Theoretical studies of structural and electronic properties of overlayers on semiconductor surfaces

In this thesis we report the results of ab initio density functional calculations of equilibrium atomic geometry, electronic states and chemical bonding for the adsorption of elemental S and H2S on chosen semiconductor surfaces. The results are in good agreement with the available experimental results and indicate the need for further experimental work. In Chapter 2 of this thesis, I describe the formalism of the ab initio pseudopotential theory and the computational procedures which are used in this thesis. In the following chapter, a few experimental techniques are discussed, which we subsequently use their results to compare with our theoretical calculated results. In Chapter 4 the passivation of S on InP(110) is investigated. Two sets of geometries are used; non-reacted geometries and reacted geometries. For non-reacted full-monolayer coverage, the epitaxially continued layer structure is found to be the most energetically favourable and it exhibits a good semiconducting nature. For an ordered reacted model with the adsorbate S atoms exchanged with their neighbouring P atoms, the average vertical distance between the top two layers is in agreement with x-ray standing wave analysis, but is characterized by a small band gap. In Chapter 5 adsorption of the H2S molecule on the InP(110), GaAs(110) and GaP(110) surfaces is investigated within a dissociative adsorption model. In general the adsorption of H2S on the three semiconductors shows similar behaviour. In Chapter 6 the adsorption of elemental S on Si(001) is investigated using three adsorption models; hemisulfide-(2 x 1) structure, monosulfide-(1 x 1) structure, and disulfide-(1 x 1) structure. An analysis of the surface free energy suggests that the monosulfide structure is more stable than the hemisulfide and disulfide structures. This result is also used to investigate the adsorption of elemental S on the Ge(001) surface. In Chapter 7, the adsorption of the H2S molecule on the Si(001) and Ge(001) surfaces are investigated within two dissociative adsorption geometries. In both geometries studied in this thesis, the fundamental band gap is free of surface states, with the highest occupied state lying below the valence band maximum. In Chapter 8 the results are summarised and we outline possibilities for future work. (author)