Theoretical studies of surfactant mediated epitaxial growth at semiconductor surfaces

This grant enabled us to appoint a post-doctoral research fellow and conduct the research project fully and beyond the original plan. Travel and subsistance funds were used to present results at national, european and international conferences. The results of this highly successfully completed project have resulted in 23 research papers, and are summarised below.

Detailed atomic and electronic structure of the asymmetric dimer model were studied for the Si(001) and Ge(001) (2×1) surfaces. The phonon spectra for these surfaces were also calculated.

It was concluded that interdiffusion for Ge growth on Si(001) becomes important only at high temperatures. Segregated structure for Si growth on Ge(001) is favoured even at low temperatures. Predicted asymmetric, non-elongated Ge dimers on Si(001)/Ge(2×1), contradicting existing (conflicting) experimental results. Calculated phonon spectrum for this surface.

Studies atomic and electronic structure of Si(001)/Sb(2×1) and Ge(001)/Sb(2×1) surfaces. Calculated phonon spectrum for Si(001)/Sb(2×1) surface.

Confirmed segregation of Sb to the surface when co-adsorbed with Ge on Si(001), attributing this to the removal of an electrostatic surface dipole which is present for the Ge-capped structure. Generated phase diagram for Ge growth on Si(001) with and without the presence of co-adsorbed Sb. The phase diagram implies an equilibrium thermodynamic explanation for the surfactant properties of Sb. The calculated phonon spectrum for the Si(001)/Ge/Sb(2×1) surface indicates the presence of quite low lying Rayleigh modes, and the absence of optical modes above the bulk continuum.

Identified the {\it overlaid above row} structure as the correct description for the c(4×4) reconstruction of the 0.25 ML Sb-covered Si(001) surface. Determined (in conjunction with experimental work performed by Dixon and McConville at Warwick) that a uniformly distributed Sb overlayer phase only exists on the sub-monolayer-covered Si(001) surface in the region of 0.25 ML coverage.

Carried out a detailed geometrical study of the GaAs(001)(2×4) structure, explaining reconstruction in terms of different behaviours of 3- and 4-fold co-ordinated Ga atoms. Occupied surface states were found to be characteristic of exposed third-layer and top-layer As dimers, while unoccupied surface states were localised on three-fold co-ordinated Ga atoms.

Studied the GaAs(001)(2×1) structures (Ga and As terminated), finding both to be metallic. As-terminated has symmetric dimers, Ga-terminated has dimers with 8° buckling. Studied GaAs(001)/Ge(2×1) and GaAs(001)/Ge(1×2) reconstructions (i.e. with Ge-As and Ga-Ge dimers respectively). The Ge-As terminated surface is semiconducting (small gap), with dimers tilted by 14.6°, while the Ga-Ge terminated surface is semiconducting (large gap), with dimers tilted by 19.0°.

Studied two models for the GaAs(001)/Sb(2×4) surface. The model suggested by Moriarty and Beton at Nottingham was found to be semiconducting, with occupied surface states characteristic of exposed third-layer As dimers and top-layer Sb dimers, and unoccupied surface states characteristic of three-fold co-ordinated Ga atoms.

Work in progress (unpublished) on adiabatic model for initial single- atom Ge adsorption on Si(001)/Sb.