Hybrid pixel detectors (HPDs) are a class of direct electron detectors that have been adopted for use in a wide variety of experimental modalities across all branches of electron microscopy. Nevertheless, this does not preclude the possibility of further improvement and optimisation of their performance for specific applications and increasing the range of experiments for which they are suitable. The aims of this thesis are two-fold. Firstly, to develop a more comprehensive understanding of the current generation HPDs using Si sensors, with a view to optimising their design. Secondly, to determine the advantages of alternative sensor materials that, in principle, should improve the performance of HPDs in transmission electron microscopy (TEM) due to their increased stopping power.
The three chapters review the relevant theoretical background. This includes the physics underpinning the performance of semiconductor-based sensors in electron microscopy as well as the operation of detectors more generally and the theory underlying the metrics used to evaluate detector performance in Chapter 1. In Chapter 2, TEM as a key tool in the study of nano- and atomic scale systems is also introduced, along with an overview of the detector technologies used in TEM. Also presented as part of the background material in Chapter 3 is a description of the experimental methods and software packages used to acquire the results presented in the latter half of the thesis.
Chapter 4, the first results chapter, presents a comparison of the performance of Medipix3 detectors with Si sensors with various combination of pixel pitch and sensor thickness for 60 keV and 200 keV electrons. In Chapter 5, simulations of the interactions of electrons with energies ranging from 30-300 keV with GaAs:Cr and CdTe/CZT, two of the most viable alternatives to Si for use in the sensors of HPDs, are compared with simulations of the interactions of electrons with Si. A comparative study of the performance of a Medipix3 device with GaAs:Cr sensor with that of a Si sensor of the same thickness and pixel pitch for electrons with energies ranging from 60-300 keV is presented in Chapter 6. Also included in this Chapter are the results of investigations into the defects present in the CaAs:Cr sensor material and how these affect device performance. These consist of confocal scanning transmission electron microscopy scans used to estimate the size and shape of individual pixels and how these relate to the linearity of pixels’ response, as well as studies of how the efficacy of a standard flat field depends on the incident electron flux. In the final results chapter, the focus shifts to preliminary measurements of the response of an integrating detector with GaAs:Cr sensor to electrons. These initial experimental measurements prompted further simulations investigating how the backside contact of GaAs:Cr sensors can be improved when using electrons
