Diameter dependence of mechanical, electronic, and structural properties of InAs and InP nanowires: A first-principles study

Semiconductor nanowires (NWs) have ideal morphologies to act as active parts and connections in nanodevices since they naturally restrict the conduction channels and periodicity to one dimension. The advantages from the reduced spatial dimension can be greatly enhanced by wisely selecting the materials composing the NWs, through the knowledge of the properties of their bulk counterparts. NW's properties can still be tailored by managing (i) internal or intrinsic characteristics as diameters, growth directions, structural phases, and the faceting or saturation of surfaces, and/or (ii) external or extrinsic influences as applied electric, magnetic, thermal, and mechanical fields. Bulk InAs has one of the lowest electron effective-masses among binary III-V semiconducting materials while bulk InP shows excellent optical properties, which make InAs and InP NWs candidates for optoelectronic materials. In this work, we use first-principles calculations to study the structural, electronic, and mechanical properties of [111] zinc-blende InAs and InP NWs as a function of diameter (ranging from 0.5 to 2.0 nm). The influence of external mechanical stress on the electronic properties is also analyzed. The axial lattice constants of the NWs are seen to decrease with decreasing diameter, as a consequence of a shorter surface lattice constant of the NWs, as compared to their bulk values. The Young's modulus of both InAs and InP NWs is determined to decrease while the Poisson's ratio to increase with decreasing diameters, with deviations from the bulk Young's modulus estimated to occur for NWs with diameters lower than 15 nm. The increase in the band-gaps with decreasing diameters is seen to be slower than the expected from simple quantum-mechanical models (1/D-2, where D is the diameter), mainly for the smallest (<1.0 nm) diameters. The electron effective-masses are seen to increase with decreasing diameters, due to a k-dependent energy shift of the conduction-band minimum. The calculated work functions for both NWs show a decrease with decreasing diameters. The change in the NWs' band-edge eigenvalues with axial strain is calculated and the band-gap deformation potentials are determined and shown to change in signal within the range of studied diameters. The influence of the mechanical strain on the electronic bands is analyzed in terms of electronic charge decompositions in directions parallel and perpendicular to the NWs' axes. Direct to indirect band-gap transitions are observed for compressive strains in very thin NWs. The hole effective-mass is seen to be lower than the corresponding electron effective-mass for the studied NWs.