Exciton-electron dynamics studied by microwave photoconductivity and photoluminescence in undoped ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ quantum wells

We report on a comparative study of the photoinduced microwave absorption (PMA) (contactless photoconductivity) at 35.6 GHz and the photoluminescence (PL) in undoped ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ quantum wells (QW’s) having various well widths (50–200 Å). While the PL and its excitation (PLE) spectra probe the excitonic transitions, the PMA intensity and its excitation (PMAE) spectra provide information on unbound electron-hole generation processes. The PMAE spectra show strong $\mathrm{}(\mathrm{e}1:\mathrm{h}\mathrm{h}1\mathrm{})\mathrm{}1S$ and $\mathrm{}(\mathrm{e}1:1\mathrm{h}1\mathrm{})\mathrm{}1S$ excitonic bands. Since these are bound electron-hole transitions that are observed to give rise to free carrier microwave absorption, exciton dissociation processes are involved. A model is presented that explains these bands in terms of Auger-like exciton dissociation at low temperatures and thermal exciton dissociation at high temperatures. We also discuss the effect of carrier and exciton localization in the spatially fluctuating QW potential.