Simulation of polarization dynamics in semi-insulating, Cr-compensated GaAs pixelated sensors under high x-ray fluxes

We developed a numerical solver for the drift–diffusion and Poisson equations in one-dimensional semiconductor detectors, including the possibility of an arbitrary number of donors and acceptors acting as trapping centers, Schottky or Ohmic contact type, velocity saturation at high electric fields, and external illumination. The temporal evolution of the system is computed by solving the full set of coupled differential equations self-consistently at each time step. An adaptive algorithm dynamically adjusts the time step allowing one to accurately track dynamics occurring at any relevant time scale within acceptable computational times. We investigated the case of semi-insulating, Cr-compensated GaAs sensors under illumination of monochromatic x-ray, showing the behavior of all the main quantities with special focus on the phenomenon of polarization, i.e., the space charge accumulation leading to the collapse of the electric field and signal loss. The model predictions are in agreement with experimental count rate efficiency data obtained with a 500 μm-thick GaAs:Cr sensor, readout by an IBEX photon counting application-specific integrated circuit (ASIC) with a pixel size of 75 µm and illuminated with 55 keV x-ray over a wide range of impinging fluxes. Finally, we investigated the maximum sustainable flux for the same detector type illuminated with monochromatic x-ray in the range 10–60 keV.