Nonpolar a-GaN films were grown by hydride vapor phase epitaxy on r-sapphire. As the thickness of the grown films was increased from 50 to over 250 mu m, the width of the double-crystal rocking curve for the (11-20) reflection decreased from 2000 to 500 arc sec, indicating a strong decrease in the dislocation density. Microcathodoluminescence mapping of the thick films suggests that the dislocation density is similar to 10(2) cm(-2), which is more than two orders of magnitude lower than for thin films. The authors also observe a corresponding decrease in the density of residual donors from 10(20) to 10(18) cm(-3), with respective mobility increase from (10-20) to 150 cm(2)/V s. The luminescence spectra of the thin films show the presence of intense defect bands attributed in literature to stacking faults. These bands are very strongly suppressed in thick films, which also show about an order of magnitude higher band edge luminescence intensity.