Stable low-resistance ohmic contact on GaN via an oxynitride interlayer over 20–200 °C: Mechanisms and performance

Achieving reliable, low-resistance ohmic contacts in GaN-based devices is fundamentally limited by interfaces with high interface-state densities, which are a direct result of essential fabrication steps. For example, etching to expose the n-GaN layer in laser diodes introduces surface defects, while regrown n+-GaN surfaces in selective-area regrowth (SAG) HEMTs exhibit high interface states due to growth kinetics. These high-frequency and high-power devices typically operate at elevated junction temperatures (100–200 °C), where high interface-state density exacerbates Fermi-level pinning (FLP) effect, leading to unstable contact resistance and reliability concerns. To address this issue, we propose and validate a universal interface engineering strategy: inserting an oxynitride interlayer. We systematically study Ti-based ohmic contacts on SAG n+-GaN from 20 to 200 °C, comparing untreated interfaces with those modified by TiOxNy or GaOxNy interlayers. Through correlated microstructural analysis, temperature-dependent electrical measurements, and conductance-based interface-state characterization, we show that the interlayers effectively suppress interface-state density, mitigate FLP, and stabilize the Schottky barrier height. While untreated contacts display non-monotonic and degrading resistivity with temperature, interlayer-modified contacts exhibit a stable, monotonic decrease in specific contact resistivity. The GaOxNy interlayer yields the best performance, achieving ∼10−8 Ω cm2 order above 100 °C. This work elucidates the critical role of interface states in contact thermal instability and provides a viable materials solution for stable ohmic contacts in GaN-based devices that operate at high temperatures with damaged or low-quality contact interface.