Development of Radiation Hard Radiation Detectors, Differences between Czochralski Silicon and Float Zone Silicon

The purpose of this work was to develop radiation hard silicon detectors. Radiation detectors made ofsilicon are cost effective and have excellent position resolution. Therefore, they are widely used fortrack finding and particle analysis in large high-energy physics experiments. Silicon detectors willalso be used in the CMS (Compact Muon Solenoid) experiment that is being built at the LHC (LargeHadron Collider) accelerator at CERN (European Organisation for Nuclear Research). This work wasdone in the CMS programme of Helsinki Institute of Physics (HIP).Exposure of the silicon material to particle radiation causes irreversible defects that deteriorate theperformance of the silicon detectors. In HIP CMS Programme, our approach was to improve theradiation hardness of the silicon material with increased oxygen concentration in silicon material. Westudied two different methods: diffusion oxygenation of Float Zone silicon and use of high resistivityCzochralski silicon.We processed, characterised, tested in a particle beam, and irradiated silicon detectors and teststructures. Samples were processed at the clean room facilities of Helsinki University of TechnologyMicroelectronics Centre (MEC) where our group has the status of a member laboratory. Electricalcharacterisations were done mainly at CERN at the premises of our collaborators from CERN RD39and RD50 research and development programmes, where our group is participating as a memberinstitute. Defect characterisations were carried out using PCD (Photoconductivity Decay) and SPV(Surface Photovoltage) methods at Helsinki University of Technology Electron Physics Laboratory.Detection performance was measured with a Helsinki Silicon Beam telescope at CERN using muonbeam. Radiation hardness was studied in irradiation tests at Jyväskylä University AcceleratorLaboratory.Our research on the radiation hardness of diffusion oxygenated Float Zone silicon resulted in severalpreviously unreported findings. We found an evident correlation between silicon oxygenation anddetector leakage current after irradiations. Additionally, we found that the oxygenation has a positiveeffect on the long-term stability of irradiated silicon. Furthermore, we successfully applied alternativemethods for the characterisation of silicon detectors, i.e. PCD (Photoconductivity Decay) and SPV(Surface Photovoltage).The most important results of our research were obtained in our work on high resistivity Czochralskisilicon. Although the advantages of Czochralski silicon had been known for some time, we were thefirst group to process, characterise, test in a particle beam, and irradiate full-size Czochralski silicondetectors. In proton irradiations, Czochralski silicon was found to be more radiation hard than anyother silicon material.