  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 two vendors, CiS and
Hamamatsu, 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
surface 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 surface density of
oxide charges 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
surface density of oxide charges 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. Finally, annealing studies have
been performed at 60 and 80 degree C on MOS capacitors and gate-controlled
diodes irradiated to 5 MGy and the annealing kinetics of oxide charges and
surface current determined.
