• Crum, Jarrod V.
• Sundaram, S. K.
• Riley, Brian J.
• Seifert, Carolyn E.
• Van Ginhoven, Renee M.
• Henager, Charles H.
• Rockett, Angus
• Aquino, Angel
• Johnson, Bradley R.

Abstract

This is the annual project report for the ASGRAD project - Amorphous Semiconductors for Gamma Radiation Detection. We describe progress in the development of new materials for portable, room temperature, gammaradiation detection at Pacific Northwest National Laboratory. High Z, high resistivity, amorphous semiconductors are being designed for use as solid-state detectors at near ambient temperatures; their principles of operation are analogous to single-crystal semiconducting detectors. Compared to single crystals, amorphous semiconductors have the advantages of rapid, cost-effective, bulk-fabrication; nearnet-shape fabrication of complicated geometries; compositional flexibility; and greater electronic property control. The main disadvantage is reduced-charge carrier mobility. The focus of this project is to develop optimized amorphous semiconductor materials for gamma detection applications that leverage their material advantages while mitigating their limitations. During the second year of this project, several important milestones were accomplished. Major accomplishments were: (1) Significant processing - property and composition - property correlations were determined for Cd-Ge-As glasses; (2) Radiation response testing was successfully demonstrated on three different amorphous semiconductor materials (Cd-Ge-As, As-Se, and As-Se-Te systems) at ambient and near ambient temperatures; (3) Advanced, enabling Schottky contacts were developed for Cd-Ge-As compounds, this will allow these materials to perform at ambient temperatures; and (4) The collaborative working relationship developed with Prof. Angus Rockett at the University of Illinois at Urbana-Champaign (UIUC) has continued to grow, and they are credited with several of the materials characterization and contact development successes achieved this year. The development of Schottky barrier contacts in amorphous semiconductors by simply using different types of metal contacts is very significant. This is because these structures allow us to fabricate functional diodes using the Schottky barrier as the 'blocking' contact, similar to the familiar silicon surface barrier detector. In device terms, lower resistivity materials with better charge carrier mobility properties can be used and operated under reverse bias to create a low-noise, high-resistivity condition that can be switched to a low-resistivity, high conductivity condition under exposure to radiation events, and then off again. In the future, lithographic techniques could be used to create an array of FET's, and thereby build pixilated detectors that would have 2D position-sensitive radiation detection capability. Thus, the third year of the project will be dedicated to building diode-based devices from these amorphous semiconductors. The major challenges to date with the Schottky contacts have been a large variation in device behavior from contact to contact, and the relative fragility of the contacts. We are developing strategies to solve these issues. Additionally, particular emphasis will be directed towards characterizing device performance in response to ionizing radiation, which is key to demonstrating the viability of amorphous semiconductor-based detectors.

Topics

• impedance spectroscopy
• surface
• compound
• single crystal
• amorphous
• resistivity
• mobility
• glass
• semiconductor
• glass
• Silicon
• field-effect transistor method