Effect of the isotopic mass of gallium on the indirect gap of GaP

We have obtained pair recombination luminescence spectra of four GaP crystals with different gallium isotopic compositions. The frequency shifts measured vs. average isotopic mass allow us to determine dE0/dM=0.19(1) meV/amu, where E0 is the indirect gap and M the average gallium isotopic mass. From this mass derivative, a contribution of the gallium vibrations to the zero-point mass renormalization of the gap of 27 meV is estimated. Subtracting this contribution from the total zero point renormalization, as determined from the temperature dependence of the indirect gap, yields a contribution of 59 meV of the phosphorus vibrations to the gap renormalization. These results are discussed in comparison with similar data for group IV, III–V, and II–VI semiconductors. A comparison with theoretical calculations is also presented.