Ph.D. degree thesis
accepted by Hamburg University, Inst.f.Experimantal Physics, Germany

Julian Becker

Supervisor: Robert Klanner

Signal development in silicon sensors used for radiation detection

 Keywords:

• Detector modelling and simulations II (electric fields, charge transport, multiplication and induction, pulse formation, electron emission, etc)

 Abstract:
This work investigates the charge collection properties in silicon sensors.

In order to perform the investigations a setup for measurements utilizing the Transient Current Technique (TCT) has been designed and built. Optical lasers with different wavelengths and short pulses (FWHM~$<$~100~ps) have been used to create charge carriers in the sensor volume.

A new parameterization of charge carrier mobilities in bulk silicon as function of electric field and temperature was derived for two different crystal orientations from investigations on pad sensors with low charge carrier densities.

In the course of these investigations a simulation program for current pulses was developed. The program simulates current pulses, which are induced by drift and diffusion of charge carriers for pad sensors, and approximately for strip and pixel sensors.

The simulation program could be used to describe the current pulses of irradiated sensors. Additionally, using the simulation program, it was shown that impact ionization is a possible reason for the recently reported charge multiplication effects in highly irradiated sensors.

The central topic of this work is the investigation of effects of high charge carrier densities, so called plasma effects. In this work plasma effects were created by focusing the lasers.

The measurements of the plasma effects on pad sensors were used as reference measurements for simulations performed by WIAS in Berlin. It was shown that using charge transport models accepted in literature, the observed plasma effects cannot be described.

Measurements on strip sensors were performed with regards to the detector development for the European XFEL.

Measurements of peak currents and charge collection times as function of photon intensity and applied bias voltage allowed the determination of optimum operation parameters of the Adaptive Gain Integration Pixel Detector (AGIPD), which will be used at the European XFEL.

Utilizing position sensitive measurements on strip sensors, the spatial distribution of charge clouds with high charge carrier densities could be determined. These distributions allowed estimations of the influence of plasma effects on the charge sharing behavior of two different pixel sizes for the AGIPD.