Strain relaxation process of undoped and Si-doped semipolar Al_xGa_1-xN grown on (2 0 (2)over-bar 1) bulk GaN substrate

The strain relaxation process of undoped and Si-doped AlxGa1-xN with various compositions and thicknesses on semipolar (2 0 (2) over bar 1) bulk GaN substrate was investigated via the cathodoluminescence (CL) and high-resolution x-ray diffraction (XRD) analyses. CL analyses showed that the strain relaxation occurred through three different mechanisms. At first, the relaxation was initiated by the basal-plane (BP) misfit dislocations (MDs) along [1 1 (2) over bar 0] followed by the crack formation along [1 0 ( 1 ) over bar 4]. As the Al composition and/or thickness of AlGaN layer further increased, the crack density saturated and non-basal plane (NBP) MDs almost parallel to [1 0 (1) over bar 4] started emerging due to the prismatic slip. Observed crack spacing was 40-60 mu m at which the energy of exposing new surfaces from the cracks is higher than Peierls potential associated with the prismatic slip. By introducing Si (2.0-6.0 x 10(18) cm(-3)) into the film, the formation of both BP and NBP MDs could be suppressed within our growth conditions. However, Si doping was not effective in suppressing the crack formation but there was no significant change of its density either.