Influence of radiation induced defect clusters on silicon particle detectors

Alexandra Junkes
Jul 2011

Thesis Type:

The Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN) addresses some of today’s most fundamental questions of particle physics, like the existence of the Higgs boson and supersymmetry. Two large general-purpose experiments (ATLAS, CMS) are installed to detect the products of high energy proton- proton and nucleon-nucleon collisions. Silicon detectors are largely employed in the innermost region, the tracking area of the experiments. The proven technology and large scale availability make them the favorite choice. Within the framework of the LHC upgrade to the high-luminosity LHC, the luminosity will be increased to $L = 10^{35}\mbox{ cm^{−2} s^{−1}}$. In particular the pixel sensors in the innermost layers of the silicon trackers will be exposed to an extremely intense radiation field of mainly hadronic particles with fluences of up to $\Phi_{eq} = 10^{16}\mbox{ cm^{−2}}$.

The radiation induced bulk damage in silicon sensors will lead to a severe degradation of the performance during their operational time. This work focusses on the improve- ment of the radiation tolerance of silicon materials (Float Zone, Magnetic Czochralski, epitaxial silicon) based on the evaluation of radiation induced defects in the silicon lattice using the Deep Level Transient Spectroscopy and the Thermally Stimulated Current methods. It reveals the outstanding role of extended defects (clusters) on the degrada- tion of sensor properties after hadron irradiation in contrast to previous works that treated effects as caused by point defects.

It has been found that two cluster related defects are responsible for the main gener- ation of leakage current, the E5 defects with a level in the band gap at $E_C-0.460\mbox{ eV}$ and E205a at $E_C-0.395\mbox{ eV}$ where $E_C$ is the energy of the edge of the conduction band. The E5 defect can be assigned to the tri-vacancy ($V_3$) defect. Furthermore, isochronal annealing experiments have shown that the $V_3$ defect exhibits a bistability, as does the leakage current. In oxygen rich material the defect transforms ($V_3$ activation energy for migration $E_a = 1.77 ± 0.08\mbox{ eV}$) to the L defect, which can be assigned to the $V_3O$ defect.

In the second part of this work, it is demonstrated that the radiation induced effective doping concentration can be attributed to the generation of three deep acceptors (H(116K), H(140K), H(151K)), two donors (BD defect and E(30K)) and the vacancy- phosphorus defect VP. The reverse annealing of the effective doping concentration is presented for samples irradiated with neutrons for fluences up to $\Phi = 10^{15}\mbox{ cm^{−2}}$. From defect concentrations it is possible to reproduce the effective doping concentration as extracted from capacitance-voltage characteristics.
The last part of this work deals with the characterisation of Float Zone pad sensors in the frame of the CMS tracker upgrade programme. Due to a new production process, several material defects were introduced in the sensors. They explain unexpected electrical properties in thin sensors.

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