Bandgap and optical absorption edge of GaAs1−xBix alloys with 0 < x < 17.8%

The compositional dependence of the fundamental bandgap of pseudomorphic GaAs1−xBix layers on GaAs substrates is studied at room temperature by optical transmission and photoluminescence spectroscopies. All GaAs1−xBix films (0 ≤ x ≤ 17.8%) show direct optical bandgaps, which decrease with increasing Bi content, closely following density functional theory predictions. The smallest measured bandgap is 0.52 eV (∼2.4 μm) at 17.8% Bi. Extrapolating a fit to the data, the GaAs1−xBix bandgap is predicted to reach 0 eV at 35% Bi. Below the GaAs1−xBix bandgap, exponential absorption band tails are observed with Urbach energies 3–6 times larger than that of bulk GaAs. The Urbach parameter increases with Bi content up to 5.5% Bi, and remains constant at higher concentrations. The lattice constant and Bi content of GaAs1−xBix layers (0 < x ≤ 19.4%) are studied using high resolution x-ray diffraction and Rutherford backscattering spectroscopy. The relaxed lattice constant of hypothetical zincblende GaBi is estimated to be 6.33 ± 0.05 Å, from extrapolation of the Rutherford backscattering spectrometry and x-ray diffraction data.