Strain Relief in Cu-Pd Heteroepitaxy

TL;DR

This study investigates how misfit strain relaxes in Cu-Pd heteroepitaxial films, revealing a significant asymmetry between tensile and compressive strain behaviors through experiments and simulations.

Contribution

It provides new experimental and theoretical insights into the asymmetric strain relaxation mechanisms in Cu-Pd heterostructures, challenging existing elasticity theories.

Findings

Compressive strain in Pd/Cu relaxes via misfit dislocations after a few monolayers.

Tensile strain in Cu/Pd remains coherent up to about 9 monolayers.

Molecular Dynamics simulations confirm the tensile-compressive asymmetry observed experimentally.

Abstract

We present experimental and theoretical studies of Pd/Cu(100) and Cu/Pd(100) heterostructures in order to explore their structure and misfit strain relaxation. Ultrathin Pd and Cu films are grown by pulsed laser deposition at room temperature. For Pd/Cu, compressive strain is released by networks of misfit dislocations running in the [100] and [010] directions, which appear after a few monolayers already. In striking contrast, for Cu/Pd the tensile overlayer remains coherent up to about 9 ML, after which multilayer growth occurs. The strong asymmetry between tensile and compressive cases is in contradiction with continuum elasticity theory, and is also evident in the structural parameters of the strained films. Molecular Dynamics calculations based on classical many-body potentials confirm the pronounced tensile-compressive asymmetry and are in good agreement with the experimental data.