Design of semimetal electronics relying on the quantum confinement effect

Loading...
Thumbnail Image
Files
Date
2021
Authors
König, Christian
Journal Title
Journal ISSN
Volume Title
Publisher
University College Cork
Published Version
Research Projects
Organizational Units
Journal Issue
Abstract
This thesis investigates the properties of bismuth (Bi) nanostructures on a fundamental level and in the context of nanoelectronic devices. Due to its unique electronic properties, the material has been studied intensely in the physics community and is well known for its pronounced quantum confinement effect. A semimetal-to-semiconductor transition has been anticipated and discussed for a long time. Since the limitations of conventional silicon technology on the nanometer scale has triggered the search for new materials and device concepts for the semiconductor industry, bismuth has thus attracted even more attention. This thesis is dedicated to studying different aspects of the crystal and electronic structure of bismuth, in particular when grown as a thin film. We discuss how the structure of thin Bi films is affected by the termination with different chemical species. Since the surface-to-volume ratio in nanometer size devices is increased with respect to the macroscopic world, a suitable termination has to be found which can passivate metallic surface states. Otherwise, a band gap which may have opened due to confinement could effectively be closed and disguised. Close to the surface, the particular bilayered structure of the films is perturbed by strong covalent bonds with adsorbates like hydrogen and oxygen. For very thin films a complete reorientation was observed which may prove problematic for applications. We furthermore present a model for the native oxide which has no detrimental effect on the crystal structure of underlying bismuth atomic layers. We investigate how the electronic structure of the bulk material depends on the crystal structure. A good understanding of how the overlap of the valence and conduction states changes as a function of the shape of the unit cell is a useful tool in the interpretation of the electronic structure of thin Bi films which are subject to, e.g., substrate induced strain. In addition to standard methods, we also calculate many-body corrections which are known to vastly improve the prediction of band gaps. Thus, we contribute to the topical and controversial discussion of the band topology in bismuth. The effect of various kinds of surface termination on the electronic structure is discussed. Our calculations show that only the passivation with a single hydrogen atom under idealized conditions is able to open a larger band gap in a very thin (three bilayer thick) film. The oxide interacts only very little with the film. The negligible band gap of the unterminated film however increases with the inclusion of many-body corrections via a G0W0 calculation. These calculations are considered to be more accurate, however we find that the surface states at the M point of the Brillouin zone are better described by density functional theory alone, which may be due to a substrate effect. By means of a constant relaxation time model we show the onset of electronic transport in films with up to 30 bilayers thickness as a function of temperature.
Description
Keywords
DFT , GW , Semimetal , Quantum confinement , Topological , Transistor , Bismuth , First principles , Density functional theory , Electronic transport , Surface state , Surface passivation , Nanostructure , Many-body calculation
Citation
König, C. A. 2021. Design of semimetal electronics relying on the quantum confinement effect. PhD Thesis, University College Cork.