STATISTICAL-MODEL EXPLAINING THE FINE-STRUCTURE AND INTERFACE PREFERENCE OF LOCALIZED EXCITONS IN TYPE-II GAAS ALAS SUPERLATTICES/

Raman scattering experiments have allowed the determination of the spa tial distribution of the thicknesses of GaAs and AlAs layers in a grad ient GaAs/AlAs superlattice. A statistical model is developed which is consistent with all the data and ways to improve interface quality ar e suggested. The fine structures of X Gamma and Gamma Gamma excitons o bserved in photoluminescence and differential reflection are found to be governed by the fractional parts of the average thickness of the la yer (in monolayers). We conclude that each structure has two scales of fluctuations which form the relief of the AlAs surface. The largest f luctuations repeat the relief of the GaAs surface. The second scale ha s the size of a typical X Gamma exciton Bohr radius. The smaller fluct uations disappear when the thickness of the AlAs layer is equal to an integer number of monolayers, which provide interfaces of the quality. The correlation of macro-rough fluctuations on the surface of AlAs an d GaAs causes an asymmetry in the densities of states of type II excit ons located at either AlAs-on-GaAs or GaAs-on-AlAs interfaces. Hence t he lowest PL line is formed by excitons localized across the AlAs-on-G aAs interface. On the other hand, in structures with micro-rough but u ncorrelated AlAs surfaces, the lowest energy state is expected to be o ccupied by excitons localized across the GaAs-on-AlAs interface.