Rahmi O. Pak

Large Area

Cd_0.9Zn_0.1Te (CZT) based pixelated radiation detectors have been fabricated and characterized for gamma ray detection. Large area CZT single crystals has been grown using a tellurium solvent method. A 10 ×10 guarded pixelated detector has been fabricated on a ~ 19.5 ×19.5 ×5 mm³ crystal cut out from the grown ingot. The pixel dimensions were 1.3 ×1.3 mm² and were pitched at 1.8 mm. A guard grid was used to reduce interpixel/inter-electrode leakage. The crystal was characterized in planar configuration using electrical, optical and optoelectronic methods prior to the fabrication of pixelated geometry. Current-voltage (I-V) measurements revealed a leakage current of 27 nA at an operating bias voltage of 1000 V and a resistivity of ~ 3.1 ×10¹⁰ Ω-cm. Infrared transmission imaging revealed an average tellurium inclusion/precipitate size less than 8 μm. 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 high as 1.8%.

{\rm Cd}_{0.9}{\rm Zn}_{0.1}{\rm Te}

Deep levels were investigated by the capacitance mode deep-level transient spectroscopy (C-DLTS) on 4H-SiC Schottky barrier diodes fabricated on 50 μm-thick n-type 4HSiC epitaxial layers. C-DLTS scans from 80 K to 800 K revealed the presence of Ti(c), Z1/2, EH5, and EH6/7 defect levels in the energy range from 0.17 to 1.6 eV below the conduction band edge. The annealing out of primary defects and generation of secondary defects were investigated by systematic and thorough C-DLTS studies from prior and subsequent isochronal annealing in the temperature range from 100 °C to 800 °C. The capture cross-section of Ti(c) was observed to decrease up to 400 °C and remained unchanged at higher annealing temperatures. Defect densities were shown to decrease up to 200 °C and gradually increase at higher temperatures. The Z1/2 and EH6/7 defect parameters showed similar variation for the temperature range studied. The thermal evolutions of these deep levels in n-type 4H-SiC epitaxial layers are analyzed and discussed for the first time.

Pixelated Detector: Fabrication and Characterization

Schottky barrier radiation detectors were fabricated on the Si-face of 50 μm thick detector grade n-type 4H-SiC epitaxial layers. The junction properties of the fabricated detectors were investigated by current-voltage (I-V) and capacitancevoltage (C-V) measurements. The radiation detector performances were evaluated by alpha pulse height spectroscopy using a 0.1 μCi 241Am radiation source. Deep level transient spectroscopy (DLTS) measurements were carried out to identify and characterize the electrically active defect levels present in the epitaxial layers. The performance of the detector was found to be limited by the presence of electrically active defect centers in the epilayer. Deep level defects were reduced significantly by isochronal annealing. Surface passivation studies were conducted on n-type 4H-SiC epilayers for use on radiation detectors for the first time. Energy resolution of the detector was found to have improved after passivation and the life time killing defects that were responsible for preventing full charge collection were reduced significantly. Systematic and thorough C-DLTS studies were conducted prior and subsequent to isochronal annealing to observe evolution of the deep level defects.

Deep Levels in n-Type 4H-Silicon Carbide Epitaxial Layers Investigated by Deep-Level Transient Spectroscopy and Isochronal Annealing Studies

CdTe and Cd0.9Zn0.1Te (CZT) crystals have been studied extensively for various applications including x- and γ-ray imaging and high energy radiation detectors. The crystals were grown from zone refined ultra-pure precursor materials using a vertical Bridgman furnace. The growth process has been monitored, controlled, and optimized by a computer simulation and modeling program developed in our laboratory. The grown crystals were thoroughly characterized after cutting wafers from the ingots and processed by chemo-mechanical polishing (CMP). The infrared (IR) transmission images of the post-treated CdTe and CZT crystals showed average Te inclusion size of ~10 μm for CdTe and ~8 μm for CZT crystal. The etch pit density was ≤ 5×104 cm-2 for CdTe and ≤ 3×104 cm-2 for CZT. Various planar and Frisch collar detectors were fabricated and evaluated. From the current-voltage measurements, the electrical resistivity was estimated to be ~ 1.5×1010 Ω-cm for CdTe and 2-5×1011 Ω-cm for CZT. The Hecht analysis of electron and hole mobility-lifetime products (μτe and μτh) showed μτe = 2×10-3 cm2/V (μτh = 8×10-5 cm2/V) and 3-6×10-3 cm2/V (μτh = 4- 6×10-5 cm2/V) for CdTe and CZT, respectively. Detectors in single pixel, Frisch collar, and coplanar grid geometries were fabricated. Detectors in Frisch grid and guard-ring configuration were found to exhibit energy resolution of 1.4% and 2.6 %, respectively, for 662 keV gamma rays. Assessments of the detector performance have been carried out also using 241Am (60 keV) showing energy resolution of 4.2% FWHM.

Surface passivation and isochronal annealing studies on n-type 4H-SiC epitaxial layer

We report the development of an edge termination by depositing thin Si3N4 passivating film on 4H-SiC epilayer based radiation detector. The edge termination method is shown to be very effective for improving both the detector leakage current and radiation detection performance compared with that of a conventional detector fabricated from the same parent wafer. The detector leakage current was found to have improved two orders of magnitude. Significant improvement in radiation detection performance was shown from alpha spectroscopy measurements prior and subsequent to Si3N4 edge termination. Deep Level Transient Spectroscopy (DLTS) measurements revealed a reduction in life-time killing defects of detectors with Si3N4 edge termination which could be related to the observed improvements in radiation detection performance.

Characterization of high-resistivity CdTe and Cd0.9Zn0.1Te crystals grown by Bridgman method for radiation detector applications

Schottky barrier radiation detectors were fabricated on 12 μm n-type 4H-SiC epitaxial layers grown on a 4° off-axis highly doped 4H-SiC substrate (0001). Schottky barrier junction properties were characterized through current-voltage (I-V) and capacitance-voltage (C-V) measurements. A diode ideality factor of 1.29 and Schottky barrier height of 1.10 eV were determined from the forward I-V characteristics using a thermionic emission model. A built-in potential of 1.91 V and effective carrier concentration of 1.03 × 1015 cm-3 was calculated from a Mott-Schottky plot of the C-V measurements. Radiation detector performance was evaluated by alpha pulse height spectroscopy (PHS) in terms of energy resolution expressed in full-width at half maxima (FWHM) and charge collection efficiency (CCE). The energy resolution was determined to be 166 keV with a CCE of 22.6% for 5.486 MeV alpha particles. Deep level transient spectroscopy (DLTS) measurements were carried out to investigate the deep levels in the detector active region. An electrically active defect level Z1/2 related to carbon vacancies was identified and characterized. The concentration and capture cross-section of Z1/2 were determined to be 1. 58 × 1012 cm-3 and 9.12 × 10-16 cm2, respectively. Electronic noise analysis in terms of equivalent noise charge (ENC) was carried out to study the effect of various noise components that contribute to the total electronic noise in the detection system.

Improved radiation detectors on 4H-SiC epilayers by edge termination

Large volume single crystals of Cd_0.9Zn_0.1Te (CZT) have been grown by vertical Bridgman technique using in-house zone refined precursor materials (Cd, Zn, and Te). The grown semi-insulating CZT crystals have shown high promise for high-resolution room-temperature radiation detectors due to their high dark resistivity (~4×10¹⁰ Ω-cm), good charge transport properties [(μτ)_e = (6-8) × 10^-3 cm²/V], and relatively low cost. The grown CZT single crystals (~2.8 cm diameter and >10 cm long) have demonstrated a very low radial Zn concentration deviation, low dislocation densities and Te precipitate/inclusions, and high infrared transmission. Details of the CZT single crystal growth, their physical and chemical analysis, surface processing, nuclear radiation detector fabrication, and testing of these devices are presented.

Investigation of 12 μm 4H-SiC epilayers for radiation detection and noise analysis of front-end readout electronics

Amorphous selenium (a-Se) alloy materials doped with lithium (Li), boron (B), arsenic (As), and chlorine (Cl), were synthesized for room temperature radiation detection applications using an optimized alloy composition for enhanced charge transport properties. A two-step synthesis process has been implemented to first synthesize the a-Se (As) and a-Se (Cl) master alloys from zone-refined Se (~7N), and then synthesize the final mixed optimized alloys. The alloy material was used for thin-film deposition on oxidized aluminum (Al) and indium tin oxide (ITO) coated glass substrates. Material purity, morphology, and compositional analysis of the a-Se alloy materials were investigated by glow discharge mass spectroscopy (GDMS), x-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and x-ray photoelectron spectroscopy (XPS). Different metals of various work functions (Ni, W, Au, Pd, In, Cu, Sn, Cu, Ag, Al, Cr, Zn and Mo) were selected for Schottky barrier detector performance studies. Current-voltage (I-V), capacitance voltage (C-V), and current transient measurements were performed at different temperatures to investigate the metal contacts and contact stability characteristics. Single and multi-element detectors with and without various blocking contacts (electron and holes) have been fabricated and tested and the results show promising characteristics for x-ray and high energy nuclear radiations with its high dark resistivity (~1012 Ω-cm) and large-area scalability.

Characterization of Cd 0.9 Zn 0.1 Te single crystals for radiation detectors

Semi-insulating Cd0.9Zn0.1Te nuclear detector grade crystals were grown by a low temperature solution method from in-house zone refined (~7N) precursor materials. The processed crystals from the grown ingot were thoroughly characterized by using a non-destructive electron beam induced current (EBIC) contrast imaging method. The EBIC results were correlated with the infrared (IR) transmittance mapping, which confirms the variation of contrasts in EBIC is due to non-uniform distribution of tellurium inclusions in the grown CZT crystal. Electrical characteristics of defect regions in the fabricated detectors were further investigated by I-V measurements, and thermally stimulated current (TSC) measurements. Finally, to demonstrate the high quality of the grown CZT crystals, pulse height spectra (PHS) measurements were carried out using gamma radiation sources of 241Am (59.6 keV) and 137Cs (662 keV).

Investigation of metal contacts on high-resistivity large-area amorphous selenium alloy films

High barrier Schottky contact has been fabricated on 50 μm n-type 4H-SiC epitaxial layers grown on 350 μm thick substrate 8° off-cut towards the [11̅20] direction. The 4H-SiC epitaxial wafer was diced into 10 x 10 mm2 samples. The metal-semiconductor junctions were fabricated by photolithography and dc sputtering with ruthenium (Ru). The junction properties were characterized through current-voltage (I-V) and capacitance-voltage (C-V) measurements. Detectors were characterized by alpha spectroscopy measurements in terms of energy resolution and charge collection efficiency using a 0.1 μCi 241Am radiation source. It was found that detectors fabricated from high work function rare transition metal Ru demonstrated very low leakage current and significant improvement of detector performance. Defect characterization of the epitaxial layers was conducted by deep level transient spectroscopy (DLTS) to thoroughly investigate the defect levels in the active region. The presence of a new defect level induced by this rare transition metal-semiconductor interface has been identified and characterized.