Strain-induced shift in the elastically soft direction of epitaxially grown fcc metals

TL;DR

This paper extends the theory of epitaxial strain energy for fcc metals beyond the harmonic approximation, revealing how different crystallographic directions soften under tensile or compressive biaxial strain, impacting epitaxial growth and alloy behavior.

Contribution

It introduces a non-harmonic model for epitaxial strain energy in fcc metals, predicting direction-dependent softening under various strains, which was not previously understood.

Findings

<001> and <111> soften under tensile strain

<110> and <201> soften under compressive strain

<201> becomes softest under sufficient compression

Abstract

The theory of epitaxial strain energy is extended beyond the harmonic approximation to account for large film/substrate lattice mismatch. We find that for fcc noble metals (i) directions <001> and <111> soften under tensile biaxial strain (unlike zincblende semiconductors) while (ii) <110> and <201> soften under compressive biaxial strain. Consequently, (iii) upon sufficient compression <201> becomes the softest direction (lowest elastic energy), but (iv) <110> is the hardest direction for large tensile strain. (v) The dramatic softening of <001> in fcc noble metals upon biaxial tensile strain is caused by small fcc/bcc energy differences for these materials. These results can be used in selecting the substrate orientation for effective epitaxial growth of pure elements and A/sub p/B/sub q/ superlattices, as well as to explain the shapes of coherent precipitates in phase separating alloys.