Correlation between the light- and heavy-hole free-exciton fine structure inAlxGa1

Abstract

Sharp line structure associated with both the light-hole free exciton (LHFE) and heavy-hole free exciton (HHFE) in multiple-quantum-well structures of GaAs-AI"Ga& "As in photoluminescence and re6ection spectra has been deconvoluted by using photoluminescence excitation spectroscopy.A correlation is established between particular LHFE fine-structure components and specific HHFE fine-structure components. A model is developed to account for the LHFE and HHFE fine structure in these samples which exploits the nonrandom character of the observed spectra. The physical location of the excitons is demonstrated to be in regions of the well(s) with essentially identical interfacial microstructure. Evidence of difusion from effectively-narrow-well regions to wide-well regions is presented.The nature of the interfacial structure in GaAsAl"oa& "As quantum wells has been studied by a number of investigators. ' " In Refs. 1-9 the interface structure, associated with growth islands, resulted in monolayer fluctuations in the well size. The current paper and Refs. 10 and 11 deal primarily with interface structure that produces effective submonolayer fluctuations in well size. The submonolayer Nuctuations in well size result in very narrow exciton lines; emission lines as narrow as 0.02 meV have been observed, with very small energy separations. Very sharp line structures associated with both the heavy-hole free excitons (HHFE's) and the light-hole free excitons (LHFE's) in multiple-quantumwell (MQW) structures in photoluminescence (PL) and re6ection spectra have been reported. ' '" It was proposed that this fine structure (FS) in these MQW structures is related to the microscopic quality of the interfaces which depends on the growth process. The interpretation of these results has evolved from Ref. 10 to the current paper. The data in Ref. 10 correlated with an effective one-half-monolayer fluctuation in well size. It was concluded that the one-half-monolayer structure resulted from interlayer rather than intralayer fluctuations. Subsequent data, reported in Ref. 11, could not be accounted for by this model. In this case, a model consisting of a random distribution of intralayer interfacial mierostructure was proposed. The most recent results show that a nonrandom distribution of interfacial microstructure provides a better explanation of the experimental data. In this paper, we present the results of photoluminescence excitation (PLE) spectroscopy on samples which exhibit FS. A correlation is established between the FS observed in the LHFE spectra and that observed in the HHFE spectra. For example, it is shown that a specific component of the LHFE spectra predominately excites a specific component of the HHFE spectra. Evidence for exciton diffusion from the narrow wells to the wider wells is also presented.The formation of the interface which controls the details of the FS is directly related to the growth process. During the growth of a heterostructure sample the Ga and arsemc (As& and/or As4) beams impinge continuously on the ...