AVS 52nd International Symposium
    Surface Science Monday Sessions
       Session SS2-MoM

Paper SS2-MoM2
Thermodynamic Stability of Alumina Films on NiAl(110) and the Dynamics of Their Extended Defects

Monday, October 31, 2005, 8:40 am, Room 203

Session: Oxide Surface Structure and Characterization
Presenter: K.F. McCarty, Sandia National Laboratories
Authors: K.F. McCarty, Sandia National Laboratories
N.C. Bartelt, Sandia National Laboratories
J.P. Pierce, Sandia National Laboratories
C.B. Carter, University of Minnesota
Correspondent: Click to Email
A well-studied oxide surface is the thin-film alumina formed by oxidizing a NiAl(110) surface [J. Libuda et al., Surf. Sci. 318 (1994) 61]. Here, we show how low-energy electron microscopy (LEEM) can determine the thermodynamic stability of these alumina films and image the formation and time evolution of extended crystallographic defects. At sufficiently high temperature, discrete islands of crystalline oxide form when NiAl is exposed to oxygen. Analysis by electron diffraction and scanning tunneling microscopy establish that these oxide islands are the same alumina produced by the literature "recipe." We directly measure the thermodynamic stability (Gibbs formation energy) of the alumina -- at a fixed temperature, the pressure of oxygen in equilibrium with the oxide is determined by finding the pressure at which individual alumina islands neither shrink nor grow. We find that the equilibrium oxygen pressure of the thin-film alumina is many orders of magnitude greater than bulk alumina. Analysis suggests that strain is the cause of the remarkable instability of the alumina film. We also investigate how two types of planar defects in the films, boundaries between rotational and translation domains, originate and evolve. Typically, domains in films are thought to originate from the nucleation stage of film growth. That is, domain boundaries occur where rotated or translated islands impinge. Indeed, we observe that rotational boundaries form in this manner. In contrast, translation ("antiphase") boundaries are observed to nucleate, grow, and even move within isolated oxide islands. The fact that translation boundaries form within isolated alumina islands strongly suggests that the boundaries are introduced to relieve strain. We will discuss how formation of translation domains reduces film strain. This work was supported by the Office of Basic Energy Sciences, Division of Materials Sciences of the U.S. DOE under Contract No. DE-AC04-94AL85000.