Photoreflectance and photoluminescence spectroscopy of low-temperature GaAs grown by molecular-beam epitaxy

Photoreflectance (PR) and photoluminescence (PL) measurements have been carried out on low-temperature GaAs (LT GaAs) and LT-GaAs/AlGaAs modulation-doped photodetector (MODPD) structures grown by molecular-beam epitaxy. Samples with LT GaAs grown at 350 degrees C show several PL lines associated with (i) transitions involving two-dimensional electron gas in LT GaAs near the interface, (ii) band-edge transitions in the bulk LT GaAs, and (iii) transitions involving deep level defect complexes in LT GaAs. In addition a PL emission band at 1.65 eV observed in all the MODPD structures is attributed to a crossover transition at the LT-GaAs/AlGaAs interface. PR spectra of these modulation-doped structures show Franz-Keldysh oscillations which are attributed to high electric fields (> 10(5) V/cm) at the LT-GaAs/AlGaAs interface. PR measurements on bare LT-GaAs layers suggest that the Eo transition in LT GaAs is about 20 meV above that of the normal GaAs E(o) gap. (C) 1996 American Institute of Physics.