Atomic layer epitaxy

Abstract

This review discusses the development and present status of atomic layer epitaxy (ALE), a technology for growing layers of crystalline and polycrystalline materials one atomic layer at a time. Atomic layer epitaxy was originally developed to meet the needs of improved ZnS thin films and dielectric thin films for electroluminescent thin film display devices. Accordingly, early work on ALE was mainly carried out for thin films. During the 80s there has been a growing interest in applying ALE in the growth of single crystals of III–V and II–VI compounds and ordered heterostructures such as layered superalloys and superlattices. ALE has also been extended to the growth of elemental single crystals. A basic advantage of atomic layer epitaxy is in the increased surface control of the growth. This is achieved by combining a sequential reactant interaction with a substrate at a temperature which prevents condensation of individual reactants on the growing surface. This results in a stepwise process where each reactant interaction is typically saturated to a monolayer formation. Accordingly, the rate of the growth in an ALE process is determined by the repetition rate of the sequential surface reactions, and the thickness of the resulting layer is determined by the number of reactant interaction cycles. This self-controlling feature of atomic layer epitaxy ensures excellent uniformity of the thickness over large substrate areas even on non-planar surfaces. Owing to its principle of operation, ALE is especially suitable for producing layered structures of III–V and II–VI compounds. Superlattice structures of both these material groups have already been demonstrated. As a limiting case of superlattices, layered superalloys have also been grown. In ALE, chemical reactions producing a material, are divided into separate subreactions between a vapor and a solid surface, each of which results in a new atomic layer of the material. From the theoretical point of view ALE offers a unique link between theoretical and experimental chemistry by permitting direct observations of subreactions under conditions where the chemical environment is more precisely determined than in conventional continuous reactions.