Epitaxial Films and Nanostructures

这项拟议的研究计划将探索最近两项意想不到的发现，这些发现与用于支持外延结构的氧化物衬底的两个特定例子有关。第一个案例涉及在尖晶石衬底上形成外延金纳米颗粒的金膜。与金尖晶石表面相关的表面能很大。这种能量驱动黄金在适度的温度下去湿，金膜如预期的那样分解成纳米级的颗粒。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，从而使它们与下面的衬底对齐。在更高的温度下，发生自限化学反应，形成氧化金的外延双层。有强有力的证据表明，这种双层膜代表了一种平衡态，即它提供了金和尖晶石晶态之间最低的自由能界面。证据是，在更高的温度或更长的时间下，双层似乎不会生长，金纳米颗粒的附着力显著增加，从接触角估计表面张力的能力大大降低。如果能够在绝缘层和金属化层的界面引入或近似平衡络合，那么许多半导体的可靠性就会提高。*第二个发现是，对于特定的取向，化合物半导体似乎能够很容易地从氧化物衬底上机械剥离。对于石墨烯和其他二维材料来说，这并不令人惊讶。然而，在这种情况下，我们预计形成薄膜的初始原子和衬底之间会有很强的成键。我们的结果与这种结合随着薄膜变厚而变弱的情况是一致的。这一结果也得到了模拟的证实，尽管其机理尚不清楚。在蓝宝石上生长CdTe和ZnTe的特殊情况下，有可能生长出单晶状的薄膜。该项目将扩展已测量的材料，但也将开始开发充分利用这种意想不到的现象的设备设计和处理方法的过程。这种提离方法允许无限期地重复使用原始衬底。目前，衬底的成本是传感器、太阳能电池和其他设备成本的主要组成部分，因此多次重复使用衬底将大大降低成本。**

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.**