Strain And Electric Field Effects On Adsorption Energetics And Structures On Si Surfaces

One important technique of surface process, which has a wide range of potential applications, is epitaxial growth. Both homoepitaxy and heteroepitaxy in growing crystalline films on a crystalline substrate play a key role for many surface and interface phenomena, since the growth offers the opportunity to create artificial materials with novel structural, physical and chemical properties of extremely well characterized surfaces on atomic scale. The general trend to tackle complexity in atomistic processes in the early stages of growth is gained in methodology. Controlling surface processes such as surface diffusion and surface growth may be seen as a challenging work, since it indeed involves in microscopic surface processes, such as making and breaking of chemical bonds and atom dynamics, and since it is inevitably influenced by various factors, such as deposition rate, substrate temperature, surface reconstruction and external stress. Many efforts have been made in this aspect throughout the last 30 years.It is well known that surfactants can be used to facilitate layer-by-layer growth. In the first part of the thesis we focus on this aspect. In the heteroepitaxy lattice mismatch between deposit and substrate will produce strain in the epitaxial layer. It is well known that an external stress imposed on a heteroepitaxial film can drive a transition from one kind of surface reconstruction to the other. Using the scattering theoretical approach based on the semi-empirical tight binding Green function method, we identified the large stress anisotropy as the primary driving force of defects on Sb/Si(001) (Chapter 3). The Sb/Si(001) 2×1 surface was found to be under a tensile stress of 1.0 eV/(1×1 cell) along the dimer bond and a compressive stress of-1.1 eV/(1×1 cell) along the dimer row. Calculations of the surface stress for Sb/Si(001) 2×4, 2×5 and 2×6 reconstructions, which are formed by shifting dimers in the dimer row to the adjacent trench, showed a significant relief of the compressive contribution to the stress along the dimer row comparing with the case of the 2×1 reconstruction, and thus a decrease of the stress anisotropy. We refer stress relief to the formation mechanism of antiphase domain boundaries, which breaks up the long-range 2×1 order of the Sb/Si(001) surface. It was further confirmed by sequent ab initio calculations (Chapter 4). The ab initio calculations indicated that stress relief should be responsible for the (2×n) reconstruction of Sb and Bi on the Si(001) surface, and the surface stress anisotropy canbe reversed with the formation of dimer vacancies, which constructs the (2*n) reconstruction.However, in the technological application of surfactants, even at elevated temperatures the floating adsorbed overlayer is difficulty removed from the deposited film after growth process completion. For this point of view, as a good way to mediate surface processes, the key feature is whether it can be easily removed after growth process completion. It is well known that the electric field between STM tip and sample can influence kinetics of adsorbat diffusion on surfaces and the new scan technique allows the STM tip to track the diffusion of adsorbats on surfaces.Thus, in the remaining part of the thesis we will focus on electric field effects on surface diffusion. We will demonstrate a localized electric field is indeed an extra degree of freedom that may be exploited to modify or control surface adsorption energetics, and thereby surface processes such as diffusion. Due to its importance for a wide range of surface phenomena (STM and FIM), understanding the influence of the localized electric field on surface properties is not only essential for interpreting some experimental results, but also may be utilized as a way for controlling surface processes.Based on first principles calculations, we found that surface adsorption energetics of Ag on Si(l 11) surface can be modified by imposing a localized electric field (Chapter 5). The preferred adsorption sites are found to be strongly field-dependent. The most energetically favorable adsorption site of Ag on the Si(lll) surface will undergo a transition from the fee site to the top site with increasing the field strength, and thus the diffusion of Ag on Si(l 11) could be enhanced or suppressed, depending on the strength of the applied electric field. The most stable adsorption site without a field is located at the fee site, which is 0.54 eV/adatom lower in energy than at the top site. When the field reaches about 1.2 V/A, the difference of the adsorption energy of Ag on Si(lll) between the fee and top site is almost zero, then, the top site becomes the most favorable site for Ag adsorption on the Si(l 11) surface with further increase of the field. As the field strength reaches 2.0 V/A, the energy difference between the top and fee site is -0.49 eV/adatom. Thus, Ag atoms favor in energy to stay in the top site. That the lower coordinated top site is favored in a strong positive field can be explained that it has a stronger induced dipole than the higher-coordinated sites. Corresponding to an intuitive notion, the electric field may be used as a new technique to tailor evolving morphology during the thin-film growth. We expected similar behaviors for Au, Na and Al on the Si(lll) surface. We also found that the growth of these atoms on Si(lll)...