Germanium tin for use in semimetal electronics

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O'Donnell, Conor
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University College Cork
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As electronic devices shrink to sub 5 nm dimensions, issues such as dopant variability as well as quantum confinement begin to hamper traditional metal oxide silicon field effect transistor (MOSFET) designs. A proposed alternative design, the confinement modulated gap transistor (CMGT), employs semimetals to overcome these hurdles. By taking advantage of the quantum confinement effect inducing a band gap in confined semimetals, a thick-thin-thick configuration can be used to create a set of monomaterial back-to-back Schottky barriers, which can then be gated. Ge1−xSnx, a material which has garnered much interest in the photonics community for its indirect to direct band gap transition with rising Sn content, has been shown to become semimetallic as Sn content is further increased. The aim of this thesis is the investigation of Ge1−xSnx alloys in terms of their miscibility as well as their electronic structure, to assess which compositions and strains would be of use in the fabrication of semimetal-based devices. We employ LDA DFT to calculate the alloy structural properties and meta-GGA to calculate band structures. First the miscibility and structural properties of the alloy are studied. The evolution of the equilibrium lattice constant, the elastic constants, and the bond lengths are predicted across the full alloy composition range. Through use of the cluster expansion formalism and Monte Carlo simulations, an equilibrium phase diagram of the solid phase is generated. The formation energies of these alloys are also investigated and decomposed into their constituent components, and this is then repeated for biaxially strained bulk cells, which are allowed to relax perpendicular to the strain direction. We consider three virtual substrates: Ge, ZnTe and CdTe. The critical thicknesses of these alloys are also predicted using an elastic continuum model. The electronic structure of the relaxed alloys is investigated, and the evolution of the band gap found to agree well with previous experimental results. This is then repeated for the same biaxially strained cells previously discussed, to understand how strain affects the band gap, and the semiconducting to semimetallic transition which occurs as Sn is added. The semiconducting to semimetallic transition is then plotted for increasing tensile strain, as well as the critical thickness at the required strain and composition. Deformation potential theory is employed to understand the behaviour of the band gap as strain is added. This is then used to generate a model which predicts the band gap over the composition range for a large tensile strain range.
Quantum confinement , Germanium tin , GeSn
O'Donnell, C. 2021. Germanium tin for use in semimetal electronics. PhD Thesis, University College Cork.