Investigation of radiation damage induced effects on capacitance measurements of silicon particle detectors

Future particle physics experiments are facing a growing demand for fast and radiation hard particle detection possibilities which can be accomplished with silicon strip and pixel sensors. To increase their radiation hardness, several attempts of sensor modification are investigated. They all have in common that the irradiated sensor under test has to be evaluated for its key parameters for operation. One of the measurement techniques used for that is the capacitance-voltage measurement (CV) where the capacitance of the sensor is measured as a function of the applied reverse bias voltage. It is evaluated and part of the standard sensor test procedure for unirradiated and lightly irradiated sensors for many years. This work investigates the effects occuring when measuring medium and highly irradiated sensors. First, a setup capable of cooling the sensors down to -40 °C while keeping them in dry atmosphere had to be designed and assembled. Due to the small sensors, it was of special importance to reduce statistical and systematic noise in the low frequency range as far as possible. In the following, measurements for different temperatures had to be conducted and analysed. The dependence of the capacitance on the measurement frequency turned out to be an effective lever for understanding which properties of the sensor contribute to its capacitance. The high frequency range is dominated by the influence of the bias resistor and the mid frequency range exhibits the effect of different resistivities in the depleted and non depleted layer. After the discovery of a current induced capacitance component in the low frequency range, a model was constructed enabling to fit the entire frequency range of measurements. The depletion width obtained from these fits could be compared to the outcome of applying the traditional way of CV measurement analysis. This lead to the identification of three effects impairing the traditional way when applied to irradiated measurements. Several parameters that can be measured with other techniques could be used to ascertain the consistency of the model. Furthermore, the bulk resistivities which had been previously inaccessible could be examined in detail for the first time. It could be shown that deep trap levels cause the strong increase of the resistivity while the physical mechanism causing the current induced capacitance increase could not be completely understood.