Current-Voltage Characteristics of High Resistivity CdTe
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Current-voltage characteristics of high resistivity CdTe Y.Iwase, R.Ohno and M.Ohmori Research and Development Group, Nikko Kyodo Co., Ltd. 3-17-35, Niizo-Minami, Toda, Saitama, 335 Japan
Current-voltage characteristics of high resistivity CdTe, with consideration to Schottky barrier height at the contact and defect levels in the band gap were investigated by experimental and numerical simulation methods. From the electron injection investigation, the density of electron traps corresponded to the doped Cl concentration in the crystal. Numerical simulation of current-voltage characteristics was made based on Schockley Read Hall (SRH) statistics. In the low bias region, calculated characteristics were in fairly close agreement with the experimental results for Pt/CdTe/Pt diode. The electric field was not uniform along the biased direction and an almost neutral region was present in the vicinity of anode contact even when the bias was applied.
INTRODUTION Cadmium Telluride (CdTe) radiation detector has been developed for more than 20 years stage for broader applications such as medical diagnostics and nonand is in the practical destructive sensing.1- 2 The signal acquisition methods are classified to pulse count mode which can obtain the energy distribution and current mode suitable to high dose measurement. In either case, the requirement for the detector is carrier collection according to radiation energy and dose. This is the most fundamental characteristic of a semiconductor detector and has been the principal target of the detector development. Since the carrier collection efficiency strongly depends on the product of carrier mobility, carrier lifetime and electric field (4itE), the enhancement of the carrier lifetime and the electric field is essential to achieve higher charge collection efficiency. The increment of applied voltage leads to large leakage current resulting in the degradation of signal/noise ratio. The current leakage was almost entirely due to the lower barrier height (for electron at cathode and hole at anode) and the existence of defect levels in the band gap, which is responsible for the reduction of barrier height under bias and give rise to generationrecombination centers. The carrier lifetime is also limited by the properties of defects in the crystal. Those properties have been mainly investigated by spectroscopic methods like thermally3 5stimulated current (TSC) and photo induced current transient spectroscopy (PICTS). - On the other hand, the current - voltage characteristics are expected to exhibit the6 defect related information of trap density and their energy levels with appropriate electrode. Therefore, the detailed analysis of current-voltage characteristics is effective for understanding the mechanism of current leakage and carrier collection. In this paper, current-voltage characteristics of high resistivity CdTe, with consideration to Schottky barrier height at contact and defect levels in the band gap were investigated by experimental and numerical simulation methods. EXPERIM
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