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

摘要：用于电子器件的MoTe2衍生半导体-金属相变材料的生长、修饰和封装。非技术：层状半导体具有较强的面内成键和较弱的平面间非共价相互作用，可以在保持良好电子性能的同时减少到单分子层的厚度。因此，这些材料能够最终缩小微电子设备的长度范围。MoTe2在这类新型半导体中特别令人感兴趣，因为它具有理想的带隙，而且重要的是，存在结构相变，使材料能够从半导体转变为金属。因此，MoTe2相的局部切换使金属/半导体器件能够完全由单一元素组合物构成。在这个项目中，我们的目标是克服材料工程方面的挑战，从而使MoTe2的特殊相变特性能够在器件结构中得到利用。首先，我们将建立晶片规模的单层或几层MoTe2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改变这种相变的方法。第三，我们讨论了MoTe2的化学不稳定性，这导致了容易被氧化。将开发将其封装在保护层中的策略，这对于使MoTe2可用于设备至关重要。该项目被嵌入南佛罗里达大学的二维材料研究活动中，并将在一个与未来技术相关的领域为研究生和本科生提供教育。技术：范德华半导体MoTe2的禁带宽度相对较小，约为1 eV，是一种很有前途的双极场效应器件材料。MoTe2在整个可见光光谱范围内的强烈光吸收使其对光伏也很感兴趣，其带隙使其成为近红外光电子学的候选材料。此外，MoTe2从半导体到金属的热诱导相变为材料工程问题提供了可能的解决方案，即与范德华半导体进行电接触。在本项目中，我们将研究和优化分子束外延生长MoTe2的方法。将研究通过合金化对MoTe2进行改性，以实现带隙和相变行为的调谐。用扫描探针显微镜/光谱和光电子能谱对薄膜的生长和性质进行了初步表征，从而在纳米尺度上对薄膜的生长和相变机理有了更深入的了解。MoTe2的一个主要缺点是其化学稳定性较差，导致其在常温下降解。为了克服这一稳定性问题，将研究通过氧化物盖层来保护这些MoTe2衍生材料。比较了有盖层和无盖层时的相变行为，特别是考虑到相变温度对Te缺乏的依赖关系。最后，利用MoTe2的相变特性制作了MoTe2与不同TMD之间的面内界面，以实现与其他TMD半导体的欧姆接触。因此，该项目将研究MoTe2是否可以作为一种通用材料，用于解决与二维范德华半导体进行电接触的一般问题。

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.