Charge-Controlled Energy Optimization of the Reconstruction of Semiconductor Surfaces: sp 3 - sp 2 Transformation of Stoichiometric GaN(0001) Surface to (4 × 4) Pattern.

It was demonstrated by ab initio calculations that energy optimization in the reconstruction of semiconductor surfaces is controlled by the global charge balance. The charge control was discovered during simulations of the influence of heavy doping in the GaN bulk, which changes sp 3 to sp 2 ratio in the reconstruction of stoichiometric GaN(0001), i.e., a Ga-polar surface. Thus, the reconstruction is not limited to the charge in the surface only; it can be affected by the charge in the bulk. The discovered new reconstruction of the GaN(0001) surface is (4 × 4), which is different from the previously reported (2 × 1) pattern. The undoped GaN reconstruction is surface charge controlled; accordingly, (3/8) top-layer Ga atoms remain in a standard position with sp 3 hybridized bonding, while the remaining (5/8) top-layer Ga atoms are shifted into the plane of N atoms with sp 2 hybridized bonding. The change in the charge balance caused by doping in the bulk leads to a change or disappearance of the reconstruction pattern.