Films Stabilized by Long-Range Forces

Original entry by Hyerim Hwang, AP 226, Spring 2012. not finished

Reference

Zhigang Suo and Zhenyu Zhang, "Epitaxial Films Stabilzed by Long-Range Forces", Physical Review B 1998 58, 5116-5120

Keywords

Dynamic forces, Colloidal suspensions, Interfacial deformation, Static surface forces

Introduction

Stability of an epitaxial film on a substrate of a different material is determined by the competition between surface tension and stress. This paper presents that thermodynamic driving forces of different physical origins which act over longer ranges than atomic length, can be strong. It shows that dispersion forces compete with the stress in thin films and the electrons in a metal film mediates a long-ranged force which can stabilize metal films of atomic layers.

Figure 1. An epitaxial film on a substrate. The film surface is perturbed into a wavy shape of wavelength and amplitude.

Figure 2. Two representative long-range interactions.

Figure 3. The top figure shows the qualitative shape of the function W(D) for Ag on GaAs, where the inflection point is marked by a small circle. The bottom figure shows the qualitative shape of the curvature.

Results

Figure 1 illustrates a film thickness on a semi-infinite substrate. The surface tension of the film is taken to be isotropic and the stress in the film results from the difference of the film and the substrate. The model is based on a stability analysis.

All energies are computed for one period of the system, per unit thickness in the direction normal to the plane, to the leading order in the perturbation amplitude. We can know that the perturbation increases the surface energy and elastic energy stored in the system decreases when the surface undulates.

Discussion

We can know from this paper that long-range interactions can stabilize epitaxial films against stress. Dispersion forces can compete with the stress in thin films. We can know the dependence of the transition thickness on stress in the presence of the long-range forces.