DETERMINATION OF INTERFACE PREFERENCE BY OBSERVATION OF LINEAR-TO-CIRCULAR POLARIZATION CONVERSION UNDER OPTICAL ORIENTATION OF EXCITONS INTYPE-II GAAS ALAS SUPERLATTICES/

A comprehensive investigation of exciton optical orientation and align ment in type-II GaAs/AlAs(001) superlattices in longitudinal and trans verse magnetic fields is presented. We observe the previously predicte d type-II orientation-to-alignment conversion induced by a longitudina l field due to the anisotropic exchange splitting of the localized-exc iton radiative doublet. A theory of polarized exciton photoluminescenc e under polarized excitation is developed, that takes into account the fine structure of the excitonic quartet, the exciton spin relaxation, and the difference in the lifetimes of electric-dipole-active and ina ctive states. By comparing experimental and theoretical curves, the ma in parameters characterizing the level splitting, the recombination ra tes, and the spin relaxation of localized excitons are deduced. Field- induced polarization conversion provides an effective method to measur e the interface preference, i.e., the difference between the fractions of excitons localized on AlAs/GaAs and GaAs/AlAs interfaces. We have also observed the field-induced transformation from [100] to [110] Lin ear polarization, which is direct evidence of:exciton cascade kinetics .