Here we examined the use of AlN buffer layers of various thicknesses to improve the crystalline quality of nonpolar a-plane GaN (a-GaN) grown on an r-plane sapphire substrate. Three types of AlN buffer layers were used: sputtered AlN buffer layers (sp-AlNs) with or without annealing, and epitaxially grown AlN buffer layers (ep-AlNs). Buffer layer thicknesses of 30, 90, and 180 nm were used. We found that the surface morphological transitions with increasing thickness were different between the sp-AlNs and the ep-AlNs, and that the sp-AlNs had poorer crystallographic orientations than did the ep-AlNs. Annealing caused marked changes to occur in the surface morphologies and crystallographic orientations of the sp-AlNs; however, the positive effect of annealing was limited because the in-plane crystallographic orientation degraded with increasing layer thickness. The optimal buffer layer was found to be the 30-nm-thick annealed sp-AlN, which was composed of uniformly arranged oval grains with better crystallographic orientation than the other sp-AlNs and annealed sp-AlNs. The crystalline quality of the a-GaN epilayer grown on 30-nm-thick annealed sp-AlN had a narrower X-ray rocking curve-full width at half maximum for both the on-and off-axis planes compared with that grown on any other AlN buffer layers. (C) 2017 Elsevier B.V. All rights reserved.