Considerable attention has been devoted recently to the dependence of the widths of the Raman phonons of semiconductors on pressure and on isotopic mass. The dependence on pressure is usually small and monotonic unless the phonon happens to be close to a singularity of the two-phonon density of states (DOS) which determines its width. In the latter case, strong nonmonotonic dependences of the phonon width on pressure and on isotopic mass can appear. We have investigated the E2high phonons of ZnO crystals with different isotopes and observed a wide range of FWHM depending on isotopic masses. Ab initio calculations of the two-phonon DOS provide an explanation for this variation of the FWHM: the E2high frequency falls on a sharp ridge of the 2-DOS corresponding to combinations of TA and LA phonons. Changes in isotopic mass result in a motion of the E2high frequency up and down that ridge which produces the changes in FWHM. These phenomena suggest a decrease of the FWHM with pressure which seems to be present in existing data obtained at 300 K. Similar phenomena are discussed for the E2low phonons. Applications of the isotope and pressure techniques to the elucidation of two-phonon spectra will be presented.
