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Abstract: X-ray detection underpins a wide range of applications in medicine, security, industrial inspection, scientific research for non-destructive imaging, and material analysis. The rapid development of Ga2O3-based x-ray detectors offers a promising pathway toward next-generation detectors with high sensitivity, low noise, and harsh environment applications, benefiting from its intrinsic material properties, such as high density, wide bandgap energy, and high thermal–chemical stability. However, the underlying device operating mechanisms, including both carrier excitation and transport processes, have not yet been adequately studied, largely due to the misuse of x-ray sources in previous studies. In addition, benchmarking of device characteristics has been problematic due to experimental or data analysis issues, as well as misunderstandings of the applied equations associated with parameter definitions. In this work, we have designed and performed instructive research based on epitaxial β–Ga2O3:Si and its planar Schottky detectors, measured with energy-tunable monochromatic x-ray beams on a synchrotron beamline, clarifying the device excitation and carrier transport mechanisms with properly benchmarked device performance. In the end, we propose a set of protocols for correctly measuring and analyzing the device performance. The proposed protocols are broadly applicable and can be readily extended to other semiconductor x-ray detectors.

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Abstract: A new class of 3rd generation, bimorph deformable, X-ray mirrors have been developed, which are UHV “bakeable” to 200°C and provide diffraction-limited performance for achromatic focusing and wavefront correction of high-intensity photon beams. Optical metrology was used to reduce slope errors to ∼ 42 nrad rms and height errors to ∼ 200 pm rms for concave, flat, and convex elliptical curvatures. Curved X-ray mirrors with slope errors < 50 nrad rms and height errors < 500 pm rms are required for nano-focusing and coherence applications at low-emittance synchrotron light and free electron laser facilities. In recent years, impressive technical progress has been made to fabricate fixed-curvature X-ray mirrors, approaching diffraction-limited performance. However, for many scientific applications, active optics with a deformable surface profile are required to intermittently change the focal distance or size of the X-ray beam, or to make fine adjustments to the X-ray wavefront. What we believe to be a new class of high-grade, actively deformable optics have been developed, which provide diffraction-limited performance for achromatic focusing and wavefront correction of X-ray beams. 3rd generation, bimorph deformable, X-ray mirrors have piezoelectric PZT actuators bonded to the silicon substrate using silver nano-particles. They can be safely thermally annealed to 200°C and are ultra-high vacuum compatible, making them suitable for a wide range of X-ray energies, including soft X-rays. We present a comprehensive optical metrology study of a 32-channel, 3rd generation bimorph mirror mounted in an opto-mechanical holder to assess suitability for routine beamline operation. Fizeau interferometry and slope profilometry were performed to characterize the range, drift, stability, repeatability, and resolution of bending. Voltages to individual electrodes were optimized to minimise surface errors, based on metrology feedback and a constrained, linear algebra solver. Slope errors of ∼ 42 nrad rms and height errors ∼ 200 pm rms were achieved for three different curvatures (concave, flat, and convex). Metrology testing also demonstrated the extreme resolution of bending (2 nm changes in the height profile by incrementally applying 0.1 V shifts to all piezo actuators) and long-term curvature stability of 0.1% rms over 16 hours. Hysteresis, creep, and short-term drift of the bimorph’s profile were observed, which will be the subject of future research.

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Abstract: A novel aberration-free X-ray compound refractive kinoform lens design based on the Cartesian oval curve is presented, designated as the OVAL-OK (OVAL Overlap Kinoform) lens. Material infilling of the kinoform step structure maintains focal spot dimensions while reducing focal intensity and reproducibility of structures. A SU-8 OVAL-OK lens fabricated through X-ray lithography achieved vertical focal sizes of 70.8 nm (knife-edge scanning) and 56 nm (wavefront propagation analysis) under 15 keV X-ray illumination, using a 120 μm × 200 μm (horizontal × vertical) aperture and 40.8 mm working distance. The lens exhibits a horizontal structural depth of 170 μm and a minimum feature size of 5 μm. The observed discrepancy between direct knife-edge measurements and wavefront-derived values is attributable to the combined effects of geometric, diffraction, coherence, instrumental instability, etc. These results demonstrate the potential for achieving sub-50 nm 2D focusing in future iterations through enhanced structural depth and expanded aperture dimensions.

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Abstract: High-performance synchrotron X-ray focusing mirrors require aspheric surfaces with nanoradian slope accuracy, which are challenging to provide for many manufacturers. Conventional grinding and polishing techniques typically introduce tooling marks to the surface during the fabrication process, and these marks can impose significant challenges for the deterministic figuring processes used as the final step in mirror fabrication, ultimately limiting the slope errors of the mirrors. To overcome this limitation, we present an alternative approach by using ion beam shaping to form an elliptical sub-microfocusing mirror from a flat mirror, followed by ion beam figuring to improve the remaining errors. Two mirrors fabricated using this approach achieved tangential slope errors below 200 nrad root mean squared (rms), in contrast to a conventionally pre-shaped mirror with identical ellipse parameters that was limited to > 500 nrad rms after ion beam figuring due to residual tooling marks. In X-ray focusing tests, the ion beam shaped mirrors are able to realise a spot size below 250 nm (full width half maximum), compared to 404 nm for the pre-shaped mirror. This demonstrates the excellent potential of ion beam shaping as a method for rapid, efficient and high-quality X-ray mirror fabrication.

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Abstract: In this paper, results are presented from the characterisation of a 2 mm thick Redlen Technologies high-flux-capable Cadmium Zinc Telluride (HF-CZT) sensor hybridised to the small-pixel, spectroscopic-imaging HEXITEC_MHz ASIC. Dynamic datasets were taken on the B16 Test Beamline at the Diamond Light Source to study a previously-identified 'excess-leakage-current' phenomenon in HF-CZT, where additional leakage current was temporarily generated upon the application of an X-ray flux. A study of the response of the detector as a function of X-ray intensity demonstrated a measurable excess leakage current signal above 105 ph s-1 mm-2. At a 20 keV flux of 7.81 × 106 ph s-1 mm-2, this effect contributed a signal equivalent to 3.79 ± 1.59 nA mm-2in addition to the expected photocurrent. On removal of X-rays at this flux, this excess leakage current took ∼ 10 s to decay below the noise floor of the detector. This long lifetime has implications for detectors required to operate at high frame rates and fluxes. The use of a small-pixel detector also allowed the spatial variation of this effect to be studied. A per-pixel comparison between the magnitude of the excess leakage current and the spectroscopic performance of the pixel showed no correlation. This suggests that the phenomenon is less likely to be a bulk-crystal effect and more likely the result of the properties of the CZT surface or metal/semiconductor interface. An Arrhenius analysis of the temperature-dependence of the dark and excess leakage currents in the detector yielded values of 0.69 ± 0.04 eV and 0.13 ± 0.01 eV respectively. The change in dark current with temperature is consistent with deep levels pinning the Fermi level close to the mid band gap, whilst the activation energy of the excess leakage current suggests shallower defects at the metal-semiconductor interface are responsible.