Low-Level Interband Absorption in Phosphorus-Rich Gallium Arsenide-Phosphide

The optical absorption edge of GaP is influenced in two principal ways when a few percent of the phosphorus atoms are replaced by arsenic. Besides the uniform decrease of the indirect band gap, the mixed crystal exhibits an extra absorption component. It is shown that this component is due to the no-phonon creation of free excitons and free-electron-hole pairs. Momentum is conserved through scattering at the arsenic impurities. The cross section for this process apparently decreases with increasing kinetic energy of the free excitons. In addition, the matrix element for indirect transitions assisted by the emission of LA phonons is enhanced by the decrease in the $X_{{}_{}{}^1{}_{}{}^{}c}-{\Gamma }_{{}_{}{}^1{}_{}{}^{}c}$ energy separation in the mixed crystal. The replacement of phosphorus by arsenic in GaP is a relatively minor perturbation. Unlike the case of nitrogen in GaP, there is no bound state. The near-threshold absorption due to the arsenic-induced creation of free particles is only ∼0.1% of that due to nitrogen substituents.