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，具有很强的面内键，面之间只有弱的非共价相互作用，可以减少到单分子层厚度，同时保持良好的电子性能。因此，这些材料使微电子器件的长度尺度最终减小。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，它存在结构相变，可以将材料从半导体转变为金属。因此，MoTe2相的局部开关可以使金属/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方面的挑战，从而在器件结构中利用MoTe2的特殊相变特性。首先，我们将建立在晶圆尺度上的MoTe2的单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改变这种相变的方法。第三，我们解决了MoTe2的化学不稳定性，这导致容易氧化。将其封装在保护层中的策略将被开发出来，这对于使MoTe2可用于设备至关重要。该项目是南佛罗里达大学二维材料研究活动的一部分，将为研究生和本科生提供未来技术相关领域的教育。技术：范德华半导体MoTe2具有较小的带隙，约为~ 1eV，是一种很有前途的双极场效应器件材料。在整个可见光谱上的强光吸收使MoTe2对光伏发电也很感兴趣，其带隙值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范德华半导体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，我们将研究和优化分子束外延法生长MoTe2。通过合金化改性MoTe2将被研究，其目标是使带隙和相变行为的调谐成为可能。这些薄膜的生长及其性质主要通过扫描探针显微镜/光谱和光电发射来表征，从而我们获得了纳米尺度上的生长和相变机制的见解。MoTe2的一个主要缺点是其相对较差的化学稳定性，导致其在环境条件下降解。为了克服这种稳定性问题，将研究氧化盖层对这些mote2衍生材料的保护。比较了有和没有盖层的相变行为，特别是考虑到转变温度与te缺乏量的关系。最后，为了利用MoTe2的相变特性与其他TMD半导体进行欧姆接触，制作了MoTe2与不同TMD之间的平面内界面。因此，这个项目将研究MoTe2是否可以作为一种通用材料来解决与二维范德华半导体电接触的一般问题。

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.