Year

2003

Degree Name

Doctor of Philosophy

Department

Department of Engineering Physics - Faculty of Engineering

Abstract

This thesis describes an experimental study into the radiation hardness of high purity silicon. This material is principally used in the manufacture of silicon based microstrip detectors and other similar devices. Radiation detector test structures which had been fabricated on the base of different types of silicon were exposed to ~1 MeV neutrons. This was done to determine the role of different impurities in the formation of radiation induced crystallographic defects within the silicon lattice. Oxygenated silicon, nitrogenised silicon and silicon containing the standard residual impurities was investigated. The effect of the deep level states associated with the defects on the detector electrical properties was also studied. At the relatively high neutron fluence employed, up to 7.5x10(superscript 13) n.cm(superscript -2), the conventional capacitance based Deep Level Transient Spectroscopy (DLTS) technique is not applicable. In order to detect and measure the properties of the defects a new technique was used known as Optical Deep Level Transient Conductance Spectroscopy (ODLTCS). Spectral features identified in the ODLTCS spectra were attributed to known radiation induced defects in silicon through the comparison of the measured energy levels of the associated deep level states and the measured introduction rates with data contained in the literature. Using ODLTCS the kinetics of the growth and contraction of particular defect concentrations in each of the irradiated detector types was measured as a function of room temperature annealing. Correlation in the evolution of the radiation induced C(subscript i)-O(subscript i) and the short term annealing of the effective impurity concentration (N[subscript eff]) was observed. Based on this finding a microscopic explanation for the improved radiation hardness of oxygenated silicon is described. Other possible mechanisms of defect engineering were also investigated. No deep level defect identified from the ODLTCS spectra could be attributed to the long term reverse anneal of N(subscript eff). This suggested that the responsible defect had a energy state outside the ODLTCS detection limit of less than (0.16 eV) as measured from either the conduction or valence band edge. Significant reduction in the production rate of the V-O defect was observed in nitrogenised silicon. Evidence supporting possible metastability of the V-O defect was also obtained. Another important aspect of this research was the development of technologies for use in the on-line monitoring of radiation damage to silicon devices in mixed radiation fields. It is shown that a PIN Dosimeter diode which has been calibrated in an epithermal neutron beam in terms of (?, see abstract in 01Front file) can be used to measure (?, see abstract in 01Front file) in a fast neutron field. This finding supports the use of a PIN Dosimeter Diode for measuring (?, see abstract in 01Front file) in neutrons fields with any arbitrary energy spectra. The response of the PIN Dosimeter Diode in a high energy electron field in terms of (?, see abstract in 01Front file) is studied. Based on experimental findings it is reasoned that PIN Dosimeter Diode can provide a universal means of measuring dose associated with Non Ionising Energy Loss (NIEL) in silicon when exposed to any mixed radiation field in terms of (?, see abstract in 01Front file). Sensors for measuring does due to Ionising Energy Loss (IEL) in SiO(subscript 2) when exposed to mixed radiation fields were also investigated. It is shown that an IEL sensor based on photodetector is not suitable in a radiation environment containing NIEL type radiations. An alternative sensor in the form of MOSFET is found to suitably radiation hard against dose associated with NIEL and able to measure IEL over a wide range of response. Based on the MOSFET and PIN Dosimeter Diode results a Radiation Damage Monitoring System is designed for the measurement of damage to electronic devices in mixed radiation fields. The system was implemented in the Belle experiment at the KEK B-Factory in Japan, and within the lepton collider at SLAC in the USA.

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