Pseudodirect Excitons in GaAs/AlAs Type-II Superlattices

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I have investigated optical properties of a pseudodirect exciton consisting of an AlAs-X_Z electron and a GaAs-G heavy hole in GaAs/AlAs type-II superlattices (SLs) from the following viewpoints : (1) the oscillator strength of the pseudodirect exciton resulting from a G-X mixing effect in the conduction band, and (2) high density excitation effects such as the formation of biexcitons.All samples were grown on a (001) semi-insulating GaAs substrate grown by molecular-beam epitaxy : (GaAs)_m/(AlAs)_m[(m, m)]SLs with m=8-13 monolayrs and (GaAs)_<10>/(AlAs)_n[(10, n)]SLs with n=10-20 monolayrs.I have obtained the following research results.(1) The relative oscillator strength of the pseudodirect exciton to the direct exciton as a function of the layr thickness is estimated from the experimental results obtained by photoluminescence (PL), PL-excitaion, and PL-decay-time measurements. The values of the relative oscillator strength in the (m, m) SLs are around 1x10^<-4> and independent of the layr thickness.In the (10, n) SLs the relative oscillator strength decreases from-1x10^<-4> to -1x10^<-5> with the increase of n.The G-X mixing factor producing the oscillator strength is estimated by using a first order perturbation theory.It is concluded that the G-X mixing factor is determined by the overlap of the envelope functions of the G and X_z electrons.(2) From the excitation-power dependence of the pseudodirect-exciton PL,it has been found for the first time that the biexcitons (exciton molecules) are sufficiently formed at an extremely low excitation power (-1 mW/cm^2) which is two-or three-order lower than in direct-transition-type GaAs quantum wells.The line shape analysis of the biexction PL band indicates that the biexciton-binding energy is about 3 me V.The observed decay time of the biexciton PL is about a half of that of the free exciton, whitch is a typical property of the biexciton dynamics.