Electrochemical porosification of n-GaN across acidic–neutral–alkaline electrolytes: From morphology to surface chemistry

Electrochemical etching is a scalable route to engineer nanoporous architectures in doped GaN, but cross-comparative investigations spanning the full electrolyte chemical window are still very limited. Here, we explore the electrochemical etching of Si-doped n-GaN in acidic, alkaline, and neutral electrolytes and correlate morphology, optical properties, and surface chemistry by using various characterization and simulation methodologies. Our findings reveal that the electrolyte with different pH values not only affects the pore size and density but also alters the dominant etching mode, which leads to morphologies ranging from interconnected “sponge-like” three-dimensional pore networks to intact surfaces with slightly and anisotropically etched pores beneath. We have established semiquantitative models to elaborate the changes in near-band edge absorption due to disorders of the pores and to describe the surface oxidation/hydroxylation, through respective optical and x-ray photoelectron spectroscopy measurements. The models are further corroborated with the Raman observations, supporting a unified picture in which porosification increases disorder/local strain fluctuations. We envisage that this work offers important information on the electrochemical etching of III-nitrides using electrolytes in full pH ranges and proposes new analytical methods, which are valuable in developing power-efficient GaN optoelectronic devices.