Deep-level Defects And Their Effects On The Photoelectric Properties Of CdZnTe Crystals

Cadmium zinc telluride (CdZnTe, CZT), with excellent electrical and opticalproperties, have been considered as one of the most promising compoundsemiconductors for the applications as room-temperature nuclear radiation detectors.As-grown CZT crystals inevitably contain a large number of impurities and defects andthe corresponding trap levels in the bandgap, which would interfere with therecombination, trapping and scattering of charge carriers, thus affecting the transportproperties (e.g., carrier mobility, lifetime) and the consequent detector performances(e.g., energy resolution, charge collection efficiency). In this study, we are concernedwith CZT crystals for X-ray and γ-ray detectors. Deep-level defects in the material weresystematically characterized to reveal the effects of trap levels on the material propertiesand the detector performances. We aimed at obtaining high-quality CZT crystals andhigh-performance CZT detectors, by controlling the generation of deep-level defectsduring the crystal growth and the detector fabrication processes.Thermally stimulated current spectroscopy (TSC) and thermoelectric effectspectroscopy (TEES) were used to investigate the energy distribution of trap levels inthe bandgap, which are associated with defect states in the crystal lattice. Thesimultaneous multiple peak analysis (SIMPA) method was adopted to determinetrap-related parameters, e.g., activation energy, capture cross section, trap density. Thus,the characterization of deep-level defects in CZT crystals was achieved. Moreover, theexperimental parameters would strongly affect the testing results in TSC and TEESmeasurements. It’s necessary to determine the most appropriate parameters andprocedures.Defect levels in CZT crystals from the tip, middle and tail regions of the as-growningots were investigated by using TSC measurements, in order to reveal the axialdistribution of the defect levels. Current-Voltage (I-V) characteristics and Hallmeasurements were used to reveal the axial distribution of the electrical and opticalproperties. The concentration of Te antisite-related deep donors is higher in CZTcrystals from the tail region than that from the tip, due to Te enrichment in the melt assolidification proceeds. Hall mobility for electrons shows a slight decrease from~467 cm2V-1s-1in the tip to~394cm2V-1s-1in the tail, due to higher concentration of Teantisite-related deep donors in the tail. The difference in the defect levels betweenMVB-grown and THM-grown CZT crystals was studied by applying TSCmeasurements, in order to reveal the effects of crystal growth methods on deep-leveldefects and electrical properties of CZT crystals. Te antisite-related deep donor isdominant in MVB-grown CZT crystals, and Te interstitial-related deep acceptor isdominant in THM-grown CZT crystals. The compensation between donor and acceptordefects results in net free electrons with concentration of~7.13×105cm-3inMVB-grown CZT crystals, thus obtaining n-type conduction and high resistivity of~1.34×1010Ωcm. The compensation between donor and acceptor defects results in netfree holes with concentration of~5.31×107cm-3in THM-grown CZT crystals, thusobtaining p-type conduction and high resistivity of~1.92×109Ωcm.The effects of deep-level defects on the carrier mobility of CZT crystals werestudied experimentally and theoretically. The total density of donor and acceptor defectsin CZT crystals with high electron mobility (~848±42cm2/Vs) and low mobility(~337±17cm2/Vs) are about2.0×1016cm-3and3.8×1017cm-3, respectively, asdetermined from TSC results. The total electron mobility of the two crystals,considering the contributions from a variety of scattering mechanisms, was estimatedbased on Matthiesen’s rule to be~1004cm2/Vs and~352cm2/Vs, respectively.Polar-optical phonon scattering is found to be the dominant scattering mechanismlimiting the mobility at room temperature with the total defect density lower than1.0×1015cm-3in CZT crystals, and ionized impurity scattering will be the dominantwith defect density higher than1.0×1017cm-3.The trapping and de-trapping processes of charge carriers through the deep trapunder sub-bandgap illumination were described by applying the modified SRH model,thus demonstrating the dynamics of illumination effects in CZT crystals. The ionizationprobability of the deep donor shows an increase from~0.62in the dark to~0.98undersub-bandgap illumination. The current transport properties in MSM structure of CZTdetectors could be comprehensively interpreted by the combination of Ohm’s law,Diffusion model and ITD theory. We obtain the decrease of bulk resistivity (from~3.0×1010Ωcm to~4.7×109Ωcm) and the increase of space charge density (from~2.5×1010cm-3to~2.2×1012cm-3) under illumination. Energy spectra would directlyreflect the effects of sub-bandgap illumination on detector performances. The shift of the photopeak position towards higher channel number demonstrates better chargecollection efficiency, and the increase of electron mobility-lifetime product from6.7×10-4cm2V-1to1.03×10-3cm2V-1demonstrates better carrier transport property.Radiation damage mechanism for high-energy high-dose γ-ray in CZT crystals andCZT detectors involves a series of energy transfer processes, i.e., Compton scattering,Rutherford scattering and collision cascade, in which a total number of1010~1011Cdvacancies and defect complexes would be induced according to the Kinchin-Peasemodel. TSC measurements were carried out to characterize the radiation induced defectlevels in CZT crystals. The considerable increase in the trap density (from1.7×1011cm-3to3.0×1012cm-3) of the Cd vacancy related trap levels contributes to the variation of theelectrical compensation ratio from~1.9×106cm-3net free electrons before radiation to~1.7×1012cm-3net free holes after radiation, and the conversion of conduction typefrom n to p, as determined by Hall results. The emission intensity ratioI(DComplex)/I(D0,X)in PL spectra increases from3.26before radiation to10.38afterradiation. The decrease of the electron mobility-lifetime product in energy spectra from1.08×10-3cm2V-1before radiation to7.12×10-4cm2V-1after radiation, clearly indicatesthe worsening of carrier transport properties and the deterioration of CZT detectorperformances.