B. MacEvoy

Bulk damage effects in irradiated silicon detectors due to clustered divacancies

High resistivity silicon particle detectors will be used extensively in experiments at the future CERN Large Hadron Collider. The detectors will be exposed to particle fluences equivalent to ∼1014 (1 MeV neutrons)/cm2, causing significant atomic displacement damage. A model has been developed to estimate the evolution of defect concentrations and the electrical behavior of irradiated silicon detectors using Shockley–Read–Hall (SRH) semiconductor statistics. The observed increases in leakage current and doping concentration changes can be described well after 60Co-gamma irradiation but less well after fast neutron irradiation. A possible non-SRH mechanism is considered, based on the hypothesis of charge transfer between clustered divacancy defects in neutron damaged silicon detectors. This leads to a large enhancement over the SRH prediction for V2 acceptor state occupancy and carrier generation rate which may resolve the discrepancy between the model and neutron damage data.

Defect evolution in irradiated silicon detector material

Abstract A numerical model based on experimental data has been used to investigate the evolution of atomic defects in high resistivity detector material during neutron and gamma irradiation to levels expected at the CERN LHC. The complexes V 2 O and V 3 O have been identified as candidates for deep-level acceptor states which give rise to experimentally observed changes in the effective doping concentration. The phosphorus dopant is removed by production of VP centres but at a rate lower than previously hypothesised and not fully, even after heavy irradiation. The importance of initial oxygen and carbon impurity concentrations is demonstrated in determining the radiation tolerance of the detectors. A hypothesis for the long term annealing behaviour via the thermal annealing of a trivacancy (V 3 ) state during neutron and heavy particle irradiation is modelled and shown to be a possible explanation of experimental observations.

Radiation damage studies of field plate and p-stop n-side silicon microstrip detectors

Abstract We present results from studies of the properties of dedicated n-side microstrip structures before and after irradiation, with photons to 7 Mrad and fast neutrons to 8 × 10 13 ncm −2 . Both p-stop and field plate devices were investigated, each having a range of strip geometries in order to determine optimal configurations for long-term viability and performance.

Defect evolution in silicon detector material

Abstract A numerical kinetics model has been used to investigate the evolution of complex defects in high resistivity silicon detector material during fast neutron irradiation to levels expected at the CERN LHC. The complex V 2 O is identified as a candidate for a deep-level acceptor state which gives rise to experimentally observed changes in the effective doping concentration. The importance of the initial oxygen impurity concentration in determining the radiation tolerance of the detectors is demonstrated. The characteristics of devices heavily irradiated with 60 Co photons are modelled satisfactorily by using a semiconductor simulation in conjunction with the kinetics model. It is postulated that inter-defect transitions between divacancy states in the terminal damage clusters are responsible for apparent discrepancies in the modelling of data from neutron-irradiated devices. This mechanism (if correct) may have important consequences for the prospects of “defect-engineering” a radiation hard device.

A microscopic explanation for type inversion and the annealing behaviour of radiation damaged silicon detectors

Abstract A semiconductor device model, DLTS measurements and defect kinetics considerations lead us to propose an explanation for the major changes in the macroscopic properties of silicon detectors caused by neutron irradiation.

Beam test performance of the APV5 chip

Abstract The performance of the latest prototype of the radiation hard front end chip to be used by the CMS collaboration for analogue readout of the microstrip tracker has been evaluated with a silicon microstrip detector in a beam at CERN. The circuit, developed by the RD20 collaboration, consists of 128 channels of amplifier, pipeline memory, analogue signal processor and a serial multiplexer. As a result of these studies improvements in the circuit design have been devised which will be implemented in the next version.

Performance of a prototype of the CMS central detector

A prototype of the barrel Tracking Detector of the Compact Muon Solenoid (CMS) experiment proposed for LHC was built and tested in a beam and in a magnetic field of up to 3 T. It contained six microstrip gas chambers, 25 cm long, and three double-sided silicon microstrip detectors, 12.5 cm long. We report some preliminary results on the performance of the chambers.

Beam test of a prototype readout system for precision tracking detectors at LHC

Abstract A prototype of a readout system developed for high spatial precision tracking detectors at LHC has been tested in a beam at CERN. It is based on a radiation hard CMOS front-end chip which includes signal amplification, storage in an analogue delay line and a deconvolution filter. Data were transferred from the front-end chip using an analogue fibre optic link employing a novel reflective electro-optic modulator and continuous laser light source remote from the detector. This is the first time such a system has been used in an experimental environment and is the basis of the system proposed for the CMS experiment at LHC.

Further experimental tests of the deep-acceptor model (*)

SummaryDeep-acceptor states have been hypothesised to be responsible for the observed changes in effective doping concentration in bulk damaged silicon detectors. A hypothesis for anti-annealing as due to the break-up of multi-vacancy defects, such as V3, in the damage clusters has also been proposed. This paper presents data from silicon photodiodes irradiated in steps up to 200 Mrad with60Co photons. Type inversion but no anti-annealing was observed in these devices, based on capacitance and signal collection measurements. The data support both the deep-acceptor model and the anti-annealing hypothesis. The presence of a deepacceptor was confirmed by illuminating the diode duringC-V measurements.

Defect kinetics in silicon detector material

SummaryA numerical model based on experimental data has been used to investigate the evolution of atomic defects in high-resistivity dtector material during neutron irradiation to levels expected at the CERN LHC. The complexes V2O and V3O have been identified as candidates for deep-level acceptor states which give rise to experimentally observed changes in the effective doping concentration. The phosphorus dopant is removed by production of VP centres but at a rate lower than previously hypothesised and not fully, even after heavy irradiation. The importance of initial oxygen and carbon impurity concentrations is demonstrated in determining the radiation tolerance of the detectors. A hypothesis for the long-term annealing behaviour via the thermal annealing of a trivacancy (V3) state during heavy-particle irradiation is modelled and shown to be a possible explanation of experimental observations.