Studies On The Defects And Annealing Treatment Of CdZnTe Crystals

Cd1-xZnxTe (CdZnTe or CZT，x≈0.1) detector has attracted great attention in the fieldsof spectroscopy measurement and medical imaging, owing to its high sptial and energyresolution and high detector efficiency for hard X andγray in the range of10-100KeV at roomtemperature. The structure of CZT detector system is relatively simple but with high capableintegration. However, large volume detector-grade single crystals are usually hard to obtain dueto the tendency of defect formation during the crystal growth. To improve the properties ofsingle crystals, the structural defects should be clearly understood and carefully controlled. Inthis work, the second phase particles and the coupling defects were studied first, and then thepost-growth annealing processing was designed to improve the physical properties of CZTcrystals, aiming to obtain large-volume detector-grade crystals. Different vapors wereintroduced to modify the defect structure during annealing. The dependence of the optical andelectrical properties on the microstructural defects were studied.By analyzing the formation mechanism and morphology feature of inclusions, the realshape of these bulk defects in three dimension were obtained. The shape of Te inclusions couldbe described by the14-hedrons models, which was also suitable to interpret the inclusionmorphology in other Te-based II-VI group compound semiconductors. The actual equilibriumshape of Te inclusions was regular tetrahedron consisting of {111}B facets. The whole shapeevolution process has also been proposed. By utilizing the defect selective etching method thehexagram dislocation-rich regions around Te inclusions were observed for the first time. Thespatial arrangement of the defects around Te inclusions were confined to a stellated octahedron.The in-situ TEM sample preparation based on the FIB lift-out technology has been used. Largenumber of extended dislocations and stacking fault dipoles accompanying with the stackingfault tetrahedron and Te precipitate were found. The formation mechanism of these defects hasbeen discussed. According to the low temperature SEM-CL results, the dislocations likelyinteracted with the native point defects during their motion and finally led to the formation ofnative deep level defects (Te antisites). Both the crystal growth conditions (cooling rate) andstoichiometry ratio of melt influenced the morphology and induced dislocation patterns aroundinclusions.Gradient temperature annealing experiments were carried out to reduce Te inclusions. Thestatistical results over the wafers were compared before and after annealing. The migrationbehavior of Te inclusions was investigated quantitatively, and the annealing process for eliminating Te inclusions in the wafers with thickness up to5mm was optimized. A liquiddroplet migration model in one dimension was established. A series of gradient annealing underCd/Zn and Te vapors were implemented, where the temperature, annealing time andtemperature gradient range were650~750C,120~300h and5~8K/cm, respectively. We foundthat all the annealing results could be summarized into three categories according to theelimination efficiency η. Ruling out the influence of insufficient annealing temperature and time,η always tends to be larger than70%with the average value around85%. Both size andvoid effects have great influence on the elimination efficiency and migration behavior of Teinclusions. The optical and electrical properties before and after Cd/Zn and Te vapor gradientannealing were studied. The status of bulk dislocations and dislocations induced by inclusionsand Te precipitates before and after annealing were analyzed.To improve the optical and electrical properties and detector performance, crystals withdifferent states were used for annealing under Te vapor and mixed vapors. The annealingconditions for as-grown low resistivity crystals were optimized finally. For the low resistivitycrystals with reduced Te inclusions, PL measurements indicated that the concentration ofvacancy-type complexes increased after annealing. The bulk resistivity was increased from106Ω cm to7.1×109Ω cm, however, the detector performance for241Am@59.5KeV gamma raydeteriorated, probably owing to the severe multiplication of dislocations after annealing,especially around Te inclusions. For the as-grown medium resistivity crystals, the resistivitywas increased after Te vapor annealing. Because the rise of resistivity resulted in lower noiselevel of the detectors, the energy resolution was able to be improved significantly. For as-grownlow resistivity crystals, five ingots with different growth conditions were chosen forexperiments. A series of constant annealing were implemented, with the temperature of300~600C and annealing time of10~300h, respectively. The optical and electrical propertiesand detector performance were investigated. For a specific annealing, when the applied voltagewas between350and500V, FWHM for241Am@59.5KeV gamma ray obtained by the planardetectors was up to5.42%~5.90%and the electron mobility-lifetime product was between0.83×10-3to1.12×10-3cm2/V. Furthermore, the mixed vapor annealing equipment was designed.After HT3and HT1annealing, the crystal transport properties were improved, and after HT1annealing, the electron mobility-lifetime product increased by nearly90%. Finally, the as-grown wafers with size of60mm were also annealed. The specific annealing ampoule andsealed connector were prepared. After annealing, the crystal resistivity increased to3.3×108Ω cm.To elucidate the mechanisms under different annealing atmospheres and better understand the annealing processing, the point defect evolution under Cd and Te vapor annealing has beendiscussed. The simple compensation model was established to analyze the resistivitydependence on deep donor defects in doped crystals. According to the defect equilibrium onCd-rich side, the diffusion coefficient ofCd2+idefects at640C was evaluated to be(0.50~1.33)×10-7cm2/s through EBIC tests. Because of the chemical diffusion of, thepoint defect structure in crystals evolved from vacancy-dominated type to interstitial-dominatedtype, the in-situ PL tests also supported this view. The ionized defect levels change during Cdvapor annealing was summarized. The diffusion of VCddefects resulted in the formation ofvacancy-complex, such as[V C d-In Cd]and[V C d-2In Cd]. For GC0710wafers, after10~200hannealing, the resistivity variation was consistent with the chemical diffusion of VCddefectsunder Te vapor, which implied that VCddefects played an important role on the formation ofTe Cddefects. The formation temperature ofTe Cdduring annealing was around400Cthrough the300~500C annealing. For the annealed CZT wafers with high resistivity, thethermal activation energy of dark current near room temperature was about0.78eV, which wasclose to the+/++energy level ofTe Cd. The variation of ionized defect levels change during Tevapor annealing was summarized. Finally, the variation of IR transmittance and PL spectrumbefore and after annealing related to the interstitial and vacancy defects was discussed in detailaccording to the annealing mechanism.