Symmetry dependence of optical transitions in group 4B transition metal dichalcogenides

Near-normal-incidence reflectivity measurements have been made over the energy range 0.5 to 5 eV at room temperature and 77K for the following layered materials: TiS2, TiSe2, ZrS2, HfSe2 and HfTe2. Both freshly cleaved basal planes (E perpendicular to c; k//c) and as-grown edge planes (E perpendicular to c, E//c; k perpendicular to c) have been studied and a strong dependence on polarisation was observed. Scattering from surface imperfections and dust particles on the as-grown edges has been taken into account. Data for Zr and Hf compounds have been analysed in terms of Lorentzian oscillators which were then used to provide the basis for the assignment of optical transitions. Trends in the spectra lead to the prediction of metallic behaviour for all the tellurides of group 4B dichalcogenides.