Localized indirect excitons in a short-period GaAs/AlAs superlattice

Abstract

We have studied the optical properties of a short-period superlattice composed of 20.4-A GaAs 0 and 14.7-A AlAs layers. The superlattice behaves as an indirect-gap material. A slow and nonexponential decay of the luminescence can be interpreted as the emission from the A indirect excitons localized at the GaAs/A1As interfaces. The temperature dependence of the exciton decay time can be explained in terms of a transition by phonon-assisted tunneling, followed by a nonradiative transition.Recent advances in crystal growth techniques, such as the molecular-beam-epitaxy (MBE) method, have made it possible to produce semiconductor crystals consisting of alternating layers of two difterent semiconductors, i.e. , superlattice structures. These superlattices are expected to form a new device and have been extensively studied, especially the GaAs/AlAs system. A number of experiments on GaAs/A1As superlattices of large layer thickness have been carried out and analyzed successfully by means of the Kronig-Penney model. ' There have been a few works on ultrathin layered GaAs/A1As superlattices. However, considerable confusion exists in the interpretation of the electronic structure of these superlattices, especially of the short-period superlattices with nearly equal GaAs and A1As layer thickness and with periods ranging from -10 to -60 A. Some groups have claimed that these short-period superlattices behave as indirect-gap materials, ' while others have concluded that these materials are direct gap.In order to clarify the electronic structure of short-period superlattices, we have studied the op-0 tical properties of a superlattice composed of 20.4-A GaAs and 14.7-A A1As layers. Special attention is given to the dynamics of photoexcited carriers. The experimental results indicate that our sample behaves as an indirect-gap material and that the emission near the band edge is due to localized indirect excitons.The GaAs/A1As sample used in this study consisted of 250 periods of 20.4-A GaAs/14. 7-A A1As grown by MBE at 550'C on a (100) semi-insulating CxaAs substrate. The Al content in the sample was 42%, which is near the direct-indirect crossover value of -40% in the Al Ga~As alloy system. ' In order to make absorption measurements, the GaAs substrate was removed by preferential etching over a region of 1&1 mm, leaving only the MBE-grown film. The cw photoluminescence and excitation measurements were made by a DCM dye laser pumped by an argon-ion laser. For time-resolved spectra, the sample was excited by a cavity-dumped mode-locked cw dye (DCM) laser synchronously pumped by a mode-locked argon-ion laser. The transmitted light and luminescence were monitored by a double monochromator equipped with a cooled photomultiplier. Lifetime measurements were made by the time-correlated singlephoton counting technique. The sample was immersed in liquid helium. At elevated temperatures, it was exposed to an atmosphere of helium gas.The absorption spectrum at 4.2 K is presented in Fig. 1. The absorption coefficient a(fico) was obtained...

Recombination Processes in Short‐Period GaAsAlAs Superlattices of Type II

“…6. d) Although we did not compute time-resolved spectra, there is a link between the overall efficiency and the time decay, provided T is not too low; so, the model provides also a To value from a plot of In (LIP) versus T , thus giving some theoretical support to experimental results [21]. From Fig.…”

“…Increasing T causes a faster decay related to detrapping of localized excitons [21]. The decrease of the decay time (Fig.…”

Time-resolved spectra and kinetics of the exciton photoluminescence in different types of GaAs/AlAs superlattices

“…Time-resolved PL spectra of GaAs/AlAs SLs have been presented and discussed in a number of publications [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. In particular, it was shown [9][10][11][12] that the measured radiative lifetime of free excitons in direct-gap SLs lies in the nanosecond time regime, which significantly exceeds the expected value [16,17].…”

“…The PL signal in such SLs exhibits very long decay times. At the same [1][2][3][4][5][6][7][8], and the decay curves can be well fitted using a model, which was originally developed by Klein et al [18] for disorder-induced zero-phonon recombination of indirect excitons in Al x Ga 1−x As. However, with increasing temperature, the PL decay time becomes much shorter in quasi-direct SLs due to the increasing influence of scattering centers at the interfaces.…”

Kinetics of exciton photoluminescence in type-II semiconductor superlattices

“…The possibility to apply the theory 1 for superlattices has been discussed by F. Minami et al. 2 Authors suppose the short-range scatterers are distributed along the plane boundary. This assumption justifies the application of Eq.…”