Gallium nitride (GaN) doped with germanium at a level of 10²⁰ cm⁻³ is proposed as a viable material for cladding layers in blue- and green-emitting laser diodes. Spectral reflectometry and ellipsometry are used to provide evidence of a reduced index of refraction in such layers. The refractive-index contrast to undoped GaN is about 0.990, which is comparable to undoped aluminium gallium nitride (AlGaN) with an aluminium composition of 6%. Germanium-doped GaN layers are lattice-matched to native GaN substrates; therefore, they introduce no strain, cracks, and wafer bowing. Their use, in place of strained AlGaN layers, will enable significant improvements to the production process yield.

Various experimental approaches of the wet nanoscale treatment have been proposed to account for features of the InAs, InSb and GaAs, GaSb semiconductor dissolution process in the (NH₄)₂Cr₂O₇–HBr–EG etching solution. Etching kinetics data showed that a crystal dissolution has diffusion-determined nature. The lowering of the solvent concentration from 80 to 0 vol.% in the solution was accompanied by a significant increase in the semiconductor etching speed. Depending on the solution composition, we have studied two types of crystal surface morphology, polished and passivated by the film, which was formed after chemical-dynamic (CDP) and/or chemical-mechanic polishing (CMP) in the solution, saturated by solvent and by oxidant, accordingly. It was found that in the polished etchants both CDP and CMP procedures lead to the formation of the mirror-like and super-smooth surface with nanoscale roughness less than 1 nm. The obtained results of surface state indicate that the (NH₄)₂Cr₂O₇–HBr–EG etchants could be used successfully for controllable CDP and CMP treatment of III–V semiconductors and formation of super-smooth surface.

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.

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_c < 1 × 10⁻⁴ Ω·cm²) was achieved with samples treated with an (NH₄)₂S-isopropanol solution, whereas the highest resistivity (ρ_c = 3.3 × 10⁻⁴ Ω·cm²) and surface roughness (R_a = 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.