| Mercury indium telluride (Hg3In2Te6, MIT) possesses excellent photoelectric property and high radiation resistance, thus MIT is believed to be a promising material for short-wave infrared detectors, especially in the field of fiber-optic communication and in the radiation inspection environment. For MIT, the structural defects, such as vacancies or interstitials, dislocations, twin boundaries and precipitates, are unavoidable, depending on the growth technology. Defects can directly or indirectly affect the material property and device performance. In order to reduce or eliminate harmful defects and acquire high-quality MIT crystals and excellent devices, it is required to characterize and analyze accurately defects in MIT. In this research, defects in MIT were characterized and analyzed systematically. The types, characteristics and formation mechanisms of main defects were obtained.High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) results reveal at least two new ordered phases in MIT crystals. Their corresponding superstructure spots are1/3{422} and1/6(422}, respectively. The powder X-ray diffraction (XRD) pattern indicates that the volume fractions of the ordered phases are lower than5%. The presence of small-volume ordered phases implies that disorder-order phase transformation still happens in small portions of ingots, since the thermal conductivity of MIT disordered phase is small and the practical cooling rate of furnace cooling is not fast enough. The structure of ordered phase1is presented by the space group P3ml with lattice parameters a=b=(?)/2, c=2(?), α=β=90°, γ=120°. Its structural vacancies are entirely ordered. The structure of ordered phase2is presented by the space group P3ml with the lattice parameters a=b=(?), c=(?), α=β=90°, γ=120°. Its structural vacancies are partially ordered with the order degree of1/2. With the development of phase transformation, first, the ordered phase2forms. Afterwards, the ordered phase2transforms into the ordered phase1. Finally, the ordered phase1grows and then the vacancy discs in the ordered phase1collapse.HRTEM and SAED results show that HgIn2Te4and Hg5In2Te8precipitates coexist in the crystals, which confirms the decomposition reaction2Hg3In2Te6=Hg5In2Te8+HgIn2Te4. The powder XRD pattern indicates that the volume fractions of the precipitates are lower than5%, so only a small portion of Hg3In2Te6crystals has been decomposed. Based on the dark-field micrograph and HRTEM image, HgIn2Te4precipitates are found to exhibit rod-shaped morphologies and contain three equivalent variants. The preferred growth directions of the three types of variants are found to be<100> family of crystal directions of Hg3In2Te6matrix. HRTEM images reveal that HgIn2Te4precipitates are coherent with Hg3In2Te6matrix and nointerface dislocation exists. Electron irradiations suggest that the above decompositionreaction is thermodynamically spontaneous. Finally, the reaction was confirmed further byhigh-temperature XRD experiments.Field emission scanning electron microscope (FE-SEM) observations reveal that etchpits on MIT (111) face are mainly line, shuttle and date shape. Black lines, which are (111)plane projections of grooves in the latter two etch pits, are oriented in three directions, whichhave a rotation relationship of120°with each other. The orientations of the black lines arecalculated to be [112],[121] and [211] by electron backscatter diffraction (EBSD),respectively. The formation of the three etch pits is explained as follows. Under theconditions of high-temperature, screw dislocation was mainly hindered by structuralvacancies and their derivative defects and then the cross-slip happens. This can explain whythe morphology of etch pits in MIT is evidently from that in typical HgTe-based crystals withzinc-blende structure, such as HgCdTe and HgMnTe. Energy disperse spectroscopy (EDS)results show that, for the lattice distortion regions near the dislocations, the corrosion ofindium is easier than that of mercury, which indicates that In-Te bond becomes weaker thanHg-Te bond. The morphology of etch pits in MIT is hardly affected by stirring during thecorrosion process, the species of the polish solution and etchant rate. HRTEM observationshows that a large number of1/6[121] dislocations exist and their distributions are uneven.Three types of twins are identified in MIT through EBSD observation and single-sectiontrace analysis. The first type of twins has a pair of parallel twin boundaries, which are vicinalto coherent. The second one has a pair of parallel and incoherent twin boundaries. The thirdone does not have parallel twin boundaries. The second and third types were more commonlyobserved than the first one, which implies that most twin boundaries are incoherent. Thedimension of twins is small and the parallel alignment of twins has not been found. Analysisshows that twins in MIT are deformation twins. Curved twin boundaries in MIT arecomposed of long coherent twin boundaries, short incoherent steps and curved segments. Theexistence of high-density structural vacancies leads to the formation of high-density structuralvacancy clusters, Frank partial dislocations and ordered phase precipitations. The abovedefects hinder the glide of twinning dislocations and affect the coherence and shape of twinboundaries. The more the number of defects encountered by twins, the larger the deviationfrom the twin morphology of parallel and coherent. HRTEM and EBSD analyses point outthat the twinning plane and direction are not changed by the presence of structural vacancies.The residual strain of most twins is higher than that of the corresponding matrices. Based on Rietveld refinement, the concentration of Hg vacancy in the middle part of theingot is about9.1×1019cm-3, which is the lowest and that in the tail part is the highest. EBSDresults reveal that the lengths of most low-angle grain boundaries are in the range of5~70μmand the misorientation angles are in the range of2~4°. However, the length of the longestlow-angle grain boundary observed in MIT is1600μm and the misorientation angle is in therange of7.5~9°. Σ43c and Σ31b boundaries usually exist with twin boundaries. Based onetching method, a Te-rich phase with the width of1.9μm can be observed and the phaseextends towards the surrounding in a line. Tellurium particles can often be observed on twinboundaries and coincidence site lattice boundaries. Impurities gather mainly on boundariesand have different morphologies and micro-structures. Some impurities are Si-rich phases orO-rich phases. |
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