InGaP (GaInP) Mesa P-i-n Photodiodes for X-ray Photon Counting Spectroscopy

In this paper, for the first time an InGaP (GaInP) photon counting X-ray photodiode has been developed and shown to be suitable for photon counting X-ray spectroscopy when coupled to a low-noise charge-sensitive preamplifier. The characterisation of two randomly selected 200 μm diameter and two randomly selected 400 μm diameter InGaP p-i-n mesa photodiodes is reported; the i-layer of the p-i-n structure was 5 μm thick. At room temperature, and under illumination from an Fe radioisotope X-ray source, X-ray spectra were accumulated; the best spectrometer energy resolution (FWHM) achieved at 5.9 keV was 900 eV for the 200 μm InGaP diameter devices at reverse biases above 5 V. System noise analysis was also carried out and the different noise contributions were computed.