Precise thickness control in recess etching of AlGaN/GaN hetero-structure using photocarrier-regulated electrochemical process

Abstract

The photocarrier-regulated electrochemical (PREC) process was developed for fabricating recessed-gate AlGaN/GaN high-electron-mobility transistors (HEMTs) for normally off operation. The PREC process is based on photo-assisted electrochemical etching using low-energy chemical reactions. The fundamental photo-electrochemical measurements on AlGaN/GaN heterostructures revealed that the photo-carriers generated in the top AlGaN layer caused homogeneous etching of AlGaN with a smooth surface, but those generated in the GaN layer underneath caused inhomogeneous etching that roughens the surface. The concept of the PREC process is to supply the photo-carriers generated only in the AlGaN layer by selecting proper conditions on light wavelength and voltage. The phenomenon of self-termination etching has been observed during the PREC process, where the etching depth was controlled by light intensity. The recessed-gate AlGaN/GaN HEMT fabricated with the PREC process showed positive threshold voltage and improvement in transconductance compared to planar-gate AlGaN/GaN HEMTs.

Photoelectrochemical Etching Technology for Gallium Nitride Power and RF Devices

“…We also confirmed that the bumps can be removed by the HCl-H2O2 post cleaning process accompanied with removing the Ti mask. Furthermore, in contactless PEC etching, the etched depth showed a self-stopping feature similar to that observed in conventional PEC etching [12,17]. The dependence of the offset between the SiO2 mask and the Ti cathode pad in terms of the fineness of the etched patterns, and the lifetime of the electrolyte remains unclear.…”

“…This oxide dissolves in acids or bases. An anodic oxidation process therefore serves as the basis of PEC etching of GaN [12,14,18,19]. The overall etching reaction for GaN, in which…”

“…Another application of PEC etching is the gate-recess etching for AlGaN/GaN HEMTs [17]. In particular, PEC etching has many advantages from the viewpoint of its self-stopping feature in the gate-recess etching process through modulation of the photoirradiation intensity [12,17]. Here, we report the use of PEC etching in the fabrication of deep trench structures in a GaN-on-GaN epilayer.…”

“…Photoelectrochemical (PEC) etching might solve these problems [12][13][14][15] as it has excellent etching selectivity against the etching mask and is less likely to cause damage, because it is a plasma-free process. PEC etching has recently been applied to the mesa fabrication of GaN pn-junction diodes (PNDs) with high VB values [16].…”

Low-Damage Etching for AlGaN/GaN HEMTs Using Photo-Electrochemical Reactions

“…After early studies by Minsky [5] and Youtsey [6], various PEC etching processes have been reported for GaN [7][8][9] and AlGaN [10] with growing interest in damage-less etching. Our PEC etching [11][12][13][14], which is carried out using the pulsed bias, is a cyclic process consisting of anodic oxidation and subsequent dissolution of the resulting oxide in an electrolyte. We reported that the dry-etching damage was effectively removed from n-GaN surfaces with the PEC process without causing any additional damage [11].…”