Characteristics of the improved a-plane GaN films grown on r-plane sapphire with two-step AlN buffer layer

Structural, electrical and optical properties of an improved a  -plane GaN films grown on (11¯02)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 10×10 μm2 scan area, and there were no pits or other deformation appeared at the surface. The films were solely (112¯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 [11¯00] 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 cm2/Vs measured at room temperature. All of these indicated that the as-grown a-plane GaN films were of high quality.