Study Of Characteristics Of Defects In CdZnTe Materials And The Annealing Technique

With the rapid development of third generation HgCdTe infrared detectors andwidespread application of nuclear detectors, the large size and high quality CdZnTe singlecrystals are required. The diameter of CdZnTe ingot is usually larger than100mm atpresent. One of the main problems is that the defects of the materials cannot be eliminated.The quality of CdZnTe materials is degraded and the size of obtained wafer is limited bythe defects. The application of the detectors based on CdZnTe materials is also limited.Although many researches about the characteristics of defects in CdZnTe materials havebeen done, the characteristics of precipitates in CdZnTe matarials and the correlationbetween the defects in HgCdTe epilayer and CdZnTe substrate are unclear at present. Thestudy about annealing technique of CdZnTe materials is not comprehensive and has notbeen converted into a practical technology to improve CdZnTe materials.In this paper, the morphologies of precipitate etch pits and the structures ofprecipitates and also the origins of defects in CdZnTe materials were studied first. Base onthe evaluation technique of defects, the evolution rules of defects during annealing processwere studied. The suitable annealing conditions to reduce the sizes of defects were foundand a practical annealing technique was developed. The detailed researches and results arepresented as follows:1, The morphologies of etch pits on {111} B surface of CdZnTe materials and theircorresponding defects were studied. The origins of common triangular pyramid etch pitsand triangular etch pits with flat bottom were discussed. The triangular pyramid etch pitoriginates from dislocations. The triangular etch pits with flat bottom are formed after thedislocations or micro-precipitates are removed.2, The study results show that the etch pits of precipitates on {111} B surface ofCdZnTe materials have different morphologies from dislocation etch pits. The etch pits ofTe-rich precipitates have the shape of triangule or hexagon with a rough bottom. This kindof defects looks like a crater under the low magnification （～50x）of optical microscope that it can be identified easier than dislocation etch pits. The etch pits of Cd-richprecipitates are always enwrapped by large quantities of dislocation etch pits which form acluster morphology. They can also be identified easily under the low magnification ofoptical microscope. Compared with observation by infrared transmission microscope, theetch pit observation method for evaluating precipitates has many advantages. It is moreeffective by using etch pit observation method. The areal densities of precipitates can bedirectly measured in a large surface region. The areal densities of the substrate defects aremore valuable parameters for the control of surface defects of HgCdTe epilayers.3, The bulk defects in Te-rich and Cd-rich CdZnTe materials were observed andclassified respectively. The precipitates with random distribution, line distribution and areadistribution have different morphologies in both kinds of materials. Te-rich precipitateshave tetrahedron and octahedron structures. The surfaces of Te-rich precipitates are {111}faces because they have lowest surface energy in the crystals with Zinc blende structure.The morphologies of Cd-rich precipitates are covered by dislocation clusters withstar-shaped distribution. It was found the star-shaped morphologies are formed by theprojection of24dislocation cluster regions extending along <211> directions, which iscalled as dislocation cluster arms in this paper. The dislocation cluster arms have {110}sheet structure. The absorption and scattering of infrared light threading these arms havegreat difference with the change of incident direction of the light. As a result, no more thansix dislocation cluster arms can be observed under the infrared transmission microscope.Base on this result, a3D model of the star-shaped defects was obtained for the first time.4，The influence of annealing temperature, vapor pressure and annealing time ondefects in CdZnTe materials was studied systematically. The result shows that the etch pitdensity of Te-rich materials increases after the annealing under the conditions (annealingtemperature lower than750℃). The defect multiplication can be avoided by improving theannealing condition. The etch pit density of Cd-rich materials do not change obviouslyafter annealing under750℃. When the annealing temperature is higher than750℃, theetch pit densities of both Te-rich and Cd-rich materials will increase obviously. The precipitates will also change under different annealing conditions. Te-rich precipitates canbe eliminated by Cd-rich annealing and Cd-rich precipitates can be eliminated by Te-richannealing from the infrared transmission morphology of CdZnTe materials. After Cd-richannealing, dislocation clusters will form around Te-rich precipitates. Further study showsthat such dislocation multiplication can be avoided by using two-step annealing technique(a Te-rich annealing followed by a Cd-rich annealing).5, The influence of annealing on the full width at half maximum of X-ray diffractioncurve, resistivity and infrared transmission of CdZnTe materials was also studied. Theresult shows that the lattice structural integrity of Te-rich materials has a little degrationafter annealing while that of Cd-rich materials does not change. The resistivity of Te-richmaterials increases3orders of magnitude after Cd-rich annealing. The density of Cdvacancies in CdZnTe materials will increase after Te-rich annealing and the infraredtransmission of materials will decrease. By using Cd-rich annealing, the infraredtransmission of materials can increase above60%.6, Open-tube type annealing equipment and technique used for batch annealing oflarge size CdZnTe wafers was studied. The difficulties to realize open-tube annealing wereovercome. The research results obtained from annealing studies in closed tube wereapplied to the open-tube annealing technique. The results of batch annealing in open-tubeequipment show that the size of precipitates can be reduced and the infrared transmissionof materials can be improved after the annealing. This technique has been used in themanufacture process of CdZnTe substrates and a good benefit has been obtained.7, The morphologies of HgCdTe surface defects formed in liquid-phase epitaxy andtheir correlations with the precipitates on the substrate surface were studied. Statistic datashow that there is a positive correlation between the HgCdTe surface defects and CdZnTesurface precipitates. The epilayers grown on the annealed substrates have lower surfacedefect densities than those grown on the unannealed substrates. It indicates that theannealing of substrates is an effective technique to reduce surface defects of HgCdTematerials. In addition, it was found that a part of the dislocations in the clusters around Cd-rich precipitates can extend into HgCdTe epilayer. But the dislocations in the clusterscaused by Cd-rich annealing in Te-rich CdZnTe materials will not extend into the epilayer.It has been a difficult work to study the characteristics and origins of defects inCdZnTe materials. This work provides some new experimental evidences for the betterunderstanding of the defects and their evolution roles during annealing process. Theresearch results can be used to reduce the defect sizes and improve the photoelectricproperties of CdZnTe materials by selecting suitable annealing conditions. The studyresults of the correlation between HgCdTe surface defects and CdZnTe substrate defectscan be used to improve the quality of HgCdTe materials. Some research results in thiswork have been confirmed and used in our manufacturing process of CdZnTe substratesand HgCdTe epilayers.