The band gap of GaAsBi epitaxial layers as a function of bismuth concentration up to 3.6% is determined. The optical transitions were measured by modulated electroreflectance. The energy of the band gap decreases at a linearized rate of 88 meV/% Bi, or 83 meV/% Bi for the heavy hole to conduction band transition for GaAsBi strained to GaAs. The valence-band splitting increases faster than that of GaAs under similar compressive strain whereas the temperature dependence of the observed GaAsBi transitions is similar to that of GaAs.
The temperature dependence of the optical-absorption edge (Urbach edge) of GaAs is measured in semi-insulating and n-type GaAs (n=2X 10" cmm3) over the temperature range from room temperature to 700 "C. Both the optical absorption and the temperature are measured using a diffuse reflectance technique. The characteristic energy of the exponential absorption edge is found to increase linearly with temperature, from 7.5 meV at room temperature to 12.4 meV at 700 "C, for semi-insulating GaAs. The temperature dependent part of the width of the Urbach edge for semi-insulating GaAs is six times smaller than predicted by the standard theory where the edge width is proportional to the phonon population.
The temperature dependences of the optical absorption edges of semi-insulating GaAs and InP have been measured from room temperature to 905°C and 748°C, respectively, with accuracies of Ϯ1°C at room temperature and Ϯ5°C at 900°C. The temperature dependence of the optical absorption edge is adequately reproduced by an Einstein model although the Varshni model gives an improved fit to the band gap. Finally, the widths of the absorption edges are correlated with ionicity.
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