Investigation of Semiconductor and Metal-Metal Oxide Surface Nanostructures by Scanning Tunneling Microscopy/Spectroscopy

A surface is where all physical and chemical interaction between the material and environment takes place. The interactions between a surface and its environment are complex and often hard to predict. Scanning tunneling microscopy and spectroscopy (STM/STS) is a powerful tool that provides information on the atomic and electronic structure of surfaces and has been used in this work to study the adsorption of atoms or molecules, the structure of thin films and structural changes during controlled reactions. In this dissertation, growth of surface nanostructures and changes in the surface structure during controlled chemical reactions are studied using STM/STS. The dissertation is divided into two parts: in the first part, the evolution of the atomic and electronic structure of Ni-Cr alloys during oxidation is presented while the second part deals with the growth of 1D and 2D nanostructures on silicon surfaces.
NiCr-based superalloys are ubiquitous in high temperature applications such as turbine blades and steam reactors that require a combination of high oxidation resistance and good mechanical strength. The initial stages of oxidation of alloys are studied to understand the changes in the nucleation and growth of the oxides that are induced by Cr and other minor alloying elements and to determine the factors that control the formation of a passive oxide layer on the surface. In order to systematically vary the composition of the alloy while retaining the same starting surface structure, strategies for the growth of smooth Ni-Cr and Ni-Cr-Mo alloy thin films on MgO(001) are successfully developed. The alloy thin films (5-35 wt.% Cr) are oxidized at 300 °C in incremental steps and the evolution of atomic and electronic structure of the surface is captured. No oxides are formed on pure Ni surfaces in ultrahigh vacuum at 300 °C and the addition of just 5 wt.% Cr results in the nucleation and growth of oxides on the surface. The oxides grow to a certain critical height beyond which the growth mode is altered and the oxides begin to grow laterally to cover the metal surface. Our results show that increasing Cr content in the alloy decreases the oxygen exposure necessary to form a continuous oxide. Extensive statistics on the oxide height, area and volume are used to discuss the nucleation and growth of oxides as a function of alloy composition. The nucleation and growth of the oxides is dependent on the supply of Cr from the bulk to the surface and an atomic-scale reaction model is proposed to explain the initial stages of oxidation. The current experimental approach also allows us to measure the local density of states of the surface during oxidation. The evolution of bandgap of the oxide is measured and correlated with the oxide dimensions. The current work serves as input for phase field, DFT and molecular dynamics simulations to describe the initial stages of oxidation of Ni-Cr alloys. The experimental framework used in this dissertation can be extended to study the interaction of ternary Ni-Cr-X alloys to determine the effect of minor alloying elements such as W or Mo on the initial stage of oxidation as well as to other A-B alloys where the alloying element (B) is either more or less reactive than the host metal (A).
In the second part of the dissertation, the adsorption of Co and Mn on a Si(100) surface and the response of elastically strained Ge wetting layers on Si(100) to thermal energy is discussed. The incorporation of Co atoms on a clean Si(100)-2×1 surface and a Mn wire-decorated Si surface is studied to understand the initial stages of adsorption of magnetic dopants and their bonding structure with the ultimate goal of achieving magnetic semiconductors for spintronics applications. The interactions between Co-Si, Co-Mn, and Mn-Mn are discussed through sequential and co-deposition of Mn and Co. Subsequently, annealing-induced formation of novel three-dimensional structures on the elastically strained Ge/Si(100) wetting layers is presented. The role of the stored misfit strain energy in determining the type of structure formed is discussed.