  Using multi-channel time-resolved current measurements (multi TCT), the
charge collection of p+n silicon strip sensors for electron-hole pairs produced
close to the Si-SiO2 interface by a focussed sub-nanosecond laser with a
wavelength of 660 nm has been studied. Sensors before and after irradiation
with 1 MGy of X-rays have been investigated. The charge signals induced in the
readout strips and the rear electrode as a function of the position of the
light spot are described by a model which allows a quantitative determination
of the charge losses and of the widths of the electron-accumulation and
hole-inversion layers close to the Si-SiO2 interface. Depending on the applied
bias voltage, biasing history and environmental conditions, like humidity,
incomplete electron or hole collection and different widths of the accumulation
layers are observed. In addition, the results depend on the time after biasing
the sensor, with time constants which can be as long as days. The observations
are qualitatively explained with the help of detailed sensor simulations.
Finally, their relevance for the detection of X-ray photons and charged
particles, and for the stable operation of segmented p+n silicon sensors is
discussed.
