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Detector grade large volume Cd 0.9 Zn 0.1 Te (CZT) single crystals were grown using a modified vertical Bridgman technique. Single crystals of dimensions 1.1 × 1.1 × 1.0 cm3were used to fabricate detectors in coplanar grid (CG) geometry. The CG-CZT detectors were characterized using various electrical, optical and spectroscopic techniques. Current-voltage (I- V) measurements were carried out in planar configuration at room temperature. The bulk resistivity was found to be 3.3 × 1010 Ω-cm. Intergrid $I$ - $V$ measurements were also carried out and the intergrid leakage current was found to be negligible under the operating conditions. IR transmission images showed the presence of tellurium (Te) inclusions with an average diameter of 8 µm, A digital data acquisition program was developed to collect the preamplifier charge pulses connected to the two grids and plot the difference spectrum to obtain single-polarity charge sensing pulse-height spectra in real time. The detector was exposed to a 137Cs radiation test source emitting 662 keV gamma rays and well-resolved digital gamma pulse height spectra were generated.

High resolution Schottky barrier detectors (SBDs) were fabricated on 150 µm thick 4H-SiC epilayers using a proprietary device design. Electrical properties of the SBD junctions were characterized by temperature-dependent current-voltage (I-V- T) measurements which showed ultra-low leakage current densities lower than 100 pA cm−2at -150 V and remained below 1 µ A.cm−2 even at 600K. Electrically active deep levels present in the epilayers were identified and characterized by deep level transient spectroscopy (DLTS) which showed the presence of three deep levels - Ti(c), Z 1/2 and EH 6/7 - with low concentrations (~1011 cm−3). The energy levels were investigated theoretically by density functional theory (DFT) calculations on intrinsic vacancies and titanium point defects. Pulse height spectra (PHS) were collected using a 241Am alpha source and a percentage energy resolution of 0.55% at 5486 keV was obtained. Further analysis of the forward bias I-V- T showed an improvement in ideality factor and barrier height at elevated temperature revealing the improvement of detection performance at higher temperature.

Availability of high-quality bulk and epitaxial 4H-SiC has revived the feasibility of fabrication of nuclear radiation detectors which are compact, operable at room or elevated temperature, physically rugged, and radiation hard. SiC detectors have been demonstrated as excellent alpha particle detectors and hence are being considered as compact neutron detectors when coupled with neutron-alpha conversion layers. Our group at University of South Carolina has successfully demonstrated the fabrication of large area (∼11 mm2) 4H-SiC epitaxial surface barrier type sensors, for detection of x-rays of energy as low as 50 eV with very high responsivity, high resolution detection of low energy gamma rays (2.1% FWHM for 59.6 keV gamma rays) and very efficient and high-resolution alpha particle detection. We have recently fabricated surface barrier detectors for alpha particles with an energy resolution of 0.29% for 5.48 MeV alpha particles, the best value reported till date, with a charge collection efficiency (CCE) ∼100%. These detectors have also shown a CCE more than 57% at zero applied bias for 5.48 MeV alpha particles which opens up the possibility of highly efficient bias-less operation, a much sought-after quality for field deployment of stand-off detection system for Homeland security applications. We have performed a noise analysis in terms of equivalent noise charge and we have found that the white series noise due to the detector capacitance has substantial effect on their spectroscopic performance. We have also studied the effect of the presence of electrically active defects on detector performance using deep level transient spectroscopy measurements.

Cd 0.9 Zn 0.1 Te (CZT) gamma radiation detectors are presently among the best semiconductor materials for room temperature operation. CZT crystals in pixilated geometry can be efficiently used in the field of Homeland security and medical diagnosis for high resolution imaging purpose and position sensitive measurements using x-rays and high energy gamma rays. Large area CZT single crystals has been grown using a tellurium solvent method. A 10×10 guarded pixilated detector has been fabricated on a ∼19.5×19.5×5 mm3 crystal cut out from the grown ingot. A full area planar Au contact was used as the cathode. Each pixel of the pixilated detector was 1.3×1.3 mm2 in area and pitched at 1.8 mm. A 100 μm wide guard grid was used to reduce inter-pixel leakage. The crystal was characterized in planar configuration using electrical and optoelectronic methods prior to the fabrication of pixilated geometry. Current-voltage (I–V) measurements revealed a leakage current of 29 nA at an operating bias voltage of 1100 V. A resistivity of ∼3.1×1010 ∖.cm was calculated from the I–V measurements. Infrared transmission imaging revealed an average tellurium inclusion/precipitate size less than 8μm. Pockels measurement has revealed a near-uniform depth-wise distribution of the internal electric field. The mobility-lifetime product in this crystal was calculated to be 6.2 × 10−3 cm2/V using alpha ray spectroscopic method. Gamma spectroscopy using a 137Cs source on the pixelated structure showed fully resolved 662 keV gamma peaks for all the pixels, with percentage resolution (FWHM) as low as 1.8 %.

Schottky barrier diode (SBD) radiation detectors on n-type 4H-SiC epitaxial layer have been fabricated and evaluated for low energy x- and γ-rays detection. The detectors were highly sensitive to soft x-rays in the 50 eV to few keV range and showed 2.1% resolution in detecting low energy y-rays of 59.6 keV from 241Am x/γ ray source. The detector's response to soft x-rays was significantly higher than that of commercial off-the-shelf (COTS) SiC UV photodiode. The devices have been characterized by current-voltage (I-V) measurements in the 94-700 K range, thermally stimulated current (TSC) spectroscopy, x-ray diffraction (XRD) rocking curve measurements, and defect delineating chemical etching. I-V characteristics of the detectors at 500 K showed low leakage current (