β-Ga2O3 Heterojunction Field-Effect Transistors Prepared via UV Laser-Assisted p-Doping of WSe2

Beta gallium oxide (β-Ga₂O₃) is a semiconductor material that has recently become the focus of research for next-generation high-power devices. The application of β-Ga₂O₃ in high-power devices is propitious due to its features including the ultra-wide bandgap energy of 4.85 eV, the large breakdown field of ~8 MV/cm (estimated), and high Baliga’s figure of merit of approximately 3214. From the device fabrication point of view, the monoclinic structure of a β-Ga₂O₃ crystal with an anisotropic unit cell allows the simple fabrication of low-scale β-Ga₂O₃ devices through mechanical exfoliation. However, p-doping of β-Ga₂O₃ is mostly unachievable due to its natural n-type conductivity of β-Ga₂O₃ attributed to the oxygen vacancies in its crystal. This limits the application of β-Ga₂O₃ in integrated devices where the semiconductor substrate requires both n- and p-doped areas. Various methods to incorporate the n-type β-Ga₂O₃ and a p-type/ambipolar material into a p-n heterojunction are being studied to address such problems. Many of the materials studied for this reason are transition metal dichalcogenides (TMDs). TMDs have a unique crystal structure composed of layers stacked on one another by interplanar van der Waals forces. The electrical and structural features stemming from such a unique structure are favorable for the fabrication of low-scale high-performing devices. These features include high in-plane carrier mobility, atomic-scale thickness, and atomically clean surface enabling the TMDs to form heterostructures with other materials via van der Waals interaction. In this work, tungsten diselenide (WSe₂) is incorporated with β-Ga₂O₃ to form the p-n heterojunction. WSe₂ exhibits ambipolar conductivity which can be modified to either n- or p-type by further processing, which includes argon plasma treatment, ultraviolet/ozone treatment, and laser-assisted oxidation. In this work, laser-assisted oxidation was used to selectively oxidize the WSe₂ and to minimize any unwanted side-effects on the β-Ga₂O₃ of the WSe₂/β-Ga₂O₃ heterojunction.<br/>In this work, β-Ga₂O₃ and WSe₂ flakes were obtained through mechanical exfoliation using an adhesive tape. β-Ga₂O₃ flakes were dry-transferred onto Si/SiO₂ substrates before Ti/Au (50/100 nm) source/drain electrodes, defined by electron-beam (e-beam) lithography, were deposited using an e-beam evaporator. Rapid thermal annealing was conducted in low vacuum (~10^-2 Torr) at 500°C for 60 s to form Ohmic Ti/β-Ga₂O₃ contacts. WSe₂ flakes were dry-transferred onto a desired position on the β-Ga₂O₃ channels. Pt/Au (20/80 nm) source/drain electrodes of the WSe₂ channel were defined by e-beam lithography and were deposited using an e-beam evaporator. Thermal annealing was conducted in high vacuum (~6×10^-6 Torr) at 200°C for 120 min to improve the Pt/WSe₂ contact. 325 nm laser with an intensity of 0.135 mW was utilized to selectively oxidize the WSe₂ in the WSe₂/β-Ga₂O₃ heterojunction field-effect transistor (HJFET).<br/>The fabricated WSe₂/β-Ga₂O₃ p-n diode exhibited a distinct rectifying behavior with an outstanding ideality factor of 1.16 and showed a five-order-of-magnitude decrease in its on-resistance after laser-assisted p-doping of WSe₂, leading to a rectification ratio of ~10⁶. The β-Ga₂O₃ HJFET with a WSe₂ top-gate exhibited excellent FET characteristics including stable current saturation properties, an on/off ratio of ~10⁷, a field-effect mobility of 10.84 cm²/Vs, and a subthreshold swing of 76.84 mV/dec. The eminent overall properties of the device can be attributed to the formation of p-n heterojunction through the selective p-doping of WSe₂ and a high-quality WSe₂/β-Ga₂O₃ interface leading to the increase of the depletion width of the β-Ga₂O₃ channel. This work demonstrates a low-scale high-quality β-Ga₂O₃ HJFET with a high switching speed and expands the potential applications of β-Ga₂O₃-based high-power devices by incorporating the n-type β-Ga₂O₃ with the p-doping controllability of WSe₂.