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Enhancement of donor-related photoluminescence intensity due to near-band-gap excitation of high purity GaAs
Solid State Communications, Vol. 70, No. 9, pp. 855-858, 1989. ~Printed in Great Britain.
0038-1098/8953.00+.00 Pergamon Press plc
ENHANCEMENT OF DONOR-RELATED PHOTOLUMINESCENCE INTENSITY DUE TO NEAR-BAND-GAP EXCITATION OF HIGH PURITY GaAs S. Zemon and G. Lambert, GTE Laboratories Incorporated, 40 Sylvan Road, Waltham, MA 02254 USA (Received 15 March 1989 by J. Tauc) Striking increases in the intensity of donor-related, photoluminescence transitions are observed in undoped (1014-1015 cm -3) GaAs for excitation energies (Ee) in the vicinity of the band-gap energy (Eg). The enhancement has maxima at E e consistent with excitation of the n=2 and n=3 states of the free exciton (Xn=2,3) and appears to be correlated to the concentration of ionized donors, suggesting that the effects are related to capture of electron-hole pairs by ionized donors through trapping of Xn=2,3. A resonant interaction may be operative as well. The enhancement decreases monotonically as E e increases to values as much as 12 meV above Eg.
pressure organometallic vapor phase epitaxy (OMVPE) and molecular beam epitaxy (MBE) on semi-insulating GaAs substrates by procedures previously described. 3-5 The OMVPE layers were lowcompensation, n-type material [donor concentration (ND) = 1014 cm3>>acceptor concentration (NA)=1013 cm-3] 4 as well as n- and p-type, compensated material (ND=NA=1014 cm-3). 3 The sharpness of the 4.2 K PL and magnetophotoluminescene (MPL) lines confirmed the high purity of the OMVPE layers.3,4, s The layer thicknesses were 1.5-17 I~m. The MBE sample was 2.5-~m thick and p-type. Typical MBE layers grown under conditions similar to those of this sample were p-type with NA=1015 cm3. 5 The low intensity of the donor-related PL features observed during nonresonant excitation of this sample suggests that NA>>ND=I 014 cm-3. The selective PL and PLE measurements were made at 4.2 K with the sample freely suspended in an immersion cryostat as well as at temperatures up to 25 K in a cold finger cryostat. The excitation source was an argon-laser-pumped dye laser (Styryl 9 dye) with a 0.1-nm bandwidth. Wavelengths were measured with a wavemeter and selected values were calibrated with a spectrometer. When required, the laser polarization was adjusted to be circularly polarized and the PL analyzed for right circularly polarized (RCP) and left circularly polarized (LCP) components. Power densities of 0.2-200 mW/cm 2 were employed. The signal was dispersed with a 0.85-m, double-grating spectrometer (0.04-nm resolution) and detected with a photon counting system using a cooled GaAs photocathode photomultiplier. The MPL system has been described in the literature, s
1. Introduction A striking enhancement has been observed in the intensity of donor-related, photoluminescence (PL) transitions in undoped (1014-1015 cm-3), epitaxial GaAs for excitation energies (Ee) in the vicinity of the band-gap energy (Eg). In some cases dramatic intensity increases of over two orders of magnitude have been found with donor-related transitions dominating acceptor-related ones in the excitonic region of even low-compensation, p-type material. At T=4.2 K the effect was maximum when pumping within the gap at Ee=Eg-0.6 meV [consistent with excitation of the n=3 state of the free exciton (Xn=3) 1] and decreased monotonically as E e was increased to values as much as 12 meV above Eg. For T>4.2 K the maximum occurred at an E e corresponding to the n=2 state of the free exciton (Xn=2). 1,2 The enhancement was most readily observed for transitions involving recombination of a free hole with an electron bound to a donor and/or radiative decay of an ionized-donor-bound exciton [(DO,h)/(D+,X)] and appears to be correlated to the concentration of ionized donors (N+o). Enhancements were also observed in a variety of other samples, including GaAs heterostructures and substrates as well as AIGaAs. We suggest that for Ee
3. Experimental Results and Discussion In Fig. 1 we show 4.2 K PL in the excitonic spectral region excited both at 804 nm (spectra labeled as 1) and 816.2 nm (spectra labeled as 2). The identifications of the features are shown at the top of the
2. Experimental Details The nominally undoped, (100)-oriented, GaAs epilayers used in this work were grown by both low855
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WAVELENGTH (nm) FIG. 1 4.2 K photoluminescence (PL) spectra in the excitonic region. (a) A low-compensation, p-type (=1015 cm -3) epilayer. (b) A thin (1.5 ~m), lowcompensation, n-type (=1014 cm-3)epilayer. Spectra 1 and 2 were excited at 804 nm and 816.2 nm, respectively. An excitation power of 200 mW/cm 2 was used in (a) and 20 mW/cm 2 in (b).
figure. 2 The laser line due to scattered pump light is indicated by an arrow in spectrum 2 of Fig. l(a). Also indicated there as a dashed line is the position of the band gap at ~.g=815.9 nm (1.5192 eV 7). Figure l(a) shows spectra for the p-type, low-compensation, MBE sample. In spectrum 1 of Fig. l(a) we observe only two peaks, one at the intrinsic free exciton feature Xn=1 and the other at the extrinsic acceptor feature (A°,X) [neutral-acceptor-bound exciton], as expected for nonresonant excitation of a high purity sample with NA (=1015 cm-3)>>Nb. No donor-related features were detected. However, for excitation at 816.2 nm in spectrum 2 a dramatic change is found to occur with the donor feature (D°,h)/(D+,X) emerging as dominant and the neutral donor feature (D°,X) becoming detectable. [Donor-to-carbon-acceptor transitions, to be discussed later, also become detectable.] Since all the donors are ionized (D +) in this p-type material, the D+ concentration N+D=ND=1014 cm -3. The ionized donors are clearly playing a crucial role in the enhancement process both by becoming photo-neutralized and, possibly, by becoming incorporated into bound excitons. In contrast, for a thick (9 p.m), n-type, low-compensation layer, where donor-related PL features dominate for nonresonant excitation (as expected), only a relatively small in-
Vol. 70, No. 9
crease in (D°,h)/(D÷,X) occurs for excitation at 816.2 nm. This reduced effect is consistent with the fact that most of the donor concentration (=1014 cm 3) is expected to be neutral. However, for a layer similar to the latter but with a reduced thickness of 1.5 #m, a substantial enhancement effect is observed. This is shown in Fig. l(b) where we note that (D°,h)/(D+,X) undergoes an increase of = 50x. (D°,X) also increases but to a lesser extent, similar to the trend found in Fig.l(a). This enhancement effect is also consistent with the picture of the importance of N+o. Here the presence of a significant ionized donor concentration is explained by the fact that the 1.5-1~mthick layer is thin enough so that depletion effects would be expected to be substantial. Enhancements of donor-related PL features were observed in a variety of other samples, e.g., (1) low compensation and compensated, n-type layers (thicknesses 0.5-2 #m) capped with AIGaAs window layers (thicknesses 0.3-7.5 I~m), (2) the 1.5-~m-thick, GaAs layer of a selectively-doped heterostructure, 8 (3) undoped GaAs substrates capped with AIGaAs, (4) GaAs grown on Si substrates, 9 and (5) AlxGal_xAS layers with x<0.02. The details will be reported elsewhere. In a magnetic field, where the degeneracy between the (D°,h) and (D+,X) peaks is substantially removed, the two types of transitions can be distinguished for samples with sufficiently sharp PL features, e.g., the high purity OMVPE epilayers, lo A compensated, OMVPE sample was tested in a magnetic field of 6.4 T at 4.2 K and an enhancement was found in (D+,X), in addition to (D°,h), for E e in the vicinity of the lowest, Landau-level, interband transition energy. When exciting the sample of Fig. l(a) with circularly polarized light (say LCP) at 4.2 K, the luminescence was unpolarized for excitation wavelengths ~.e<822 nm. However, for 822 nm<~.e<816.2 nm the RCP component of (D°,h)/(D+,X) was greater than the LCP component while the Xn= 1, (D°,X) and (A°,X) features remained unpolarized. For example, at Xe=816.2 nm (the peak of the polarized PLE spectra) the RCP component of (D°,h)/(D+,X) was =40% greater than the LCP. A similar result was observed for the sample of Fig. l(b). This polarization memory indicates that there is strong coupling between a state initially photoexcited and the corresponding emission state. 11 To examine the wavelength dependence of the enhancement in detail, we present in Fig. 2(a) a PLE spectrum of the sample of Fig. l(a) where the spectrometer is set at ~.s=819.1nm for (D°,h)/(D+,X) and, for comparison, in Fig. 2(b) at ~.s=819.6 nm for (A°,X) in the J=3/2 state. For reference the wavelength dependence of the dye laser output is included in Fig. 2(b). The major features are labeled.I, 2 As expected for the PLE of (A°,X), the spectrum of Fig. 2(b) has prominent peaks at the excitonic features Xn=2, Xn=l, (D°,X), and (A°,X) in the J=1/2 state as well as a weak feature at Xn=3.1,2 [The Xn=2 and Xn=3 peaks are observed as well on the PLE spectra for Xn=l and (D°,X).] In marked contrast, the spectrum of Fig. 2(a)
ENHANCEMENT OF DONOR-RELATED PHOTOLUMINESCENCE
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rises monotonically [i.e., the (D°,h)/(D+,X) feature grows] with increasing excitation wavelength to a maximum at 816.2 nm which is equivalent to an energy 0.6 meV below Eg [Xg is denoted by a dashed line in Fig. 2(a) and (b)] and, as discussed below, is identified with excitation of Xn=3. The full width at half maximum of the main PLE band is =2 meV, the enhancement first becoming detectable as much as 12 meV above Eg. Similar spectra were observed for power densities as low as 0.2 mW/cm 2. The prominent shoulder at =816.5 nm is better resolved in the higher purity OMVPE samples and can be assigned to excitation of Xn=2. The low-amplitude shoulder at =816.9 nm, which is in close correspondence to a PLE feature in Fig. 2(b) and a PL feature observed in Fig. l(a), appears to be = 0.2 nm longer than (D°n=2,h). Finally, low-amplitude peaks are seen for Xn= 1 and (D°,X). No resonant enhancement of the (D°,h)/(D+,X) feature was observed, in agreement with earlier observations. 12 From selectively excited PL spectra we find that pumping the Xn=2 transition has a stronger enhancement effect for (D°,h)/(D+,X) than for the other excitonic features, while the opposite is true when pumping the Xn= 1 transition. PLE data similar to Fig. 2(a) were observed for all the other homoepitaxial layers studied (with some differences for thick layers of low compensation, n-type material where the enhancement effect was small). Variations, however, occurred in the strength of the enhancement.
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Within experimental error a PLE feature at 816.2 nm is consistent not only with excitation of Xn=3 (as indicated above) but also with excitation of an electron from the valence band into an n=3 state of a donor. 13 However, little or no response was found in the PLE spectrum of (D°n=I,h)/(D+,X) when pumping at the wavelength corresponding to (D°n=2,h) at 816.8 nm [e.g., see Fig. 2(a)] or resonant excitation of the line itself. 12 Furthermore, no (D°n,h) absorption peaks have been reported in the literature for high purity GaAs. 1 Thus, we assume that direct excitation of (D°n=3,h) does not make a significant contribution to the PLE spectrum of Fig. 2(a). One explanation for the two main PLE features observed in Fig. 2(a) is that they represent enhancement of donor transitions due to trapping of Xn=2,3 by D +. Subsequently, this complex can transform into (D+,Xn=l) as well as (D°n=l,h). The trapping of Xn=3 apparently is more effective than that of Xn=2, which in turn is much more effective than Xn=1, possibly because the excitonic radius increases with n ( the Bohr radius goes as n 2 14). It is interesting to note that strong overlap exists between the line shapes of Xn=3 and (D°n=3,h) transitions (the peaks are less than 0.1 nm apart), a slightly reduced overlap between Xn=2 and (D°n=2,h) [the peaks are = 0.2 nm apart], and weak overlap between Xn=1 and (D°n=l,h) [the peaks are 1 nm apart]. Thus, a resonant interaction may be operative between X n and (D°n,h) for n=2 and 3. The Xn=3 PLE peak at 816.2 nm in Fig. 2(a) disappears by T=11 K. Presumably this is because of thermal dissociation due to the small (0.6 meV) binding energy. However, a reduced enhancement effect remains, exhibiting a peak at Xn=2 along with the tail previously observed in Fig. 2(a) at short wavelengths. By 25 K, the tail has essentially disappeared. The explanation for the enhancement effect for Ee>Eg is still under study. Perhaps it is related to creation of excitonic excited states with finite kinetic energies although such transitions are expected to be weak, requiring interactions with phonons in order to satmfy conservation of momentum. 15 A further demonstration of the enhancement of donor-related transitions can be obtained from Fig. 3. Here we show 4.2 K PL in the acceptor spectral region for an OMVPE, compensated layer with prominent, acceptor-related, PL features. The excitation wavelengths are ~.e=805 nm (spectrum 1) and 816.2 nm (spectrum 2), similar to those used in Fig. 1. The excitation power density and the signal gain were held constant. The two main peaks both involve transitions which terminate in a carbon-acceptor state (A°c), one from the conduction band [(e,A°c)] and the other from a neutral donor [(D°,A°c)], as indicated in the figure. Comparing the two spectra it can be seen that for ~.e=816.2 nm there is a substantial increase in the amplitude of (D°,A°c) while that of (e,A°c), in fact, decreases. These data along with the corresponding PLE spectra provide strong evidence that the concentration of photo-neutralized donors is increasing while that of photoexcited free electrons is decreasing as ;Le increases toward 816.4 rim.
858
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vol. 70, No. 9
We note that for the uncapped samples where the enhancement was strong, the PL intensity increased for all transitions [e.g., see Fig. l(a)], indicating that the total luminescence intensity had increased. Presumably this is doing so at the expense of competing nonradiative recombination occurring at the surface. Possibly, for sufficiently small Ee, excitations are created which do not lead to energy loss due to nonradiative processes. Instead the excitations (Xn=2,3) are efficiently trapped by localized centers (D ÷) and subsequent energy transfers to radiative states ensue. In summary, we report the observation of an increase in the intensity of donor-related PL features in a variety of undoped GaAs layers (1014-1015 c m -3) during excitation in the vicinity of the band edge. In particular, dramatic enhancements of over two orders of magnitude have been found for a low compensation, p-type sample where, over a range of excitation values, the (D°,h)/(D*,X) feature actually dominated the (A°,X) features in the excitonic region. The strong effects observed for this sample as well as for a thin, low-compensation, n-type layer have been correlated to the presence of ionized donors. The peaks observed in the PLE spectra of (D°,h)/(D+,X) are interpreted in terms of enhancement of donor-related transitions by trapping of Xn=2,3 by ionized donors. Possibly, a resonant interaction is operative between X n and (D°n,h) for n=2,3. The enhancements first become detectable for excitation energies as much as 12 meV about the band gap energy.
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FIG. 3. 4.2 K, acceptor photoluminescence (PL) for a compensated sample. Spectra 1 and 2 are excited at 805 nm and 816.2 nm, respectively. The excitation power density (200 mW/cm 2) and signal gain are kept constant.
Acknowledgement-We thank Drs. C. Jagannath and J. Lee for helpful conversations, Drs. P. Haugsjaa, H. Lockwood, and L. Andrews for their support, Drs. S. K. Shastry, P. Norris and J. BLack for the OMVPE samples, and Drs. J. Salerno and E. Koteles for the MBE sample.
REFERENCES
1. For an absorption spectrum of high purity GaAs, see in C. Weisbuch, Ph.D. thesis, University of Paris, 1977 and M. D. Sturge, in Excitons, edited by E. I. Rashba and M. D. Sturge (North-Holland, Amsterdam, 1982), Chap. 1, p.11. 2. U. Heim and P. Hiesinger, Phys. Stat. Sol. B 66, 461 (1974). 3. J. Black, P. Norris, E. Koteles, and S. Zemon, Inst. Phys. Conf. Ser. 74, 683 (1985). 4. S. K. Shastry, S. Zemon, and P. Norris, Inst. Phys. Conf. Ser. 83, 81 (1987);S. K. Shastry, S. Zemon, D. G. Kenneson, and G. Lambert, Appl. Phys. Lett. 52, 150 (1988). 5. Jack P. Salerno, E. S. Koteles, J. V. Gormley, B. J. Sowell, E. M. Brody, J. Y. Chi, and R. P. Holmstrom, J. Vac. Sci. Technol. B 3, 618 (1985). 6. S. Zemon, P. Norris, and G. Lambert, J. Cryst. Growth 77, 321 (1986). 7. D. D. Sell, Phys. Rev. B 6, 3750 (1972).
8. S. K. Shastry, S. Zemon, D. Dugger. and M. DeAngelis, Inst. Phys. Conf. Ser. 91,307 (1988). 9. S. Zemon, C. Jagannath, S. K. Shastry, W. J. Miniscalco, and G. Lambert, Appl. Phys. Lett. 53, 213 (1988). 10. S. Zemon and G. Lambert (to be published). 11. See, for example, S. Zemon, C. Jagannath, S. K. Shastry, and G. Lambert, Solid State Commun. 85, 553 (1988). 12. R. Ulbrich and B. Moreth, Solid State Commun. 14, 331 (1974). 13. G. E. Stillman, C. M. Wolfe, and J. O. Dimmock, Solid State Commun. 7, 921 (1969). 14. John O. Dimmock, in Semiconductors and Semimetals, edited by R. K. Willardson and Albert C. Beer (Academic, New York, 1967), Vol. 3, p. 259. 15. See, for example, R. J. Elliot, in Polarons and Excitons, edited by C. G. Kuper and G. D. Whitfield (Oliver and Boyd, Edinburgh, 1963), pp. 269-293.
0038-1098/8953.00+.00 Pergamon Press plc
ENHANCEMENT OF DONOR-RELATED PHOTOLUMINESCENCE INTENSITY DUE TO NEAR-BAND-GAP EXCITATION OF HIGH PURITY GaAs S. Zemon and G. Lambert, GTE Laboratories Incorporated, 40 Sylvan Road, Waltham, MA 02254 USA (Received 15 March 1989 by J. Tauc) Striking increases in the intensity of donor-related, photoluminescence transitions are observed in undoped (1014-1015 cm -3) GaAs for excitation energies (Ee) in the vicinity of the band-gap energy (Eg). The enhancement has maxima at E e consistent with excitation of the n=2 and n=3 states of the free exciton (Xn=2,3) and appears to be correlated to the concentration of ionized donors, suggesting that the effects are related to capture of electron-hole pairs by ionized donors through trapping of Xn=2,3. A resonant interaction may be operative as well. The enhancement decreases monotonically as E e increases to values as much as 12 meV above Eg.
pressure organometallic vapor phase epitaxy (OMVPE) and molecular beam epitaxy (MBE) on semi-insulating GaAs substrates by procedures previously described. 3-5 The OMVPE layers were lowcompensation, n-type material [donor concentration (ND) = 1014 cm3>>acceptor concentration (NA)=1013 cm-3] 4 as well as n- and p-type, compensated material (ND=NA=1014 cm-3). 3 The sharpness of the 4.2 K PL and magnetophotoluminescene (MPL) lines confirmed the high purity of the OMVPE layers.3,4, s The layer thicknesses were 1.5-17 I~m. The MBE sample was 2.5-~m thick and p-type. Typical MBE layers grown under conditions similar to those of this sample were p-type with NA=1015 cm3. 5 The low intensity of the donor-related PL features observed during nonresonant excitation of this sample suggests that NA>>ND=I 014 cm-3. The selective PL and PLE measurements were made at 4.2 K with the sample freely suspended in an immersion cryostat as well as at temperatures up to 25 K in a cold finger cryostat. The excitation source was an argon-laser-pumped dye laser (Styryl 9 dye) with a 0.1-nm bandwidth. Wavelengths were measured with a wavemeter and selected values were calibrated with a spectrometer. When required, the laser polarization was adjusted to be circularly polarized and the PL analyzed for right circularly polarized (RCP) and left circularly polarized (LCP) components. Power densities of 0.2-200 mW/cm 2 were employed. The signal was dispersed with a 0.85-m, double-grating spectrometer (0.04-nm resolution) and detected with a photon counting system using a cooled GaAs photocathode photomultiplier. The MPL system has been described in the literature, s
1. Introduction A striking enhancement has been observed in the intensity of donor-related, photoluminescence (PL) transitions in undoped (1014-1015 cm-3), epitaxial GaAs for excitation energies (Ee) in the vicinity of the band-gap energy (Eg). In some cases dramatic intensity increases of over two orders of magnitude have been found with donor-related transitions dominating acceptor-related ones in the excitonic region of even low-compensation, p-type material. At T=4.2 K the effect was maximum when pumping within the gap at Ee=Eg-0.6 meV [consistent with excitation of the n=3 state of the free exciton (Xn=3) 1] and decreased monotonically as E e was increased to values as much as 12 meV above Eg. For T>4.2 K the maximum occurred at an E e corresponding to the n=2 state of the free exciton (Xn=2). 1,2 The enhancement was most readily observed for transitions involving recombination of a free hole with an electron bound to a donor and/or radiative decay of an ionized-donor-bound exciton [(DO,h)/(D+,X)] and appears to be correlated to the concentration of ionized donors (N+o). Enhancements were also observed in a variety of other samples, including GaAs heterostructures and substrates as well as AIGaAs. We suggest that for Ee
3. Experimental Results and Discussion In Fig. 1 we show 4.2 K PL in the excitonic spectral region excited both at 804 nm (spectra labeled as 1) and 816.2 nm (spectra labeled as 2). The identifications of the features are shown at the top of the
2. Experimental Details The nominally undoped, (100)-oriented, GaAs epilayers used in this work were grown by both low855
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WAVELENGTH (nm) FIG. 1 4.2 K photoluminescence (PL) spectra in the excitonic region. (a) A low-compensation, p-type (=1015 cm -3) epilayer. (b) A thin (1.5 ~m), lowcompensation, n-type (=1014 cm-3)epilayer. Spectra 1 and 2 were excited at 804 nm and 816.2 nm, respectively. An excitation power of 200 mW/cm 2 was used in (a) and 20 mW/cm 2 in (b).
figure. 2 The laser line due to scattered pump light is indicated by an arrow in spectrum 2 of Fig. l(a). Also indicated there as a dashed line is the position of the band gap at ~.g=815.9 nm (1.5192 eV 7). Figure l(a) shows spectra for the p-type, low-compensation, MBE sample. In spectrum 1 of Fig. l(a) we observe only two peaks, one at the intrinsic free exciton feature Xn=1 and the other at the extrinsic acceptor feature (A°,X) [neutral-acceptor-bound exciton], as expected for nonresonant excitation of a high purity sample with NA (=1015 cm-3)>>Nb. No donor-related features were detected. However, for excitation at 816.2 nm in spectrum 2 a dramatic change is found to occur with the donor feature (D°,h)/(D+,X) emerging as dominant and the neutral donor feature (D°,X) becoming detectable. [Donor-to-carbon-acceptor transitions, to be discussed later, also become detectable.] Since all the donors are ionized (D +) in this p-type material, the D+ concentration N+D=ND=1014 cm -3. The ionized donors are clearly playing a crucial role in the enhancement process both by becoming photo-neutralized and, possibly, by becoming incorporated into bound excitons. In contrast, for a thick (9 p.m), n-type, low-compensation layer, where donor-related PL features dominate for nonresonant excitation (as expected), only a relatively small in-
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crease in (D°,h)/(D÷,X) occurs for excitation at 816.2 nm. This reduced effect is consistent with the fact that most of the donor concentration (=1014 cm 3) is expected to be neutral. However, for a layer similar to the latter but with a reduced thickness of 1.5 #m, a substantial enhancement effect is observed. This is shown in Fig. l(b) where we note that (D°,h)/(D+,X) undergoes an increase of = 50x. (D°,X) also increases but to a lesser extent, similar to the trend found in Fig.l(a). This enhancement effect is also consistent with the picture of the importance of N+o. Here the presence of a significant ionized donor concentration is explained by the fact that the 1.5-1~mthick layer is thin enough so that depletion effects would be expected to be substantial. Enhancements of donor-related PL features were observed in a variety of other samples, e.g., (1) low compensation and compensated, n-type layers (thicknesses 0.5-2 #m) capped with AIGaAs window layers (thicknesses 0.3-7.5 I~m), (2) the 1.5-~m-thick, GaAs layer of a selectively-doped heterostructure, 8 (3) undoped GaAs substrates capped with AIGaAs, (4) GaAs grown on Si substrates, 9 and (5) AlxGal_xAS layers with x<0.02. The details will be reported elsewhere. In a magnetic field, where the degeneracy between the (D°,h) and (D+,X) peaks is substantially removed, the two types of transitions can be distinguished for samples with sufficiently sharp PL features, e.g., the high purity OMVPE epilayers, lo A compensated, OMVPE sample was tested in a magnetic field of 6.4 T at 4.2 K and an enhancement was found in (D+,X), in addition to (D°,h), for E e in the vicinity of the lowest, Landau-level, interband transition energy. When exciting the sample of Fig. l(a) with circularly polarized light (say LCP) at 4.2 K, the luminescence was unpolarized for excitation wavelengths ~.e<822 nm. However, for 822 nm<~.e<816.2 nm the RCP component of (D°,h)/(D+,X) was greater than the LCP component while the Xn= 1, (D°,X) and (A°,X) features remained unpolarized. For example, at Xe=816.2 nm (the peak of the polarized PLE spectra) the RCP component of (D°,h)/(D+,X) was =40% greater than the LCP. A similar result was observed for the sample of Fig. l(b). This polarization memory indicates that there is strong coupling between a state initially photoexcited and the corresponding emission state. 11 To examine the wavelength dependence of the enhancement in detail, we present in Fig. 2(a) a PLE spectrum of the sample of Fig. l(a) where the spectrometer is set at ~.s=819.1nm for (D°,h)/(D+,X) and, for comparison, in Fig. 2(b) at ~.s=819.6 nm for (A°,X) in the J=3/2 state. For reference the wavelength dependence of the dye laser output is included in Fig. 2(b). The major features are labeled.I, 2 As expected for the PLE of (A°,X), the spectrum of Fig. 2(b) has prominent peaks at the excitonic features Xn=2, Xn=l, (D°,X), and (A°,X) in the J=1/2 state as well as a weak feature at Xn=3.1,2 [The Xn=2 and Xn=3 peaks are observed as well on the PLE spectra for Xn=l and (D°,X).] In marked contrast, the spectrum of Fig. 2(a)
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820
WAVELENGTH (nm)
FIG. 2 4.2 K photoluminescence excitation (PLE) spectra for the sample of Fig. l(a) where ~,s denotes the spectrometer setting. The wavelength dependence of the laser excitation power (=200 mW/cm 2) is included in (b).
rises monotonically [i.e., the (D°,h)/(D+,X) feature grows] with increasing excitation wavelength to a maximum at 816.2 nm which is equivalent to an energy 0.6 meV below Eg [Xg is denoted by a dashed line in Fig. 2(a) and (b)] and, as discussed below, is identified with excitation of Xn=3. The full width at half maximum of the main PLE band is =2 meV, the enhancement first becoming detectable as much as 12 meV above Eg. Similar spectra were observed for power densities as low as 0.2 mW/cm 2. The prominent shoulder at =816.5 nm is better resolved in the higher purity OMVPE samples and can be assigned to excitation of Xn=2. The low-amplitude shoulder at =816.9 nm, which is in close correspondence to a PLE feature in Fig. 2(b) and a PL feature observed in Fig. l(a), appears to be = 0.2 nm longer than (D°n=2,h). Finally, low-amplitude peaks are seen for Xn= 1 and (D°,X). No resonant enhancement of the (D°,h)/(D+,X) feature was observed, in agreement with earlier observations. 12 From selectively excited PL spectra we find that pumping the Xn=2 transition has a stronger enhancement effect for (D°,h)/(D+,X) than for the other excitonic features, while the opposite is true when pumping the Xn= 1 transition. PLE data similar to Fig. 2(a) were observed for all the other homoepitaxial layers studied (with some differences for thick layers of low compensation, n-type material where the enhancement effect was small). Variations, however, occurred in the strength of the enhancement.
INTENSITY
857
Within experimental error a PLE feature at 816.2 nm is consistent not only with excitation of Xn=3 (as indicated above) but also with excitation of an electron from the valence band into an n=3 state of a donor. 13 However, little or no response was found in the PLE spectrum of (D°n=I,h)/(D+,X) when pumping at the wavelength corresponding to (D°n=2,h) at 816.8 nm [e.g., see Fig. 2(a)] or resonant excitation of the line itself. 12 Furthermore, no (D°n,h) absorption peaks have been reported in the literature for high purity GaAs. 1 Thus, we assume that direct excitation of (D°n=3,h) does not make a significant contribution to the PLE spectrum of Fig. 2(a). One explanation for the two main PLE features observed in Fig. 2(a) is that they represent enhancement of donor transitions due to trapping of Xn=2,3 by D +. Subsequently, this complex can transform into (D+,Xn=l) as well as (D°n=l,h). The trapping of Xn=3 apparently is more effective than that of Xn=2, which in turn is much more effective than Xn=1, possibly because the excitonic radius increases with n ( the Bohr radius goes as n 2 14). It is interesting to note that strong overlap exists between the line shapes of Xn=3 and (D°n=3,h) transitions (the peaks are less than 0.1 nm apart), a slightly reduced overlap between Xn=2 and (D°n=2,h) [the peaks are = 0.2 nm apart], and weak overlap between Xn=1 and (D°n=l,h) [the peaks are 1 nm apart]. Thus, a resonant interaction may be operative between X n and (D°n,h) for n=2 and 3. The Xn=3 PLE peak at 816.2 nm in Fig. 2(a) disappears by T=11 K. Presumably this is because of thermal dissociation due to the small (0.6 meV) binding energy. However, a reduced enhancement effect remains, exhibiting a peak at Xn=2 along with the tail previously observed in Fig. 2(a) at short wavelengths. By 25 K, the tail has essentially disappeared. The explanation for the enhancement effect for Ee>Eg is still under study. Perhaps it is related to creation of excitonic excited states with finite kinetic energies although such transitions are expected to be weak, requiring interactions with phonons in order to satmfy conservation of momentum. 15 A further demonstration of the enhancement of donor-related transitions can be obtained from Fig. 3. Here we show 4.2 K PL in the acceptor spectral region for an OMVPE, compensated layer with prominent, acceptor-related, PL features. The excitation wavelengths are ~.e=805 nm (spectrum 1) and 816.2 nm (spectrum 2), similar to those used in Fig. 1. The excitation power density and the signal gain were held constant. The two main peaks both involve transitions which terminate in a carbon-acceptor state (A°c), one from the conduction band [(e,A°c)] and the other from a neutral donor [(D°,A°c)], as indicated in the figure. Comparing the two spectra it can be seen that for ~.e=816.2 nm there is a substantial increase in the amplitude of (D°,A°c) while that of (e,A°c), in fact, decreases. These data along with the corresponding PLE spectra provide strong evidence that the concentration of photo-neutralized donors is increasing while that of photoexcited free electrons is decreasing as ;Le increases toward 816.4 rim.
858
ENHANCEMENT OF DONOR-RELATED PHOTOLUMINESCENCE
(e, A~lg/Be)--] F (DO,A~)
I
(/) I-
E, >rt,"
,,¢ E IE
,,¢ >I(/) Z I,LI I-Z m
..I o,.
I
I 831
I
WAVELENGTH
(nm)
vol. 70, No. 9
We note that for the uncapped samples where the enhancement was strong, the PL intensity increased for all transitions [e.g., see Fig. l(a)], indicating that the total luminescence intensity had increased. Presumably this is doing so at the expense of competing nonradiative recombination occurring at the surface. Possibly, for sufficiently small Ee, excitations are created which do not lead to energy loss due to nonradiative processes. Instead the excitations (Xn=2,3) are efficiently trapped by localized centers (D ÷) and subsequent energy transfers to radiative states ensue. In summary, we report the observation of an increase in the intensity of donor-related PL features in a variety of undoped GaAs layers (1014-1015 c m -3) during excitation in the vicinity of the band edge. In particular, dramatic enhancements of over two orders of magnitude have been found for a low compensation, p-type sample where, over a range of excitation values, the (D°,h)/(D*,X) feature actually dominated the (A°,X) features in the excitonic region. The strong effects observed for this sample as well as for a thin, low-compensation, n-type layer have been correlated to the presence of ionized donors. The peaks observed in the PLE spectra of (D°,h)/(D+,X) are interpreted in terms of enhancement of donor-related transitions by trapping of Xn=2,3 by ionized donors. Possibly, a resonant interaction is operative between X n and (D°n,h) for n=2,3. The enhancements first become detectable for excitation energies as much as 12 meV about the band gap energy.
(e, ~ )
827
INTENSITY
835
FIG. 3. 4.2 K, acceptor photoluminescence (PL) for a compensated sample. Spectra 1 and 2 are excited at 805 nm and 816.2 nm, respectively. The excitation power density (200 mW/cm 2) and signal gain are kept constant.
Acknowledgement-We thank Drs. C. Jagannath and J. Lee for helpful conversations, Drs. P. Haugsjaa, H. Lockwood, and L. Andrews for their support, Drs. S. K. Shastry, P. Norris and J. BLack for the OMVPE samples, and Drs. J. Salerno and E. Koteles for the MBE sample.
REFERENCES
1. For an absorption spectrum of high purity GaAs, see in C. Weisbuch, Ph.D. thesis, University of Paris, 1977 and M. D. Sturge, in Excitons, edited by E. I. Rashba and M. D. Sturge (North-Holland, Amsterdam, 1982), Chap. 1, p.11. 2. U. Heim and P. Hiesinger, Phys. Stat. Sol. B 66, 461 (1974). 3. J. Black, P. Norris, E. Koteles, and S. Zemon, Inst. Phys. Conf. Ser. 74, 683 (1985). 4. S. K. Shastry, S. Zemon, and P. Norris, Inst. Phys. Conf. Ser. 83, 81 (1987);S. K. Shastry, S. Zemon, D. G. Kenneson, and G. Lambert, Appl. Phys. Lett. 52, 150 (1988). 5. Jack P. Salerno, E. S. Koteles, J. V. Gormley, B. J. Sowell, E. M. Brody, J. Y. Chi, and R. P. Holmstrom, J. Vac. Sci. Technol. B 3, 618 (1985). 6. S. Zemon, P. Norris, and G. Lambert, J. Cryst. Growth 77, 321 (1986). 7. D. D. Sell, Phys. Rev. B 6, 3750 (1972).
8. S. K. Shastry, S. Zemon, D. Dugger. and M. DeAngelis, Inst. Phys. Conf. Ser. 91,307 (1988). 9. S. Zemon, C. Jagannath, S. K. Shastry, W. J. Miniscalco, and G. Lambert, Appl. Phys. Lett. 53, 213 (1988). 10. S. Zemon and G. Lambert (to be published). 11. See, for example, S. Zemon, C. Jagannath, S. K. Shastry, and G. Lambert, Solid State Commun. 85, 553 (1988). 12. R. Ulbrich and B. Moreth, Solid State Commun. 14, 331 (1974). 13. G. E. Stillman, C. M. Wolfe, and J. O. Dimmock, Solid State Commun. 7, 921 (1969). 14. John O. Dimmock, in Semiconductors and Semimetals, edited by R. K. Willardson and Albert C. Beer (Academic, New York, 1967), Vol. 3, p. 259. 15. See, for example, R. J. Elliot, in Polarons and Excitons, edited by C. G. Kuper and G. D. Whitfield (Oliver and Boyd, Edinburgh, 1963), pp. 269-293.