Year
2010
Abstract
CdZnTe (CZT) is a promising detection medium for room-temperature semiconductor radiation- detectors. Their operational principle is based upon measuring the total number of electrons generated by incident particles. However, carrier trapping by impurity-related defects that exist in any semiconductor competes with the collection of the charge. The time the electrons remain free in the crystal and contribute to the output signals is termed the electron’s lifetime. The charge-carrier lifetime is an important characteristic of any detecting medium. For CZT detectors, knowledge of it is critical in selecting high-grade material for fabricating devices. A common procedure for evaluating carrier lifetime rests upon using the Hecht equation to fit the measured dependences of the collected charge vs. the applied bias. Notwithstanding, the sensitivity range of the Hecht equation is limited. . For example, for a typical 2-mm-thick device and a µt-product of about 10-3 cm2/V, the voltage range is extendable up to 200 V. But, if the µt-product is about 10-2 cm2/V or higher, the useful range is limited to 20 V where systematic errors specific to this region strongly affect the measurements. Here, we describe our new technique for accurately measuring the electron lifetime in a high µt-product CZT using long, >10 mm, virtual Frisch-grid CdZnTe detectors. The anode signals, digitized and processed, generate a curve of the dependence of the collected charge versus the electron-cloud’s drift time. The slope of this curve directly measures the electron lifetime, yielding a value that is more accurate than estimates obtained from applying the Hecht equation.