Growth,Defect Analysis and Characterization of CdZnTe Crystals Grown by Traveling Heater Method

Author:Zhou Bo Ru

Supervisor:jie wan qi


Degree Year:2018





Ternary compound semiconductor Cd1-x-x ZnxTe(CdZnTe or CZT)is one of the most promising materials for room temperature radiation detectors due to its excellent optoelectronic properties.However,large volume detector-grade CZT crystals are usually hard to obtain due to the presence of defects during the crystal growth,especially grain boundaries,Te inclusions,etc.In the recent years,Traveling Heater Method(THM)is deemed to be one of the most promising techniques to grow large volume CZT crystals with good quality.In this work,the accelerated crucible rotation technique(ACRT)was adopted in THM process to grow large volume CZT single crystal with good crystalline quality.The defect energy levels in CZT crystals were regulated by optimizing some parameters,such as the growth temperature,the doping concentration of element In,etc.,aiming to obtain detector-grade CZT crystals with high resistivity and high mobility-lifetime(μτ)product of electrons.Solute transport was studied in Te-rich solvent zone.The result shows that the maximum rotation rate and the holding time of ACRT and the growth temperature have a significant impact on the solute transport in Te-rich solvent zone.Based on the result,the THM process with ACRT was systematically studied and optimized,and then the optimized growth process was obtained,i.e.the growth temperature of 1123K,the growth rate of 0.4 mm/h,the length of Te-rich solvent zone of 2030 mm,the following ACRT parameters:?=40 rpm,?1=3 s and?2=30 s.Several CZT ingots with diameter of 53 mm were grown by THM with ACRT and single crystal with the volume up to 220 cm3 was obtained.The relationships among the growth parameters,the growth interface morphology and the crystalline quality were revealed by analyzing the morphology of the quenched growth interface.The results indicate that without crucible rotation,the growth interface is usually concave one in macro-scale with large curvature.When a low forced convection was introduced using ACRT,the curvature of the concave interface was significantly reduced.A flat or even slightly convex interface can be obtained if the forced convection is increased by changing the maximum rotation rate or decreasing the holding time.In addition,the constant crucible rotation can also reduce the curvature of concave interface but the effect is limited.In macro-scale,concave interfaces resulted in polycrystalline ingot with smaller grain size,and the convex or flat interface resulted in larger grain size or even single crystal ingot.In micro-scale,the growth interface with irregular cellular structure promoted the formation of Te inclusions in CZT crystals.However,an essential flat interface in micro-scale can effectively suppress the formation of Te inclusions in CZT crystals,where the density of those is only 1.74×105 cm-3 and the size of those is smaller than3μm.High-resolution transmission electron microscopy(HRTEM)and selected area electron diffraction(SAED)results indicate that the majority of Te precipitates are found to be smaller than 10 nm in size with a density of 9.2×1015 cm-3–approximately two orders of magnitude lower than those in Bridgman-grown CZT crystals.Moreover,Te precipitates with the hexagonal structure have coherent interface with CZT matrix.The orientation relationships between CZT matrix and Te precipitates are[112]M//[0001]P,(1 11)M//(1100)P and[111]M//[0001]P,(220)M//(1120)P.The preferred growth direction is along the[110]M direction in CZT matrix.Te precipitates in the irregular and faceted morphology are seen in CZT crystals with fast cooling rate(60 K/h)after growth.When the cooling rate is reduced to 5 K/h,hexagonal Te precipitates are obtained.In addition,two new ordered phases were found in CZT crystals.Their corresponding superstructure spots are 1/2{1 11}and1/2{11 1},respectively.The preferred growth direction is along the[110]M direction in CZT matrix.The preferred site of ordered phases formation is the step-like interface between CZT matrix and Te precipitates,where large strain energy promotes the formation of ordered phases.The ordered phases in CZT crystals can be eliminated by reducing the thermal stress and regulating the morphology of Te precipitates in CZT crystals.Thermally stimulated current spectroscopy(TSC)was used to investigate the trap levels in THM-grown CZT crystals.It is found that the growth temperature has a significant impact on trap levels.The density of Cd vacancy-related defects is reduced by nearly two orders of magnitude with the decrease of the growth temperature from1223 K to 1023 K.However,the growth temperature of 1023 K is too low to favor the growth of detector-grade CZT crystals.The presence of Te interstitial-related defects(Tei-/Tei2-)with high density in CZT crystals grown at low growth temperature results in the deterioration of photoelectric properties of CZT crystals,such as low resistivity,low carrier mobility,etc.Moreover,the cooling rate after growth has a significant impact on the density of second ionized deep-donor defects(TeCd2+)in CZT crystals.With the decrease of the cooling rate,the density of the deep-donor defects(TeCd2+)increase.The relationships among the density of trap levels,the carrier concentration and the resistivity were revealed by TSC,Hall and I-V measurements.Based on the TSC results,the effects of deep-level defects on the optoelectronic properties of CZT crystals were studied by TOF and energy spectrum test system.The results indicate that the dominant factor affecting the electron mobility and the detector performance is second ionized deep-donor defects(TeCd2+).The electron mobility decreases from1159±13 cm2V-1s-1 to 710±19 cm2V-1s-1 with the increase of the density ofTeCd2+defects.The density ofTeCd2+defects lower than 1013 cm-3 is beneficial to obtain detector-grade CZT crystals with high quality.Detector-grade CZT crystals with high crystalline quality and good optoelectronic properties were achieved by further optimizing the growth parameters.The density of Te inclusions in CZT crystals is 0.93.0×105 cm-3 and the size of those is smaller than 3μm.The average IR transmittance of CZT crystals is over 63%.Planar configuration CZT detectors were fabricated and showed excellent detector performances with resistivity of1010Ω·cm,(μτ)e value of 6.55×10-3 cm2V-1,(μτ)h value of 4.02×10-4 cm2V-1 and the energy resolution of 3.67%for 241Am@59.5 keV at room temperature.