STRUCTURE AND FORMATION ENERGY OF STEPS ON THE GAAS(001)-2X4 SURFACE

The energies of various steps on the As-terminated GaAs(001)-2 x 4 sur face are evaluated using a novel, approximate method of ''linear combi nation of structural motifs''. It is based on the observation that pre vious total energy minimizations of semiconductor surfaces produced in variably equilibrium structures made of the same recurring local struc tural motifs, e.g. tetrahedral fourfold Ga, pyramidal threefold As, et c. Furthermore, such surface structures were found to obey consistentl y the octet rules as applied to the local motifs. We thus express the total energy of a given semiconductor surface as a sum of (i) the ener gies {epsilon(M)} of the local structural motifs appearing in the surf ace under consideration and (ii) an electrostatic term representing th e Madelung energy of point charges resulting from application of the o ctet rule. The motif energies are derived from a set of pseudopotentia l total energy calculations for flat GaAs(001) surfaces and for point defects in bulk GaAs. This set of parameters suffices to reproduce the energies of other (001) surfaces, calculated using the same pseudopot ential total energy approach. Application to GaAs(001)-2 x 4 surfaces with steps reveals the following. (i) ''Primitive steps'', defined sol ely according to their geometries (i.e. step heights, widths and orien tations) are often unstable. (ii) Additional, non-geometric factors be yond step geometries such as addition of surface adatoms, creation of vacancies and atomic rebonding al step edges are important to lower st ep energies. So is step-step interaction. (iii) The formation of steps is generally endothermic. (iv) The formation of steps with edges para llel to the direction of surface As dimers (A steps) is energetically favored over the formation of steps whose edges are perpendicular to t he As dimers (B steps).