Development of radiation hard radiation detectors : differences between Czochralski silicon and float zone silicon

Abstract

The purpose of this work was to develop radiation hard silicon detectors. Radiation detectors made of silicon are cost effective and have excellent position resolution. Therefore, they are widely used for track finding and particle analysis in large high-energy physics experiments. Silicon detectors will also be used in the CMS (Compact Muon Solenoid) experiment that is being built at the LHC (Large Hadron Collider) accelerator at CERN (European Organisation for Nuclear Research). This work was done in the CMS programme of Helsinki Institute of Physics (HIP).

Exposure of the silicon material to particle radiation causes irreversible defects that deteriorate the performance of the silicon detectors. In HIP CMS Programme, our approach was to improve the radiation hardness of the silicon material with increased oxygen concentration in silicon material. We studied two different methods: diffusion oxygenation of Float Zone silicon and use of high resistivity Czochralski silicon.

We processed, characterised, tested in a particle beam, and irradiated silicon detectors and test structures. Samples were processed at the clean room facilities of Helsinki University of Technology Microelectronics Centre (MEC) where our group has the status of a member laboratory. Electrical characterisations were done mainly at CERN at the premises of our collaborators from CERN RD39 and RD50 research and development programmes, where our group is participating as a member institute. 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 muon beam. Radiation hardness was studied in irradiation tests at Jyväskylä University Accelerator Laboratory.

Our research on the radiation hardness of diffusion oxygenated Float Zone silicon resulted in several previously unreported findings. We found an evident correlation between silicon oxygenation and detector leakage current after irradiations. Additionally, we found that the oxygenation has a positive effect on the long-term stability of irradiated silicon. Furthermore, we successfully applied alternative methods 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 Czochralski silicon. Although the advantages of Czochralski silicon had been known for some time, we were the first group to process, characterise, test in a particle beam, and irradiate full-size Czochralski silicon detectors. In proton irradiations, Czochralski silicon was found to be more radiation hard than any other silicon material.