FILMS IN TETRAGONAL STATES PRODUCED BY EPITAXIAL STRAIN

Abstract

The tetragonal states of {001} films of cubic materials produced by coherent epitaxy on cubic {001} substrates are calculated for Cu and Fe assuming Poisson ratios for the bcc and fcc structures that are constant as the material deforms. The states lie on lines, referred to as epitaxial lines, on the plane, referred to as the tetragonal plane, whose axes are the aspect ratio c/a and volume per atom V. The results of quantitative low-energy electron diffraction (QLEED) for the bulk layer spacing of coherent epitaxial films of Cu and Fe, when plotted on the tetragonal plane, show whether the films lie on the bcc or fcc epitaxial line. Strain-energy calculations along the epitaxial lines lead to a critical value of a at which a first-order transition from one epitaxial line to the other occurs; this transition is illustrated by the epitaxial films of Fe, some of which lie on the bcc line and some on the fcc line.