Effect of charge trapping on effective carrier lifetime in compound semiconductors: High resistivity CdZnTe

Abstract

The dominant problem limiting the energy resolution of compound semiconductor based radiation detectors is the trapping of charge carriers. The charge trapping affects energy resolution through the carrier lifetime more than through the mobility. Conventionally, the effective carrier lifetime is determined using a 2-step process based on measurement of the mobility-lifetime product (μτ) and determining drift mobility using time-of-flight measurements. This approach requires fabrication of contacts on the sample. A new RF-based pulse rise-time method, which replaces this 2-step process with a single non-contact direct measurement, is discussed. The application of the RF method is illustrated with high-resistivity detector-grade CdZnTe crystals. The carrier lifetime in the measured CdZnTe, depending on the quality of the crystals, was between about 5 μs and 8 μs. These values are in good agreement with the results obtained using conventional 2-step approach. While the effective carrier lifetime determined from the initial portion of the photoresponse transient combines both recombination and trapping in a manner similar to the conventional 2-step approach, both the conventional and the non-contact RF methods offer only indirect evaluation of the effect of charge trapping in the semiconductors used in radiation detectors. Since degradation of detector resolution is associated not with trapping but essentially with detrapping of carriers, and, in particular, detrapping of holes in n-type semiconductors, it is concluded that evaluation of recombination and detrapping during photoresponse decay is better suited for evaluation of compound semiconductors used in radiation detectors. Furthermore, based on previously reported data, it is concluded that photoresponse decay in high resistivity CdZnTe at room temperature is dominated by detrapping of carriers from the states associated with one type of point defect and by recombination of carriers at one type of extended defects. The recombination at the extended defects produces long, logarithmic decay limiting substantially performance of CdZnTe detectors. This decay is associated with the “electrostatic trapping” of excess holes by the Schottky-type depletion space-charge regions formed around the defects.

A Simple Method to Differentiate between Free‐Carrier Recombination and Trapping Centers in the Bandgap of the p‐Type Semiconductor

“…Different measurement methods also give widely differing values of the free-carrier capture cross sections for the same material or device structure [5,15]. Values for the free-carrier capture cross sections ranged from 10 − 21 cm 2 to 10 − 13 cm 2 which have been reported by several researchers in silicon-based devices and different materials [6][7][8][9][10][11][12][13][14][16][17][18][19]. Dudeck and Kassing [10] determined the temperature dependence of the capture cross section of negatively charged gold centers in n-type silicon for holes in the temperature range of 55 to 340 K from the temperature-dependent breakdown voltage of gold-compensated PIN diodes showing a negative differential resistance region.…”

“…If the localized state captures free carriers temporarily and then reemits them back to their original band, the localized state is called a free-carrier trap [1][2][3][4][5]. ese properties depend on the shallowness and the deepness of the localized state in the bandgap of the material [6][7][8][9][10][11][12][13][14]. Different localized states have different capture coefficients for free carriers.…”

“…Based on a simplified analysis of the kinetics of filling of the traps in the slow regime of filling, Stievenard et al [13] presented a new technique of measuring capture cross sections in semiconductors. In his work on a new radio frequency-based pulse rise time method with a single noncontact direct measurement, Kamieniecki [14] also reported the effect of charge trapping on the transient behaviors of the effective carrier lifetime in compound semiconductors. e investigations of Hangleiter [16] on the deep levels induced by transition metal impurities in silicon and those of Schroder [19] on various recombination mechanisms in silicon are also the most reliable sources on silicon.…”

Charge Sharing in (CdZn)Te Pixel Detector Characterized by Laser-Induced Transient Currents

“…Despite evident advantages, (CdZn)Te suffers from several drawbacks, one of them being a relatively large difference between the transport properties of electrons and holes [7,8]. Low hole mobility and enhanced hole trapping reduce the charge collection efficiency (CCE) and produce long asymmetric tail at measured energy spectra (hole tailing).…”

“…Low hole mobility and enhanced hole trapping reduce the charge collection efficiency (CCE) and produce long asymmetric tail at measured energy spectra (hole tailing). Thus, poor hole mobility-lifetime product (µ h τ h ) and relevant defect structure [7,9] are critical issues in the utilization of (CdZn)Te radiation detectors.…”