  O2-plamsa-based digital etching of Al0.25Ga0.75N with a 0.8 nm AlN spacer on
GaN was investigated. At 40 W RF bias power and 40 sccm oxygen flow, the etch
depth of Al0.25Ga0.75N was 5.7 nm per cycle. The 0.8 nm AlN spacer layer acted
as an etch-stop layer in 3 cycles. The surface roughness improved to 0.33 nm
after 7 digital etch cycles. Compared to the dry etch only approach, this
technique causes less damages. It was shown to be effective in precisely
controlling the AlGaN etch depth required for recessed-AlGaN HEMTs.
