Anomalous Conductivity Switch Observed in Treated Hafnium Diselenide Transistors

Layered hafnium diselenide (HfSe₂) is an emerging Van der Waals semiconductor in which a hafnium layer is sandwiched between two selenium layers. Owning to its indirect band gap with magnitudes close to silicon's band gap and high predicted carrier mobility, hafnium diselenide material is a strong candidate for device applications. Here, the effect of laser treatment on 2H- HfSe₂ devices is shown in ambient conditions using µ-Raman spectroscopy. It is shown that an emerging Raman peak evolves with increasing laser exposure time. It is also shown that top-down fabricated 2H-HfSe₂ devices exhibit an anomalous p-type behavior post laser treatment, with I_on/I_off ratio as high as 10³. This anomalous conductivity change can be observed after thermal and electrical annealing. For bottom-up devices, it is observed that p-type conductivity with remarkable I_on/I_off ratio reaching 10⁴. This conductivity switch can also be shown on 1T-HfSe₂ devices post laser irradiation and high V_ds bias treatments. Based on the circuit model, this conductivity switch is attributed to contact doping caused by an increase in the Schottky barrier height at each contact, which shifts the Fermi energy closer to the valance band. These results demonstrate a unique conductivity switching mechanism for HfSe₂-FET devices.