Tuning and protecting MoTe2 derived phase change materials for electronic device fabrication

摘要：非技术性：层状半导体具有很强的面内键合和平面之间只有很弱的非共价相互作用，可以减少到单个分子层的厚度，同时保持良好的电子性能。因此，这些材料能够最终减小微电子器件的长度尺度。MoTe 2在这类新型半导体中特别令人感兴趣，因为其具有理想的带隙，并且重要的是，其存在能够将材料从半导体转变为金属的结构相变。MoTe 2的相的局部切换因此能够使金属/半导体器件完全由单一元素组成构建。在这个项目中，我们的目标是克服材料工程的挑战，从而使利用特殊的相变性能的MoTe 2的设备结构。首先，我们将建立在晶圆级的MoTe 2的单个或几个分子层的合成。其次，我们研究了从半导体到金属相的受控相变，并研究了通过添加其他元素来修改这种相变的方法。第三，我们解决了MoTe 2的化学不稳定性，这导致容易氧化。将开发将其封装在保护层中的策略，这对于使MoTe 2可用于设备至关重要。该项目是嵌入在南佛罗里达大学的二维材料的研究活动，并将提供教育，研究生和本科生在未来的技术相关领域。技术支持：范德华半导体MoTe 2具有相对较小的带隙，约为1 eV，是一种很有前途的双极场效应器件材料。在整个可见光谱范围内的强光吸收使得MoTe 2对于光电子学也是有趣的，并且其带隙值使其成为近红外光电子学的候选者。此外，MoTe 2从半导体到金属的热致相变为制造与货车德瓦尔斯半导体的电接触的材料工程问题提供了可能的解决方案。在这个项目中，我们将研究和优化MoTe 2的分子束外延生长。通过合金化对MoTe 2的改性将被研究，其目标是能够调节带隙以及相变行为。这些薄膜的生长和它们的性质主要是其特征在于与扫描探针显微镜/光谱和光电发射，从而我们获得的增长和相变机制在纳米尺度的见解。MoTe 2的一个主要缺点是其相对差的化学稳定性，这导致其在环境条件下降解。为了克服这种稳定性问题，将研究氧化物覆盖层对这些MoTe 2衍生材料的保护。比较了有无覆盖层的相变行为，特别是考虑到相变温度对Te缺陷的依赖性。最后，在MoTe 2和不同的TMD之间的平面界面制造的目的是利用MoTe 2的相变特性，使欧姆接触到其他TMD半导体。因此，本项目将研究MoTe 2是否可用作解决二维货车德瓦尔斯半导体电接触的一般问题的通用材料。

Abstract: Growth, modification, and encapsulation of MoTe2 derived semiconductor-to-metal phase change materials for electronic device applications.Nontechnical: Layered semiconductors, with strong in-plane bonding and only weak, non-covalent interactions between planes, can be reduced to a single molecular layer thickness while maintaining good electronic properties. Thus these materials enable the ultimate reduction in length scale for microelectronic devices. MoTe2 is of particular interest in this class of novel semiconductors because of a desirable band-gap and importantly, the presence of a structural phase change that enables transforming the material from a semiconductor to a metal. Local switching of the phase of MoTe2 thus enables to make metal/semiconductor devices entirely build out of a single elemental composition. In this project we are aiming at overcoming materials engineering challenges and thus enabling utilization of the special phase change properties of MoTe2 in device structures. First, we will establish synthesis of single or few molecular layers of MoTe2 on a wafer scale. Second, we investigate the controlled phase transformation from the semiconducting to the metallic phase and investigate approaches to modify this phase transformation by addition of other elements. Third, we address the chemical instability of MoTe2, which leads to easy oxidation. Strategies to encapsulate it in a protective layer will be developed, which is essential for making MoTe2 useable for devices. This project is embedded in the research activity on two-dimensional materials at the University of South Florida and will provide education to graduate and undergraduate students in an area of future technological relevance. Technical: The van-der Waals semiconductor MoTe2 exhibits a relatively small band gap of about ~ 1eV and is a promising material for ambipolar field effect devices. The strong light absorption over the entire visible spectrum makes MoTe2 also interesting for photovoltaics and its band gap value makes it a candidate for near infrared optoelectronics. In addition, a thermally induced phase change of MoTe2 from semiconducting to metallic provides possible solutions to materials engineering problems of making electrical contacts to van der Waals semiconductors. In this project we will investigate and optimize the growth of MoTe2 by molecular beam epitaxy. Modification of MoTe2 by alloying will be studied with the goal of enabling tuning of the band gap as well as phase change behavior. The growth of these films and their properties are primarily characterized with scanning probe microscopy/spectroscopy and photoemission and thus we gain insights of the growth and phase transformation mechanisms at the nanoscale. One main shortcoming of MoTe2 is its relatively poor chemical stability, which causes its degradation under ambient conditions. To overcome this stability-issue the protection of these MoTe2-derived materials by oxide-capping layers will be studied. Phase change behavior with and without capping layer is compared, especially in view of the dependence of the transition temperature on Te-deficiency. Finally, in-plane interfaces between MoTe2 and dissimilar TMDs are fabricated with the aim of utilizing the phase change properties of MoTe2 for making ohmic-contacts to other TMD semiconductors. Thus this project will investigate if MoTe2 could be used as a universal material for addressing the general problem of making electrical contacts to two-dimensional van der Waals semiconductors.