High performance X-ray detectors with sub-100eV energy resolution

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Soft X-ray detection is used in a wide range of applications ranging from material research, medical imaging, to industrial manufacturing inspection. This proposal aims to develop a new class of detectors for soft X-ray (<20keV) with potential to achieve better energy resolution and sensitivity than existing cooled Ge and Si detectors. These new X-ray detectors wil be made using InAs, a III-V semiconductor material which has a rare combination of properties for X-ray detection superior to Si, including high X-ray absorption coefficients, high number of carriers generated per absorbed photon, and ability to produce high gain without introducing excess noise (recently discovered at Sheffield).These InAs X-ray avalanche photodiodes (APDs) will be cooled to reduce unwanted leakage current. Our InAs APDs have the potential to achieve the fundamental Fano factor limited energy resolution rather than avalanche gain limited, thus improving on the best data from Si X-ray APDs. This could be the enabling technology to harvest the benefit of high internal gain without incurring significant energy resolution degradation, which has long been the main drawback of X-ray APDs. Its attenuation length, the distance by which the 63% of the X-ray is absorbed, is more than 6 times shorter than that of Si at 5.9keV photon energy, with increasing advantage at higher energies. This enables much shallower InAs pixel to be fabricated and could lead to a new generation of large format arrays that are compatible with digital X-ray imaging. Thus the ultimate aim of this proposal is to demonstrate shallow (<30micron) single-pixel InAs APDs with high gain and excellent energy resolution for soft X-ray detection. This new capability could underpin the next generation X-ray detection for applications such as non-destructive imaging (failure analysis and inspection in material research, electronic component/circuit board inspections and mechanical parts/assemblies), medical imaging (large area imaging of patients, angiography to selectively show blood vessels in the body), real-time imaging (to assist surgeries), and security screening of concealed objects through imaging.Developing InAs X-ray APDs is now finally feasible, owing to exciting developments in wafer growth and device fabrication at Sheffield in the recent years for InAs infrared APDs. Although there are significant different challenges in the development of InAs X-ray APDs, progress in both projects will be accelerated by the synergy. The investigator, who has years of experience with APDs and is currently developing AlGaAs X-ray detectors for room temperature operation, is well-placed to carry out the work. In addition to demonstration of high performance InAs X-ray APDs, this project aims to develop a device simulator of X-ray APDs which will be available to other X-ray detector researchers. This simulator has the unique ability to predict energy resolution limit, taking into account the statistical contribution of the avalanche gain process. In addition to the X-ray APD simulator, the proposed work will establish wafer growth conditions and device fabrication procedures appropriate for InAs X-ray APDs and eventually demonstrate devices with high gain and excellent energy resolution.