The space charge accumulation in CdZnTe crystals seriously affects the performance of high-flux pulse detectors. The influence of sub-bandgap illumination on the space charge distribution and device performance in CdZnTe crystals were studied theoretically by Silvaco TCAD software simulation. The sub-bandgap illumination with a wavelength of 890 nm and intensity of 8 × 10−8 W/cm2 were used in the simulation to explore the space charge distribution and internal electric field distribution in CdZnTe crystals. The simulation results show that the deep level occupation faction is manipulated by the sub-bandgap illumination, thus space charge concentration can be reduced under the bias voltage of 500 V. A flat electric field distribution is obtained, which significantly improves the charge collection efficiency of the CdZnTe detector. Meanwhile, premised on the high resistivity of CdZnTe crystal, the space charge concentration in the crystal can be further reduced with the wavelength of 850 nm and intensity of 1 × 10−7 W/cm2 illumination. The electric field distribution is flatter and the carrier collection efficiency of the device can be improved more effectively.
Grain boundary is one of the main defects that limiting the large-area application of CdZnTe nuclear radiation imaging detectors. In order to explore ways to improve the electric field distribution properties near grain boundary, the effect of sub-bandgap illumination on the electric field distribution in CdZnTe detectors with grain boundary was studied by Silvaco TCAD simulation technique. The grain boundary potential barrier and electric field dead zone were found in simulation results that significantly affected the carrier transport process in CdZnTe detectors. The electric field dead zone caused by the grain boundary was disappeared under the bias with sub-bandgap illumination. Thus the electric field distribution tended to be linear. Meanwhile, the effects of different wavelengths and intensities of sub-bandgap illumination on the electric field distribution at the grain boundary were also investigated. The results showed that the electric field of CdZnTe was distorted by sub-bandgap illumination at intensity lower than 1×10<sup>-9</sup> W/cm<sup>2</sup>. In contrast, a flatter electric field distribution was achieved at a wavelength of 850 nm and intensity of 1×10<sup>-7</sup> W/cm<sup>2</sup>. The carriers can be transported by drifting, reducing the probability of being captured or recombined by defects during transport, thus improving the charge collection efficiency of the detector.<br>In addition, the microscopic mechanism of the modulation of the electric field distribution by sub-bandgap illumination and the energy band model of CdZnTe crystal containing grain boundary were proposed. Due to the existence of the grain boundary, two space charge regions were formed near the grain boundary. The energy band at the grain boundary was bent upward. Meanwhile, the metal-semiconductor contact formed a Schottky barrier, and the energy band near the electrode was bent upward. When the bias voltage was applied, the energy band structure of the CdZnTe tended to tilt from the cathode to the anode. The sub-bandgap illumination can lower the energy band barrier at the grain boundary and regulate the energy band on both sides of the grain boundary. It is believed that these discussions will also make some contributions to understand the influence of illumination and grain boundary in other types of optoelectronic devices, especially the application of thin films in solar cell and detector.
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