Controlled Fabrication of Uniform Columnar Porous GaN Architectures by Electrochemical Etching

Abstract

Electrochemical etching (ECE) of n-type c-plane GaN enables the formation of porous architectures capable of enhancing the performance of optoelectronic devices and broadening the range of possible GaN applications. Here, we demonstrate that the addition of NaOH to Na2C2O4 (and other non-acidic etchant solutions) directs ECE toward the formation of nanoscale, vertically aligned, highly uniform columnar pore morphologies with relatively limited surface roughening. By varying the NaOH concentration within mixed electrolyte systems, the columnar pore shape, diameter, spacing, and bulk and surface porosity can be readily tuned. In addition to manually mixing NaOH with another etchant, aqueous basic electrolytes such as Na2CO3 and Na3PO4, each inherently containing significant hydroxide concentration through hydrolysis, also produce similar columnar pore morphologies, indicating that hydroxide plays the key role in directing the vertical pore growth. Chronoamperometry and pore structure analysis reveals that the mixed hydroxide-containing electrolyte systems exhibit a distinct, new ECE mechanism rather than a simple superposition of the individual ECE behaviour of each anion present. This work provides significant insight into the ECE mechanism for porous GaN, which is proposed to contain multiple reaction steps where hydroxide anions are involved in the formation of a reactive intermediate. These findings establish a versatile and controllable route for engineering columnar porous GaN architectures, offering significant advancement in applications requiring high porosity, ordered, and bespoke structures.