Gap vibration modes of mass defects in the cubic compound semiconductor ZnS

Abstract The gap vibration modes caused by mass defects in the cubic semiconductor ZnS are investigated using the Green's function method and the rigid-ion model for the host phonon spectra, with some surprising results. (i) The gap modes are generally less localized than the high frequency localized modes, and the vibrations can be optic or acoustic, anion-like or cation-like, depending on the defect site and the defect mass. (ii) The gap mode amplitudes do not necessarily peak at the defect site or fall monotonically with distance from the site, as once believed. (iii) A mode which is caused by replacing a heavier host atom (Zn) with an even heavier impurity atom is the least localized one, with only a few per cent of the normalized vibration amplitude on the impurity site. Probably this is the reason that this mode, predicted theoretically, has never been observed experimentally.