Doping and Contact Engineering of CsPbBr3 for Gamma-Ray Spectroscopy

The development of low-cost detector materials that enable the fabrication of high energy resolution, high efficiency gamma-ray detectors supports the office of defense nuclear non-proliferation’s (DNN’s) mission to detect, identify, locate, and characterize: 1) foreign nuclear material production and weapons development activities; 2) movement and illicit diversion of special nuclear materials; and 3) global nuclear detonations. The current state-of-the-art room-temperature semiconductor gamma-ray detector material is cadmium zinc telluride (CZT). Despite decades of research, however, this material still suffers from structural defects that reduce yield of spectrometer grade material and keep detector costs high. Recently, the perovskite semiconductor CsPbBr3 (CPB) has demonstrated promise as a lower-cost replacement for CZT. This material has higher stopping power than CZT and the mobility-lifetime products for holes and electrons are comparable, making high-performance planar devices possible. Pixelated CPB devices and capacitive Frisch grid (CFG) devices exhibit an energy resolution less than 2% (FWHM at 662keV) without corrections, however, the devices may exhibit large leakage currents and may require “conditioning” to stabilize the performance.To address this issue, RMD aims to improve CsPbBr3 device performance and stability by reducing dark current by reducing the concentration of the dominant mobile defects and reducing their mobility by “pinning” vacancies. To address this challenge, we plan to apply defect and transport models to predict the conductivity in CsPbBr3 as a function of temperature and dopant level. This defect engineering approach was successfully applied to TlBr which is now being grown with an order of magnitude higher resistivity than previously. Another objective involves engineering detector surfaces and electrodes. The goal is to minimize surface leakage current, reduce reverse bias dark current, reduce any device “conditioning” time and stabilize electrodes against parasitic electrochemical reactions induced by ionic conduction.The Phase I effort develops a suitable doping process and a protocol to optimize electrode deposition conditions so that CPB devices exhibit excellent performance in terms of stability. The goal of the doping process is to reduce the number of reactive vacancies, while research related to forming a buffer layer between the semiconductor and electrode will be developed to prevent electrochemical reactions at the electrodes while permitting charge transport for readout. The Phase I effort demonstrates the feasibility of developing a material and device fabrication process.Nuclear security applications, such as spectroscopic dosimeters and Radio-Isotope Identification Instruments (RIIDs), will benefit from the availability of these CsPbBr3 semiconductor detectors. There are also applications in nuclear and space physics and nuclear medicine (e.g., PET and SPECT imaging) where high performance high-Z spectrometers will have beneficial applications. High-performance room temperature semiconductor gamma spectrometers are also useful for nuclear material accounting applications. This work will also lead to advancements of new nuclear detection technologies to address the nation’s future energy needs and maintain our nation’s technological edge over competing nations.