AlGaAs two by two pixel detector for electron spectroscopy in space environments

A prototype monolithic 2 × 2 square pixel Al0.2Ga0.8As p+-i-n+ mesa photodiode array (each photodiode of area 200μm by 200μm, with a 3μm i layer) has been investigated for its utility as a detector for direct detection electron (β- particle) spectroscopy. Each photodiode was electrically characterised and its response to illumination from a 63Ni radioisotope β- particle source was investigated at 20 °C. The percentage of electron energy absorbed in the active layer (i layer), Eabs, of the photodiode and the spectrum expected to be detected, were calculated via Monte Carlo simulations. Comparisons between the simulated and detected 63Ni β- particle spectra are presented and demonstrate uniformity in response across the two by two pixel array. The percentage of electron energy absorbed in the active layer of the detector was at a maximum of 0.53 ± 0.04 for electrons with an energy of 38 keV; the percentage of electron energy absorbed in the active layer of the detector reduced to 0.29 ± 0.02 at 66 keV. To increase the percentage of electron energy absorbed in the active layer of the detector at high energies, inactive Al absorption layers placed atop the detecting structure were investigated as part of the modelling work. These additional Al layers partially attenuated the β- particles’ energy, thus reducing the incident particles’ energy to values more readily detected by the relatively thin photodiode. Al layers of 20μm, 100μm, and 500μm thickness were investigated and found, by modelling, to increase the percentage of electron energy absorbed in the active layer of such a spectrometer by 22 % at 100 keV; 46 % at 200 keV; and 20 % at 500 keV, respectively.