Charge Transportation Properties Of Detector-grade CdZnTe Crystals

CdZnTe is regarded as the most promising material for room temperature radiation detectors due to its excellent electrical and optical properties. However, the charge transportation properties are barely satisfactory because of variant crystal defects during the crystal growth and device fabrication process, which limit the performance and the yield of detectors. The aim of this study is to elucidate the effect of defects on the charge transport process and detector performace in semi-insulating CdZnTe:In single crystals, to give guidance to the fabrication of high-quality CZT crystals and high-performance imaging devices.We first developed a laser beam induced transient current (LBIC) setup to test the mobility and other charge transport parameters in high resistivity semiconductor. The fundamental physical principles of LBIC measurement were discussed, and a method to directly determine trapping, de-trapping time, mobility and space charge density according to the shape of LBIC waveform was presented. The LBIC measurements were carried out under various bias voltages for CdZnTe crystals. The results suggest that the electric field strengthens carrier emission from trap centers, associated with the Poole-Frenkel effect. The typical electron mobility was calculated to be 950 cm2/Vs and the corresponding electron mobility-lifetime product was found to be 1.32×10-3 cm2/V by using a modified Hecht equation with consideration of the surface recombination effect.The defect levels and carrier lifetime in CdZnTe:In crystal were characterized with photoluminescence (PL), thermally stimulated current measurements (TSC), as well as contactless microwave photoconductivity decay (MWPCD) technique. An evaluation equation to extract the recombination lifetime and the reemission time from MWPCD signal is developed based on Hornbeck-Haynes trapping model. An excellent agreement between defect level distribution and carrier reemission time in MWPCD signal reveals that the tail of the photoconductivity decay is controlled by the defect level reemission effect. The results demonstrate that high density of A-centers reduces the bulk recombination lifetime and the electron mobility. The defect levels (Teed, Tei) with higher defect concentration and activation energy possibly intervene in the extension of carrier reemission time. Combining 241Am gamma ray radiation response measurement and LBIC measurement, it is predicted that defect level with the reemission time shorter than the collection time could lead to better charge collection efficiency of CdZnTe detectors.The effects of sub-bandgap illumination on the charge transport process and detector performance of CdZnTe were experimentally studied. According to the resulting bulk resistivity and photocurrent response under sub-bandgap illumination, the variation of the deep-level occupation fraction was identified based on a modified Shockley-Read-Hall model. From LBIC measurements, a decrease of negative space charge density and consequently flattening of the electric field distribution were found under the external sub-bandgap illumination, demonstrating a reduction of the active trap concentration. Furthermore,241Am gamma-ray spectroscopy response measurements confirmed that simultaneous incidence of sub-bandgap light could significantly improve energy resolution and charge collection efficiency (from 8.59×10-4 cm2/V to 1.17×10-3 cm2/V) of CdZnTe detectors.Transient current techniques using alpha particle source were utilized to study the influence of extended defects on the electron drift time and the detector performance of CdZnTe crystals. Different from the case of trapping by isolated point defect, a barrier controlled trapping model was applied to explain the mechanism of carrier trapping at the extended defects. The effect of extended defects on the photoconductivity was studied by LBIC measurement. The results demonstrate that the Schottky-type depletion space charge region is introduced at the vicinity of the extended defects, which further distorts the internal electric field distribution. Persistent drift time distortion was observed in the crystal with grain boundary even under high bias voltage, because electron clouds move across these regions with different velocity. By tracing the carrier trajectory inside the crystal, it is demonstrated that extended defects act as recombination-activated regions, causing inhomogeneous charge collection of detectors.Temporal response of CdZnTe detectors under ultrafast-pulsed X-ray irradiation was evaluated at room temperature. The temporal response processes have been modeled at the microscale level and described by the multi-trapping model. The influences of irradiation flux and bias on the transient currents were reported. It was demonstrated that the temporal response processes were affected by the defect level occupation fraction. A fast photon-current can be achieved at intense X-ray flux up to 2.78×109 photons mm-2s-1 Meanwhile, high bias voltage is desirable to enhance carrier de-trapping and prevent charge capture by structure defects, which further improves the temporal response of CdZnTe detectors.Radiation damage created by high dose 60Co γ-ray in CdZnTe crystals was studied. The mechanisms of radiation damage, including energy transfer and defect structure formation processes were discussed. Based on Kinchin-Pease model, after primary Compton scattering, the maximum energies transfered to Cd, Zn and Te atoms were calculated to be 45.42 eV、 78.11 eV and 40.01 eV, respectively. The ratio of atomic displacement cross-sections σ(Cd), σ(Zn) and σ (Te) were found to be 1:3.9:1.12 in terms of Mott-McKinley-Feshbach classical equation. With increasing the radiation dose, the increase of A-center and AOX peaks’ concentration and decrease of DAP peak’s concentraion were found, which demonstrated the formation of Cd vacancy during high dose radiation. The effects of radiation damage on charge transport process and detector performance were evaluated by LBIC and 241Am gamma-ray spectroscopy response measurements. The electron mobility was found to decrease due to the increase of ionized impurity scattering. The empirical equation for evaluating electron mobility damage was deduced to be1/μ=1/μ0+5.489×10-4Φ-0.02676. The decrease of the electron mobility-lifetime product from 1.36×10-3 cm2/V before radiation to 7.56×10-4 cm2/V after radiation was found, which clearly indicates the worsening of carrier transport properties and the deterioration of CdZnTe detector performances during the radiation.