We report on the characteristics of a-plane GaN films directly grown on optimized silicon-dioxide-patterned r-plane sapphire substrates. Various shapes and sizes of silicon dioxide patterns were considered with the aim of achieving fully coalescent a-plane GaN films with a smooth surface and high crystalline quality. The omega full widths at half maximum of the (11-20) X-ray rocking curve values of optimized a-plane GaN films with regular hexagonal patterns of 1 mu m window width and 6 mu m mask width were measured to be 597 arc sec along the c-axis direction and 457 arc sec along the m-axis direction. Atomic force microscopy images revealed a significant reduction in the density of submicron pits in the mask region. Plan-view and cross-sectional transmission electron microscopy images showed that basal stacking faults and threading dislocation densities were reduced from similar to 5.7 x 10(5) cm(-1) and similar to 1 x 10(9) cm(-2) in the window region to similar to 1.8 x 10(5) cm(-1) and similar to 2.1 x 10(8) cm(-2) in the mask region, respectively. (C) 2013 Elsevier B. V. All rights reserved.