Transport phenomena in X and ? ray semi-insulator detector: a new charge correction approach

This research is part of the broader project of study and application of II-VI semi-insulating materials and especially of Cadmium and Zinc Telluride (CdZnTe or CZT). The current interest about these ternary compounds, such as semi-insulating materials for high energy photon detectors is mainly due to their high energy-gap that makes these materials ideal for applications at room temperature avoiding noise problems due to leakage current. Within this class of materials CZT is particularly appreciated both for the high-stopping power, due to the high atomic number of its components and its crystal structure, and for the high transport properties if compared to those of similar semiconductors. For these reasons CZT arises as an ideal candidate for high energy detector. The applications are numerous in several areas as security and environmental monitoring, storage of radioactive materials, medical instrumentation, space applications, astrophysics and cosmology. The problems of purity and homogeneity of the material are still far from being resolved. For this reason transport properties are still limited as compared to those of silicon, germanium and gallium arsenide, also in relation to the size of developed sensors (even several cubic centimeters). On increasing the photon energy the mean absorption depth arises with detriment of charge collection and spectroscopic property, with a consequent line broadening. In addition still persist difficulties regarding passivation and realization of contacts that could ensure low noise and an efficient charge collection. The main purpose of this work is to study charge collection processes and signal deterioration causes, improving the growth process and identifying appropriate methodologies for charge deficit correction, in order to create an electronic circuitry for data acquisition and signal correction.\\ The experimental activity was focused on the study of material grown by the Technology Group of IMEM-CNR Institute of Parma. This material have been grown with Boron Oxide Vertical Bridgman technique and used to create high energy detectors (10-700keV). The work can be divided into three main parts: 1) The material characterization, by means of photo-induced current, I-V characteristics and X and ? spectroscopy, to characterize the material and in particular to analyze bulk an superficial defects, impurity levels and the consequent transport properties in devices made by technology group of IMEM Institute. 2) The second one consists of theoretical model assessment to describe the material photo-response and the electronic read-out chain in order to obtain both the shape of the electronic signal and transport property informations. These model could be useful to correct the charge deficit through the information concerning the photon absorption depth in the crystal. 3) The third part concerns the development of data acquisition, filtering system and data elaboration. After a brief introduction about the fundamental issues involved, we make a careful analysis of these three aspects mentioned above. Finally we will discuss the thesis conclusions and the possible developments of this research. Other complementary activities, that play a minor role in the this research, can be found in the appendix.