Performance Non-uniformity Of CdZnTe Nuclear Radiation Detectors

Owning to its superior photoelectric performance,Cd_1-xZn_xTe?CdZnTe or CZT?semiconductor detector has been highly valued in the application fields of hard X-and?-ray detection with a preferred 10 keV¹ MeV energy range,i.e.nuclear medical imaging,security check and radiation monitoring.With the adcances of the crystal growth and post-processing technologies,large volume??20×20×15 mm³?CZT crystal with high electron mobility-lifetime product??10^-22 cm²/V?has now been commercially available.However,response non-uniformity is emerging as one of the critical issues limiting the CZT detector applications,in particular.In this work,two main factors leading to the CZT detector response non-uniformity,namely,Te inclusion and ion-induced radiation damage are studied.Their quantitative effect on detector photoelectric performance was characterized under different work conditions.Then,the mechanism of carrier transport property degradation is revealed,based on which the methods aiming to improve detector response uniformity are proposed.The features that Te inclusions affect carrier transport process is analyzed using?particle induced charge technique,and a modified Hecht equation considering the influence of spatial distribution and induced charge loss of extended defects is proposed.Besides,we developed an ion-beam induced charge?IBIC?technique in cooperation with the Institute of Modern physics,Fudan University,on basis of which a direct corresponding relationship between Te inclusion locations and the low charge collection efficiency?CCE?area is revealed.It was found that when detector bias was higher than 200 V,CCE in Te inclusion degraded regions decreases with increasing applied bias,and the reason therein is explained.Moreover,it is suggested that an applied bias of about 200 V is optimal to reduce Te inclusion induced CCE non-uniformity.I-V test for 400?m-and 200?m-sized pixelated CZT detector shows that when Te inclusions exist at the interface of pixel electrode,corresponding leakage current will increase up to 250%.By simulating the electrical field distribution as well as analyzing the energy band diagram,it was found electrical field at the electrode in this case is increased,which impaired the potential barrier height for holes,and thus lead to an increase of hole injection.By measuring the surface potential distribution of CZT crystals using Kelvin probe force microscopy?KPFM?,Te inclusions are revealed to act as lower potential centers in CZT crystals,with its potentials 0.1⁰.39 V lower than the surrounding CZT matrix.Since cleaved CZT surface is rarely affected by the roughness,stoichiometric composition derivation and absorptions,the measured Te/CZT potential difference is suggested to approach the ones in practical volume condition.The electrical properties of Te/CZT interface is studied by analyzing the measured Te/CZT potential profile using Poisson equation.It is shown that the electrical field at Te/CZT interface reaches up to 2000 V/cm,and there is positive space charge with a density of 6×10¹22 q₀/cm³ existing in CZT crystal surrounding Te inclusion.This is about 2³ orders of magnitude higher than the normal value,suggesting high density of defects in this region.Further,the mechanism why Te inclusions degrade carrier transport properties is revealed by analyzing the energy band diagram at Te/CZT interface.Calculated potential distribution around Te inclusions shows that Te inclusion-induced charge loss increases significantly with electrical field in the range of 500⁷00 V·cm^-1,since the potential barrier height for electrons is smaller than its thermal activation energy at room temperature.The conclusion agrees well with IBIC experimental results.Te/CZT potential difference is found to depend on Te inclusion shape significantly as revealed by KPFM.Measured potential profile along Te/CZT interface and first-principle calculated results by CASTEP show that potential differences along{100}A_CZT?47?{0001}_Te-and{111}A_CZT?47?{01-11}_Te-oriented interfaces are higher than that of{111}B_CZT?47?{01-11}_Te-oriented interface.A model correlating Te/CZT potential difference and the potential difference as well as its area of individual oriented Te/CZT interfaces is established.It is proposed that the overall Te/CZT potential difference is determined by all the composed oriented Te/CZT interfaces.Since profound difference exists among the potential difference of individual oriented Te/CZT interfaces,the change of Te inclusion shape will lead to the variation of relative area between oriented interfaces,thus contribute to the shape-controlled potential difference of Te inclusions.Considering the fact that the potential barrier surrounding Te inclusion plays an important role in affecting electron transport properties,it is suggested that Te inclusion with its shape deviating from the equilibrium morphology induces less charge loss and benefits the improvement of detector response uniformity.Measured damage threshold of 2 MeV protons for CZT detector is about 3.3×10¹¹p/cm².PL test shows that when radiation dose reaches up to 10¹33 p/cm²,high density of dislocation will be generated in CZT crystals,which act as non-radiative recombination centers and leads to a reduced luminous efficiency of PL spectra.By analyzing TSC spectra with SIMPA method,it is suggested that the density of the acceptor complex A center with an activation energy of 0.115-0.133 eV and dislocation-related defects with 0.159 eV activation energy increases profoundly after 2 MeV proton radiation,which may contribute to the performance degradation of CZT detector.Moreover,it is found that even local radiation damage may lead to the overall degradation of energy spectrum and charge collection uniformity.This effect is more serious under low applied bias,suggesting increasing detector applied bias is an efficient way to reduce the effect of radiation damage on detector response uniformity.When radiation dose reaches up to 4×10¹11 p/cm²,2.08 GeV Kr⁺ions induced local radiation damage significantly degrades the overall?-ray energy spectra of CZT detector.This is mainly attributed to the radiation-induced polarization field with an internal voltage of opposite polarity,which increase the trapping possibility of radiation generated carriers.Therefore,increasing the applied bias will reduce the impact of the polarization field and improve the detector performance.Temperature-dependent PL test shows that the bandgap of CZT crystal increases about 2.5 meV,due to the growing lattice constant resulting from radiation-enhanced stress.Moreover,it is suggested that 2.08 GeV Kr⁺ions induced radiation damage will generate amounts of dislocations and A centers,increasing the possibility of non-radiative recombination and further resulting in the degraded integral intensity of PL spectra.Besides,a comparison of the TSC spectra before and after irradiation proved that the intensites of three type defects increase significantly,namely,Te_Cdd with an activation energy of 0.400 eV,a dislocation-related defect with an activation energy of about 0.160 eV increase and second-ionized Cd interstitial Cdⁱ⁺⁺with an activation energy of 0.604 eV.The remarkable increase of deep level defects also contributes to the transfer of E_DDD located 0.714eV below the conduction band bottom to 0.753 eV.