Morphology evolution and luminescence properties of porous GaN generated via Pt-assisted electroless etching of hydride vapor phase epitaxy GaN on sapphire

Porous gallium nitride (PGaN) is produced by Pt-assisted electroless etching of hydride vapor phase epitaxy (HVPE)–GaN. Ultrathin Pt films are sputtered onto the GaN surface, and etching is carried out in a 1:2:1 solution of CH3OH:HF:H2O2. The evolution of the morphology proceeds by first forming a network of small pores, after which a ridge-trench morphology evolves, with ridges separated by a porous network in trenches between the ridges. As the etch progresses further the ridges evolve to a maximum size and then start to disappear. The formation and evolution of the ridge-trench morphology is explained by the presence of two different etch rates, an enhanced etch rate which generates the porous network and a slower etch rate that leads to the terraces of the ridge morphology. The rate at which the morphology evolves depends on the carrier concentration, with more heavily doped samples etching faster. In all cases, the final depth of the trenches between ridges is independent on the thickness of the starting GaN film. Cathodoluminescence (CL) spectroscopy of the unintentionally doped and the Si doped HVPE materials produce PGaN which shows only band gap emission at 368 nm before and after etching with only small shifts in the wavelength of maximum emission. The intensity of CL emission decreases with etch time as the GaN is consumed. CL spectroscopy and imaging show the ridges to be optically inactive, suggesting that the ridges might arise from grain boundaries or dislocations present in the starting GaN material.