EPITAXIAL STRAIN AND THE GROWTH OF CU(001) ON FE(001)

In order to study the role of epitaxial strain in stabilizing a body-centered cubic (bcc) modification of Cu, we have made high-resolution, in situ measurements of the in-plane elastic film strain associated with epitaxial growth of Cu(001) on Fe(001). Using grazing-incidence x-ray scattering with synchrotron radiation, we observe in-plane elastic strains in the Cu as large as 7.6% occurring at a coverage of 2 equivalent monolayers. These tensile strains tetragonally distort the face-centered cubic (fcc) Cu lattice, resulting in a body-centered tetragonal structure. A true bcc structure is not observed. As the film grows thicker, the epitaxial strain decays in a continuous, monotonic manner resulting in a slightly strained fcc structure by 10 equivalent monolayers. The observed strain relaxation agrees with equilibrium theory, suggesting that the growth of bcc Cu is contingent upon kinetic barriers to misfit dislocation nucleation and/or movement.