Al0.2Ga0.8As X-ray photodiodes for X-ray spectroscopy

Three custom-made Al0.2Ga0.8As p-i-n mesa X-ray photodiodes (200 µm diameter, 3 µm i layer) were electrically characterised and investigated for their response to illumination with soft X-rays from an 55Fe radioisotope X-ray source (Mn Kα = 5.9 keV; Mn Kβ = 6.49 keV). The AlGaAs photodiodes were shown to be suitable for photon counting X-ray spectroscopy at room temperature. When coupled to a custom-made low-noise charge-sensitive preamplifier, a mean energy resolution (as quantified by the full width at half maximum of the 5.9 keV photopeak) of 1.24 keV was measured at room temperature. Parameters such as the depletion width (1.92 µm at 10 V), charge trapping noise (61.7 e− rms ENC at 5 V, negligible at 10 V) and the electronic noise components (known dielectric noise (63.4 e− rms), series white noise (27.7 e− rms), parallel white noise (9.5 e− rms) and 1/f series noise (2.2 e− rms) at 10 V reverse bias) affecting the achieved energy resolution were computed. The estimated charge trapping noise and mean energy resolution were compared to similar materials (e.g. Al0.8Ga0.2As) previously reported, and discussed. These results are the first demonstration of photon counting X-ray spectroscopy with Al0.2Ga0.8As reported to date

Gallium Arsenide detectors for X-ray and electron (beta particle) spectroscopy

Results characterizing GaAs p+-i-n+ mesa photodiodes with a 10 µm i layer for their spectral response under illumination of X-rays and beta particles are presented. A total of 22 devices, having diameters of 200 µm and 400 µm, were electrically characterized at room temperature. All devices showed comparable characteristics with a measured leakage current ranging from 4 nA/cm2 to 67 nA/cm2 at an internal electric field of 50 kV/cm. Their unintentionally doped i layers were found to be almost fully depleted at 0 V due to their low doping density. 55Fe X-ray spectra were obtained using one 200 µm diameter device and one 400 µm diameter device. The best energy resolution (FWHM at 5.9 keV) achieved was 625 eV using the 200 µm and 740 eV using the 400 µm diameter device, respectively. Noise analysis showed that the limiting factor for the energy resolution of the system was the dielectric noise; if this noise was eliminated by better design of the front end of the readout electronics, the achievable resolution would be 250 eV. 63Ni beta particle spectra obtained using the 200 µm diameter device showed the potential utility of these detectors for electron and beta particle detection. The development of semiconductor electron spectrometers is important particularly for space plasma physics; such devices may find use in future space missions to study the plasma environment of Jupiter and Europa and the predicted electron impact excitation of water vapor plumes from Europa hypothesized as a result of recent Hubble Space Telescope (HST) UV observations

Soft X-ray detection and photon counting spectroscopy with commercial 4H-SiC Schottky photodiodes

The results of electrical characterisation and X-ray detection measurements of two different active area (0.06 mm2 and 0.5 mm2) commercial 4H-SiC Schottky photodiodes at room temperature are reported. The devices exhibited low dark currents (less than 10 pA) even at a high electric field strengths (403 kV/cm for 0.06 mm2 diodes; 227 kV/cm for 0.5 mm2 diodes). The results of the X-ray measurements indicate that the diodes can be used as photon counting spectroscopic X-ray detectors with modest energy resolutions: FWHM at 5.9 keV of 1.8 keV and 3.3 keV, for the 0.06 mm2 and 0.5 mm2 devices, respectively. Noise analysis of the photodiodes coupled to a custom low noise charge sensitive preamplifier is also presented

4H-SiC Schottky diode arrays for X-ray detection

Five SiC Schottky photodiodes for X-ray detection have been electrically characterized at room temperature. One representative diode was also electrically characterized over the temperature range 20°C to 140 °C. The performance at 30 °C of all five X-ray detectors, in both current mode and for photon counting X-ray spectroscopy was investigated. The diodes were fabricated in an array form such that they could be operated as either a 2×2 or 1×3 pixel array. Although the devices showed double barrier heights, high ideality factors and higher than expected leakage current at room temperature (12 nA/cm2 at an internal electric field of 105 kV/ cm), they operated as spectroscopic photon counting soft X-ray detectors uncooled at 30 °C. The measured energy resolution (FWHM at 17.4 keV, Mo Kα) varied from 1.36 to 1.68 keV among different diodes

Characterization of gallium arsenide X-ray mesa p-i-n photodiodes at room temperature

Two GaAs mesa p+-i-n+ photodiodes intended for photon counting X-ray spectroscopy, having an i layer thickness of 7 μm and diameter of 200 μm, have been characterized electrically, for their responsivity at the wavelength range 580 nm to 980 nm and one of them for its performance at detection of soft X-rays, at room temperature. Dark current and capacitance measurements as a function of applied forward and reverse bias are presented. The results show low leakage current densities, in the range of nA/cm2 at the maximum internal electric field (22 kV/cm). The unintentional doping concentration of the i layer, calculated from capacitance measurements, was found to be <1014 cm−3. Photocurrent measurements were performed under visible and near infrared light illumination for both diodes. The analysis of these measurements suggests the presence of a non-active (dead) layer (0.16 μm thickness) at the p+ side top contact interface, where the photogenerated carriers do not contribute to the photocurrent, possibly due to recombination. One of the diodes, D1, was also characterized as detector for room temperature photon counting X-ray spectroscopy; the best energy resolution achieved (FWHM) at 5.9 keV was 745 eV. The noise analysis of the system, based on spectra obtained at different shaping times and applied reverse biases, showed that the dominant source of noise is the dielectric noise. It was also calculated that there was at least (165±24) eV charge trapping noise at 0 V

Relevance of VEGF and Nephrin Expression in Glomerular Diseases

The glomerular filtration barrier is affected in a large number of acquired and inherited diseases resulting in extensive leakage of plasma albumin and larger proteins, leading to nephrotic syndrome and end-stage renal disease. Unfortunately, the molecular mechanisms governing the development of the nephrotic syndrome remain poorly understood. Here, I give an overview of recent investigations that have focused on characterizing the interrelationships between the slit diaphragm components and podocytes-secreted VEGF, which have a significant role for maintaining the normal podocyte structure and the integrity of the filtering barrier

Measurement of the electron–hole pair creation energy in Al0.52In0.48P using X-ray radiation

The average energy consumed in the generation of an electron–hole pair (ε AlInP ) in Al 0.52 In 0.48 P was experimentally measured across the temperature range −20 °C to 100 ∘ C, using a custom AlInP X-ray-photodiode, an 55 Fe radioisotope X-ray source, and custom low-noise charge-sensitive preamplifier electronics. ε AlInP was found to linearly decrease with increasing temperature according to the equation ε AlInP = (-0.0033 eV/K ± 0.0003 eV/K)T + (6.31 eV ± 0.10 eV). At room temperature (20 °C), ε AlInP = 5.34 eV ± 0.07 eV

Prototype GaAs X‐ray detector and preamplifier electronics for a deep seabed mineral XRF spectrometer

Work towards developing a prototype GaAs based X-ray fluorescence spectrometer focusing on the detector-preamplifier system for in situ characterisation of deep seabed minerals is presented. Such an instrument could be useful for marine geology and provide insight into hydrothermal processes. It would also be beneficial for deep sea mining applications. The GaAs photodiode was electrically characterised at 4 °C (ambient seawater temperature) and 33 °C. A system energy resolution (full width at half maximum) at 5.9 keV of 580 eV at 4°C, limited by the dielectric noise, broadening to 680 eV at 33°C, was recorded. The spectral performance of the system was characterised across the energy range 4.95 keV to 21.17 keV, at 33°C, using high-purity X-ray fluorescence calibration samples excited by a Mo target X-ray tube. The charge output from the system was found to be linear with incident photon energy. The energy resolution was found to broaden from 695 eV at 4.95 keV to 735 eV at 21.17 keV, attributed to the increasing Fano noise with energy. The same X-ray tube was used to fluoresce an unprepared manganese nodule (revealing the presence of Mn, Fe, Ni, Cu, Zn, Pb, Sr, and Mo) and a black smoker hydrothermal vent sample (containing Fe, Co, Ni, Cu, Zn, Pb, and Mo). Such a spectrometer may also find use in future space missions to study the hydrothermal vents that are believed to exist in the oceans of Jupiter's moon Europa

A silicon drift detector-CMOS front-end system for high resolution X-ray spectroscopy up to room temperature

A system constituted by a Silicon Drift Detector (SDD), fabricated with an innovative technology for minimizing the anode current, and a new CMOS charge sensitive preamplifier (CSA), designed for ultimate low noise performance, has been realized and experimentally characterized. The SDD is hexagonal with an active area of 13 mm2. The current density measured at the anode with the detector in operating condition is 25 pA/cm2 at +20°C. The CSA - named SIRIO - has intrinsic Equivalent Noise Charge (ENC) ranging from 2.9 to 1.5 electrons r.m.s. at 0.8 μs and 11 μs peaking times at room temperature, respectively. With the SDD-SIRIO system at +21°C, an energy resolution of 141 eV FWHM on the 55Fe line at 5.9 keV and 74 eV FWHM on the pulser line with a noise threshold of 170 eV have been measured at 0.8 μs peaking time. The system has been tested from -30°C to +30°C with energy resolution from 124 eV to 148 eV FWHM at 5.9 keV. A moderate cooling at +10°C is sufficient to reach 133 eV FWHM at 5.9 keV