The effect of pixel geometry on spatial and spectral resolution in a CdZnTe imaging array

We analyze the various factors contributing to the spatial and spectral resolution of a focal plane imager made from a 25 mm square slab of CdZnTe patterned into an array of pixels. Each pixel is connected to a charge-sensitive preamplifier that integrates the current flowing through the pixel electrode for a frame time of one millisecond. The fraction of charge collected by the integrating circuit is influenced by two trends. Some signal is lost to neighboring pixels due to nonlocalized generation and subsequent diffusion of charge carriers. An opposing phenomenon we term the "near-field effect" has a greater influence on charge collection as well as spatial resolution. We show using electrostatic theory that the smaller the pixel dimension, the more electron transport contributes to the induced signal, and the less hole transport matters. The result is an improved pulse-height spectrum with less tailing due to trapped charges. Monte Carlo simulations and measurements on test pixels confirm the improvement in pulse height spectrum with smaller pixel dimension. Numerical calculation of the potential distribution within the detector volume allows us to include the effect of inter-pixel spaces in the simulations.