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

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.

The InAs X-ray APD has the potential to supersede current commercial solid state X-ray detectors with superlative energy resolution and sensitivity so can have transformative effects on a number of X-ray applications, some of which are described below. Novel scanners at border and security checkpoints: Security screening systems based on metal detectors and chemical sniffers are the common techniques employed for detection of weapons and explosives respectively. However there is a wide range of plastic and ceramic weapons developed using modern technology that is transparent to these systems. X-ray scanners are now being used for these applications for more comprehensive screening. The use of low dosage X-rays is essential to protect the personnel and travellers, resulting in a trade-off between achieving high image resolution, speed and low radiation exposure. InAs X-ray APDs could lead to new, sensitive and high resolution scanners requiring very low radiation levels. Single photon counting camera: Single-photon counting hybrid pixel detectors utilising Si-based readout electronics and compound semiconductors such as GaAs and CdTe has attracted much attention recently. The excellent avalanche gain behaviour in InAs can provide extremely high gain without incurring significant excess noise so can provide orders of magnitude improvement in the photon detection efficiency. Such InAs based photon counting cameras are attractive for space applications (see letter of support from David Lumb, ESA Scientist) since InAs APDs have several advantages including; i) low operation voltage of <30V (Si APDs typically operating at >100V, requiring high voltage supplies), ii) potential large, dense arrays, iii) faster speed (shorter depletion region than Si and the avalanche process is fast since only electrons can impact ionise), iv) better X-ray stopping power, v) high X-ray energy resolution, and vi) better radiation tolerance. X-ray fluorescence: Emission of X-ray spectrum has been widely used for elemental and chemical analysis for characterisation of materials such as metal, glass, and ceramics, as well as for research in geochemistry, forensic science and archaeology. High energy resolution detectors are essential to resolve the X-ray signature(s) of the material being studied. InAs APDs with sub-100eV resolution can improve the resolution of current energy and wavelength dispersive spectrometers. The analysis of forensic samples is often carried out using a combination of techniques, including X-ray and infrared microscopy, to extract as much information as possible. Therefore InAs has the potential to not only provide very high resolution but also provide added detection in the infrared spectrum up to 3 micron leading a new class of microscope. UK companies such as Centronic Ltd, Photek, e2v Scientific Instruments, Rapiscan Systems and Kromek that are likely to benefit from our novel InAs APD development and provide possible a exploitation route. This promising Sheffield invention, when fully developed, can have significant economic impact and propel the UK as the leader in high energy resolution X-ray detectors. The PI is also collaborating with Centronic Ltd in a STFC-funded X-ray project. The PI is currently collaborating with Dr John Lees and Centronic Ltd in a STFC-funded X-ray project so progress made in the proposed work will be communicated to Centronic Ltd in the STFC project meetings. The PI will make contact with the UK-based companies after some progress in InAs X-ray detectors has been made, seeking to communicate progress through presentations during visits. For companies which the PI has already had contacts with, dissemination of progress reports will increase their awareness of the development of InAs X-ray detectors.