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Quaternary Cd0.9Zn0.1Te1– y Se y (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.

In this letter we investigate for the first time the behavioral differences of transport properties of electrons and holes observed in high-resolution radiation detectors fabricated on high-purity single crystalline diamond using biparametric (BP) correlation studies. The chemical vapor deposition (CVD) grown $500~\mu \text{m}$ thick diamonds exhibited an excellent energy resolution of 0.5% for 5486 keV alpha particles for both electron and hole transport in a vertical electrode geometry. Interestingly, the behavior of the electron and hole transport were found to be vastly different at bias voltages less than 100 V. No discernable detector signal was observed due to electron transit below an applied bias of −100 V. The hole transit on the other hand, resulted in resolvable pulse height spectrum (PHS) from bias voltages as low as +10 V. Digital biparametric correlation studies of the rise-times and induced charge from each interaction of the incident alpha particles, revealed that the observed difference is due to the excessive electron trapping below bias voltages of −100 V. At lower biases, the width of the alpha peak in the PHS due to hole movement, was hardly affected by trapping and was observed to be limited due to lower hole drift velocities. The developed method with new insights of correlating the biparametric plots and the detector performance can be extended to vast range of semiconductor detectors.

In this letter we report a direct correlation between the space charge limited current flow and radiation detection properties of Cd x Zn1- x Te1- y Se y (CZTS) detector. CZTS is a recently discovered quaternary wide bandgap semiconductor for room-temperature gamma photon detection. The CZTS single crystals have been grown in-house using a modified Bridgman technique. Planar detectors were fabricated and characterized in terms of charge carrier flow and radiation detection measurements. An anomalous current flow falling outside the Lampert’s triangle in $\textit {logJ}-\textit {logV}$ plot, has been observed which is not generally seen in semiconductor detectors governed by space-charge limited current flow mechanism. The anomalous behavior has been attributed to the presence of electron traps. The devices were tested for their radiation detection properties by acquiring pulse height spectra using 241Am and a 137Cs isotopes as a source of alpha particles and gamma photons, respectively. It was further noticed that the only detectors which worked satisfactorily as radiation detectors, did not exhibit the anomalous current flow mechanism.

Large-volume detector-grade Cd0.9Zn0.1Te (CZT) single crystals were grown using a vertical Bridgman technique. The bulk resistivity of the as-grown crystal was found to be $\sim 5\times 10^{10}\,\,\Omega $ -cm from current–voltage measurements. A $19.0\times 19.0\times5.0$ mm3 block of the crystal was used to fabricate a detector with $10\times 10$ pixelated anode configuration integrated with interpixel guard rings. The detector was tested for its radiation detection properties using 662-keV gamma rays from a 137Cs source. The pixels exhibited well-resolved gamma pulse-height spectra (PHS) with percentage energy resolution ~1.6% at 662 keV. A few pixels exhibited tailing of the photopeak on the lower energy side indicating the presence of hole traps. Biparametric plots (BPs) were obtained from digitally recorded preamplifier pulses. The BPs showed anomalous behavior, which was correlated with the gamma interactions in the active region of the detector in virtual Frisch grid configuration. The BPs also enabled to extract PHS free from the extensive tailing of the photopeak.

Reactor dosimetry environments require radiation detectors that are capable of operating at high temperatures in extremely high neutron and gamma-ray dose rates. Silicon carbide (SiC) is one of the most promising wide bandgap semiconductors (3.27 eV) for harsh environment applications due to its radiation hardness, high breakdown voltage, high electron saturation velocity, and high thermal conductivity. In this paper, we summarize the prospect of Schottky barrier radiation detectors, fabricated on highly crystalline low-defect detector-grade n-type 4H-SiC epitaxial layers with thickness ranging from 20 to 250 lm, for harsh environment applications. A comprehensive discussion on the characterization of the parameters that influence the energy resolution has been included. The usage of electrical and radiation spectroscopic measurements for characterizing the junction and rectification properties, minority carrier diffusion lengths, and energy resolution has been elaborated. Characterization of crucial factors that limit the energy resolution of the detectors such as charge trap centers using thermally stimulated transient techniques is summarized. Finally, the effect of neutron fluence on the performance of the 4H-SiC detectors is discussed.

We report the implementation of a deep convolutional neural network to train a high-resolution room-temperature CdZnTeSe based gamma ray spectrometer for accurate and precise determination of gamma ray energies for radioisotope identification. The prototype learned spectrometer consists of a NI PCI 5122 fast digitizer connected to a pre-amplifier to recognize spectral features in a sequence of data. We used simulated preamplifier pulses that resemble actual data for various gamma photon energies to train a CNN on the equivalent of 90 seconds worth of data and validated it on 10 seconds worth of simulated data.

Silicon carbide (SiC) is the only wide-bandgap semiconductor to possess native oxide layer thus favoring efficient fabrication of metal-oxide-semiconductor (MOS) devices. 4H-SiC MOS structure has recently been demonstrated as improved radiation detector compared to the conventional Schottky barrier architecture. We report the fabrication of vertical Au/SiO2/n-4H-SiC MOS capacitors for radiation detection, by dry-oxidation of 20 μm thick n-type 4H-SiC epitaxial layer in air at 1000°C. Charge-carrier traps (defects) are known to limit the performance of semiconductor devices. In order to characterize the defects, capacitance mode deep level transient spectroscopy (C-DLTS) was carried out. Apart from regular electron-traps e.g., Ti-impurity and Z1/2 sites, we have also observed the carbon-interstitial related hole traps HK3. While studying defect centers in these devices using a filling pulse peaking to 0 V from a quiescent reverse gate voltage VG = -4 V, we observed a robust positive peak centered around 650 K. Positive peaks in C-DLTS scan indicates minority-carrier trapping, although above-mentioned type of filling pulses does not populate minority-carrier trap centers normally. The activation energy of the observed trap, most likely a carbon vacancy (HK3), was calculated to be 1.27 eV above the valence band edge.

Thick 4H-SiC epitaxial layers are essential for high-resolution detection of x- and gamma-rays in harsh environment. In this work, we have fabricated high-resolution Ni/n-4H-SiC Schottky barrier radiation detectors on 250 μm epitaxial layers, the highest thickness ever reported. Capacitance-voltage (C-V) measurements showed a low-carrier concentration of ≈2 × 1014 cm-3 which based on simulations of the electric field allow the detectors to be fully depleted without break down. Current-voltage (I-V) characteristics displayed low leakage currents of 10 μm. Both the long minority carrier diffusion length and high resolution were correlated to the low concentration of lifetime killing defects Z1/2 and EH6/7 (both associated with different charge states of carbon vacancy) found in the detector’s DLTS spectra

While CdZnTe (CZT) is one of the best materials for room-temperature radiation detection, they are not quite suitable for high-temperature or harsh-environment applications. This chapter discusses the fabrication and characterization of high-resolution 4H-SiC epitaxial detectors, which are appropriate for use in harsh environments like high temperature, chemically reactive and corrosive environments, and most importantly high-dose nuclear radiation environments. Schottky barrier diodes on thin (≤20 μm) 4H-SiC epitaxial layer detectors have been found to be a very promising device for charged particle detection at room and elevated temperatures. While such thin epitaxial layers are sufficient to stop energetic charged particle like alpha particles, highly penetrating radiations such as X- and γ-rays need thicker epitaxial layers to enable substantial photon absorption. This chapter discusses the fabrication of Schottky barrier radiation detectors in n-type 4H-SiC epitaxial layers with different thicknesses (20, 50, and 150 μm); their characterization in terms of alpha, X-rays, and γ-detection; and evaluation of the factors like deep-level defects which regulate their performance as radiation detectors.

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.

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.

We report on the fabrication and characterization of a high-resolution Schottky barrier radiation detector for high temperature (HT) applications using high quality 150 𝜇m thick n-4H-SiC epitaxial layers with an ultra-low micropipe density of ≤0.1 cm-2. To evaluate the depletion region parameters of the detector, it was first characterized temperature- dependent capacitance voltage (C-V-T) measurements which showed barrier heights Φ𝐵 ranging from 2.09 to 2.24 eV and a carrier concentration of ~2 x 1014 cm-3 at room temperature (RT) which linearly increased at a rate of 9.65 x 1010 cm-3/K. From the depletion region simulation studies, it was shown that the change in doping concentration with temperature (T) can increase the needed bias to fully capture charged particles by up to 9V for 241Am and over 30V for a higher energy particles from 213Po. To examine the barrier properties and leakage currents of the detectors, we have systematically characterized temperature-dependent current - voltage measurements (I-V-T). The forward bias characteristics showed two linear regions - a low voltage region corresponding to a Shockley-Read-Hall (SRH) recombination center at 𝐸𝐶 − 0.93 eV and an upper region which corresponded to the barrier height of 2.02 eV. The reverse bias leakage currents were measured to be ~6 pA at ~65V at 300K implying a high signal to noise ratio (SNR) at RT and exhibited a current of less than 1 nA until 500K suggesting that detector should be operable with high SNR at ~200 oC. The Arrhenius plot of the reverse I-V-T showed an activation energy of 0.11 eV up to 400K and then 0.73 eV for 450K to 600K suggesting that most of the excess current at HT is derived from deep level state Z1/2 recombination center. Deep level transient spectroscopy (DLTS) results showed low defect concentrations of ~1011 cm-3 confirming that charge loss from trapping is negligible. Density functional theory (DFT) calculations suggested that the measured trap levels corresponded primarily to carbon (C) vacancies while the level at 0.93 eV corresponded to Si vacancies near the surface. The pulse height spectra (PHS) of the detector showed an excellent RT energy resolution of 0.55% FWHM at 5486 keV alpha particles after gaussian correction owed gamma response at 59.54 keV.

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.

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% ( ΔE/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.

Schottky barrier detectors (SBDs) require larger surface areas than conventional electronics to increase the detection efficiency although such SBDs manifest large diode ideality factors due to inhomogeneous areal distribution of surface barrier height (SBH). Inhomogeneous SBH distributions lead to various current flow mechanisms in SBDs, which need to be identified to optimize detector performance. In this Letter, we identify the current flow mechanism in large area Schottky barrier diodes for radiation detection fabricated on 150 μm thick n-4H–SiC epitaxial layers. The analysis of temperature-dependent forward current–voltage (I–V–T) characteristics of SBDs revealed two linear regions in current–voltage curves up to 450 K, one corresponding to the current flow through a low barrier patch, while the other corresponds to that of average barrier distribution. Applying a SBH distribution model to the reverse I–V–T characteristics, an activation energy of 0.76 eV for the current flow over the Schottky barrier was calculated. The activation energy did not directly correspond to any of the defect levels observed from the deep level transient spectroscopy (DLTS). Above 450 K, a Schottky type barrier lowering suggested a current flow through a low barrier patch of ≈ 0.8 eV. The absence of any SBH lowering below 450 K indicated that the current corresponded to a neutrally charged trap level at ≈ 0.6 eV below the conduction band edge, which was consistent with DLTS measurements revealing the presence of an electron trap level Z1/2 at 0.59 eV below the conduction band edge.

The application of Cd0.9Zn0.1Te (CZT) single crystals, the primary choice for high-resolution, room-temperature compact gamma-ray detectors in the field of medical imaging and homeland security for the past three decades, is limited by the high cost of production and maintenance due to low detector grade crystal growth yield. The recent advent of its quaternary successor, Cd0.9Zn0.1Te1−ySey (CZTS), has exhibited remarkable crystal growth yield above 90% compared to that of ~33% for CZT. The inclusion of Se in appropriate stoichiometry in the CZT matrix is responsible for reducing the concentration of sub-grain boundary (SGB) networks which greatly enhances the compositional homogeneity and growth yield. SGB networks also host defect centers responsible for charge trapping, hence their reduced concentration ensures minimized charge trapping. Indeed, CZTS single crystals have shown remarkable improvement in electron charge transport properties and energy resolution over CZT detectors. However, our studies have found that the overall charge transport in CZTS is still limited by the hole trapping. In this article, we systematically review the advances in the CZTS growth techniques, its performance as room-temperature radiation detector, and the role of defects and their passivation studies needed to improve the performance of CZTS detectors further.

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.

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

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.

The effect of trapping of charge carriers on the shape of the charge pulses from room temperature nuclear detectors, based on a recently discovered quaternary semiconductor Cd0.9Zn0.1Te0.98Se0.02 (CZTS), has been studied. We present a charge-trapping model that explains the pulse shapes for its entire duration from radiation detectors containing multiple defect types. A piecewise continuous model, based on the movement of a large number of charge carriers rather than a single charge, has been proposed to explain the pulse shapes from planar room temperature nuclear detectors. An experimental method based on the model has been described to extract charge-trapping information. This model can be applied to similar semiconductor detectors with electrically active trap centers, in general, to extract information like pre-trapping drift times. Charge pulses from a CZTS planar detector, exposed to 5486 keV alpha particles emitted by a 241Am source, were acquired using a digitizer, and the pulse shapes were explained based on the proposed model. The pre-trapping drift times for electrons as well as holes in CZTS were calculated and studied as a function of operating bias voltage, and a contrast between their bias dependent behaviors was noticed, indicating the behavioral difference of electron and hole traps.

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 E c -0.6 eV (Z 1/2 ) and at E c -1.6 eV (EH 6/7 ),both related to carbon vacancies, affected the detector performance the most.

Cd0.9Zn0.1Te (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%.

Nanocrystalline non-stoichiometric cadmium ferrite (CdFe2O4) has been synthesized by high-energy ball milling the mixture (1:1mol%) of CdO and α-Fe2O3 at room temperature. Formation of nanocrystalline CdFe2O4as normal spinel structure has been noticed after ball milling the mixture for 5h. The structural and microstructural evolution of CdFe2O4 caused by milling is investigated by X-ray powder diffraction. The relative phase abundances, particle sizes, r.m.s. strains, lattice parameter changes of different phases have been estimated employing Rietveld powder structure refinement analysis of XRD data. It appears that solid state diffusion of nanocrystalline CdO into α-Fe2O3 particles results in formation of nanocrystalline CdFe2O4 spinel. XRD data analysis indicates that the ferrite prepared by ball milling is a non-stoichiometric Cd-riched phase. Positron annihilation lifetime measurements on the milled samples indicate rise of defect density up to 5h milling time and supports the formation of non-stoichiometric CdFe2O4 phase.

In this article we report the fabrication and characterization of large area, room-temperature operable and very high resolution Schottky barrier detectors for alpha particles using 20 μm thick n-type 4H–SiC epitaxial layers. Schottky barriers were fabricated by depositing circular nickel contacts of ~11 mm2 area on the 4H–SiC epitaxial layers. Room temperature current–voltage measurements revealed very high Schottky barrier height of 1.6 eV and extremely low leakage current of 3.5 pA at an operating reverse bias of −90 V. We also report an energy resolution of 0.29%, which is the best resolution obtained so far for uncollimated 5.48 MeV alpha particles in 4H–SiC epitaxial detectors with such a large area. Very low micropipe density (

Schottky barrier diode (SBD) radiation detectors have been fabricated on n-type 4H-SiC epitaxial layers and evaluated for low energy x- and γ-rays detection. The detectors were found to be highly sensitive to soft x-rays in the 50 eV to few keV range and showed 2.1 % energy resolution for 59.6 keV gamma rays. The response to soft x-rays for these detectors was significantly higher than that of commercial off-the-shelf (COTS) SiC UV photodiodes. 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 ( nA at 200 V) revealing a possibility of high temperature operation. The XRD rocking curve measurements revealed high quality of the epitaxial layer exhibiting a full width at half maximum (FWHM) of the rocking curve ~3.6 arc sec. TSC studies in a wide range of temperature (94-550 K) revealed presence of relatively shallow levels ( ~0.25 eV) in the epi bulk with a density ~7×1013 cm-3 related to Al and B impurities and deeper levels located near the metal-semiconductor interface.

Detector grade Cd0.9Zn0.1Te single crystals were grown using a tellurium solvent method. Single crystal blocks of volume ~1 cm3 were prepared for detector fabrication and characterization. The grown crystals were characterized using infra-red transmission imaging and Pockel's effect measurements. Two detectors in single-polarity charge sensing configurations viz., small pixel, and virtual Frisch grid were fabricated on two crystals obtained from the same section of the ingot. Current-voltage measurements performed in planar configuration exhibited a very low leakage current of ~5 nA at 1000 V and resistivities of the order of 8.5×1010 Ω·cm. Electron drift mobilities of the order of 840 cm2/V.s and electron mobility-lifetime products of the order of 2.7×10-3 cm2/V were calculated from alpha spectroscopy using detectors in planar configuration. The small pixel and the virtual Frisch grid detector showed similar energy resolution of 3.7% for 662 keV gamma rays however, the virtual Frisch grid configuration revealed a better overall performance with a peak-to-Compton ratio of 2.8. A digital spectrometer and related software has been developed using a digitizer card and used to employ offline correction schemes to compensate for the charge loss effects, resulting in significant improvement of the 662 keV peak resolution (1.8% as compared to 3.7% without correction) obtained in the case of small pixel detector.

Detector grade CZT single crystals have been grown from zone refined Cd, Zn, and Te (∼7 N) precursor materials using a tellurium solvent method. Infrared transmission imaging has shown an average tellurium inclusion size of ∼8 μm in the as-grown crystal. Radiation detectors were fabricated in planar and virtual Frisch grid geometry and characterized for their spectroscopic performance. Charge transport properties revealed a high drift mobility of∼1200 cm 2 /V s and a mobility–lifetime product of ∼2.8×10 −3 cm 2 /V. A detector with planar configuration (dimensions 6.9×6.9×4.8 mm 3 ) showed an energy resolution of 4.7% for 59.5 keV gamma rays. The detector in a Frisch grid configuration (dimensions 4.2×6.2×6.5 mm 3 ) exhibited an energy resolution of 1.4% for 662 keV gamma rays. Digital spectroscopic measurements were carried out using a high-resolution digitizer card. A biparametric correlation scheme was employed to study the effect of charge loss on energy resolution of the planar detector. Based on this correlation scheme, a digital correction method was applied and improved pulse-height spectrum was obtained for 662 keV gamma rays using the planar detector with spectral features comparable to those of the virtual Frisch grid detector.

Schottky barrier detectors have been fabricated on 50 μm n-type 4H-SiC epitaxial layers grown on 360 μm SiC substrates by depositing ∼10 nm nickel contact. Current–voltage ( I – V ) and capacitance–voltage ( C – V ) measurements were carried out to investigate the Schottky barrier properties. The detectors were evaluated for alpha particle detection using a 241 Am alpha source. An energy resolution of ∼2.7% was obtained with a reverse bias of 100 V for 5.48 MeV alpha particles. The measured charge collection efficiency ( CCE ) was seen to vary as a function of bias voltage following a minority carrier diffusion model. Using this model, a diffusion length of∼3.5 μm for holes was numerically calculated from the CCE vs. bias voltage plot. Rise-time measurements of digitally recorded charge pulses for the 5.48 MeV alpha particles showed a presence of two sets of events having different rise-times at a higher bias of 200 V. A biparametric correlation scheme was successfully implemented for the first time to visualize the correlated pulse-height distribution of the events with different rise-times. Using the rise-time measurements and the biparametric plots, the observed variation of energy resolution with applied bias was explained.

Zn-ferrite nanoparticles were synthesized at room temperature by mechanical alloying the stoichiometric (1:1 mol%) mixture of ZnO and α-Fe2O3 powder under open air. Formation of both normal and inverse spinel ferrite phases was noticed after 30 minutes and 2.5 hours ball milling respectively and the content of inverse spinel phase increased with increasing milling time. The phase transformation kinetics towards formation of ferrite phases and microstructure characterization of ball milled ZnFe2O4 phases was primarily investigated by X-ray powder diffraction pattern analysis. The relative phase abundances of different phases, crystallite size, r.m.s. strain, lattice parameter change etc. were estimated from the Rietveld powder structure refinement analysis of XRD data. Positron annihilation lifetime spectra of all ball milled samples were deconvoluted with three lifetime parameters and their variation with milling time duration was explained with microstructural changes and formation of different phases with increase of milling time duration.

Recent advances in the growth of CdZnTe material have allowed the development of small pixel, spectroscopic, X-ray imaging detectors. These detectors have applications in a diverse range of fields such as medical, security and industrial sectors. As the size of the pixels decreases relative to the detector thickness, the probability that charge is shared between multiple pixels increases due to the non zero width of the charge clouds drifting through the detector. These charge sharing events will result in a degradation of the spectroscopic performance of detectors and must be considered when analyzing the detector response. In this paper charge sharing and charge loss in a 250 $\mu$ m pitch CdZnTe pixel detector has been investigated using a mono-chromatic X-ray beam at the Diamond Light Source, U.K. Using a 20 $\mu$ m beam diameter the detector response has been mapped for X-ray energies both above (40 keV) and below (26 keV) the material $K$ -shell absorption energies to study charge sharing and the role of fluorescence X-rays in these events.

We have synthesized a series of nanocrystalline ferrite samples with the composition MnxCu1-xFe2O4 (x=0.2, 0.4, 0.6, 0.8) by an advanced sol-gel auto-combustion method. The X-ray diffraction patterns confirm the existence of single-phase cubic spinel crystal structure of ferrites with lattice parameter ranges from 8.395 ? to 8.45 ?. We report the equilibrium radii for tetrahedral and octahedral sites in the unit cells and the estimated cation distribution over the two sites of nanocrystalline MnxCu1-xFe2O4. We also estimate the oxygen positional parameter as 0.389. The positron annihilation life time spectroscopic studies were carried out for all the samples and analyzed the variation of life time parameters ?1; I1, ?2; I2 and the mean life time ?m to elucidate the defect structure of the nanocrystalline MnxCu1-xFe2O4. We found that the overall vacant type defects fill up as the Mn2+ ion concentration, x, increases. The value of ?1 varies from 150 ps to 170 ps and that of ?2 varies from 295 ps to 335 ps, which are the characteristic values for nanocrystalline samples, indicating the presence of intergranular as well as surface-diffused vacancies in the crystal structure.

Inter University Consortium for DAE Facilities Silicon surface barrier (SSB) detectors have been fabricated with oxygen enriched, high-resistivity, detector grade, n-type FZ silicon. Oxygen enrichment of the wafer was done by high-energy (140 MeV) oxygen ion implantation. Annealing of the irradiated silicon wafer was done to minimize the irradiation-induced defect concentration. Positron annihilation lifetime studies were used to select the annealing temperature. A comparative study was carried out among various SSB detectors made from as-grown, as-irradiated and annealed silicon wafers. Detector made from the annealed wafer worked satisfactorily and its performance was comparable with that of a detector made from as-grown crystal.

The surface leakage current of high-resolution 4H-SiC epitaxial layer Schottky barrier detectors has been improved significantly after surface passivations of 4H-SiC epitaxial layers. Thin (nanometer range) layers of silicon dioxide (SiO2) and silicon nitride (Si3N4) were deposited on 4H-SiC epitaxial layers using plasma enhanced chemical vapor deposition (PECVD) on 20 m thick n-type 4H-SiC epitaxial layers followed by the fabrication of large area (~12 mm2) Schottky barrier radiation detectors. The fabricated detectors have been characterized through current-voltage (I-V), capacitance-voltage (C-V), and alpha pulse height spectroscopy measurements; the results were compared with that of detectors fabricated without surface passivations. Improved energy resolution of ~ 0.4% for 5486 keV alpha particles was observed after passivation, and it was found that the performance of these detectors were limited by the presence of macroscopic and microscopic crystal defects affecting the charge transport properties adversely. Capacitance mode deep level transient studies (DLTS) revealed the presence of a titanium impurity related shallow level defects (Ec-0.19 eV), and two deep level defects identified as Z1/2 and Ci1 located at Ec-0.62 and ~ Ec-1.40 eV respectively.

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 %.