Original citationSanchez-Soares, A., O'Donnell, C. and Greer, J. C. (2016) Electronic structure properties of nanowires (NWs) with diameters of 1.5 and 3 nm based on semimetallic α − Sn are investigated by employing density functional theory and perturbative GW methods. We explore the dependence of electron affinity, band structure, and band-gap values with crystallographic orientation, NW crosssectional size, and surface passivants of varying electronegativity. We consider four chemical terminations in our study: methyl (CH 3 ), hydrogen (H), hydroxyl (OH), and fluorine (F). Results suggest a high degree of elasticity of Sn-Sn bonds within the Sn NWs' cores with no significant structural variations for nanowires with different surface passivants. Direct band gaps at Brillouin-zone centers are found for most studied structures with quasiparticle corrected band-gap magnitudes ranging from 0.25 to 3.54 eV in 1.5-nm-diameter structures, indicating an exceptional range of properties for semimetal NWs below the semimetal-to-semiconductor transition. Band-gap variations induced by changes in surface passivants indicate the possibility of realizing semimetal-semiconductor interfaces in NWs with constant cross-section and crystallographic orientation, allowing the design of novel dopant-free NW-based electronic devices.

We calculate the electronic structure of germanium-tin (Ge1−xSnx) binary alloys for 0 ≤ x ≤ 1 using density functional theory (DFT). Relaxed alloys with semiconducting or semimetallic behaviour as a function of Sn composition x are identified, and the impact of epitaxial strain is included by constraining supercell lattice constants perpendicular to the [001] growth direction to the lattice constants of Ge, zinc telluride (ZnTe), or cadmium telluride (CdTe) substrates. It is found that application of 1% tensile strain reduces the Sn composition required to bring the (positive) direct band gap to zero by approximately 5% compared to a relaxed Ge1−xSnx alloy having the same gap at Γ. On the other hand, compressive strain has comparatively less impact on the alloy band gap at Γ. Using DFT calculated alloy lattice and elastic constants, the critical thickness for Ge1−xSnx thin films as a function of x and substrate lattice constant is estimated, and validated against supercell DFT calculations. The analysis correctly predicts the Sn composition range at which it becomes energetically favourable for Ge1−xSnx/Ge to become amorphous. The influence of stoichiometry and strain is examined in relation to reducing the magnitude of the inverted ("negative") Γ − 7 -Γ + 8 band gap, which is characteristic of semimetallic alloy electronic structure. Based on our findings, strategies for engineering the semimetal-to-semiconductor transition via strain and quantum confinement in Ge1−xSnx nanostructures are proposed.

The thermodynamic stability of germanium tin G e 1 − x S n x alloys is investigated across the composition range 0 ⩽ x ⩽ 1 by applying density functional theory together with the cluster expansion formalism (CE). It is known that GeSn alloys are immiscible and that non-equilibrium growth techniques are required to produce metastable films and nanostructures. Insight into the driving forces behind component segregation is gained by investigating the equilibrium thermodynamics of GeSn systems. The alloy free energy of mixing is computed by combining enthalpies from the CE with entropy terms for configurational and vibrational degrees of freedom. Volume deformations due to the large mismatch in ionic radii are readily found to be the key driving force for immiscibility at all temperatures of relevance. This leads to a study of epitaxial stabilisation by employing latticed matched substrates to favour growth of alloys with fractional compositions of x = 0, approximately x = 0.5 and x = 1. The reduction in the free energy of mixing due to epitaxial strain in thin films is quantified for each substrate leading to indicators for growth of kinetically stable films.

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