Growing epitaxial films when the misfit between film and substrate is large

摘要

A method is described for reducing the defect density in a crystalline film grown on a substrate with which it has a substantial misfit. The principle of the method is to grow the film, not directly from the substrate, but from a layer of small islands previously deposited onto the substrate. The technique has been fully investigated for the growth of Ag (and then Au) films on NaCl, a substantial improvement in the quality of the overgrown film being obtained when an intermediate layer of Ni islands is deposited on the NaCl prior to the deposition of the Ag. It has been demonstrated that it is important for the intermediate islands to bePP(a) generally epitaxially aligned with the substrate, even if as a result of their misfit with the substrate, they are partially incoherent;PP(b) weakly bonded to the substrate so that they can move on this substrate during the deposition of the upper layer upon them;PP(c) approximately hemispherical and small so that elastic strains in the overgrown layer decay rapidly with distance from the island and so that the preferred adatom site density is large on the islands compared with the substrate despite a relatively low coverage of islands on the substrate of about 10%;PP(d) be of intermediate misfit with the substrate compared with the crystal layer to be overgrown. PPWhile these principles have been demonstrated for the NaCl/Ni island (Ag/Au) system, it is to be expected that the use of this "multiple aligned seed island" technique could be applied to a number of other systems, such as the growth of GaAs on Si or GaP on GaAs or Si on Al.sub.2 O.sub.3, provided that the material and general specifications of the intermediate seed islands met those listed above.