Physica B 219&220 (1996) 59 -61
Effect of nonequilibrium acoustic phonons on exciton states in interrupted grown GaAs/Alo.33Gao.67As quantum wells E.S. Moskalenko a'*, A.V. Akimov a, A.A. Kaplyanskii a, A.L. Zhmodikov a, L.J. Challis b, T.S. Cheng b, C.T. Foxon b aA.F. Io~e Physical-Technical Institute, Russian Academy of Science, 26 Polytechnicheskava, 194021 St. Petersburg, Russian Federation bDepartment of Physics, University of Nottingham, Nottingham NG7 2RD. UK
Abstract We present the first studies of the effect of heat pulses on the exciton luminescence from GaAs/Alo.33Gao.67As quantum wells (QW) prepared using growth interrupts to reduce the interface step density. We have observed the redistribution of excitons between the main thinner part and islands one monolayer thick of the QW induced by the heat pulses. The effect strongly depends on the relative position of heater and exciton gas and is discussed in terms of the drag of free excitons produced by "phonon wind".
1. Introduction Experiments with nonequilibrium acoustic phonons provide valuable information on the interaction of phonons with free carriers and excitons as well as on the nature of the electronic excitations themselves. One of the interesting aspects of this interaction is that the phonon flux can induce motion of free excitations in crystals. This phonon wind effect  has been investigated in detail for the free excitons in bulk semiconductors  and for 2D electrons in two-dimensional semiconductor structures [3, 4]. We report here the first observation of the exciton flow in a single GaAs QW produced by phonon wind. In contrast to the case of bulk semiconductors where excitons can move distances ~100 lam , the typical diffusion length of free excitons in GaAs QWs is only 1 gm and it is difficult to detect directly their lateral motion. To overcome this problem we have used GaAs QWs in which the growth was interrupted to allow atoms * Corresponding author.
to redistribute in the plane of the interface. The most noteworthy feature is that large flat islands where the QW has an additional thickness of one monolayer is separated by several gm . The lateral motion of the excitons produced by phonon wind leads to a measurable redistribution of the exciton population between the main thinner part of the QW and the island region.
2. Samples and experimental arrangement The samples were obtained from a wafer NU959 which was grown at a substrate temperature of 630C. The (00 1) semi-insulating LEC GaAs substrate had a thickness of 0.4 mm. The growth was interrupted for 60 s at each interface during the production of five single QWs of different thicknesses L= =2.5, 5.1, 10.2, 19.5 and 29.7nm separated from each other by 20.4nm Alo.33Ga0.67 As barriers. The experimental arrangement is shown in Fig. 1. The sample is mounted in a helium flow cryostat whose temperature could be varied above 4.5 K and a 100 gm
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E.S. Moskalenko et al. /Physica B 219&220 (1996) 59-61
LE ii FE.~
0.6 Fig. 1. Experimental setup.
CW laser beam (HeNe, 2 = 632.8 nm) creates two-dimensional exciton gases (2DExGs) in the QWs of sheet density Nex up to 101° cm -2. A phonon generator (h) consisting of a 1.5 x 1 mm 2 metal film 20 nm thick is evaporated on the polished face of the substrate opposite the QW structure. The film is heated by 200 ns current pulses producing pulses of nonequilibrium phonons which propagate through the substrate and strike the 2DExGs (the transit time for TA phonons is 100 ns). The changes in photoluminescence (PL) that result depend strongly on the mean angle of incidence of the phonons on the 2DExGs which could be varied by moving the laser illuminated area relative to the heater.
3. Experimental procedure and results The solid lines in Fig. 2 show PL spectra for the 10.2 nm QW obtained at a bath temperature of 4.5 K at N e x ~ 1010 cm-2 and are typical of data at high values of Nex. The spectra are essentially independent of the position of the laser illuminated area relative to the heater and Fig. 2(a) and (b) shows data with the laser spot immediately opposite the heater and one shifted to one side of the heater. It is seen that there are two sharp lines, at 1.5460 and 1.5444 eV, which can be attributed to excitons in QWs differing in thickness by one monolayer. The higher energy line is evidently from free excitons (FEs) in the main thinner part of the QW while the lower energy line is due to so-called "localized" excitons (LEs) in the thicker parts where there are islands one monolayer thick. The existence of a sharp LE line is evidence that the islands have lateral dimensions large compared with the exciton diameter (13 nm for Lz = 10.2 nm ). In narrower QWs only the LE line was observed at T = 4.5 K. In wider QWs LE and FE lines are not resolved. The dotted lines in Fig. 2 show the effect of the phonon pulses. They show in fact the PL measured during
0.4 0.2 0.0 1.544 1.548 Energy, eV Fig. 2. PL spectra from 2DExG measured with phonon generator off (solid curve) and on (dotted curve) for normal (a) and oblique (b) phonon incidence on the 2DExG.
a 500 ns interval gated to coincide with the beginning of the 200 ns current pulse through the heater film. It can be seen that the effects differ markedly between Fig. 2(a) and (b). In Fig. 2(a), which shows data for essentially normal incidence of the phonons on laser excited area, the intensity from the LE excitons falls while that of the FE rises. Care is taken to ensure that the excitons are over the centre of the heater. However, exactly the opposite occurs when the phonons are incident obliquely (Fig. 2(b)).
4. Discussion Since the area of the islands is only a small fraction of the total it seems probable that most of the excitons are generated in the main part of the QW and that the relatively large population of LE excitons is the result of capture from the diffusing FE excitons. The efficiency of this FE ~ LE capture depends on the ratio of the mean distance L~ between the islands and the diffusion length of the FE, Ld = (DTR) 1/2, where D is the diffusion coefficient which, at high /Vex, is governed mostly by exciton-exciton (exciton-electron) scattering and rR is the lifetime of the FE. A pulse of normally incident phonons evidently excites LE excitons out of the lower energy island regions and transfers them to the higher energy FE regions. That results in the observed decrease of LE and increase of FE luminescence intensity (Fig. (2(a)). It is clear however that when the same phonons are incident obliquely another more effective process takes place. The
E.S. Moskalenko et al. / Physica B 219&220 (1996) 59 61
phonons now have momentum parallel to the plane of the 2DExG and we believe the transfer of momentum to the excitons by phonon wind effectively enhances FE diffusion rate and so increases the probability of FE capture in the island regions. This process is clearly more effective than the excitation LE ~ FE process out of the island regions and leads to the observed decrease of FE and increase of LE intensity (Fig. 2(b)). The effect of phonon wind which is observed at high :Vex ~ 10 l° cm -2 becomes appreciably smaller for N~,x < 109 cm -2, when exciton exciton scattering becomes less important  and excitons can easily reach islands without contribution of phonon wind. The effect of phonon wind in our experiments can in fact only be observed in particular macroscopic ( < 10 ram) regions of the whole wafer. This would appear to indicate that the islands are inhomogeneously distributed in the QWs grown with interruption. It can be shown by simple calculations that the effect of phonon wind decreases if Li is large or small compared with an optimum value.
Acknowledgements We gratefully acknowledge financial support from the Russian Foundation for Basic Research (93-02-2560), the
Royal Society, INTAS-94-395 and the International Science Foundation NUC000.
References  L.V. Keldysh and N.N. Sibeldin, in: Nonequilibrium Phonons in Nonmetallic Crystals, eds. W. Eisenmengerand A.A. Kaplyanskii (North-Holland, Amsterdam, 1986) p. 455.  N.N. Zinov'ev, I.P. Ivanov, V.I. Kozub and I.D. Yaroshetskii, Soy. Phys. JETP 57 (1983) 1027; A.V. Akimov, A.A. Kaplyanskii, E.S. Moskalenko and R.A. Titov, Sov. Phys. JETP 67(11) (1988) 2348.  H. Karl, W. Dietsche, A. Fischer and K. Ploog, Phys. Rev. Lett. 61 (1988) 2360. [47 L.M. Smith, J.S. Preston, J.P. Wolfe, D.R. Wake, J. Klein, T. Henderson and H. Morcoc, Phys. Rev. B 39 (1989) 1862.  For a reviewsee: M.A. Herman, D. Bimbergand J. Christen, J. Appl. Phys. 70 (1991) R1.  E.S. Moskalenko, A.L. Zhmodikov, A.V. Akimov, A.A. Kaplyanskii, L.J. Challis, T. Cheng and O.H. Hughes, Phys. Solid State 36 (1994) 1668; Ann. Phys. 4 (1995) 127.