Charge Collection Measurements Research Articles

Characterisation of strip silicon detectors for the ATLAS Phase-II Upgrade with a micro-focused X-ray beam

The planned HL-LHC (High Luminosity LHC) in 2025 is being designed to maximise the physics potential through a sizable increase in the luminosity up to 6·1034 cm−2s−1. A consequence of this increased luminosity is the expected radiation damage at 3000 fb−1 after ten years of operation, requiring the tracking detectors to withstand fluences to over 1·1016 1 MeV neq/cm2. In order to cope with the consequent increased readout rates, a complete re-design of the current ATLAS Inner Detector (ID) is being developed as the Inner Tracker (ITk).Two proposed detectors for the ATLAS strip tracker region of the ITk were characterized at the Diamond Light Source with a 3 μm FWHM 15 keV micro focused X-ray beam. The devices under test were a 320 μm thick silicon stereo (Barrel) ATLAS12 strip mini sensor wire bonded to a 130 nm CMOS binary readout chip (ABC130) and a 320 μm thick full size radial (end-cap) strip sensor - utilizing bi-metal readout layers - wire bonded to 250 nm CMOS binary readout chips (ABCN-25).A resolution better than the inter strip pitch of the 74.5 μm strips was achieved for both detectors. The effect of the p-stop diffusion layers between strips was investigated in detail for the wire bond pad regions.Inter strip charge collection measurements indicate that the effective width of the strip on the silicon sensors is determined by p-stop regions between the strips rather than the strip pitch.

Neutron Energy Reconstruction and Fluence Determination at 27 keV With the LNE-IRSN-MIMAC MicroTPC Recoil Detector

The aim is to characterize the energy distribution of neutron fluence in the energy range 8 keV–5 MeV based on a primary standard: the LNE-IRSN/MIMAC microTPC. The microTPC is a time projection chamber. Time projection chambers are gaseous detectors able to measure charged particles energy and to reconstruct their track. The gas is used as a (n, p) converter in order to detect neutrons down to few keV. The neutron energy is reconstructed event by event thanks to proton scattering angle and proton ionization energy measurements. The scattering angle is deduced from the 3-D track. The proton energy is obtained by charge collection measurements, knowing the ionization quenching factor. The fluence is reconstructed thanks to the detected events number and the simulation of the detector response. The microTPC is a new reliable detector able to measure energy distribution of the neutron fluence without unfolding procedure or prior neutron calibration contrary to usual gaseous counters. The microTPC is characterized at the AMANDE facility, with neutron energies going from 8 keV to 565 keV. This work shows the first direct reconstruction of neutron energy and fluence, simultaneously, at 27.2 keV in a continuous irradiation mode.

Recent technological developments on LGAD and iLGAD detectors for tracking and timing applications

This paper reports the latest technological development on the Low Gain Avalanche Detector (LGAD) and introduces a new architecture of these detectors called inverse-LGAD (iLGAD). Both approaches are based on the standard Avalanche Photo Diodes (APD) concept, commonly used in optical and X-ray detection applications, including an internal multiplication of the charge generated by radiation. The multiplication is inherent to the basic n++–p+–p structure, where the doping profile of the p+ layer is optimized to achieve high field and high impact ionization at the junction.The LGAD structures are optimized for applications such as tracking or timing detectors for high energy physics experiments or medical applications where time resolution lower than 30ps is required. Detailed TCAD device simulations together with the electrical and charge collection measurements are presented through this work.

Charge collection studies in irradiated HV-CMOS particle detectors

Charge collection properties of particle detectors made in HV-CMOS technology were investigated before and after irradiation with reactor neutrons. Two different sensor types were designed and processed in 180 and 350 nm technology by AMS. Edge-TCT and charge collection measurements with electrons from 90Sr source were employed. Diffusion of generated carriers from undepleted substrate contributes significantly to the charge collection before irradiation, while after irradiation the drift contribution prevails as shown by charge measurements at different shaping times. The depleted region at a given bias voltage was found to grow with irradiation in the fluence range of interest for strip detectors at the HL-LHC. This leads to large gains in the measured charge with respect to the one before irradiation. The increase of the depleted region was attributed to removal of effective acceptors. The evolution of depleted region with fluence was investigated and modeled. Initial studies show a small effect of short term annealing on charge collection.

Trapping in proton irradiated p+-n-n+ silicon sensors at fluences anticipated at the HL-LHC outer tracker

The degradation of signal in silicon sensors is studied under conditions expected at the CERN High-Luminosity LHC. 200 μm thick n-type silicon sensors are irradiated with protons of different energies to fluences of up to 3 · 1015 neq/cm2. Pulsed red laser light with a wavelength of 672 nm is used to generate electron-hole pairs in the sensors. The induced signals are used to determine the charge collection efficiencies separately for electrons and holes drifting through the sensor. The effective trapping rates are extracted by comparing the results to simulation. The electric field is simulated using Synopsys device simulation assuming two effective defects. The generation and drift of charge carriers are simulated in an independent simulation based on PixelAV. The effective trapping rates are determined from the measured charge collection efficiencies and the simulated and measured time-resolved current pulses are compared. The effective trapping rates determined for both electrons and holes are about 50% smaller than those obtained using standard extrapolations of studies at low fluences and suggest an improved tracker performance over initial expectations.

A Proposal for the Upgrade of the Muon Drift Tubes Trigger for the CMS Experiment at the HL-LHC

A major upgrade of the readout and trigger electronics of the CMS Drift Tubes muon detector is foreseen in order to allow its efficient operation at the High Luminosity LHC. A proposal for a new L1 Trigger Primitives Generator for this detector is presented, featuring an algorithm operating on the time of charge collection measurements provided by the asynchronous readout of the new TDC system being developed. The algorithm is being designed around the implementation in state-of-the-art FPGA devices of the original development of a Compact Hough Transform (CHT) algorithm combined with a Majority Mean-Timer, to identify both the parent bunch crossing and the muon track parameters. The current state of the design is presented along with the performance requirements, focusing on the future developments.

Charge Photogeneration in Organic Photovoltaics: Role of Hot versus Cold Charge‐Transfer Excitons

The role of excess excitation energy on long‐range charge separation in organic donor/acceptor bulk heterojunctions (BHJs) continues to be unclear. While ultrafast spectroscopy results argue for efficient charge separation through high‐energy charge‐transfer (CT) states within the first picosecond (ps) of excitation, charge collection measurements suggest excess photon energy does not increase the current density in BHJ devices. Here, the population dynamics of charge‐separated polarons upon excitation of high‐energy polymer states and low‐energy interfacial CT states in two polymer/fullerene blends from ps to nanosecond time scales are studied. It is observed that the charge‐separation dynamics do not show significant dependence on excitation energy. These results confirm that excess exciton energy is not necessary for the effective generation of charges.

Radiation effects in Low Gain Avalanche Detectors after hadron irradiations

Novel silicon detectors with charge gain were designed (Low Gain Avalanche Detectors - LGAD) to be used in particle physics experiments, medical and timing applications. They are based on a n++-p+-p structure where appropriate doping of multiplication layer (p^+) is needed to achieve high fields and impact ionization. Several wafers were processed with different junction parameters resulting in gains of up to 16 at high voltages. In order to study radiation hardness of LGAD, which is one of key requirements for future high energy experiments, several sets of diodes were irradiated with reactor neutrons, 192 MeV pions and 800 MeV protons to the equivalent fluences of up to Φeq=1016 cm−2. Transient Current Technique and charge collection measurements with LHC speed electronics were employed to characterize the detectors. It was found that the gain decreases with irradiation, which was attributed to effective acceptor removal in the multiplication layer. Other important aspects of operation of irradiated detectors such as leakage current and noise in the presence of charge multiplication were also investigated.

First measurements of segmented silicon tracking detectors with built-in multiplication layer

Silicon sensors with a built-in multiplication layer, also known as Low Gain Avalanche Detectors (LGAD), are a new technology of potentially radiation hard silicon sensors developed at CNM (Centro Nacional de Microelectrónica) in the framework of the CERN-RD50 collaboration. The concept of the LGAD technology is to produce a tracking sensor with an intrinsic low gain due to charge multiplication in order to enhance the signal-to-noise ratio. This makes LGAD detectors appealing for the high energy physics community which foresees a possible application in harsh radiation environments such as the inner detectors of the experiments at the High Luminosity LHC, where a fluence of 2×1016 neq/cm2 is expected in the innermost pixel layer of the tracker.LGAD pad diodes with a multiplication factor of 10 at 200V before irradiation have been produced and studied but it still has to be proven that this technology can be applied to segmented devices. The electric field profile at the p–n junction plays a crucial role for the multiplication mechanism that could be affected by implantation edge effects in highly segmented devices.In this paper the results of the first measurements of segmented LGAD devices from 285μm thick float zone wafers will be discussed. Time resolved Transient-Current-Technique analysis has been performed on strip devices and charge collection measurements have been carried out on pixel detectors to test the LGAD technology on highly segmented devices.

Long term performance stability of silicon sensors

The HL-LHC investigations on silicon particle sensor performance are carried out with the intention to reproduce the harsh environments foreseen, but usually in individual short measurements. Recently, several groups have observed a decrease in the charge collection of silicon strip sensors after several days, in particular on sensors showing charge multiplication. This phenomenon has been explained with a surface effect, the increase of charge sharing due to the increment of positive charge in the silicon oxide coming from the source used for charge collection measurements. Observing a similar behaviour in other sensors for which we can exclude this surface effect, we propose and investigate alternative explanations, namely trapping related effects (change of polarization) and annealing related effects. Several n-on-p strip sensors, as-processed and irradiated with protons and neutrons up to 5×1015neq/cm2, have been subjected to charge collection efficiency measurements for several days, while parameters like the impedance have been monitored. The probable stressing conditions have been changed in an attempt to recover the collected charge in case of a decrease. The results show that for the investigated sensors the effect of charge sharing induced by a radioactive source is not important, and a main detrimental factor is due to very high voltage, while at lower voltages the performance is stable.

Characterization of an Alternative Diamond Based Microdosimeter Prototype

A diamond microdosimeter prototype with sensitive volumes separated through laser milled trenches is characterized. It is proposed that this design will offer a new and alternative means of creating a diamond based microdosimeter. The device has been characterized by means of ion beam induced charge collection (IBIC) measurements. In this work, it is shown that laser milled trenches have a significant effect upon the lateral confinement of charge collection essentially creating a pixelated diamond detector. Specifics of the results and an update on the current status of the project are presented.

Ion-Induced Charge-Collection Transients in p-Channel AlGaSb/InGaSb Heterojunction Field-Effect Transistors

Ion-induced, time-resolved charge-collection measurements for p-channel AlGaSb/InGaSb field-effect transistors are reported for a range of gate and drain bias conditions. The transient response reveals two distinct contributions: a faster initial response (<;1 ns) followed by a slower, >10 ns, relaxation. The slower contribution depends sensitively on the applied gate bias, is suppressed when the gate is made more positive toward depletion, and is identified with an enhanced source-drain current in which more charge is collected than is deposited by the ion. A model consistent with the experimental measurements suggests that the dynamics of the enhancement processes are associated with the trapping and detrapping dynamics of electrons in the AlGaSb barrier materials.

A charge collection study with dedicated RD50 charge multiplication sensors

This study investigates the charge collection efficiency of silicon strip detectors, produced by MICRON Semiconductor Co., Ltd. within the CERN RD50 collaboration, designed specifically to understand the effect of design parameters on the onset and magnitude of charge multiplication. Charge collection measurements are performed before and after irradiation with a proton fluence of 1×1015 1MeVneq/cm2 (neq/cm2) and neutron fluence ranging from 1–5×1015neq/cm2. Structures on these devices include varying diffusion times and energies for the implantation process, different sensor thicknesses, the use of intermediate biased or floating strips between the readout strips, and several different strip width and pitch geometries. The charge collection for these devices is studied as a function of the bias voltage, looking for indications of charge multiplication. Results are compared to standard float zone 300μm thick silicon strip sensors having a strip width of 25μm and pitch of 80μm.

The Significance of High-Level Carrier Generation Conditions for Charge Collection in Irradiated Devices

Pulsed-laser induced charge-collection measurements in a bulk silicon diode are used to investigate charge collection mechanisms during high-level carrier generation conditions. The recently developed Ambipolar Diffusion with a Cutoff (ADC) model for charge collection is used to emphasize the significance of the carrier densities, electric fields, and potential throughout the entire device in response to high-level carrier generation events. Device-level simulations are used to show that for high carrier density, short-track events, carriers are confined close to the junction by a strong electric field that forms underneath the region of high carrier density. These simulations are used to explain why, for some of the experimental data shown here, almost all of the generated charge can be collected, even if the peak carrier generation is located several micrometers away from the depletion region boundary. The physical framework for charge collection provided by the ADC model is compared to other analytical charge collection models, such as the funnel model, to emphasize that the more general approach it provides makes it well suited to situations where other analytical charge-collection models cannot be directly applied.

Schottky barrier detectors on 4H-SiC n-type epitaxial layer for alpha particles

Schottky barrier detectors have been fabricated on 50μm n-type 4H-SiC epitaxial layers grown on 360μm SiC substrates by depositing ∼10nm 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 241Am alpha source. An energy resolution of ∼2.7% was obtained with a reverse bias of 100V for 5.48MeV 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∼3.5μm for holes was numerically calculated from the CCE vs. bias voltage plot. Rise-time measurements of digitally recorded charge pulses for the 5.48MeV alpha particles showed a presence of two sets of events having different rise-times at a higher bias of 200V. 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.

Charge collection measurements on slim-edge microstrip detectors

We have generated slim edges on manufactured silicon strip detectors by cleaving the non-active edge material and passivating the very smooth edge with a thin coat of silicon oxide. We report a comparison of I-V measurements and charge collection and noise measurements on two identical sensors, one with and one without slim edge treatment. The current voltage measurements of the entire sensor and individual strips indicate that the large current increase due to the treatment is confined to the guard ring, while the strips show essentially no increase in leakage currents. The noise on all strips, including the one adjacent to the slim edge, is not changed by the cut. The signal from a beta source before and after cutting is the same within 4%.

Performance of novel silicon n-in-p planar pixel sensors

The performance of novel n-in-p planar pixel detectors designed for future upgrades of the ATLAS Pixel system is presented. The n-in-p silicon sensors technology is a promising candidate for the pixel upgrade thanks to its radiation hardness and cost effectiveness that allow for enlarging the area instrumented with pixel detectors. The n-in-p modules presented here are composed of pixel sensors produced by CiS connected by bump-bonding to the ATLAS read-out chip FE-I3.The characterization of these devices has been performed before and after irradiation up to a fluence of 5×1015 1MeVneqcm−2. Charge collection measurements carried out with radioactive sources have proven the functioning of this technology up to these particle fluences. First results from beam test data with a 120GeV/c pion beam at the CERN-SPS are also discussed, demonstrating a high tracking efficiency of (98.6±0.3)% and a high collected charge of about 10ke for a device irradiated at the maximum fluence and biased at 1kV.

Performance of n-in-p Pixel Detectors Irradiated at Fluences up to 5x1015 neq/cm2 for the Future ATLAS Upgrades

Abstract We present the results of the characterization of novel n-in-p planar pixel detectors, designed for the future upgrades of the ATLAS pixel system. N-in-p silicon devices are a promising candidate to replace the n-in-n sensors thanks to their radiation hardness and cost effectiveness, that allow for enlarging the area instrumented with pixel detectors. The n-in-p modules presented here are composed of pixel sensors produced by CiS connected by bump-bonding to the ATLAS readout chip FE-I3. The characterization of these devices has been performed with the ATLAS pixel read-out systems, TurboDAQ and USBPIX, before and after irradiation with 25 MeV protons and neutrons up to a fluence of 5x10 15 neq/cm2. The charge collection measurements carried out with radioactive sources have proven the feasibility of employing this kind of detectors up to these particle fluences. The collected charge has been measured to be for any fluence in excess of twice the value of the FE-I3 threshold, tuned to 3200 e. The first results from beam test data with 120 GeV pions at the CERN-SPS are also presented, demonstrating a high tracking efficiency before irradiation and a high collected charge for a device irradiated at 10 15 neq/cm2. This work has been performed within the framework of the RD50 Collaboration.

Accelerated annealing of n+-p strip detectors irradiated with pions

Charge collection measurements with silicon detectors with implanted n-type readout strips in p-type silicon bulk (n+-p) are presented. Detectors were irradiated with 191 MeV pions at the Paul Scherrer Institute (PSI) in Villigen in Switzerland. Signals induced by electrons from 90Sr source were measured with SCT128 chip. Collected charge and detector current were measured after several annealing steps summing up to over 10000 minutes at 60°C. It was observed that irradiation of these detectors with pions results in only ∼ 30% of the increase of Vfd seen after irradiation with neutrons to the same NIEL equivalent fluence. Charge multiplication effects in pion irradiated detectors were seen only after long accelerated annealing time. Both effects are consistent with smaller space-charge introduction rates after irradiation with charged hadrons, characteristic for oxygenated detector material. It was confirmed that, at sufficient bias voltage, reverse annealing after pion irradiation does not represent a problem for application of these detectors in trackers at upgraded LHC.

Update of the annealing scenario for irradiated silicon p-in-n microstrip sensors

The configuration of almost all silicon microstrip sensors instrumenting the tracker detectors of the current experiments in LHC is with readout from implanted p-strips on n-type silicon bulk (from 300 to 400μm thick). The performance of these sensors will change with the irradiation and the time after irradiation (annealing). The effect of the annealing on the performance of the tracking devices has been a source of concern for the experiments due to the rise of the full depletion voltage with time. The experiments have anticipated to suppress annealing effects by keeping the sensors at low temperature also outside operation time. In fact, a scenario that allows a certain amount of annealing could be advantageous for running the silicon sensors towards the end of the experiment lifetime when the readout signal starts to be sensitively degraded. A new set of measurements of the full depletion voltage and charge collection as a function of annealing time with miniature silicon microstrip sensors is presented here and discussed in view of possible annealing scenarios.