where photoluminescence lines have been associated with these entities by various authors.46 It is observed in the present work that the exciton line broadens with increased temperature, as as would be expected, to a full width at half maximum of nearly 1 meV at 4.2°K. The line 1.5141eV, however, does not change width up to 4.2°K. This is the behaviour expected for a bound exciton, in agreement with the identification above. Further, the variation of absorption coefficient at 1.5141eV between the 53,~imsample and the thinner ones indicates that this is an impurity-related line, and that the concentration was not the same for the two samples. If one takes an average of the integrated neutral donor-exciton absorption for these samples, an oscillator strength7 of about 170 is obtained using the neutral donor concentration
Vol. 11, No. 9
NEAR BAND-EDGE OPTICAL ABSORPTION IN PURE GaAs
as 3 x 1013 cm~.The accuracy of this figure is probably limited to ±50 per cent by the lack of better knowledge of the neutral donor density. Such ‘giant’ oscillator strengths have been 8 predicted for excitons bound to neutral donors and observed for CdS.7 Further, a radiative lifetime of 0.05 nsec can be inferred9 on the basis of this oscillator strength. For CdS the lifetime predicted on this basis have been confirmed by more direct measurements.9 The lifetime for the exciton-neutral donor state at these temperatures has been measured as 1.07 nsec by Hwang and Dawson. 10 The reason for disagreement of this magnitude is not understood. In order to obtain reliable data for high absorption coefficients, thinner samples are necessary. Thus, samples in the thickness range of 1—6~imhave been prepared on conducting substrates and measured. However progressively thinner samples show the high-energy end of the spectrum depressed with respect to the low energy portion. This depression is such that a 5~Lm sample shows the n — 2 peak reduced to about 1/2 the value observed for the n = 1 line, while a sample = 1 micron thick exhibits no n = 2 absorption peak, even though the n = 1 peak is reasonably strong and the neutral donor related line is still clearly visible. Thus the data for thin samples will not consistently fit that for the thicker ones over the range of energy under consideration. These thinner samples also show that both the continuous portion of the absorption and the n = 2 line decrease in strength with respect to the n = 1 peak. Sell and Dingle did not report on any specimens thicker than l0~m and did not calculate absorption coefficient from their data. From their figures it is clear that they observe the same effect. The absorption not related to discrete exciton effects can be estimated by comparison with the broken line of Fig. 1 depicting the discrete exciton portion of the theory of Glioski et a!. ~ which includes a broadening parameter EB. The non-exciton portion of the continuous absorption could (hi.i/E be an Urbach-type band edge, a 12exp In 0), asE is found in less pure GaAs. this case 0 would be less than 1meV, in contrast to values of 2 meV and greater 12 found for less pure samples.
1189
These thin-sample effects are very pronounced, but the precise reasons for them are not clear. The effect of finite sample thickness relative to the various exciton radii has been estimated theoretically13 and is not sufficient to account for these effects. Therefore, surface effects are very likely play an important role. These could be caused either by effects near the substrate interface or near the air interface. Since similar effects seem to occur on samples with or without substrates2 it is likely that at least some problem is related to the air interface surface. Substrate interface effects could reduce the continuous portion of the spectrum by a Burstein shift of the absorption edge, if the carrier concentration is somewhat higher in this region. Regardless of the exact nature of these effects, it is reasonable that the n = 2 lines should be affected before the other lines, since this line is related to the most loosely bound particle observed. The fit of the data with the above theory represented by the broken line in Fig. 1 requires Eg 1.51917eV, R = 4.1meV and EB = 0.17meV. The less pure samples shown would require changing EB to about 1 meV, while the data of Sturge can be fitted with an EB of almost 3 meV, indicating the strong dependence on impurity content. Mthough the polariton nature of the excitonj4 is not accounted for by this theory, this correction2 would still leave the value of E 0 = 1.5 192 ±0.0002 in agreement with Dingle and Sell. The value of the exchange energy seems not definitely established 2, 14 and could change E~ if the larger values are correct. The theory also gives us a means of examining the relative magnitude of the absorption for the n = 1 and 2 exciton for the case of thick samples, where surface effects are not large. If one subtracts an estimate of the observed continuous absorption from that observed for the n = 2 excitori one gets a peak absorption coefficient of about 9 x lOBcm1. The calculated curve, which was fitted to3cm the n for the 1 line, n = gives 2 a value of about 1.6 x lO line, as one would expect from the 1/na dependence of simple theory. Thus the observed absorption strength for the n 2 exciton is nearly six times greater than expected relative to the
1190
NEAR BAND-EDGE OPRICAL ABSORPTION IN PURE GaAs
Vol. 11, No.9
n - 1 line. This difference is not understood at the present.
The fourth line is seen only as a shoulder in absorption and may have been undetectable in
The line at 1.5174eV has been tentatively 2 The two associated with impurity effects. smaller lines at about 0.2 and 0.4 meV higher energies are observed for the first time in this work. If these lines are donor related, one might expect the absorption for the 53.2~m sampie to differ from that for the thinner samples as observed for the neutral donor-excitori line. However, this is not the case. A pair of photoluminescence lines in this region have been associated with the n 2 and 3 excitons. ~ This identification is very likely not correct.
In summary, this paper presents data on the absorption coefficient of pure GaAs which shows the absorption lines of the n 1 and 2 exciton, the exciton bound to a neutral donor and several other features not reported previously. The inferred exciton binding energy is 4.1 ±0.2meV, and the band gap is 1.5192eV ±0.0002. An absorption strength for the n - 2 excitori of about 6 times that predicted by theory is reported. A ‘giant’ oscillator strength of 170 for the excitori bound to a neutral donor is found and a corresponding radiative lifetime of 0.05 nsec is calculated. Finally a supported absorption sample is used which is much simpler to prepare and easier to handle, and yet yields data which are as good as those for the unsupported sample.
The remaining features of Fig. 1 have not been previously reported. They are the four small lines on the low energy side of the exciton peak whose widths are slit-width limited. These are almost certainly related to the neutral donor since the strongest three occur at the same position, within experimental error, as photoluminescence lines identified with the neutr al donor by Rossi, Wolf, Stillman and Dimmock. ~
Acknowledgement — The author would like to express his appreciation to Dr. R. Walline and J. Holtz for providing the excellent samples, to Dr P.T. Bailey for helpful discussions, and to Dr. D.D. Sell for access to his manuscript prior to publication.
REFERENCES 1.
STURGE M.D., Phys. Rev. 127, 768 (1962.
2. 3.
SELL D.D. and DINGLE R., Bull. Am. Phys. Soc. Series II, 17, 325 (1972), paper EE2, and SELL D.D., to be published. WOLFE C.M., STILLMAN G.E. and DIMMOCK J.O., J. appi. Phys. 41, 504 (1970).
4.
SELL D.D., DINGLE R., STOKOWSKI S.E. and DILORENZO J.V., Phys. Rev. Lett. 27, 1644 (1971).
5.
ROSSI J.A., WOLFE C.M., STILLMAN G.E. and DIMMOCK J.O., Solid State Commun. 8, 2021 (1970).
6.
GILLEO MA., BAILEY PT. and HILL D.E., Phys. Rev. 174, 898 (1968); BOGARDUS E.H. and BEBB H.B., Phys. Rev. 176, 993 (1968); SHAH J.,LIETE R.C.C. and NAHORY E., Phys. Rev. 184, 811 (1969).
7.
THOMAS D.G. and HOPFIELD
8.
RASHBA E.I. and GURGENISHVILI G.E., Soviet Phys. Solid State, 4, 759 (1962).
9.
HENRY C.H. and NASSAU K., Phys. Rev. Bi, 1628 (1970).
J.J.,
Phys. Rev. 175, 1021 (1968).
10.
HWANG C.J. and DAWSON L.R., Solid State Commun. 10, 443 (1972).
11.
GLINSKY R.G., LONG KS. and WOOLLEY J.C., Phys. Status Solidi (b) 48, 815 (1971).
12.
PANKOVE J.I., Phys. Rev. 140, A2059 (1965).
13.
BENDOE B., Phys. Rev. B3, 1999 (1971).
14.
GILLEO, M.A., BAILEY P.T. and HILL D.E., J. Luminescence 1, 2, 562 (1970).
15.
BIMBERG D. and SCHAIRER W., Phys. Rev. Lett. 28, 442 (1972).
Vol. 11, No.9
NEAR BAND-EDGE OPTICAL ABSORPTION IN PURE GaAs
Das optische Absorptionsspektrum eines reinen epitaxialen GaAs zeigt bei 1.38°K und bei 4.2°K starke, an n = 1 und 2 exciton- an das neutrale donatorexciton-verwandte Banden. Es werden auch sechs schwachere Absorptionslinienmeine unerwartet n = 2 Excitonabsorption und eine neutrales Donartorexciton mit der Oscillatorstarke von 170 berichtet. Durchlassigkeitsproben mit Burstein-verschobenen, durchsichtigen GaAs-Substraten werden verwendet und sie geben den selbsttragenden Proben equivalente Werte.
1191