Improvement of β-Ga2O3 Junction Field-Effect Transistors By Employing WSe2/β-Ga2O3 p-n Heterojunction Via Laser-Assisted Oxidation

Abstract

Beta gallium oxide (β-Ga2O3) is a semiconductor material that has been receiving a lot of attention during the past years owing to its suitability for next-generation high-power devices. The attributes of β-Ga2O3 that make it attractive for use in high-power devices include ultra-wide bandgap energy of 4.9 eV, large breakdown field of ~8 MV/cm (estimated), and high Baliga’s and Johnson’s figures of merit of approximately 3214 and 2844, respectively. From the device fabrication point of view, β-Ga2O3 crystal having a monoclinic structure with an anisotropic unit cell facilitates the effortless fabrication of low-scale β-Ga2O3 devices through simple mechanical exfoliation. However, the nature of β-Ga2O3 that it is n-doped by its oxygen vacancies and its difficulties in p-doping limit the application of β-Ga2O3 in integrated structures such as the CMOS (complementary metal-oxide-semiconductor), where the semiconductor substrate is required to have both n- and p-doped areas. Various studies, with the aim of finding alternative solutions to this problem, are being conducted on the fabrication of p-n heterojunctions composed of n-type β-Ga2O3 and p-type or ambipolar semiconductor materials. Among them, transition metal dichalcogenides (TMDs) are studied for the p-n heterojunctions involving the n-type β-Ga2O3. TMDs have a crystal structure composed of layers, consisting of in-plane repeating units with two to three atoms, attached to one another by interplanar van der Waals interaction which gives rise to favorable electrical and structural characteristics, including high in-plane carrier mobility, atomic-scale thickness, and atomically clean surface allowing TMDs to form van der Waals heterostructures without strain from heteroepitaxial growth. In this work, tungsten diselenide (WSe2), which has a moderate bandgap of ~1.2 to 1.7 eV (bulk to monolayer, respectively), is utilized to form the p-n heterojunction with β-Ga2O3. WSe2 exhibits ambipolar transport characteristics in nature and shows excellent doping controllability through processes such as Ar plasma treatment, ultraviolet/ozone treatment, and laser-assisted oxidation. In this work, laser-assisted oxidation was utilized for the selective p-doping of WSe2 in the WSe2/β-Ga2O3 heterojunction area.In this work, β-Ga2O3 and WSe2 flakes were mechanically exfoliated from single crystal bulks with an adhesive tape, respectively. β-Ga2O3 flakes were transferred via dry transfer method onto Si/SiO2 substrates, followed by electron-beam lithography to define the β-Ga2O3 channels. Ti/Au (50/100 nm) source/drain electrodes were deposited by electron-beam evaporator. Rapid thermal annealing was performed in low vacuum (~10-2 Torr) at 500℃ for 60 s to form Ohmic contacts. Selected WSe2 flakes were transferred onto a specific position of the β-Ga2O3 channels by PDMS-film-assisted dry transfer method. Source/drain electrodes of the WSe2 channel were defined using electron-beam lithography, followed by Pt/Au (20/80 nm) electrode deposition with an electron-beam evaporator. Fabricated devices were thermally annealed in a high vacuum condition (~6×10-6 Torr) at 200℃ for 120 min to improve the contact. 325 nm laser with an intensity of 0.135 mW was utilized for the selective oxidation of WSe2 in the WSe2/β-Ga2O3 heterojunction.The fabricated device exhibited distinct rectifying behavior with an ideality factor of 1.16 and an on/off ratio of ~106, where the on-resistance decreased by approximately five orders of magnitude after WSe2 oxidation. β-Ga2O3 JFET with a WSe2 top gate showed outstanding characteristics with a subthreshold swing of 76.84 mV/dec and an on/off ratio of ~107. This work demonstrates low-scale, high-quality β-Ga2O3 JFETs with WSe2, paving the way to the potential applications of high power β-Ga2O3 device with high switching speed and small device footprint.