Khai V. Nguyen

Effect of Z1/2, EH5, and Ci1 deep defects on the performance of n-type 4H-SiC epitaxial layers Schottky detectors: Alpha spectroscopy and deep level transient spectroscopy studies

Spectroscopic performance of Schottky barrier alpha particle detectors fabricated on 50 μm thick n-type 4H-SiC epitaxial layers containing Z1/2, EH5, and Ci1 deep levels were investigated. The device performance was evaluated on the basis of junction current/capacitance characterization and alpha pulse-height spectroscopy. Capacitance mode deep level transient spectroscopy revealed the presence of the above-mentioned deep levels along with two shallow level defects related to titanium impurities (Ti(h) and Ti(c)) and an unidentified deep electron trap located at 2.4 eV below the conduction band minimum, which is being reported for the first time. The concentration of the lifetime killer Z1/2 defects was found to be 1.7 × 1013 cm−3. The charge transport and collection efficiency results obtained from the alpha particle pulse-height spectroscopy were interpreted using a drift-diffusion charge transport model. Based on these investigations, the physics behind the correlation of the detector properties viz., ...

Correlation of Deep Levels With Detector Performance in 4H-SiC Epitaxial Schottky Barrier Alpha Detectors

High resolution Schottky barrier detectors for alpha particles were fabricated using 20 μm thick detector grade n-type 4H-SiC epitaxial layer. The Schottky barrier detectors were characterized through current-voltage (I-V) and capacitance-voltage (C-V) measurements. Deep level transient spectroscopic (DLTS) measurements were carried out to identify and characterize the electrically active defect levels present in the epitaxial layers. The detection properties of the Schottky detectors were characterized in terms of alpha particle peak widths in pulse height spectra obtained using a standard alpha emitting radioisotope source. The differences in the performance of different detectors were correlated on the basis of the barrier properties and the deep level defect type, concentration, and capture cross-section. Varying degree of the presence of deep level defects was found to be the reason behind the leakage current variation and the difference in the ultimate detector performance observed among the detectors. From the DLTS data it was found that at least two defect centers located at Ec-0.6 eV (Z1/2) and at Ec -1.6 eV (EH6/7),both related to carbon vacancies, affected the detector performance the most.

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}

A new edge termination with SiO2 and Si3N4 passivating layers has been developed and is shown to be a very effective method for improving energy resolution of 20 μm n-type 4H-SiC epilayer Schottky barrier radiation detectors. The junction properties of the fabricated detectors before and after edge termination were studied by current-voltage (I-V) and capacitance-voltage (C-V) measurements. A thermionic emission model applied to the forward I-V characteristics showed surface barrier height of ~ 1.4 eV and diode ideality factor of ~ 1.1. The C-V measurements showed a doping concentration of 1.8 ×1014 cm - 3 which ensured a fully depleted ( ~ 20 μm) detector at bias voltages as low as ~ 70 V. Alpha spectroscopy measurements revealed an improved energy resolution from ~ 0.7 % to ~ 0.4% for 5.48 MeV alpha particles. Deep level transient spectroscopy (DLTS) measurements have shown a decreased concentration of Z1/2 defect levels in detectors following edge termination.

Pixelated Detector: Fabrication and Characterization

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.

Improved n-Type 4H-SiC Epitaxial Radiation Detectors by Edge Termination

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

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.

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

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).