Enhanced Properties of CdMgTe Crystals by Multi-Step Annealing Method
Introduction Cadmium magnesium telluride(CdMgTe)crystal is an ideal material for room-temperature radiation detection.However,there are still many defects in as-grown crystals,which degrade the detector performance.At present,a single annealing method is unable to achieve the effect of both eliminating defects and maintaining or increasing resistivity.The multi-step combined annealing method can improve the crystal quality and the detector performance.There are a few studies on the annealing of CdMgTe crystals.In this work,a multi-step combined annealing method was used,i.e.,the CdMgTe crystals grown under Te-rich condition were annealed in Cd atmosphere and Te atmosphere.The effect of annealing temperature on the defects,mechanics,photoelectric properties and detector performance of CdMgTe crystals was investigated.Methods High purity(7N)cadmium(Cd)and tellurium(Te)were selected as raw materials.The selected slices for annealing were derived from In-doped Cd0.95Mg0.05Te ingot grown by a modified vertical Bridgman method under Te rich condition,with the size of 5 mm×5 mm×2 mm.The annealing in Cd atmosphere was mainly divided into two steps,i.e.,high and low temperature annealing.In the high temperature annealing step,the temperatures of the slices were 873,923 K and 973 K,respectively.The corresponding temperatures of Cd source were 901,946 K and 986 K,respectively.The annealing time was 120 h.After high temperature annealing,the temperature of the slices and Cd source reduced to 623 K,and the annealing time was also 120 h.After annealing in Cd atmosphere,it continued annealing in Te atmosphere.The annealing temperature of both slices and Te source was selected to 773 K,and the annealing time was also 120 h.The CdMgTe crystals before and after annealing were characterized by infrared transmission microscopy,scanning electron microscopy,ultraviolet-visible-near infrared(NIR)spectroscopy,infrared transmittance spectroscopy,Raman spectroscopy,X-ray photoelectron spectroscopy(XPS),Vickers indentation analysis and current-voltage(I-V)measurement.Au/CdMgTe/Au planar detectors were fabricated by an evaporation method,and the energy spectrum was collected in ORTEC test system.The radiation source was 241Am α particle source with a non-collimated energy of 5.48 MeV.Results and discussion The band gap of the as-grown crystal is 1.510 eV,and the maximum band gap is increased by 0.021 eV after annealing.After annealing in Cd atmosphere,Te inclusions in the crystals are greatly reduced.At the annealing temperature of 973 K,Te inclusions can be completely eliminated.The IR transmittance and resistivity are reduced.The density of Te inclusions has little change after annealing in Te atmosphere.However,the IR transmittance and resistivity improve and are better than those of the as-grown crystals.The optimum IR transmittance and resistivity are 63％and 2.41×1010 Ω·cm,respectively.After multi-step combined annealing,there is no extra oxidation on the crystal surface,and the microhardness is increased by 16％.The Raman spectra show that the crystal quality is improved.Au/CdMgTe/Au planar room temperature radiation detectors are fabricated with the crystals obtained under the optimum annealing conditions.The time-of-flight(TOF)measurements show a significant increase in electron mobility,up to 526.86 cm2/(V·s).The optimum energy resolution and electron mobility lifetime product of the detector are 15.6％and 1.89×10-4 cm2/V,respectively,using 241Am α particles with an energy of 5.48 MeV as a radiation source.Conclusions CdMgTe crystals were annealed by a multi-step combined method.After annealing,there was no additional oxidation on the crystal surface,and Te inclusions decreased significantly.The band gap,microhardness,IR transmittance and resistivity of the crystals were all improved,indicating the improvement of crystal quality.The optimum annealing conditions were in Cd atmosphere at high(i.e.,923 K)and low(623 K)temperatures for 120 h,and then in Te atmosphere at a low(i.e.,623 K)temperature for 120 h.The optimum performance of CdMgTe detector was electron mobility(μe)of 526.86 cm2/(V·s),the energy resolution of 15.6％,and the mobility lifetime product(μτ)e of 1.89×10-4 cm2/V.
cadmium magnesium telluride crystalmulti-step combined annealingcrystal qualityresistivityroom temperature radiation detector
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