Interfacial Reactions and Smooth Etching Strategy of n-type Gallium Nitride Photoanodes

Abstract

While photoelectrochemical etching is the only efficient wet-etching method for manufacturing n-type gallium nitride (GaN)-based devices, the current technology cannot simultaneously achieve nanoscale surface roughness (Ra) and flatness. We proposed a smooth etching strategy through the depth study of the interfacial reactions of GaN photoanodes in organic deep eutectic solvents (DESs) and aqueous electrolytes. The results show that with the increase of GaN photoanode potential, first hydroxyl anions and then H2O molecules capture ultraviolet (UV)-photogenerated holes (h + ) at the GaN surface/interface, initiating GaN oxidation by generating hydroxyl radical (OH·) surface intermediates. The etching results from the synergic effects of interfacial reactions, including GaN oxidation, oxide film formation and dissolution, and side reactions. Only the synergic effects in weak acidic electrolytes (e.g., pH = 2.3) can minimize roughening influences without bending the oxide film. Such etching can prepare the most typical groove structure of GaN-based devices (e.g., insulated gate bipolar transistor) with a nanoscale surface flatness. The Ra reaches 3.28 nm, and the etching rate attains 10.78 nm·min−1.

High-Efficiency and High-Quality Photogalvanic Etching of the Silicon Doped N-Type Gallium Nitride Using Potassium Peroxomonosulfate Oxidant

“…The photocurrent of the two curves passed through a turning potential at around +0.3 V to reach an approximate plateau value, half of that of u-GaN examined in our previous PEC study. 32 These suggest that a high carrier density of the si-GaN would slow its etching.…”

“…Unexpectedly, the etching rate reached 12.0 nm•min −1 , equivalent to the PEC etching rate of u-GaN that we previously measured in 0.1 M K 2 SO 4 (pH = 2.3). 32 Such a rapid rate seems to violate the law that high carrier density slows the etching. However, considering that the PEC etching rate of u-doped GaN attains 35.14 nm•min −1 in 0.1 M H 2 SO 4 , this rate observed for si-GaN is still reasonable.…”

“…Concretely speaking, our previous PEC study results have demonstrated that a lower electrolyte pH value brings etching the features of the step-preferred orientation and higher efficiency, but the as-formed oxide layer is too thin to eliminate the roughening influence. 32 There is a balance between the etching rate and Ra, depending on the thickness of the formed oxide layer. For example, when the photogalvanic etching of si-GaN was conducted in an electrolyte favorable for the oxide dissolution (e.g., 20 mM PMS + 2 mM H 2 SO 4 (pH = 2.3)), the etching rate was found to have increased to 18.3 nm•min −1 and the Ra attained 14.34 nm (5 × 5 μm 2 ) simultaneously.…”

“…Previously, we have demonstrated that the PEC etching of the unintentionally doped n-type GaN (u-GaN) in a weak acid electrolyte can prepare high-quality grooves. 31,32 In principle, photogalvanic etching can also achieve high quality in the same electrolyte.…”

Deep eutectic solvents as green solvents for materials preparation

“…Another example showcases the typical process in the semiconductor industry, realizing nanoscale surface roughness with desirable flatness. 178 Hu and coworkers proposed a smooth etch strategy by in-depth study of the interfacial reactions of GaN photoanodes in DES and other aqueous electrolytes. With the increase of the anodic potential, hydroxyl anions and H 2 O molecules first react with holes (h + ) on the GaN interface, initiating GaN oxidation by generating hydroxyl radicals (OH • ).…”