Attractive interactions between like-oriented surface steps from an ab initio perspective: Role of the elastic and electrostatic contributions

In this work, we show that by using the density functional approach, in which the electronic degrees of freedom are separated by the ionic ones, it is possible to individuate and separately study the elastic and electrostatic step interactions, traditionally introduced in the literature as the only two kinds of step interaction expected at T=0. We have applied the method to the technologically important GaAs(001) surface and found some unexpected results for the relatively short step distances accessible to the ab initio approaches, contradicting those of the continuum models so far employed for the study of the elastic step interactions: (i) the sign of the step interaction depends on the step termination and is due to the electrostatic interaction; (ii) the elastic interaction does not contribute to the step interaction, contrary to the common belief of a strong elastic repulsive interaction between like-oriented steps. We show that this is due to the electron behavior. When considering only ion displacements and pointlike steps as in the continuum theories, we recover the classical results and repulsive step elastic interactions; (iii) the experimentally observed Ab step termination shows a weakly attractive step interaction whereby attractive step interactions between like-oriented steps on an unstrained surface are believed not to exist. The proposed method of separating elastic and electrostatic interactions for further analysis of their dependence on the configurational degrees of freedom can be extended to other defective situations.