Effect of Deep-Level Defects on the Performance of CdZnTe Photon Counting Detectors

Abstract

The effect of deep-level defects is a key issue for the applications of CdZnTe high-flux photon counting devices of X-ray irradiations. However, the major trap energy levels and their quantitive relationship with the device’s performance are not yet clearly understood. In this study, a 16-pixel CdZnTe X-ray photon counting detector with a non-uniform counting performance is investigated. The deep-level defect characteristics of each pixel region are analyzed by the current–voltage curves (I–V), infrared (IR) optical microscope photography, photoluminescence (PL) and thermally stimulated current (TSC) measurements, which indicate that the difference in counting performance is caused by the non-uniformly distributed deep-level defects in the CdZnTe crystals. Based on these results, we conclude that the CdZnTe detectors with a good photon counting performance should have a larger Te cd 2 + and Cd vacancy-related defect concentration and a lower A-center and Tei concentration. We consider the deep hole trap Tei, with the activation energy of 0.638–0.642 eV, to be the key deep-level trap affecting the photon counting performance. In addition, a theoretical model of the native defect reaction is proposed to understand the underlying relationships of resistivity, deep-level defect characteristics and photon counting performance.

Quantitative Imaging of Defect Distributions in CdZnTe Wafers Using Combined Deep-Level Photothermal Spectroscopy, Photocarrier Radiometry, and Lock-In Carrierography

“…The quantitative trap density images are outcomes of the theoretical model of eqs –, which describes a general two-trap system consistent with the DLPTS data shown in Figures and . While the N T2 trap densities are generally in agreement with activation energies in the 0.10–0.18 eV range, which have been reported to be ∼(1.23–8) × 10 15 cm –3 , ,, those of the N T1 densities are on the high side, or higher than, published deep-level trap densities in Cd 1– x Zn x Te. There are three possible explanations for the possibility of measuring higher than expected trap densities: (1) The known ionization energies of the main intra-band-gap defects and impurities in Cd 1– x Zn x Te within the range of the activation energies shown in Figure b represent defects/traps comprising Cd native vacancies, V Cd , in the 0.13–0.21 eV range, various acceptor impurity clusters in the 0.05–0.35 eV range, and pairing of Cd vacancies with a group III or group VII donor (designated as A centers), which are shallow acceptor complexes with a single ionization level.…”

“…The proximity of these energetically adjacent defect cluster states is such that thermal energetic overlap cannot be excluded in our DLPTS measurements, which thus may have led to increased effective trap densities when evaluated by the kinetic model of eqs –. This possibility suggests carrying out more highly resolved temperature thermal scans to identity overlapped adjacent peaks . (2) In the context of the highly nonequilibrium steady-state heterodyne optical excitation, with large fluctuating numbers of free excess carriers roaming at, or near, the band edge, impurity states emptied by the laser beam modulated at one frequency may act as extra traps of free carriers generated by the other, phase lagged, laser beam, thereby rendering the effective trap densities N T1 and N T2 functions of time.…”

“…The level, E v + 0.14 eV, and A 1 level, E v + 0.15 eV, which have also been observed in PL investigations of CdZnTe 33 and are believed to be related to complexes involving a V Cd and a V Te vacancy, respectively. Very recently, a much more spectrally resolved study of thermally stimulated current peaks (TSC) from CdZnTe crystals 34 4−6, with the temperature of the sample kept at 100 K for an optimal signal-to-noise ratio (SNR). A heterodyne PCR amplitude depression (notch or dip) phenomenon recently observed in Si wafers with origins in destructive interference among trap-captured and -released carrier density waves due to the nonlinear nature of eqs 1−3 27 appeared in the HeLIC image pixel frequency response in the intermediate frequency range over the entire area of the sample around 7.9 kHz, at I = I max , as shown in Figure 4.…”

“…Castaldini et al used cathodoluminescence (CL), photoinduced current transient spectroscopy (PICTS) and photo-DLTS (P-DLTS) to identify more highly resolved activation energies of the A centers: the A level, E v + 0.14 eV, and A 1 level, E v + 0.15 eV, which have also been observed in PL investigations of CdZnTe and are believed to be related to complexes involving a V Cd and a V Te vacancy, respectively. Very recently, a much more spectrally resolved study of thermally stimulated current peaks (TSC) from CdZnTe crystals has attributed traps in the 0.154–0.159 eV range to dislocation-related defect complexes and traps in the range 0.174–0.179 eV to O Te – V Cd –/2– complexes. These authors identified the A center-associated traps lying in the 0.106–0.107 eV activation energy range, a different energetic location in the band gap from the earlier reports.…”

“…The importance of quantitative trap density images lies in the fact that these densities and their distributions under optical excitation conditions control the CdZnTe optoelectronic behavior and properties when this material is used as a substrate for radiation detectors and high-efficiency solar cells. , In our measurements, the values of N T1 and N T2 calculated in the mappings of Figure , also consistent with HePCR measurements, Table , were found to be consistent with conventional homodyne PCR measurements . This agreement implies that possibility #2 must be discarded, while possibility #1 may be the most likely cause due to adjacent trap overlaps in the 0.05–0.35 eV intra-band-gap range for N T1 , while the other, more isolated N T2 defect state in the 0.10–0.18 eV range, represented by the HeLIC images, are likely to yield accurate trap densities, also in agreement with literature values. ,, A study of trap densities from HeLIC imaging and the underlying mechanism for the possibility of overestimation is currently under further investigation using CdZnTe crystals with known defect concentrations as validation standards.…”