Microstructure, electrical properties, and thermal stability of Au-based ohmic contacts to p-GaN

The work described in this paper is part of a systematic study of ohmic contact strategies for GaN-based semiconductors. Gold contacts exhibited ohmic behavior on p-GaN when annealed at high temperature. The specific contact resistivity (ρ_c) calculated from TLM measurements on Au/p-GaN contacts was 53 Ω · cm² after annealing at 800 °C. Multilayer Au/Mg/Au/p-GaN contacts exhibited linear, ohmic current-voltage (I-V) behavior in the as-deposited condition with ρ_c = 214 Ω · cm². The specific contact resistivity of the multilayer contact increased significantly after rapid thermal annealing (RTA) through 725 °C. Cross-sectional microstructural characterization of the Au/p-GaN contact system via high-resolution electron microscopy (HREM) revealed that interfacial secondary phase formation occurred during high-temperature treatments, which coincided with the improvement of contact performance. In the as-deposited multilayer Au/Mg/Au/p-GaN contact, the initial 32 nm Au layer was found to be continuous. However, Mg metal was found in direct contact with the GaN in many places in the sample after annealing at 725 °C for 15 s. The resultant increase in contact resistance is believed to be due to the barrier effect increased by the presence of the low work function Mg metal.