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

非技术描述：原子薄半导体，被称为二维（2D）半导体，最近成为下一代低功耗和高性能电子产品的潜在候选材料。2D半导体的原子平整度、表面平整度和厚度均匀性使得包括晶体管在内的电子设备可以比目前的硅技术做得更小、更密集。此外，2D材料优异的机械强度和拉伸性也为下一代柔性电子产品开辟了新的机会。晶体管的一个关键部件是金属-半导体电触点，因为载流子是通过这些触点注入和提取的。尽管二维半导体具有优异的电子和机械性能，但目前基于二维半导体的原型电子器件的性能仍然受到非理想金属-半导体接触的严重限制。隔离和理解限制金属电极和2D半导体之间电接触的因素仍然是一个主要的材料挑战，并阻碍了2D半导体的实际电子应用。为了解决这一知识差距，本研究使用新方法来可控地修改和表征金属二维半导体界面的特性。该项目通过一系列活动，如高中生暑期研究、指导奥林匹克科学竞赛团队和实验室参观，将研究、教育和社区推广工作紧密结合在一起。这个项目的教育部分的重点是通过积极招收来自代表性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，合理的高迁移率，优异的机械强度和高柔韧性，是超越硅基场效应晶体管缩放限制的下一代柔性电子产品的有前途的通道材料。然而，二维半导体（如过渡金属二硫族化合物）电子应用的一个主要瓶颈是它们倾向于与大多数金属在其接触处形成一个实质性的肖特基势垒，这主要是由于费米水平钉住效应。这是一个很大的缺点，因为实际设备需要低电阻欧姆触点。该项目的目标是建立对金属/ 2d半导体界面上费米能级钉钉机制的基本理解，并随后开发全面的接触工程策略来消除肖特基障。本项目以可忽略费米水平钉钉的二维范德华接触为基线，可控地引入各种可能的钉钉因素，定量了解它们在费米水平钉钉中的作用。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，将与电输运测量一起进行，以将材料性质与肖特基势垒高度联系起来。从本研究中获得的知识有望帮助设计有效的接触工程策略，以实现二维半导体的超低电阻欧姆接触，并为二维电子和光电子材料的基础研究开辟新的途径。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.