Growth And Characterization Of Cdznte Single Crystals

Cd0.96Zn0.04Te (CdZnTe) single crystal has been regarded as the preferred substrate forlong-wavelength infrared (LWIR) and very long-wavelength infrared (VLWIR) mercury cadmiumtelluride (MCT) focal-plane arrays. And it had other applications, such as radiation detectors andsolar cells. More attention has been attracted for growing high quality CdZnTe single crystals withexcellent quality. Some properties of CdZnTe, such as high growth temperature, low thermalconductivity, low stacking fault energy and Zn component segregation, limited the growth of CdZnTesingle crystals. So it was very difficult to obtain large-size CdZnTe single crystal. Many kinds of crystaldefects were observed in CdZnTe single crystals. For decades, a lot of research focused on growinglarge-diameter crystals and improving the crystal quality.In order to meet the urgent demand of large-size and high quality CdZnTe substrate forgrowthing MCT epilayers by liquid phase epitaxy (LPE). CdZnTe single crystals growth by the verticalBridgman method have been further developed in this paper, based on years of work in ourlaboratory. Factors affecting the growth of CdZnTe single crystals and the formation mechanism ofthe defects have been discussed. By optimizing the crystal growth technique, increasing crystaldiameter, improving crystal singularity and reproducibility and inhibiting crystal defects, CdZnTesubstrates with large-size and high quality have been obtained.The solid-liquid interface is the key point of CdZnTe crystal growth. By studying the formationmechanism of three kinds of typical ingot tail shape, it was found that the shapes of the tail wererelated to the relative location between the ingot tail and the freezing point when the growthended. Hence, the location of the solid-liquid interface could be judged by the ingot tail end shape,which provides an important reference for controlling the solid-liquid interface. And measuring thegrowth stipe enable us to control the shape of the solid-liquid interface.The effect of asymmetry thermal field on the growth of CdZnTe crystal was studied. The resultsshowed that asymmetry thermal field would cause polycrystals. The ingot surface morphology wasclosely related to the roughness of the crucible inner wall. However, no evidences indicatenucleation came from the wall of the crucible. Rotating the crucible was an effective method toreduce the effects of asymmetry thermal field, as a result, ingots with a whole large grain wereobtained.Etch pit density (EPD), usually considered related to the dislocation desity, was one of the keyparameters for evaluating CdZnTe substrate quality. The results showed that the EPD of CdZnTegrown by pyrolytic boron nitride (pBN) crucible was generally less than5×104cm-2, at least half anorder of magnitude lower than that grown by carbon coating quartz crucible. It suggested thatduring the cooling process, the vapor pressure decreased and the quartz crucible contracted. Thusthe ignot was compressed by the crucible and dislocations in the crystal multiplicated. Therelationship between cooling rate and EPD was further studied. It showed that the rapid coolingprocess did not result in dislocation multiplicating in the crystals grown by quartz crucible or pBN crucible. It suggested that as the cooling rate was increased, the thermal stress increased, but thetime of the crucible compressing the ingot decreased. Effects of cooling rate on EPD are thecombination of thermal stress and the crucible compression.Inclusion was one of the main defects in CdZnTe substrate. The relationships between inclusiondensity and process factors were studied and results indicated that non-stoichiometry was the keyfactor. So controlling stoichiometry strictly is the most fundamental solution to eliminate inclusion,by which inclusion-free CdZnTe single crystals were obtained in this paper.To avoid the impact of the crucible wall on growing CdZnTe single crystal, great efforts havebeen made to develop dewetted method. This paper developed a new dewetted method utilizingthe property of volume reducing when CdZnTe melt freezing. By growing the crystal downward in aneck-shape crucible, crystal growth without contacting crucible wall can be achieved. This methodwas especially suitable for growing large single crystals. In this paper, Φ50mm CdZnTe single crystalswere grown by this dewetted method and large size grains were obtained after severalimprovements. Substrates with maximum size of100×40mm2were fabricated. The inclusion sizewas less than3um and the density was about13cm-2. EPD was about1.3×105cm-2.In a substrate processing, an efficient chemical etching method was developed to remove thedamage layer of sliced wafer, which replaced the traditional mechanical polishing method. Using thechemical etching method, a batch of wafers up to16pcs could be processed at the same time. Theoperating time was reduced to2hours from6days, which greatly improved the efficiency andreduced the cost.Generally, the defects in the substrates were asymmetric and had some characteristic. Thesubstrate application paid more attention to the evaluation of the full substrate. To obtain theethced surface morphology images of the whole substrate, photographic method was applied. Themethod has the advantages of fast testing and small data storage and it becomes the primarytechnical means of substrate quality evaluation. An optical microscopy with mapping function wasdeveloped to obtain fine etching morphology of the substrate and inclusion distribution map. Themicroscopy could work both in reflection and near-infrared transmission modes. And the softwarecould map more than10000pictures. Moreover, the value of defect density counting by softwarewas more reliable than manual counting method. X-ray diffraction topography was an effective andnon-destructive method for investigating structural integrity and homogeneity of CdZnTe substrateas well as MCT epitaxial layer. In our work, the latest X-ray diffraction topography in reflection hasbeen applied to evaluate CdZnTe substrate quality. The development of above methods enrichedthe research tools to explore the relationship between CdZnTe substrate and MCT epitaxial layer.Results of MCT epitaxial layer grown on CdZnTe substrate by LPE showed that, crystal structuredefects such as sub-grain boundaries of CdZnTe substrate were "copied" into the epitaxial layer.MCT epitaxial layer quality was highly correlated to that of the substrate, which indicated substratewith perfect structure was the basis for preparing high quality MCT epitaxial layer.Based on the work of this paper, optimal singlarity of Φ60mm ingot grown by the vertical Bridgman method was up to90%and the average singlarity of Φ90mm was about56%. The (111)substrate with the size of30×40mm2,40×40mm2,40×60mm2and50×50mm2can be massproduced. EPD of CdZnTe substrate is less than5×104cm-2. CdZnTe ingots with large diameter of120mm were grown, from which60×60mm2substrate could be obtained. The developeddewetted technique provides a promising method for growing large diameter CdZnTe crystal. Theformation mechanism of crystal defects were studied by using and developing crystal qualityevaluation method. CdZnTe substrate with large size and high quality provides firm foundation fordeveloping high performance LWIR MCT focal-plane arrays.