AVS 47th International Symposium
    Surface Science Monday Sessions
       Session SS2+NS+TF-MoM

Paper SS2+NS+TF-MoM3
Thermodynamics of Thin Film Alloys: Experimental and Theoretical Study of Ag and Co on Ru(0001)

Monday, October 2, 2000, 9:00 am, Room 209

Session: Nucleation and Growth
Presenter: G.E. Thayer, University of California, Davis
Authors: G.E. Thayer, University of California, Davis
A.K. Schmid, Sandia National Laboratories
V. Ozolins, Sandia National Laboratories
N.C. Bartelt, Sandia National Laboratories
S. Chiang, University of California, Davis
R.Q. Hwang, Sandia National Laboratories
Correspondent: Click to Email
It has long been known that the structure of materials in thin film configurations can differ significantly from their bulk phase. This is particularly true of alloy films. It has recently been shown that strain induced by the lattice mismatch between substrate and film can lead to the formation of novel alloys that do not exist in the bulk. One prototypical example is the system of one monolayer Ag/Co films grown on Ru(0001). In the bulk, Ag and Co are immiscible. However, on the Ru surface alloy phases of distinct stoichiometries are formed. We have investigated the phase diagram of 1ML AgCo/Ru(0001) films using scanning tunneling microscopy (STM) and first principles calculations. For Ag rich films, segregation between a pure Ag phase containing dislocations and a pseudomorphically strained alloy of stoichiometry Co@sub 0.6@Ag@sub 0.4@ is found. The driving force for the phase segregation is the competition between two strain relief mechanisms: dislocation formation and alloying. In the Ag saturated Co@sub 0.6@Ag@sub 0.4@ alloy, atomic resolution STM images show that the alloy consists of a Co film containing disordered, elongated Ag droplets with an average size of 30 atoms separated by an average distance of 10 Co atoms. As the composition of Ag in the film is decreased, the Ag droplets become more uniform in size and decrease to an average minimum size of 10 atoms. Simultaneously, the distribution of the droplets becomes uniform and dilute. This behavior has been predicted using first principles, local density approximation (LDA) calculations. These calculations, together with atomically resolved STM images, have quantified configuration energies for various stoichiometries of this system. This investigation has led to a quantitative interpretation of the competition between the chemically repulsive interaction of Ag and Co (also seen in their bulk miscibility gap) and the strain fields in the thin film alloy that they form.