A simple setup for electrodeless photo-assisted electrochemical (PEC) etching was discussed from the viewpoint of the experimental geometry, in which the sample was dipped into the electrolyte under ultraviolet (UV) irradiation. Sulfate radicals ( ) were produced from K2S2O8 with UV light as the oxidizing agent; this consumed the extra UV photogenerated electrons, making it electrodeless. The transmittances were measured for various concentrations of K2S2O8 (aq.) to adjust the electrolyte depth. The effect of tetramethylammonium hydroxide post-treatment was also examined. The results indicate that damage-free PEC etching is feasible for everyone, even those who are not familiar with electrochemistry.

We investigated the ability of a photo-assisted electrochemical (PEC) etching process to remove the damage that dry etching causes in the near-surface region of GaN samples. The process consists of anodic oxidation of the GaN surface and subsequent dissolution of the oxide with a chemical treatment, and the extent of the PEC reactions depends on the total charge density transferred in them. The PEC process was conducted for samples prepared with various dry-etching conditions followed by fabrication of Schottky barrier diodes (SBDs) and metal-insulator-semiconductor (MIS) capacitors. The PEC process greatly improved the barrier height, ideality factor, and reverse leakage current of SBDs. Capacitance-voltage measurements of MIS capacitors revealed that the densities of interface states and discrete traps were both reduced by the PEC process. The results obtained here show that the PEC process can remove dry-etching damage from the GaN surface.

Electrodeless photo-assisted electrochemical etching was successfully demonstrated using a H3PO4-based solution containing S2O82– ions. The pH value of the solution changed under UVC illumination, clearly showing that SO4·– radicals were produced from S2O82– ions by absorbing UVC light. The production rate of SO4·– radicals maintained a constant value over the wide pH range of the solution, leading to etching rates and surface roughness comparable to those obtained in KOH-based solutions. The positive-type photoresist was applicable as the etching mask for the H3PO4-based solution. This finding will contribute to the development of a simple wet etching process suitable for the manufacturing of GaN-based devices.

Contactless photo-electrochemical (PEC) etching was successfully demonstrated on AlGaN/GaN heterostructures using a K2S2O8 aqueous solution. The etching was conducted by a simple method such as just dipping the sample with Ti-cathode pads into the solution under UVC illumination. The etching morphology of the AlGaN surface was very smooth with an root mean square roughness of 0.24 nm. The etching was self-terminated in the AlGaN layer, whose residual thickness was 5 nm uniformly throughout the etched region. These contactless PEC etching features are promising for the fabrication of recessed-gate AlGaN/GaN high-electron-mobility transistors with high recessed-gate thickness reproducibility.

Photoelectrochemical (PEC) etching was used to fabricate deep trench structures in GaN-on-GaN epilayers grown on n-GaN substrates. The width of the side etching was less than 1 µm, with high accuracy. The aspect ratio (depth/width) of a 3.3-µm-wide trench with a PEC etching depth of 24.3 µm was 7.3. These results demonstrate the excellent potential of PEC etching for fabricating deep trenches in vertical GaN devices. Furthermore, we simplified the PEC etching technology to permit its use in a wafer-scale process. We also demonstrated simple contactless PEC etching technologies for the manufacture of power and RF devices. A trench structure was fabricated in a GaN-on-GaN epilayer by simple contactless PEC etching. The role of the cathodic reaction in contactless PEC etching is discussed in relation to the application of a GaN HEMT epilayer on a semi-insulating substrate. Fortunately, the GaN HEMT structure contains an ohmic electrode that can act as a cathode in contactless PEC etching, thereby permitting the recess etching of a GaN HEMT epilayer grown on a semi-insulating SiC substrate. These results indicate that PEC etching technologies are becoming suitable for use in the fabrication of practical GaN power and RF devices.