GaN films were grown on c-plane (0001), a-plane (1120) and r-plane (1102) sapphire substrates by the ECR-assisted MBE method. The films were grown using a two-step growth process, in which a GaN buffer is grown first at relatively low temperatures and the rest of the film is grown at higher temperatures. RHEED studies indicate that this growth method promotes lateral growth and leads to films with smooth surface morphology. The epitaxial relationship to the substrate, the crystalline quality and the surface morphology were im·estigated by RHEED, X-ray diffraction and SEM studies. INTRODUCTION Recently Electron-Cyclotron-Resonance Microwave Plasma-assisted Molecular Beam Epitaxy (ECR-MBE) has emerged as one of the most versatile methods for the growth of GaN and other III-V nitrides[l-5). In this method, films are grown at relatively low temperatures under ultrahigh vacuum conditions. This allows for control of the film stoichiometry[6) and reduction of unwanted impurities[7). Moreover, our group demonstrated that significant improvement in film quality can be attained if film growth takes place in two steps[2,3,5]. In this process, a GaN buffer is grown first at relatively low temperatures and the rest of the film is grown at higher temperatures. Using this method, we ha,·e demonstrated the epitaxial stabilization of single crystalline GaN films in the zincblcnde structure on (001) silicon substrates[2,3). In this paper, we report on a comparative study of GaN films grown on c-plane, a-plane and r-plane sapphire substrates by the ECR-MBE method, using the two-step growth process. EXPERIMENTAL 11ETHODS The ECR-MBE method was used for the growth of GaN films on c-plane, a-plane and r-plane sapphire substrates. The details of the deposition method haYe been published elsewhere[2,3). The substrates were subjected to the following cleaning steps, prior to the growth of the GaN films. They were sequentially cleaned in ultrasonic baths of trichloroethylene, acetone and isopropanol for removal of hydrocarbon residues from the surface, etched in H3 P04 : H2S04 (1:3) for the removal of surface contaminants and mechanical damage due to polishing and finally rinsed in de-ionized water. After these steps the substrates were blown dry with nitrogen, mounted on a molybdenum block and transferred to the introduction chamber of the MBE system. In the preparation chamber, the substrates were heated to 850°C for approximately half an hour and then transferrecl to the growth chamber, where they were su!,jected to bombardment by a nitrogen plasma for approximately half an hour at 700°C. The GaN films were grown in two steps at different temperatures. First, a thin GaN buffer (about 300A) was i;rown at 400°C 500°C and then the rest of the film was grown at 600 800°C. The film growth rate was 2000 3000.{jh and the overall film thickness was 1 211m. Mal. Res. Soc. Symp. Proc. Vol. 242. ' 1992 Materials Research Society