Germanium tin for use in semimetal electronics
Loading...
Files
Full Text E-thesis
Date
2021-05-01
Authors
O'Donnell, Conor
Journal Title
Journal ISSN
Volume Title
Publisher
University College Cork
Published Version
Abstract
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.
Description
Keywords
Quantum confinement , Germanium tin , GeSn
Citation
O'Donnell, C. 2021. Germanium tin for use in semimetal electronics. PhD Thesis, University College Cork.