Epitaxial Films and Nanostructures

提出的研究计划将探讨最近两个意想不到的发现，这些发现与用于支持外延结构的两个特定氧化物衬底有关。第一种情况涉及在尖晶石衬底上将金薄膜演变成外延金纳米颗粒。有一个大的表面能与金尖晶石表面有关。这种能量在适当的温度下驱动金的脱湿，金膜如预期的那样分解成纳米级颗粒。然而，金和尖晶石具有接近的晶格匹配，因此在稍高的温度下，金纳米颗粒旋转，使它们与底层衬底对齐。在更高的温度下，会发生自我限制的化学反应，形成氧化金的外延双层。有强有力的证据表明，这种双分子层代表了一种平衡的肤色，即它在金和尖晶石晶体状态之间提供了最低的自由能界面。证据是，在更高的温度或更长的时间下退火，双分子层似乎没有生长，金纳米颗粒的附着力急剧增加，并且从接触角估计的表面张力大大降低。如果能在绝缘层和金属化层的界面处引入或近似引入平衡络合物，则可以提高许多半导体的可靠性。

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.