  Imaging experiments at the European X-ray Free Electron Laser (XFEL) require
silicon pixel sensors with extraordinary performance specifications: Doses of
up to 1 GGy of 12 keV photons, up to 10^5 12 keV photons per pixel of 200 \mum
\times 200 \mum arriving within less than 100 fs, and a time interval between
XFEL pulses of 220 ns. To address these challenges, in particular the question
of radiation damage, the properties of the SiO_2 layer and of the Si-SiO_2
interface using MOS capacitors manufactured on high resistivity n-type silicon
irradiated to X-ray doses between 10 kGy and 1 GGy, have been studied.
Measurements of Capacitance/Conductance-Voltage (C/G-V) at different
frequencies, as well as Thermal Dielectric Relaxation Current (TDRC) have been
performed. The data can be described by a radiation dependent oxide charge
density and three dominant radiation-induced interface states with
Gaussian-like energy distributions in the silicon band gap. It is found that
the densities of the fixed oxide charges and of the three interface states
increase up to dose values of approximately 10 MGy and then saturate or even
decrease. The shapes and the frequency dependences of the C/G-V measurements
can be quantitatively described by a simple model using the parameters
extracted from the TDRC measurements.
