Spontaneous formation of surface antisite defects in the stabilization of the Sb-rich GaSb(001) surface

This Letter solves the long-standing puzzle [Phys. Rev. Lett. 79, 693 (1997)] of why GaSb(001) apparently violates the electron counting rule (ECR) in forming a reconstruction featuring long Sb-dimer chains, rather than the c(4x4) reconstruction found in all other arsenide and antimonide III-V compounds in the V-rich regime. We find that an alternative strategy, that in fact satisfies the ECR, is followed by the Sb-rich GaSb(001) surface, whereby long Sb-dimer chains are stabilized by randomly distributed subsurface Ga antisite defects. The excess of surface Sb drives the defect formation that in turn stabilizes the surface in a metastable phase. The transition to the c(4x4) reconstruction, where the ECR is instead satisfied through missing dimers, is therefore inhibited. Our conclusions are supported by ab initio simulations of experimental reflectance anisotropy spectra.