Impact of intrinsic trap states on the electrical and optoelectronic behavior of ReS₂ and ReSe₂

Rhenium-based two-dimensional transition metal dichalcogenides, such as ReS₂ and ReSe₂, have attracted considerable interest for optoelectronic applications—including polarization-sensitive photodetectors—due to their weak interlayer coupling, strong in-plane anisotropy, and monolayer-like direct bandgaps. However, performance inconsistencies such as hysteresis, often induced by both intrinsic and extrinsic trap states, remain a challenge in TMD-based devices. In this study, we present a comparative analysis of the scattering mechanisms, hysteresis, and photoresponsivity of ReS₂ and ReSe₂ phototransistors by examining their transfer characteristics under varying temperatures and illumination conditions. Trap-state distributions were systematically investigated using wavelength-resolved sub-bandgap illumination, which enables emission of carriers from deeper trap states. The ReS₂ device exhibits electron–phonon scattering as the dominant transport mechanism, along with minimal hysteresis, attributed to a lower density of intrinsic traps with widely spaced trap states. In contrast, the ReSe₂ device is characterized by charge-impurity scattering and pronounced hysteresis, arising from a higher trap density with closely spaced trap states. Additionally, ReSe₂ displays a more gradual decline in the power-law exponent (γ) across a broader gate-voltage range and lower compared to ReS₂. These distinctions are linked to the higher formation energy of chalcogen vacancies in ReS₂, which makes it more resistant to defect formation than ReSe₂.