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Thermal-neutron detectors based on 4H-SiC semiconductor and 10B converter reactions have many advantages for neutron dosimetry and monitoring applications. These radiation-resistant detectors are capable of stable operation in elevated-temperature environments up to 700 °C for extended periods. The recent development of SiC detectors where the 10B is incorporated into the detector by ion implantation or diffusion leads to interesting application-specific design possibilities. The design of boron-diffused detectors is discussed as well as ways to optimize their design.

In this article, we demonstrate the radiation detection performance of vertical metal-oxide-semiconductor (MOS) capacitors fabricated on 20 μm thick n-4H-SiC epitaxial layers with the highest energy resolution ever reported. The 100 nm SiO2 layer was achieved on the Si face of n-4H-SiC epilayers using dry oxidation in air. The Ni/SiO2/n-4H-SiC MOS detectors not only demonstrated an excellent energy resolution of 0.42% (Δ E / E × 100) for 5.48 MeV alpha particles but also caused a lower enhancement in the electronic noise components of the spectrometer compared with that observed for the best high-resolution Schottky barrier detectors. The MOS detectors also exhibited a high charge collection efficiency (CCE) of 96% at the optimized operating bias despite the presence of the oxide layer. A drift-diffusion model applied to the CCE vs gate bias voltage data revealed a minority (hole) carrier diffusion length of 24 μm. Capacitance mode deep level transient spectroscopy (C-DLTS) scans in the temperature range 84–800 K were carried out to identify the resolution limiting electrically active defects. Interestingly, the C-DLTS spectra revealed both positive and negative peaks, indicating the simultaneous presence of electron (majority) and hole (minority) trap centers. It has been inferred that at the steady-state bias for the C-DLTS measurement, the MOS detector operates in the inversion mode at certain device temperatures, causing holes to populate the minority trap centers and, hence, manifests minority carrier peaks as well. [ABSTRACT FROM AUTHOR]

Advances in the growth processes of 4H-SiC epitaxial layers have led to the continued expansion of epilayer thickness, allowing for the detection of more penetrative radioactive particles. We report the fabrication and characterization of high-resolution Schottky barrier radiation detectors on 250 μm thick n-type 4H-SiC epitaxial layers, the highest reported thickness to date. Several 8 × 8 mm2 detectors were fabricated from a diced 100 mm diameter 4H-SiC epitaxial wafer grown on a conductive 4H-SiC substrate with a mean micropipe density of 0.11 cm−2. From the Mott–Schottky plots, the effective doping concentration was found to be in the range (0.95–1.85) × 1014 cm−3, implying that full depletion could be achieved at ∼5.7 kV (0.5 MV/cm at the interface). The current-voltage characteristics demonstrated consistently low leakage current densities of 1–3 nA/cm2 at a reverse bias of −800 V. This resulted in the pulse-height spectra generated using a 241Am alpha source (5486 keV) manifesting an energy resolution of less than 0.5% full width at half maximum (FWHM) for all the detectors at −200 V. The charge collection efficiencies (CCEs) were measured to be 98–99% with no discernable correlation to the energy resolution. A drift-diffusion model fit to the variation of CCE as a function of bias voltage, revealed a minority carrier diffusion length of ∼10 μm. Deep level transient spectroscopy measurements on the best resolution detector revealed that the excellent performance was the result of having ultralow concentrations of the order of 1011 cm−3 lifetime limiting defects—Z1/2 and EH6/7. [ABSTRACT FROM AUTHOR]

Incorporation of Se into Cd1−xZnxTe (CZT) to form the quaternary compound semiconductor Cd1−xZnxTe1–ySey (CZTS) has proven to be an effective solution for compensating the major flaws associated with CZT, including poor homogeneity and high concentrations of electronically active deep levels that limit the performance of CZT detectors. In order to investigate how deep levels are affected by the Se concentration in CZTS, we performed photoinduced current transient spectroscopy (PICTS) measurements on CZTS crystals grown by the traveling heater method (THM) with 10% atomic Zn and varying atomic percentage of Se from 1.5% to 7.0%. The PICTS scans for up to 4% Se showed an exponential reduction in the capture cross section of deep levels associated with Te secondary phases in conjunction with an increase in a deep level positioned near the mid-gap, which initially increases the electron trapping time before degrading again at higher Se concentrations. The PICTS peaks present in 7% Se were anomalous relative to the other crystals and are expected to originate from transition metal impurities found in the lower-purity CdSe precursor material. [ABSTRACT FROM AUTHOR]

Recent advances in the development of thick 4H-SiC epitaxial layers for the fabrication of surface barrier radiation detectors have been paving the way for their use in highly penetrating radiation detection. Challenges still exist to achieve full depletion all the way to the epilayer width, while maintaining a low leakage current at high reverse bias conditions. We report the fabrication of high-resolution and low leakage current Schottky barrier alpha particle detectors with a large active area of 11 mm2 on 50 μm thick n-type 4H-SiC epitaxial layers, which can be fully depleted. The detectors were cut out of large substrates of 100 mm diameter with a micropipe density <1 cm−2 in the epilayers. Mott–Schottky plots obtained from the capacitance–voltage measurements revealed an effective doping concentration of 1.9 × 10 14 c m − 3 . A parallel plate capacitor model implied that a reverse bias of ∼440 V was needed to achieve a full-depletion width (50 μm). The current–voltage characteristics demonstrated a very low leakage current of 0.45 nA at a reverse bias of −450 V, which is low enough for the detector to be operated at full-depletion bias. In fact, pulse height spectroscopy using a 241Am alpha source, with the detector biased at −120 V, exhibited a well-resolved alpha spectrum with an energy resolution of 0.8% for the alpha peak corresponding to 5486 keV. This energy resolution was maintained consistently up to a full-depletion bias of −440 V. The ultra-stable performance of the detector has been attributed to the remarkably low concentration of carrier lifetime affecting deep-level defects like Z 1 / 2 and E H 6 / 7 , which were found to be of the order of 10 12 c m − 3 or less using capacitance mode deep-level transient spectroscopy measurements. [ABSTRACT FROM AUTHOR]

CdxZn1−xTeySe1−y (CZTS) has emerged as a next-generation compound semiconductor for high energy gamma-ray detection. In the present study, we report for the first time the hole transport property measurements in CZTS based gamma-ray detectors in the planar configuration. Current–voltage measurements revealed a bulk resistivity of ∼5 × 108 Ω cm and the fabricated detectors produced well-resolved 5486 keV alpha particle peaks, for both electrons and holes drifting alike, when pulse-height spectra (PHS) were recorded using a 241Am radiation source. The PHS measurements were enabled to measure the charge transport properties for both the charge carriers. The mobility–lifetime product (μτ) for electrons and holes was calculated to be 6.4 × 10−4 cm2 V−1 and 8.5 × 10−5 cm2 V−1, respectively, using a single polarity Hecht plot regression method. The pre-amplifier pulses were also recorded and processed digitally to obtain electron and hole drift mobilities of 692 cm2 V−1 s−1 and 55 cm2 V−1 s−1, respectively, 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 using fabricated detectors that showed tailing of the 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. Digital correction procedures were applied to generate well-resolved PHS with an energy resolution of ∼2% for 662 keV γ-rays. [ABSTRACT FROM AUTHOR]

The effect of super-cadmium neutron-induced defects on radiation detection performance of Schottky barrier diodes, fabricated on 250- $\mu \text{m}$ -thick 4H-SiC epitaxial layers with ultralow intrinsic defect concentration, has been studied. The epilayers were irradiated in a TRIGA Mark II nuclear reactor dry instrumentation tube [intrareflector irradiation system or (IRIS)] in the net neutron fluence range 1010–1013 cm−2. Current–voltage (I–V) and capacitance–voltage (C–V) characteristics revealed that the detectors irradiated up to a fluence of 1012 cm−2 maintained a Schottky diode behavior. Radiation detection measurements showed an energy resolution of 28 keV (0.5%) full-width at half-maximum (FWHM) when exposed to 5486-keV alpha particles for the epilayers irradiated up to a neutron fluence of 1011 cm−2, which broadened to 42 keV (0.8%) FWHM for a fluence of 1012 cm−2. I–V and C–V measurements revealed substantial donor compensation in the epilayer irradiated at a fluence of ~1013 cm−2; however, the detector still worked satisfactorily with an energy resolution of 76-keV (1.8%) FWHM. The degradation in the detector performance with increased neutron dose was attributed to the trapping of charge carriers in the radiation-induced trap centers. Deep-level transient spectroscopy studies in the detector irradiated with a fluence of 1010 cm−2 revealed the formation of EH5 centers along with an unidentified deep electron trap located at 1.8 eV below the conduction band edge, both usually absent in as-grown 250- $\mu \text{m}$ 4H-SiC epilayers. A drift-diffusion model of charge transport showed a degradation in hole diffusion length from 10 $\mu \text{m}$ at a neutron fluence of 1010 cm−2 to $2.6~\mu \text{m}$ at 1012 cm−2 indicating formation of hole-trap centers as well. [ABSTRACT FROM AUTHOR]

We report the effect of EH6/7 electron trap centers alone on the performance of high-resolution radiation detectors fabricated on n-type 4H–SiC epitaxial layers. A Schottky barrier detector (SBD) and a metal-oxide-semiconductor (MOS) capacitor detector fabricated using two sister samples derived from the same 50 μm 4H–SiC parent wafer exhibited widely different energy resolutions of 0.4% and 0.9% for 5486 keV alpha particles. An equivalent noise charge model analysis ruled out the effect of the detector capacitance and the leakage current on the resolution of the detectors. Deep level transient spectroscopic studies revealed the presence of two trapping centers in each detector within the temperature scan range 240–800 K. The Z1/2 center, a potential electron trap, was detected in both the detectors in equal concentration, which suggested that the observed difference in the energy resolution is due to the presence of the other defect, the EH6/7 center, in the SBD. The capture cross section of the EH6/7 center was calculated to be three orders of magnitude higher than the second defect [a carbon antisite vacancy (CAV) center] observed in the MOS detector with an activation energy of 1.10 eV, which accounted for the enhanced electronic trapping in the SBD leading to its poor energy resolution. It has been proposed that the EH6/7 centers in the SBD have likely been reconfigured to CAV pairs during the thermal growth of the silicon dioxide layer in the MOS detector. The proposed formation mechanism of CAV, a stable qubit state for quantum information processing, addresses the outstanding questions related to the role of defect dynamics in their formation. [ABSTRACT FROM AUTHOR]

We report a modified vertical Bridgman method to grow Cd0.9Zn0.1Te0.97Se0.03 (CZTS) single crystals using in-house zone-refined 7 N (99.99999%) purity elemental precursors for room-temperature radiation detection. CZTS is an economic yet high performance alternative to expensive CdZnTe (CZT) detectors for room-temperature gamma-ray detection. Radiation detector in planar geometry has been fabricated on an 11.0 × 11.0 × 3.0 mm3 CZTS single crystal. A bulk resistivity of 1010 Ω.cm has been achieved without using any compensating dopant. The elemental composition of the grown crystal has been examined using energy-dispersive X-ray (EDX) analysis. Powder X-ray diffraction (XRD) showed formation of zincblende phase with a lattice constant of 6.447 Å, and sharp peaks confirmed the formation of highly crystalline single-phase CZTS crystals. A modified Vegard's law has been applied to calculate the atomic percentage of Se in the grown crystals from the XRD patterns and compared with the intended and the measured stoichiometry. The electron mobility-lifetime (μτ) product and the drift mobility have been calculated to be 1.5 × 10–3 cm2/V and 710 cm2/V.s, respectively, using alpha spectroscopy. The presented vertical Bridgman growth method uses a single pass through the controlled heating zone in contrast to the previously reported multiple pass growth techniques, thus, reducing the growth duration by two third which would help to further reduce the cost of production of CZTS-based room-temperature detectors. [ABSTRACT FROM AUTHOR]

Quaternary Cd0.9Zn0.1Te1–ySey (CZTS) single crystals, a novel room-temperature nuclear radiation detector semiconductor material, have been grown using a modified vertical Bridgman method (VBM) and a traveling heater method (THM). The percentage concentration of selenium in the VBM-grown crystal was 3% and that in the THM-grown crystal was 2%. While the THM Frisch collar detector ($4.4\times 4.4\times10.7$ mm $^{3}$) produced a highly resolved pulse height spectra (PHS) with a resolution of ~1.06% for 662-keV gamma rays without any correction, the VBM-grown detector ($10\times 10\times 2$ mm $^{3}$) offered a high-energy resolution of ~2% after the application of a digital bi-parametric correction. The high-resolution performance of these detectors has been attributed to the addition of Se in the Cd0.9Zn0.1Te (CZT) matrix. Ab-initio calculations based on density functional theory (DFT) also confirmed that the addition of Se in the CZT matrix helps to reduce the formation of TeCd and the TeZn antisites. The VBM-grown crystals were characterized using powder X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). While the XRD results revealed sharp diffraction peaks confirming the crystalline nature of the grown crystal, the EDS results confirmed the targeted stoichiometry of the elemental composition. The bulk resistivity of the grown crystal was calculated to be $\sim 3\times 10^{10}\,\,\Omega$ -cm from the current–voltage characteristics recorded at room temperature in a planar configuration, ensuring that the grown CZTS crystals have low dark current as required for detector-grade crystals. [ABSTRACT FROM AUTHOR]