Studies of the structure and bonding of interfaces in multilayer heteroepitaxy of barium titanate, barium oxide, and barium disilicide on (001) silicon

Formation of epitaxial oxide films possessing non-linear optical properties on silicon substrates presents new issues in the field of hetero-epitaxy. The oxide, which is often a perovskite, is nearly always a compound of at least three or more elements derived from different regimes of the periodic table. The bonding in the oxide is complex while silicon is primarily covalent, the lattice mismatch is often substantial, and silicon tends form glassy silicates by reacting with the oxide.; The objective of this research was to investigate the atomic structure and bonding of interfaces in a multilayer architecture that provides a graded transition from the silicon substrate to the epitaxial barium titanate overlayer. In the present study, these intermediate interfaces are formed between barium silicide and silicon, barium oxide and silicon, and barium titanate and barium oxide.; The three principal techniques used to study the interfacial structure and bonding were high-resolution transmission electron microscopy (HRTEM), reflection high energy electron diffraction (RHEED), and X-ray photoelectron spectroscopy (XPS). The films were grown by molecular beam epitaxy (MBE) with unactivated molecular oxygen in an ultrahigh vacuum (UHV) chamber fitted with barium and titanium effusion cells. In situ RHEED and XPS were used to characterize the transitions in bonding and structure during monolayer-by-monolayer construction of the interfaces.; Barium metal was found to react at room temperature with silicon to form a barium silicide; thick orthorhombic BaSi{dollar}sb2{dollar} forms epitaxially on Si(001) with two variants, and the BaSi{dollar}sb2{dollar}/Si(001) interface is sharp and complex. Oxidation of barium disilicide involves establishment of Si-O bonding, and consequently Ba-O bonding. Monolayers of barium oxide shows two epitaxial relationships with the Si(001) surface; an "explosive" crystallization of BaO may occur under conditions of low atomic mobility. Steady state growth conditions exist that favor only the "cube-on-cube" epitaxy of BaO/Si(001). Atomic models of the sharp BaO/Si(001) interface with Si-O bonding are proposed. Initial formation of BaTiO{dollar}sb3{dollar} on BaO(001) indicates an abrupt interface, but heating of the system induces formation of an intermediate glassy layer, possibly Ba{dollar}sb2{dollar}TiO{dollar}sb4{dollar}.