Mohsin Sajjad

Date of Award

Summer 2020

Document Type

Open Access Dissertation

Department

Electrical Engineering

First Advisor

Krishna C. Mandal

Abstract

In recent years, there have been considerable interests in nuclear radiation detectors for national security and nuclear safeguard including monitoring special nuclear materials and nuclear waste characterization, for medical imaging devices using nuclear medicine such as positron emission tomography (PET), and for NASA space astronomy. One of the most promising semiconductors being considered for the field-deployable nuclear detector is cadmium zinc telluride (CdZnTe or CZT). CZT is a ternary semiconductor that can efficiently stop and detect high-energy gamma-rays at room temperature. CZT has desirable properties for efficient nuclear radiation detection and imaging due to high atomic number (Z ~ 50, high stopping power), high density (5.8 g/cm3), high resistivity (≥ 1010 Ω- cm), wide bandgap (~1.6 eV at 300K), low leakage current, high gamma-ray absorption coefficient, and good electron transport properties. However, there are considerable difficulties in growing large volume, homogeneous, and defect-free CZT single crystals. These difficulties enhance the cost of CZT detectors for industrial mass-production. Furthermore, CZT suffers from poor hole transport properties, which creates significant problems as a high-energy gamma-ray detector.

In the present work, a comprehensive investigation is undertaken to develop a successful growth method for CZT at the University of South Carolina (UofSC). The method is called vertical Bridgman solvent-growth technique which reduces the complexity of growing detector-grade CZT single crystals at high temperature. The new

method we developed is a low-temperature growth using tellurium (Te) as a solvent while maintaining crystal homogeneity and took the advantages of other modern CZT growth techniques. Large diameter (up to 3.0-inch diameter and 4.8 inches in length) detector grade CZT boules (≥ 75% singularity) have been grown using in-house zone-refined precursor materials loaded into carbon-coated quartz ampoules. Ampoules were sealed under ultra-high vacuum and crystal growth was performed using temperature profiled furnaces built in-house at UofSC. The grown ingots were cut into wafers and chemo- mechanically polished for structural, optical, electrical, and spectroscopic characterization. The bulk resistivity of the as-grown crystal was found to be ~ 5×1010 W-cm from current- voltage (I-V) measurements. Infrared transmission imaging revealed an average tellurium inclusion size of < 8 microns. Nuclear radiation detectors were fabricated and characterized using analog and digital radiation detection systems integrated with front- end readout electronics developed at UofSC. Block of CZT crystals of dimensions ~20 × 20 × 5 mm3 were used to fabricate detectors with 10 × 10 pixelated anode configuration integrated with inter-pixel guard rings and were tested for their radiation detection properties using 662 keV gamma rays from 137Cs gamma sources. The pixels exhibited well-resolved gamma pulse-height spectra with energy resolution ~ 1.5% at 662 keV. A few pixels exhibited tailing of the photopeak on the lower energy side indicating the presence of hole traps. Biparametric plots (BP) were generated from digitally recorded pre-amplifier pulses. The BPs showed anomalous behavior, which was correlated to the gamma interactions in the active region of the detector. The biparametric plots also enabled to extract the pulse-height spectra free from the extensive tailing of the photopeak. A Matlab software code was developed to apply correction schemes to the biparametric plots and to improve the results acquired from the detectors.

CdxZn1−xTeySe1−y (CZTS) has emerged in the last couple of years as a next- generation high-yielding material for the fabrication of room temperature high energy gamma detection. We report for the first time, the hole transport property measurements in CZTS based gamma-ray detectors in planar configuration. I-V measurements revealed detector grade bulk resistivity and the fabricated detectors produced well-resolved 5486 keV alpha peaks, for both electrons and holes drifting alike, when PHS was recorded using a 241Am radiation source. The PHS measurements enabled to measure the charge transport properties for both the charge-carriers. The mobility-lifetime product (µτ) for electrons and holes were calculated using a single-polarity Hecht plot regression method. The pre- amplifier pulses were recorded and processed digitally to obtain electron and hole drift mobilities using a time-of-flight method. The measured transport properties indicated the hole lifetime to be greater than the electron lifetime by a factor of ∼1.5. Gamma-ray PHS were recorded on the fabricated detectors which showed tailing of the 137Cs (662 keV) photopeak due to hole-trapping effects. Depth dependent PHS were digitally generated from 2D biparametric plots to reveal the effects of hole trapping on the gamma PHS at different detector depths and multiple defect energy levels. A digital correction procedure was applied to generate well-resolved PHS with an energy resolution of ≤2% for 662 keV γ-rays and to investigate the kinetics of charge trapping. Finally, a novel charge-trapping model has been developed for the first time which explained the entire duration of the pulse-shapes from CZTS based radiation detectors and could be applied to other wide band- gap semiconductors.

Rights

© 2020, Mohsin Sajjad

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