Full Depletion Voltage Research Articles

Performance of silicon pad detectors after mixed irradiations with neutrons and fast charged hadrons

A large set of silicon pad detectors produced on MCz and FZ wafer of p- and n-type was irradiated in two steps, first by fast charged hadrons followed by reactor neutrons. In this way the irradiations resemble the real irradiation fields at LHC. After irradiations controlled annealing started in steps during which the evolution of full depletion voltage, leakage current and charge collection efficiency was monitored. The damage introduced by different irradiation particles was found to be additive. The most striking consequence of that is a decrease of the full depletion voltage for n-type MCz detectors after additional neutron irradiation. This confirms that effective donors introduced by charged hadron irradiation are compensated by acceptors from neutron irradiation.

Annealing study of a bistable cluster defect

This work deals with the influence of neutron and proton induced cluster related defects on the properties of n-type silicon detectors. Defect concentrations were obtained by means of Deep Level Transient Spectroscopy (DLTS) and Thermally Stimulated Current (TSC) technique, while the full depletion voltage and the reverse current were extracted from capacitance–voltage ( C– V) and current–voltage ( I– V) characteristics. The annealing behaviour of the reverse current can be correlated with the annealing of the cluster related defect levels labeled E 4 a and E 4 b by making use of their bistability. This bistability was characterised by isochronal and isothermal annealing studies and it was found that the development with increasing annealing temperature is similar to that of divacancies. This supports the assumption that E 4 a and E 4 b are vacancy related defects. In addition we observe an influence of the disordered regions on the shape and height of the DLTS or TSC signals corresponding to point defects like the vacancy-oxygen complex.

Analytical model for 3D detectors parameterization

A simplified model of operation of the cylindrical p–n junctions in silicon 3D detectors is proposed. To do this, two distinct fragments are recognized in the p–n junction column: a cylindrical space charge region whose radius increases with bias and a semispherical “dead” tip with a maximal electric field due to the focusing effect. For description of the detector operation, three main parameters, namely the pinch-off voltage, the full-depletion voltage and the maximal operational voltage, are introduced and evaluated analytically using the detector material properties and geometry. This approach makes it possible to extrapolate these critical parameters to 3D detector operation in the upgraded LHC. By using possible combinations of two simple elements, the p–n junction and the ohmic column, the different configurations of 3D detectors are analyzed and tabulated.

Studies of the Characteristics of a Silicon Neutron Sensor

Electrical characteristics and neutron dosimetry properties of silicon based p-i-n diodes are presented in support of the applications in the sensors for beam monitoring and medical physics. Both the current-voltage (I-V) and capacitance-voltage (C-V) characteristics of silicon planar p-i-n diode sensors with cylindrical geometry have been theoretically modeled and experimentally measured. The shifts of the forward and reverse diode characteristics of the sensors versus the neutron dose have been obtained. It is shown that the neutron irradiation caused shift of the forward voltage of the p-i-n diodes is proportional to the current at which it is measured in the case of the low level injection or to the square root of the current in the case of the high level injection. The C-V characteristics and the full depletion voltages of the diodes have been estimated and experimentally verified. It is shown that the sensitivity of planar cylindrical structures as neutron sensors can be optimized by the selection of the device geometry and the current at which the measurement is performed.

Development of Radiation Hard ${\rm N}^{+}$-on-P Silicon Microstrip Sensors for Super LHC

Radiation tolerance up to 1015 1-MeV neq/cm2 is required for the silicon microstrip sensors to be operated at the Super LHC experiment. As a candidate for such sensors, we are investigating non-inverting n+-on-p sensors. We manufactured sample sensors of 1 times 1 cm in 4 and 6 processes with implementing different interstrip electrical isolation structures. Industrial high resistive p-type wafers from FZ and MCZ growth are tested. They are different in crystal orientations lang100rang and lang111rang with different wafer resistivities. The sensors were irradiated with 70-MeV protons and characterized in views of the leakage current increase, noise figures, electrical strip isolation, full depletion voltage evolution, and charge collection efficiency.

Radiation Hard Silicon for Medical, Space and High Energy Physics Applications

The objective of this paper is to give an overview on how silicon particle detector would survive operational in extremely harsh radiation environment after luminosity upgrade of the CERN LHC (Large Hadron Collider). The Super-LHC would result in an integrated fluence 1×1016 p/cm2 and that is well beyond the radiation tolerance of even the most advanced semiconductor detectors fabricated by commonly adopted technologies. The Czochralski silicon (Cz-Si) has intrinsically high oxygen concentration. Therefore Cz-Si is considered as a promising material for the tracking systems in future very high luminosity colliders. The fabrication process issues of Cz-Si are discussed and the formation of thermal donors is especially emphasized. N+/p-/p+ and p+/n-/n+ detectors have been processed on magnetic Czochralski (MCz-Si) wafers. We show measurement data of AC-coupled strip detectors and single pad detectors as well as experimental results of intentional TD doping. Data of spatial homogeneity of electrical properties, full depletion voltage and leakage current, is shown and n and p-type devices are compared. Our results show that it is possible to manufacture high quality n+/p-/p+ and p+/n-/n+ particle detectors from high resistivity Czochralski silicon.

Characterization of Radiation Hard Silicon Materials

Segmented silicon detectors are widely used in modern high-energy physics (HEP) experiments due to their excellent spatial resolution and well-established manufacturing technology. However, in such experiments the detectors are exposed to high fluences of particle radiation, which causes irreversible crystallographic defects in the silicon material. Since 1990’s, considerable amount of research has gone into improving the radiation hardness of silicon detectors. One very promising approach is to use magnetic Czochralski silicon (MCz-Si) that has been found to be more radiation hard against charged hadrons than traditional Float Zone silicon material (Fz-Si) used in the current HEP applications. Other approaches include operating the devices at cryogenic temperatures and designing special detector structures such as p-type detectors or semi-3D detectors. In order to demonstrate that the developed technologies are suitable for the HEP experiments, it is necessary to extensively characterize the potentially radiation hard detectors. We have an excellent instrument for this, the Cryogenic Transient Current Technique (C-TCT) measurement setup, which is an effective research tool for studying heavily irradiated silicon detectors. With the C-TCT setup it is possible to extract the full depletion voltage, effective trapping time, electric field distribution and the sign of the space charge in the silicon bulk in the temperature range of 45-300 K. This article

Radiation damage effects in Si materials and detectors and rad-hard Si detectors for SLHC

Silicon sensors, widely used in high energy and nuclear physics experiments, suffer severe radiation damage that leads to degradations in sensor performance. These degradations include significant increases in leakage current, bulk resistivity, space charge concentration, and free carrier trapping. For LHC applications, where the total fluence is in the order of 1 × 1015 neq/cm2 for 10 years, the increase in space charge concentration has been the main problem since it can significantly increase the sensor full depletion voltage, causing either breakdown if operated at high biases or charge collection loss if operated at lower biases than full depletion. For LHC Upgrade, or the SLHC, however, whit an increased total fluence up to 1 × 1016 neq/cm2, the main limiting factor for Si detector operation is the severe trapping of free carriers by radiation-induced defect levels. Several new approaches have been developed to make Si detector more radiation hard/tolerant to such ultra-high radiation, including 3D Si detectors, Current-Injected-Diodes (CID) detectors, and Elevated temperature annealing.

Effect of bias voltage on the space charge in irradiated silicon detectors

Full depletion voltage of irradiated silicon pad detectors was observed to increase with time after applied bias voltage. The increase of V fd , obtained from C– V measurements, is proportional to the fluence and is independent of the irradiation particle type, space charge sign, silicon material, and thickness. Upon switching off the bias voltage, the V fd returns to those expected values for an unbiased sample. However, the leakage current is not affected. The same behavior is observed in the measurements of the charge collection efficiency for the minimum ionizing particles. As the time constants of the increase and decrease of V fd are of the order of 10 h at − 8 ∘ C , the effect can play an important role in the future high-energy physics experiments.

CCE measurements and annealing studies on proton-irradiated p-type MCz silicon diodes

Magnetic Czochralski (MCz) silicon has recently been investigated for the development of radiation tolerant detectors for future high-luminosity HEP experiments. A study of p-type MCz Silicon diodes irradiated with 24 GeV / c protons up to a fluence of 10 16 p cm - 2 has been performed by means of Charge Collection Efficiency (CCE) measurements as well as standard CV/IV characterizations. The changes of CCE, full depletion voltage and leakage current as a function of fluence are reported. A subsequent annealing study of the irradiated detectors shows an increase in effective doping concentration and a decrease in the leakage current, whereas the CCE remains basically unchanged. Two different series of detectors have been compared differing in the implantation dose of p-spray isolation as well as effective doping concentration ( N eff ) of the p-type bulk presumably due to a difference in thermal donor (TD) activation during processing. The series with the higher concentration of TDs shows a delayed reverse annealing of N eff after irradiation.

Comparison of irradiated stFZ silicon sensors using LHC speed front-end electronics

Results from irradiated detector modules assembled with sensors made out of standard p-in-n FZ silicon will be presented. The modules were read out using LHC speed front-end electronics and characterised with different techniques. Two complementary methods to generate the charge inside the sensor were used: An IR laser set-up with a wavelength of λ = 982 nm and a β set-up utilising a 90 Sr source allowing for both relative and absolute CCE measurements. As the aim of our measurement programme is to develop silicon detectors able to operate at the sLHC, these modules were irradiated with three different fluences with the highest corresponding roughly to the fluence expectations for an LHC luminosity upgrade at a radial distance of r ≈ 35 cm [M. Huhtinen, First CMS Upgrade Workshop, CERN, February 26–27, 2004. 〈 http://agenda.cern.ch/fullAgenda.php?ida=a036368 〉 ]. The sensors were characterised before and after irradiation with 26 MeV protons. After irradiation the modules were stored at temperatures below - 40 ∘ C and operated at temperatures around - 5 ∘ C to limit the leakage current and annealing effects. The full depletion voltage V fd and the charge collection efficiency have been measured.

Low-temperature TCT characterization of heavily proton irradiated p-type magnetic Czochralski silicon detectors

n +/p −/p + pad detectors processed at the Microelectronics Center of Helsinki University of Technology on boron-doped p-type high-resistivity magnetic Czochralski (MCz-Si) silicon substrates have been investigated by the transient current technique (TCT) measurements between 100 and 240 K. The detectors were irradiated by 9 MeV protons at the Accelerator Laboratory of University of Helsinki up to 1 MeV neutron equivalent fluence of 2×10 15 n/cm 2. In some of the detectors the thermal donors (TD) were introduced by intentional heat treatment at 430 °C. Hole trapping time constants and full depletion voltage values were extracted from the TCT data. We observed that hole trapping times in the order of 10 ns were found in heavily (above 1×10 15 n eq/cm 2) irradiated samples. These detectors could be fully depleted below 500 V in the temperature range of 140–180 K.

The Cryogenic Transient Current Technique (C-TCT) measurement setup of CERN RD39 Collaboration

The CERN RD39 Collaboration has constructed a Transient Current Technique (TCT) measurement setup, which is capable to operate below liquid nitrogen temperatures. By analyzing the current transients, it is possible to extract the full depletion voltage, effective trapping time, electric field distribution and the sign of the space charge in the silicon bulk. Our results show that the effective space charge and trapping can be manipulated by charge injection and temperature. This might allow significantly higher Charge Collection Efficiency (CCE) compared to the detectors operating under normal reverse bias and at temperatures from 0 to - 30 ∘ C .

Characterization of [formula omitted] thick epitaxial silicon detectors from different producers after proton irradiation

Epitaxial (EPI) silicon has recently been investigated for the development of radiation tolerant detectors for future high-luminosity HEP experiments. A study of 150 μ m thick EPI silicon diodes irradiated with 24 GeV / c protons up to a fluence of 3 × 10 15 p / cm 2 has been performed by means of Charge Collection Efficiency (CCE) measurements, investigations with the Transient Current Technique (TCT) and standard CV / IV characterizations. The aim of the work was to investigate the impact of radiation damage as well as the influence of the wafer processing on the material performance by comparing diodes from different manufacturers. The changes of CCE, full depletion voltage and leakage current as a function of fluence are reported. While the generation of leakage current due to irradiation is similar in all investigated series of detectors, a difference in the effective doping concentration can be observed after irradiation. In the CCE measurements an anomalous drop in performance was found even for diodes exposed to very low fluences ( 5 × 10 13 p / cm 2 ) in all measured series. This result was confirmed for one series of diodes in TCT measurements with an infrared laser. TCT measurements with a red laser showed no type inversion up to fluences of 3 × 10 15 p / cm 2 for n-type devices whereas p-type diodes undergo type inversion from p- to n-type for fluences higher than ≈ 2 × 10 14 p / cm 2 .

Development and test of silicon strip detector for international linear collider

We have been designing and developing DC- and AC-coupled double-sided and single-sided silicon strip detectors for tracking device in the international linear collider (ILC). We present the electrical characteristics of the fabricated sensors such as the leakage currents and the capacitances as a function of reverse bias voltage. We also discuss results of the radiation damage test with 45 MeV proton beam of a cyclotron in Korea Institute of Radiological and Medical Science (KIRAMS) in Seoul, Korea. The leakage currents and the full depletion voltage were measured to be less than 10 nA/strip and about 60 V, respectively. We conclude that developed sensors have an excellent safety margin of radiation hardness appropriate for the ILC. The results of the signal-to-noise ratio (SNR) of the fabricated silicon strip sensors with 90Sr radioactive source and the proton beam of the KIRAMS are also presented. The SNRs of the strip sensor are measured to be 25.0 and 6.7–16.4 for the source test and for the proton beam test, respectively.

P-Bulk silicon microstrip sensors and irradiation

Anticipating the requirement for highly radiation-tolerant silicon microstrip sensors suitable for the SLHC application, we have fabricated n-in-p microstrip sensors in p-FZ and p-MCZ industrial wafers which we then irradiated with 70 MeV protons. Studies were made of the leakage current, onset of microdischarge, body capacitance, charge collection efficiency, and n-strip isolation at the fluences of nil, 0.7×10 14, and 7×10 14 1-MeV neutrons equivalent (neq)/cm 2. The bias and edge structure achieved holding the bias voltages up to 1000 V. The full depletion voltages were about 160, 250, and 600 V in the p-FZ and 1190, 500, and 840 V in the p-MCZ at nil, low, and high fluences, respectively. The radiation damage helped to reduce the density of electron accumulation layer. The strip isolation in the p-MCZ sensors was found to be much better than in the p-FZ sensors; even the no-isolation structure isolated the strips at nil fluence at bias voltage above 50 V. The lower density of electron accumulation layer in the p-MCZ could be attributed to an order less interface trap density in the 〈1 0 0〉 surface than that of 〈1 1 1〉, the negative potential in the inter-strip region by the bias voltage, and the possible effect of high oxygen content in the MCZ bulk.

Magnetic Czochralski silicon as detector material

The Czochralski silicon (Cz-Si) has intrinsically high oxygen concentration. Therefore Cz-Si is considered as a promising material for the tracking systems in future very high luminosity colliders. In this contribution a brief overview of the Czochralski crystal growth is given. The fabrication process issues of Cz-Si are discussed and the formation of thermal donors is especially emphasized. N +/p −/p + and p +/n −/n + detectors have been processed on magnetic Czochralski (MCz-Si) wafers. We show measurement data of AC-coupled strip detectors and single pad detectors as well as experimental results of intentional TD doping. Data of spatial homogeneity of electrical properties, full depletion voltage and leakage current, is shown and n and p-type devices are compared. Our results show that it is possible to manufacture high quality n +/p −/p + and p +/n −/n + particle detectors from high-resistivity Cz-Si.

Characterization of non-uniformly irradiated silicon micro-strip sensors

We describe a method we used to characterize micro-strip sensors, which were non-uniformly irradiated up to a fluence of ∼ 10 14 1 MeV equivalent neutrons per cm 2 . The method allows for a complete bidimensional mapping of the sensor characteristics over the entire active area. Information is gathered through the Q – V characteristic, measured scanning the sensor with an infra-red laser source. Q – V characteristics are then fitted to a simple analytical model, which returns local full-depletion voltages, carrier lifetimes, etc. With the present method one can even obtain the profile of the absorbed fluence. The development and tuning of the present method have been done in the context of the R&D programs for the micro-strip forward tracker of the BTeV experiment at the Tevatron.

Localized energy levels generated in Magnetic Czochralski silicon by proton irradiation and their influence on the sign of space charge density

The microscopic damage produced in diodes made of n-type Magnetic Czochralski (MCz) silicon by 24 GeV and 26 MeV protons, up to the fluence of 1.3 × 10 15 cm - 2 1 MeV equivalent neutrons, has been investigated and results are compared to the damage produced in devices made of standard Floating Zone (STFZ) silicon. It is found by means of Thermally Stimulated Currents (TSC) that the production of a radiation induced charged defect is enhanced in MCz, and might be in part responsible for the differences observed in the two materials at room temperature. The influence of defects on the sign of the space charge density has been studied by current transients at constant temperature i ( T , t ) and by Transient Current Technique (TCT). Type inversion is not revealed up to the highest investigated fluence. Full depletion voltage V dep measurements versus fluence exhibits a minimum close to 2 × 10 14 cm - 2 1 MeV equivalent neutrons; at the same fluence, V dep measured as a function of annealing time changes its initial slope from positive to negative. It is shown by numerical simulations that these features can be accounted by the formation of a double junction, even in absence of type inversion.

Numerical Simulation of Radiation Damage Effects in p-Type and n-Type FZ Silicon Detectors

In the framework of the CERN-RD50 Collaboration, the adoption of p-type substrates has been proposed as a suitable mean to improve the radiation hardness of silicon detectors up to fluencies of 1times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sup> n/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . In this work two numerical simulation models will be presented for p-type and n-type silicon detectors, respectively. A comprehensive analysis of the variation of the effective doping concentration (N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> ), the leakage current density and the charge collection efficiency as a function of the fluence has been performed using the Synopsys T-CAD device simulator. The simulated electrical characteristics of irradiated detectors have been compared with experimental measurements extracted from the literature, showing a very good agreement. The predicted behaviour of p-type silicon detectors after irradiation up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sup> n/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> shows better results in terms of charge collection efficiency and full depletion voltage, with respect to n-type material, while comparable behaviour has been observed in terms of leakage current density