Germanium Photodiode Arrays for Hard X-Ray Imaging

Project Overview

The detection and imaging of high-energy (22–75 kiloelectron-Volt (keV), e.g., "hard") x rays is critical to successfully diagnosing and improving the understanding of inertial confinement fusion, a core component of NNSA's stockpile stewardship mission and which may be used for clean energy production in the future. The two major weaknesses in current techniques are (1) suitable stopping power in detectors to quantitatively detect hard x rays satisfactorily and (2) sufficient speed in the detectors to accurately measure the temporal dynamics of x-rays produced during a fusion event (which transpire on a nanosecond scale.) A third corollary is to use suitable detectors, if they can be made, in a two-dimensional imaging device that allows physicists to analyze the dynamic evolution of the plasma of an inertial confinement fusion (ICF) or other high-energy-density (HED) experiments. These activities are necessary to carrying out the National Ignition Facility (NIF) mission.

Our approach was to design, model, fabricate, and measure detectors and imaging arrays that used high-quality germanium (Ge) as the active material in the electronic devices. This was a pioneering effort, since germanium had never been explored for this purpose, although the material has been thought to be suitable for use in imagers. Our efforts produced device designs and fabricated test structures that were measured and provided encouraging proof-of-concept. The devices demonstrated good stopping power at the energies measured, and speeds that indicated promise as a hard x-ray imaging resource. The first-year results informed the pathway for second and third year activities in design, fabrication, and modeling as well as epitaxial deposition methods resulting in the highest-quality and purest germanium suitable for large-area imager fabrication. All data indicate the performance metrics in the detectors will satisfactorily integrate with available high-speed imaging electronics such that superior hard x-ray imagers will be viable using this technology.

Mission Impact

NIF diagnostic platforms supporting HED and ICF can benefit from Ge-based image sensors. Rayleigh–Taylor growth and Compton radiography experiments could utilize a multiframe nanosecond-gated, higher device quantum efficiency (DQE), hard x-ray imager with nanosecond temporal resolution and enhanced background and neutron radiation tolerance to improve signal-to-noise and background rejection, and to explore the evolution of a process in any given single-shot experiment. Specifically, for Rayleigh–Taylor growth experiments, a detector with a DQE of 7–10% for 50 KeV x-rays could serve as a replacement for the High-Energy Imaging Diagnostic (HEID) that supports Stockpile Stewardship Program (SSP) experiments. For Compton radiography experiments, a detector with 5% DQE could replace the AXIS (Advanced Radiographic Capability X-ray Imaging System) diagnostic that supports the ICF program. Furthermore, due to germanium's small bandgap, it has good near-infrared quantum efficiency that can be used to diagnose high-power laser wavefront performance as well as nanosecond-gated laser three-dimensional imaging for national security applications. Ge pixilated diodes could also be used for portable radiation detection, measurement, and imaging. Finally, several laboratories have expressed interest in this work, notably the Army Research Office (N. K. Dhar, Science & Technology Division, Night Vision and Sensors), and collaborators at SLAC/Linac Coherent Light Source, Los Alamos, and Sandia National Laboratories' Ultrafast X-ray Imager (UXI) group.

Publications, Presentations, and Patents

Hunt, C., Carpenter, A.C., Voss, L., Scott, R., Shao, Q., Looker, Q., Garafalo, A., Mistyuk, S., Durand, C., Kumar, A., Stroud, J.P., Van Benthem, K. 2020. "p-i-n High-Speed Photodiodes for X-Ray and Infrared Imagers Fabricated by In-Situ-Doped APCVD Germanium Homoepitaxy," IEEE Transactions on Electron Devices, Vol. 67, No. 8, August 2020 p. 3235.