Epitaxy And Single Crystal Growth

Improvements in optoelectronic materials have resulted from the strict control of orientation in macroscopic assemblies of electroactive materials. The two main parts of this chapter are devoted to the different methodologies which increase order in thin or ultra thin films of organic materials from low molecular weight molecules and from macromolecules. The orientation of small or nonpolymeric molecules and the orientation of polymeric structures are however quite different. The orientation of small molecules is carried out by crystal growth techniques, whether solution growth, melt growth or sublimation growth. The orientation in polymeric structures is often produced by an average orientation of the long macromolecular chains by stretching, rubbing or similar techniques which depend upon the physico-chemical and mechanical properties of the polymers. The growth of organic layers by epitaxy has proved to be a promising way of orienting both low molecular weight molecules and monomers able to polymerize in the solid state. Numerous examples of oriented growth by a crystalline substrate have been reported concerning both second and third order materials. The epitaxial growth of organic layers on a crystal substrate is based on the control of lattice interactions. Organic crystals are generally bonded by van der Waals forces and it is not neccessary for the thin film to match perfectly with the substrate. It has been shown that, when the substrate differs from an ionic substrate, the epitaxy method could produce ordered films with a relatively small dislocation content.