DYNAMICS OF ELECTRIC-FIELD DOMAINS AND OSCILLATIONS OF THE PHOTOCURRENT IN A SIMPLE SUPERLATTICE MODEL

A discrete model is introduced to account for the time-periodic oscillations of the photocurrent in a superlattice observed by Kwok et al. in an undoped 40-period AlAs/GaAs superlattice. The basic ingredients are an effective negative-differential resistance due to the sequential resonant tunneling of the photoexcited carriers through the potential barriers, and a rate equation for the holes that incorporates photogeneration and recombination. The photoexciting laser acts as a damping factor ending the oscillations when its power is large enough. The model explains (i) the known oscillatory static I-V characteristic curve through the formation of a domain wall connecting high- and low-electric-field domains, and (ii) the photocurrent and photoluminescence time-dependent oscillations after the domain wall is formed. In our model, they arise from the combined motion of the wall and the shift of the values of the electric field at the domains. Up to a certain value of the photoexcitation, the nonuniform field profile with two domains turns out to be metastable: after the photocurrent oscillations have ceased, the field profile slowly relaxes toward the uniform stationary solution (which is reached on a much longer time scale). Multiple stability of stationary states and hysteresis are also found. An interpretation of the oscillations in the photoluminescence spectrum is also given.