Structural and chemical properties of epilayers in heteroepitaxy
Résumé
A possible definition of heteroepitaxy is the growth of an A crystal (the so-called epilayer) onto a B crystal (the so-called substrate) where the B crystal imposes to the A crystal at least one element of crystalline symmetry. Keeping in mind this definition, we will review the physical origin of the structural and chemical properties of an epilayer compared to the bulk state.
A and B crystals can differ by their crystalline lattice, their lattice parameter and/or their chemical composition. Many of the structural properties of the epilayer come from the necessary accommodation of the A crystal to the B crystal through an interface. For instance, a suitable epitaxial relationship can allow the crystalline system of A to accommodate that of B, if different. The difference of lattice parameters (lattice misfit or mismatch) is accommodated by elastic strain of the epilayer and/or plastic relaxation through misfit dislocations located at interface. The difference of chemical composition induces specific interfacial bonding and/or intermixing at interfaces. In addition, as epitaxial growth is an out-of-equilibrium process, the epi-layers can adopt metastable phases which do not exist in bulk state. Finally interfaces and surfaces them-selves play a major role in the growth modes, in particular through the classical concepts of wetting/dewetting involving energies of the A and B surfaces and of the A/B interface. Due to the crystalline character of the materials, faceting also has an important impact on the morphology.
The talk will be illustrated by examples taken in various systems, most often studied by transmission electron microscopy, which is one of the most powerful and versatile tool to study these properties at different scales.