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

非技术：基于氮化镓/铝氮化镓{(Al,Ga)N}系统的异质结双极晶体管长期以来一直是电信和传感的紧凑型高频功率器件的兴趣。这些器件很难实现，因为无法产生薄的、高导电性的基础层，这是高电流增益和工作频率所必需的。热电子晶体管（HETs）可以避免这个问题。het中的载流子输运是通过将热电子从发射极注入基极调制的集电极而发生的。为了实现高性能，需要超薄的底座来实现超短的传输时间。原子薄的二维材料，如过渡金属二硫族化物，是作为HET基电极的理想材料。然而，这将需要在保持高质量界面的同时，在氮化物异质结构中加入2D基层。以前的研究已经使用层传递方法将二维层纳入高温热交换体中，但这种方法引入了界面杂质，并且难以大面积扩展。该项目将侧重于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基片，并使用超宽带隙六方氮化硼作为发射层。关于外延生长，材料特性和电流传输的基本问题将在一个逐步的过程中进行研究，重点是发射器-基极和基极-集电极界面。这将提供对整体HET设备性能的洞察和进一步优化的建议。研究成果将纳入本科和研究生课程。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。技术方面：提出了一种二维（2D）层基热电子晶体管（HET），以克服iii -氮化物基异质结双极晶体管（hbt）所面临的困难。由于其基材的掺杂值有限，这些材料在表现出良好的电特性方面受到严重阻碍。在晶体管基板中使用的二维层由于其高导电性将导致低基板电阻，而超薄基板应该允许注入其中的电子载流子不受传输时间限制地移动。二维过渡金属二硫族化合物（TMDs），如WS2和WSe2，它们与GaN和AlGaN收集器几乎晶格匹配。为了避免引入界面杂质和在二维层转移技术中常见的大面积缩放困难，研究将重点放在金属有机化学气相沉积（MOCVD）的直接生长上。这将用于生长六方氮化硼（hBN）发射体和TMD单层和氮化铝镓（(Al,Ga)N）集热器上的少层基膜。它还将允许高质量的2D/氮化物接口。这些研究将有助于理解表面能、晶格错配和缺陷在（Al,Ga）N上二维薄膜的成核和外延生长中的作用，以及界面上的缺陷及其与材料性能和生长的关系。基本的材料和器件块将被研究，以获得对生长和传输机制的基本理解，并允许对高温超导进行优化。最重要的是理解电流通过导致HET器件及其优化的各个层的传输。从该项目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和公众。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。

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.