Adsorption of Indium on a InAs wetting layer deposited on the GaAs(001) surface

In this work we perform a first-principles study of the adsorption properties of an In adatom deposited on 1.75 monolayers (ML) InAs, forming a wetting layer on GaAs$(001)$ with the $\alpha_2 (2\times4)$ or $\beta_2 (2\times4)$ reconstruction. The structural properties of these reconstructions have been studied: we determine the equilibrium geometry of the surfaces and their stability for various growth conditions. We have then carried out a detailed study of the potential energy surface (PES) for an In adsorbate, finding the minima and the saddle points. The main characteristics of the PES and the bonding configurations of the In adatom on the surface are analyzed by comparing with analogous studies reported in the literature, trying to extract the effects due to: (i) the compressive strain to which the InAs adlayer is subjected, (ii) the particular surface reconstruction, and (iii) the wetting layer composition. We found that, in general, stable adsorption sites are located at: (i) locations besides the As in-dimers, (ii) positions bridging two As in-dimers, (iii) between two adjacent ad-dimers (only in $\beta_2$), and (iv) locations bridging two As ad-dimers. We find also other shallower adsorption sites which are more reconstruction specific due to the lower symmetry of the $\alpha_2$ reconstruction compared to the $\beta_2$ reconstruction.