X-ray Radiation Damage Studies and Design of a Silicon Pixel Sensor for Science at the XFEL

Doctoral thesis English OPEN
Zhang, Jiaguo (2013)
  • Publisher: DESY
  • Subject: Dissertation

Experiments at the European X-ray Free Electron Laser (XFEL) require silicon pixel sensors which can withstand X-ray doses up to 1 GGy. For the investigation of X-ray radiation damage up to these high doses, MOS capacitors and gate-controlled diodes built on high resistivity n-doped silicon with crystal orientations <100> and <111> produced by four vendors, CiS, Hamamatsu, Canberra and Sintef, have been irradiated with 12 keV X-rays at the DESY DORIS III synchrotron-light source. Using capacitance/conductance-voltage, current-voltage and thermal dielectric relaxation current measurements, the densities of oxide charges and interface traps at the Si-SiO2 interface, and the surface-current densities have been determined as function of dose.Results indicate that the dose dependence of the oxide-charge density, the interface-trap density and the surface-current density depend on the crystal orientation and producer.In addition, the influence of the voltage applied to the gates of the MOS capacitor and the gate-controlled diode during X-ray irradiation on the oxide-charge density, the interface-trap density and the surface-current density has been investigated at doses of 100 kGy and 100 MGy. It is found that both strongly depend on the gate voltage if the electric field in the oxide points from the surface of the SiO2 to the Si-SiO2 interface.To verify the long-term stability of irradiated silicon sensors, annealing studies have been performed at 60 degree C and 80 degree C on MOS capacitors and gate-controlled diodes irradiated to 5 MGy as well, and the annealing kinetics of oxide charges and surface current were determined.Moreover, the macroscopic electrical properties of segmented sensors have also been investigated as function of dose. It is found that the defects introduced by X-rays increase the full depletion voltage, the surface leakage current and the inter-electrode capacitance of the segmented sensor. An electron-accumulation layer at the Si-SiO2 interface is observed. Its width increases with dose and decreases with applied bias voltage. The electron-accumulation layer is relevant for the change of the electrical properties of segmented sensors. Finally, according to the optimum parameters of silicon pixel sensors from TCAD simulations taking the damage-related parameters into account, a radiation-hard silicon pixel sensor for the AGIPD Project has been designed.
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