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1. Ammonium Fluoride Passivation of CdZnTeSe Sensors for Applications in Nuclear Detection and Medical Imaging

   https://doi.org/10.3390/s19153271  

   Egarievwe, Stephen U.; Roy, Utpal N.; Goree, Carmella A.; Harrison, Benicia A.; Jones, Jeanette; James, Ralph B. (August 2019, Sensors)

   Cadmium zinc telluride selenide (Cd1−xZnxTe1−ySey or CZTS) is one of the emerging CdTe-based semiconductor materials for detecting X- and gamma-ray radiation at or near room temperature (i.e., without cryogenic cooling). Potential applications of CZTS sensors include medical imaging, X-ray detection, and gamma-ray spectroscopy. Chemical passivation of CZTS is needed to reduce the conductivity of Te-rich surfaces, which reduces the noise and improves the device performance. In this study, we focus on the effect of surface passivation of CZTS using a 10% aqueous solution of ammonium fluoride. The effects of the chemical treatment were studied on the leakage current, charge transport measured as the electron mobility-lifetime (µτ) product, and the spectral resolution measured as the full-width at half-maximum (FWHM) of specific peaks. After passivation, the leakage current increased and began to decrease towards pre-passivation levels. The energy resolutions were recorded for eight applied voltages between −35 V and −200 V. The results showed an average of 25% improvement in the detector’s energy resolution for the 59.6 keV gamma peak of Am-241. The electron µτ product was unchanged at 2 × 10−3 cm2/V. These results show that ammonium fluoride is effective for chemical passivation of CZTS detectors. 

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2. Fabrication and Characterization of CdZnTeSe Nuclear Detectors

   https://doi.org/10.1109/NSS/MIC42101.2019.9059726  

   Egarievwe, Stephen U.; Roy, Utpal N.; Harrison, Benicia A.; Goree, Carmella A.; Savage, Emmanuel K.; Jones, Jeanette; James, Ralph B. (October 2019, 2019 IEEE Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC))

   Cadmium zinc telluride selenide (CdZnTeSe) is emerging as a promising semiconductor material for low-cost production of nuclear and radiological detection systems capable of operating at room temperature. This paper presents studies of the fabrication process and detector response for gamma-ray detectors produced from high-quality CdZnTeSe crystals grown by the traveler heater method (THM). The resistivity of the CdZnTeSe is on the order of 10 GΩ-cm. The electron mobility-lifetime (μτ) product characterizing the charge-transport properties is on the order of 0.001 square-cm per volt. Energy resolutions as good as 6.5% FWHM for the 59.5-keV gamma-peak of 241 Am were recorded for planar detectors with gold contacts. 

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3. Characterization of CdMnTe Planar Nuclear Detectors Grown by Vertical Bridgman Technique

   https://doi.org/10.1109/NSS/MIC42101.2019.9059615  

   Egarievwe, Stephen U.; Roy, Utpal N.; Agbalagba, Ezekiel O.; Dunning, Keiandrea L.; Okobiah, Oghaghare K.; Israel, Mordecai B.; Drabo, Mebougna L.; James, Ralph B. (October 2019, 2019 IEEE Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC))

   Cadmium manganese telluride (CdMnTe) crystals are expected to be homogeneous in structure due to the segregation coefficient of Mn in CdTe, which is about 1.0. This could translate in the growth of large-volume CdMnTe crystals free of defects that currently limit X-ray and gamma-ray detection efficiencies. The present characterization experiments show results on CdMnTe planar detectors grown by the vertical Bridgman technique. The CdMnTe crystal used in the experiments was mostly free of tellurium inclusions and high angle grain boundaries. We recorded an energy resolution of 9.2% FWHM for the 59.5-keV gamma-peak of Am-241 for the planar detector. We also resolved peaks at energies below the 59.5-keV peak. 

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4. Observation of time-dependent internal charge amplification in a planar germanium detector at cryogenic temperature

   https://doi.org/10.1140/epjc/s10052-023-11432-y  

   Acharya, P.; Fritts, M.; Mei, D. -M.; Mandic, V.; Wang, C. -J.; Mahapatra, R.; Platt, M. (April 2023, The European Physical Journal C)

   Abstract For the first time, time-dependent internal charge amplification through impact ionization has been observed in a planar germanium (Ge) detector operated at cryogenic temperature. In a time period of 30 and 45 min after applying a bias voltage, the charge energy corresponding to a baseline of the 59.54 keV$$\gamma $$ $\gamma$rays from a$$^{241}$$ ${}^{241}$Am source is amplified for a short period of time and then decreases back to the baseline. The amplification of charge energy depends strongly on the applied positive bias voltage with drifting holes across the detector. No such phenomenon is visible with drifting electrons across the detector. We find that the observed charge amplification is dictated by the impact ionization of charged states, which has a strong correlation with impurity level and applied electric field. We analyze the dominant physics mechanisms that are responsible for the creation and the impact ionization of charged states. Our analysis suggests that the appropriate level of impurity in a Ge detector can enhance charge yield through the impact ionization of charged states to achieve extremely low-energy detection threshold (< 10 meV) for MeV-scale dark matter searches if the charge amplification can be stabilized. 

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5. Comparative Studies of CdZnTe, CdMnTe, and CdZnTeSe Materials for Room-Temperature Nuclear Detection Applications

   https://doi.org/10.1109/NSS/MIC44867.2021.9875638  

   Egarievwe, Stephen U.; Soto, Stephan D.; Sykes, Simeon W.; Fuller, Leslie J.; Alsbrooks, Quentin J.; Alim, Mohammad A.; Roy, Utpal N.; Agbalagba, Ezekiel O.; Drabro, Mebougna L.; James, Ralph B. (October 2021, 2021 IEEE Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC))

   Cadmium telluride (CdTe) and its ternary and quaternary compounds have found applications in the development of X-ray and gamma-ray detectors used in nuclear detection and medical imaging applications. Example of these detectors include CdZnTe (CZT), CdMnTe (CMT), and CdZnTeSe (CZTS). These nuclear detectors can operate at room temperature without cryogenic cooling. This paper presents comparative studies of these semiconductor material. The properties studied include detector resistivity, Te inclusions, grain boundary networks, mobility/lifetime of the charge carriers, and energy resolution. The effects of passivation with chemicals such as KOH and NH4F, are also presented. X-ray photoelectron spectroscopy (XPS) studies showed increase in the quantity of TeO2 on surfaces of these materials after passivation in KOH and NH4F. While CZT detector has wide commercial availability, it has more Te inclusions and grain boundary network compared to CZTS. CMT and CZTS have better crystal uniformity than CZT. The comparatively low presence of Te inclusions and grain boundary network in CZTS gives it a higher crystal growth yield for detector-grade material. 

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