Pros and Cons of (NH₄)₂S Solution Treatment of p-GaN/Metallization Interface: Perspectives for Laser Diode

The impact of wet treatment using an (NH₄)₂S-alcohol solution on the interface state of the p-GaN/Ni/Au/Pt contact system and laser diode processing was investigated. Sulfur wet cleaning resulted in reduced surface roughness and contact resistivity. The lowest specific contact resistance (ρ < 1 × 10^-4 Ω·cm²) was achieved with samples treated with an (NH₄)₂S-isopropanol solution, whereas the highest resistivity (ρ = 3.3 × 10^-4 Ω·cm²) and surface roughness (R = 16 nm) were observed in samples prepared by standard methods. Annealing the contact system in an N₂ + O₂ + H₂O atmosphere caused degradation through species inter-diffusion and metal-metal solid solution formation, irrespective of the preparation method. Standard prepared substrates developed a thin GaN-Au intermediate layer at the interface after heat treatment. Enhanced adhesion and the absence of GaN decomposition were observed in samples additionally cleaned with the (NH₄)₂S-solvent solution. Complete oxidation of nickel to NiO was observed in samples that underwent additional sulfur solution treatment. The intensity of metal species mixing and nickel oxidation was influenced by the metal diffusion rate and was affected by the initial state of the GaN substrate obtained through different wet treatment methods.