AlInP X-ray photodiodes without incomplete charge collection noise

Abstract

Previously, an Al0.52In0.48P p+-i-n+ spectroscopic photon counting X-ray photodiode with 2μm thick i layer (200μm diameter) was shown to suffer from energy-dependent incomplete charge collection noise (Lioliou et al., 2019). Subsequent measurements on a larger (400μm diameter) Al0.52In0.48P p+-i-n+ photodiode (reported here) revealed the presence of even greater incomplete charge collection noise. Given these findings, an expectation would have been that thicker Al0.52In0.48P structures (which would be required for efficient absorption of all but the softest X-rays) would have a greater incomplete charge collection noise contribution, thus suggesting that thick Al0.52In0.48P photodiodes may be of limited practicality as high performance detectors for photon counting X-ray spectroscopy. However, two new Al0.52In0.48P p+-i-n+ photodiodes (with 6μm i layers) were fabricated from material grown by the same technique (metalorganic vapour phase epitaxy) in the same reactor, and are now shown here to exhibit no signs of detectable incomplete charge collection noise under the illumination of X-ray photons of energy 4.95 keV to 21.17 keV. As such, now that greater experience has been built with Al0.52In0.48P, concerns about incomplete charge collection noise in X-ray detectors made from the material appear to have been unwarranted; the path towards thick Al0.52In0.48P X-ray detectors is now clear.