Introduction

Surface energy is an underlying concept in understanding thin-film process. By definition, thin film has a very large surface-to-volume ratio. Surface energy controls the nucleation as well as the heterogeneous epitaxial growth processes. It also plays a key role in many applications of thin films, for example, in MEMS devices. Generally speaking, surface energy is the extra energy expended to create a surface, so surface energy is positive. It is important to know that metals have high surface energies and oxides have low surface energies, so a native oxide can grow on a metal.

The surface energy determines whether or not one material wets another and forms a uniform adherent layer as in heterogeneous epitaxial growth. A material with a very low surface energy will tend to wet a material with a higher surface energy so that epitaxial growth is possible. On the other hand, if the material to be deposited has a higher surface energy than the substrate surface, it tends to form clusters (“ball up”) on the low-surface-energy substrate. The epitaxial growth of a superlattice structure of ABABAB requires that in addition to a good lattice parameter match between A and B, the surface energy of A and B should be nearly the same. There is a well-known wetting principle that if A wets B, B will not wet A but ball up on A. In order to have A wetting B and B wetting A in growing the superlattice, their surface energies should be the same.