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PACS 71.55. Gs, 72.20.Jv,72.40.+w Polycrystalline CdZnTe thick films were grown by thermal evaporation method using CdZnTe and CdZnTe:Cl source. Non-doped CdZnTe thick films have Te-rich stoichiometry having resistivity 3.3 × 10 9 Ωcm. The Cl-doped CdZnTe thick films have 1.2 × 10 10 Ωcm in resistivity. To evaluate the role of Cl in the increasement of the resistivity, the density of localized states were calculated from the analysis of transient TOF (time of flight)current. From the comparison of Cl-doped CZT thick films with non-doped and O2 annealed CdZnTe thick films, we have found that Cl doping is related to the formation of states at EV + 0.75 eV region and oxygen reduces grain boundary related defects through formation TeO2 or CdTeO3. IntroductionThe CdZnTe has an inherently high stoping power, an excellent carrier transport property, and a relatively wide band gap energy. Therefore detectors using this materials have the potential for sufficient X-ray sensitivity and DQE(detective quantum efficiency) at a sufficiently low leakage current. Although the many research results have been presented on single crystal CdTe and CdZnTe detectors with small sized silicon readout devices, it would be difficult to apply these results to large-area flat-panel radiation detectors. As an alternative of single crystal, we have grown the large area polycrystalline CdZnTe thick films having high resistivity by thermal evaporation method [1].Energy resolution of X-ray semiconductor detector is deteriorated by the charge trapping and leakage current noise. Detectors have been traditionally aimed at the maximal electrical resistance, in order to minimize energy broadening by leakage noise.Recent reports have identified the deep level defects which are likely to be associated with semiinsulating property [2,3]. In this paper, we have investigated the relations between the high resistivity and the density of localized states from the analysis of transient TOF (time of flight) currents. The grainboundary defects are analyzed quantitatively by comparison of measured values with the literature data for single crystals and by monitoring the variation of the deep levels spectrum with changing the grain boundary properties.

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