Local Atomic Arrangements and Phase Stability in Ultrathin Metal Alloy Films

Northwestern University, Evanston IL

Investigators

Abstract

In this research surface-diffuse-scattering experiments and atomic-scale computational modeling are closely coupled in order to investigate the nature of the interatomic interactions governing the energetics and thermodynamic properties of ultra-thin metal-alloy films. The long-term goal is the development of a theoretical framework for predicting how the atomic structure and phase stability of specific alloy systems are affected by variations in temperature, the composition and thickness of the film, and the chemistry and crystalline geometry of the substrate. This experimental effort focuses on the investigation of chemical short-range order and local atomic displacements in thin-film alloys through measurements of surface x-ray scattering diffuse intensities. These experiments are carried out primarily at the Advanced Photon Source, ANL. In order to derive quantitative structural information from these experiments it is necessary to measure the diffuse intensity on an absolute scale. For this purpose, a number of new techniques are explored for providing the intensity of the direct beam for surface diffuse scattering, which is the evanescent wave traveling along the surface. Once data is obtained on an absolute scale at many points in reciprocal space, chemical short-range-order and atomic-displacement contributions to the diffuse scattered intensity of ultra-thin metallic alloy solid solutions can be extracted. Such detailed structural information provides important insights into the nature of the interatomic interactions governing phase stability in thin-film alloys. The theoretical effort focuses on the development of a microscopic framework for calculating both chemical and elastic contributions to the energetics of ordered and disordered structures in ultra-thin epitaxial alloy films. This approach makes use of the results of electronic structure calculations of the total energies and relaxed atomic structures of ordered alloy films to extract interatomic interaction parameters and force constants from first principles. These parameters form the basis for efficient Monte-Carlo simulations from which thin-film alloy structural and thermodynamic properties are calculated. In order to validate and further guide the development of the computational approach, detailed comparisons are performed between the structural information obtained from these simulations and from surface diffuse-scattering measurements. %%% Bulk composition-temperature phase diagrams provide only limited insight into the atomic structures which form in ultra-thin (i.e., a single to a few monolayers in thickness) alloy films. In particular, it has been observed commonly that two elements, which are immiscible in bulk crystalline phases, form stable mixed alloy phases in ultra-thin films. Furthermore, it has been demonstrated recently that ultra-thin films consisting of alloys of immiscible metals display a unique form of lateral, nanometer-scale compositional ordering not observed in the bulk. The potentially novel catalytic and magnetic properties associated with these new metallic alloy phases have given rise to increasing interest in their atomic structures and thermodynamic stability. ***