Influence of transport properties on the excitation spectra of GaAs/Al_xGa_1-xAs superlattices and bulk layers

In GaAs/Al_xGa_1-xAs superlattices, GaAs and AlGaAs epitaxial layers, the results of photoluminescence excitation spectroscopy are shown to depend strongly on carrier mobilities when, at a given distance from the surface, an escape mechanism exists, i.e. a possibility for carriers to escape from the active layer after moving through it. When there is no escape mechanism close enough to the surface, excitation spectra are roughly flat apart from the excitonic resonances. When an escape mechanism is present at a distance of around 1 mu m from the surface, if the mobility is high enough as in GaAs and superlattices with periods smaller than about 70 AA (for x approximately 0.3 and nearly equal well and barrier widths), the excitation spectra exhibit a decreasing high-energy tail; on the contrary in AlGaAs and superlattices with greater periods the excitation spectra have the same shape as in similar samples without an escape mechanism. Superlattices with small periods constitute better structures than equivalent AlGaAs layers for device applications.