On the basis of accurate ab initio calculations, we propose a model for predicting the stability of III-V nanowires (NW) having different side walls and ridge configurations. The model allows us to obtain the NW formation energies by performing calculations only on relatively "small" systems, small diameter NWs and flat surfaces, to extract the contributions to the stability of each structural motif. Despite the idea illustrated here for the case of hexagonally shaped GaAs NWs grown along the [111]/[0001] direction, the method can also be applied generally to other differently shaped and oriented III-V NWs. The model shows that NW surfaces (side walls plus ridges) mainly determine the NW stability, so the changes to the surface structure (e.g., induced by defects or different growth conditions) would modify the final NW structure in a remarkable way. We find that wurtzite and zinc blende nanowires have similar energies over a wide diameter range, thus explaining the observed polytypism. Furthermore, new more stable ridge reconstructions are proposed for zinc blende nanowires. The surface-related structural motifs also have clear fingerprints on the NW electronic structure. We find that the more stable nanowires are all semiconducting. The band gaps are ruled by surface states and do not follow the trend mandated by the quantum confinement effect. Small diameter wurtzite nanowires have an indirect band gap, but for some of them, an indirect to direct transition can be foreseen to occur at larger diameters. Surface states have a larger impact on the zinc blende NW band gaps than on the wurtzite NW ones. Zinc blende nanowire band gaps reduce significantly with increasing nanowire radius, reaching the bulk value at a diameter of about 30 Å. The surface structure and the high surface related DOS below the conduction band are going to affect the nanowire dopant incorporation and efficiency when doping is carried out during the NW growth.

A pronounced anisotropy is observed in the low-temperature mobility of a two-dimensional electron gas formed in an In 0.75 Ga 0.25 As/ In 0.75 Al 0.75 As quantum well grown on a GaAs substrate. We show that the mobility differences along ͓011͔ and ͓011͔ directions are mainly due to In concentration modulations. Spatially resolved photoemission measurements show an asymmetric indium concentration modulation, correlated with the surface morphology observed by atomic force microscopy. A theoretical model considering conduction band energy modulations agrees well with the transport measurements. The identification of this mobility limiting mechanism allowed us to design and grow higher quality two-dimensional electron gases, needed for high indium content InGaAs device fabrication.

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 α 2 (2 × 4) or β 2 (2×4) 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 β 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 α 2 reconstruction compared to the β 2 reconstruction.

In this paper we present a study of In surface diffusion on InAs wetting layers deposited on the ͑001͒ surface of GaAs. The ␣ 2 ͑2 ϫ 4͒ and ␤ 2 ͑2 ϫ 4͒ reconstructions stabilized by a high In concentration are considered. The low symmetry of the ␣ 2 ͑2 ϫ 4͒ reconstruction allowed us to understand the effect of the wetting-layer symmetry on the adsorbate diffusion. We find that ͑i͒ the diffusion coefficient value is larger for In motion on the ␣ 2 reconstruction than on the ␤ 2 reconstruction. This is due to the presence on ␤ 2 of an additional As dimer that rises locally the potential energy surface and offer an additional site to which the In adatom can bind strongly. ͑ii͒ The In adsorption sites located within the As dimers have to be taken into account properly for these specific reconstructions, since they greatly affect the value of the diffusion coefficient. This is in contrast to what happens for the other reconstructions reported in the literature. ͑iii͒ The adsorbate diffusion is highly anisotropic with the ͓110͔ direction favored over the ͓110͔ direction, due to the presence of low-potential channels along ͓110͔. ͑iv͒ The anisotropy is slightly smaller on the ␣ 2 reconstruction than on the ␤ 2 reconstruction because on the ␤ 2 there is an additional diffusion channel along the ͓110͔ direction.

By using density functional theory (DFT) calculations of the potential energy surface in conjunction with the analytical solution of the master equation for the time evolution of the adatom site distribution, we study the diffusion properties of an isolated In adatom on In(x)Ga(1-x)As wetting layers (WL) deposited on the GaAs(001). The WL reconstructions considered in this study are, listed in the order of increasing In coverage: c(4 × 4), (1 × 3), (2 × 3), α(2)(2 × 4) and β(2)(2 × 4). We analyze the dependence of the diffusion properties on WL reconstruction, composition and strain, and find that: (i) diffusion on the (2 × N) reconstructions is strongly anisotropic, owing to the presence of the low barrier potential in-dimer trench, favoring the diffusion along the [Formula: see text] direction over that along the [110] direction; (ii) In diffusion at a WL coverage θ = 2/3 monolayers (ML; with composition x = 2/3) is faster than on clean GaAs(001) c(4 × 4), and decreases at θ = 1.75 ML (x = 1; e.g. InAs/GaAs(001)); (iii) diffusion and nucleation on the (2 × 4) WL is affected by the presence of adsorption sites for indium inside the As dimers; (iv) the approximation used for the exchange-correlation potential within DFT has an important effect on the description of the diffusion properties.