Novel electric field effects in gallium arsenide-(aluminum,gallium)arsenide superlattices.

This thesis presents results of optical and transport measurements of electric field effects in GaAs-(Al,Ga)As superlattices. Our research focuses on two phenomena: the quantum well Pockels effect and electric field domains in superlattices. We report on the first observation of giant in-plane optical anisotropy in quantum well structures. The quantum well Pockels effect refers to the biaxial response of photoluminescence to electric fields. For fields along (001), emission due to nominally forbidden excitons exhibits substantial differences between (110) and (110) polarizations. Allowed transitions show no noticeable anisotropy. In terms of symmetry, our observations relate to the linear electro-optic (Pockels) effect. A distinction compared to previous quantum well effects is that the relevant dependence on the field is odd, as opposed to even. The field dependence of the anisotropy is unusual in that it decreases with increasing field. Results can be qualitatively accounted for by perturbative analyses. At high excitation powers, nominally undoped superlattices spontaneously break into electric field domains characterized by the alignment of levels between neighboring wells. Photoluminescence, photocurrent and Raman measurements were used to characterize the domains. Raman scattering by intersubband transition is a precise way to determine the field strength under given resonant tunneling conditions. The intensity of the photoluminescence is a sensitive probe of the behavior of the photocurrent in the vicinity of resonant tunneling. The photoluminescence data suggest that domain formation do not necessarily require exact alignment between the subbands. Based on various optical studies, we also propose a physical picture describing the transition region in voltage-current measurements, where the current rises abruptly. Optical data provide strong evidence that bistability of domains is intrinsic. We report on the first kinetic studies of domains in superlattices. The characteristic time of domain formation was determined by time-resolved studies. Results reveal a complex behavior with parameters that depend strongly on the bias voltage. Oscillations correspond to the transient motion of domain boundaries approaching the equilibrium position. Finally, we present theoretical simulation results of the oscillations.