Radiation induced defects
The interaction of traversing particles with the silicon lattice causes displacement of lattice atoms, known as Primary Knock-on Atoms (PKAs) The kinetic energy transferred to PKAs varies significantly based on the type and energy of the incoming particle PKAs lose energy through further displacements and ionization, with those exceeding 35 keV creating dense clusters of displacements, while lower-energy displacements affect only a few atoms Displaced atoms are termed Interstitials (I), while the resulting gaps are referred to as Vacancies (V) Both interstitials and vacancies are mobile within the silicon lattice, engaging in various reactions with impurities and other radiation-induced defects.
Radiation damage in detectors
Three main macroscopic effects are seen in high-resistivity silicon detectors following energetic hadron irradiation (see e.g [ 6 , 7 ]) These are:
• Change of the effective doping concentration with severe consequences for the operating voltage needed for total depletion (see Figure 1).
• Fluence proportional increase in the leakage current, caused by creation of generation/recombination centers (see Figure 2).
• Deterioration of charge collection efficiency due to charge carrier trapping leading to a reduction of the effective drift length both for electrons and holes
"p - type" n - type n - type type inversion type inversion
The electrical properties of various n-type and p-type materials are crucial for semiconductor applications N-type float zone (FZ) silicon exhibits resistivities ranging from 3 KΩ cm to 25 KΩ cm, with specific values including 3 KΩ cm, 4 KΩ cm, 7 KΩ cm, 410 Ω cm, 780 Ω cm, and 130 Ω cm In contrast, p-type epitaxial (EPI) silicon shows resistivities of 2 KΩ cm, 4 KΩ cm, and 380 Ω cm Understanding these resistivity ranges is essential for optimizing semiconductor performance in electronic devices.
Figure 1.: Example for the change of the depletion voltage with increasing particle fluence [ 8 ].
Figure 2.: Increase of leakage current with fluence for different types of materials measured after an annealing of 80 min at 60 °C
The primary impact on current detectors at the LHC is significant, as the depletion voltage (Vdep) required to fully extend the electric field across the depth of an asymmetric junction diode, such as a silicon detector, is directly linked to the effective doping concentration (N eff) of the bulk material.
In a non-irradiated n-type detector, the depletion voltage (V dep) is influenced by the concentration of shallow donors, typically phosphorus, resulting in a positive effective space charge (N eff) However, exposure to energetic hadron irradiation alters V dep, initially causing a decrease due to donor removal, followed by a type inversion where the effective space charge becomes negative As fluence continues to increase, V dep rises again, potentially surpassing the device's operational voltage, necessitating that the detector operates below full depletion This condition results in incomplete charge collection, leading to reduced signals from minimum ionizing particles (mips) Post-irradiation, V dep exhibits complex annealing behavior, notably reverse annealing, which significantly increases V dep over time unless the detector is maintained below approximately 0 °C This requirement imposes strict limitations on the maintenance of high-energy physics (HEP) detectors, which must be serviced at lower temperatures or for minimal durations Despite these precautions, it remains impossible to eliminate the temperature and time-independent damage effects.
The second and third effects mentioned impact the signal-to-noise (S/N) ratio, leading to increased power dissipation and reduced spatial resolution in mips detection Operating the detector at moderately low temperatures, around -10 °C, significantly minimizes leakage current, ensuring lower noise and power dissipation While the trapping effects are manageable for current LHC experiments, they may pose limitations for future high luminosity colliders, as discussed in the following section.
Present limits of operation
Recent research on radiation-hardened silicon detectors has concentrated on their performance following exposure to neutron or charged hadron fluences reaching 10^15 cm^-2 At this fluence level, significant alterations in the macroscopic parameters of the detectors have been observed.
The effective drift lengths for electrons and holes are approximately 150 µm and 50 µm, respectively Additionally, the material experiences a significant inversion of its conduction type, attributed to vacancy-related radiation-induced deep acceptors, which initiate a depletion process beginning at the n-contact.
– Fluence proportional increase of leakage current per unit volume due to the presence of radiation induced generation/recombination centers (I/V ≈ 30 mA/cm 3 at 20 °C).
The negative space charge increases to 10^12 cm^-3, necessitating approximately 1000 Vdep for full depletion at 300 µm Additionally, the phenomenon of reverse annealing is observed, characterized by an increase in negative space charge following extended annealing at room temperature.
– Deterioration of the charge collection efficiency due to a combination of trapping and incomplete depletion, both for pixels and simple non-segmented pad structures
Objectives
The main objective of the R&D program is:
To develop radiation hard semiconductor detectors that can operate beyond the limits of present devices These devices should withstand fast hadron fluences of the order of
10 16 cm -2 , as expected for example for a recently discussed luminosity upgrade of the LHC to
This article aims to provide recommendations for optimizing the materials, device structures, and operational conditions of radiation detectors, along with essential quality control procedures to enhance radiation tolerance These recommendations will be substantiated by tests conducted on a generic demonstrator detector system, evaluated under realistic operational conditions.
To enhance our understanding of radiation damage in silicon and other semiconductors used in detectors, we aim to support and collaborate with high-energy physics (HEP) research focused on radiation effects This includes efforts to develop radiation-resistant detector materials for linear collider programs.
Strategy
Recent advancements in CERN's research and development projects, along with breakthroughs in radiation-hardened semiconductor devices, have led to the identification of three essential strategies for creating more resilient tracking detectors.
Material engineering involves the intentional alteration of bulk materials in detectors A key method in this field is defect engineering of silicon, which entails enhancing the silicon base material by introducing elements such as oxygen, oxygen dimers, or other impurities to improve its properties.
Another approach is the use of other semiconductor materials than silicon (Section 9) like e.g silicon carbide
This strategy focuses on enhancing current planar detector structures through modifications in electrode configuration and bulk material thinning, as well as the advancement of innovative detector geometries like 3D detectors.
The changes of the detector operational conditions include for example the operation of silicon detectors at low temperatures or under forward bias (Section 8).
To significantly enhance the radiation hardness of detectors, we anticipate that each of the three proposed strategies will contribute positively; however, the optimal outcome may be achieved through a strategic combination of these methods A deep understanding of the physics behind radiation-induced degradation and the charge collection efficiency of various detector types is crucial for the success of our research program.
With basic studies we mean the characterization of microscopic defects as well as the parameterization of macroscopic detector properties in dependence of different irradiation and annealing conditions
• Defect modeling and device simulation
Computer simulations comprehensively analyze the entire radiation damage process, starting with the initial interactions of damaging particles with the semiconductor lattice These simulations examine the subsequent formation of defects and their structural and electrical characteristics Additionally, they assess how these defects influence the macroscopic properties of detectors and include simulations of the overall device performance in the presence of these defects.
To evaluate the detector performance under realistic operational conditions, a substantial part of the tests will be performed on segmented devices:
• Test of segmented devices and detector systems
We aim to assess the impact of radiation damage on segmented test structures, such as mini-strips or pixels Concurrently, we will conduct tests on basic detector systems to evaluate key performance metrics, including speed, signal-to-noise ratio, spatial resolution, efficiency, and sensor power dissipation, while minimizing multiple scattering effects.
The proposed project's three-year work plan, detailed in Section 11, aims to investigate all relevant materials and technologies necessary for a high luminosity collider, specifically focusing on very high gamma and charged hadron fluences This exploration will encompass both fundamental structures essential for material studies and advanced segmented devices, such as pixel and microstrip technologies.
The research will culminate in a recommendation for the best detector material, design, and operational conditions, leveraging advanced readout electronics Additionally, a follow-up program will be proposed to enhance the most promising technologies and facilitate their integration into experimental applications.
Project phases and collaborationsCollaborations with other R&D projects
Two project phases are foreseen (for details see Sec.11):
• Phase I – Evaluation of radiation hardening concepts and possibilities
In Phase I of the project, we will investigate various materials and technologies essential for a high luminosity collider, focusing on extreme gamma and charged hadron fluences This phase will encompass both fundamental structures vital for material analysis and advanced segmented devices, such as pixel and microstrip detectors, paired with cutting-edge readout electronics The research will be complemented by data modeling to enhance the direction of our studies, setting the stage for Phase II of the program.
• Phase II – Applied research on most promising technologies
At the conclusion of Phase I, the most viable and promising technologies will be identified Phase II will concentrate on these selected technologies, facilitating their integration into experimental applications.
In order to share resources and scientific results the research program will be performed in close collaboration with other R&D efforts on detector and electronics radiation hardness issues:
• Research for the Linear Collider Program
Radiation damage in linear collider detectors is primarily caused by fast lepton interactions, leading to point defects, whereas hadron collider detectors experience both point defects and clustered defects Investigating electron damage allows for a focused study on point defects, which is essential for understanding the more complex hadronic damage Although the hadronic radiation damage in linear colliders is minimal, it remains significant and poses a considerable challenge for radiation hardness Consequently, developing defect-engineered materials that exhibit greater radiation tolerance to hadron damage is crucial for linear colliders A notable example is oxygen-enriched silicon, which not only enhances resistance to charged hadron irradiation but also significantly improves radiation hardness against gamma and electron irradiation.
While the detector requirements and anticipated radiation fields in linear collider experiments differ significantly from those in hadron collider experiments, there are shared interests in fundamental research and defect engineering of silicon.
CERN has made significant advancements in various R&D projects, including the cryogenic operation of silicon detectors (RD39), diamond detectors (RD42), and radiation-tolerant electronics (RD49) These initiatives aim to foster collaboration and knowledge sharing through joint testing and workshops.
Defect engineering refers to the intentional introduction of impurities or defects into silicon bulk material at various stages of detector processing This approach aims to mitigate the creation of microscopic defects that negatively impact the overall performance of detectors during or after exposure to radiation By addressing the radiation damage issue at its core, defect engineering effectively enhances the reliability and functionality of silicon-based detectors.
Oxygen enriched silicon
The CERN RD48 (ROSE) Collaboration has introduced oxygen-enriched silicon, known as Diffusion Oxygenated Float Zone Silicon (DOFZ), to the High Energy Physics (HEP) community This innovative technique, first utilized by Zheng Li et al on high resistivity Float Zone (FZ) silicon, involves the diffusion of oxygen into the silicon bulk from an oxide layer, achieved through a standard oxidation process at 1150°C for 24 hours Oxygen depth profiles in various DOFZ samples have been measured using the Secondary Ion Emission Spectroscopy (SIMS) method, showcasing the effectiveness of this approach.
O -c on ce nt ra tio n [a to m s/ cm 3 ]
10 18 HTLT diffusion, 6days/1200 o C HTLT diffusion, 6days/1200 o C enhanced diffusion, 24h/1150 0 C enhanced diffusion, 24h/1150 0 C enhanced diffusion, 12h/1100 o C enhanced diffusion, 12h/1100 o C standard Oxide, 6h/1100 o C standard Oxide, 6h/1100 o C
DOFZ process: diffusion oxygenation of bulk silicon
Carbon-enriched (P503) Standard (P51) O-diffusion 24 hours (P52) O-diffusion 48 hours (P54) O-diffusion 72 hours (P56)
Figure 3.: Oxygen depth profile as measured by
SIMS after different oxygen diffusion processes
[Error: Reference source not found] (HTLT High Temperature Long Time)
Figure 4.: Influence of Carbon and Oxygen on the depletion voltage Vdep [Error: Reference source not found].
In 1998, RD48 revealed that oxygenated materials exhibit significantly enhanced radiation tolerance when compared to charged hadrons The key characteristics of oxygen-enriched silicon are outlined in previous studies and can be summarized as follows:
The increase in negative space charge, indicated by the rise in depletion voltage following type inversion, is diminished by approximately threefold for high-energy charged hadrons, as illustrated in Figure 4.
Recent studies have shown that 23 GeV protons and 192 MeV pions lead to a significant reduction in damage, with low energy protons (16-27 MeV) demonstrating a reduction factor of about 2 However, neutron irradiation did not yield similar improvements Notably, after exposure to high fluences of 23 GeV protons and 192 MeV pions, oxygenated silicon exhibited a saturation in reverse annealing amplitude, resulting in a reduction factor of up to 3 for DOFZ diodes, with a time constant at least double that of previous observations These enhancements provide a considerable safety margin for anticipated effects during warm-up maintenance periods Additionally, the leakage current remains unaffected by oxygen content, and the DOFZ process does not alter surface and interface properties.
The DOFZ technique has been adopted in the detector processes of manufacturing companies, initiated by RD48, resulting in significant experience among various detector producers Both the ATLAS-Pixel and CMS-Pixel collaborations are utilizing DOFZ silicon, although there remain numerous unresolved questions about optimizing the technology and understanding the oxygen effect.
• Why does oxygen improve the radiation tolerance?
The microscopic mechanisms behind the oxygen effect remain unclear, with the specific defects yet to be identified Further studies on defect characterization are essential A promising avenue for exploration is the production and irradiation of a 17 O-doped DOFZ, which has not been previously attempted.
The study of radiation-induced defects in silicon can be significantly enhanced by utilizing 28 silicon samples, particularly through the detailed examination of 17 O atoms and their associated defects using pulsed EPR techniques.
• Quantitative correlation between oxygen content and radiation hardness?
Numerous experiments have confirmed the positive impact of oxygen enrichment on the radiation tolerance of detectors from various manufacturers However, a definitive quantitative relationship between oxygen content and enhanced radiation hardness remains elusive This suggests that the unique manufacturing processes of different producers play a significant role, which is not yet fully understood Addressing these questions is crucial for the future and current applications of oxygenated silicon.
• Reliable characterization of Oxygen and Carbon profiles and simulation of O-diffusion
To accurately measure oxygen and carbon profiles in high-resistivity detector-grade silicon using Secondary Ion Emission Spectroscopy (SIMS), it is essential to operate near the detection limit, necessitating precise absolute calibration Additionally, simulations of oxygen diffusion profiles are crucial for interpreting the shape of the oxygen profiles obtained through SIMS measurements.
The DOFZ process has been analyzed within a temperature range of 1150°C for 16 hours to 1200°C for 8 days SIMS measurements indicate that the low in-diffusion process results in a heterogeneous oxygen distribution, whereas the higher temperature process yields a nearly uniform depth profile Identifying the optimal process that balances radiation hardness and cost-effectiveness is essential.
• More detailed characterization of oxygenated detectors
The oxygenation process significantly suppresses the reverse annealing This needs to be understood, as more improvement may be possible
• High resistivity Czochralski Silicon (CZ)
New developments in the silicon manufacturing technology make high resistivity CZ possible This material might be cheaper than DOFZ and exhibit the same or even better radiation tolerance.
Oxygen dimer in silicon
Recent research has demonstrated the conversion of oxygen interstitials (Oi) to oxygen interstitial dimers (O2i) in silicon This advancement is significant for the future development of radiation-tolerant detectors for two key reasons.
O2i demonstrates greater effectiveness than Oi in enhancing radiation tolerance, as evidenced by the electrically neutral nature of V2O2, in contrast to V2O, which is considered an acceptor situated near the mid-gap.
O2i diffuses more quickly through silicon compared to Oi, with migration energies of 1.8 eV for O2i and 2.54 eV for Oi This highlights a potential method for rapidly oxygenating silicon wafers, as detailed in Section 10.1.1 on Electron Paramagnetic Resonance (EPR).
2 is to introduce Oi into the surface of the wafer by a short high temperature diffusion, convert this
The process involves introducing oxygen (O2i) into the wafer and thermally diffusing it at a significantly lower temperature, which shortens the diffusion time and reduces the furnace temperature needed for oxygenated silicon material preparation Additionally, the vacancy oxygen (VO) can act as both an electron and hole trap depending on its charge state, while VO2 remains electrically neutral Notably, VO is considered the primary charge trap during cryogenic temperature forward bias operations, which restricts the maximum charge collection efficiency at high fluences in this operational mode.
Oxygen dimer silicon diodes have been produced with 10 15 /cm 3 carbon, low (10 15 /cm 3 ) and high
The silicon diodes used in this study were n-type, with a resistivity of 4 kΩ-cm and an oxygen concentration of 10^17/cm^3 To induce dimerisation, the diodes were irradiated at 350°C using a Cobalt-60 gamma source, which uniformly produces interstitial-vacancy pairs throughout the silicon This method offers an advantage over previous attempts using 2 MeV electrons, and also facilitates the formation of divacancies, a crucial aspect of the dimerisation process.
V2 are produced a factor 50 less than single vacancies V [ 23 ]
The quasi-chemical reactions that are thought to lead to Oxygen dimer formation are [ 24 ]:
The successful outcome of the process is evidenced by the absence of the DLTS VO (Vacancy Oxygen) peak (E(90)) in both low and high oxygen samples following proton irradiation Additionally, the presence of the E(170) peak, associated with VOH, was observed after the dimerization process at a concentration of 5×10^11 cm^-3 This concentration remains unchanged after proton irradiation and is insufficient to impact the overall concentration of radiation-induced defects.
C o n ce n tr at io n ( cm -3 )
Figure 5 DLTS spectra of high (309) and low (366) oxygen content silicon diodes D indicates that the sample underwent dimerization process In both 366D and 309D sample the VO (Vacancy-Oxygen) E(90) peak has disappeared [ 25 ].
Deep Level Transient Spectroscopy (DLTS) is a technique used to analyze defects in materials, as described in Section 10.1.1 The term E(90) refers to a specific defect that emits an electron, resulting in a peak in the DLTS spectrum observed at a maximum temperature of 90 K.
Reverse annealed samples measured at –50 °C exhibit a reduction in charge build-up, which is linked to the intensity of the DLTS peak E(226) related to the di-vacancy V2 cluster, particularly in the low oxygen dimered sample.
Recent discoveries have highlighted the potential of this material for radiation hardness applications To fully comprehend the influence of oxygen dimers on defect formation and device performance, systematic studies are essential Comprehensive measurements will be crucial in gaining a deeper understanding of these dynamics.
• Optimization of dose rate and exposure time during material processing
• Defect formation (Infra Red Absorption, DLTS, Electron Parametic Resonance (EPR), positron lifetime)
• Space Charge and Reverse Annealing
• Low temperature and forward bias behavior
In a semiconductor tracking detector, signals are generated on segmented electrodes when the electric field lines from charge carriers change due to their motion This signal formation is governed by the Ramo-Shockley theorem, which utilizes the weighting potential Vw(r), derived from Laplace's Equations, to describe the potential at the signal electrode In highly segmented detectors, the weighting potential exhibits an exponential increase toward the collecting electrode, leading to the dominance of carriers moving toward this electrode in the resulting signal.
After irradiation, the drift of carriers is constrained by charge trapping at radiation-induced defects, leading to an effective drift length defined as Leff = τt ⋅ Vdrift, where τt represents the carrier trapping time and Vdrift is the drift velocity Measurements and simulations indicate that the effective drift lengths are approximately 150 Å for electrons and 50 Å for holes in an electric field of 1 V/Å following a fluence of 1 × 10^15 particles/cm² Considering charge trapping, the signal at an electrode within the depleted region from a charge pair +q-q released at a distance x can be expressed in an approximate manner.
37 where Vw(x) is the weighting potential in the point x and Leff is the effective drift length for the carriers moving towards the electrode It follows that a segmented detector after
• Collect electrons and not holes;
• Have an optimized electrode configuration and detector thickness.
The 3D radiation hard properties are fundamentally geometric, with enhancements typically amplifying material improvements A key feature of the 3D detector design involves creating p and n electrodes as narrow columns throughout the bulk, rather than depositing them parallel to the detector surface In contrast to traditional silicon sensors, which require high voltages for complete depletion and charge collection across the entire wafer thickness (usually 250 - 300 µm) and may become less effective after exposure to high radiation levels, this innovative approach benefits from the reduced distance between collecting electrodes.
This allows at the same time very fast collection times, very low full depletion bias voltage (~10 V), low noise and the full 25 000 e/h provided by the 300 àm detector active thickness
Figure 6.: Schematic, three-dimensional view of part of a sensor with 3D electrodes penetrating through the substrate The front border of the figure is drawn through the center of three electrodes.
Figure 7 3D detectors signal after irradiation with a fluence of 1ã10 15 55 MeV protons/cm 2 The hardness factor corresponds to 1.7ã10 15
Proton irradiation at doses up to 1×10^15 55 MeV protons/cm² results in a sensor with a 100 µm n-n separation becoming fully depleted at 105 V, maintaining a plateau up to 150 V Unirradiated sensors exhibit leakage currents ranging from approximately 0.1 to 1 nA/mm³ of depleted silicon The observed increase in leakage current due to irradiation aligns with the behavior noted in similar planar detectors.
Thin detectors
Thin detectors share similar advantages to 3D detectors, primarily due to their efficient use of the effective drift length of charge carriers combined with a low full depletion voltage This design not only minimizes the material budget but also enhances the overall momentum resolution of particles.
The planar 300 µm silicon detectors strike a balance between signal-to-noise ratio, silicon availability, and mechanical handling ease Future experiments could greatly benefit from thin, low-mass semiconductor trackers, as demonstrated by CCDs at SLAC, which offer enhanced tracking precision, improved momentum resolution, and better timing capabilities These thin detectors also require lower operating voltages, exhibit reduced leakage currents, and demonstrate superior radiation hardness Even after high radiation doses, electrons and holes can still be collected effectively in a p+n segmented diode structure for a thin detector However, the minimum ionizing particle (m.i.p.) signal from a 50 µm silicon sensor layer yields approximately 3,500 electron-hole pairs, characterized by a broad Landau distribution To achieve sufficiently low noise with nanosecond timing, advanced small pixel concept readout electronics are essential.
Technical challenges arise in processing and thinning thin silicon devices, which are of significant interest to the industry for applications in credit and smart cards Collaboration with industry is essential to develop low-cost, reliable samples for testing, whether or not they include readout electronics At this point, providing an accurate cost estimate is challenging without further research and development.
The RD39 collaboration is investigating the Charge Collection Efficiency (CCE) recovery of heavily irradiated planar standard silicon detectors at approximately 130 K, a phenomenon known as the "Lazarus Effect." They are also exploring methods to mitigate space charge polarization effects at low temperatures, specifically the time-dependent reduction of CCE due to charge trapping, through constant charge injection This can be achieved via forward bias operation, as demonstrated by the Lancaster group, or through short wavelength illumination Additionally, operating the detector at low temperatures helps manage space charge Experimental results and simulations from RD39 indicate that utilizing the exponential dependence of energy level occupancy on temperature (~exp(–Et/kT)) effectively controls the charge state of radiation-induced deep traps.
Operating highly irradiated detectors under forward bias or employing alternative techniques to introduce free charge into the detector bulk presents a promising approach for enhancing performance in low-light conditions.
Testing new materials and devices at temperatures around 130 K is anticipated to enhance detector performance and radiation tolerance Close collaboration with RD39 will be pursued to leverage their expertise and optimize resources during these tests.
The RD42 collaboration has investigated the radiation hardness of diamond detectors for the LHC, while other materials have emerged as promising candidates for radiation hard applications A selection of these materials, along with their fundamental radiation-resistant characteristics, is presented Their inclusion in the final proposal will depend on the interest expressed by institutions with relevant expertise in further exploring their potential as radiation hard particle detectors.
Silicon Carbide
Semi-insulating 4H-SiC is a promising candidate for radiation-hard particle detection due to its intrinsic properties With a large band gap of 3.3 eV, it generates approximately 5100 electron-hole pairs per 100 micrometers per minimum ionizing particle (MIP) and exhibits low carrier density These characteristics contribute to minimal leakage current and an impressive initial resistivity, making 4H-SiC an ideal material for high-performance particle detectors.
The current wafer size is 30 mm, but the detector-processing yield is restricted due to a high concentration of as-grown defects in the material After exposure to approximately 4 x 10^14 cm^-2 of 8 MeV protons, the measured charge reached around 2000 electrons with a bias voltage of 500V Additionally, polarization was noted, exhibiting a time constant of approximately 14 minutes and resulting in a final charge accumulation.
Amorphous Silicon
Amorphous silicon, particularly in its hydrogenated form (a-Si:H), is widely utilized in solar cells, flat panel displays, and optical scanners due to its ability to passivate intrinsic dangling bonds caused by missing atoms in its structure These dangling bonds can hinder the material's effectiveness as radiation detectors, as they serve as recombination centers for electrons Currently, Metal Insulator Semiconductor (MIS) and PIN structures have been developed with thicknesses reaching tens of microns through Radio-frequency Plasma Enhanced Chemical Vapor Deposition (PECVD) Notably, the Charge Collection Efficiency (CCE) for 0.8 MeV alpha particles has been recorded at 3%.
GaN-based materials
Gallium Nitride (GaN) has been widely researched for its optical properties and is effectively used in blue laser fabrication Advanced growth techniques, such as Molecular Beam Epitaxy and Chemical Vapor Deposition, enable the production of substrates with low trap density The large band gap of AlGaAs, ranging from 3.4 to 6.2 eV, contributes to low leakage current and high intrinsic resistivity Additionally, its high breakdown voltage of 300 V/µm and the potential for internal gain through electron avalanches present promising opportunities for enhancing charge collection efficiency.
10 Basic Studies, Modeling and Simulations
Radiation-induced alterations in macroscopic properties of silicon detectors, such as leakage current, depletion voltage, and charge collection efficiency, stem from electrically active microscopic defects A thorough understanding of detector degradation due to radiation requires an investigation into these microscopic defects, their reaction and annealing kinetics, and their correlation with macroscopic damage parameters Additionally, modeling defect formation and conducting device simulations are essential for comprehending the complex mechanisms behind defect formation and the functionality of irradiated structured devices.
Studies focused on semiconductor-based devices, such as sensors and electronics, in irradiation environments are crucial for advancing technology To leverage the collective expertise of various research groups, we plan to collaborate closely with the RD39, RD42, and RD49 collaborations at CERN, as well as the LCFI collaboration involved in the TESLA project This partnership will facilitate joint research activities and regular updates on research progress during collaboration meetings.
Basic Studies
In the last years many measurements on irradiation induced microscopic defects in high resistivity
FZ silicon has been extensively studied, yet the specific defects causing macroscopic radiation damage remain unclear To address this gap in understanding, we propose further investigation into the nature of these defects.
• Defect characterization with various different techniques
In recent years, various techniques have been employed for defect characterization in detector silicon, including Deep Level Transient Spectroscopy (DLTS), Thermally Stimulated Current (TSC), and Transient Charge Technique (TCT) Additionally, methods such as Photo Luminescence (PL), Electron Paramagnetic Resonance (EPR), and Fourier Transform Infrared (FTIR) spectroscopy have also been utilized to enhance the understanding of defects in silicon detectors.
Deep Level Transient Spectroscopy (DLTS) is a crucial technique for analyzing the electrical properties of defects in semiconductors This method provides insights into the deep-level defects that can significantly affect semiconductor performance Additionally, Electron Paramagnetic Resonance (EPR) is another essential technique used to investigate the structural properties of these defects Together, DLTS and EPR offer valuable information for understanding the complex behavior of defects in semiconductor materials.
Monitoring capacitance transients by pulsing voltage across semiconductor junctions at various temperatures generates a spectrum with distinct peaks for each deep level The peak height indicates trap density, while its sign differentiates between electron and hole traps The peak's position on the temperature axis helps determine key defect parameters, including defect concentration (Nt), capture cross sections for holes (σh,t) and electrons (σe,t), and the energy level (Et) within the band gap These parameters are crucial for understanding thermal emission and charge carrier capture, enabling the calculation of defect-induced trapping times.
EPR (Electron Paramagnetic Resonance) spectroscopy is a crucial technique that analyzes the absorption of microwave radiation by atoms with unpaired electron spins, significantly advancing our understanding of atomic structures and paramagnetic interactions Recent advancements in EPR instrumentation and theory have improved its resolution and sensitivity, enabling detailed studies of atomic arrangements, bond angles, and dynamic phenomena in solids We advocate for the use of innovative pulse EPR and pulse ENDOR (Electron-Nuclear Double Resonance) techniques to characterize radiation-induced damage in silicon and other semiconductors By recording and analyzing two pulse EPR spectra, we can assess Electron Spin Echo Envelope Modulation (ESEEM) spectra, revealing information about the neighboring nuclear spins This analysis will provide insights into the type, quantity, and proximity of nuclei surrounding unpaired states, allowing for a comprehensive examination of defect environments in irradiated semiconductors over distances up to 40 Å.
• Irradiations at different temperatures – online measurements at low temperatures
At room temperature, both vacancies and interstitials migrate rapidly, making it challenging to directly observe the formation of most defects To gain a clearer understanding of the defect formation process, researchers can conduct irradiations at low temperatures, effectively "freezing" the migration process for detailed analysis.
41 measurements have either to be performed on the beam line (irradiation facility) or the samples have to be transported cold to the measurement setup.
Comprehensive studies on the radiation-induced alterations in detector properties are crucial, as they reveal the relationship between microscopic defects and detector performance These experiments must consider factors such as particle fluence, type, energy, temperature, and annealing time Understanding these variables is essential for accurately predicting radiation damage effects in tracker experimental environments Key areas for further investigation include the intricate dynamics of these interactions.
Most systematic studies on radiation-induced changes in effective doping concentration have primarily relied on depletion voltage measurements derived from Capacitance-Voltage (CV) assessments However, to gain a more comprehensive understanding of trapping behavior, it is essential to conduct systematic charge collection measurements, either using lasers—though this method poses challenges for absolute calibration—or employing minimum ionizing particles (MIPs).
• Comparison between pixels, full-size or mini strip detectors and simple test structures
Such measurements are closely related to investigations on the dependence of CCE and electric field distribution on the device structure.
• Irradiations under bias at operating temperatures (e.g -10°C)
Most irradiations to date have been conducted without applied bias and at room temperature Research indicates that irradiation under bias affects the changes in depletion voltage Given that detectors function under bias and at temperatures lower than ambient, it is essential to explore these effects more thoroughly.
• Establishment of comparable measurement procedures
There exists no agreed common measurement procedure for irradiated detectors Detector treatments after irradiation (annealing procedure) differ strongly from community to community and are making inter-comparison very difficult
• Systematic investigations on the particle and energy dependence (NIEL)
The ROSE collaboration has conclusively shown that the "NIEL-Hypothesis" does not apply universally to all damage parameters, indicating the need for irradiation tests across a broader spectrum of particle energies.
• Combined investigation with state of the art radiation hard electronics
Radiation hard electronics are essential for the optimal performance of silicon trackers The RD49 collaboration has successfully established effective design strategies for current LHC experiments and is actively working on enhancements for future projects.
39 high luminosity scenario Close contacts are foreseen with the RD49 groups and combined tests are planned to evaluate small-scale radiation-hard modules.
Modeling and Simulation
Modeling defect formation is crucial for understanding radiation damage and creating new defect-engineered materials Ab-initio calculations, widely utilized in theoretical solid-state research, can predict the structure, energy levels, and charge states of defects, thereby enhancing our comprehension of these processes.
Recent advancements using both commercial and in-house software have significantly enhanced our understanding of the limitations of irradiated silicon devices Device simulators play a vital role in developing new device structures, optimizing key performance metrics such as signal formation, charge collection efficiency, signal-to-noise ratio, power dissipation, and device thickness.
11 Work Plan, Time Scale and Milestones
Work Plan
The collaborating institutes bring together a diverse range of expertise, including theoretical and applied solid state physics, device and material processing, detector systems, detector design, and defect simulation Table 1 provides a summary of the research interests represented by these institutes.
Radiation studies on full systems 4
Our diverse interests and expertise enable us to advance multiple facets of our research program simultaneously The initial phase of our work plan will involve the formation of specialized working groups, each focusing on specific aspects of the proposed research Fundamental studies and simulations will be integral to this phase Subsequently, a follow-up project may be initiated to refine and test the most effective detector designs and materials, adhering to the approved experimental designs and readout electronics.
This study aims to investigate the impact of oxygen and oxygen dimers on silicon under high luminosity radiation conditions involving neutrons, hadrons, and gammas The research will encompass both microscopic and macroscopic testing of simple structures across various operational conditions, with data supported by simulations Concurrently, existing oxygenated segmented structures, previously fabricated for baseline LHC experiments, will undergo testing at the same radiation levels This comparison is essential for correlating microscopic material characteristics with segmented electric field distributions and fast electronics, ultimately assessing the effects of high radiation fluence on signal-to-noise ratios, power dissipation, and signal speed Special attention will be paid to the influence of radiation under bias and varying operational conditions, such as temperature.
• Evaluation of other detector structures.
Short drift length design detectors will be processed in specialized laboratories within the collaboration, with the resulting structures distributed to other members for irradiation testing and performance evaluations Material selection will focus on options available to the collaboration, including oxygenated silicon and various semiconductors, based on processing limitations.
Institutes with access to radiation-hardened materials will serve as distributors to coordinate a comprehensive evaluation of the technology This evaluation will focus on assessing the formation of radiation-induced defects and other critical aspects of the technology.
The research focuses on the macroscopic response under various operational conditions, emphasizing the necessity for seamless integration with advanced multi-channel electronics To achieve this, continuous collaboration with high-energy physics electronic design teams will be established Additionally, computer simulations will play a crucial role in the research process.
Timescale
The timescale foreseen to complete the Phase I of the research plan is 3 years.
Milestones
• Share of responsibilities within the collaboration’s working groups
• Design and fFabrication of common test structures
• Irradiations of simple and segmented structures (oxygenated and non-oxygenated) up to
10 16 cm n, p and very high γ -2 and e doses
• Agreement on post-irradiation detector handling and measurement procedures
• II Workshop on radiation hard semiconductor detectors for very high luminosity collidersand
• Full comparative characterization of simple and segmented structures with oxygenated and non-oxygenated silicon.
• Improved understanding of the “proton/neutron puzzle” and the microscopic mechanisms leading to the beneficial effect of oxygen in silicon.
• Design and fabrication of segmented structures using dimered silicon and other semiconductor materials
• Processing of short drift length structures using other semiconductors and segmented devicesDesign and fabrication of thin and 3D detectors
• III Workshop and on radiation hard semiconductor detectors for very high luminosity colliders
• Tests of segmented structures and short drift length structures with high speed readoutFull comparative characterization of above described devices including fast electronics
• IV Workshop and finalon radiation hard semiconductor detectors for very high luminosity colliders
• Final report, containing recommendations for: o detector material o detector structure o operational conditions o further research work
The structure of the R&D collaboration will be established during the initial meetings of the Collaboration Board, and this will be formally recorded in a separate document that includes the agreed-upon publication policy.
In the following we describe a preliminary organization, which was discussed during the workshop on “Radiation hard semiconductor devices for very high luminosity colliders”, held at CERN in November 2001[ 37 ]
The collaboration consists of 150 members from 30 institutes, necessitating the formation of specialized research teams to manage its extensive scientific program Each team is led by a Research Team Convener, while the Spokesperson oversees the overall coordination of the research efforts.
Figure 8: The participating institutes form research teams focused on specific activities Each team is co-ordinated by a Team Convener.
The Collaboration Board (CB) serves as the main decision-making entity for the collaboration, with each institute represented by a single member The CB is responsible for electing a chairperson, a deputy, a spokesperson, and an additional deputy Furthermore, the spokesperson is tasked with nominating the Research Term Conveners and the Budget Holder for the Common Fund, who are subsequently appointed by the CB.
Defect Engineering New Structures Basic Studies /
Figure 9: The participating institutes form research teams focused on specific activities Each team is co-ordinated by a Team ConvenerRole of Collaboration Board and Spokesperson within the collaboration structure.
Participating institutes are responsible for organizing the necessary resources for their research activities within their own laboratories By integrating into a CERN-approved R&D project, they can seek national funding for financial and manpower support The collaboration includes multiple institutes with access to irradiation sources, such as reactors and accelerators, as well as clean room and sensor processing facilities Additionally, a diverse array of specialized equipment for sensor and material characterization is available, detailed in appendix A.
A low volume Common Fund will be established, requiring each institute to contribute a minimum annual amount This fund will support collaboration workshops, cover rental costs for shared equipment, and facilitate other activities of mutual interest For specific project-related investments, such as developing common test structures or acquiring specialized equipment, additional contributions may be solicited from the participating institutes.
CB chairperson + deputy (2 years) elects
Research Team Conveners appoints nominates
Budget Holder of Common Fund appoints nominat es
The new R&D collaboration aims to utilize existing infrastructure and equipment at CERN, specifically accessing lab space in building 143 for irradiated detector characterization, building 28 for general work, and the future Silicon Facility in hall 186, which offers clean space A total of approximately 50 m² will be required temporarily for this initiative.
The collaboration aims to utilize the existing test beams and irradiation facility at CERN's PS complex, which provides 24 GeV/c protons and neutrons The EP-TA1/SD section will oversee this facility and offer essential support for sample preparation, irradiation, and dosimetry, as well as assistance with wire bonding and sensor mounting However, the anticipated workload is expected to be minimal, and some additional support from the EP-MIC, EP-ED, and EP-ESS sections could be beneficial.