EAGER: Understanding Carrier Multiplication in Black Phosphorus for High-Gain MWIR Avalanche Photodiodes

黑磷是一种新兴的“二维”半导体材料，具有许多非凡的光学和电子特性。特别是，黑磷在中红外波长范围内具有很强的光吸收能力，具有很高的载电流能力，并且可以以仅几纳米厚的层转移到各种衬底上。由于这些原因，黑磷可能成为自适应红外成像仪和光通信系统的革命性平台。然而，为了实现这一承诺，必须了解载流子成倍增加以产生增益的过程。该项目旨在了解一种这样的增益机制，雪崩倍增通过冲击电离，迄今为止，还没有研究过黑磷。这种基本的物理机制将通过制造纳米级器件结构来研究，这些器件结构将使用稳态和随时间变化的电测量以及光学技术进行测试。与最先进的解决方案相比，这些研究可以提供对撞击电离的基本理解，这对于实现具有更高灵敏度和可调性以及更低成本的新型红外成像系统至关重要。该项目的学习成果也将广泛适用于其他使用黑磷的设备，包括光源、逻辑和存储设备，甚至传感器。通过这种方式，这项工作可以帮助实现高性能、低成本柔性成像仪和电子产品的变革性技术平台。该计划还将纳入纳米电子学研究生和本科生的培训，并为小学和中学教育水平的学生提供动手活动的自然机会，以说明纳米科学概念。本研究计划的技术目标是评估和了解黑磷的冲击电离过程，并分析使用该材料制成的光电二极管的雪崩增益机制。这些目标将通过使用高精度电子束光刻技术在脱落的黑磷上制造金属-半导体-金属器件结构，然后进行高场输运测量以提取电子和空穴的电离系数来实现。黑磷中的雪崩增益也将通过用近带边缘光照射器件和使用空间映射光电流表征来表征。这项工作在智力上具有重要意义，因为它有望为黑磷中的高电场输运提供广泛的基本见解。特别是，它将使我们能够理解撞击电离，这是一个基本的过程，对于几乎任何由这种材料制成的实用设备的操作都是至关重要的，包括光电探测器，还有逻辑、存储器和传感晶体管。此外，这个项目将有助于确定由电子和空穴联合雪崩产生的多余噪声可以在黑磷中以类似于文献报道锗的方式被抑制的方法。与目前的解决方案相比，这种学习最终可能会带来突破性的、低成本的、多光谱通信和成像系统，并且速度和灵敏度都有所提高。

Black phosphorus is an emerging "two-dimensional" semiconductor material with many extraordinary optical and electronic properties. In particular, black phosphorus has strong optical absorption in the mid-infrared wavelength range, has high electrical current carrying capacity, and can be transferred onto a variety of substrates in layers only a few nanometers thick. For these reasons, black phosphorus could be a revolutionary platform for adaptable infrared imagers and optical communications systems. However, in order to achieve this promise, the process by which charge carriers multiply to create gain must be understood. This project seeks to understand one such gain mechanism, avalanche multiplication through impact ionization, which to date, as not been studied in black phosphorus. This basic physical mechanism will be studied by fabricating nanoscale device structures which will be tested using steady-state and time-dependent electrical measurements as well as optical techniques. These studies can provide fundamental understanding of impact ionization that will be important to realize new types of infrared imaging systems with higher sensitivity and tunability, as well as lower cost, compared to state-of-the-art solutions. The learning from this project will also be broadly applicable to a wide range of other devices using black phosphorus, including light emitters, logic and memory devices and even sensors. In this way, this work could help to realize a transformative technological platform for high-performance, low-cost flexible imagers and electronics. The program will also incorporate training for graduate and undergraduate students in nanoelectronics, and provides natural opportunities for hands-on activities for students at the primary and secondary educational level to illustrate nanoscience concepts.The technical goals of this research program are to evaluate and understand the process of impact ionization in black phosphorus and to analyze the avalanche gain mechanism in photodiodes made using this material. These goals will be met by using high-precision electron-beam lithography to fabricate metal-semiconductor-metal device structures on exfoliated black phosphorus and then performing high-field transport measurements to extract the ionization coefficients for both electrons and holes. Avalanche gain in black phosphorus will also be characterized by illuminating the devices with near-band-edge light and using spatially-mapped photocurrent characterization. This work is intellectually significant in that it is expected to provide extensive fundamental insight into high-electric-field transport in black phosphorus. In particular, it will allow for an understanding of impact ionization, a fundamental process that is critical for the operation of nearly any practical device made from this material, including photodetectors, but also logic, memory and sensing transistors. Furthermore, this project will help to determine the means by which excess noise created from the combined avalanching of electrons and holes can be suppressed in black phosphorus in an analogous way to literature reports on germanium. This learning could ultimately lead to ground-breaking, low-cost, multi-spectral communication and imaging systems with improved speed and sensitivity compared to current solutions.