In this paper, we investigate the effect of Pd thickness and heat treatment on Pd/Ni/Au/p-GaN metal contacts. The as-deposited samples exhibit a smooth morphology and non-linear I-V characteristics. Heat treatment in a N2 atmosphere leads to degradation of the contact microstructure, resulting in diffusion of Ga, void formation on the interface and mixing of metals. Annealing in a mixture of N2 and O2 improves adhesion and reduces contact resistance. However, this process also induces GaN decomposition and species mixing. The mixing of metal-Ga and metal-metal remains unaffected by the method of thermal treatment but depends on gas composition for thin Pd contacts. To achieve low-resistance contacts (≈1 × 10-4 Ω cm2), we found that increasing the Pd thickness and using N2 + O2 as the annealing environment are effective measures. Nevertheless, the degradation effect of the annealed contact microstructure in the form of the void generation becomes evident as the thickness of Pd increases. Laser diodes (LDs) with optimized palladium-based contacts operate at a voltage of 4.1 V and a current density of 3.3 kA/cm².
Keywords: diffusion; gallium nitride (GaN); gold (Au); laser diode (LD); nickel (Ni); ohmic contact; palladium (Pd); solid-state solution; specific contact resistance (ρ); thermal evaporation.