arXiv subjects

Eckhart Fretwurst

Publications and source records attributed to Eckhart Fretwurst.

At least 19 recordsLinked to original sources

arXiv2026

Gain-Layer Project

Gain-layer degradation from exposure to radiation limits the use of Low-Gain Avalanche Diodes (LGADs) in high energy particle physics detector experiments. Proper understanding of how the gain-layer is destroyed is not available on a defect level. Only measurements for materials with much lower effective doping concentrations are available. The direct study of the gain-layer is not possible with typical defect spectroscopy measurements like Thermally Stimulated Currents (TSC) and Deep-Level Transient Spectroscopy (DLTS). To combat this problem and gain a better understanding of the processes which degrade LGADs, the Gain-Layer Project was started. This project produced 19050 diodes with various Boron, Phosphorus, Oxygen and Carbon concentrations. The material used is low-resistivity p-type Silicon. The effective doping concentrations are in the order of a LGAD gain-layer. These diodes will serve the defect community in the coming years for various studies. This article introduces this project with detailed descriptions of the diodes, their flavours and their processing, and reports on results from I-V, C-V, SIMS and DLTS measurements on unirradiated diodes.

physics.ins-det↗

arXiv2025

Defects and acceptor removal in 60Co γ-irradiated p-type silicon

Boron-doped silicon detectors used in high radiation environments like the future HL-LHC show a degradation in device performance due to the radiation induced deactivation of the active boron dopant. This effect, known as the so-called Acceptor Removal Effect (ARE), depends on particle type, particle energy and radiation dose and is usually explained by the formation of boroninterstitial - oxygen-interstitial (BiOi) defects that induce a donor-type defect level in the upper part of the Si band gap. Here we present defect characterization studies using Thermally Stimulated Current technique (TSC) and Deep Level Transient Spectroscopy (DLTS) on a set of epitaxially grown p-type silicon diodes of different resistivity, irradiated with 60Co γ-rays. We used the defect parameters (activation energy, charge carrier capture cross sections and defect concentration) obtained from DLTS experiments for modeling the corresponding TSC spectra, and subsequently compared those with the experimental TSC results. This approach shows that the di-vacancy which is well characterized by DLTS correlates with the so-far unspecified charge emission signal of the X-defect that partially overlaps with the BiOi peak in TSC spectra. Additionally, in order to evaluate the impact of BiOi defect formation on the macroscopic properties of the device, we compared the BiOi defect concentration with the change in the effective carrier concentration Neff obtained from C-V measurements. It shows that the variations in Neff are about twice the changes in the BiOi concentration, which is in perfect consistency with the assumption of boron deactivation by the formation of the BiOi donor in irradiated p-type Si.

hep-ex↗

arXiv2023

Investigation of high resistivity p-type FZ silicon diodes after 60Co γ-irradiation

In this work, the effects of $^\text{60}$Co $γ$-ray irradiation on high resistivity $p$-type diodes have been investigated. The diodes were exposed to dose values of 0.1, 0.2, 1, and \SI{2}{\mega Gy}. Both macroscopic ($I$--$V$, $C$--$V$) and microscopic (Thermally Stimulated Current~(TSC)) measurements were conducted to characterize the radiation-induced changes. The investigated diodes were manufactured on high resistivity $p$-type Float Zone (FZ) silicon and were further classified into two types based on the isolation technique between the pad and guard ring: $p$-stop and $p$-spray. After irradiation, the macroscopic results of current-voltage and capacitance-voltage measurements were obtained and compared with existing literature data. Additionally, the microscopic measurements focused on the development of the concentration of different radiation-induced defects, including the boron interstitial and oxygen interstitial (B$_\text{i}$O$_\text{i}$) complex, the carbon interstitial and oxygen interstitial C$_\text{i}$O$_\text{i}$ defect, the H40K, and the so-called I$_\text{P}^*$. To investigate the thermal stability of induced defects in the bulk, isochronal annealing studies were performed in the temperature range of \SI{80}{\celsius} to \SI{300}{\celsius}. These annealing processes were carried out on diodes irradiated with doses of 1 and \SI{2}{\mega Gy} and the corresponding TSC spectra were analysed. Furthermore, in order to investigate the unexpected results observed in the $C$-$V$ measurements after irradiation with high dose values, the surface conductance between the pad and guard ring was measured as a function of both dose and annealing temperature.

physics.ins-det↗

arXiv2023

Investigation of the Boron removal effect induced by 5.5 MeV electrons on highly doped EPI- and Cz-silicon

This study focuses on the properties of the B$_\text{i}$O$_\text{i}$ (interstitial Boron~-~interstitial Oxygen) and C$_\text{i}$O$_\text{i}$ (interstitial Carbon~-~interstitial Oxygen) defect complexes by \SI{5.5}{\mega\electronvolt} electrons in low resistivity silicon. Two different types of diodes manufactured on p-type epitaxial and Czochralski silicon with a resistivity of about 10~$Ω\cdot$cm were irradiated with fluence values between \SI{1e15}{\per\square\centi\meter} and \SI{6e15}{\per\square\centi\meter}. Such diodes cannot be fully depleted and thus the accurate evaluation of defect concentrations and properties (activation energy, capture cross-section, concentration) from Thermally Stimulated Currents (TSC) experiments alone is not possible. In this study we demonstrate that by performing Thermally Stimulated Capacitance (TS-Cap) experiments in similar conditions to TSC measurements and developing theoretical models for simulating both types of B$_\text{i}$O$_\text{i}$ signals generated in TSC and TS-Cap measurements, accurate evaluations can be performed. The changes of the position-dependent electric field, the effective space charge density $N_\text{eff}$ profile as well as the occupation of the B$_\text{i}$O$_\text{i}$ defect during the electric field dependent electron emission, are simulated as a function of temperature. The macroscopic properties (leakage current and $N_\text{eff}$) extracted from current-voltage and capacitance-voltage measurements at \SI{20}{\celsius} are also presented and discussed

physics.app-ph↗

arXiv2023

Study of the V$_2^0$ state in neutron-irradiated silicon using photon-absorption measurements

Pieces of $n$-type silicon with 3.5 k$Ω\cdot $cm resistivity have been irradiated by reactor neutrons to fluences of (1, 5 and 10) $\times 10^{16}$ cm$^{-2}$. Using light-transmission measurements, the absorption coefficients have been determined for photon energies, $E_γ$, between 0.62 and 1.30 eV for the samples as irradiated and after 15 min isochronal annealing with temperatures between 80°C and 330°C. The radiation-induced absorption coefficient, $α_\mathit{irr}$, has been obtained by subtracting the absorption coefficient for non-irradiated silicon. The $E_γ$-dependence of $α_\mathit{irr}$ shows a resonance peak, which is ascribed to the neutral divacancy, V$_2^0$, sitting on a background, and $α_\mathit{irr} (E_γ)$ is fitted by a Breit-Wigner line shape on a parameterized background. It is found that at an annealing temperature of 210°C the V$_2^0$ intensity is reduced by a factor 2, and that at the meV level, the position and the width of the fitted Breit-Wigner do not change with irradiation dose and annealing.

cond-mat.mtrl-sci↗

arXiv2022

Defect characterization studies on neutron irradiated boron-doped silicon pad diodes and Low Gain Avalanche Detectors

High-energy physics detectors, like Low Gain Avalanche Detectors (LGADs) that will be used as fast timing detectors in the High Luminosity LHC experiments, have to exhibit a significant radiation tolerance. Thereby the impact of radiation on the highly boron-doped gain layer that enables the internal charge multiplication, is of special interest, since due to the so-called Acceptor Removal Effect (ARE) a radiation-induced deactivation of active boron dopants takes place. In this paper we present defect-spectroscopy measurements (Deep-Level Transient Spectroscopy and Thermally Stimulated Current technique) on neutron irradiated p-type silicon pad diodes of different resistivity as well as LGADs irradiated at fluences up to 1 x 10^15 neq/cm2. Thereby we show that while for the silicon pad diodes irradiated with electrons, neutrons or protons the determination of defect electronic properties and defect introduction rates is straightforward, DLTS and TSC measurements on LGADs are strongly influenced by the impact of the gain layer. It is shown that the measurability of the capacitance of the gain layer shows a strong frequency and temperature dependence leading to a capacitance drop in DLTS and non-reliable measurement results. With TSC defects formed in the LGADs can be very nicely observed and compared to the defects formed in the silicon pad diodes. However the exact assignment of defects to the gain layer or bulk region remains challenging and the charge amplification effect of the LGADs impacts the exact determination of defect concentrations. Additionally, we will demonstrate that depending on the TSC measurement conditions defect induced residual internal electric fields are built up in the irradiated LGADs that are influencing the current signal of carriers emitted from the defect states.

physics.ins-det↗

arXiv2019

A new model for the TCAD simulation of the silicon damage by high fluence proton irradiation

For the high-luminosity phase of the Large Hadron Collider (HL-LHC), at the expected position of the innermost pixel detector layer of the CMS and ATLAS experiments, the estimated equivalent neutron fluence after 3000 fb$^{-1}$ is 2$\cdot$10$^{16}$ n$_{eq}$/cm$^2$, and the IEL (Ionizing Energy Loss) dose in the SiO$_2$ 12 MGy. The optimisation of the pixel sensors and the understanding of their performance as a function of fluence and dose makes a radiation damage model for TCAD simulations, which describes the available experimental data, highly desirable. The currently available bulk-damage models are not able to describe simultaneously the measurements of dark current (I-V), capacitance-voltage (C-V) and charge collection efficiency (CCE) of pad diodes for fluences $\ge 1\cdot 10^{15}$ n$_{eq}$/cm$^2$. Therefore, for the development and validation of a new accurate bulk damage model we use I-V, C-V and CCE measurements on pad diodes available within the CMS-HPK campaign and data from samples irradiated recently with 24 GeV/c protons. For the determination of the radiation-induced damage parameters we utilise the "optimiser" of Synopsys TCAD, which allows the minimisation of the difference between the measured and simulated I-V, C-V and CCE. The outcome of this optimisation, the Hamburg Penta Trap Model (HPTM), provides a consistent and accurate description of the measurements of diodes irradiated with protons in the fluence range from 3$\cdot$10$^{14}$ n$_{eq}$/cm$^2$ to 1.3$\cdot$10$^{16}$ n$_{eq}$/cm$^2$.

physics.ins-det↗

arXiv2018

Study of point- and cluster-defects in radiation-damaged silicon

Non-ionising energy loss of radiation produces point defects and defect clusters in silicon, which result in a signifcant degradation of sensor performance. In this contribution results from TSC (Thermally Stimulated Current) defect spectroscopy for silicon pad diodes irradiated by electrons to fluences of a few $10^{14}$ cm$^{-2}$ and energies between 3.5 and 27 MeV for isochronal annealing between 80 and 280°C, are presented. A method based on SRH (Shockley-Read-Hall) statistics is introduced, which assumes that the ionisation energy of the defects in a cluster depends on the fraction of occupied traps. The dfference of ionisation energy of an isolated point defect and a fully occupied cluster, $ΔE_a$, is extracted from the TSC data. For the VOi (vacancy-oxygen interstitial) defect $ΔE_a = 0$ is found, which cofirms that it is a point defect, and validates the method for point defects. For clusters made of deep acceptors the $ΔE_a$ values for different defects are determined after annealing at 80°C as a function of electron energy, and for the irradiation with 15 MeV electrons as a function of annealing temperature. For the irradiation with 3.5 MeV electrons the value $ΔE_a = 0$ is found, whereas for the electron energies of 6 to 27 MeV $ΔE_a > 0$. This agrees with the expected threshold of about 5 MeV for cluster formation by electrons. The $ΔE_a$ values determined as a function of annealing temperature show that the annealing rate is different for different defects. A naive diffusion model is used to estimate the temperature dependencies of the diffusion of the defects in the clusters.

physics.ins-det↗

arXiv2014

Design and First Tests of a Radiation-Hard Pixel Sensor for the European X-Ray Free-Electron Laser

The high intensity and high repetition rate of the European X-ray Free-Electron Laser, presently under construction in Hamburg, requires silicon sensors which can stand X-ray doses of up to 1 GGy for 3 years of operation at high bias voltage. Within the AGIPD Collaboration the X-ray-radiation damage in MOS Capacitors and Gate-Controlled Diodes fabricated by four vendors on high-ohmic n-type silicon with two crystal orientations and dif- ferent technological parameters, has been studied for doses between 1 kGy and 1 GGy. The extracted values of oxide-charge and surface-current densi- ties have been used in TCAD simulations, and the layout and technological parameters of the AGIPD pixel sensor optimized. It is found that the op- timized layout for high X-ray doses is significantly different from the one for non-irradiated sensors. First sensors and test structures have been de-livered in early 2013. Measurement results for X-ray doses of 0 to 10 MGy and their comparison to simulations are presented. They demonstrate that the optimization has been successful and that the sensors fulfill the required specifications.

physics.ins-det↗

arXiv2013

Study of X-ray radiation damage in the AGIPD sensor for the European XFEL

The European X-ray Free Electron Laser (XFEL), currently being constructed in Hamburg and planning to be operational in 2017 for users, will deliver 27,000 fully coherent, high brilliance X-ray pulses per second with duration less than 100 fs. The unique features of the X-ray beam pose major challenges for detectors used at the European XFEL for imaging experiments, in particular a radiation tolerance of silicon sensors for doses up to 1 GGy for 3 years of operation at an operating voltage above 500 V. One of the detectors under development at the European XFEL is the Adaptive Gain Integrating Pixel Detector (AGIPD), which is a hybrid detector system with ASICs bump-bonded to p+n silicon pixel sensors. We have designed the silicon sensors for the AGIPD, which have been fabricated by SINTEF and delivered in the beginning of February of 2013. To demonstrate the performance of the AGIPD sensor with regard to radiation hardness, mini-sensors with the same pixel and guard-ring designs as the AGIPD together with test structures have been irradiated at the beamline P11 of PETRA III with 8 keV and 12 keV monoenergetic X-rays to dose values up to 10 MGy. The radiation hardness of the AGIPD sensor has been proven and all electrical properties are within specification before and after irradiation. In addition, the oxide-charge density and surface-current density from test structures have been characterized as function of the X-ray dose and compared to previous measurements for test structures produced by four vendors.

physics.ins-det↗

arXiv2013

Time dependence of charge losses at the Si-SiO2 interface in p+n-silicon strip sensors

The collection of charge carriers generated in p+n strip sensors close to the Si-SiO2 interface before and after 1 MGy of X-ray irradiation has been investigated using the transient current technique with sub-nanosecond focused light pulses of 660 nm wavelength, which has an absorption length of 3.5 um in silicon at room temperature. The paper describes the measurement and analysis techniques used to determine the number of electrons and holes collected. Depending on biasing history, humidity and irradiation, incomplete collection of either electrons or holes is observed. The charge losses change with time. The time constants are different for electrons and holes and increase by two orders of magnitude when reducing the relative humidity from about 80 % to less than 1 %. An attempt to interpret these results is presented.

physics.ins-det↗

arXiv2013

Study of the accumulation layer and charge losses at the Si-SiO2 interface in p+n-silicon strip sensors

Using the multi-channel Transient Current Technique the currents induced by electron-hole pairs, produced by a focussed sub-nanosecond laser of 660 nm wavelength close to the Si-SiO2 interface of p+n silicon strip sensors have been measured, and the charge-collection efficiency determined. The laser has been operated in burst mode, with bursts typically spaced by 1 ms, each consisting of 30 pulses separated by 50 ns. In a previous paper it has been reported that, depending on X-ray-radiation damage, biasing history and humidity, situations without charge losses, with hole losses, and with electron losses have been observed. In this paper we show for sensors before and after irradiation by X-rays to 1 MGy (SiO2), how the charge losses change with the number of electron-hole pairs generated by each laser pulse, and the time interval between the laser pulses. This allows us to estimate how many additional charges in the accumulation layers at the Si-SiO2 interface have to be trapped to significantly change the local electric field, as well as the time it takes that the accumulation layer and the electric field return to the steady-state situation. In addition, results are presented on the change of the pulse shape caused by the plasma effect for high charge densities deposited close to the Si-SiO2 interface.

physics.ins-det↗

arXiv2012

Charge losses in segmented silicon sensors at the Si-SiO2 interface

Using multi-channel time-resolved current measurements (multi TCT), the charge collection of p+n silicon strip sensors for electron-hole pairs produced close to the Si-SiO2 interface by a focussed sub-nanosecond laser with a wavelength of 660 nm has been studied. Sensors before and after irradiation with 1 MGy of X-rays have been investigated. The charge signals induced in the readout strips and the rear electrode as a function of the position of the light spot are described by a model which allows a quantitative determination of the charge losses and of the widths of the electron-accumulation and hole-inversion layers close to the Si-SiO2 interface. Depending on the applied bias voltage, biasing history and environmental conditions, like humidity, incomplete electron or hole collection and different widths of the accumulation layers are observed. In addition, the results depend on the time after biasing the sensor, with time constants which can be as long as days. The observations are qualitatively explained with the help of detailed sensor simulations. Finally, their relevance for the detection of X-ray photons and charged particles, and for the stable operation of segmented p+n silicon sensors is discussed.

physics.ins-det↗

arXiv2012

Challenges for Silicon Pixel Sensors at the European XFEL

A systematic experimental study of the main challenges for silicon-pixel sensors at the European XFEL is presented. The high instantaneous density of X-rays and the high repetition rate of the XFEL pulses result in signal distortions due to the plasma effect and in severe radiation damage. The main parameters of X-ray-radiation damage have been determined and their impact on p+n sensors investigated. These studies form the basis of the optimized design of a pixel-sensor for experimentation at the European XFEL.

physics.ins-det↗

arXiv2012

Investigation of X-ray induced radiation damage at the Si-SiO2 interface of silicon sensors for the European XFEL

Experiments at the European X-ray Free Electron Laser (XFEL) require silicon pixel sensors which can withstand X-ray doses up to 1 GGy. For the investigation of X-ray radiation damage up to these high doses, MOS capacitors and gate-controlled diodes built on high resistivity n-doped silicon with crystal orientations <100> and <111> produced by two vendors, CiS and Hamamatsu, have been irradiated with 12 keV X-rays at the DESY DORIS III synchrotron light source. Using capacitance/conductance-voltage, current-voltage and thermal dielectric relaxation current measurements, the surface densities of oxide charges and interface traps at the Si-SiO2 interface, and the surface-current densities have been determined as function of dose. Results indicate that the dose dependence of the surface density of oxide charges and the surface-current density depend on the crystal orientation and producer. In addition, the influence of the voltage applied to the gates of the MOS capacitor and the gate-controlled diode during X-ray irradiation on the surface density of oxide charges and the surface-current density has been investigated at doses of 100 kGy and 100 MGy. It is found that both strongly depend on the gate voltage if the electric field in the oxide points from the surface of the SiO2 to the Si-SiO2 interface. Finally, annealing studies have been performed at 60 and 80 degree C on MOS capacitors and gate-controlled diodes irradiated to 5 MGy and the annealing kinetics of oxide charges and surface current determined.

physics.ins-det↗

arXiv2012

Study of radiation damage induced by 12 keV X-rays in MOS structures built on high resistivity n-type silicon

Imaging experiments at the European X-ray Free Electron Laser (XFEL) require silicon pixel sensors with extraordinary performance specifications: Doses of up to 1 GGy of 12 keV photons, up to 10^5 12 keV photons per pixel of 200 \mum \times 200 \mum arriving within less than 100 fs, and a time interval between XFEL pulses of 220 ns. To address these challenges, in particular the question of radiation damage, the properties of the SiO_2 layer and of the Si-SiO_2 interface using MOS capacitors manufactured on high resistivity n-type silicon irradiated to X-ray doses between 10 kGy and 1 GGy, have been studied. Measurements of Capacitance/Conductance-Voltage (C/G-V) at different frequencies, as well as Thermal Dielectric Relaxation Current (TDRC) have been performed. The data can be described by a radiation dependent oxide charge density and three dominant radiation-induced interface states with Gaussian-like energy distributions in the silicon band gap. It is found that the densities of the fixed oxide charges and of the three interface states increase up to dose values of approximately 10 MGy and then saturate or even decrease. The shapes and the frequency dependences of the C/G-V measurements can be quantitatively described by a simple model using the parameters extracted from the TDRC measurements.

physics.ins-det↗

arXiv2011

Study of X-ray Radiation Damage in Silicon Sensors

The European X-ray Free Electron Laser (XFEL) will deliver 30,000 fully coherent, high brilliance X-ray pulses per second each with a duration below 100 fs. This will allow the recording of diffraction patterns of single complex molecules and the study of ultra-fast processes. Silicon pixel sensors will be used to record the diffraction images. In 3 years of operation the sensors will be exposed to doses of up to 1 GGy of 12 keV X-rays. At this X-ray energy no bulk damage in silicon is expected. However fixed oxide charges in the insulating layer covering the silicon and interface traps at the Si-SiO2 interface will be introduced by the irradiation and build up over time. We have investigated the microscopic defects in test structures and the macroscopic electrical properties of segmented detectors as a function of the X-ray dose. From the test structures we determine the oxide charge density and the densities of interface traps as a function of dose. We find that both saturate (and even decrease) for doses between 10 and 100 MGy. For segmented sensors the defects introduced by the X-rays increase the full depletion voltage, the surface leakage current and the inter-pixel capacitance. We observe that an electron accumulation layer forms at the Si-SiO2 interface. Its width increases with dose and decreases with applied bias voltage. Using TCAD simulations with the dose dependent parameters obtained from the test structures, we are able to reproduce the observed results. This allows us to optimize the sensor design for the XFEL requirements.

physics.ins-det↗

arXiv2010

Measurements of charge carrier mobilities and drift velocity saturation in bulk silicon of <111> and <100> crystal orientation at high electric fields

The mobility of electrons and holes in silicon depends on many parameters. Two of them are the electric field and the temperature. It has been observed previously that the mobility in the transition region between ohmic transport and saturation velocities is a function of the orientation of the crystal lattice. This paper presents a new set of parameters for the mobility as function of temperature and electric field for <111> and <100> crystal orientation. These parameters are derived from time of flight measurements of drifting charge carriers in planar p^+nn^+ diodes in the temperature range between -30°C and 50°C and electric fields of 2x10^3 V/cm to 2x10^4 V/cm.

physics.ins-det↗