Investigation of the Anomalous Raman Line Shape of the TO Phonon in GaP

The Raman spectrum of the transverse optical (TO) phonon in GaP is known to exhibit an asymmetric line shape at room temperature, broadened to lower energies. High resolution Raman measurements on GaP crystals at low temperature reveal a characteristic fine structure in the line shape of the TO phonon that has been studied systematically by variation of the isotopic composition of the gallium atoms. Calculations of the Raman spectra based on third-order anharmonic phonon–phonon coupling account for the experimental findings. In this paper we give an extended view into the nature of the line shape. The understanding of the low temperature line shape is necessary for the interpretation of changes in the spectra with rising temperature. The temperature dependence of the TO Raman line of natural GaP is presented as well as a qualitative explanation for the broadening in terms of anharmonic effects.