CdxZn1-xTe1-ySey (CZTS) is an emerging wide bandgap semiconductor material for the applications of high-resolution room-temperature x-ray and gamma-ray detectors. Large volume Cd0.9Zn0.1Te1-ySey single crystal ingots were grown by vertical Bridgman technique with composition 𝑦 = 0.02, 0.03, 0.05, and 0.07. Several planar detectors were fabricated on single grain cut out from the grown ingots and characterized. Current-voltage (I-V) measurements revealed very low leakage current (≤ 1 nA) at an operating bias voltage of ≥ 100 V and a bulk resistivity of ~1010 Ω-cm. X-ray diffraction (XRD) results showed sharp diffraction peaks, which confirmed a highly crystalline structure of the grown crystals. Energy dispersive x-ray spectroscopy (EDX) showed uniform elemental distribution over a large area and confirmed the stoichiometry of the samples. While all the detectors showed response to alpha particles, the detector with composition 𝑦 = 0.03 showed very good 137Cs (662 keV) gamma response. The drift mobility and mobility-lifetime product in all the samples of those compositions were calculated for both electrons and holes. Pulse height spectroscopy using 137Cs on the fabricated detectors showed fully resolved 662 keV gamma peaks with an energy resolution of ~2%. A one-to-one correlation between the space charge limited current (SCLC) flow and radiation detection properties was found to exist in these crystals. An anomalous current flow mechanism, falling outside the comprehension of SCLC flow mechanism, has been observed in the rest of the samples. The anomalous behavior has been attributed to the presence of electron traps.
Cd0.9Zn 0.1Te (CZT) based pixelated radiation detectors have been fabricated and characterized for high-energy gamma ray detection. Large-volume ingots with large single crystalline grains have been grown and several 10 × 10 pixilated detectors have been fabricated from the grown crystals for imaging studies. Guard ring grids have been designed and fabricated to reduce cross-talk and inter-pixel/inter-electrode leakage. The grown crystals were initially characterized in planar configurations through electrical and optical methods prior to the fabrication of pixilated structure. Current-voltage (I-V) measurements revealed very low leakage current in the range of nA at an operating bias voltage of 1000 V and a resistivity of ~ 6×1010 Ω-cm was obtained. Infrared transmission imaging revealed average tellurium inclusion size of <8 micron. Measurements through Pockels’ effect has revealed a near-uniform depth-wise distribution of the internal electric field. Gamma-ray spectroscopy using 137Cs (662 keV) radiation source on the monolithic pixelated structure showed excellent detection performance for all the 100 pixels, with a resolution ~1.5 % FWHM.
Schottky barrier radiation detectors have been fabricated on n-type 4H-SiC epitaxial layers (Eg = 3.27 eV at 300 K) grown on low-resistive n-type 4H-SiC bulk substrates. Current-voltage (I-V) and capacitance-voltage (C-V) measurements were carried out, and using a thermionic emission model, Schottky barrier height of ~1.42 eV was obtained. Capacitance mode deep level transient spectroscopy (DLTS) revealed the presence of deep levels along with two shallow level defects related to titanium impurities (Ti(c)) and an unidentified deep electron trap located at 1.42 eV below the conduction band minimum. The concentration of the lifetime killer Z1/2 defect was found to be ~1013 cm-3. A 0.1 μCi 241Am radiation source was used to investigate the detector performance including charge collection efficiency (CCE) by pulse height spectroscopy (PHS), and an energy resolution of ~ 0.38% full-width half maxima (FWHM) was observed for alpha particles at ~ 5447 keV. The average diffusion length (Ld) of holes (minority carriers) were calculated to be ~ 13.6 μm using a drift-diffusion model and MATLAB code. An electronic noise analysis of front-end readout electronics in terms of equivalent noise charge revealed that the white series noise due to the detector capacitance has substantial effect on their spectroscopic detection performance.
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