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Novel, printed contacts for single crystal CVD grown diamond particle detectors are reported. Silver contacts with guard-rings were formed by three-dimensional metal printing. The resulting devices exhibit similar properties to the current state-of-the-art diamond detectors employing carbide layer contacts, yet detectors with the printed contacts retain high charge collection efficiency even after exposure to 450 °C (unlike aluminum contact devices for instance). The printed silver contacts do not require high temperature annealing steps as carbide based contacts. The contact printing process is performed within a few minutes in ambient conditions. The novel printed contact method enables contact deposition without expensive vacuum equipment; furthermore, contact patterning can be designed and performed instantaneously, without design and fabrication of expensive photolithography masks and without the need for clean room facilities.

Highly resistive undoped p-type gallium oxide samples were subjected to cumulative proton irradiation with energies ranging from 25 to 70 keV and doses in the 1.6 × 1014–3.6 × 1014 cm−2 range. Proton irradiation resulted in up to a factor of 2 reduction of minority electron diffusion length in the samples for temperatures between ∼ 300 and 400 K. Electron injection into the samples under test using a scanning electron microscope beam leads to pronounced elongation of diffusion length beyond the pre-irradiation values, thus demonstrating stable (days after injection) recovery of adverse radiation impact on minority carrier transport. The activation energy of 91 meV estimated from the temperature dependent diffusion length vs electron injection duration experiments is likely related to the local potential barrier height for native defects associated with the phenomenon of interest.

In this work, we evaluate a novel jetink printing method with silver nano particle ink for fabrication of metal–semiconductor–metal structures based on Cd 1 − x Zn x Te for gamma-ray detector applications. Contact printing technology offers several advantages in terms of infrastructure and prototype cycle time. For comparison and reference similar samples with thermally evaporated silver and indium contacts were processed and analyzed. The MSM devices were investigated by various static and dynamic characterization methods including current–voltage (I–V), power spectral density of noise (PSD), transient current technique (TCT), and gamma-spectroscopy. In addition, impact of thermal annealing up to 200 °C was studied. The devices with the printed contacts exhibit comparable performance with the reference benchmark detectors. Preliminary study indicate that the printed contacts are immune to moderate thermal treatment, and moreover, show signs of improvement with annealing.

Cd1-xMnxTe is a promising material that has been introduced as a potential successor for the popular Cd1-xZnxTe material in the field of room-temperature gamma-ray detection. In this work, we demonstrate a planar array on a 5 mm thick Cd1-xMnxTe detector with spectroscopy performance similar to that of industrial Cd1-xZnxTe devices of the same thickness. The uniformity of Cd1-xMnxTe was theoretically predicted and experimentally demonstrated to be superior to that of Cd1-xZnxTe due to a better solubility of manganese. The main challenge until now was to exhibit large area, thick Cd1-xMnxTe arrays with satisfactory spectroscopy performance. In this Letter, static as well as spectroscopic properties of thick Cd1-xMnxTe devices are compared to those of conventional Cd1-xZnxTe detectors, grown by the same technology (horizontal Bridgman).

In this study we compared three diamond substrate grades: polycrystalline, optical grade single crystal, and electronic grade single crystal for detector application. Beside the bulk type, the choice of contact material, pre-treatment, and sputtering process details have shown to alter significantly the diamond detector performance. Characterization of diamond substrate permittivity and losses indicate grade and crystallinity related, characteristic differences for frequencies in 1 kHz–1 MHz range. Substantial grade related variations were also observed in surface electrostatic characterization performed by contact potential difference (CPD) mode of an atomic force microscope. Study of conductivity variations with temperature reveal that bulk trap energy levels are also dependent on the crystal grade.

Surface passivation in detectors is designed to improve the performance and stabilize the characteristics over time and ambient. In Cd 1−x Zn x Te radiation detectors, oxidation by plasma is a well-known passivation method, but its physics is not fully understood. This study focuses on the macroscopic and microscopic effects of plasma treatment. It is shown that plasma oxidation before contact deposition causes a considerable decrease in the device’s dark current and that this reduction is in a strong correlation with the lowering of the surface potential. X-ray photoelectron spectroscopy (XPS) measurements revealed a significant increase in TeO 2 fraction on the plasma-treated surface. The main conclusion of this study is that the plasma-related effects are not confined to the surface layer. Furthermore, the deep penetration of plasma treatment byproduct takes place over long time periods, which may lead to variations in device characteristics. The effects were corroborated by several characterization techniques. DC current–voltage measurements combined with layer-by-layer stripping indicate plasma signature deep into the bulk. Transient current technique (TCT) results demonstrate that the plasma treatment affects the electric field distribution millimeters into the bulk. Photo-induced current transient spectroscopy (PICTS) and thermoelectric emission spectroscopy (TEES) measurements reveal plasma-related traps in-depth of the CdZnTe crystals.

This paper presents analyses of polarization effects in ohmic and junction sensors by detailed three-dimensional modeling. It is shown for the example of compensated ${{\rm Cd}_{1 - {\rm x}}}{{\rm Zn}_{\rm x}}{\rm Te}$ detectors that the compensating deep-levels can be also responsible for both bias-induced and ionization-induced polarization phenomena. The capture cross sections of the deep levels are shown to play a dominant role in the formation of the polarizing space charge. Calculations show that bias-induced polarization is not expected to occur in ohmic MSM structures as long as the mobility remains constant. Ionization-induced polarization is expected to occur equally in ohmic and Schottky devices, but the sign and the distribution of the polarizing space charge depend on deep level cross sections.

This article focuses on the impact of polishing of CdZnTe crystals on the electrical properties of the resulting detectors. It is shown that the standard rough polishing of the crispy crystals by 5 μ m size alumina powder increases the effective surface area by a factor 3 and introduces in-depth defects. The defects affect the static current–voltage dependence and were also observed by crystallographic characterization. The depth of the defected layer was determined to be 20– 25 μ m by a consecutive stripping of surface layers. The crystallographic results, indicating structural imperfections, fully correspond to the electrical characteristics. The results are explained by limited CdZnTe/In contact recombination velocity and enhanced generation–recombination velocity of the defected (polycrystalline) layer. The hypothesis was corroborated by a finite element computation.

GaN is a robust and stable wide bandgap material with a bandgap of 3.4eV. It is extensively studied for applications in light emitting diodes, high-power electronics and optoelectronics. Radiation hardness is another attractive attribute of GaN due to its atomic structure and wide bandgap with possible electronic applications in lower Earth satellite orbits. Like other wide bandgap materials, GaN also suffers from short diffusion length, which has been earlier reported to be ~100-300 nm. Minority carrier transport properties such as carrier lifetime (τ) and diffusion length (L) are of vital importance in bipolar and minority-carrier devices. Previously, it was shown that low energy electron beam irradiation (up to 30 keV) for extended duration (up to 3000 seconds) can be used to enhance L by a factor of 4 at room temperature in other wide bandgap materials like p-ZnO and β-Ga2O3 [1-3] . It was suggested that the increase in L is associated with trapping of non-equilibrium electrons on meta-stable trap levels acting as recombination centers. Dependence of L on τ is given by the relation L=√(Dτ) , where D is the diffusion coefficient. Therefore, removal of recombination centers, due to electron trapping on them would, in turn, increase τ, and hence L. Another effect of low energy electron irradiation, apart from increasing τ, is manifested in decrease in the continuous CL intensity. In this work, unintentionally doped GaN was studied using Electron Beam-Induced Current (EBIC), Continuous (CL) and Time-Resolved Cathodoluminescence (TRCL) techniques to independently measure L and τ, and verify the validity of the proposed model[4]. Measurements were conducted on GaN Schottky rectifiers (~400 µm GaN layer grown by Halide Vapor Phase Epitaxy on sapphire) with electron concentration of ~1x1017 cm-3. L was found to increase from 306 nm to 347 nm for a maximum electron injected charge density of 117.5 nC/µm3 and saturating at 60 nC/µm3. For the same maximum and saturation injected charge densities, τ was found to increase from 77 ps to 101 ps. Fig. 1a shows a plot of TRCL streaks for bare region and the region with maximum injected charge density. Slower decay of streak signal indicates longer lifetime in the electron-injected region. Fig. 1b depicts the plot of L and √τ as a function of injected charge density at room temperature. A clear linear dependence (L=√(Dτ)) is seen till saturation charge density of ~60 nC/µm3. Furthermore, mobility values (from the diffusion coefficient) for various charge injection doses were obtained for the ultrafast regime of TRCL spectroscopy. References: L. Chernyak, A. Osinsky, and A. Schulte, Solid State Electron. 45(9), 1687 (2001). O. Lopatiuk-Tirpak, L. Chernyak, F. X. Xiu, J. L. Liu, S. Jang, F. Ren, S. J. Pearton, K. Gartsman, Y. Feldman, A. Osinsky, and P. Chow, J. Appl. Phys. 100(8), 086101 (2006). S. Modak, L. Chernyak, S. Khodorov, I. Lubomirsky, J. Yang, F. Ren, and S. J. Pearton, ECS J. Solid State Sci.Technol. 8(7), Q3050 (2019). S. Modak, L. Chernyak, M. Xian, F. Ren, S.J. Pearton, S. Khodorov, I. Lubomirsky, A. Ruzin, and Z. Dashevsky, J. Appl. Phys. 128(8), 085702 (2020). Figure Caption: Fig. 1 (a) Recorded streak signal for the bare region (τ=77 ps) and electron injected region with maximum dose (τ=101 ps). Lifetime is higher for the injected region with lower decay rate. Inset shows photon counts in a raw streak image. (b) Linear dependence between measured L (left) and √τ (right) as a function of injected charge density. Both parameters saturate at ~60 nC/µm3 and further charge deposition produces no effect. Figure 1

The impact of electron injection on minority carrier (hole) diffusion length and lifetime at variable temperatures was studied using electron beam-induced current, continuous, and time-resolved cathodoluminescence techniques. The hole diffusion length increased from 306 nm to 347 nm with an electron injection charge density up to 117.5 nC/μm3, corresponding to the lifetime changing from 77 ps to 101 ps. Elongation of the diffusion length was attributed to the increase in the non-equilibrium carrier lifetime, which was determined using ultrafast time-resolved cathodoluminescence and related to non-equilibrium carrier trapping on gallium vacancy levels in the GaN forbidden gap.

In the last decade Cd 1−x Mn x Te (CMT) has been considered as a very promising material for room-temperature radiation detectors. Manganese has a better segregation compared to that of zinc in Cd 1−x Zn x Te (CZT), in addition MnTe has a wider bandgap than ZnTe enabling broader band “tunability” in the ternary alloy . In this work, we provided a general comparison of these materials grown in similar conditions, focusing, on material composition uniformity and electric properties under dark conditions. Results of material uniformity analyses clearly confirm better homogeneity of CMT crystals, both laterally and in-depth. In addition to material studies, the current–voltage (I–V) characteristics of both In/CZT/In and In/CMT/In metal–semiconductor–metal (MSM) structures were characterized. The results revealed significantly lower variations in contact-to-contact currents in CMT compared to that in CZT (10% and 25% variance, respectively). The temperature behavior of I–V in In/CZT/In and In/CMT/In devices was investigated with calculation of dark current activation energy. The activation energies of dark current mechanisms in CMT were found to be similar at both polarities, whereas there was a 0.2 eV polarity-related difference in CZT based structures.

In this work, we focused on studying the impact of different surface treatments for Cd 1−x Zn x Te (CZT) and Cd 1−x Mn x Te (CMT) samples with indium evaporated contacts, specifically: mechanical polishing (MP), mechano-chemical polishing (MCP) and mechanical polishing followed by the chemical etching (MP+Et). We examined 3 samples in each group of Cd 0.85 Zn 0.15 Te and 3 samples in each group of Cd 0.93 Mn 0.07 Te. Devices were consecutively characterized by current–voltage dependence (I–V). The measured characteristics revealed that for CMT both surface treatments, including chemical exposure to bromine ethylene glycol in MCP or bromine methanol in etching, resulted in linear I–V characteristic consistent with the bulk resistance. At the same time, chemical treatments of CZT samples dramatically increased their current densities (up to several μ A/cm2).

Current increase due to edge effect in ohmic contacts was calculated by finite-element software in three-dimensional devices. The emphasis in this study is on semi-intrinsic (SI) and compensated high resistivity semiconductors. It was found that the enhanced electric field around the contact edges may cause about twofold increase in the total contact current. For contact radii larger than the device thickness and nano scale contacts the impact is considerably reduced. In nanoscale contacts the edge effect does not control the electric field under the entire contact, but rather decreases. The introduction of velocity saturation model has a limited impact, and only in compensated semiconductors.

We report the effect of extended duration electron beam exposure on the minority carrier transport properties of 10 MeV proton irradiated (fluence ∼1014 cm−2) Si-doped β-Ga2O3 Schottky rectifiers. The diffusion length (L) of minority carriers is found to decrease with temperature from 330 nm at 21 °C to 289 nm at 120 °C, with an activation energy of ∼26 meV. This energy corresponds to the presence of shallow Si trap-levels. Extended duration electron beam exposure enhances L from 330 nm to 726 nm at room temperature. The rate of increase for L is lower with increased temperature, with an activation energy of 43 meV. Finally, a brief comparison of the effect of electron injection on proton irradiated, alpha-particle irradiated and a reference Si-doped β-Ga2O3 Schottky rectifiers is presented.

This paper tackles the issue of downscaling of an ohmic contact from the infinite approximation to nanometer dimensions. Using the finite-element simulation program, it is shown that small-size ohmic contacts on a wide-bandgap semiconductor exhibit nonlinear current-voltage dependence in case velocity saturation is introduced. Furthermore, the dependence becomes asymmetrical around zero bias. In addition, it is shown that in small-size contacts, a nonlocal tunneling is bound to occur even in pure ohmic contacts. This may explain the absence of linear I-V curves in the reported experiments with nanometer-scale contacts.

Understanding complex correlations between the macroscopic device performance (largely dependent on the character of the metal–semiconductor contact) and the metallurgy of contact formation on the atomic level in cadmium zinc telluride (CdZnTe) radiation detectors remains a formidable challenge. In this work, an effort towards bridging that macro–nano knowledge gap is made by conducting a series of controlled experiments aimed at correlating electrical properties of the In contact to n-type CdZnTe(110) surface with the step-by-step process of contact formation. This can only be achieved by using high spatial resolution techniques, capable of conducting highly localized measurements on the nano- and sub-nanoscale, such as scanning probe microscopy. Scanning tunneling microscopy is used in situ to monitor the behavior of various In atom coverages on an atomically flat and ordered CdZnTe surface under well-controlled molecular beam epitaxial conditions in ultra-high vacuum. Electrical derivatives of atomic force microscopy are used to measure the electrical contact properties, such as contact potential difference and spreading resistance in torsion resonance tunneling mode. It is concluded that In atoms preferentially reacted with Te atomic-rows already at room temperature, forming nanometric patches of indium–telluride Schottky-type contacts. The methods developed in this study, in terms of both nanocontact fabrication and characterization (especially in terms of electrical properties) should benefit basic and applied research of any metal–semiconductor system.

Various sputter–anneal treatments in-vacuum were applied to cleaved Cd 0.9 Zn 0.1 Te(1 1 0) surfaces. The morphology, stoichiometry, orientation and the atomic structures were investigated on “as-cleaved” as well as on sputter–annealed surfaces. The studies were conducted by in-situ scanning probe microscopy, in scanning tunneling and atomic force modes, low- and reflection high-energy electron diffraction, and Auger electron spectroscopy. Additional ex-situ investigation methods, such as electron back scattered diffraction and energy dispersive spectroscopy in a scanning electron microscope were applied. Such treatments resulted in a clean, unreconstructed (1 × 1) surface, which was structurally ordered, even if somewhat rough on the atomic scale. The optimal treatment found in this work consists of several cycles of 1.0 keV sputtering followed by 2 h annealing at 180 °C, despite some compositional changes, most notably Cd enrichment at the surface. The morphology of the surface was shown to depend on the sputtering energy, unlike the (1 × 1) atomic structure.

Electrical properties of irradiated p-type, PIN, Si 1− x Ge x ( x = 0.028 ) devices were compared to the similar non-irradiated devices, which were fabricated on the same wafer. Differences in effective doping concentration, current generation mechanisms and the process activation energies were observed. Effective doping decrease with increasing irradiation fluence was attributed to the acceptor removal process. Material resistivity was measured, for both irradiated and non-irradiated samples, showing radiation-induced increase in material resistivity, which correlated with the acceptor removal hypothesis. Changes in current generation mechanism were attributed to the radiation-induced generation-recombination centers creation. Dominance of the Space Charge Region (SCR) current generation mechanism was assumed and confirmed for both irradiated and non-irradiated devices. The activation energy of the generation process was shown to be near the theoretical mid-gap. Deep level traps were examined using a current Deep Level Transient Spectroscopy (DLTS) technique. A trap level, with activation energy of 0.43 eV, was detected in both irradiated and non-irradiated samples. The irradiated samples exhibited two additional trap levels with activation energies of 0.12 and 0.27 eV, the levels were attributed to corresponding dislocation types. The 0.12 eV level attribution to B I 0 / + trap correlated with the radiation-induced boron removal hypothesis.

The superior performance of certain polycrystalline (PX) solar cells compared to that of corresponding single-crystal ones has been an enigma until recently. Conventional knowledge predicted that grain boundaries serve as traps and recombination centers for the photogenerated carriers, which should decrease cell performance. To understand if cell performance is limited by grain bulk, grain surface, and/or grain boundaries (GBs), we performed high-resolution mapping of electronic properties of single GBs and grain surfaces in PX p-CdTe/n-CdS solar cells. Combining results from scanning electron and scanning probe microscopies, viz., capacitance, Kelvin probe, and conductive probe atomic force microscopies, and comparing images taken under varying conditions, allowed elimination of topography-related artifacts and verification of the measured properties. Our experimental results led to several interesting conclusions: 1) current is depleted near GBs, while photocurrents are enhanced along the GB cores; 2) GB cores are inverted, which explains GB core conduction. Conclusions (1) and (2) imply that the regions around the GBs function as an extension of the carrier-collection volume, i.e., they participate actively in the photovoltaic conversion process, while conclusion (2) implies minimal recombination at the GB cores; 3) the surface potential is diminished near the GBs; and 4) the photovoltaic and metallurgical junction in the n-CdS/p-CdTe devices coincide. These conclusions, taken together with gettering of defects and impurities from the bulk into the GBs, explain the good photovoltaic performance of these PX cells (at the expense of some voltage loss, as is indeed observed). We show that these CdTe GB features are induced by the CdCl 2 heat treatment used to optimize these cells in the production process.

Morphology and conductivity (σ) of the non-tarnishing electroless Ag(W) films for interconnect were studied as a function of thickness (d) by Atomic Force Microscopy (AFM) and Tunneling Atomic Force Microscopy methods. For d ≤ 100 nm the conductivity dependence on thickness can be modeled as percolation of the electrical transport while for thicker d > 100 nm layers it was independent on d (σ = σ0). A simple electrical circuit model that described the experimental dependence σ(d) for both thin and thick layers was proposed. The AFM study has shown that the small network changes in the film morphology, due to vacuum annealing cause the significant (few orders of magnitude) improvements in electrical conductivity. Although, near bulk conductivity was achieved for the thicker sample, vacuum annealing was not sufficient to achieve such conductivity for very thin Ag(W) films.

In this study we present computer simulations and experimental results of electric field distribution inside silicon PIN devices before and after the introduction of radiation-induced damage. The investigated devices were irradiated with 24 GeV/ c protons and characterized using atomic force microscopy based methods. The topic of “type inversion” of irradiated silicon devices is simulated using commercial TCAD software. The results of surface potential difference measured by atomic force microscopy on non-irradiated devices are in good agreement with the simulated results for low interface charge case.

Results of p+−i−n+-like (PIN) devices fabricated with a single crystal, Czochralski grown silicon germanium are reported. It is shown that the material has a high potential for x-ray detector applications. The devices were characterized under dark conditions as well as under infrared laser illumination and gamma-ray photons. The Si1−xGex semiconductor contains ∼5% (atomic concentration) of germanium and has a measured resistivity of ∼100 Ω cm. The devices show diode behavior with frequency and temperature independent capacitance–voltage characteristics. The deep level transient spectroscopy results reveal a substantial concentration of trapping centers, some possibly related to the boron-vacancy complex. The devices were operated with x- and gamma-ray photons in spectroscopy mode at low temperatures and the results clearly show the beneficial effect of the added germanium on the absorption coefficient, which increased by a factor 2–3 compared to silicon. The results of the transient current characterizati...

Thermal activation energies of bulk and interface-generated currents for high-resistivity silicon detectors are presented and compared. In non-irradiated devices it is shown that the bulk generation activation energy is strongly dependent on the processing technology, with no significant influence of material resistivity. The activation energies of the interface-generated currents on the other hand are shown to be process, resistivity, and orientation independent. The concentration cross-section products of both bulk and interface-generation centers are shown to be process dependent. Samples with resistivity range of 2–28 kΩ cm and orientations 〈1 1 1〉 and 〈1 0 0〉 were used. The activation energies of the current generating centers of proton-irradiated samples are presented.

This study presents a comparison of dark characteristics of PIN (p + –semi-intrinsic–n + ) structures fabricated with several technologies on high-resistivity silicon wafers. The study considers the temperature dependence of the various current mechanisms. Significant and consistent differences were observed in the reverse current–voltage characteristics of the various samples, while capacitance–voltage profiles remain similar, indicating no process related variations in the effective space charge distribution. The results show that the activation energy of the reverse current in samples processed by various technologies differs, which may indicate that the Shockley Read Hall generation occurs through centers at different energies. Specifically, in some samples the reverse currents are dominated by generation–recombination centers located ∼0.2 eV from the mid-gap, while in other samples the currents are dominated by the ‘standard’ mid-gap generation–recombination centers. The difference is shown to be mostly technology related with little dependence on the starting material. To emphasize the comparative nature of this study, similar wafers and masks were used in all the reported technologies.

This article presents the calculated current–voltage characteristics of symmetric Metal–Semiconductor–Metal configurations for Schottky, Ohmic, and injecting-Ohmic contacts on high resistivity CdTe. The results clearly demonstrate that in the wide band-gap, semi-insulating semiconductors, such as high resistivity CdTe, the linearity of the I–V curves cannot be considered a proof of the ohmicity of the contacts. It is shown that the linear I–V curves are expected for a wide range of contact barriers. Furthermore, the slope of these linear curves is governed by the barrier height, rather than the bulk doping concentration. Therefore the deduction of bulk's resistivity from the I–V curves may be false.

This is a report of preliminary results of Atomic Force Microscopy and Kelvin Probe Force Microscopy measurements performed on high-resistivity silicon particle detectors. All the measured devices were PIN structures. The measurements were performed on cleaved surfaces of non-irradiated as well as irradiated devices. The results indicate that the electric field under the junction contact is non-uniform at thermal equilibrium. The results also show a drastic variation in Contact Potential Difference after irradiation.

In this study lithium atoms were diffused into single crystal, Czochralski grown silicon–germanium. It is shown that the diffusion coefficient has a considerable dependence on germanium concentration. In addition the surface concentration of lithium in silicon–germanium is significantly higher than the values reported for float zone grown pure silicon crystals. The study compares direct and indirect characterization methods used to determine the lithium profile in silicon–germanium. The Si 1− x Ge x semiconductors used in this study contain 2.6% and 5.4% (atomic concentration) of germanium and have measured resistivities of 100–200 Ω cm.

Radiation hardness results of silicon detectors fabricated on bulk float-zone (FZ) material enriched either by carbon or by oxygen are reported. The detectors were irradiated by nuclear reactor neutrons, by 24 GeV/ c protons and by 192 MeV pions. It has been shown that the leakage current increases with fluence equally in all silicon detectors under study, regardless of the oxygen or carbon concentrations. The variations in the space-charge density were found to be very different in the oxygen and carbon-enriched materials in the case of irradiation by charged particles, yet rather similar in case of neutrons.

The current status of the research carried out by the RD-48 (ROSE) Collaboration is presented. Results of macroscopic and microscopic experimental studies are described as well as the progress in modeling. Detectors with improved radiation hardness were achieved by oxygen enrichment of silicon. The oxygen concentration required to obtain the improvement is ∼1017 cm−3.

Radiation produces lattice damage in silicon by displacing the atoms from their original positions and thereby generating the corresponding defects. As a result, new states are created in the semiconductorforbidden band gap, negatively affecting the electrical performance of the devices. Endurance to radiation can be improved by having a high oxygen concentration in the silicon. For detector fabrication, high resistivity silicon is also needed, thus float zone wafers are preferred; however, this kind of material exhibits a low oxygen concentration. Although different ways to incorporate oxygen in float zone silicon have been proposed, all of them imply modifications during the ingot growth. Thermal diffusion from SiO 2 layers on polished wafers is an interesting alternative to improve their oxygen content. Different thermal processes aimed at obtaining oxygen enriched silicon for the fabrication of radiation hard detectors have been tested. Attention has also been paid to carbon introduction during processing since, high concentrations of this element has been proved deleterious.

The tolerance of silicon detectors to hadron irradiation can be improved by the introduction of a high concentration of oxygen into the starting material. High-resistivity Floating-Zone (FZ) silicon is required for detectors used in particle physics applications. A significantly high oxygen concentration (>10 17 atoms cm −3 ) cannot readily be achieved during the FZ silicon refinement. The diffusion of oxygen at elevated temperatures from a SiO 2 layer grown on both sides of a silicon wafer is a simple and effective technique to achieve high and uniform concentrations of oxygen throughout the bulk of a 300 μm thick silicon wafer.

Recent results on the radiation hardness of silicon detectors fabricated on epitaxial and float zone bulk silicon enriched by various impurities, such as carbon, oxygen, tin and platinum are reported. A new methodology of measurements of electrical properties of the devices has been utilized in the experiment. It has been shown that in the case of irradiation by protons, oxygen enriched silicon has better radiation hardness than standard float zone silicon. The carbon enriched silicon detectors, on the other hand, exhibited significantly inferior radiation hardness compared to standard detectors. This study shows for the first time, a violation of the widely used narmalization technique of the various particle irradiations by NIEL coefficients. The study has been carried out in the framework of the RD48 (ROSE) collaboration, which studies the radiation hardening of silicon detectors.

Detectors for high-energy particles sustain a substantial amount of structural defects induced by the particles during the operation period. Some of the defects have been found to be electrically active, degrading the detector's performance. Understanding the mechanisms of the electrical activities and learning to suppress their influence are essential if long “lifetime” detectors are required. This work reports about radiation hardness of silicon P-I-N devices fabricated from oxygen enriched high resistivity material. The high and nearly uniform concentration of oxygen in Float Zone silicon has been achieved by diffusion of oxygen from SiO 2 layers.

Systematic measurements of dark noise spectra of CdZnTe x- and γ-ray spectrometers, correlated with the dc I-V characteristics and detector technology, are reported. The dark noise of two innovative CdZnTe spectrometer configurations are studied: metal-semiconductor-metal (MSM) resistive detectors with three terminals as well as heterostructure PIN detectors with thermally evaporated n+ CdS and p+ ZnTe contacts, which are fabricated on high pressure Bridgman CdZnTe (Zn=10%) crystals. The two innovative CdZnTe spectrometer configurations presented here exhibit very low dark (leakage) current. By reducing the dc value of the dark (leakage) current below 1 nA, shot noise becomes the dominant noise mechanism and the contribution of 1/f noise becomes negligible. The use of non-injecting contacts (evaporated gold) for the MSM detectors and the operation of the PIN detector in the reverse bias mode prevent generation-recombination noise which becomes dominant with injecting contacts (for example MSM detectors with evaporated indium and titanium contacts) or when operating the PIN detector in the forward bias mode. Surface leakage is reduced by applying surface passivation but is eliminated only by using the three terminal MSM configuration which exhibits simple shot noise instead of the suppressed shot noise observed in the two terminal MSM spectrometers. The noise measurements are useful for optimizing detector technology.

The effect on energy resolution of dark noise currents experimentally observed in state of the art CdZnTe spectrometers is quantitatively analyzed. Expressions for the energy resolution determined by shot noise, generation-recombination noise, and 1/f noise are discussed in turn and are compared with the linewidth due to incomplete charge collection. The effect of the dark noise upon energy resolution is directly measured by monitoring the broadening of peaks provided by a pulser, due to biased, non-irradiated CdZnTe spectrometers that are connected in parallel. The noise characteristics of the spectrometers under study were measured separately and their dominant noise mechanism is characterized. In this manner, the effect on resolution of the dark noise is correlated not only with the dc value of the dark current but also with the noise spectral behavior which is determined by the noise mechanism.

The basic understanding as well as the technology of wide band-gap semiconductors in II–VI group in general; and in CdZnTe in particular, are still at an early stage of investigation and are not established. In this research new models were developed to describe the charge collection in semiconductor spectrometers for X- and γ-rays as well as for particles. For the first time a model was developed to describe analytically or numerically the variance in the charge collection efficiency as a function of energy. A methodology has been developed, allowing to evaluate electrical properties of the semiconductor based on spectroscopy measurements. The issue of passivation for wide band-gap CdZnTe spectrometers has been found to be of great importance and we report it for the first time. We have shown that the leakage current in resistive CdZnTe spectrometers is mainly a surface effect. For the first time the noise power spectral density of the spectrometers was characterized and correlated to the spectrometer technology including contacts, crystal growth technique and passivation.

A novel methodology is presented for extraction of the semiconductor electron and hole mobility-lifetime products by x- and γ-ray spectroscopy analysis. The methodology is based on the analysis of spectroscopy results, namely the maximal and the average charge collection efficiencies as discussed below, for different photon energies at various bias voltages. The methodology enables the evaluation of both electron and hole mobility-lifetime products without the need to use α particle measurements. The evaluation is carried out by a single parameter fitting of the models analyzed in this study. Mobility-lifetime products for CdZnTe substrates grown by Bridgman and high pressure Bridgman (HPB) methods are reported and compared. Typical values of μe⋅τe=2⋅10−3 and μh⋅τh=1⋅10−5 cm2/V are extracted for HPB grown CdZnTe with 10% Zn concentration. Values of μe⋅τe=9⋅10−4 and μh⋅τh=1⋅10−7 cm2/V are obtained for CdZnTe with 10% Zn concentration grown by the Bridgman method.

Charge collection efficiency and the variance in the collected charge of semiconductor spectrometers are modeled. The model is based on a statistical approach and the extended Ramo theorem. The model yields an expression for variance in charge collection efficiency as a function of photon energy, bias voltage, and semiconductor parameters. These calculations as a function of absorption depth are particularly important in semiconductors with high atomic numbers, such as CdZnTe, since in these materials a uniform absorption cannot be assumed for a wide range of energies. Three different spectrometer configurations were considered: resistive, partially depleted Schottky barrier, and fully depleted Schottky barrier. An analytical model for the resistive configuration is presented and the results are compared to numerically obtained results of the Schottky configuration.

This study characterizes, for the first time, contacts to CdZnTe radiation detectors by measuring the dark noise spectra as a function of the applied bias. The noise currents are correlated with the dc dark current-voltage characteristics of CdZnTe x-ray and gamma-ray detectors. In order to identify and separate the role of the contacts in the overall performance, the measured noise phenomena is correlated with detector configuration and contact design as well as the growth method of the CdZnTe crystals, contact technology, and passivation. Several contact technologies (electroless gold, and a number of evaporated metallic contacts including gold, indium, zinc, titanium, aluminum, and platinum contacts) are compared. Contacts to CdZnTe crystals grown by high pressure Bridgman are compared with contacts to CdZnTe crystals grown by modified Bridgman. Contacts of resistive detectors as well as of Schottky detectors are reported. Large area symmetric contacts are compared with small area pixelized contacts. The role of the metallization used for contacts, the role of surface effects and passivation, and the role of contact design are discussed.

The charge collection efficiency of CdZnTe radiation detectors with two different configurations: aSchottky diode detector and aresistive detector are compared. The average charge collection efficiencies for three different directions of irradiation (negative electrode, positive electrode and perpendicular to the electric field) are calculated. The mobility-lifetime product of the CdZnTe substrates is evaluated from the dependence of the measured spectra upon detector bias voltage. The measurement of the average charge collection efficiency is based on monitoring the shift of the peak channel with bias voltage in an experimental setup which is well calibrated. Two types of radiation are used:gamma photons from several radioactive sources andalpha particles from an241Am source. The models for the evaluation of mobility-lifetime product from the measured data for the two types of detector configurations as well as for the two types of radiation sources, are compared and discussed. The CdZnTe (Zn = 10%) substrates under study are obtained commercially and are grown by the high pressure Bridgeman method. The mobility-lifetime products and specific resistivity of the two types of detectors are evaluated and compared. A lower resistivity material has a narrower depletion region and behaves like a thinner detector thus exhibiting better collection efficiencies. Therefore, medium resistivity material which is completely inadequate for resistive detectors can still yield high performance Schottky detectors. The preferred direction of irradiation, i.e. from the negative electrode, is possible only in the case of n-type material which is reverse biased by negative voltages applied to the Schottky gate. The mobility-lifetime products that are derived on both the resistive detector (with specific resistivity of ≈1.1010 ω.cm) and the Schottky diode (with specific resistivity of ≈1.106 Ω.cm) are μnτn ≈-4.10−4 cm2V−1 and μpτp≅ 8.10−5 cm2V−1.

In this paper, we report on the design, fabrication, and performance of the first CdZnTe Schottky photodiode arrays for radiation detection. High pressure Bridgman-grown CdZnTe substrates with bulk resistivities in the range 108 to 1010 ohm-cm were used. CdZnTe Schottky photodiodes were formed with In and Ti/Au contacts. Diode arrays with pixel sizes from 1000 × 1000 (im to 100 × 100 urn were fabricated. The diode’s I-V characteristics exhibited low leakage current and high bulk resistivity; leakage current decreased as diode pixel size was reduced. Response of these detector arrays to high energy photons was uniform and their energy resolution improved with smaller pixel size.

In this study we present the dark properties of ‘pin’ devices fabricated with Czochralski grown Si 0.95 Ge 0.05 bulk single crystals. The growth of such material is most challenging because of the constitutional supercooling effect. The potential advantages of Si 1− x Ge x to be used for X- and gamma-ray detection applications are overviewed. At room temperature the generation current in the devices is too high for spectroscopy applications, but enables transient current technique (TCT) measurements. The current however drops significantly with moderate cooling. The effective majority carrier concentration is shown to be ∼2×10 14 cm −3 , and hole mobility ∼320 cm 2 /V s.