Epitaxial Films and Nanostructures

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The proposed research program will explore two recent unexpected discoveries associated with two particular examples of oxide substrates being used to support epitaxial structures. The first case involves a gold film evolving into epitaxial gold nanoparticles on a spinel substrate. There is a large surface energy associated with the gold spinel surface. This energy drives the dewetting of the gold at modest temperatures with the gold film breaking up into nanometer scale particles as expected. However, gold and spinel have a near lattice match so that at somewhat higher temperatures the gold nanoparticles rotate such that they are aligned with the underlying substrate. At yet higher temperatures, a self-limited chemical reaction takes place, forming an epitaxial bilayer of gold oxide. There is strong evidence that this bilayer represents an equilibrium complexion i.e. that it provides the lowest free energy interface between the gold and spinel crystalline states. The evidence is that with annealing at higher temperatures or longer times the bilayer does not appear to grow, the adhesion of the gold nanoparticle increases dramatically and that the estimate of the surface tension from the contact angle is greatly reduced. If equilibrium complexions could be introduced or approximated at the interface of insulating layers and metallization layers then the reliability of many semiconductor could be improved. The second discovery is that for specific orientations, it appears that compound semiconductors are able to be easily peeled mechanically from oxide substrates. This would not be surprising for graphene and other 2 dimensional materials. However, it this case we would expect strong bonding between the initial atoms forming the film and the substrate. Our results are consistent with this bonding becoming weaker as the film grows thicker. This result is also borne out by simulations although the mechanism is not understood. In the specific case of CdTe and ZnTe on sapphire, it is possible to grow single crystal-like films. This project will expand the materials that have been measured, but also begin the process of developing a device design and processing approach that takes full advantage of this unexpected phenomena. This lift-off approach allows the original substrate to be reused indefinitely. Current the cost of the substrate is a major component of the cost of sensors, solar cells and other devices so reusing the substrate multiple times would dramatically reduce costs.