arXiv subjects

Michael Moll

Publications and source records attributed to Michael Moll.

At least 19 recordsLinked to original sources

arXiv2026

Event-by-Event Space-Charge Gain Suppression in LGADs and Its Impact on Time Resolution

Low Gain Avalanche Detectors (LGADs) achieve time resolutions below 50 ps through internal charge multiplication, yet the gain is not a fixed device property as the space-charge of the multiplied electron-hole density screens the gain layer field, which suppresses further impact ionization. While TCAD simulations reproduce the average gain suppression, their deterministic treatment of a continuous charge density cannot capture the event-by-event fluctuations that determine the timing performance. We present a Monte Carlo study of space-charge-induced gain suppression in LGADs using the Garfield++ framework, in which the dynamic field of the drifting carriers, computed from a coarse-grained system of charged rings, is superimposed on TCAD-computed static field maps of Centro Nacional de Microelectr\'onica (CNM) and Hamamatsu Photonics (HPK) produced sensors while preserving single carrier avalanche statistics. The simulated gain reproduces published two-photon absorption-transient current technique measurements across two orders of magnitude in injected charge, including the dependence on the injection depth. For minimum ionizing particles, the event-by-event coupling between deposited charge and gain compresses the Landau tail of the multiplied charge, narrowing the signal arrival time distribution and improving the intrinsic time resolution of the HPK-S1 sensor investigated in this work from 53.9 $\pm$ 1.0 ps to 44.5 $\pm$ 0.9 ps. Treating the primary ionization as a stochastic sequence of charge clusters, it is further shown analytically that any gain suppression that decreases with the cluster size narrows the centroid time distribution.

physics.ins-det↗

arXiv2026

Novel Strip-like Readout Geometries in Resistive AC-coupled Silicon Detectors (RSD / AC-LGAD)

Resistive Silicon Detectors (RSD/AC-LGAD) are novel silicon detectors capable of both precise spatial and temporal resolution. Such sensors will be essential for the next generation of particle colliders (EIC, FCC-ee, CEPC, FCC-hh) and would enable the possibility of a 4D tracker. RSD sensors are typically fabricated with a pixel-like geometry that provides excellent spatial resolution in the x and y directions. However, in regions further from the interaction point, high spatial resolution in one direction (strip-like geometry) is often preferred to reduce the number of readout channels. For example, strip AC-LGADs are now the default option for the US electron ion collider (EIC). The second production of RSD sensors by Fondazione Bruno Kessler includes sensors with unconventional readout pad shapes that act as a mixture between strip-like and pixel-like readout. This work presents the first characterization of these new pad designs using the Transient Current Technique (TCT). The measurements demonstrate exceptional one-dimensional spatial resolution, confirming the potential of novel strip-like RSDs for future tracking systems.

physics.ins-det↗

arXiv2026

TPA-TCT analysis of the RD50-MPW4 monolithic pixel particle detector

The RD50-MPW4, a Depleted Monolithic Active Pixel Sensor (DMAPS) was analyzed using a Two Photon Absorption Transient Current Technique (TPA-TCT). This technique provides sensitivity maps with micrometer-scale spatial resolution, enabling the resolution of the boundaries of the detector's sensitive volume, even for small-area pixels (62 x 62 $\mu$m$^2$ in this study). With a 3D resolution, the depletion depth, the boundaries of the detector electric field, the 3D hit detection efficiency and the charge sharing between neighboring pixels were measured. The RD50-MPW4, a multi-project wafer chip developed by the HV-CMOS working group within the CERN RD50 collaboration, features a 64 x 64 DMAPS pixel matrix. Illuminating the chip from the backside, the TPA-TCT technique can characterize any pixel element in the matrix because silicon is transparent for near infrared laser light (1550 nm). Electron-hole pairs are generated only around the light focal point, deep in the silicon, so that any charge collected is precisely only from the focal point. With the TPA-TCT technique, the RD50-MPW4 was found to be have a 100% hit detection efficiency under specified conditions and an effective depletion depth of 226 $\mu$m. It was also found that part of the charge in the periphery of the pixel was collected in the neighboring pixel. A 3D map of the sensor clearly shows the in-pixel electronics and the limits of the depletion region.

physics.ins-det↗

arXiv2026

Proton Energy Dependence of Radiation Induced Low Gain Avalanche Detector Degradation

Low Gain Avalanche Detectors (LGADs) are key components for precise timing measurements in high-energy physics experiments, including the High Luminosity upgrades of the current LHC detectors. Their performance is, however, limited by radiation induced degradation of the gain layer, primarily driven by acceptor removal. This study presents a systematic comparison of how the degradation evolves with different incident proton energies, using LGADs from Hamamatsu Photonics (HPK) and The Institute of Microelectronics of Barcelona (IMB-CNM) irradiated with 18 MeV, 24 MeV, 400 MeV and 23 GeV protons and fluences up to 2.5x10^15 p/cm2. Electrical characterization is used to extract the acceptor removal coefficients for different proton energies, whereas IR TCT measurements offer complementary insight into the gain evolution in LGADs after irradiation. Across all devices, lower energy protons induce stronger gain layer degradation, confirming expectations. However, 400 MeV protons consistently appear less damaging than both lower and higher energy protons, an unexpected deviation from a monotonic energy trend. Conversion of proton fluences to 1 MeV neutron-equivalent fluences reduces but does not eliminate these differences, indicating that the standard Non-Ionizing Energy Loss (NIEL) scaling does not fully account for the underlying defect formation mechanisms at different energies and requires revision when considering irradiation fields that contain a broader spectrum of particle types and energies.

physics.ins-det↗

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

On the nature and charge state of the X-Defect, a radiation-induced Silicon defect with field-enhanced charge carrier emission

The elusive X-Defect, a defect found in low-resistivity $p$-type Silicon after irradiation, observed as a low-temperature shoulder of the $\mathrm{B}_\mathrm{i}\mathrm{O}_\mathrm{i}$ defect (Boron-interstitial-Oxygen-interstitial complex) in Thermally Stimulated Current (TSC) measurements, was investigated to determine its properties, matching them with those of a previously identified defect. Through a combination of TSC, Deep-Level Transient Spectroscopy (DLTS), Difference-DLTS (DDLTS), numerical simulations of field-enhanced charge carrier emissions in TSC measurements and a comparison to literature, the X-Defect was identified as the singly positively charged Silicon di-vacancy $\mathrm{V}_2(+/0)$. This assignment is supported by an agreement in activation energy, capture cross-section, trap type and charge emission process, as well as simulations comparing the effects of phonon-assisted tunnelling (PAT) and Poole-Frenkel (PF) mechanisms on TSC spectra. DDTLS measurements revealed a quadratic dependence of the activation energy on the electric field strength, confirming PAT as the prevailing mechanism over PF in the case of the radiation-induced X-Defect. Assigning the X-Defect to an electrically neutral defect in the space charge region resolves previous contradictions regarding its deficiency in impacting on the effective doping concentration.

cond-mat.mtrl-sci↗

arXiv2025

Studies on the effect of low-fluence proton and neutron irradiation on n-type LGADs

The presented study investigates the effects of low fluences from $5\times10^{12}$ up to $1\times10^{14}$ particles/cm$^{2}$ of 60MeV proton and neutron irradiation on n-type Low Gain Avalanche Detectors (nLGADs). An nLGAD is a silicon sensor with a highly doped gain layer that enables controlled charge multiplication via impact ionization. In contrast to the well-established p-type LGADs for high-energy physics (HEP) applications, nLGADs are optimized for the detection of low-penetrating particles such as UV photons and soft X-rays. In addition to studying their potential application in environments with radiation backgrounds, these novel devices also enable the exploration of the underlying phenomenology arising from the combination of n-type bulk material with a gain layer, which degradation was previously studied predominantly in the context of p-type LGADs. The irradiation effects were characterized through measurements of the leakage current and capacitance with increasing bias voltage (I-V and C-V), revealing systematic and fluence-dependent behavior related to space charge sign inversion (SCSI) of the n-type bulk material, which especially alters the electric field in the sensor and thus the depletion behavior. Additionally, annealing studies were performed to assess both beneficial and reverse annealing regimes with isothermal and isochronal annealing. The findings are consistent with previous high-energy proton studies and contribute to a deeper understanding of the fundamental behavior of nLGADs under irradiation.

physics.ins-det↗

arXiv2025

Defects and acceptor removal in 60Co {\gamma}-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 {\gamma}-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↗

arXiv2025

Characterisation of Crystalline Defects in 4H Silicon Carbide using DLTS and TSC

Future hadron collider experiments will require sensing materials that withstand stronger radiation fields. Therefore, either a frequent replacement of detectors, a significant increase in radiation hardness of Silicon, or a shift to different materials is needed. Wide-bandgap materials are a natural choice, due to their significantly reduced leakage currents, even after irradiation. In recent years, substantial progress in the production of high-quality monocrystalline Silicon Carbide of the 4H polytype has led to a renewed interest in this material. In this article, a study of electrically active defects in a n-type epitaxial 4H Silicon Carbide diode is presented. By employing spectroscopical measurement methods, like Deep-Level Transient Spectroscopy (DLTS) and Thermally Stimulated Currents (TSC), energy levels in the bandgap are investigated. Defect parameters like concentration, activation energy and capture cross-section are stated. A simulation framework was utilised to compare and match the results from the two methods. This study is made in the context of a study of radiation hardness of 4H Silicon Carbide sensors. Other studies investigating macroscopic properties of the material, like their charge collection efficiency after irradiation, were performed on the same kind of diodes. This study provides a first set of measured defect parameters in state-of-the-art 4H-SiC material, from defects present prior to irradiation. These defects are intrinsic, such as vacancies, related to impurities and doping imperfections, or are growth related. The $Z_\text{1/2}$ defect and a Nitrogen related defect were identified.

physics.ins-det↗

arXiv2025

Exploring the Design and Measurements of Next-Generation 4H-SiC LGADs

This contribution presents the design, production, and initial testing of newly developed 4H-SiC Low Gain Avalanche Detectors (LGADs). The evaluation includes performance metrics such as the internal gain layer's efficiency in enhancing signal generation. Initial laboratory and Transient Current Technique (TCT) measurements provide insight into the device's stability and response to the signal. Due to the increase of availability provided by the industry, 4H-SiC is emerging as a strong candidate for the next-generation of semiconductor detectors. Such sensors are promising due to the inherent radiation tolerance of 4H-SiC and its stable operation across a wide temperature range. However, due to the wider-bandgap of 4H-SiC compared to standard silicon, and difficulty to produce high-quality layers thicker than 50 \textmu m, an internal charge multiplication layer needs to be introduced. The presented 4H-SiC LGADs, fabricated by onsemi, are optimized for an N-type substrate and epi wafer. The initial TCT and laboratory test results demonstrate fast charge collection and uniform multiplication across multiple samples produced on a single wafer.

physics.ins-det↗

arXiv2025

Investigating irradiation effects and space charge sign inversion in n-type Low Gain Avalanche Detectors

Low Gain Avalanche Detectors built on n-type substrate (nLGADs) have been developed by IMB-CNM to enhance the detection of low-penetrating particles, with a wide range of applications from medicine, industry to synergies with developments for high-energy physics (HEP). In this work, irradiation effects on nLGADs were investigated through proton irradiation at the CERN PS-IRRAD facility up to proton fluences of $10^{14}\,\mathrm{cm}^{-2}$. Electrical characterization before and after irradiation reveals space charge sign inversion of the n-type bulk, leading to significant modifications in the depletion behavior and electric field distribution. Utilizing UV TCT and TPA-TCT measurements, the impact of irradiation on the electric fields and the gain are studied in more detail, confirming a change of sensor depletion and a reduced electric field in the gain layer. The results suggest that donor removal in nLGADs is stronger pronounced already at lower fluences compared to acceptor removal in traditional p-type LGADs. These findings provide not only first insights into the effects of irradiation on nLGADs but also contribute to the development of methods to quantify the donor removal.

physics.ins-det↗

arXiv2024

TCAD Simulation of Two Photon Absorption -- Transient Current Technique measurements on Silicon Detectors and LGADs

Device simulation plays a crucial role in complementing experimental device characterisation by enabling deeper understanding of internal physical processes. However, for simulations to be trusted, experimental validation is essential to confirm the accuracy of the conclusions drawn.In the framework of semiconductor detector characterisation, one powerful tool for such validation is the Two Photon Absorption - Transient Current Technique (TPA-TCT), which allows for highly precise, three-dimensional spatially-resolved characterisation of semiconductor detectors. In this work, the TCAD framework Synopsys Sentaurus is used to simulate depth-resolved TPA-TCT data for both p-type pad detectors (PINs) and Low Gain Avalanche Detectors (LGADs). The simulated data are compared against experimentally measured TPA-TCT results. Through this comparison, it is demonstrated that TCAD simulations can reproduce the TPA-TCT measurements, providing valuable insights into the TPA-TCT itself. Another significant outcome of this study is the successful simulation of the gain reduction mechanism, which can be observed in LGADs with increasing densities of excess charge carriers. This effect is demonstrated in an p-type LGAD with a thickness of approximately 286 um. The results confirm the ability of TCAD to model the complex interaction between carrier dynamics and device gain.

physics.ins-det↗

arXiv2023

Investigation of high resistivity p-type FZ silicon diodes after 60Co {\gamma}-irradiation

In this work, the effects of $^\text{60}$Co $\gamma$-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~$\Omega\cdot$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

Gain layer degradation study after neutron and proton irradiations in Low Gain Avalanche Diodes

The high-luminosity upgrade of the ATLAS and CMS experiments includes dedicated sub-detectors to perform the time-stamping of minimum ionizing particles (MIPs). These detectors will be exposed up to fluences in the range of 1.5 - 2.5e15 neq/cm2 at the end of their lifetime and, Low Gain Avalanche Diode (LGAD) has been chosen as their baseline detection technology. To better understand the performance of LGAD detectors in these environments, a gain layer degradation study after neutron and proton irradiations up to a fluence of 1.5e15 neq/cm2 was performed. LGADs manufactured at Hamamatsu Photonics (HPK) and Centro Nacional de Microelectr\'onica (CNM-IMB) were chosen for this study and, a comparison in the gain layer degradation after exposure to reactor neutrons at the Jozef Stefan Institute (JSI) in Ljubjana and 24 GeV/c protons at the CERN-PS is presented here.

physics.ins-det↗

arXiv2022

Study of impact ionization coefficients in silicon with Low Gain Avalanche Diodes

Impact ionization in silicon devices has been extensively studied and several models for a quantitative description of the impact ionization coefficients have been proposed. We evaluate those models against gain measurements on Low Gain Avalanche diodes (LGADs) and derive new parameterizations for the impact ionization coefficients optimized to describe a large set of experimental data. We present pulsed IR-laser based gain measurements on 5 different types of $50\mu m$-thick LGADs from two different producers (CNM and HPK) performed in a temperature range from $-15^oC$ to $40^oC$. Detailed TCAD device models are conceived based on SIMS doping profiles measurements and tuning of the device models to measured C-V characteristics. Electric field profiles are extracted from the TCAD simulations and used as input to an optimization procedure (least squares fit) of the impact ionization model parameters to the experimental data. It is demonstrated that the new parameterizations give a good agreement between all measured data and TCAD simulations which is not achieved with the existing models. Finally, we provide an error analysis and compare the obtained values for the electron and hole impact ionization coefficients against existing models.

physics.ins-det↗

arXiv2022

Techniques for the investigation of segmented sensors using the Two Photon Absorption -- Transient Current Technique

The Two Photon Absorption - Transient Current Technique (TPA-TCT) was used to investigate a silicon strip detector with illumination from the top. Measurement and analysis techniques for the TPA-TCT of segmented devices are presented and discussed using a passive strip CMOS detector as an example. The influence of laser beam clipping and reflection is shown and a method that allows to compensate these intensity related effects for the investigation of the electric field is introduced and successfully employed. Besides, the mirror technique is introduced, which exploits reflection at a metallised back side to enable the measurement directly below a top metallisation, while illuminating from the top.

physics.ins-det↗

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↗