The thickness of low temperature AlGaN buffer layers grown on r -sapphire substrates has been found to directly affect the crystalline structure of the buffer layer as well as the structural and optical properties of subsequently grown a -plane GaN films. A buffer layer with a thickness of 30 nm results in randomly distributed fine domains without extended defects. Increasing the thickness to 90 nm leads to a uniform and largely coalesced crystalline structure, with well-defined stacking faults. GaN films grown on the thinner buffer layer contain a lower density of larger stacking faults, and exhibit brighter stacking-fault luminescence as compared to films grown on thicker buffer layers. Our studies indicate that the optimum buffer layer thickness for growth of a -plane GaN is about 30 nm. © 2008 American Institute of Physics.