In-plane anisotropy of the optical and electrical properties of ReS2 and ReSe2 layered crystals

ReS2 and ReSe2 belong to the family of transition-metal dichalcogenides cry stallized in the distorted octahedral layer structure of triclinic symmetry . Crystals with triclinic symmetry are optically biaxial. The interband tra nsitions of ReS2 and ReSe2 are expected to chow anisotropic character for l inearly polarized light, incident normal to the basal plane. The optical an isotropic effects for E parallel tob and E perpendicular tob polarizations were studied through polarization-dependent piezoreflectance (PzR) and opti cal :absorption measurements. The polarization dependence of the PzR spectr a provides conclusive evidence that the features associated with the interb and excitonic transitions are originated from different origins. The result s of absorption measurements indicate that ReS2 and ReSe2 are indirect semi conductors, in which E parallel to b-axis polarization exhibits a smaller b and gap and a single phonon makes an important contribution in assisting th e indirect transitions. The electrical conductivity along the b-axis was sh own to be several times higher than that perpendicular to b-axis for both c rystals. (C) 2001 Elsevier Science B.V: All rights reserved.