Formation of V-grooves on the (Al,Ga)N surface as means of tensile stress relaxation☆

Abstract In this paper we present a study of the relaxation mechanism of the top Al 0.30 Ga 0.70 N layer grown on GaN, as used in High Electron Mobility Transistor (HEMT) structures. We show that the initial mechanism for relaxation of strain is by means of formation of V-grooves on the surface of the Al 0.30 Ga 0.70 N. It is also demonstrated that a thin Si 3 N 4 layer, grown in-situ, immediately after the Al x Ga 1− x N can “freeze-in” the surface structure. Using tapping mode Atomic Force Microscopy (AFM) it can be observed that immediately after termination of the growth of the thin Al 0.30 Ga 0.70 N layer, the steps on the surface show round shape and spiral-like features. After about 1 min of annealing time under NH 3 flow the surface structures become straighter. Upon prolonged annealing a V-groove pattern is observed. These V-grooves run along the 〈−1–120〉 directions. Although some other facets can be observed, from cross-sectional Transmission Electron Microscopy (TEM) images we can infer that the side walls of the grooves are {1–101} planes and that the grooves do not penetrate till the Al 0.30 Ga 0.70 N/GaN interface. Therefore, we come to the conclusion that the initial relaxation of a thin Al 0.30 Ga 0.70 N layer does not occur via a dislocation glide mechanism leading to the formation of an array of misfit dislocations at the Al 0.30 Ga s.70 N/GaN interface. Instead, we propose that the mechanism is by surface instability leading to V-groove formation.