Pin down the mechanism of Fermi-level pinning in metal/2D-semiconductor contacts

非技术描述：原子薄型半导体，称为二维(2D)半导体，最近已成为下一代低功耗和高性能电子产品的潜在候选材料。2D半导体的原子平坦度、表面光滑度和厚度均匀度使包括晶体管在内的电子设备比目前的硅技术更小、更密集。此外，2D材料优异的机械强度和延伸性也为下一代柔性电子产品开辟了新的机遇。晶体管的一个关键部件是金属-半导体电接触，因为载流子是通过这些接触注入和提取的。尽管2D半导体具有优异的电学和机械性能，但目前基于2D半导体的电子器件原型的性能仍然受到非理想金属-半导体接触的严重限制。隔离和了解哪些因素限制了金属电极和2D半导体之间的电接触仍然是一项重大的材料挑战，并阻碍了2D半导体的实际电子应用。为了解决这一知识鸿沟，本研究使用新的方法来可控地修改和表征2D半导体与金属的界面性质。该项目通过一系列活动，如高中生暑期研究、指导科学奥林匹克团队和实验室之旅，将研究、教育和社区推广工作紧密结合在一起。该项目教育部分的重点是通过积极招收代表不足的少数群体的研究生和本科生来促进多样性。技术描述：二维(2D)半导体具有合适的带隙、相当高的迁移率、优异的机械强度和高灵活性，有望成为超越硅基场效应晶体管规模限制的下一代柔性电子产品的沟道材料。然而，过渡金属二卤化物等2D半导体在电子应用中的一个主要瓶颈是，由于费米能级钉扎效应，它们容易与大多数金属在接触处形成实质性的肖特基势垒。这是一个很大的缺点，因为现实的器件需要低电阻的欧姆接触。该项目的目标是建立对金属/2D-半导体界面费米能级钉扎机制的基本了解，并随后开发全面的接触工程策略来消除肖特基势垒。这个项目使用可以忽略费米能级钉扎的2D van der Waals接触作为基线，并可控制地引入各种可能的钉扎因子，以定量地了解它们在费米能级钉扎中的作用。各种材料表征技术，如光电子能谱、原子力显微镜和扫描隧道显微镜/光谱学，将与电输运测量相结合，以将材料特性与肖特基势垒高度相关联。从这项研究中获得的知识有望帮助设计有效的接触工程策略，以实现对2D半导体的超低电阻欧姆接触，并为2D电子和光电子材料的基础研究开辟新的途径。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Nontechnical Description: Atomically thin semiconductors, referred to as two-dimensional (2D) semiconductors, have recently emerged as potential candidate materials for next-generation low-power and high-performance electronics. 2D semiconductors’ atomic flatness, surface-smoothness and thickness-uniformity allow electronic devices, including transistors, to be made smaller and more densely-packed than is currently possible with silicon technology. Furthermore, the excellent mechanical strength and stretchability of 2D materials also open up new opportunities for next-generation flexible electronics. A critical component of transistors is the metal-semiconductor electrical contacts because charge carriers are injected and extracted through these contacts. In spite of the superb electronic and mechanical properties of 2D semiconductors, the performance of current prototype 2D-semiconductor-based electronic devices is still severely limited by the non-ideal metal-semiconductor contacts. Isolating and understanding which factors limit electrical contacts between metal-electrodes and 2D semiconductors remains a major materials challenge and prevents practical electronic application of 2D semiconductors. To address this knowledge gap, this research uses new methods to controllably modify and characterize the properties of 2D semiconductor interfaces with metals. This project tightly integrates research, education, and community outreach efforts through a series of activities such as summer research for high school students, coaching Science Olympiad teams and lab tours. The emphasis of the education component of this project is placed on promoting diversity through actively recruiting graduate and undergraduate students from underrepresented minority groups. Technical Description:Two-dimensional (2D) semiconductors with a suitable bandgap, a reasonably high mobility, excellent mechanical strength and high flexibility are promising channel materials for next generation flexible electronics beyond the scaling limit of silicon-based field-effect transistors. However, a major bottleneck in electronic applications of 2D semiconductors such as transition metal dichalcogenides is their tendency to form a substantial Schottky barrier with most metals at their contacts largely due to Fermi-level pinning effects. This is a strong disadvantage because low-resistance ohmic contacts are needed for realistic devices. The goal of this project is to establish a fundamental understanding of the Fermi-level pinning mechanism at metal/2D-semiconductor interfaces and subsequently develop comprehensive contact-engineering strategies to eliminate the Schottky barrier. This project uses 2D van der Waals contacts with negligible Fermi-level pinning as the baseline and controllably introduce various possible pinning factors to quantitatively understand their roles in Fermi-level pinning. A variety of materials characterization techniques such as photoelectron spectroscopy, atomic force microscopy and scanning tunneling microscopy/spectroscopy will be performed in conjunction with electrical transport measurements to correlate the materials properties with the Schottky barrier height. The knowledge gained from this study is expected to help design effective contact-engineering strategies to achieve ultralow-resistance ohmic contacts to 2D semiconductors, and open up new avenues for basic research on 2D electronic and optoelectronic materials.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.