Electronic transport and polarization-dependent photoresponse in few-layered hafnium trisulfide (HfS3) nanoribbons

Abstract

We report on the electrical and optoelectronic characterization of field-effect transistor (FET) devices based on few-layered HfS₃ nanoribbons that were mechanically exfoliated from bulk crystals. According to theoretical calculations, bulk HfS₃ crystals require small energies for exfoliation along the (001) planes that are comparable to the cleavage energies of graphene layers from graphite. If measured in air, the devices show a p-type response, which is likely caused by physisorbed and chemisorbed oxygen species. In a vacuum, the devices exhibit n-type conductivity and a large photoresponse to white light and several lasers with wavelengths in the visible range of the spectrum. The device photocurrent exhibited a strong dependence on the direction of polarization, which is related to the highly anisotropic quasi-1D crystal structure of HfS₃. Optical absorption spectroscopy indicates a direct optical band gap of about 2.3 eV and an indirect band gap of about 2 eV. The indirect gap is supported by the band structure that was calculated using density functional theory (DFT). According to the DFT results, direct and indirect band gaps are present in both monolayered and few-layered HfS₃ crystals and decrease with an increase in the number of layers. The absence of strong photovoltaic charging in X-ray photoemission indicates mobile hole carriers. The effect supports the contention that in the presence of light, the photocarriers include both electrons and holes, hence enhancing the photocurrent of the devices. X-ray absorption spectroscopy indicates the S-p–Hf-d hybridization at the conduction band minimum, which is consistent with the calculated band structure.