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Simulation of semiconductor-oxide interface atomic and electronic structure

The silicon-silicon dioxide interface is a vital ingredient in modern digital electronics as it has formed the basis of the metal oxide semiconductor field effect transistor (MOSFET). Generation of these interfaces with a very low defect density has made the PC possible. However, reduction in transistor dimensions to the point where the silicon dioxide layer is only a few atomic dimensions thick led, in 2007, to the introduction of oxides related to hafnia (hafnium dioxide) as the MOSFET gate oxide. Further development of MOSFETs is believed to be possible through replacement of silicon by so-called III-V materials, such as InGaAs, which have a higher electron mobility and possibly higher switching speed than silicon.
Unpassivated Si(001)-aSiO2 interface and H passivated Si(001)-aSiO2

The chemistry of the silicon-silicon dioxide interface is ideal from the point of view of low interface defect density. Any dangling electrons at the interface can be tied up by adding hydrogen so that the number of electron states in the band gap is small. Energy levels for the silicon-hydrogen bond lie well below the silicon valence band edge.

Density of States for unpassivated Si(001)-aSiO2

Atom-projected densities of states for the Si(001)-aSiO2 interface

The chemistry of the III-V oxide interfaces is poorly understood. Adding hydrogen is not a good solution for removing electronic states from the III-V band gap as arsenic-hydrogen bond energy levels, etc. lie around the III-V valence band edge.

In this project we aim to understand the electronic behaviour of silicon-silicon dioxide and silicon-hafnium dioxide interfaces using a combination of classical molecular dynamics and density functional theory. The figures below show structures generated for these interfaces using a combination of these techniques.

Defect wave functions

Electronic wave functions for dangling electron defects at the Si(001)-aSiO2 interface

The figure above shows that Si dimers form spontaneously at the interface but are not ordered as they are at clean Si(001) surfaces. This leads to a high defect density at the interface unless these are passivated by terminating dangling bonds by H atoms.

This work is sponsored by the Irish Higher Education Authority under the PRTLI-V grant.