Atomistic simulation and analysis of novel group IV semiconductor alloys and devices
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Date
2023-01-01
Authors
Dunne, Michael D.
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University College Cork
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Abstract
A long held goal of the semiconductor community is the development of a direct gap silicon (Si) compatible material to enable the seamless integration of optical and electronic components on a single chip. The dominant elemental group-IV semiconductors silicon and germanium are the mainstays of current microelectronics, but their fundamental indirect gaps pose a roadblock to the development of active photonic components. The alloying of germanium with other group-IV elements, such as tin or carbon, has come into focus in recent years in pursuit of developing a direct gap alloy. Band engineering of germanium is attractive owing to the small difference between the indirect L6c-Γ8v and direct Γ7c-Γ8v band gaps of germanium which is only 140 meV. Alloying opens the possibility of reducing the Γ7c state below that of the L6c state leading to a direct gap alloy. Initial work on Ge1−xCx alloys have predicted the formation of a direct gap upon incorporation of dilute quantities of C (<1%), though there has not yet been an experimental demonstration of direct gap behaviour. Ge1−xSnx alloys have attracted greater research interest owing to the experimental demonstration of direct gap behaviour for a range of Sn compositions. Previous theoretical work suggested the transition from indirect to direct band gap occurs in a composition range of 6-11% Sn, while recent research indicates that a direct band gap emerges continuously with increasing x due to alloy band mixing. This atomistic effect, which is neglected in the widely-employed virtual crystal approximation (VCA), results in the alloy conduction band (CB) edge possessing hybridised character that evolves continuously from indirect (Ge L6c-like) to direct (Ge Γ7c-like) with increasing x. In this thesis we present a theoretical analysis of electronic structure evolution in the highly- mismatched dilute carbide group-IV alloy Ge1−xCx by adopting an atomistic approach encompassing calculations of the electronic structure using the semi-empirical tight-binding method. We demonstrate that C incorporation strongly perturbs the conduction band (CB) structure by driving hybridisation of A1-symmetric linear combinations of Ge states lying close in energy to the CB edge. These calculations describe the emergence of a “quasi-direct” alloy band gap, which retains a significant admixture of indirect Ge L-point CB edge character. The trends identified by our calculations are markedly different to those expected based on a recently proposed interpretation of the CB structure based on the band anti-crossing model.
For Ge1−xSnx alloys we are interested in the impact of the previously overlooked alloy effects have on the band to band tunneling in the alloy. We achieve this using non-equilibrium Green’s function (NEGF) band-to-band tunneling (BTBT) calculations based on atomistic tight-binding electronic structure calculations. We then extend this analysis to look at the effect of Sn incorporation on the
current characteristics of TFET devices. We demonstrate that CB mixing strongly modifies the complex band structure, driving complex band anti-crossing that – for Sn compositions at which the band gap is assumed indirect in the VCA – strongly increases the BTBT generation rate G. Our results highlight the importance of atomistic effects in determining the electrical properties of Ge1−xSnx alloys
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Keywords
Semiconductor , LASER , TFET , GeC , GeSn , Condensed matter theory , Electronic structure , Group-IV
Citation
Dunne, M. D. 2023. Atomistic simulation and analysis of novel group IV semiconductor alloys and devices. PhD Thesis, University College Cork.