G. Lindstroem

A new method of carrier trapping time measurement

Abstract A new method of measuring carrier trapping time by a simple analysis of the current pulse shape is proposed and demonstrated for irradiated silicon detectors. This method which we call Exponentiated Charge Crossing (ECC) requires no knowledge of either the electric field profile in the detector or of the relation between the carrier drift velocity and the electric field. It is general enough to be valid not only for solid-state particle detectors but also for other devices such as some gaseous and liquid detectors. The results obtained by the proposed method are consistent with those obtained by an earlier method.

Thermally stimulated current method applied on diodes with high concentration of deep trapping levels

We propose an improved method of thermally stimulated currents (TSC) spectra analysis in the case of diodes having a concentration of traps higher than that of doping impurities. Beside the calculation of trap concentrations from TSC peaks analysis, the method allows us to evaluate the density and the type of the very deep trapping level which, due to the contribution of leakage current, can not be detected in a real TSC experiment. The proposed method is applied to a p+-n Silicon diode irradiated with 1.82×1013neutrons/cm2.

Radiation induced point- and cluster - related defects with strong impact to damage properties of silicon detectors

This work is focusing on the investigation of those radiation induced defects causing degradation effects of Silicon detector performance. Comparative studies of the defects induced by irradiation with Co60- γ rays, 23 GeV protons and 1 MeV equivalent reactor neutrons revealed the existence of some point defects and cluster related centers having a strong impact to damage properties of Si diodes. The detailed relation between the “microscopic” reasons as based on defect analysis and their “macroscopic” consequences for detector performance are presented. In particular, it is shown that the changes in the Si device properties (depletion voltage and leakage current) after exposing to high levels of Co60- γ irradiation can be completely understood by the microscopically investigated formation of two point defects: i) a defect formed via a second order process (I p ) that can be associated with the long searched for V 2 O complex or with a Carbon related center and is the cause for the observed type inversion effect in Oxygen lean material; ii) a bistable donor (BD) created during irradiation that is strongly generated in Oxygen rich material, associated with one of the earlier thermal donors in Si. It is the cause for the observed positive space charge induced by irradiation in oxygenated Si diodes. Specific for hadron irradiation are the annealing effects which decrease resp. increase the originally observed damage effects as seen by the changes of the depletion voltage (effects known as “beneficial” and “reverse” annealing, respectively). A group of four cluster related defects proved to be responsible for these annealing effects. Their formation is not affected by the Oxygen content or Si growth procedure suggesting that they are complexes of multi-vacancies located inside extended disordered regions.

The local hardening effect on electromagnetic showers. A way for signal equalization in Si/high-Z hadron calorimeters

Abstract The condition for obtaining the linear response of a calorimeter to hadronic showers and an energy resolution improving as the incident energy increases is the equalization between the electromagnetic and the hadronic signals. This equalization is obtained within a new approach exploiting a local hardening effect that is realized by inserting low-Z absorbers next to the silicon readout detectors. In this way, the calorimeter response to the electromagnetic component of the hadronic shower is reduced. A systematic investigation of the visible energy response for electromagnetic showers in Si/U and Si/W calorimeters has been carried out for incoming electron energies of 2, 4, and 6 GeV. The insertion of low-Z material (G10 plates) in front or at the rear of the silicon detectors allows a fine tuning of the calorimeter response.

Silicon sampling hadronic calorimetry: A tool for experiments at the next generation of colliders

Abstract The SICAPO Collaboration project to build a perfectly compensating hadron calorimeter using silicon as the active medium, is described. The insertion of low-Z material (G10 plates) in front or at the rear of the silicon detectors allows fine tuning of the calorimeter response to electromagnetic showers. This is a new approach to obtaining compensation. The tuning can be exploited to obtain e/π = 1 (compensation condition). The expected performance ranks this calorimeter among the best candidates to face the severe constraints requested by the next generation of colliders.

Investigation of point and extended defects in electron irradiated silicon—Dependence on the particle energy

This work is focusing on generation, time evolution, and impact on the electrical performance of silicon diodes impaired by radiation induced active defects. n-type silicon diodes had been irradiated with electrons ranging from 1.5 MeV to 27 MeV. It is shown that the formation of small clusters starts already after irradiation with high fluence of 1.5 MeV electrons. An increase of the introduction rates of both point defects and small clusters with increasing energy is seen, showing saturation for electron energies above ∼15 MeV. The changes in the leakage current at low irradiation fluence-values proved to be determined by the change in the configuration of the tri-vacancy (V3). Similar to V3, other cluster related defects are showing bistability indicating that they might be associated with larger vacancy clusters. The change of the space charge density with irradiation and with annealing time after irradiation is fully described by accounting for the radiation induced trapping centers. High resolution ...

DEFECT GENERATION IN CRYSTALLINE SILICON IRRADIATED WITH HIGH ENERGY PARTICLES

Abstract High resistivity silicon with different concentrations of the impurities oxygen and carbon were irradiated with neutrons and charged particles. The deep level transient spectroscopy (DLTS) method is used to determine the defect parameters. During irradiation of silicon with particles lattice atoms are displaced and the primary defects silicon interstitials and vacancies form the impurity defects Ci, CiCs, CiOi and VOi. In the dense displacement regions mainly divacancies VV are formed. The radiation-induced defects change the macroscopic parameters of silicon detectors. During irradiation with neutrons mainly clusters are created. During irradiation with charged particles the generation of single isolated displacements is enhanced due to Coulomb scattering. This is the main difference between irradiation damage after charged particle and neutron irradiation. The higher radiation tolerance of oxygen enriched silicon after charged particle irradiation is related to the higher introduction rates of impurity defects, because only the reaction kinetic of point defects is influenced by the impurity content. The cluster damage is less particle dependent and the threshold energy at which a recoiled silicon atom starts to create a cluster is estimated to be 300 eV.

Compensation condition in Si/U hadronic calorimeter

The first prototype of a Si/U hadronic calorimeter was designed and developed. It consists of silicon mosaics, located next to uranium plates 10-mm thick and 50*50 cm/sup 2/ in area. The total calorimeter depth is six interaction lengths. Measurements performed with the Si/U calorimeter at the CERN PS (at 2, 4 and 6 GeV) support the view that the signal equalization, which provides the condition e/h=1, can be obtained with a silicon readout by tuning the calorimeter response to the electromagnetic component of the hadronic shower. This can be achieved by inserting, in front and/or back of the silicon detector, low-Z absorber (G10 plates). The fiberglass absorbs soft electrons, thus reducing the total energy sensed. The low-Z absorber is used to reduce the response of the electromagnetic component in a hadronic shower, while the pure hadronic component is expected to be only slightly affected. >

The annealing of interstitial carbon atoms in high-resistivity n-type silicon after proton irradiation

The annealing of interstitial carbon Ci after 7-10 MeV and 23 GeV proton irradiations at room temperature in high resistivity n-type silicon is investigated. Deep level transient spectroscopy is used to determine the defect parameters. The annealing characteristics of the impurity defects Ci, CiCs, CiOi and V Oi suggest that the mobile Ci atoms are also captured at divacancy V V sites at the cluster peripheries and not only at Cs and Oi sites in the silicon bulk. The deviation of the electrical filling characteristic of Ci from the characteristic of a homogeneously distributed defect can be explained by an aggregation of Ci atoms in the environment of the clusters. The capture rate of electrons into defects located in the cluster environment is reduced due to a positive space charge region surrounding the negatively charged cluster core. The optical filling characteristic of Ci suggests that the change of the triangle shaped electric field distribution in a reverse biased p + n junction due to charged clusters is negligible.

Kinetics of cluster-related defects in silicon sensors irradiated with monoenergetic electrons

This work focuses on the kinetic mechanisms responsible for the annealing behavior of radiation cluster-related defects with impact on the electrical performance of silicon sensors. Such sensors were manufactured on high resistivity n-type standard float-zone (STFZ) and oxygen enriched float-zone (DOFZ) material and had been irradiated with mono-energetic electrons of 3.5 MeV energy and fluences of 3 × 1014 cm−2 and 6 × 1014 cm−2. After irradiation, the samples were subjected either to isochronal or isothermal heat treatments in the temperature range from 80 °C to 300 °C. The specific investigated defects are a group of three deep acceptors [H(116 K), H(140 K), and H(152 K)] with energy levels in the lower half of the band gap and a shallow donor E(30 K) with a level at 0.1 eV below the conduction band. The stability and kinetics of these defects at high temperatures are discussed on the basis of the extracted activation energies and frequency factors. The annealing of the H defects takes place similarly in both types of materials, suggesting a migration rather than a dissociation mechanism. On the contrary, the E(30 K) defect shows a very different annealing behavior, being stable in STFZ even at 300 °C, but annealing-out quickly in DOFZ material at temperatures higher than 200 °C , with a high frequency factor of the order of 1013 s−1. Such a behavior rules out a dissociation process, and the different annealing behavior is suggested to be related to a bistable behavior of the defect.This work focuses on the kinetic mechanisms responsible for the annealing behavior of radiation cluster-related defects with impact on the electrical performance of silicon sensors. Such sensors were manufactured on high resistivity n-type standard float-zone (STFZ) and oxygen enriched float-zone (DOFZ) material and had been irradiated with mono-energetic electrons of 3.5 MeV energy and fluences of 3 × 1014 cm−2 and 6 × 1014 cm−2. After irradiation, the samples were subjected either to isochronal or isothermal heat treatments in the temperature range from 80 °C to 300 °C. The specific investigated defects are a group of three deep acceptors [H(116 K), H(140 K), and H(152 K)] with energy levels in the lower half of the band gap and a shallow donor E(30 K) with a level at 0.1 eV below the conduction band. The stability and kinetics of these defects at high temperatures are discussed on the basis of the extracted activation energies and frequency factors. The annealing of the H defects takes place similarly ...