Optical Absorption by Impurities inp-Type Gallium Phosphide

We present the results of a detailed study of optical absorption due to the oxygen donor and the Zn-O and Cd-O nearest-neighbor impurity complexes in $p-\mathrm{G}\mathrm{a}\mathrm{P}$. In solution-grown crystals doped for optimum luminescence efficiency we find that the absorption coefficient below the band gap over the visible region ($1.7 \mathrm{eV}<h\nu <2.3\mathrm{} \mathrm{eV}$) is typically in the range 2-4 ${\mathrm{cm}}^{-1\mathrm{}}$. Although a major fraction of the absorption in this region results from the O and the Zn-O or Cd-O centers, we find that other inadvertent impurities (e.g., Si, Cu, S, and C) also contribute to the absorption. The temperature dependence of the individual absorption bands due to Zn-O and Cd-O are found to be well described by a semiclassical single-linear-mode model for phonon-coupled impurity absorption. In contrast, the highly skewed oxygen absorption band requires (on theoretical grounds) a hybridization of the standard configuration-coordinate approach with a model which takes into account the continuum nature of the initial (valence band) state of the free-to-bound absorption. Qualitative agreement is found between the hybridized model and the shape of the oxygen band. Because of the large degree of lattice coupling associated with the oxygen transitions, we find that the oscillator strength in absorption is a factor 4-12 times stronger than that in emission. This effect is attributed to differences in the lattice configuration depending on whether the oxygen donor is in a neutral or ionized state. For the Zn-O transitions, a factor-of-3 enhancement of absorption to emission is found.