Defect Characterization of CdTe Bulk Crystals Doped with Heavy Elements and Rare Earths
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Defect Characterization of CdTe Bulk Crystals Doped with Heavy Elements and Rare Earths Svetlana Neretina1, N.V. Sochinskii2, Peter Mascher1, and E. Saucedo2,3 1
Department of Engineering Physics, McMaster University, Hamilton, Ontario, Canada Instituto de Microelectronica de Madrid, CNM-CSIC, Parque Techologico de Madrid, Madrid, Spain 3 Departamento de Fisica de Materialses, Univesidad Automona de Madrid, Madrid, Spain
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ABSTRACT: The doping level in Cadmium Telluride (CdTe) is of the utmost importance for many applications. In this work, we have characterized CdTe crystals doped with Tl, Bi, or Yb as well as a crystal co-doped with Ge and Yb. The crystals were characterized using low-temperature Photoluminescence (PL), Positron Lifetime Spectroscopy (PLS), resistivity and a terahertz pump-probe technique that can determine carrier lifetimes. The properties of the crystals were also studied before and after a rapid thermal anneal (RTA) as well as after a longer conventional anneal. The results obtained using the various dopants vary widely. It will be shown, however, that the above mentioned dopants can form complexes with Cd vacancies (vacancy-impurity pairs). As a result, these Cd vacancies can play a key role in determining the resistivity and carrier lifetimes. INTRODUCTION Cadmium Telluride (CdTe) has been successfully employed as a room temperature x-ray and gamma-ray detector material [1] in applications such as medical and industrial imaging, security and monitoring, nuclear safeguards, non-destructive testing, and astrophysics. These applications require a high resistivity material so as to obtain a low dark current with long carrier lifetimes [2]. Attempts have been made to increase the efficiency of these detector materials as well as enhance the absorption by doping the CdTe with elements having high atomic numbers. In particular, rare earth elements have been incorporated in order to increase the carrier lifetime [3]. Adding these dopants can alter the transport properties of the CdTe by introducing deep impurity levels or through the creation of defect complexes such as those associated with vacancyimpurity complexes. These deep levels can also act as carrier trapping and recombination centers leading to a reduction of the mobility-lifetime products of the charge carriers, and thus, deterioration in detector performance. If doped CdTe crystals are to be used in device applications then it becomes important to determine the influence that anneals will have on their properties. This is a consequence of the fact that anneals are an inevitable step in the fabrication of devices. To this end, anneals were performed in a rapid thermal anneal (RTA) furnace as well as in a conventional furnace. Both types of anneals are of interest as RTA anneal provides information about surface defects, while a long duration anneal effects the bulk properties. In this paper, we have shown that both doping and annealing affect the defect structure of CdTe to the point where there are significant changes to the e
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