Band-edge properties of layered germanium dichalcogenides

The band-edge characteristic of germanium dichalcogenides Ge(Se,S)(2) has been characterized using thermoreflectance (TR) and resistivity measurements at 300 K. Two band-edge excitonic transitions denoted as E-A and E-B are found in each TR spectrum of Ge(Se1-xSx)(2) (0 <= x <= 1). A Rydberg excitonic series that contains two excitonic levels of n=1 and n=2 are simultaneously detected in the E-A and E-B transitions of GeSe2. For the other layered crystals, only the ground-state level (n=1) is observed. Angular-dependent and polarization-dependent TR measurements of Ge(Se1-xSx)(2) were carried out to identify the transition origins of the band-edge transitions. The experimental TR results show that Ge(Se1-xSx)(2) with x=0, 0.2, 0.4, 0.6, and 0.8 are crystallized in the high-temperature crystalline phase, while the GeS2 belongs to the high-temperature and low-temperature mixed phases. The high in-plane resistivities of the Ge(Se1-xSx)(2) indicate the high purity of the layer crystals.