Surface charge induced $sp^3$-$sp^2$ transformation: new energy optimization universal mechanism of (4x4) reconstruction of stoichiometric GaN(0001) surface
Ab initio calculations were applied to large size slabs simulations for determination of the properties of the principal structures of the stoichiometric GaN(0001) surface. The results are different from published previously: stoichiometric GaN(0001) surface structure is characterized by mixed structure in which 3/8 top layer Ga atoms remain in standard position with $sp^3$ hybridized bonding while the remaining 5/8 top layer Ga atoms is located in plane of N atoms with $sp^2$ hybridized bonding. This involves charge transfer to the previous one, entailing energy optimization, therefore Ga-s and Ga-p energy difference is driving force for the transition. In large size surfaces Ga atoms create (4 x 4) reconstruction which is additionally stabilized by strain optimization. Heavy doping in the bulk changes $sp^3$ to $sp^2$ ratio thus confirming the charge transfer mechanism of the reconstruction. The charge transfer energy optimization is universal for all surfaces terminated by $sp^3$ bonded atoms.