Vertical Van Der Waals Heterojunction Diodes Comprising 2D Semiconductors on 3D β-Ga₂O₃

Monocyclic Gallium oxide β-Ga₂O₃ is a promising candidate for high-power and high-temperature electronics and UV-range optoelectronic devices. Its advantageous properties include an ultra-wide bandgap, high critical electric field, and affordable bulk single crystal melt-growth techniques at the large production scale. However, single crystalline β-Ga₂O₃ substrates are seldom explored, in part due to the uncertain effects of the semiconductor’s proven anisotropy. In this study, we fabricated 2D/3D β-Ga₂O₃ vertical heterojunctions and comprehensively assessed the effect of the choice of 2D material, contact metal and substrate crystalline orientation on their performance. Three orientations of degenerately n-doped β-Ga₂O₃ were considered: (001), (010) and (-201). Tungsten diselenide (WSe₂), tungsten disulfide (WS₂) and black phosphorus (BP) were chosen as 2D semiconductor materials for their popularity in high-performing optoelectronic devices, field-effect transistors (FETs) and light-emitting diodes (LEDs) among others. We compared Electron beam (E-beam) evaporated titanium (Ti), molybdenum trioxide (MoO₃) and palladium (Pd) as candidates for electrical contacts, with thicknesses based on previous reports of low-resistance: Ti/Au (10/90nm), MoO₃/Au (3/30nm), Pd/Au (30/70nm). The structure and properties of our fabricated devices were then evaluated using current-voltage (I-V) measurements at varying temperatures, atomic-force microscopy (AFM) techniques and technology computer-aided design (TCAD) simulations. Scanning transmission electron microscopy (STEM) cross-sectional images verified the clean interfaces of the heterostructures. We were able to realize high-performing diodes, whose ideality factors ranged from 1 to 3. Our findings suggest that heterojunction performance is optimized by the β-Ga₂O₃ orientation (-201), combined with 2D WS₂ exfoliated layers and Ti metal contacts, and show record rectification ratios (> 10⁶) concurrently with ON current density (> 10³ A/cm²) for application in power rectifiers. The lowest ideality factors along the (-201) orientation were 1.18 (BP & Ti/Au contacts), 1.22 (WS₂ & Ti/Au contacts) and 1.43 (WS₂ & Pd/Au contacts) in line with some of the best 2D/3D van der Waals heterojunction diodes reported in the past. The TCAD simulations were able to verify this, and accurately mimic the (-201) heterostructures’ behavior. At large biases and before the junctions reach saturation, the exponential current density increase matches the strongly rectifying property of ideal diodes. In such a linear regime, the devices are driven by the diffusion current across the internal 2D/3D potential barrier. Moreover, the reverse and low forward bias regions present current densities that are both very low (<10^-10 A/µm²) and independent of the applied voltage. Our findings demonstrate a facile fabrication of strongly rectifying and high ON current density Ga₂O₃-based heterojunction diodes and enables this material’s potential for new avenues in solid state devices.