Atomic Layer Epitaxy

Abstract

Atomic layer epitaxy (ALE) is a relatively new and generally poorly understood thin film growth technique which is likely to find application in high technology industries during the next decade. It can be used to grow high quality thin solid films with specific crystal structures or orientations and with very fine control of filn1 thicknesses to one atomic layer. Most thin film deposition techniques have, so far, simply involved material transfer from a suitable source to a suitable substrate via the vapour phase, e.g. vacuum evaporation, sputtering, or chemical vapour deposition. Nucleation takes place on the substrate surface, the molecules seeking the most energetically favourable position relative to one another and fonning n1icrocrystals that in later phases control thin film growth. This results in uncontrolled crystal boundaries, producing films whose average density can be considerably below that of the bulk crystal. Annealing is often employed to improve the quality of the deposited film. In ALE the compound thin film immediately achieves its final crystal form through sequential surface reactions in which one element of the con1pound reacts with the growing surface. This n1ethod is a self controlled process that automatically accepts only one atomic layer at a time.