PHYSICAL REVIEW B VOLUME 46, NUMBER 12 15 SEPTEMBER 1992-II Particle localization and phonon sidebands in GaAs/Al„Ga& „As multiple quantum wells I. Brener, * M. Olszakier, ~ E. Cohen, and E. Ehrenfreund Solid State Institute and Physics Department, Technion, Haifa 32000, Israel Arza Ron Solid State Institute and Chemistry Department, Technion, Haifa 32000, Israel L. Pfeiffer A T&T Bell Laboratories, Murray Hill, New Jersey 07974 (Received 19 August 1991; revised manuscript received 9 March 1992) We use time-resolved spectroscopy of the LO-phonon sidebands to study the in-plane localization of carriers and excitons in undoped GaAs/Al„Ga& „As multiple quantum wells at low temperatures. We Snd three distinct populations contributing to the radiative recombination (excluding shallow back- ground impurities): (a) weakly localized excitons, their localization dimension being larger than the exci- ton Bohr radius, (b) tightly localized excitons, (c) separately localized electrons and holes that decay ra- diatively on a microsecond time scale. The radiative recombination in undoped multiple quantum wells (MQW's) is due to excitons confined in the wells. In most commonly studied MQW's, the lumines- cence band is Stokes shifted with respect to its excitation spectrum. This is generally understood to be due to exci- tons localized in the plane of the QW, either by interface roughness [e. g. , in GaAs/Al& „As MQW's (Ref. 1)] or by alloy concentration fiuctuations [e. g. , in In& „Ga„As/InP MQW's (Ref. 2)]. Localization effects have mainly been studied by energy transfer between lo- calized exciton sites ' or by exciton dephasing process- es. It has been shown ' that the exciton radiative recombination rate in QW's is enhanced by the fact that the exciton in-plane motion is coherent over an area of dimension A „h & A, (where A, is the wavelength of the ex- citon radiation). In this paper we examine the effects of in-plane localization on the LO-phonon sidebands (PSB's) of the luminescence of GaAs/Al„Ga, „AsMQW's, by using the method of time-resolved spectroscopy. PSB's have long been related to the degree of e or h lo- calization. Hopfield has shown that in a direct gap semi- conductor, an optical transition involving particles with a well-defined k has a vanishing small LO-PSB intensity relative to that of its no-phonon (NP) band. This is a direct result of momentum conservation. Localized par- ticles, on the other hand, relax the k conservation rule and, therefore, are expected to show stronger PSB's. These PSB's are sharp, since the Frohlich interaction is strong only for k & 10 cm ', a small fraction of the Bril- louin zone in which the LO-phonon dispersion curve is Hat. We study the LO-PSB's photoluminescence (PL) and their excitation spectra at different time intervals after excitation by a laser pulse. We show that the results can be interpreted in terms of the radiative recombination of three different types of localized e hpairs: (a) weak-ly lo- calized excitons which do not show any LO-PSB's; (b} tightly localized excitons; and (c) electrons and holes which are separately localized at different sites in the QW (much like the case of donor-acceptor pairs in bulk semi- conductors). The last two groups of localized e hpairs- show strong LO-PSB's. The temperature dependence of the 1LO-PSB intensity, which is typical of a localized particle with a small binding energy, and the nonex- ponential decay curve of that PSB, lends further support to the interpretation in terms of separate localization of electrons and holes. The MQW's studied are GaAs/Al„Ga, „Assamples with x=0. 3 or 1 and with well widths in the range of 40-70 A. They were selectively excited by 6-psec dye laser pulses and the PL was measured with a temporal resolution of 350 ps. A time correlation technique was used for the time-resolved measurements: a specific delay time (t) was set and the spectrum was continuously recorded with a temporal window of 350 psec. The sam- ples were either immersed in liquid He or in cold He va- por. Figure 1 shows the time-resolved PL spectrum of a GaAs/A1As MQW consisting of 100 wells, each of 70 A width separated by 200-A barriers. The PL was excited at 1. 632 eV. Similar spectra were observed for all the other MQW's under study. The most intense feature [designated (a}] is the NP band of the (e 1:hhl)1S exciton. The spectrum shown in Fig. 1(b) was measured at t = 300 psec. This is the delay time at which this PL is maximal. LO& and LO2 denote the PSB's associated with GaAs-like and A1As-like LO phonons, respectively. The phonon energies were measured by Raman scattering. Thus it was found that hco2 corresponds exactly to the A1As-like LO phonon of the Al Ga, „As MQW barrier material. Figure 1(c) shows the spectrum measured at t =3.8 nsec, a much longer delay time than the excitonic radiative de- cay time measured for this MQW ( &400 psec). The 1LO-PSB now appears blueshifted by 2 meV. The same shift is observed in the higher-order PSB's. From this temporal behavior we conclude that the PSB's consist of two components. (1) A fast one, which follows the (el:hhl) NP exciton luminescence decay; the 1LO& fast 46 7927 1992 The American Physical Society