Characterization of CdZnTe crystals and radiation detectors

J. Butcher, M. Hamade, M. Petryk, A. E. Bolotnikov, G. S. Camarda, Y. Cui, R. Gul, A. Hossain, K. H. Kim, G. Yang, and R. B. James Brookhaven National Laboratory, Upton, NY, USA ABSTRACT Data obtained with BNL's National Synchrotron Light Source (N SLS) has helped to elucidate, in detail, the roles of non-uniformity and extended defects on the performance of CZT detectors, as well as the root cause of device polarization during exposure to a high flux of incident X-rays . Measurements of carrier traps will be reported, including their nature and relationships to different growth methods (conventional Bridgman, high-pressure Bridgman, traveling heater, and floating zone methods). Most findings will be co rrelated with the performance of spectrometer-grade CZT X-ray and gamma detectors, and new directions to re solve the material defici encies will be offered. Keywords: CdZnTe detectors, crystal defects, synchrotron light source 1. INTRODUCTION CdZnTe (CZT) is a leading semiconductor material for the future room-temperature nuclear radiation sensors capable of detecting different types of radiological threats by measuring pulse-height spectra from gamma rays, which are the most prominent signature of nuclear materials [1,2]. The capability to measure the pulse-height (energy) spectra with high resolution and efficiency are two key characteristics of any detector used for measuring the gamma rays. From this point of view, the CZT detectors have many advantages to provide much clearer energy spectra with high resolution and efficiency than many conventional detectors currently used for such tasks. CZT detectors represent a sustained, but still developing technology. The performance of todays CdZnTe (CZT) detectors depends on the quality of the original crystals, which often contain point and extended defects such as impurities, secondary phases (Te inclusions), twins and subgrain boundaries with different geometrical shapes. The randomly distributed point defects continuously trap free carriers generated by incident particles inside the detector volumes as they drift toward the collecting electrodes. The amount of the charge trapped is proportional to the electron cloud drift time and can be corrected by using pulse-processing techniques. In contrast, Te inclusions, which are generally randomly distributed inside crystals, can trap locally significant fractions of carriers proportional to their sizes. Such large fluctuations of the carriers lost are impossible to correct. The presence of subgrain boundaries is the most serious problem affecting todays commercial CZT material, regardless of the growth techniques employed, or the different vendors who supply them. The main mechanism by which subgrain boundaries degrade carrier transport seemingly lies in the accumulation in the distorted areas around the subgrain boundaries of impurities and secondary phases that trap the carriers and cause local variations of the electric field [3-5]. It is well known that subgrain boundaries consist of linear dislocations arranged in planes, or in more complex three-dimensional surfaces. Based on their densities, they are classified as large-angle boundaries with high dislocation densities, to networks of polygonized dislocations, also called dislocation walls [6,7]. Such walls of dislocations accumulate impurities with local concentrations significantly exceeding the c oncentration of point defects in the bulk. In addition, the wall of dislocations can affect the local distribution of the inte rnal electric field. Another way to look at this problem is to treat the walls of dislocations as a source of local variations of the electron mu-tau product which, in turn, cause uncorrelated fluctuations of the output signals. It is a difficult, expensive task for vendors to identify subgrain boundaries since IR transmission microscopy, their main screening technique, is inadequate for revealing dislocation-related defects; the most effective techniques are white X-ray beam diffrac tion topography and chemical etching of the crystals surfaces (etch-pit density analysis). The most important characteristic of a gamma-ray detector is its spectral response. In CZT detectors, the crystal defects play very important deteriorating roles. Their roles in degradation of the detector spectral responses depend on