Transient Current Technique Research Articles

Modelling of gas network transient flows with multiple hydrogen injections and gas composition tracking

Blending hydrogen into the natural gas (NG) network could provide an efficient pathway for decarbonising the NG system through power-to-gas technologies. However, due to the presence of potentially multiple and intermittent hydrogen injection sources, the gas blended throughout the network would be neither homogenous nor at a constant mole fraction. The above features are not captured by the current transient modelling techniques. To bridge this gap, this work presents a transient analysis model that enables the tracking of gas compositions and particularly hydrogen fractions in real-world meshed networks with multiple NG sources, non-pipe elements, and multiple and intermittent hydrogen injection sources. A time-varying compressibility factor is also introduced to account for the variable gas composition across the network. Moreover, numerical techniques are adopted for improving the stability of the Eulerian numerical calculation, and a specific grid size threshold Δxmax is introduced for selecting the stable mesh grid to alleviate convection-dominated oscillations caused by the hydrogen fraction tracking. The case study based on the well-known 20-node Belgian gas network validates the effectiveness of the method in solving practical-scale problems, whereas the unsuitability of steady-state models is also discussed and highlighted. The results clearly demonstrate the effect and importance of introducing variable compressibility factor, hydrogen fraction tracking, and variable gas demand. The impacts of hydrogen blending on pressures and linepack of the network are further investigated.

Parameterization of charge transport process with avalanche multiplication in irradiated Si p-i-n structures at T = 1.9 K

The study is devoted to the treatment of in situ radiation tests results for silicon p-i-n detectors of relativistic protons, which showed the two-stage process of charge transport with avalanche multiplication at a temperature of 1.9 K. The goal of the work is to extract the carrier transport parameters from the experimental data obtained by transient current technique. For that, the impact of a spatial nonuniformity of carrier generation by the laser and spreading of the drifting carrier cloud due to diffusion on the current pulse response formation were considered. The mathematical procedure proposed for the current pulse simulation showed a key contribution of avalanche multiplication in the signal formation and allowed direct estimation of the multiplication factor from the experimental pulses. It is found that this factor only slightly depends on the bias voltage, which suggests the electric field inside the detector to be affected by the space-charge-limited current.

High‐Performance Single‐Crystal Diamond Detector for Accurate Pulse Shape Discrimination Based on Self‐Organizing Map Neural Networks

Herein, a high‐performance single‐crystal diamond (SCD) detector (4.5 × 4.5 ×0.3 mm3) to achieve accurate pulse shape discrimination, which is critical for source tracking in harsh and complex radiation conditions, is demonstrated. Enabled by a deep learning algorithm based on self‐organizing map (SOM) neural networks, and using the transient current technique (TCT) for sampling the detector's response to γ, α, and neutron radiation fields, the SCD detector achieves high recognition accuracy of 97.51%. The SCD detector exhibits a low leakage current of 0.75 pA mm−2 under an electric field of 0.51 V μm−1, and its response to 238Pu α‐rays shows that the charge collection efficiency for electrons and holes is as high as 99.2 and 98.8% respectively, with an energy resolution as low as 1.42%. The results indicate that the high‐performance SCD detector assisted by the machine learning algorithm can effectively distinguish α‐particles and γ‐rays with a potential application in separating the neutron and γ events as well.

Towards GaAs thin-film tracking detectors

Silicon-based tracking detectors have been used in several important applications, such as in cancer therapy using particle beams, and for the discovery of new elementary particles at the Large Hadron Collider at CERN. III-V semiconductor materials are an attractive alternative to silicon for this application, as they have some superior physical properties. They could meet the demands for fast timing detectors allowing time-of-flight measurements with ps resolution while being radiation tolerant and cost-efficient. As a material with a larger density, higher atomic number Z and much higher electron mobility than silicon, GaAs exhibits faster signal collection and a larger signal per μm of sensor thickness. In this work, we report on the fabrication of n-in-n GaAs thin-film devices intended to serve next-generation high-energy particle tracking detectors. Molecular beam epitaxy (MBE) was used to grow high-quality GaAs films with doping levels sufficiently low to achieve full depletion for detectors with an active thickness of 10 μm. The signal collection speed of the detector structures was assessed using the transient current technique (TCT). To elucidate the structural properties of the detector, Kelvin probe force microscopy (KPFM) was used, which confirmed the formation of the junction in the detector and revealed residual doping in the intrinsic layer. Our results suggest that GaAs thin films are suitable candidates to achieve thin and radiation-tolerant tracking detectors.

Printed silver contacts for Cd[formula omitted]Zn[formula omitted]Te radiation detectors

In this work, we evaluate a novel jetink printing method with silver nano particle ink for fabrication of metal–semiconductor–metal structures based on Cd1−xZnxTe 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.

Modeling the impact of defects on the charge collection efficiency of a Cadmium Telluride detector

Cadmium telluride is a favorable material for X-ray detection as it has an outstanding characteristic for room temperature operation. It is a high-Z material with excellent photon radiation absorption properties. However, CdTe single crystals may include a large number of extended crystallographic defects, such as grain boundaries, twins, and tellurium (Te) inclusions, which can have an impact on detector performance. A Technology Computer Aided Design (TCAD) local defect model has been developed to investigate the effects of local defects on charge collection efficiency (CCE). We studied a 1 mm thick Schottky-type CdTe radiation detector with transient-current technique by using a red laser at room temperature. By raster scanning the detector surface we were able to study signal shaping within the bulk, and to locate surface defects by observing their impact on the CCE. In this paper we present our TCAD model with localized defect, and compare the simulation results to TCT measurements. In the model an inclusion with a diameter of 10 μm was assumed. The center of the defect was positioned at 6 μm distance from the surface. We show that the defect has a notable effect on current transients, which in turn affect the CCE of the CdTe detector. The simulated charge collection at the position of the defect decreases by 80 % in comparison to the defect-free case. The simulations show that the defects give a characteristic shape to TCT signal. This can further be used to detect defects in CdTe detectors and to estimate the overall defect density in the material.

Spotlight on the Effect of Electrolyte Composition on the Potential of Maximum Entropy: Supporting Electrolytes Are Not Always Inert.

The influence of electrolyte pH, the presence of alkali metal cations (Na+, K+), and the presence of O2 on the interfacial water structure of polycrystalline gold electrodes has been experimentally studied in detail. The potential of maximum entropy (PME) was determined by the laser‐induced current transient (LICT) technique. Our results demonstrate that increasing the electrolyte pH and introducing O2 shift the PME to more positive potentials. Interestingly, the PME exhibits a higher sensitivity to the pH change in the presence of K+ than Na+. Altering the pH of the K2SO4 solution from 4 to 6 can cause a drastic shift in the PME. These findings reveal that, for example, K2SO4 and Na2SO4 cannot be considered as equal supporting electrolytes: it is not a viable assumption. This can likely be extrapolated to other common “inert” supporting electrolytes. Beyond this, knowledge about the near‐ideal electrolyte composition can be used to optimize electrochemical devices such as electrolyzers, fuel cells, batteries, and supercapacitors.

Charge Transport and Space-Charge Formation in Cd1−xZnxTe1−ySey Radiation Detectors

The electron- and hole-transport properties in cadmium zinc telluride selenide (CZTS) crystals are studied using a laser-induced transient-current technique with pulsed and dc bias. The internal electric field profile and velocity of surface recombination are determined by Monte Carlo simulations of electron and hole transient currents combined with a numerical solution of the drift-diffusion equation coupled with Poisson's equation. Electron and hole drift mobilities of ${\ensuremath{\mu}}_{e}$ = 830 ${\mathrm{cm}}^{2}$/Vs and ${\ensuremath{\mu}}_{h}$ = 40 ${\mathrm{cm}}^{2}$/Vs, respectively, are determined. We also develop a simple technique for evaluating surface recombination directly from measured current waveforms without the need for numerical simulation. The good quality of the prepared detector at pulsed bias, with electron- and hole-mobility-lifetime products of $(\ensuremath{\mu}\ensuremath{\tau}{)}_{e}$ = 1.9 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}3}$ ${\mathrm{cm}}^{2}$/V and $(\ensuremath{\mu}\ensuremath{\tau}{)}_{h}$ = 1.4 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$ ${\mathrm{cm}}^{2}$/V, respectively, are observed. The formation of a positive space charge, originating from hole injection combined with a recombination level, is found. We observe a significant position dependence of the lifetime of electrons and holes in dc bias due to hole injection. The experiment is successfully fitted by a simple model dominated by a single deep recombination level with an energy of ${E}_{t}={E}_{C}\ensuremath{-}0.73\phantom{\rule{0.2em}{0ex}}\mathrm{eV}$; concentration of 7.3 \ifmmode\times\else\texttimes\fi{} ${10}^{11}$ ${\mathrm{cm}}^{\ensuremath{-}3}$; and electron- and hole-capture cross sections of 3.5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}14}\phantom{\rule{0.1em}{0ex}}{\mathrm{cm}}^{2}$ and 6.5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}14}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{2}$, respectively.

Characterization of magnetic Czochralski silicon devices with aluminium oxide field insulator: effect of oxygen precursor on electrical properties and radiation hardness

Aluminium oxide (Al2O3) has been proposed as an alternative to thermal silicon dioxide (SiO2) as field insulator and surface passivation for silicon detectors, where it could substitute p-stop/p-spray insulation implants between pixels due to its negative oxide charge, and enable capacitive coupling of segments by means of its higher dielectric constant.Al2O3 is commonly grown by atomic layer deposition (ALD), which allows the deposition of thin layers with excellent precision.In this work, we report the electrical characterization of single pad detectors (diodes) and MOS capacitors fabricated on magnetic Czochralski silicon substrates and using Al2O3 as field insulator. Devices are studied by capacitance-voltage, current-voltage, and transient current technique measurements. We evaluate the influence of the oxygen precursors in the ALD process, as well as the effect of gamma irradiation, on the properties of these devices. We observe that leakage currents in diodes before the onset of breakdown are low for all studied ALD processes. Charge collection as measured by transient current technique (TCT) is also independent of the choice of oxygen precursor. The Al2O3 films deposited with O3 possess a higher negative oxide charge than films deposited by H2O, However, in diodes a higher oxide charge is linked to earlier breakdown, as has been predicted by simulation studies. A combination of H2and O3 precursors results in a good compromise between the beneficial properties provided by the respective individual precursors.

Effects of oxygen plasma treatment on Cd[formula omitted]Zn[formula omitted]Te material and devices

Surface passivation in detectors is designed to improve the performance and stabilize the characteristics over time and ambient. In Cd1−xZnxTe 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.

In and Al Schottky Contacts Comparison on P-Type Chlorine-Doped CdTe.

The performance of the CdTe radiation detectors heavily relies on the method of contact preparation. A convenient research method addressing this problem is the laser-induced transient current technique. In this paper, we compare the performance of two CdTe crystals which underwent different metallization processes. We showed that appropriately designed Au/Al contacts induce much less bulk polarization than commercial Pt/In electrodes under the same working conditions and can thus provide a convenient alternative to the industry standard. The comparison was based on the monitoring of the time-dependent sensor polarization measuring transient currents excited by above-bandgap laser illumination complemented by the Am 241 gamma spectroscopy. The theoretical analysis of current waveforms and radiation spectra enabled us to determine the charge carrier mobility, mobility-lifetime products of electrons and holes, and temporal and bias dependence of the space charge formation.

Development of a Tabletop Setup for the Transient Current Technique Using Two-Photon Absorption in Silicon Particle Detectors

The transient current technique (TCT) is widely used in the field of silicon particle detector development. So far, only laser wavelengths with a photon energy larger than or similar to the silicon bandgap (single photon absorption) were used. Recently, measurements using two-photon absorption (TPA) for silicon detector testing have been carried out for the first time. Excess carriers are only created at the focal point of the laser beam and thus resolution in all three spatial directions could be achieved. The resolution perpendicular to the incident laser beam could be increased roughly by a factor of 10. First measurements using this new method were performed at the Singular Laser Facility of Universidad del País Vasco (UPV)/Euskal Herriko Unibertzitatea (EHU). Following the initial success of the method, a compact TPA-TCT setup is under development. A first description of the setup and laser system is presented in this article.

Charge carrier properties of single-crystal CVD diamond up to 473 K

The drift behavior of charge carriers, generated by α-particles of a reference 241Am-source, in electronic grade, single crystal chemical vapor deposition (scCVD) diamond was investigated by the transient current technique (TCT) from room temperature up to ≈473K. Furthermore, the α-spectroscopic behavior was analyzed in terms of charge collection and spectroscopic resolution for the same temperature range. All conducted measurements revealed complete charge collection up to the maximum temperature. The electron–hole-pair creation energies were derived from the TCT as well as from the spectroscopic measurements. The herein presented results imply that high temperature α-spectroscopy with diamond-based semiconductor solid state detectors, using presently available scCVD sensor substrates, is feasible at least up to 473K. Only at the highest applied temperature, the conducted TCT measurements showed distorted signal traces, indicating a uniform positive space charge built-up.

AC-coupled n-in-p pixel detectors on MCz silicon with atomic layer deposition (ALD) grown thin film

We report initial characterization of our novel sensor process solutions with AC-coupled n+/p−/p+ pixel detectors made on 150 mm diameter p-type Magnetic Czochralski silicon (MCz-Si) wafers. The pixels were segmented in a 52 × 80 dual column array and designed to be AC capacitive coupled. The resistive coupling between pixels, allowing quality assurance probing prior the flip chip bonding, was realized with thin film metal-nitride resistors fabricated by sputtering deposition. This approach allows us to omit punch-through resistor structures, which reduces the overall process complexity. Moreover, our previous studies have emphasized that applying ALD Aluminum Oxide (Al2O3) field insulator and passivation layer results in negative net oxide charge and thus additional p-spray or p-stop surface current termination structures are not necessary. Our focused application is a radiation-hard ALD AC-coupled pixel detector to be used in future particle physics experiments, such as the High-Luminosity Large Hadron Collider (HL-LHC), as well as photon counting applications. The pixel detectors were tested at Helsinki Institute of Physics (HIP) Detector laboratory and Ruđer Bošković Institute (RBI). We show measurement data of pixel detectors and other test structures. For the TiN resistors surrounding pixels, the resistance values were measured to be about 15kΩ. Data of electrical properties, full depletion voltage and leakage current are shown as well. Our Transient Current Technique (TCT) measurements indicated clear pixel segmentation with excellent homogeneity. For further study, AC-coupled sensors were hybridized to PSI46dig read out chips (ROC) by flip-chip interconnection technique and tested with a radioactive source.

Space charge formation in chromium compensated GaAs radiation detectors

Space charge formation in chromium-compensated GaAs sensors is investigated by the laser-induced transient current technique applying pulsed and DC bias. Formation of non-standard space charge manifested by an appearance of both negatively and positively charged regions in DC biased sensors was revealed during 5 ms after switching bias. Using Monte Carlo simulations of current transients we determined enhanced electron lifetime τ = 150 ns and electron drift mobility μd = 3650 cm2 Vs−1. We developed and successfully applied a theoretical model based on fast hole trapping in the system with spatially variable hole conductivity.

In situ Probing of Mn2O3 Activation toward Oxygen Electroreduction by the Laser-Induced Current Transient Technique

Electrochemical transformation of Mn4+ into Mn3+ in Mn2O3 bixbyite structure is believed to activate this oxygen reduction catalyst for O2 electrosorption. The actual mechanism, however, still rema...

Study of interpad-gap of HPK 3.1 production LGADs with Transient Current Technique

The Phase-2 upgrade of the Large Hadron Collider (LHC) to High-Luminosity LHC (HL-LHC) allows an increase in the operational luminosity value by a factor of 5–7 that will result in delivering 3000 fb−1 or more integrated luminosity. Due to high luminosity, the number of interactions per bunch crossings (pileup) will increase up to a value of 140–200. To cope with high pileup rates, a precision minimum ionising particles (MIPs) timing detector (MTD) with a time resolution of ∼30–40 ps and hermetic coverage up to a pseudo-rapidity of |η|=3 is proposed by the Compact Muon Solenoid (CMS) experiment. An endcap part (1.6<|η|<3) of the MTD, called the endcap timing layer, will be based on low-gain avalanche detector (LGAD) technology. LGADs provide a good timing resolution due to a combination of a fast signal rise time and high signal-to-noise ratio. The performance of the ETL depends on optimising the crucial features of the sensors, namely; gain, signal homogeneity, fill factor, leakage current, uniformity of multiple-pad sensors and long term stability. The paper mainly focuses on the study of the fill factor of LGADs with varying temperature and irradiation at varying proton fluences as these sensors will be operated at low temperatures and are subjected to a high radiation environment.The 3.1 production of LGADs from Hamamatsu Photonics K.K. (HPK) includes 2x2 sensors with different structures, in particular, different values of narrower inactive region widths between the pads, called the no-gain region. In this paper, the term interpad-gap is used instead of no-gain region in order to follow the conventional terminology. These sensors have been designed to study their fill factor, which is the ratio of the area within the active region (gain region) to the total sensor area. A comparative study on the dependence of breakdown voltage with the interpad-gap width for the sensors has been carried out. Using infrared light (as the electron–hole pair creation by IR laser mimics closely to the traversing of MIPs) from the Scanning-Transient Current Technique (Scanning-TCT) set-up shows that the fill factor does not vary significantly with a variation in temperature and irradiation at high proton fluences.

First characterisation of 3D pixel detectors irradiated at extreme fluences

Highly irradiated silicon sensors are currently being investigated within the scenario of the most powerful particle accelerator proposed for the post-LHC era, i.e the Future Circular Collider (FCC), for which radiation tolerance up to a level of 8 × 1017 neq/cm2 is required. In this paper, 3D strip and pad detectors irradiated with neutrons up to a fluence of 3 × 1017 neq/cm2 (1 MeV neutron equivalent) are characterised for the first time: the charge collection measurements with Transient Current Technique (TCT) are carried out at several voltages and the results are reported.

Micrometric laser characterization of a 300μm fully-depleted monolithic active pixel sensor in standard 110 nm CMOS technology

This work presents the characterization of a customized 110 nm CMOS technology fully depleted monolithic active pixel sensor test structure through micrometric laser beam. The test structures, with removed electronics, enabled transient current technique evaluation of the collected charge with two laser wavelengths. High charge collection uniformity and full depletion are proved using front-side illumination in the sensitive areas.

Investigation of nitrogen enriched silicon for particle detectors

This article explores the viability of nitrogen enriched silicon for particle physics application. For that purpose silicon diodes and strip sensors were produced using high resistivity float zone silicon, diffusion oxygenated float zone silicon, nitrogen enriched float zone silicon and magnetic Czochralski silicon. The article features comparative studies using secondary ion mass spectrometry, electrical characterization, edge transient current technique, source and thermally stimulated current spectroscopy measurements on sensors that were irradiated up to a fluence of 1015 neq/cm2. Irradiations were performed with 23 MeV protons at the facilities in Karlsruhe (KIT), with 24 GeV/c protons at CERN (PS-IRRAD) and neutrons at the research reactor in Ljubljana. Secondary ion mass spectrometry measurements give evidence for nitrogen loss after processing, which makes gaining from nitrogen enrichment difficult.