Luminescence Properties of Periodic Disordered Thin Layer GaAs/AlAs Superlattices
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LUMINESCENCE PROPERTIES OF PERIODIC DISORDERED THIN LAYER GaAs/AlAs SUPERLATTICES RUTH Y. A. ZHANG*, J. STROZIER**, C. HORTON* AND A. IGNATIEV* *Space Vacuum Epitaxy Center, University of Houston, Houston, Tx 77204-5507 "**EmpireState College, SUNY, Stony Brook, NY 11794 ABSTRACT Disordered thin layer GaAs/AlAs superlattices with various disorder periods have been grown using MBE. The disorder was introduced by varying the thickness of GaAs and AlAs layers in the growth direction in various specific but randomly generated finite disorder sequences. The photoluminescence spectra of these disordered superlattice samples showed a sharp peak at the high energy side and a broad peak at the low energy side. The temperature, excitation, and disorder sequence dependence of the photoluminescence spectra indicates that the sharp peak is due to the pseudo-direct exciton emission, and the broad peak is strongly related to the localized states induced by disorder. In addition, the results demonstrate that the luminescence intensity of disordered superlattices can be improved by up to two orders of magnitude over that of ordered short period superlattices. Finally, we propose a kinetic model for the state population and find that the photoluminescence spectra can be well described by this model. INTRODUCTION Disordered superlattices (SLs) are semiconductor heterostructures in which layers of different materials follow a given nonperiodic sequence. As an example, Fig. 1 shows the arrangement of layers of different materials in a specific disordered GaAs/AlAs SL. The structure of an ordered =t (GaAs) 2 (AIAs) 2 SL is also shown for comparison. The disordered SL shown in the figure represents a sequence a (GaAs)I(AIAs) 2(GaAs) 3(AIAs)I(GaAs) 2 (AIAs) 3 .., where the layer thickness is randomly selected from the set of numbers [1,2,3]. It is clear from the figure that the structure of a disordered SL can be considered as a non-periodic (a) (b) succession of different layers in the growth direction, in contrast to the periodic stacking of an ordered SL. GaAs (IML) AIAs (IML) In 1989, Sasaki et al. I reported that disordered thin layer Fig. 1. Schematic illustration of layer sequence GaAs/AlAs SLs exhibit markedly stronger luminescence as for disordered SL (a), and ordered SL (b). IML compared to ordered SLs. Further studies 2,3,4, 5 also showed is defined as a layer of As atoms plus a layer of that disordered SLs exhibit enhanced luminescence intensity Ga (or Al) atoms. and reduced band gap in comparison with the ordered SL, whose period is the same as the averaged layer thickness of the disordered SLs. Both experimental 6 and theoretical 7 work have indicated that the band structure of thin layer (AIAs)n/(GaAs)n SLs is indirect when n
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