Abstract

There has been intense interest in the past few years in photogenerated carrier escape mechanisms from quantum wells¹. This process is not only fundamental to understanding carrier dynamics in quantum confined systems, but has direct relevance to speed and intensity saturation limits for quantum well waveguide photodetectors and electroabsorption modulators², reverse biased laser structures³, and optical SEED devices⁴. Only recently¹, however, have escape times for both electrons and holes from single quantum wells been simultaneously and unambiguously measured with sub-picosecond resolution, and the results appeared to contradict simple existing theories for escape rates based on thermionic emission⁵ and tunneling. In this paper, we directly compare the results of our theory for photogenerated carrier escape rates as a function of applied field from single GaAs/AlGaAs quantum wells, with the experimental results from [1]; that is, for wells with x=0.2 and x=0.4 barriers at room temperature. We include thermionic emission (and for electrons include the effects of indirect conduction band minima in the well), thermally assisted, and direct tunneling. Our expressions for thermionic emission reduce, in the limit of large well width, to those⁵ which assume a 3D density of states. We assume that carriers in the well are in thermal equilibrium with each other and with the lattice, so “thermally assisted tunneling” refers here to tunneling from thermally occupied upper sub-bands - the 2D equivalent of Fowler-Nordheim tunneling. We also assume parabolic bands for holes within the quantum well, accounting for light / heavy hole mixing by using effective in-plane masses taken from the literature. Tunneling lifetimes are calculated using a standard⁶ transfer matrix approach to obtain the linewidths and energy levels of the quasi-bound states as a function of applied field. Under these assumptions the thermionic emission rate for electrons is

© 1994 Optical Society of America