Microstructural properties of some oxide thin films studied by X-ray diffraction and extended X-ray absorption fine structure
Abstract: The microstructure of some high-temperature superconductorrelated oxide thin films was studied by X-ray Diffraction (XRD)and Extended X-ray Absorption Fine Structure (EXAFS)techniques. A general one-dimensional kinematic x-raydiffraction model was further developed to quantitativelyanalyze the structural properties of complex layered films.Microstructural information was obtained by fitting thesimulated XRD profile to the measured x-ray diffraction spectraof thin films grown by molecular beam epitaxy (MBE). A totalelectron yield technique, that is commonly used for EXAFSstudies of surface structures, was applied for microstructuralanalysis of thin film samples.The overall structural quality of MBE-grown Bi2Sr2CaCu2Oy(Bi-2212) films, analyzed by reflectionhigh-energy electron diffraction (RHEED), conventional XRD andRutherford back-scattering spectroscopy (RBS), was observed tobe rather high. A quantitative comparison between measured XRDspectra and calculated x-ray diffraction profiles revealed somedisorder within the unit cell. The average interplanar CuO2-CuO2distances were found to be expanded and theSrO-CuO2distances were found to be contracted in filmswhilec-axis lattice parameter of the unit cell was notaltered and exhibited the same value as bulk Bi-2212 compound.It was shown that these changes are related to the cationicsubstitutions mainly between Sr2+and Ca2+. The cationic disorder is expected to be causedby growth kinetics during MBE synthesis. It is suggested thatthe structural disorder results in a suppressed zerosuperconducting transition temperature, Tc,zero, and broad superconducting transition width,?Tc.The XRD analysis of Bi-2212 films with excess or deficiencyof the Sr, Ca and Cu atoms revealed a certain number ofstacking faults (SF) of adjacent Bi2Sr2CuOy(Bi-2201) and Bi2Sr2Ca2Cu3Oy(Bi-2223) phases in the film matrix. If the ratiobetween stacking faults of Bi-2201 and Bi-2223 in Bi-2212 filmis not equal to unity, the x-ray diffraction spectra exhibitedbroadened and shifted XRD peaks. The peak shift is not observedwhen the number of stacking faults of Bi-2201 phase is equal tothe amount of SF of Bi-2223 phase. The stacking faults areexpecteaffect the superconducting properties of the films sincedifferent phases have different Tc,zerovalues.The fitting results of the MBE-grown Bi-2201/Bi-2212multilayers revealed two distinct types of discrete disorderpresent in the film structure: stacking faults of adjacentphases randomly distributed within the layers and interfaceroughness of half unit cell height (i.e. layer thicknessfluctuations). Both types of disorder are expected to influencethe superconducting properties of Bi-2201/Bi-2212 multilayersdifferently and therefore structural quality of the films hasto be evaluated before the transport properties areanalyzed.Structures of Bi2Sr2Can-1CunOywith n=8,9, 10, 11 were epitaxially stabilized using layer-by-layerMBE. Due to the absence of such structures in bulk form,microstructural quality of grown films was examined bycomparison of measured XRD spectra with calculated XRDprofiles. RHEED and X-ray diffraction analysis revealed thatnearly single-phase films were synthesized. Resistivity versustemperature measurements did not show superconductingtransition, possibly due to insufficient charge carrier dopingof Cu02planes."Infinite-layer" (IL) Ca1-xSrxCuO2films heteroepitaxially grown on SrTiO3(100) substrates were studied by RHEED, XRD andEXAFS. The misfit strain between substrate and film induced bya compressive stress for films with x>0.66 caused bucklingof Cu02planes that is associated with a partial relief ofthe strain from CuO bonds. Thin films with x<0.66 were foundto be affected by tensile stress and CuO2planes remained planar. The observed higherconductivity of IL films grown under compressive stress isprobably due to a release of the stress from Cu-O bonds thatconsiderably promote the ability of CuO2sheets to be optimally charged.Microstructure of Ca1-xSrxCuO2films grown on LaAlO3and SrTiO3(100) substrates was analyzed by XRD and EXAFS.Since the in-plane lattice constant of LaAlO3is lower that that for Ca1-xSrxCuO2compound within 0<x<1, IL films are expectedto be under compressive stress. Films on SrTiO3substrate experience tensile stress when x<0.66and compressive stress for x>O.66. EXAFS and XRD dataanalysis revealed that IL films grown under compressive stressexhibit buckled Cu02sheets and rather large critical thickness,hc,while for films under tensile stress CuO2planes remain planar andhcvalues are lower. It is proposed that oxygendisplacements alongc-axis relieve the strain from Cu-O bonds and thusoverall strain values in such films are reduced thus the filmstructure remain coherently strained.Hg-based thin films grown on SrTiO3 substrates usuallyexhibit lowerc-axis lattice parameter and lower Tc,zeroas compared to bulk. The local structure ofHgBa2CaCu2O6+?films was examined by EXAFS. Themicrostructural studies showed that the smallerc-axislattice parameter is mainly caused by cationicdisorder:CuO2-BaO interplanar distances were found to becontracted, Ca-site was found to be partially occupied by Ba2+. The observed local unit cell disorder issuggested to be induced by a lattice mismatch between substrateand the film. Besides other growth defects the cationicdisorder is expected to be additional source for lower Tc,zerovalues.Keywords:oxide thin films, high-temperaturesuperconductor, heteroepitaxy, molecular beam epitaxy, x-raydiffraction, extended x-ray absorption fine structure, cationdisorder, stacking faults, misfit strain, CuO2planes.
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