Fabrication of vertical GaN Schottky barrier diodes on free-standing GaN substrates and their characterization

Vertical GaN Schottky barrier diodes (SBDs) on free-standing GaN substrates have garnered significant attention in recent years. Using a free-standing GaN substrate with a low defect density, a vertical GaN-on-GaN SBD can be operated under higher power-ratings and show a lower on-resistance and higher breakdown strength, compared to its lateral-current-flow counterpart. In this study, a vertical GaN SBD was fabricated with an epitaxially grown 10-μm-thick lightly doped n-type drift layer of GaN on a 400-μm-thick GaN substrate. An ohmic contact of a Ti/Al bilayer with a specific contact resistivity of 1.9 × 10⁻³ Ω cm² was fabricated on the N-polarity GaN bottom surface. Sequentially, Ni electrode-based Schottky contacts were formed on the Ga-polarity top surface. Low reverse currents were observed, with Schottky barrier heights of approximately 0.9–1.0 eV. A mesa-etch process was employed to define a channel width in the SBDs. The dependence of the mesa-etch depth on the breakdown voltage revealed a gradual increase in the breakdown voltage of the vertical GaN SBDs with the increase in mesa depth.