Photoionization of electrons and holes at oxygen donors in gallium phosphide

The spectral dependence of the photoionization cross-section for the oxygen donor in GaP is studied in the temperature range between 90 and 410°K by steady state photocurrent measurements using two different radiation sources. From a fit of Lucovsky's model to the experimental results, accurate information is obtained about the energy position of the oxygen donor at all temperatures. It is found that the oxygen level is pinned to the valence band for temperatures above 295 K and that at these temperatures the Franck-Condon shift is very small. This may indicate that there is little interaction between the impurity level and the surrounding lattice.