Study On Defect Formation And Regulation Of CdZnTe Crystals And Its Photoelectric Properties

Cadmium zinc telluride（CZT）crystals due to its advantages such as wide band gap width,high energy resolution and adjustable lattice constant,have great research significance and application value in infrared detection and room temperature nuclear radiation detection fields.However,due to the low thermal conductivity of CZT crystals,the mass and heat transfer process is difficult,introducing defects such as vacancy and precipitation.These defects not only destroy the lattice periodicity of CZT crystal,but also absorb free carriers,reducing charge collection and transfer efficiency,and finally weaken the performance of photodetectors.Therefore,it is of great significance to study the defect formation,defect regulation and photoelectric properties of CZT crystals for the preparation of high performance,large size and strategic grade CZT single crystal.In this paper,the structural characteristics and formation mechanism of defects in the CZT crystals were studied.The effect of CdTe gas and Cd Cl₂ liquor as annealing medium on the defect microstructure and photoelectric properties of the crystal was compared,and the annealing mechanism under different annealing medium was obtained.The research results are as follows:（1）The structural characteristics and formation mechanism of defects in CZT crystal were revealed.The twins in CZT crystal are symmetric about the{111}plane,and the interface between twin and matrix is completely coherent.The dislocation and stacking faults are gathered around twin boundaries.The fluctuation of temperature gradient and concentration gradient at the crucible wall result in the formation of twins.The Te rich droplets are wrapped at the growth interface and formed micron Te precipitations after solidification.The two-dimensional morphology of micron Te precipitation is triangular and hexagonal,and its spatial structure is composed of{111}and{100}planes.Nano Te precipitations is formed by over saturation point defects condensation.The nano-precipitations is hexagonal and its equilibrium morphology is polyhedron composed of（111）plane with the lowest interfacial energy.Minimization of local lattice strain energy results in the formation of needle-like ordered phases in the crystals,with width of 1～3 nm,and its superlattice spots located along[111]and[111]direction.（2）The effect of annealing in CdTe atmosphere on the defect microstructure and photoelectrical properties of CZT crystal was obtained.After annealing,the Te precipitations larger than 5μm was eliminated,but high concentration of nano-precipitations still existed in the crystal.Micron Te precipitations can diffuse out of the crystal through thermal migration.However,the Te nano-precipitations are difficult to remove due to the migration rate decreases under the inhibition of large surface tension.The density of dislocation pits decreased,but dislocation proliferation occurs on the surface due to the thermal migration of Te atoms.When the annealing time arrives 180 h,the transmittance increases from 40%to 65%,and the bulk resistivity increases from 1.60×10⁹Ω·cm to 9.86×10¹¹Ω·cm.The vapor phase annealing can fill the internal V_Cdand improve the photoelectric properties by chemical diffusion of electroactive defects.（3）The evolution of internal defects and photoelectric properties of CZT crystals annealed in Cd Cl₂ solution were illustrated.Cd Cl₂ crystals were discovered on the surface of the annealed crystal,and the crystal size increases exponentially with the increase of annealing time.The liquid phase annealing process is mild,resulting in less damage on crystal surface.After annealing for 30 h,the infrared transmittance arrives 55%and the resistivity increases to 7.37×10¹⁰Ω·cm.The V_Cd in the crystal can be filled by the diffusion reduction of Cd²⁺,and the diffusion of Cl^-can introduce the deep donor doping level,so that the Fermi level is closer to the center of the band gap,and the high resistance CZT material is obtained.