New developments in CdZnTe semiconductors for X and Gamma-ray detection

Abstract

Semiconductor ionizing radiation detectors have experienced a rather rapid development in the last years. Recently, a substantial international effort has been invested in developing a range of compound semiconductors with wide band gap and high atomic number for X-ray and Gamma ray detectors. Among the compound semiconductors Cadmium Zinc Telluride (CZT) is the most promising material for radiation detectors with good energy resolution, high detection efficiency and room temperature operation. In parallel to the main activity on the tuning of grown technique to produce CZT crystals with the required performance, such as high resistivity, good charge collection efficiency and low tellurium inclusions density; several efforts have been also dedicated on the development of a reliable method to metalize the surface for the realization of efficient contacts and to control surface resistivity. They are still an open challenge in the detector preparation technology. Undersigned’s phd work fits in this background as will be exposed in the following. First of all the problem of inclusions in CZT crystals is treated. A new diagnostic technique based on near infrared transmission microscopy is proposed by the author of this thesis in order to characterize tellurium inclusions in CZT crystals. By means of the described technique it was possible to perform an interesting study on the role played by these inclusions on the performance of CZT based detectors and on the influence of the latter on transport properties of photo-generated carriers inside the material. Then a new approach to eliminate tellurium inclusion in CZT crystals is proposed. A novel laser-induced thermo-migration system is proposed and evaluated. Then importance of metallization process during the CZT device production was take into account. A very interesting technique to realize metal deposition on CZT surfaces is proposed and a special metal deposition equipment was developed. Reliability of the process was demonstrate by means of a study on the influence of the contact deposition technique on the detection performance, in term of charge collection efficiency. Finally results reached in the preparation and characterization of four CZT detectors developed in range of “3D CZT High Resolution Detectors” was discussed. This project, funded by European Space Agency (ESA), was aimed to the development of a novel type of CZT based detector: the drift stripes 3D detector. Aim of this project was to demonstrate that the good energy resolution of the CZT drift strips detector can be combined with 3D sensing capabilities, very important features for X and Gamma ray detectors for high energy astrophysics missions. IMEM institute was involved in some important parts of the project such as detector preparation, detector characterization and data analysis. The detector characterization was performed at European Synchrotron Radiation Facility (ESRF) in Grenoble.