An interface is an internal surface, the boundary between two media which may be crystalline, amorphous solid, or liquid. Its close similarity with a surface, a solid-vacuum boundary, suggests that many of the powerful techniques available for studying surfaces might be applied to the interface structure problem. The extent to which this is possible is the subject of this article.
The techniques to be discussed in this article include low energy electron diffraction (LEED), medium energy ion scattering (MEIS), x-ray diffraction, and x-ray reflectivity. (The most widely used method, transmission electron microscopy (TEM), is the subject of a separate article in this issue of the MRS BULLETIN.) To summarize what we will find, surface methods were developed to be nonpenetrating in order to have surface sensitivity. This works against us in the interface situation by requiring the use of extremely thin samples, at least on one side of the interface. This means special handling of samples in some cases and raises the possibility of artifac-tual results. Of the three methods, x-ray diffraction is the most penetrating and least surface sensitive; it probably has the greatest potential for widespread use in interface science.
This article defines structure as “atomic structure” for this purpose: we are interested in the coordinates of atoms at the interface and their relation to bulk structures on one or both sides. For this reason, we will consider only interfaces that are crystalline on at least one side. Since crystals are by far our strongest structural reference point, much less can be said about other interfaces. We will also consider the morphology of an interface, defined as the boundary of the crystal(s) that demarcates the interface, also at the atomic level. This is most apparent in the form of interface roughness. The roles of strain and misfit dislocations in interface formation, also studied by these techniques, are outside the scope of this article.