Evolution of surface roughness of AlN and GaN induced by inductively coupled Cl2/Ar plasma etching

Abstract

We study the effects of plasma etching on the evolution of surface roughness of GaN and AlN. The etch-induced roughness is investigated using atomic force microscopy by systematically varying plasma power, chamber pressure, and Cl2/Ar mixture gas composition. GaN etches three to four times more rapidly than AlN for identical plasma conditions. For both GaN and AlN, we find that the surface roughness is correlated to etch rate. Induced roughness remains comparable to the as-grown value provided etching is carried out below rates 400 (GaN) and 90 nm/min (AlN). Above these cutoff etch rates, the roughness increases in proportion to etch rate. This result is independent of plasma parameters varied to produce the higher etching rates. By analyzing the surface properties through the power spectral density (PSD), we correlate roughness with the formation of fine-scale features present as a consequence of more aggressive etching. The cutoff etch rates and spatial-frequency dependence of the PSD are interpreted using the theory of kinetic roughening. A low-frequency range exhibits saturation corresponding to large-scale feature sizes, and a high-frequency regime exhibits scaling properties.