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

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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.