Charge collection efficiency and space charge formation in CdTe gamma and X-ray detectors

A new extended model for the charge collection efficiency in CdTe gamma and X ray detectors is presented which allows to derive from apparent experimental gamma spectra of a quasi-monochromatic source, an Am-241 source in the present case, not only the mu tau products of electrons and holes individually but also the sign, spatial distribution, and temporal evolution of the net space charge accumulated in the detector. Resistive CdTe and CdZnTe as well as CdTe Schottky detectors are studied. While the resistive type is stable in time and exhibits higher mu tau products, the Schottky type shows space charge accumulation which approaches saturation after about 1 h at several 10(11) cm(-3). This is attributed to efficient majority carrier depletion, Fermi level shift, and trap filling. Resistive detectors thus appear optimized to the needs of gamma spectroscopy even at low bias voltage, while Schottky types need higher bias to overcome the space charge. They are suited to both, gamma spectroscopy and X-ray detection in analog current mode, where they operate more stably due to the higher bias. From the point of view of materials characterization, gamma spectroscopy with Schottky detectors probes and reveals the trap density near the Fermi level (several 10(12) cm(-3) eV(-1)). We find a basically homogeneous spatial distribution suggesting the trap origin being in crystal growth rather than surface processing. Capture of photogenerated charges in traps is detrimental for current-mode operation under high X-ray flux because delayed emission from traps limits the detector's ability to respond to fast signal changes. (C) 1998 Elsevier Science B.V. All rights reserved.