CHARGE COLLECTION EFFICIENCY AND SPACE-CHARGE FORMATION IN CDTE GAMMA-RAY-DETECTOR AND X-RAY-DETECTOR

A new extended model for the charge collection efficiency in CdTe gamm a and X ray detectors is presented which allows to derive from apparen t experimental gamma spectra of a quasi-monochromatic source, an Am-24 1 source in the present case, not only the mu tau products of electron s 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 stu died. While the resistive type is stable in time and exhibits higher m u tau products, the Schottky type shows space charge accumulation whic h approaches saturation after about 1 h at several 10(11) cm(-3). This is attributed to efficient majority carrier depletion, Fermi level sh ift, and trap filling. Resistive detectors thus appear optimized to th e 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 poin t of view of materials characterization, gamma spectroscopy with Schot tky detectors probes and reveals the trap density near the Fermi level (several 10(12) cm(-3) eV(-1)). We find a basically homogeneous spati al distribution suggesting the trap origin being in crystal growth rat her than surface processing. Capture of photogenerated charges in trap s is detrimental for current-mode operation under high X-ray flux beca use delayed emission from traps limits the detector's ability to respo nd to fast signal changes. (C) 1998 Elsevier Science B.V. All rights r eserved.