Photoluminescence of two-dimensional excitons in an electric field: Lifetime enhancement and field ionization in GaAs quantum wells

Two-dimensional excitons in GaAs/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As single quantum wells exposed to electric fields perpendicular to the layers are studied by means of time-resolved photoluminescence. The temporal decay of the excitonic emission distinguishes two characteristic field regimes: In the small-field regime, luminescence lifetime increases with increasing field, and a pronounced Stark shift is additionally observed. In the high-field domain \ensuremath{\ge}50 kV/cm, the lifetime decreases with increasing field because of excitonic field ionization, leading to carrier tunneling through the barriers. We discuss these features within the framework of a simple semiclassical model. Quantitative agreement is obtained for quantum wells of different well widths and barrier thicknesses with respect to lifetime, luminescence intensity, and tunneling current. Thus a consistent description of the dynamics of two-dimensional excitons exposed to an electric field is obtained.