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Optimal contact geometry for CdZnTe pixel detectors

We are developing CdZnTe pixel detectors for use as focal plane sensors in astronomical hard X-ray telescopes. To optimize the spectral response and imaging performance, we are investigating the effect of contact geometry on charge collection. Specifically, we have studied contact designs with orthogonal thin strips placed between pixel contacts. We apply a negative bias on the grid with respect to the pixel potential to force charge to drift toward the contacts. The grid bias is selected to be just sufficient to avoid charge collection on the grid, while increasing the transverse electric field on the surface between contacts. In contrast to focusing electrodes designed to force field lines to terminate on the pixel contact, our approach allows us to overcome the effects of charge loss between the pixels without significant increase of the leakage current, improving the overall energy resolution of the detector. In this paper we describe the performance of a CdZnTe pixel detector containing a grid electrode, bonded to a custom low-noise VLSI readout. We discuss the advantages of this type of detector for high spectral resolution applications.