High temperature N2 annealing for N-polar bulk AlN substrates

High temperature annealing treatments with two different wafer configurations under N2 atmosphere were performed to improve the N-polar surface quality of bulk AlN substrates and to mitigate its impact on the morphology and structural properties of subsequently grown homoepitaxial films. After N2 annealing at 1700–1850 °C, scratches and particles that remained on the as-CMP N-polar surface completely disappeared. The annealed N-polar surface consisted of a step-and-terrace structure with step heights of ∼0.5 nm and uniform terrace widths. For single wafer configuration anneals, the etching rate increased from 0.32 μm/h to 2.54 μm/h with annealing temperature, while the step retreat velocity estimated from etching increased from 18.4 to 155.6 nm/s over the same temperature range. An activation energy for etching of 5.0 eV for the single wafer annealing configuration was calculated. Although surface pits were observed after N2 annealing in the single wafer configuration, the use of a proximity cap configuration reduced the pit density to less than 0.3 mm−2 and produced smooth surfaces. Furthermore, AlN films grown by MOCVD on annealed substrates exhibited a significant reduction in the density of surface hillocks as compared to those grown on unannealed substrates. This study provides a general AlN pre-epi wafer annealing approach to improve the morphology and surface quality of N-polar AlN substrates and subsequently grown homoepitaxial films.