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Inter-quantum-well diffusion in semiconductor superlattices.

The diffusion of a minority carrier in the growth direction in a semiconductor superlattice is studied theoretically. Both coherent band diffusion and incoherent well-to-well tunneling processes are considered in an ordered superlattice. The formalism is employed to calculate the diffusion length of a heavy hole in In/sub x/Ga/sub 1-//sub x/As/GaAs superlattices by accounting for scattering by acoustic and polar optical phonons and using the Kronig-Penney model in the temperature range 70--300 K. It is found that the diffusion takes place mainly via activation to broad-subband states above the barriers in superlattices with thick barriers (e.g., approx.115 A). For superlattices with thinner barriers (e.g., approx.50 A), we find a crossover from over-barrier diffusion at high temperatures to tunneling diffusion through low-lying narrow-subband states at low temperatures. For even thinner barriers (e.g., less than or equal to40 A) tunneling diffusion prevails at all temperatures. For a light hole, the crossover is found to occur in superlattices with much thicker barriers (e.g., approx.115 A). The relevance of these results to recent data is discussed. Finally, the effect of the disorder-induced localization is discussed.