Critical issues need to be overcome to produce high performance Schottky diodes on gallium nitride (GaN). To activate dopant, high temperature thermal treatments are required but damage GaN surface where hexagonal pits appear and prevent any device processing. In this paper, we investigated the efficiency of cap-layers on GaN during thermal treatments to avoid degradation. Aluminum nitride (AIN) and silicon oxide (SiOx) were grown on GaN by direct current reactive magnetron sputtering and plasma-enhanced chemical vapor deposition, respectively. AIN growth parameters were studied to understand their effect on the grown layers and their protection efficiency. Focused ion beam was used to measure AIN layer thickness. Crystalline quality and exact composition were verified using X-ray diffraction and energy dispersive X-ray spectroscopy. Two types of rapid thermal annealing at high temperatures were investigated. Surface roughness and pits density were evaluated using atomic force microscopy and scanning electron microscopy. Cap-layers wet etching was processed in H3PO4 at 120 degrees C for AIN and in HF (10%) for SiOx. This work reveals effective protection of GaN during thermal treatments at temperatures as high as 1150 degrees C. Low surface roughness was obtained. Furthermore, no hexagonal pit was observed on the surface. (C) 2015 Elsevier B.V. All rights reserved.