Radiation Damage in Silicon - Defect Analysis and Detector Properties -

Frank Hönniger
Jan 2008

Thesis Type:

Silicon microstrip and pixel detectors are vital sensor-components as particle tracking detectors for present as well as future high-energy physics (HEP) experiments. All experiments at the Large Hadron Collider (LHC) are equipped with such detectors. Also for experiments after the upgrade of the LHC (the so-called Super-LHC), with its ten times higher luminosity, or the planned International Linear Collider (ILC) silicon tracking detectors are forseen. Close to the interaction region these detectors have to face harsh radiation fields with intensities above the presently tolerable level. Defect engineering of the used material, e.g. oxygen enrichment of high resistivity float zone silicon and growing of thin low resistivity epitaxial layers on Czochralski silicon substrates has been established to improve the radiation hardness of silicon sensors.

This thesis will focus mainly on the investigation of radiation induced defects and their differences observed in various kinds of epitaxial silicon material. Comparisons with other materials like float zone or Czochralski silicon are added.

Deep Level Transient Spectroscopy (DLTS) and Thermally Stimulated Current (TSC) measurements have been performed for defect characterization after gamma-, electron-, proton- and neutron-irradiation.

The differences in the formation of vacancy and interstitial related defects as well as so-called clustered regions were investigated for various types of irradiation. In addition to the well known defects $VO_i$, $C_iO_i$, $C_iC_s$, $VP$ und $V_2$ several other defect complexes have been found and investigated. Also the material dependence of the defect introduction rates and the defect annealing behavior has been studied by isothermal and isochronal annealing experiments. Especially the $IO_2$-defect which is an indicator for the oxygen-dimer content of the material has been investigated in detail.

On the basis of radiation induced defects like the bistable donor (BD) defect and a deep acceptor, a model has been introduced to describe the radiation induced changes in macroscopic detector properties as affected by the microscopic defect generation. Finally charge collection measurements have been performed at high radiation doses.

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