Calculation of Carrier Distribution, Space Charge Accumulation, and Internal Electric Field Distribution in CdZnTe Detectors under Extremely High X-Ray Doses

Abstract

Photon counting CdZnTe (or Cadmium Zinc Telluride) detectors have been accepted as a new generation device for X-ray imaging in the last decades [1]. In this case, CdZnTe detector may work under extremely high X-ray dose [2-4]. For example, in medical CT, X-ray flux can be as high as 108 Photon/s/mm2 to obtain images with enough contrast [5-7]. However, CdZnTe crystal was found to have polarization effect under such high doses of X-ray [7,8]. On the other side, the amplitude of the charge signal generated by a detector can be extremely low, even at the same level of noise [9,10]. In this case, the detector will be in a “stagnation” state and cannot work properly [11-13]. Therefore, the application requires X-ray detector to possess high resolution at low X-ray dose as well as to bear high X-ray flux without deterioration. Such a task of CZT X-ray detectors is usually hindered by the existence of highdensity non-equilibrium carriers in the crystal, particularly holes with a low mobility lifetime, which are trapped by deep level defects [14-16]. The electrostatic repulsion force of the crystal defects accumulates space charge. When the accumulated space charge reaches a certain value, the electric field will be distorted, which will severely impede the transport process of carriers and deteriorate the performance of the detector [17-19]. In the present paper, the space charge accumulation behaviors and their effects on the detector performance will be evaluated by analyzing the trap and de-trap process of X-ray stimulated charges. Methods and Calculations