Bandgap and optical absorption edge of GaAs_1-xBi_x alloys with 0 < x < 17.8%, M. Masnadi-Shirazi, R. B. Lewis, V. Bahrami-Yekta, T. Tiedje, M. Chicoine, P. Servati, J. Appl. Phys. 116, 223506 (2014)

The compositional dependence of the fundamental bandgap of pseudomorphic GaAs_1-xBi_x layers on GaAs substrates is studied at room temperature by optical transmission and photoluminescence spectroscopies. All GaAs_1-xBi_x 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 GaAs_1-xBi_x bandgap is predicted to reach 0 eV at 35% Bi. Below the GaAs_1-xBi_x 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 GaAs_1-xBi_x 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 A, from extrapolation of the Rutherford backscattering spectrometry and x-ray diffraction data.