A study of the charge collection, storage and processing in pixelated semiconductor detectors

The new generation of accelerator-based X-ray sources, the so-called free electron lasers (FEL), generate extremely intense, coherent and ultra-brilliant radiation in femtosecond pulses that allow to study nano-structures and ultra-fast processes which were previously inaccessable. In order push the limits of the new radiation sources, detectors are needed that can resolve the enormous contrasts in the experimental data reaching from single photons to 100 000 photons with an energy of 12 keV per pixel per pulse. The pnCCD (pn-junction Charge-Coupled Device) meets the challenges of the new light sources since it can handle high radiation intensities at high readout speeds. However, extremely high photon intensities can result in pixel saturation and charge spilling into neighboring pixels. This charge blooming effect reduces the spatial information for FEL diffraction experiments preventing a reliable image reconstruction.
In this thesis, the collection, storage and processing of signal charges in the pixelated pnCCD semiconductor detector was studied. Experimentally, a laser setup was designed and built to inject different amounts of charges in individual pixels and to scan the pixel structure with high spatial resolution. Numerical device simulations were used to model the electric conditions inside the detector during charge collection, storage and transfer. The simulation results were compared with the experimental data and both were used to establish a physical model for the charge handling capacity of pixelated detectors based on the full depletion of the semiconductor substrate. The results enabled a significant increase of the charge handling capacity and therefore, the dynamic range of the pnCCDs.
Furthermore, it was studied how to remove excess charges from the pixel structure before they spill into neighboring pixels. Since pnCCDs have no closed oxide layer at the register side, there is a direct electric acccess to the semiconductor that allows surplus charges to drain from the device. It was analyzed, if it is possible to establish charge drains in the electric potential of the pnCCD by applying the appropriate operation conditions without modifying the pixel layout or fabrication process. This novel mode of operation opens up a new field in imaging with photons but also with electrons and other charged particles with high intensities.