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 (rho(c)) calculated from TLM measurements on Au/p-GaN contacts was 53 Ohm . cm(2) after annealing at 800 degrees C. Multilayer Au/Mg/Au/p-GaN contacts exhibited linear, ohmic current-voltage (I-V) behavior in the as-deposited condition with rho(c) = 214 Ohm . cm(2). The specic contact resistivity of the multilayer contact increased significantly after rapid thermal annealing (RTA) through 725 degrees 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 degrees 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.