Structural, electrical and optical properties of an improved a-plane GaN films grown on (1 (1) over bar 0 2) r-plane sapphire by metalorganic chemical vapor deposition (MOCVD) with a low-temperature and a high-temperature AlN buffer layers were revealed. It was found that the qualities of a-plane GaN film grown on r-plane sapphire substrate with this two-step AlN buffer layer were greatly improved compared with the a-plane GaN films grown by MOCVD with usually used one-step low-temperature GaN or high-temperature AlN buffer. The as-grown films had a smooth surface with a root mean square (RMS) roughness of 1.40 nm for a 10 x 10 mu m(2) scan area, and there were no pits or other deformation appeared at the surface. The films were solely (1 1 (2) over bar 0) a-plane oriented, and the full width at half maximums (FWHMs) of on-axis diffraction were 697 and 1100 arcsecs with the incident X-ray beam parallel to the [0 0 0 1] and [1 (1) over bar 0 0] direction of the as-grown GaN films, respectively. Strong emissions from free exciton A and B were observed at 21.7 K, and the films gave an electron mobility 15 cm(2)/Vs measured at room temperature. All of these indicated that the as-grown a-plane GaN films were of high quality. (C) 2007 Elsevier B.V. All rights reserved.