Abstract
Two commercial-off-the-shelf (COTS) 4H-SiC UV photodiodes have been investigated for their suitability as low-cost high temperature tolerant X-ray detectors. Electrical characterisation of the photodiodes which had different active areas (0.06mm2 and 0.5mm2) is reported over the temperature range 0°C to 140°C together with measurements of the X-ray photocurrents generated when the detectors were illuminated with an 55Fe radioisotope X-ray source. The 0.06mm2 photodiode was also investigated as a photon counting spectroscopic X-ray detector across the temperature range 0°C to 100°C. The depletion widths (at 120V reverse bias) of the two diodes were found to be 2.3µm and 4.5µm, for the 0.06mm2 and 0.5mm2 detectors respectively, at 140°C. Both devices had low leakage currents (<10 pA) at temperatures ≤40°C even at high electric field strengths (500kV/cm for 0.06mm2 diode; 267kV/cm for 0.5mm2 diode). At 140°C and similar field strengths (514kV/cm for 0.06mm2 diode; 269kV/cm for 0.5mm2 diode), the leakage currents of both diodes were <2nA (corresponding to leakage current densities of 2.4µA/cm2 and 0.3µA/cm2 for each diode respectively). The results demonstrated that both devices could function as current mode detectors of soft X-rays at the temperatures <80°C and that when coupled to a low noise charge sensitive preamplifier, the smaller diode functioned as a photon counting spectroscopic X-ray detector at temperatures ≤100°C with modest energy resolution (1.6keV FWHM at 5.9keV at 0°C; 2.6keV FWHM at 5.9keV at 100°C). Due to their temperature tolerance, wide commercial availability, and the radiation hardness of SiC, such detectors are expected to find utility in future low-cost nanosatellite (cubesat) missions and cost-sensitive industrial applications.
Highlights
Many materials have been investigated for use in high temperature photon counting X-ray spectrometers
The energy resolutions at 5.9 keV at high temperature were not as good as some wide bandgap devices designed for X-ray detection, e.g. GaAs mesa photodiodes (1.5 keV at 80 °C, area of 0.03 mm2 [3]; 840 eV at 60 °C, area of 0.03 mm2 [2]) and the best results obtained with custom-made SiC X-ray detectors (233 eV at 100 °C, area of 0.4 mm2) [21], the full width at half maximum (FWHM) of the device was comparable to Al0.8Ga0.2As X-ray detectors at 80 °C (2.0 keV, area of 0.03 mm2) [5]
The electrical characterisation of different areas (0.06 mm2 and 0.5 mm2) of commercial 4H-SiC Schottky photodiodes at varying temperatures has been reported
Summary
Many materials have been investigated for use in high temperature photon counting X-ray spectrometers. With high quality material available and well-developed fabrication technology, SiC detectors are especially attractive for such applications. The ultra-low leakage current achievable with SiC detectors has stimulated the development of ultra-low-noise charge-sensitive preamplifiers [22] Whilst such results are superb and demonstrate the suitability and high technology readiness level of the material and the excellent quality of the researchers’ preamplifiers many researchers may not have access to such facilities, yet still desire to make photon counting X-ray spectrometers using low-cost commercial-off-the-shelf components for applications such as university-led Cubesat missions and industrial monitoring devices. Results have been reported showing that commercialoff-the-shelf 4H-SiC photodiodes (sold as UV photodetectors by a standard electronics retailer) could be repurposed as X-ray detectors for use at room temperature [23]. The calculated depletion widths were 2.5 μm (0.06 mm diode) and 4.5 μm (0.5 mm diode) based on the devices’ capacitances at 150 V reverse bias
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