First-Principles Study On Surface Kinetics And Metallic Quantum Well Systems

Ongoing miniaturization of microelectronic devices spurs wide interest in understanding, controlling, and modifying the growth of nanoscale-size structures on various surfaces. In this nanoscale regime, quantum size effect and quantum interference properties of electrons give rise to many novel and marvelous phenomena. The thesis is devoted to the study of surface kinetics and electronic structures in metallic quantum well systems from first-principles.The thesis consists of seven chapters. Chapter one and two are introductions, which review some important experimental and theoretical developments in pertinent fields. Chapter three gives a brief description of the density functional theory based first-principles calculation methods used in the thesis.In chapter four, we have studied the strain dependence of adatom binding energies and diffusion barriers in homo- and hetero-epitaxies of Si and Ge on (001) surface using first-principles calculations. In general, Si adatom binding energies and diffusion barriers are larger on Si(001) and Ge(001) surfaces than Ge adatom, in accordance with decreasing bond strength from Si-Si to Si-Ge and to Ge-Ge bond. The overall surface diffusion anisotropy of Si and Ge adatoms is found to be comparable on both Si(001) and Ge(001). The essentially linear dependence of binding energies and diffusion barriers on external strain are reproduced in all the cases, giving strong evidence for a priori quantitative prediction of the effect of external strain on adatom binding and surface diffusion.Chapter five concerns the surface mobility difference between Si and Ge and its effect on growth of SiGe alloy films and islands. Based on first-principles calculations of surface diffusion barriers, we show that on a compressive Gc(001) surface, Ge diffuses 10~2-10~3 times faster than Si in the temperature range of 300to 900 K; while on a tensile surface, the Ge and Si diffusivities are comparable. Consequently, growth of a compressive SiGe film is rather different from that of a tensile film. The diffusion disparity between Si and Ge is also greatly enhanced on the strained Ge islands compared to that on the Ge wetting layer on Si(001), explaining the experimental observation of Si enrichment in the wetting layer relative to that in the islands.Chapter six deals with the electronic structures in metallic quantum well systems. Using first-principles calculations within density functional theory, we carry out the first comprehensive study of how a symmetry gap around the Fermi surface develops towards its bulk value as the thickness of Ag films grown on Au(lll) substrate increases layer by layer. We show that, contrary to prevailing assumptions, the symmetry gap in ultrathin Ag films is much wider than the gap of the Au substrate along the [111] direction. As a result, the first and second quantum well states (QWS's) can be confined within the Ag films only when the film thickness reaches 10 and 19 monolayers, respectively. When Au films are grown on Ag(lll) slabs of comparable thicknesses, synergetic QWS's spanning across the whole (Au+Ag) systems are established, leading to distinct energy oscillations upon adsorption of O or CO on either the Au or Ag surfaces.The last chapter presents a summary of this thesis and some prospects for the ongoing investigations.