The Chemisorption Of Au On Si(001) Surface And Fe On Au-passivated Si(001) Surface

The chemisorption of Au on the Si(001) surface and the interaction of Fe withthe Au-passivated Si(001) surface are studied by using the self-consistenttight-binding linear muffin-tin orbital (TB-LMTO) method in this thesis.The adsorption of one monolayer Au atoms on an ideal Si(001) surface isstudied by using a standard supercell model. This supercell model includes nineSi atomic layers. A monolayer Au atoms are placed on each side of the slab. In theregion of the slab, there are five atomic layers vacuum introduced in the supercell,and this supercell has mirror symmetry with respect to the central layer of thesolid films. Adsorption energies of a Au atom on different sites are calculated.The layer projected density of states for Au atoms covered Si(001) surface arestudied and compared with that of the clean surface. The charge transfer areinvestigated. It has been found that the adsorbed Au atoms may sit at the top siteabove the surface and that a passivation layer of Au atoms may passivate theSi(001) surface effectively. It is possible for the adsorbed Au atoms to sit belowthe Si(001) surface at the bridge site, resulting in a Au-Si mixed layer at theAu/Si(001) interface , which explains theoretically the problem that the Au-Siintermixing cannot easily be observed and the thickness of silicide cannot easilybe confirmed experimentally. These results are in agreement with theexperimental results.For the Au adatoms chemisorption on Au-pasivated Si(001) surface, amonolayer of Au atoms is used to saturate the dangling bonds both sides of thesupercell surfaces. A standard supercell model includes eight Si atomic layers andone adsorbed monolayer of Fe, and five layers of vacuum. The Fe adatomschemisorption on an ideal Si(001) surface is also considered for comparison. Amonolayer of Au atoms is used to saturate the dangling bonds one side of thesupercell surfaces. The supercell includes eight silicon layers, a monolayer of Auatoms to saturate one face of the slab to simplify the calculation procedures, oneadsorbed monolayer of Fe atoms, and six layers of vacuum. The standard supercellmodel includes sixteen atomic layers. In an attempt to hinder silicide formationeffectively, a monolayer of Au atoms is introduced to saturate the dangling bondson the slab. This method will be expected to improve the slab calculation and tobe able to deal with the present problem theoretically.The chemisorption of Fe adatoms on the ideal Si(001) surface andAu-passivated Si(001) surface are considered. Energies of adsorption systems of aFe atom on different sites are calculated, and the layer project density of states(LPDOS), the charge transfer are investigated. It is found that for the adsorbed Feadatoms on the ideal Si(001) surface, the Fe atoms sit at fourfold hollow site anda Fe-Si mixed layer may exist at the Fe/Si interface. While for Fe adsorption onthe Au-passivated Si(001) surface, the Fe atoms are favourable on the fourfoldhollow and Fe atoms might sit below the Au surface. Therefore, a Au-Fe mixedlayer may exist in the Fe/Au-Si (001) interface region. The adsorbed Fe atomscannot exist below the Si surface and there is no Fe-Si mixed layer. From theabove discussion, we can conclude that using a Au buffer layer can effectivelyhinder the Fe-Si intermixing at the interface, which proves the possibility ofgrowing high-quality magnetic expitaxial Fe films on noble metal-passivationSi(001) surface theoretically.