EAGER Collaborative Research: Fundamentals of Tunneling, Heterojunction-based 2D-Hot Electron Transistors

非技术性：基于氮化镓/氮化铝镓{(Al,Ga)N}系统的异质结双极晶体管长期以来一直受到用于电信和传感的紧凑型高频功率器件的关注。由于无法生产对于高电流增益和工作频率至关重要的薄且高导电的基层，这些器件很难实现。热电子晶体管（HET）可以解决这个问题。 HET 中的载流子传输是通过将热电子从发射极注入到由基极调制的集电极来实现的。需要超薄底座来实现超短传输时间以实现高性能。 原子薄的二维材料（例如过渡金属二硫属化物）是用作 HET 基极的理想材料。然而，这需要将二维基层合并到氮化物异质结构内，同时保留高质量的界面。先前的研究已经使用层转移方法将二维层合并到 HET 中，但这种方法引入了界面杂质，并且难以扩展到大面积。 该项目将重点关注通过金属有机化学气相沉积在 (Al,Ga)N 集电极上直接外延生长超薄 TMD 基层，并使用超宽带隙六方氮化硼作为发射极层。有关外延生长、材料表征和电流传输的基本问题将在逐步过程中进行研究，重点关注发射极-基极和基极-集电极界面。 这将提供对 HET 器件整体性能的深入了解以及进一步优化的建议。研究成果将纳入本科生和研究生课程。该项目还将为来自不同背景的本科生和高中生提供研究机会。技术：提出了一种二维（2D）层基热电子晶体管（HET），以克服基于III族氮化物的异质结双极晶体管（HBT）所面临的困难。由于其基极的掺杂值有限，这些材料在表现出良好的电气特性方面受到严重阻碍。晶体管基极中使用的 2D 层由于其高导电性而导致基极电阻低，而超薄基极应允许注入其中的电子载流子在没有渡越时间限制的情况下传播。将探索与 GaN 和 AlGaN 集电极几乎晶格匹配的 WS2 和 WSe2 等二维过渡金属二硫属化物 (TMD) 用于基极。 为了避免引入界面杂质以及二维层转移技术中常见的大面积缩放困难，该研究将重点关注金属有机化学气相沉积（MOCVD）的直接生长。这将用于在氮化铝镓 ((Al,Ga)N) 集电极上生长六方氮化硼 (hBN) 发射极和 TMD 单层和几层基膜。它还将允许高质量的二维/氮化物界面。这些研究将有助于了解表面能、晶格失配和缺陷对 (Al,Ga)N 上二维薄膜的成核和外延生长的作用，以及界面处的缺陷及其与材料性能和生长的相关性。我们将研究基本材料和器件模块，以更好地理解生长和传输机制的基本原理，并优化 HET。最重要的是了解 HET 器件各层的电流传输及其优化。从该项目中获得的知识将影响电信和传感领域，从而有助于提高生活质量。教育/推广活动将针对研究生、本科生和高中生以及 K-12 学生和公众。该奖项反映了 NSF 的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

Non-technical:Heterojunction bipolar transistors based on the gallium nitride/aluminum gallium nitride {(Al,Ga)N} system have long been of interest for compact high frequency power devices for telecommunications and sensing. These devices are difficult to realize due to the inability to produce a thin, highly conductive base layer that is essential for high current gain and frequency of operation. Hot electron transistors (HETs) could circumvent this problem. Carrier transport in HETs occurs via the injection of hot electrons from an emitter to a collector modulated by a base electrode. An ultra-thin base is needed to enable ultra-short transit time for high performance. Atomically thin 2D materials such as transition metal dichalcogenides are ideal materials to serve as the base electrode in a HET. This will, however, require incorporating the 2D base layer within the nitride heterostructure while retaining high quality interfaces. Previous studies have used layer transfer methods to incorporate 2D layers in HETs, but this approach introduces interfacial impurities and is difficult to scale to large areas. This project will focus instead on direct epitaxial growth of ultra-thin TMD base layers on an (Al,Ga)N collector via metal organic chemical vapor deposition along with the use of ultra-wide bandgap hexagonal boron nitiride as the emitter layer. Fundamental issues concerning the epitaxial growth, material characterization and current transport will be investigated in a step-wise process focusing on the emitter-base and base-collector interfaces. This will provide insight into the overall HET device performance and suggestions for further optimization. Research outcomes will be incorporated into undergraduate and graduate course curriculum. The project will also provide research opportunities for undergraduates and high school students from diverse backgrounds.Technical:A Two-Dimensional (2D) layer base Hot Electron Transistor (HET) is proposed for overcoming the difficulties faced by III-Nitride-based Heterojunction Bipolar Transistors (HBTs). These are severely handicapped in demonstrating good electrical characteristics due to the limited doping values of their base. The 2D layer used in the transistor base will lead to low base resistance due to its high electrical conductivity while an ultrathin base should allow the electron carriers injected into it to travel without transit time limitations. 2D transition metal dichalcogenides (TMDs) such as WS2 and WSe2, which are nearly lattice-matched to GaN and AlGaN collectors will be explored for the base. To avoid introduction of interfacial impurities and the difficulties involved in scaling to large areas commonly encountered in 2D layer transfer techniques, the research will focus on direct growth by Metalorganic Chemical Vapor Deposition (MOCVD). This will be employed for growth of Hexagonal Boron Nitride (hBN) emitters and TMD monolayer and few-layer base films on Aluminum Gallium Nitride ((Al,Ga)N) collectors. It will also allow high quality 2D/nitride interfaces. The studies will allow understanding of the role of surface energy, lattice mismatch and defects on the nucleation and epitaxial growth of 2D films on (Al,Ga)N, as well as defects at the interfaces and their correlation to material properties and growth. Basic material and device blocks will be investigated to gain good understanding of the fundamentals of growth and transport mechanisms and allow optimization of HETs. Of major importance is the understanding of current transport through the various layers leading to HET devices and their optimization. The knowledge obtained from the project will impact the fields of telecommunications and sensing thereby contributing to improving quality of life. Educational/outreach activities will be targeted at graduate, undergraduate and high school students as well as K-12 and the public.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.