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The influence of exciton migration on photoluminescence lifetime in growth-interrupted GaAsAlAs single quantum wells
MA11mlALS SCIENCE& ENGINEERING Materials Scienceand EngineeringB35 (1995) 125-128
ELSEVIER
B
The influence of exciton migration on photoluminescence lifetime in growt]h-interrupted GaAs/A1As single quantum wells ......
Haiping Yu, Ray Murray
Interdisciplinary Research Centrefor Semiconductor Materials, The Blackett Laboratory, Imperial College, London SW7 2AZ, UK
Abstract We have used resonant and non-resonant, low temperature, picosecond time-resolved spectroscopy to investigate exciton migration in a GaAs/A1As single quantum well grown by molecular beam expitaxy with growth interruption and post-growth hydrogen passivation. Excitons migrate with a time constant around 1500ps from the narrower to the wider well regions depopulating the narrower regions in favour of the wider regions and this results in a faster decay time. This process is also demonstrated by the photoluminescence spectrum with a large enhancement in the emission intensity of the wider regions when the narrower well regions are resonantly excited. Keywords: Exciton migration; GaAs/A1As; Resonant enhancement
I. Introduction The behaviour of excitons is of great importance for the operation of some electro-optic devices. Over the last decade there have been many investigations of exciton behaviour in quantum wells (QWs) [1-9]. Since excitons are readily trapped by impurities and interface defects, an investigation of intrinsic excitonic properties requires high quality QWs with smooth interfaces. It has been demonstrated in the GaAs/(AI,Ga)As system that growth interruption (GI) results in smooth, abrupt interfaces [10-15], although usually at the expense of impurity incorporation, ttowever, a post-growth anneal in a hydrogen plasma recovers the internal quantum efficiency to values comparable with those of conventionally grown material by passivating CAs acceptors and deep levels associated with O impurities incorporated during GI [14-17]. This would appear to provide ideal material for a study of exciton behaviour. In this paper we report time-resolved photoluminescence (PL) measurements, obtained: from a GI hydrogenated GaAs/A1As single QW (SQW), which exhibits distinct emission peaks from regions of the QW differing in width by a monolayer (ML). The best fit to the decay curves is found by assuming excitons migrate from the narrower to the wider IVlL regions. There have been 0921-5107/95/$09.50 © 1995-- Elsevier ScienceS.A. All rights reserved SSDI 0921-5107(95)01417-9
previous reports of exciton migration in GI GaAs/ (AI,Ga)As QWs using up-conversion [18] or time-correlated single-photon counting [19] but values of the time constant associated with the migration differ by more than an order of magnitude.
2. Experimental details The GaAs/AIAs SQWs were grown by molecular beam epitaxy (MBE) in a Vacuum Generators V80H machine and consist of a nominal 2 2 M L (62.3/~) GaAs well with 27 ML AlAs barriers grown on a 6 ML GaAs/6 ML AlAs superlattice at 630 °C. The reflection high energy electron diffraction conditions were carefully chosen so that a maximum in the reflected signal corresponded to the completion of an ML. This sample was GI for 120 s at both the inverted (GaAs on AlAs) and normal (AlAs on GaAs) interfaces and a sample cut from the wafer was then annealed at 280 °C in a hydrogen plasma. After this treatment the luminescence signal increased by an order of magnitude, comparable with the values of a conventionally (continuously) grown sample, but the narrow, discrete PL lines attributed to ML splitting were unchanged in energy and relative intensity, showing that the hydrogenation treatment does not degrade the interfaces.
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H. Yu, R. Murray / Materials Science and Engineering B35 (1995) 125-128
Fig. 1 shows the low temperature (10 K) PL obtained from this sample. Three well-resolved, narrow (full width at half-maximum, about 1 meV) peaks are observed, indicative of atomically smooth interface regions which extend over areas large compared with the exciton diameter (about 150/~) separated by ML steps. Using a finite square well model we attribute these peaks to regions of the SQW with thicknesses of 20, 21 and 22 MLs. We have grown nominally identical samples on different growth runs and in different MBE machines and always find the peak energies to be the same to better than 1 meV. The relative intensities of the peaks change at different points on the sample because the laser spot (about 100/~m) probes different average thicknesses owing to unavoidable flux variations in the MBE machine even when the substrate is rotated. An Ar ÷ pumped continuous-wave Ti:sapphire (Spectra-Physics model 3900) or mode-locked Ti:sapphire (Spectra-Physics Tsunami) laser system was used as an excitation light source for the time-integrated and timeresolved measurements. The sample was held at 10 K in a closed-cycle helium cryostat and a 0.85 m doublegrating spectrometer (SPEX model 1404) was used to disperse the luminescence which was detected by a thermoelectrically cooled GaAs photomultiplier using standard photon counting techniques. Decay time and time-resolved PL (TRPL) measurements were carried out using time-correlated single-photon counting with the system time resolution estimated to be about 30 ps. Details of the sample preparation and experimental set-up can be found elsewhere [15]. A well-established deconvolution program was used to analyse the decay curves [20].
21 ML 22 ML
A
d • E
C
20 ML
m
..I
J
I 1.60
I
I 1.61
I
I 1.62
I
I
I
1.63
I 1.64
Emission Energy (eV) Fig. 1. Low temperature (10 K) PL spectrum obtained from GI, hydrogenated GaAs/A1As SQW showing peaks due to recombination in the 20, 21, 22 ML regions of the QW. PL excitation measurements (not shown) show there is a negligible Stokes shift, indicating that the excitons are delocalized.
that the difference in the decay times is not simply the migration time. For the excitation energy used here (1.675 eV), electron-hole pairs are created in the well only and rapidly (less than 20ps) lose energy by the emission of I0000
c
3. Results and discussion
Fig. 2 shows the decay curves at 10 K for the PL emission peaks shown in Fig. 1. An analysis gives values of radiative recombination time rr at this temperature of 160ps, 240ps and 360ps for the 20 ML, 21 ML and 22 ML regions respectively of the SQW. Since the emission occurs in the same SQW differences for the ML regions cannot be attributed to interface trapping [8,9] or non-radiative recombination which is known to be negligible in these samples at temperatures below 70K [15]. Also, the differences of about 100 ps ML -1 cannot be accounted for by differences in the QW thickness which are expected to amount to only 10 ps ML -1 [7,21] and this casts further doubt on a simple analysis based on only radiative recombination and/or thermalization processes. It has been suggested that differences in the decay times for ML flat QWs are caused by excitons migrating from the narrower to the wider well regions [18], but it should be pointed out
I000
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lOO
_e .J n
io
I
I
I
I
I
I
1
2
3
4
5
Time (ns) Fig. 2. Decay curves from the 20 ML (curve a), the 21 ML (curve b) and the 22 ML (curve c) region emissions for the same sample at 10K.
H. Yu, R. Murray / Materials Science and Engineering B35 (1995) 125-128
phonons, forming excitons with large Kii, the in-plane momentum vector [1]. The consensus is that the exciton population then forms a thermalized distribution at the lattice temperature. Several groups have attempted to measure the time constant associated with this step and values in the range 80-.400 ps [1,18] at low temperatures have been reported. Only a fraction of the thermalized population with Kib <~nCOo/C are able to recombine [7] with a radiative lifetime which depends on the sample temperature but is predicted to be about 300ps at 10K [6]. However, the time-resolved data should contain information on all the processes leading to exciton decay, including migration, which depopulates the narrower regions in favour of the wider regions and leads to a fast decay time. Thus a global analysis of the three decay curves has been applied [24]; I , ( t ) = A , e -~/~th - B , e -~/~r -- Cne-r/Vmig (n = 20, 21, 22 MLs), where A, B and C reflect the relative importance of each process to the decay, which gives the 10 K values of thermalization time constant "tthand radiative recombination time constant zr of 190ps and 315ps respectively. These values apply to all three decay curves and are in good agreement with previous reports [1,7,8,18,21,23], while the value of rm~gof 1500 ps, associated with exciton migration, is in reasonable agreement with that found earlier by Kohl et al. (2100 ps) [19] but is considerably larger than that reported by Deveaud et al. (150ps) [18]. The latter result was obtained from QW of similar width (17 MLs) and so differences in ~th cannot be attributed to thickness effects. To confirm the above global analysis, a TRPL experiment was performed. Fig. 3 shows the plot of the TRPL spectra at different delay times after the laser pulse (1.675 eV). Close to zero delay, the spectrum consists of two weak peaks at 1.613 and 1.62 eV, corresponding to the 21 and 22 ML regions. With increasing delay time the intensities of both peaks rise as the excitons thermalize to the radiative state close to KII ~ O, but the strength of the 21 ML peak increases more rapidly than that of the 22 ML peak. After about 1000 ps, the two peaks are of comparable intensity and ratio of the intensities c f the 22 ML and 21 ML peaks reaches a maximum of about 2 after 1500 ps. The ratio remains almost constant with further increasing delay, although an accurate e.stimate of the ratio is difficult since the PL intensities are now weak. The changes in the relative peak intensities with delay in Fig. 3 are consistent with exciton migration from the 21 ML to the 22 ML regions, and this process is substantially complete after about 1500 ps, which is the time constant for the migratiorL time at low temperature deduced from the global a~aalysis of all three decay curves. An estimate of the lateral size of the ML regions may be obtained using the Einstein relation assuming a value for the exciton diffusion constant. Reported val-
22 ML ~.ex=740nm ~
127
21 ML ~
20 .ML ~
~t (ps) 2880 2640 2400 2160
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1920 1680
~
1440
~
1200 ~
960 720 480
240 0
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I
1.605 1.61 1.615 1,62 1.625 1.63 Emission Energy (eV) Fig. 3, TRPL spectra (10 K) for the same sample as in Fig. 2 demonstrating exciton migration from the narrower to the wider well regions. Note that the ratio of PL intensities for two peaks (22 to 21 ML) reaches its maximum after a 1500 ps delay.
ues vary from 15cms -~ [22] to 50cms -~ [25] and imply lateral size in the range 1.5-2.5 Ixm in good agreement with recent scanning tunnelling microscopy studies [26]. Exciton migration is also evident under the conditions of resonant excitation. Fig. 4 shows the polarized PL spectra at different excitation energies with an z(x, y)~ configuration, in which a nearly backscattering geometry was used: the laser polarization was parallel to the (011) direction (i.e. x) and analyser parallel to (011) (i.e. y). The very narrow peak (0.2meV) is the laser scattering signal through the analyser for reference, which is denoted by an asterisk. Under non-resonant excitation (Fig. 4, spectrum a), two PL peaks of nearly equal intensity are. observed at 1.612 and 1.605 eV, corresponding to exciton recombination in the 22 and 23 ML well regions. (This is not the same area of the wafer as that in Figs. 1 and 2, where three peaks of 20, 21 and 22 ML are seen. This difference is due to the flux variation across the wafer during MBE growth.) Here the laser excitation energy was about 1.625 eV (indicated by an asterisk), some 13 meV higher in energy than the 22 ML QW region band gap. Using laser excitation at 1.612 eV, resonant with the 22 ML peak as shown in Fig. 4, spectrum b, the PL signal from the 22 ML well is enhanced by a factor of about 20 (there is a minor contribution from the scattered light which might be about 10% from consideration of spectrum a). Here the experimental conditions
128
H. Yu, R. Murray / Materials Science and Engineering B35 (1995) 125 128
are identical to those for spectrum a except for the excitation wavelength. Importantly, the PL emission from the 23 M L regions has also been enhanced by a factor of about 15. For the 22 M L luminescence, we attribute the enhancement to the efficient generation of cold excitons (in-plane wavevector KIj ~ 0) in this well region by the laser, compared with the case of spectrum a where the excitons created by the laser were " h o t " (gll > 0). Only cold excitons with Kit ~ 0 can recombine [7]. Enhancement of the 23 M L peak can only occur if there is exciton migration from the narrower to the wider well width regions. In addition, a new feature appears, about 1.5 meV lower in energy than the main free exciton peaks. This may arise from bound excitons trapped by impurities such as carbon. However, these features are not evident in spectrum a of Fig. 4. Alternatively, recombination of biexcitons may be responsible for this extra feature.
4. Conclusions In conclusion we have demonstrated exciton migration in a very high quality GaAs/A1As SQW using decay time measurements, T R P L and resonant PL. The time constant associated with this process has a value of 1500 ps and has a significant effect on the decay times for the different M L regions, which could be misinterpreted as a change in the radiative lifetime.
,a,
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I 1.59
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I 1.60
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Emission Energy (eV)
Fig. 4. PL spectra and decay times for z(x, y)5 configurations at exciton energies Eex detuning to the QW band gap Eg at E~x- E22ML= 13 meV (spectrum a) and Eex- E22ML= 0 meV (spectrum b), the resonant excitation. The experimental conditions are kept identical for both spectrum a and spectrum b, except the exciting wavelength. The very narrow peak (0.2 meV) is from the laser scattering through the analyser, which is denoted by an asterisk.
Acknowledgements We would like to thank Dr. G. Rumbles, Dr. J.J. Harris, Dr. I. Galbraith and Professor A. Miller for stimulating discussions. Dr. B. Crystall and Dr. M. Carey are thanked for help with the data analysis. EPSRC (Grant no. GR/J97540) are thanked for financial support.
References [1] T.C. Damen, J. Shah, D.Y. Oberli, D.S. Chemla, J.E. Cunningham and J.M. Kuo, Phys. Rev. B, 42 (1990) 7434. [2] B. Deveaud, B. Sermage and D.S. Katzer, J. Phys. (Paris) IV, 3 (1993) 11. [3] A. Vinattieri, J. Shah, T.C. Damen, D.S. Kim, L.N. Pfeiffer, M.I. Maialle and L.J. Sham, Phys. Rev. B, 50 (1994) 10808. [4] M.J. Snelling, P. Perozzo, D.C. Hutchings, I. Galbraith and A. Miller, Phys. Rev. B, 49 (1994) 17160. [5] T.C. Damen, K. Leo, J. Shah and .I.E. Cunningham, Appl. Phys. Lett., 58 (1991) 1902. [6] L.C. Andreanni, F. Tassone and F. Bassani, Solid State Commun., 77 (1991) 641. [7] J. Feldmann, G. Peter, E.O. Gfbel, P. Dawson, K. Moore, T. Foxon and R.J. Elliot,' Phys. Rev. Lett., 59 (1987) 2337. [8] M. Colocci, M. Gurioli and A. Vinattieri, J. Appl. Phys., 68 (1990) 2809. [9] D.S. Citrin, Solid State Commun., 84 (1992) 281; Phys. Rev. B, 47 (1993) 3832. [10] H. Sakaki, M. Tanaka and T. Matsusue, Jpn. J. Appl. Phys., 24 (1985) IAI7. [11] R.C. Miller, C.W. Tu, S.K. Sputz and R.F. Kopf, Appl. Phys. Lett., 49 (1986) 1246. [12] K. Fujiwara, K. Kanamoto and N. Tsukada, Phys. Rev. B, 40 (1989) 9689. [13] A. Yoshinaga, P.B. Mookherjee, R. Murray, J.H. Neave and B.A. Joyce, J. Cryst. Growth, 127 (1993) 788. [14] D. Bimberg, D. Mars, J.N. Miller, R. Bauer and D. Oertel, J. Vac. Sci. Technol. B, 4 (1986) 1014. [15] H. Yu, P.B. Mookherjee, R. Murray and A. Yoshinaga, J. Appl. Phys., 77 (1995) 1217. [16] T. Someya, H. Akiyama, Y. Kadoya, T. Noda, T. Matsusue, H. Noge and H. Sakaki, Appl. Phys. Lett., 63 (1993) 1924. [17] M. Gurioli, A. Vinattieri, M. Colocci, C. Deparis, J. Massies, G. Neu, A. Bosacchi and S. Franchi, Phys. Rev. B, 44 (1991) 3115. [18] B+ Deveaud, T.C. Damen, J. Shah and C.W. Tu, Appl. Phys. Lett., 51 (1987) 828. B. Deveaud, A. Chomette, N. Roy, B. Sermage and D.S. Katzer, Surf Sci., 267 (1992) 199. [19] M. Kohl, D. Heitmann, S. Tarucha, K. Leo and K. Ploog, Phys. Rev. B, 39 (1989) 7736. [20] H. Yu, E. ,ioslin, B. Crystall, W. Sinclair, T. Smith and D. Phillips, J. Phys. Chem., 97 (1993) 8146. [21] ,i. Martinez-Pastor, A. Vinattieri, L. Carraresi, M. Colocci, Ph. Roussignol and G. Weimann, Phys. Rev. B, 47 (1993) 10456. [22] ,i. Hegarty, L. Goldner and M.D. Sturge, Phys. Rev. Lett., 30 (1984) 7346. [23] J. Feldmann, G. Peter, E.O. G6bel, K. Leo, H.-,I. Polland, K. Ploog, K. Fujiwara and T. Nakayama, Appl. Phys. Lett., 51 (1987) 226. [24] D.V. O'Connor and D. Phillips, Time-correlated Single Photon Counting, Academic Press, London, 1984, p. 176. [25] Y. Takahashi, S.S. Kano, K. Muraki, S. Fukatsu, Y. Shiraki and R. Ito, dppl. Phys. Lett., 64 (1994) 1845. [26] J. Sudijono, M.D. Johnson, C.W. Snyder and B.G. Orr, J. Vac. Sci. Technol. B, 11 (1993) 775.
ELSEVIER
B
The influence of exciton migration on photoluminescence lifetime in growt]h-interrupted GaAs/A1As single quantum wells ......
Haiping Yu, Ray Murray
Interdisciplinary Research Centrefor Semiconductor Materials, The Blackett Laboratory, Imperial College, London SW7 2AZ, UK
Abstract We have used resonant and non-resonant, low temperature, picosecond time-resolved spectroscopy to investigate exciton migration in a GaAs/A1As single quantum well grown by molecular beam expitaxy with growth interruption and post-growth hydrogen passivation. Excitons migrate with a time constant around 1500ps from the narrower to the wider well regions depopulating the narrower regions in favour of the wider regions and this results in a faster decay time. This process is also demonstrated by the photoluminescence spectrum with a large enhancement in the emission intensity of the wider regions when the narrower well regions are resonantly excited. Keywords: Exciton migration; GaAs/A1As; Resonant enhancement
I. Introduction The behaviour of excitons is of great importance for the operation of some electro-optic devices. Over the last decade there have been many investigations of exciton behaviour in quantum wells (QWs) [1-9]. Since excitons are readily trapped by impurities and interface defects, an investigation of intrinsic excitonic properties requires high quality QWs with smooth interfaces. It has been demonstrated in the GaAs/(AI,Ga)As system that growth interruption (GI) results in smooth, abrupt interfaces [10-15], although usually at the expense of impurity incorporation, ttowever, a post-growth anneal in a hydrogen plasma recovers the internal quantum efficiency to values comparable with those of conventionally grown material by passivating CAs acceptors and deep levels associated with O impurities incorporated during GI [14-17]. This would appear to provide ideal material for a study of exciton behaviour. In this paper we report time-resolved photoluminescence (PL) measurements, obtained: from a GI hydrogenated GaAs/A1As single QW (SQW), which exhibits distinct emission peaks from regions of the QW differing in width by a monolayer (ML). The best fit to the decay curves is found by assuming excitons migrate from the narrower to the wider IVlL regions. There have been 0921-5107/95/$09.50 © 1995-- Elsevier ScienceS.A. All rights reserved SSDI 0921-5107(95)01417-9
previous reports of exciton migration in GI GaAs/ (AI,Ga)As QWs using up-conversion [18] or time-correlated single-photon counting [19] but values of the time constant associated with the migration differ by more than an order of magnitude.
2. Experimental details The GaAs/AIAs SQWs were grown by molecular beam epitaxy (MBE) in a Vacuum Generators V80H machine and consist of a nominal 2 2 M L (62.3/~) GaAs well with 27 ML AlAs barriers grown on a 6 ML GaAs/6 ML AlAs superlattice at 630 °C. The reflection high energy electron diffraction conditions were carefully chosen so that a maximum in the reflected signal corresponded to the completion of an ML. This sample was GI for 120 s at both the inverted (GaAs on AlAs) and normal (AlAs on GaAs) interfaces and a sample cut from the wafer was then annealed at 280 °C in a hydrogen plasma. After this treatment the luminescence signal increased by an order of magnitude, comparable with the values of a conventionally (continuously) grown sample, but the narrow, discrete PL lines attributed to ML splitting were unchanged in energy and relative intensity, showing that the hydrogenation treatment does not degrade the interfaces.
126
H. Yu, R. Murray / Materials Science and Engineering B35 (1995) 125-128
Fig. 1 shows the low temperature (10 K) PL obtained from this sample. Three well-resolved, narrow (full width at half-maximum, about 1 meV) peaks are observed, indicative of atomically smooth interface regions which extend over areas large compared with the exciton diameter (about 150/~) separated by ML steps. Using a finite square well model we attribute these peaks to regions of the SQW with thicknesses of 20, 21 and 22 MLs. We have grown nominally identical samples on different growth runs and in different MBE machines and always find the peak energies to be the same to better than 1 meV. The relative intensities of the peaks change at different points on the sample because the laser spot (about 100/~m) probes different average thicknesses owing to unavoidable flux variations in the MBE machine even when the substrate is rotated. An Ar ÷ pumped continuous-wave Ti:sapphire (Spectra-Physics model 3900) or mode-locked Ti:sapphire (Spectra-Physics Tsunami) laser system was used as an excitation light source for the time-integrated and timeresolved measurements. The sample was held at 10 K in a closed-cycle helium cryostat and a 0.85 m doublegrating spectrometer (SPEX model 1404) was used to disperse the luminescence which was detected by a thermoelectrically cooled GaAs photomultiplier using standard photon counting techniques. Decay time and time-resolved PL (TRPL) measurements were carried out using time-correlated single-photon counting with the system time resolution estimated to be about 30 ps. Details of the sample preparation and experimental set-up can be found elsewhere [15]. A well-established deconvolution program was used to analyse the decay curves [20].
21 ML 22 ML
A
d • E
C
20 ML
m
..I
J
I 1.60
I
I 1.61
I
I 1.62
I
I
I
1.63
I 1.64
Emission Energy (eV) Fig. 1. Low temperature (10 K) PL spectrum obtained from GI, hydrogenated GaAs/A1As SQW showing peaks due to recombination in the 20, 21, 22 ML regions of the QW. PL excitation measurements (not shown) show there is a negligible Stokes shift, indicating that the excitons are delocalized.
that the difference in the decay times is not simply the migration time. For the excitation energy used here (1.675 eV), electron-hole pairs are created in the well only and rapidly (less than 20ps) lose energy by the emission of I0000
c
3. Results and discussion
Fig. 2 shows the decay curves at 10 K for the PL emission peaks shown in Fig. 1. An analysis gives values of radiative recombination time rr at this temperature of 160ps, 240ps and 360ps for the 20 ML, 21 ML and 22 ML regions respectively of the SQW. Since the emission occurs in the same SQW differences for the ML regions cannot be attributed to interface trapping [8,9] or non-radiative recombination which is known to be negligible in these samples at temperatures below 70K [15]. Also, the differences of about 100 ps ML -1 cannot be accounted for by differences in the QW thickness which are expected to amount to only 10 ps ML -1 [7,21] and this casts further doubt on a simple analysis based on only radiative recombination and/or thermalization processes. It has been suggested that differences in the decay times for ML flat QWs are caused by excitons migrating from the narrower to the wider well regions [18], but it should be pointed out
I000
d mall*
lOO
_e .J n
io
I
I
I
I
I
I
1
2
3
4
5
Time (ns) Fig. 2. Decay curves from the 20 ML (curve a), the 21 ML (curve b) and the 22 ML (curve c) region emissions for the same sample at 10K.
H. Yu, R. Murray / Materials Science and Engineering B35 (1995) 125-128
phonons, forming excitons with large Kii, the in-plane momentum vector [1]. The consensus is that the exciton population then forms a thermalized distribution at the lattice temperature. Several groups have attempted to measure the time constant associated with this step and values in the range 80-.400 ps [1,18] at low temperatures have been reported. Only a fraction of the thermalized population with Kib <~nCOo/C are able to recombine [7] with a radiative lifetime which depends on the sample temperature but is predicted to be about 300ps at 10K [6]. However, the time-resolved data should contain information on all the processes leading to exciton decay, including migration, which depopulates the narrower regions in favour of the wider regions and leads to a fast decay time. Thus a global analysis of the three decay curves has been applied [24]; I , ( t ) = A , e -~/~th - B , e -~/~r -- Cne-r/Vmig (n = 20, 21, 22 MLs), where A, B and C reflect the relative importance of each process to the decay, which gives the 10 K values of thermalization time constant "tthand radiative recombination time constant zr of 190ps and 315ps respectively. These values apply to all three decay curves and are in good agreement with previous reports [1,7,8,18,21,23], while the value of rm~gof 1500 ps, associated with exciton migration, is in reasonable agreement with that found earlier by Kohl et al. (2100 ps) [19] but is considerably larger than that reported by Deveaud et al. (150ps) [18]. The latter result was obtained from QW of similar width (17 MLs) and so differences in ~th cannot be attributed to thickness effects. To confirm the above global analysis, a TRPL experiment was performed. Fig. 3 shows the plot of the TRPL spectra at different delay times after the laser pulse (1.675 eV). Close to zero delay, the spectrum consists of two weak peaks at 1.613 and 1.62 eV, corresponding to the 21 and 22 ML regions. With increasing delay time the intensities of both peaks rise as the excitons thermalize to the radiative state close to KII ~ O, but the strength of the 21 ML peak increases more rapidly than that of the 22 ML peak. After about 1000 ps, the two peaks are of comparable intensity and ratio of the intensities c f the 22 ML and 21 ML peaks reaches a maximum of about 2 after 1500 ps. The ratio remains almost constant with further increasing delay, although an accurate e.stimate of the ratio is difficult since the PL intensities are now weak. The changes in the relative peak intensities with delay in Fig. 3 are consistent with exciton migration from the 21 ML to the 22 ML regions, and this process is substantially complete after about 1500 ps, which is the time constant for the migratiorL time at low temperature deduced from the global a~aalysis of all three decay curves. An estimate of the lateral size of the ML regions may be obtained using the Einstein relation assuming a value for the exciton diffusion constant. Reported val-
22 ML ~.ex=740nm ~
127
21 ML ~
20 .ML ~
~t (ps) 2880 2640 2400 2160
"~
1920 1680
~
1440
~
1200 ~
960 720 480
240 0
I
I
I
I
L
I
1.605 1.61 1.615 1,62 1.625 1.63 Emission Energy (eV) Fig. 3, TRPL spectra (10 K) for the same sample as in Fig. 2 demonstrating exciton migration from the narrower to the wider well regions. Note that the ratio of PL intensities for two peaks (22 to 21 ML) reaches its maximum after a 1500 ps delay.
ues vary from 15cms -~ [22] to 50cms -~ [25] and imply lateral size in the range 1.5-2.5 Ixm in good agreement with recent scanning tunnelling microscopy studies [26]. Exciton migration is also evident under the conditions of resonant excitation. Fig. 4 shows the polarized PL spectra at different excitation energies with an z(x, y)~ configuration, in which a nearly backscattering geometry was used: the laser polarization was parallel to the (011) direction (i.e. x) and analyser parallel to (011) (i.e. y). The very narrow peak (0.2meV) is the laser scattering signal through the analyser for reference, which is denoted by an asterisk. Under non-resonant excitation (Fig. 4, spectrum a), two PL peaks of nearly equal intensity are. observed at 1.612 and 1.605 eV, corresponding to exciton recombination in the 22 and 23 ML well regions. (This is not the same area of the wafer as that in Figs. 1 and 2, where three peaks of 20, 21 and 22 ML are seen. This difference is due to the flux variation across the wafer during MBE growth.) Here the laser excitation energy was about 1.625 eV (indicated by an asterisk), some 13 meV higher in energy than the 22 ML QW region band gap. Using laser excitation at 1.612 eV, resonant with the 22 ML peak as shown in Fig. 4, spectrum b, the PL signal from the 22 ML well is enhanced by a factor of about 20 (there is a minor contribution from the scattered light which might be about 10% from consideration of spectrum a). Here the experimental conditions
128
H. Yu, R. Murray / Materials Science and Engineering B35 (1995) 125 128
are identical to those for spectrum a except for the excitation wavelength. Importantly, the PL emission from the 23 M L regions has also been enhanced by a factor of about 15. For the 22 M L luminescence, we attribute the enhancement to the efficient generation of cold excitons (in-plane wavevector KIj ~ 0) in this well region by the laser, compared with the case of spectrum a where the excitons created by the laser were " h o t " (gll > 0). Only cold excitons with Kit ~ 0 can recombine [7]. Enhancement of the 23 M L peak can only occur if there is exciton migration from the narrower to the wider well width regions. In addition, a new feature appears, about 1.5 meV lower in energy than the main free exciton peaks. This may arise from bound excitons trapped by impurities such as carbon. However, these features are not evident in spectrum a of Fig. 4. Alternatively, recombination of biexcitons may be responsible for this extra feature.
4. Conclusions In conclusion we have demonstrated exciton migration in a very high quality GaAs/A1As SQW using decay time measurements, T R P L and resonant PL. The time constant associated with this process has a value of 1500 ps and has a significant effect on the decay times for the different M L regions, which could be misinterpreted as a change in the radiative lifetime.
,a,
t
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3
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.c_ .J O.
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I 1.59
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I 1.61
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I 1.63
Emission Energy (eV)
Fig. 4. PL spectra and decay times for z(x, y)5 configurations at exciton energies Eex detuning to the QW band gap Eg at E~x- E22ML= 13 meV (spectrum a) and Eex- E22ML= 0 meV (spectrum b), the resonant excitation. The experimental conditions are kept identical for both spectrum a and spectrum b, except the exciting wavelength. The very narrow peak (0.2 meV) is from the laser scattering through the analyser, which is denoted by an asterisk.
Acknowledgements We would like to thank Dr. G. Rumbles, Dr. J.J. Harris, Dr. I. Galbraith and Professor A. Miller for stimulating discussions. Dr. B. Crystall and Dr. M. Carey are thanked for help with the data analysis. EPSRC (Grant no. GR/J97540) are thanked for financial support.
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