Study On Physical Properties And Suppression Techniques Of CdZnTe Defects

Cd_1-xZn_xTe?CdZnTe,CZT?crystal is an excellent and widely used ternary compound semiconductor material,which is mainly used as the substrate of Hg CdTe epitaxy material and the photoelectric material of?-Ray detector.Cd_1-xZn_xTe?x=0.04?matches the lattice of HgCdTe well,and is the optimum substrate material for preparing the high performance HgCdTe infrared focal plane detector.However,CZT crystal has many defects,which influences the quality of the epitaxy HgCd Te material,therefore restricts the further development of the advanced Infrared focal plane detector?small pixel,lightweight,high performance,low power consumption and low cost?.With the continuous improvement of the CZT crystal growth technologies,the size of the ingot is increasing?^maximum diameter of?120mm?,and the yield of the substrate has been improved significantly?³5%?.However,the dislocation density of CZT material is still between10³cm^-2-10⁴cm^-2,and a considerable number of amorphous phase defects,line defect cluster and the point defect cluster also exist in the crystal.These defects decrease the yield of substrate seriously,which make the cost of the CZT substrate is still much higher than the affordable price of the civil market.Many researches on the physical characteristics and evaluation technologies of these defects have been accumulated,but some properties and formation mechanisms of these defects still have not been understood clearly so far.So it sets up obstacles of developing effective technologies for eliminating these defects.In order to solve the key problems which inhibit the advancement of CZT,this work aims at improving the CZT crystal growth technology and refraining the defects in the CZT crystals.Some physical analysis methods were used to investigate these four kinds of defects?Cd vacancy defect,amorphous phase defects,grain boundary extended defect and twins?,which reduced the CZT substrate yield production significantly.By researching and verifying the formation mechanisms of these defects,some defects suppression techniques and process methods were proposed,and the main research contents and innovative results were summarizd as follows:1.Research on characteristics and control technology of Cd vacancy defects in CZT crystals.1)Using phase diagram theory to study the origin and distribution of Cd vacancies in the crystal growth process,and analyze the influence of Cd vacancies concentration on the infrared absorption.The main factors affecting the formation of Cd vacancies were also studied.The effects of changes in the stoichiometric ratio of the mother liquor,diffusion of Cd atoms in the free space,and heat treatment processes on the Cd vacancies concentration distribution were evaluated by calculation.Studies have shown that the initial ingredient and ampoule free space determine the initial concentration of Cd vacancies,while the Cd atoms diffusion between the ampoule free space and the mother liquor also affect the distribution of Cd vacancies in the crystal tail.And the distribution of Cd vacancies in the crystals is explained reasonable by this mechanism.2)The Cd vacancy defects suppress technology was studied in the crystal growth process.Results showed that the Cd loss of the mother liquor before the start of growth was mainly affected by three factors:the size of the free space in the ampoule,the mother liquor temperature,and the mother liquor stoichiometric ratio.The amount of excess Cd determines the stoichiometric ratio of the mother liquor.The smaller the stoichiometric ratio,the lower the concentration of Cd vacancies,however,too small stoichiometric ratio led to the increase of Cd-rich amorphous defects.3)For the substrate materials with higher Cd vacancy concentration,a heat treatment technology based on multi-layer casing pipe was developed.Compared with the traditional open-tube heat treatment technology,the loss of Cd source in the heat treatment process has been reduced one order of magnitude.It was found that 570^oC Cd-rich atmosphere heat treatment technology can effectively eliminate the Cd vacancy defects in the CZT material,and the infrared transmittance of the CZT substrate can be increased to⁶5%.2.The physical properties of amorphous phase defects in CZT materials were studied.And the formation mechanisms of amorphous phase defects in CZT were analyzed theoretically in terms of both phase space and physical space.Then the corresponding defect refraining technologies were studied.1)The structure of Cd-rich amorphous defects was performed by 3D modeling analysis and a three-dimensional morphology of the amorphous Cd-rich defects was obtained.Base on this model,the infrared transmission images of Cd-rich amorphous phase defect were obtained using simulated mapping method,which were consistent with the Cd-rich amorphous phase morphologies observed by infrared transmission microscope.2)The study of the distribution of the Te-rich amorphous phase defects in the crystal showed that the larger amorphous phase defects were more likely to appear in the region where the solid-liquid interface fluctuates drastically,which means that the larger-sized amorphous phase defects could be the inclusions.And the size of the amorphous phase defects is obviously reduced in the flat region,but the defects density is obviously increased,which indicated that the formation of such amorphous phase defects may be related to the non-equilibrium solid phase precipitation.3)The formation mechanisms of the two kinds of amorphous phase defects?Cd-rich and Te-rich?were analyzed from the phase space.One of the mechanism is that the amorphous phase defects was caused by the liquid phase wrap,and the supersaturated growth of the encapsulated droplets also produced the amorphous phase defects as the temperature dropped;The other mechanism is that non-equilibrium precipitation led to the amorphous phase defects.Calculations also showed that these two formation mechanisms were consistent with the characterized results in the as–grown CZT materials.Based on these mechanisms,it was concluded that most defects with dislocation proliferation were inclusions,and defects without dislocation proliferation were precipitates.4)A casing-type tube-opening heat treatment technology suitable for mass production process was developed to suppress the two kinds of amorphous phase defects.The results showed that the Cd-rich amorphous defects with the size of10-15?m in the unpolished blank substrate could be effectively removed without causing significant dislocation proliferation by heat-treated at 700°C for 8 days.The substrate with Te-rich amorphous defects required a two-step heat treatment technique using Te-rich and Cd-rich atmosphere in order.Compared with the conventional one-step Cd-rich heat treatment technology,it can eliminate the Te-rich amorphous defects phase more effectively,without producing apparent dislocation proliferation in the material.It was found that Te-rich amorphous phase defects of about 10-15?m could be effectively eliminated by heat-treated at 700°C for one day in Te-rich atmosphere and then for 6 days in Cd-rich atmosphere.The application for LPE also showed that the substrates after heat treatment by these two techniques could obtain qualified HgCdTe epitaxial layer.Moreover,it was also found that the heat treatment process caused a certain degree of lattice distortion in the surface layer of the material,which resulted in changes of the tested Zn concentration.3.The structural characteristics and related refraining methods of the grain boundary extension defects in CZT were also studied.1)It has been found that the direction of the grain boundary extension defects always extended to the seed,which lived at the zone of lower temperature in the growth temperature field.By converting the morphology information of the defects into spatial coordinate information,a digital topography of the defect was obtained.The results showed that the grain boundary extension defects present a spatial three-dimensional continuous distribution throughout the whole crystal,the defects near the grain boundary was denser,and gradually became sparse away from the grain boundary.Moreover,the presence of grain boundary extension defects led to an obvious decrease of the short-wave infrared transmittance.2)The grain boundary extension was a region with high dislocation density.Its formation mechanism was related to stress,and dislocation was a manifestation of stress release.3)Seen from the Zn component distribution which depended on the segregation effects,there was component gradient in the crystal.The presence of composition gradient could lead to lattice mismatch in the material.It was found that there were composition fluctuations near the grain boundary where the stress was greater than other regions.The study also showed that the lattice mismatch stress caused by the component fluctuation and the thermal stress caused by the temperature gradient were superposed in the same direction.4)By reducing the temperature gradient in the process of crystal growth,the occurrence probability of the defects was indeed greatly reduced.This indicated that this defect originated from the stress release process after the grain boundary was formed.The stress was greater than the threshold required for stress release when multi-factors interactions affectted the grain boundary,which caused the dislocation to multiply and propagate toward to the side of crystal with large thermal stress.This formation mechanism reasonably explained why the grain extension defect appeared at the grain boundary.4.The characteristics of twins in CZT were studied systematically and related control techniques for suppressing twinning were obtained.1)It was found that twinning planes generally occur in groups.It was because the twin boundaries were oxophile,and the grain boundaries with low energy tended to grow large grains.The high energy twin grain boundary was easily reversed to form a flaky twin.Geometrical analysis of twinning microstructures showed that there were two kinds of corresponding relations in the twin crystal structures,including the symmetry relationship and the 60°rotation relationship.The structural characteristics indicated that there were differences in the extended direction during the formation of twins.2)The locations where twins generated easily were analyzed and summarized,which included seed,the connection area of seed and shoulder,shoulder area,polycrystalline grain boundary,and sidewall of crucible.It was found that most angles between the twin planes and the growth planes included 90°,80°,75°,70°,and 45°in the spontaneous selective growth process.The growth directions of these main grains in the ingots above were corresponding to<110>,<311>,<210>,<111>and<310>orientations.Research also indicated that the angle between the other three groups of twins and the crystal axis was 19°28?using?111?seed.And using seed with?111?plane had an equivalent suppression effect on other three twins boundaries,thus it was easier to obtain crystals with few twins or no twins ingot.3)Zn composition distribution was used to characterize the solid-liquid interface distribution during crystal growth indirectly.Research indicated that the solid-liquid interface perturbation,crystal growth rate,and the rate of change of the diameter/axial ratio of shoulder area three factors led to twinning.But the disturbance of the solid-liquid interface was the most paramount factor.4)Studies have also shown that the twin in CZT can be significantly suppressed by optimizing the distribution of temperature field,reducing the fluctuation of temperature gradient,increasing the temperature gradient reasonably?does not cause cracks?,controlling the growth rate reasonably,and using B₂O₃ liquid sealing technologies.The CZT ingot with fewer twins and a single crystal rate of>90%was obtained.5)By optimizing the seed technology,the<111>direction oriented growth technology of the?90mm CZT crystal was obtained.The influence of the shoulder angles of 35°,68°,and 90°on the<111>orientation growth was studied.It was found that the angle of the shoulder was not the key factor for twin suppression,and the most critical factor to restrict the twin production was the distribution of the temperature field during the growth process.6)The uniformity of the temperature field distribution of the equipment was optimized by improving the heating electrodes distribution and thermocouple control methods.On this basis,high-quality<111>oriented seed were used to successfully reproduce the?90mm CZT crystals with high single crystal rate and few twins.This result also verified the analysis of selecting direction of the growth plane in 2).To conclude,the progress of CZT growth technologies was promoted by above research results,which were demonstrated in the following three aspects:1)CZT crystal with large size,high single crystal ratio,few twins,and low defects were obtained,which solved the practical application of the?90mm CZT growth technology.The new technology can effectively suppress the generation of twins and grain extension defect,and can obtain CZT single crystals with single crystal ratio of more than 90%.2)A heat treatment technology capable of effectively reducing the two kinds of Cd-rich and Te-rich amorphous phase defects has been developed by studying the amorphous phase defect properties and their formation mechanisms.For Te-rich amorphous phase defect,a two-step heat treatment technology of the Te-rich first and then Cd-rich can not only reduce the defect size but also eliminate the side-effects of dislocation proliferation compared with the traditional one-step heat treatment method.At present,the heat treatment process can eliminate the Te-rich/Cd-rich amorphous phase defects with the size of less than 10-15?m without dislocation proliferation.3)A multilayer tube-type open-tube heat treatment technology that significantly reduced the amount of Cd source leakage in the heat treatment process was invented.This technology can increase the heat treatment temperature from 700°C to 850°C while maintaining the same amount of Cd source leakage,which effectively extended the application range of heat treatment technology.And the heat treatment capacity was also enhanced from 8 slices of 40 mm×40 mm to 25 slices of 50 mm×50 mm in the existing equipment.Finally,the technology has been transferred to the conventional substrate fabrication for increasing the infrared transmittance and eliminating the amorphous phase defects,which improved the quality of the CZT substrate significantly.