AVS 53rd International Symposium
    Surface Science Wednesday Sessions
       Session SS1-WeM

Invited Paper SS1-WeM1
Modes of Alloy Crystal Growth Dominated by Sub-Surface Defect Dynamics

Wednesday, November 15, 2006, 8:00 am, Room 2002

Session: Reactions on Metal & Bimetallic Surfaces
Presenter: J.P. Pierce, Sandia National Laboratories
Authors: J.P. Pierce, Sandia National Laboratories
N.C. Bartelt, Sandia National Laboratories
K.F. McCarty, Sandia National Laboratories
Correspondent: Click to Email
We show that the evolution of an alloy's surface during film deposition can be controlled by phenomena typically thought of as bulk or sub-surface processes. The response of a two-element compound, NiAl, is observed in real time with low-energy electron microscopy as one element, Al, is deposited on its surface. At low temperature (<600 K), this is a conventional heteroepitaxial system; the NiAl is a template on which increasingly Al-rich film phases grow. At higher temperatures, however, the substrate actively participates in the crystal growth by supplying a flux of Ni atoms to the surface. New layers of NiAl alloy form on the substrate even though only Al is deposited. As more Al is deposited, initially immobile bulk dislocations dissociate and move, and new atomic layers nucleate along their tracks. This behavior is unlike typical epitaxial growth in two ways. First, the compositions of the deposited and growing materials differ - Al is deposited but NiAl grows. Second, localized changes in the surface's topography and composition accompany the dislocation motion. These dynamics relate simply to the type and abundance of point defects in the near-surface region. Quantitative analysis shows that exactly half of the deposited Al atoms replace Ni in the bulk and the other half incorporate into new NiAl on the surface, establishing that Ni antisites are the point defects that initially supply Ni to the surface for NiAl growth. Analysis also shows that dislocations begin to move when the near-surface concentration of these defects is depleted to the critical composition below which Ni vacancies form. We find that the crystallographic direction of dislocation motion changes with composition. Finally, we describe how the motion of dissociated (partial) dislocations both locally enhances the crystal growth rate and changes the local composition. This work supported by U. S. DOE, OBES, Division of Materials Sciences under contract DE-AC04-94AL8500.