Performance Comparison Of CdZnTe Radiation Detectors With PIN And MSM Structure And The Study On Leakage Current Of CdZnTe

Cadmium zinc telluride（CZT）is widely used in the field of nuclear radiation detection due to its excellent optoelectronic properties,and continues to receive the attention of many researchers.The traditional CZT crystal surface passivation process often cannot completely remove the negative effects of the low-resistance,non-stoichiometric thin layer introduced by the surface treatment.Therefore,we used TCAD to establish a relevant model to explore other factors that can inhibit the surface leakage current except the passivation process.In addition,the existing surface leakage current testing methods are complicated and cannot meet the quick needs of the characterization in daily experiments.Therefore,we propose a method to obtain the surface leakage current and resistivity parameters of CZT crystals according to electrode size and current variation features.In view of the influence of different electrode contact on device performance,we designed three device models with different contact:MSM（Ohmic）,MSM（Schottky）and PIN structure by TCAD.A detailed analysis of these three structures of CZT detectors is carried out,and the influence of different electrode contact characteristics on device performance is summarized.The main findings of this paper are as follows:（1）A two-dimensional CZT model of the MSM structure with a low-resistance p-type layer on the surface was established by silvaco TCAD.Simulation suggests that a larger electrode coverage can make the surface current path dominated by（110）surface with high surface resistance rather than（111）surface with a low one,which can effectively reduce the surface leakage current;The method of depleting the surface layer to increase surface resistance and can also achieve the purpose.Therefore,in the preparation process of CZT device with MSM structure,the（111）crystal plane should be used as cathode and anode surfaces,and the（110）crystal plane should be used as the sides.（2）We propose a method to estimate the surface leakage current,bulk leakage current and bulk resistivity of CZT by preparing two electrodes of different sizes on cathode and anode surfaces,and measuring their volt-ampere characteristic curves.Compared with the results obtained by the traditional guard-ring electrode method,we confirm that this method is feasible.（3）We used silvaco TCAD to establish MSM（Ohmic）,MSM（Schottky）and PIN structure CZT detector models,and simulated their photocurrent gain under X-ray and transient current pulse underγ-ray.The simulation results show that the working bias voltage of the MSM（ohmic）photoconductive structure is very large and the polarization phenomenon is the weakest.Which means the MSM（ohmic）can a be used in a high-flux environment,but the dark current is quite large.MSM（Schottky）structure device has extremely small dark current,but the polarization phenomenon is strong,and the working bias voltage is limited.It has good energy resolution for low-energy gamma rays（<100 ke V）,and can be prepared by low-resistance（n）CZT crystals to improve single crystal utilization.In the PIN structure device,the electric field strength in the N⁺and P⁺regions is zero,which seriously affects the effective collection of charges.So the current pulse height is much smaller than the first two devices under the same conditions.For the MSM structure CZT,the existence of electrode barrier can effectively reduce the dark current,but at the same time,it also leads to the reduction of working bias,the enhancement of polarization effect and the the uniformity of internal electric field.（4）MSM（Ohmic）and MSM（Schottky）CZT devices were prepared by thermal diffusion method,and we approximated the samples with In-CZT-Au structure after thermal diffusion treatment as PIN devices.By measuring and analyzing the volt-ampere characteristic curves and the multi-channel energy distribution spectrum under ²⁴¹Am of the three,it is found that the MSM（Schottky）structure device can obtain higher energy resolution under low bias.However,due to its limited working bias voltage,the dark current will increase sharply as external bias increases to a certain level,which is affected by the space-charge-limited current,then the energy resolution gradually deteriorates.The PIN structure CZT device has relatively poor energy resolution at low bias voltage,but it gradually improves with the increase of external bias.This is due to its double-rectifying Schottky contact,which enables to make working bias range lager than the MSM（Schottky）structure;However,the MSM（ohmic）photoconductive structure has relatively strong noise.