Radiation-induced deep-level defects in CCD imaging and spectroscopy sensors

This thesis is concerned with the analysis of radiation-induced defects within the silicon lattice of a Charge-Coupled Device (CCD) image or spectroscopy sensor and particularly those defects with energy levels which can trap charge as it passes through a device, ultimately leading to image quality degradation. A novel technique for the study of certain defect levels within the silicon lattice in a CCD is introduced and developed, allowing for the analysis of individual defects and providing high spatial and temporal resolution. The technique and results both have potentially important consequences for CCD characterisation and use; particularly in the case of space-based detectors for which the radiation environment is harsh and the sensor performance specifications increasingly demanding. CCDs currently constitute the standard sensor for large-scale space-based telescopes such as the Hubble Space Telescope (HST), GAIA, EUCLID etc. Through design processes, careful device optimisation, and post-image correction algorithms the effects of radiation-induced image degradation can be mitigated to levels which have, until now, been sufficient. However the increasingly ambitious scientific goals of such missions (the upcoming EUCLID mission being a prime example) place more and more stringent demands on sensor performance. Therefore further analysis of the defects responsible for loss of image quality is required to enable more efficient and detailed correction algorithms to be implemented, as described in Chapter 3 of this thesis. Current, well-established defect analysis techniques may not have the ability to provide the accuracy required for future missions and so in Chapter 4 a relatively new technique of single trap-pumping is introduced and developed for our research aims. Using this technique both p-channel (Chapter 5) and n-channel (Chapter 7) CCDs are analysed, with the primary aim being to measure defect emission time constants (the length of time between the capture and emission of a charge carrier at a defect level) since this is the property of a defect level which most strongly affects its ability to degrade an image, depending on the operational conditions of the device. A number of important defects are studied over a range of temperatures allowing for further defect parameters, such as energy level, to be deduced. Defect emission time constants are highly temperature-sensitive and so the dominant chargetrapping defects differ depending on the operational temperature of the device and the clock timings used. The effect of device temperature during irradiation on the defect distribution within a CCD is investigated in Chapter 6 using a p-channel CCD irradiated with protons at 153K. The results show large differences between this device and one irradiated at roomtemperature, which is currently the standard practice for device characterisation and testing for all space missions. This is an important result which shows that current irradiation testing procedures may require changing as device performance specifications become more and more demanding. Further potential uses of the trap-pumping method as a defect analysis tool are outlined in Chapter 8 before the entire results are summarised.