Reconstructions of the GaN(000 ) Surface

Reconstructions of the GaN(000 ) surface are studied for the first time. Using scanning tunneling microscopy and reflection high-energy electron diffraction, four primary structures are observed: 1×1, 3×3, 6×6, and c(6×12). On the basis of first-principles calculations, the 1×1 structure is shown to consist of a Ga monolayer bonded to a N-terminated GaN bilayer. From a combination of experiment and theory, it is argued that the 3×3 structure is an adatom-on-adlayer structure with one additional Ga atom per 3×3 unit cell. Gallium nitride and other III-nitrides have attracted considerable interest recently because of their application for blue light-emitting diodes and lasers.[1] These materials have several unique properties compared to the more conventional III-V semiconductors (GaAs, InP, etc.): they exist in both cubic (zincblende) and hexagonal (wurtzite) form, they are refractory, and some of the materials have large band gaps. The relatively small size of nitrogen, compared to Ga or In, in these compounds leads to a number of unique surface structures, which have begun to be explored both experimentally and theoretically for the (001) growth surface of cubic GaN.[2,3] However, for the technologically more relevant (0001) growth surface of hexagonal GaN, very little is known concerning its structure aside from several reports of 2×2 and other reconstructions based on reflection high-energy electron diffraction (RHEED).[4] It is important to understand the surface structures of these materials, since this knowledge will impact our ability to achieve high quality epitaxial growth of the materials as required for optoelectronic applications. In this work we report the first observations of reconstructions on the GaN(000 ) surface. The reconstructions are studied using scanning tunneling microscopy (STM), RHEED, and ab initio calculations. We find four dominant reconstructions, which in order of increasing surface Ga/ N ratio are given by: 1×1, 3×3, 6×6, and c(6×12). From among a number of candidate structures, the 1×1 reconstruction is found to consist of a monolayer of Ga atoms, located in atop sites above the N atoms of a N-terminated bilayer. This is a novel structure, with no known analogue among other semiconductor surfaces. It is energetically feasible because of the much larger size of the Ga 1