Severe signal loss in diamond beam loss monitors in high particle rate environments by charge trapping in radiation‐induced defects
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2016-07-19 |
| Journal | physica status solidi (a) |
| Authors | F. Kassel, M. Guthoff, Anne Dabrowski, W. De Boer |
| Institutions | European Organization for Nuclear Research |
| Citations | 19 |
| Analysis | Full AI Review Included |
Expert Material Analysis: Rate-Dependent Polarization and Signal Loss in MPCVD Diamond Detectors
Section titled “Expert Material Analysis: Rate-Dependent Polarization and Signal Loss in MPCVD Diamond Detectors”Executive Summary
Section titled “Executive Summary”This paper investigates the critical issue of severe signal loss (reduced Charge Collection Efficiency, CCE) in high-purity Single Crystal Diamond (sCVD) detectors used for Beam Condition Monitoring Leakage (BCML) at the CERN Large Hadron Collider (LHC). The findings confirm that charge trapping in radiation-induced defects causes a highly non-linear signal reduction, a phenomenon known as diamond polarization.
- Core Challenge: High particle rates (30x higher than lab tests) accelerate the build-up of internal space charge, locally reducing the electrical field and dramatically increasing charge recombination.
- Material Performance: High-quality electronic grade sCVD diamonds (Nitrogen < 5 ppB) showed CCE dropping by a factor of five or more after low integrated luminosity, quickly approaching the poor CCE characteristics of pCVD.
- Mechanism Identified: The polarization effect is rate-dependent. Trapping rate at high fluxes exceeds the detrapping rate, collapsing the internal electric field, especially at low bias voltages (e.g., field close to zero V/cm across 50% of the bulk at 100 V bias).
- Technical Solution: An effective deep trap model (eRC1/eRC2) incorporating acceptor- and donor-like centers was developed and fitted to experimental Transient Current Technique (TCT) data.
- Simulation Validity: Quasi 3D-simulations (SILVACO TCAD) successfully extrapolated laboratory measurements (90Sr source) to the harsh LHC environment, predicting CCE drops from 65% (lab rate) to 23% (CMS rate) at 100 V bias.
- Mitigation Strategy: Potential mitigation includes high-frequency bias voltage switching or operating at significantly higher bias voltages to overcome the internal polarizing field.
Technical Specifications
Section titled “Technical Specifications”The following hard data was extracted from the experimental results and effective trap model parameters used for simulation.
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Material Grade | Electronic Grade sCVD | N/A | [N] < 5 ppB, [B] < 1 ppB |
| Initial CCE Range | 92 - 98 | % | Measured at E = 1800 V/cm |
| Active Thickness Range | 538 - 549 | µm | Detector dimensions |
| Total Fluence (Maximum) | 3 x 1012 | cm-2 | Irradiated dose, 23 MeV Protons/Neutrons |
| Electrode Composition | Titanium/Tungsten (Ti/W) | N/A | Ohmic contacts, 100 nm thick |
| Electrode Surface Area | 4 x 4 | mm2 | Active area |
| Annealing Temperature | 400 | °C | 4 min in N2 environment |
| Lab Source Ionization Rate (TCT/CCE) | 32.2 / 33.4 | MBq | 90Sr source activity used for polarization build-up |
| Simulated High Rate Environment | ~30 | Factor higher | Relative to 90Sr lab rate (CMS environment) |
| Polarized CCE (100V Bias, CMS Rate) | 23 | % | Simulated CCE drop due to polarization |
| Hole Drift Critical Field (Ec) | 5.787 ± 0.113 | kV/cm | Measured Caughey and Thomas parameter |
| eRC1 Trap Energy Level | 1.8 | eV | Effective Donor trap depth below Conduction Band (Ec) |
| eRC2 Trap Energy Level | 0.83 | eV | Effective Acceptor trap depth above Valence Band (Ev) |
| eRC2 Trap Density (Donor-like) | 1.44 x 1012 | cm-3 | Effective recombination center density |
Key Methodologies
Section titled “Key Methodologies”The study relied on high-purity sCVD diamond, precise processing, and sophisticated electrical and simulation techniques to characterize radiation damage and polarization effects.
- Material Preparation and Processing:
- Surface Etching: 1 µm of the diamond surface was removed using chlorine chemistry Reactive Ion Etching (RIE) prior to metalization to ensure pristine surface quality.
- Metalization: Ohmic electrodes composed of Titanium/Tungsten (Ti/W) were sputtered to a thickness of 100 nm onto a 4 x 4 mm2 surface area.
- Annealing: Metalized samples were annealed at 400 °C for 4 minutes in an N2 environment.
- Irradiation Campaign:
- Samples were irradiated stepwise using 23 MeV protons and neutrons (up to 10 MeV) to achieve a maximum total fluence of 3 x 1012 cm-2.
- Electrical Characterization (TCT/CCE):
- Transient Current Technique (TCT): Used 241Am (3.56 kBq, alpha particles, penetration < 15 µm) to induce electron-hole pairs on one side, enabling measurement of charge carrier drift (electrons or holes) and indirect mapping of the internal electrical field structure.
- CCE Measurement: Used 90Sr (33.4 MBq, beta particles, Minimum Ionizing Particles or MIPs) to create electron-hole pairs homogeneously throughout the 540 µm bulk for efficiency calculation (CCE = Qmeas / Qinduced).
- Polarization Measurement Protocol:
- To establish a baseline (unpolarized state), the diamond was exposed to the 90Sr source for 20 minutes without bias voltage to homogenously fill the traps.
- Bias voltage was then rapidly ramped up (tramp < 10 s), and TCT/CCE measurements were recorded over extended periods (up to t > 3000 s) until the diamond reached a stable, fully polarized state.
- Simulation and Modeling:
- Device Simulation: SILVACO TCAD (Quasi 3D-rotational symmetrical system) was used to model electrical properties and calculate CCE/TCT pulse shapes based on mobility parameters and the effective trap model.
- Effective Recombination Centers: Two effective deep traps (eRC1, eRC2) acting as acceptor- and donor-like centers were introduced into the model to match experimental data.
- Radiation Transport: FLUKA software was used to simulate the energy deposition profiles of alpha and beta particles in the diamond bulk, providing input for the TCAD model geometries.
6CCVD Solutions & Capabilities
Section titled “6CCVD Solutions & Capabilities”This research highlights the absolute necessity of ultra-high-purity, defect-minimized diamond for radiation-hard particle detection systems. 6CCVD is uniquely positioned to supply and engineer the material required to replicate, extend, or mitigate the effects observed in this study.
| Requirement from Paper | 6CCVD Capability & Solution | Value Proposition |
|---|---|---|
| Material Purity (sCVD) | Optical/Electronic Grade SCD (Single Crystal Diamond). We meet the strict purity requirements ([N] < 5 ppB, [B] < 1 ppB) necessary to minimize initial defect concentration. | Highest initial CCE and maximum potential radiation hardness before catastrophic polarization onset. |
| Custom Thickness & Dimensions | SCD/PCD up to 500 µm active thickness control, with substrates available up to 10 mm. Custom Dimensions: Plates/wafers up to 125 mm (PCD). We can supply the target 549 µm thick samples used. | Precise control over active volume thickness is crucial for consistent CCE and optimizing detector geometry. |
| Surface Preparation | Polishing Capabilities: Ultra-low surface roughness (Ra < 1 nm for SCD) combined with precision material removal (RIE/etching support). | Eliminates surface defects that can interfere with contact quality and charge carrier injection/collection efficiency. |
| Custom Ohmic Metalization | In-house Sputtering: We offer custom metalization stacks including Ti, W, Pt, Au, Pd, and Cu. We can precisely replicate the Ti/W ohmic contacts used in this study (100 nm thickness). | Ensures robust, low-resistance ohmic contacts essential for applying high bias voltage and obtaining accurate TCT/CCE signals, critical for replicating this research. |
| Replication & Engineering Support | Engineering Consulting: 6CCVD’s in-house PhD team offers support for material selection, detector design, and understanding the impact of high-rate environments on charge carrier trapping kinetics. | Accelerate R&D timelines by leveraging expert knowledge in MPCVD growth parameters, crystal quality, and radiation detection applications. |
Applicable Materials
Section titled “Applicable Materials”To replicate or extend this research concerning rate-dependent polarization in high-energy physics detectors, 6CCVD recommends:
- Electronic Grade SCD (Single Crystal Diamond): Necessary for high spatial resolution and highest inherent radiation hardness potential, serving as the baseline material studied.
- High-Purity PCD (Polycrystalline Diamond): For applications requiring larger areas (up to 125mm) where the inherent lower CCE (due to grain boundaries) is acceptable, but excellent radiation damage tolerance is still required.
Customization Potential
Section titled “Customization Potential”The experimental preparation involved removing 1 µm of surface material and applying specific 4 x 4 mm2 Ti/W electrode patterns. 6CCVD provides comprehensive custom fabrication services:
- Laser Cutting and Shaping: Delivery of diamond wafers in custom dimensions (e.g., the 5 x 5 mm2 utilized here).
- Metalization Patterning: Precise deposition and patterning of complex electrode geometries required for TCT/CCE measurements or integration into large-scale detector arrays.
Call to Action
Section titled “Call to Action”The degradation of CCE in high-rate environments by rate-dependent polarization is a major obstacle for next-generation radiation detectors. 6CCVD supplies the highest quality diamond materials essential for overcoming this challenge through optimized detector design and operation (e.g., high bias voltage operation).
For custom specifications or material consultation on high-rate, radiation-hard particle detection projects, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
The beam condition monitoring leakage (BCML) system is a beam monitoring device in the compact muon solenoid (CMS) experiment at the large hadron collider (LHC). As detectors 32 poly‐crystalline (pCVD) diamond sensors are positioned in rings around the beam pipe. Here, high particle rates occur from the colliding beams scattering particles outside the beam pipe. These particles cause defects, which act as traps for the ionization, thus reducing the charge collection efficiency (CCE). However, the loss in CCE was much more severe than expected from low rate laboratory measurements and simulations, especially in single‐crystalline (sCVD) diamonds, which have a low initial concentration of defects. After an integrated luminosity of a few corresponding to a few weeks of LHC operation, the CCE of the sCVD diamonds dropped by a factor of five or more and quickly approached the poor CCE of pCVD diamonds. The reason why in real experiments the CCE is much worse than in laboratory experiments is related to the ionization rate. At high particle rates the trapping rate of the ionization is so high compared with the detrapping rate, that space charge builds up. This space charge reduces locally the internal electric field, which in turn increases the trapping rate and recombination and hence reduces the CCE in a strongly non‐linear way. A diamond irradiation campaign was started to investigate the rate‐dependent electrical field deformation with respect to the radiation damage. Besides the electrical field measurements via the transient current technique (TCT), the CCE was measured. The experimental results were used to create an effective deep trap model that takes the radiation damage into account. Using this trap model, the rate‐dependent electrical field deformation and the CCE were simulated with the software SILVACO TCAD. The simulation, tuned to rate‐dependent measurements from a strong radioactive source, was able to predict the non‐linear decrease of the CCE in the harsh environment of the LHC, where the particle rate was a factor 30 higher.