SURFACE-ENERGY AND STABILITY OF STRESS-DRIVEN DISCOMMENSURATE SURFACE-STRUCTURES

A method is presented to obtain ab initio upper and lower bounds to su rface energies of stress-driven discommensurate surface structures, po ssibly nonperiodic or exhibiting very large unit cells. The instabilit y of the stressed, commensurate parent of the discommensurate structur e sets an upper bound to its surface energy; a lower bound is defined by the surface energy of an ideally commensurate but laterally straine d hypothetical surface system. The surface energies of the phases of t he Si(111):Ga and Ge(111):Ga systems and the energies of the discommen surations are determined within +/-0.2 eV.