Post-Annealing Temperature Effects on Electrical Characteristics of Sputtered Mo/β-Ga₂O₃ Vertical Schottky Barrier Diodes

In this study, Mo/β-Ga₂O₃ vertical Schottky barrier diodes (SBDs) were fabricated using a sputtering process, and the electrical characteristics were evaluated as a function of post-annealing temperature. β-Ga₂O₃ is an ultra-wide bandgap semiconductor (UWBG) with a bandgap of about 4.8 eV and a critical breakdown field of 8 MV/cm, a promising material for high-voltage power switching applications. Mo (molybdenum) has a higher work function (~4.95 eV) than the electron affinity (~4.0 eV) of β-Ga₂O₃, its melting point (2623 °C) is higher than Pt (1768 °C) and Ni (1455 °C), so it has excellent thermal stability, and it is selected as a Schottky metal. The device is a structure in which a Si-doped β-Ga₂O₃ epitaxial layer (10 μm, N_d-N_a = 2.2 × 10¹⁶ cm⁻³) is grown on an Sn-doped β-Ga₂O₃ substrate (415 μm, N_d-N_a = 4.5 × 10¹⁸ cm⁻³). The Ti/Au (10/40 nm) was used for the back ohmic junction and Mo (300 nm) was used for the front Schottky junction. Post-annealing treatment was performed at 400, 500, and 550 °C using a rapid thermal annealing process (RTA) in an Ar gas atmosphere, and in this process, the Schottky junction and ohmic junction were formed simultaneously. Electrical characteristics including current-voltage (I–V), capacitance-voltage (C–V), Schottky barrier height (SBH), ideality factor (n), turn-on voltage (V_on), on-resistance (R_on), on/off ratio, and breakdown voltage (BV) were evaluated. No obvious Schottky characteristics were observed before post-annealing treatment, which means that As-deposited Mo does not form a rectifying junction on the β-Ga₂O₃ without post-annealing treatment. After post-annealing treatment, the I–V curve of the Schottky rectification characteristics could be confirmed under all three conditions. Among the three conditions, the device annealed at 500 °C exhibits best performance, with an SBH of 0.96 eV, n of 1.01, V_on of 0.72 V, R_on of 13.8 mΩ·cm², an on/off ratio of 10⁹, a breakdown voltage of −474 V, and a PFOM of 16.3 MW/cm². As a result of temperature-dependent I–V measurement, as chuck temperature increased, the reverse leakage current increased and SBH decreased in all devices, which is consistent with the thermally activated carrier transport. These results demonstrate that the Mo/β-Ga₂O₃ SBDs are a thermally stable contact for power device applications.