Exciton photoluminescence (PL) is an important technique for the characterization of quantum wells (QW's). We discuss the etfect of localization on the diamagnetic energy shift of an exciton in a QW. It is shown how the diamagnetism of an electron in the ground state, with arbitrary geometry, depends on the dimensions of its wave packet. We consider the properties of free excitons in QW s, introducing an effective electron-hole interaction. We use dimensional analysis to relate the finitebarrier problem to the simpler case of perfect confinement. For a bound exciton, localization in the plane of the QW causes the diamagnetism to be smaller than for a free exciton. The efFect of localization is not important if the range is much larger than the free-exciton Bohr radius. The exciton diamagnetic shift is reduced by localization in 10 and 20 A (In, Ga)As/InP QW s grown by solidsource molecular-beam epitaxy. Uncertainty about the value of the free-exciton diamagnetic shift limits the sensitivity of the diamagnetism as a probe of exciton localization. Despite this, the method still provides valuable information on exciton localization, particularly when combined with studies of the phonon sideband of PL.

The strength of the exciton-longitudinal-optical-(LO) phonon (Frohlich) interaction as a function of sheet carrier density (n, ) is studied in Schottky-gated, modulation-doped In Gal "As-InP quantum wells. LO-phonon satellites of the heavy-hole exciton photoluminescence line, enhanced by hole-localization effects, are observed at low carrier density. The satellites become very weak for n, greater than -3&10" cm, as a result of screening of the Frohlich interaction with increasing carrier density. Clear evidence for hole localization at high n, is obtained from magnetic field measurements. Direct measurements of the variation of the exciton-longitudinal-optical-(LO) phonon interaction as a function of sheet carrier density (n, ) are reported from photoluminescence (PL) studies of Schottky-gated, modulation-dopedIn"Ga~"As-InP single quantum wells (QW's). Low-temperature PL in alloy QW's is shown to be a new method for the study of the effects of screening and carrier localization on the strength of the carrier-LO-phonon (Frohlich) coupling in two dimensions (2D). In the Schottky-gated structure, n, could be varied continuously from 0 to -10 ' cm . LO-phonon satellites of the exciton recombination, enhanced by holelocalization effects, are observed when n, =0. With increasing n, the strength of the LO-phonon satellite decreases monotonically, and is very weak for n, & 3x10" cm, because of screening of the hole-LO-phonon interaction at high n, .There has been great interest recently in the effects of reduced dimensionality and screening on the Frohlich interaction in heterojunctions and QW s. Nicholas et al. observed pinning of electron Landau levels to the energy of the transverse optical (TO) phonon rather than the LO phonon in In"Ga~-"As-InP and In"Ga~-"As-Al"In~"As heterojunctions. ' This anomalous pinning was not well understood, but it was suggested that it arose either from the boundary conditions imposed by the interface or by screening of the LO-TO splitting by the 2D electron gas.In In"Gal -"As-Al"ln|-"As QW's, by contrast, pinning to the LO-phonon energy was observed.For other systems both enhancement and reduction of polaron effects in 2D have been reported.From the theoretical point of view, an enhancement of polaron effects in 2D is expected.However, when screening is included, the enhancements are reduced, leading to a nonresonant polaron mass close to or lower than that found in three dimensions. There has also been much debate on the importance of screening for hot-carrier-LO-phonon relaxation rates in 2D systems. ' The experiments were carried out on single-sided, modulation-doped single QW's of In"Gal "As-InP (x =0.47) grown by atmospheric-pressure metal-organic chemical vapor deposition (MOCVD). "'2 The structure consists of an n+-type InP substrate, a 2000-A undoped InP buft'er layer (n -5 x 10's cm 3), a 1000-A InP layer doped with sulfur at 2x10' cm, a 100-A undoped InP spacer layer, 100-A In"Ga|-"As QW's (n -2x 10' cm ), and a 2000-A undoped InP capping layer. Electric fields up to -1.5x10 ...

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Comment on "Picosecond Raman Studies of the Frohlich Interaction in Semiconductor Alloys"In a recent Letter Kash, Jha, and Tsang 1 (henceforth KJT, with equations and figures from KJT given the prefix K) report time-domain pump-probe Raman studies of electron-phonon interactions in semiconductor alloys. They show that the kinetics (rise and decay times) of the AlAs-type and the GaAs-type LO phonons in Al x Gaix As, the GaAs-type LO phonon in Ino.1Gao.9-As, and the LO modes of GaAs are very similar. KJT then use this result to determine the relative strength of the electron-phonon interactions of the GaAs-type and AlAs-type modes in Al x Gaix As. They take the non-