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Journal of Applied Physics

Photoluminescence decay time studies of type-II GaAs/AlAs quantum well structures

Michael D. Sturge, Janet L. Mackay, C. Maloney, John K. Pribram

Published Dec 1, 1989• 10 citations
Publisher

Abstract

This is a comment on a paper with the same title by Dawson et al. [J.Appl. Phys. 65, 3606 (1989)], which argues that the nonexponential decay of X-point excitons in type-II GaAs/AlAs superlattices previously reported by one of us [E. Finkman, M. D. Sturge, and M. C. Tamargo, Appl. Phys. Lett. 49, 1299 (1986)] is due to spectral diffusion within the inhomogeneous line, and that where diffusion is absent, the decay is exponential. We report time decay measurements as a function of excitation intensity on the samples used by Dawson et al., and on similar ones from a different source. We find that at very low intensities the results agree with those of Finkman et al., while at high intensities they agree with Dawson et al. We argue that spatial fluctuations in the transition matrix element due to irregularities in the interfaces are responsible for the nonexponential behavior, and that the change at high intensity is due to delocalization of excitons and consequent spatial averaging of the transition matrix element.

DOI
10.1063/1.343675

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This is a comment on a paper with the same title by Dawson et al. [J.Appl. Phys. 65, 3606 (1989)], which argues that the nonexponential decay of X-point excitons in type-II GaAs/AlAs superlattices previously reported by one of us [E. Finkman, M. D. Sturge, and M. C. Tamargo, Appl. Phys. Lett. 49, 1299 (1986)] is due to spectral diffusion within the inhomogeneous line, and that where diffusion is absent, the decay is exponential. We report time decay measurements as a function of excitation intensity on the samples used by Dawson et al., and on similar ones from a different source. We find that at very low intensities the results agree with those of Finkman et al., while at high intensities they agree with Dawson et al. We argue that spatial fluctuations in the transition matrix element due to irregularities in the interfaces are responsible for the nonexponential behavior, and that the change at high intensity is due to delocalization of excitons and consequent spatial averaging of the transition matrix element.
We present here the first observation of quasisteady state optical nonlinearities in the absorption and photoluminescence (PL) spectra of GaAs/AlAs type II heterostructures. Using pump-probe spectroscopy we observed deep modulation, ΔT ~ 0.5 (Δα =−50,000 cm−1), due to a >40-meV blue shift and bleaching of the heavy hole exciton absorption peak. In addition, a blue shift of up to ~10 meV of the indirect PL line is observed with increasing PL excitation intensity. These nonlinearities are understood in terms of spatially separated layers of electrons and holes that are produced by the optical excitation process.
Time-resolved photoluminescence (PL) of excitonic molecules in GaAs quantum wells (QW's) reveals an initial transient characterized by a finite rise and an extremely fast nonexponential decay [H. Wang, J. Shah, and T. C. Damen et al., Solid State Commun. 98, 807 (1996)]. The transient is attributed to coherent quantum evolution towards the molecule ground state of two optically correlated excitons, ${\ensuremath{\sigma}}^{+}$ and ${\ensuremath{\sigma}}^{\ensuremath{-}},$ which undergo the Coulombic attraction. In the present paper, we develop a theory of the transient PL and fit the experimental data. The radiative decay of a quasi-two-dimensional (2D) excitonic molecule is analyzed with the giant oscillator strength model adapted to QW's and with the bipolariton model. Within the 2D bipolariton model, the main ``hidden'' channel of the radiative decay of an excitonic molecule is the resonant dissociation into two outgoing interface (surface) polaritons rather than the observable decay into the bulk radiative modes. We conclude that quasi-2D molecules dissociate already within the initial transient of PL, while the following exponential decay of the molecule-mediated PL is due to the escape of secondary interface polaritons into the bulk modes. According to the 2D bipolariton model, the sequence ``two incoming $\mathrm{photons}\ensuremath{\rightarrow}\mathrm{two}$ virtual $\mathrm{excitons}\ensuremath{\rightarrow}2\mathrm{D}$ $\mathrm{molecule}\ensuremath{\rightarrow}\mathrm{two}$ outgoing interface polaritons'' is a completely coherent process. The fit of the time-resolved molecule-mediated PL provides us with numerical estimates of the exciton-photon coupling in GaAs QW's.
We report time-resolved photoluminescence (TRPL) measurements of the radiative lifetime (τr) at 1.8K of resonantly excited heavy-hole excitons in a 27nm GaAs single quantum well as a function of exciton density. By the use of AlAs interface smoothing layers [1], the sample exhibits very little inhomogeneous broadening (≈0.15meV). The exciton centre of mass motion is therefore not significantly restricted by localisation in well width fluctuation islands. However, measurements of the exciton homogeneous linewidth (Γ h ) using time-resolved degenerate four wave mixing in the self-diffraction configuration (Fig.1) [2], additionally show that the sample imperfection scattering contribution to the Γ h of these delocalised excitons is also small (0.2meV). Thus, as indicated by the dependence of Γ h on exciton density (N x ), even at moderate exciton densities, exciton-exciton scattering is the dominant dephasing interaction within the photocreated exciton ensemble.
Photoluminescence spectra of interwell excitons in double GaAs/AlGaAs quantum wells (n-i-n structures) have been investigated (an interwell exciton in these systems is an electron-hole pair spatially separated by a narrow AlAs barrier). Under resonance excitation by circularly polarized light, the luminescence line of interwell excitons exhibits a significant narrowing and a drastic increase in the degree of circular polarization of photoluminescence with increasing exciton concentration. It is found that the radiative recombination rate significantly increases under these conditions. This phenomenon is observed at temperatures lower than the critical point and can be interpreted in terms of the collective behavior of interwell excitons.
Recent experimental and theoretical work suggests that excitons in GaAs/AlGaAs multiple quantum wells may be localized by disorder, or delocalized with Bloch-type wavefunctions1. At low temperature, localized excitons are expected to be characterized by spectral diffusion and by long optical coherence times, while delocalized excitons have short coherence times and exhibit spatial mobility2. The short optical coherence time associated with delocalized excitons results in a linear absorption profile that is primarily homogeneously broadened, while the disorder associated with localized excitons leads to an inhomogeneously broadened profile. In this work, we investigate the energy and phase relaxation of HH1 excitons in GaAs/AlGaAs MQW's at low temperature, and provide evidence that, dependent on the exciton density, both localized and delocalized excitons contribute to the nonlinear optical response.