Current Transport Mechanism Of CdZnTe Relaxation Semiconductors

The cadmium zinc telluride（CdZnTe）semiconductor used for X/γ-rays detection developed dramatically in recent years.One of the properties of the CdZnTe material is its high resistivity(larger than 10¹⁰Ω·cm)at room temperature,which can significantly reduce the leakage current in detectors.However,high resistivity semiconductor exhibits generically different carrier transport behaviors from that of a low resistivity one:it is in the relaxation regime if the charge relaxation time is larger than the carrier lifetime,otherwise it is in the lifetime regime.The former one can be called a relaxation semiconductor,and the latter one can be called a lifetime semiconductor.Because the charge relaxation time is proportional to the resistivity,relaxation semiconductors typically have high resistivity and exhibit novel current transport phenomena.However,most of the existing current transport theories of CdZnTe detectors do not take into account the relaxation regime,resulting in difficulties or even controversies in the interpretation of many current transport phenomena.In this study,boundary conditions for the relaxation and lifetime regime of CdZnTe crystals were obtained by combining the compensation model and the recombination model.One-dimensional spatiotemporal evolution of locally injected carriers was studied analytically and numerically.The steady-state and transient current transport mechanisms for CdZnTe relaxation semiconductor radiation detectors were investigated by combining simulation results with experiments.By deriving the explicit time evolution of the separation distance between locally small injections of excess electrons and holes using the center of charge approach,we proved that excess carriers will transport ambipolarly in the lifetime regime,while separate spontaneously in the relaxation regime.The intrinsic carrier concentration has to be smaller than a critical value to keep the material in the relaxation regime.However,after introducing a high concentration of traps in the material,the majority and minority carrier lifetimes are not equal.Therefore,a new intermediate regime emerges where the charge relaxation time is in between of the two lifetimes.By combining the three-energy-level compensation model and the trap-mediated recombination model,the criterion for different regimes was investigated in deep donor compensated CdZnTe semiconductors.Numerical simulation verifies the analytical results and reveals rich dynamics of carrier transport.For semiconductors with low trap concentration,the separation distance rises asymptotically to a polarization distance in the lifetime regime,while there’s a transitional sub-region near the regime boundary where majority carriers form a double-hump distribution within a period after the injection and go through a separating-ambipolar or separating-remaining transformation dynamics.This phenomenon originates from the competition between charge relaxation and external field related components of the drift current.On the regime boundary,excess electrons and holes will separate without the influence of Coulombic attraction.In the relaxation regime,majority carriers deplete because of a larger recombination rate in the minority carrier pulse region.For semiconductors with high trap concentration,excess carriers still separate in the relaxation regime and transport ambipolarly in the lifetime regime.However,storage of excess carriers in traps weakens the majority carrier depletion and enhances the internal electric field between electrons and holes,thus reduces carriers’transport velocities in the relaxation regime.In the lifetime regime,captured charges of minority carriers could pin the excess majority carriers to the injection spot.In the intermediate regime,semiconductor type and relative magnitudes of the charge relaxation time and carrier lifetimes determine the excess carrier transport behavior.For deep donor compensated CdZnTe semiconductors,different compensation ratio leads to different regimes of the material,which determines the transport dynamics of radiation-generated carriers.The mechanisms of the dark current transport in CdZnTe radiation detectors were investigated by numerically simulating the static working states in the 0.01-1000 V bias range.Major current components in various bias and barrier conditions were determined by comparing different current transport theories based on computed device working states.For back-to-back Schottky contacts with large barrier heights,the reverse-biased cathode determines the current in the low bias range,where the current is limited by carrier generation and diffusion in the depletion layer.Otherwise,the current is restricted by the high-resistivity n-type bulk material if the barrier is relatively small.However,when the device is fully depleted of electrons in the high bias range,injected holes from the forward-biased anode become dominating mobile charges and could produce a rapid current increase.For ohmic contacts where the electron barrier height is smaller than the hole barrier height,the current-voltage（I-V）curve deviates from the linear relationship in the high bias range due to the electron space-charge-limited current.From the simulation results,a double-rectifying electrode configuration was proposed,which can significantly reduce the dark current in the high bias range.Experimental I-V characteristics were compared for the Au/CdZnTe/Al and Au/CdZnTe/Au detectors,which are consistent with the simulation results.The current temporal response（I-t）in CdZnTe detectors as a function of temperature was investigated both experimentally and numerically.After biasing,the transient current increases for detectors with small barrier heights,while decreases for those with large barrier heights.A faster current transient at higher temperatures indicates the process is determined by the de-trapping of deep levels in the depletion layer.Evolution of the electric potential,electric field,and trap occupancy in the detector was simulated.For detectors with small barrier heights,the current is first limited by the high resistivity bulk material,then by the depletion layer,which increases the transient current.For detectors with large barrier heights,the current is always limited by the reverse-biased electrode.The de-trapping process further expands the depletion layer and thus leads to a descending current.Time constants of the de-trapping process were obtained from the experimental and numerical I-t curves,which were analyzed by Arrhenius fitting to calculate the activation energy and capture cross-section for deep level traps in the material.