Optical Properties of Gallium Arsenide-Phosphide

The index of refraction of Ga (${\mathrm{As}}_{1-x}{\mathrm{P}}_x$) is measured as a function of photon energy by the minimum-angle-of-deviation prism method at 300 and ${87}^{\mathrm{o}}$K. The dielectric constant in the optical region is found to vary linearly as a function of mole fraction of GaP between the values for GaAs and GaP. Curve-fit data are presented which allow calculation of index of refraction at 300 and ${87}^{\mathrm{o}}$K for a given crystal composition $x$ and photon energy. Transmission data on Ga (${\mathrm{As}}_{1-x}{\mathrm{P}}_x$) are obtained at 300 and ${77}^{\mathrm{o}}$K. The reflectivity is calculated from the index of refraction and is used to calculate the absorption constant, assuming multiple internal reflections in the crystal. In the infrared, true free-carrier absorption is observed for degenerate GaAs-rich mixed crystals, whereas absorption due to transitions from donor levels to the conduction band is dominant in GaP-rich mixed crystals. The band-to-band absorption seen by Spitzer and Whelan in GaAs is observed in GaAs-rich alloys, and is thought to be due to transitions from the (0,0,0) minimum to minima in the $〈111〉$ directions. Finally, the absorption edge is investigated, and the Burstein shift of the absorption edge to higher energies with increased doping is observed, as well as shifts to lower energies due to compensation. In all cases, the fundamental absorption edge is found to exhibit an exponential dependence on photon energy.