Isotope effects on the optical spectra of gallium phosphide and cadmium selenide

The availability of samples of the binary compound semiconductors GaP and CdS with isotopically enriched gallium and sulfur composition respectively have allowed us to measure the change in the indirect band gap of GaP due to isotopic substitution of gallium and the change in the direct band gap of CdS due to isotopic substitution of sulfur. The technique of Fourier transform photoluminescence spectroscopy has been used to measure the fine structure of the sulfur donor-carbon acceptor pair band in samples of GaP with differing isotopic composition of gallium, resulting in a measurable shift of the fine structure spectra. In the CdS samples, photoluminescence and reflectivity spectroscopy have been used to measure the donor bound exciton spectrum and the free exciton spectrum, revealing a shift in the fine structure of these spectra between the samples of different isotopic composition of sulfur. These changes in bandgap are explained in the context of the renormalization of the electronic bandgap due to the electron-phonon interaction.