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 crystallized in the distorted octahedral layer structure of triclinic symmetry. Crystals with triclinic symmetry are optically biaxial. The interband transitions of ReS2 and ReSe2 are expected to chow anisotropic character for linearly polarized light, incident normal to the basal plane. The optical anisotropic effects for E parallel tob and E perpendicular tob polarizations were studied through polarization-dependent piezoreflectance (PzR) and optical :absorption measurements. The polarization dependence of the PzR spectra provides conclusive evidence that the features associated with the interband excitonic transitions are originated from different origins. The results of absorption measurements indicate that ReS2 and ReSe2 are indirect semiconductors, in which E parallel to b-axis polarization exhibits a smaller band gap and a single phonon makes an important contribution in assisting the indirect transitions. The electrical conductivity along the b-axis was shown to be several times higher than that perpendicular to b-axis for both crystals. (C) 2001 Elsevier Science B.V: All rights reserved.