PHOTOLUMINESCENCE OF ALXGA1-XAS NEAR THE GAMMA-X CROSSOVER

Spatially resolved experiments have been performed on the luminescence of bevelled AlxGa1-xAs structures at different temperatures as a function of the alloy composition. The AlAs mole fraction (x) of the samples investigated ranges from 0.15 to 0.52. This was determined by measurements employing the electron microprobe analysis. At T=85 and 300 K the band-gap energy of direct-gap material Eg(x) has been evaluated from band-to-band optical transitions, whereas at lower temperatures (T=2 and 38 K) the peak position of the luminescence line originating from the radiative decay of excitons bound to neutral donors served as a measure for the composition dependence of the fundamental energy gap in the direct-gap as well as in the indirect-gap region. For AlAs contents lower than 40 mol.% the luminescence data show that Eg(x) varies linearly with the composition. Our observations at different temperatures suggest that the temperature coefficient of the direct band gap increases with rising x. In contrast to Dingle's results for the crossover parameters we find xc=0.374 and Ec=2.057 eV at T=38 K and xc=0.41 and Ec=1.98 eV at T=300 K.