The Morphology of Cu Clusters on SrTiO 3 (001) at Initial Stages of Metal Film Growth
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ABSTRACT Copper overlayers deposited on nearly stoichiometric SrTiO 3 (001) have been investigated with scanning tunneling microscopy (STM). Vapor deposition of Cu on a SrTiO3(001) surface at ambient temperature leads to the formation of three dimensional islands (clusters). The distribution of Cu islands appears to be inhomogeneous with two characteristic morphologies. In regions with a low density of Cu islands the Cu was always associated with step edges or defect sites. In regions with a high density of Cu islands the islands exhibit a random but nearly close packed morphology. The variation of Cu island density is indicative of diffusion of Cu clusters on the SrTiO 3 (001) surface. Diffusion was further confirmed by annealing the Cu/SrTiO3 at elevated temperatures yielding agglomeration of Cu clusters.
INTRODUCTION The fundamental interaction of metal atoms with oxide surfaces impacts a wide range of phenomena including local bonding and defect compensation, complex oxide film growth, interface bonding, and surface conduction. Since the strong metal support interaction was discovered by Tauster and coworkers in 1978 [1], deposition of metals on metal-oxide surfaces has received a tremendous amount of attention [2-6]. Much effort has been devoted to understanding the interplay between the metal-substrate reactivity and morphology in terms of the charge transfer between the metal thin film and the substrate. These studies are usually based on analysis of surface structure and reactivity. An important manifestation of metalsubstrate interactions is the geometry of the metal thin films [7,8]. With a few exceptions involving transmission electron microscopy [9] or scanning tunneling microscopy (STM) [1012], characterization of the structure of the metal thin films has been indirect, i.e. determined from Auger electron spectroscopy peak heights or sputter composition profile measurements. This approach can characterize a film or overlayer in terms of its growth mode; i.e. whether it grows layer-by-layer as would a wetting film or grows in a 3-D morphology as would a non wetting film. However, nothing can be learned from such measurements about the lateral arrangement of clusters or islands, the in-plane interactions between clusters, and the consequences to film morphology. The objective of this work is to use STM as a real space probe to study the morphological development of a non interacting metal thin film on the surface of a stoichiometric, i.e. fully oxidized, transition metal oxide. Copper is used as the metal because previous photoemission measurements have shown that charge transfer does not occur between Cu and most oxides [13]. After a brief description of the experimental procedure, STM of SrTiO3(001) surfaces deposited with various amounts of Cu will be presented. The effect of annealing on the topographic structure of Cu/SrTiO3 will also be shown. Our results indicate that macroscopic motion of the Cu clusters on the surface occurs at room temperature yielding a spatial variation of the density of Cu
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