Comprehensive Study Of Composition Gradient Strontium Doped Lanthanum Manganite Thin Films Deposited By Magnetron Co-sputtering

Abstract

In this body of work, the examination of three different types of Strontium doped lanthanum manganite thin films synthesized by magnetron sputtering in the Surface and Nano Engineering Laboratory (SaNEL) allowed for the development of a detailed understanding between the processing conditions, the nano-scale structure and strain relaxation of the as-deposited thin films. Pure Lanthanum manganite and strontium doped lanthanum manganite thin films which were epitaxially grown on (001) single crystalline lanthanum alumina (LAO) and magnesium oxide (MgO) were fabricated by using single target deposition. The optimum deposition conditions were explored as preparation for synthesis epitaxial composition gradient strontium doped lanthanum manganite thin films by using co-sputtering deposition later. Surface morphology and microstructure of as-deposit thin films were characterized by atomic force microscopy (AFM), x-ray diffraction (XRD) and high resolution transmission electronic microscopy (HRTEM). Pseudo-cubic structure was identified in epitaxial lanthanum manganite thin film on LAO and strontium doped lanthanum manganite on MgO substrate, while orthorhombic structure was obtained in epitaxial strontium lanthanum manganite on LAO and lanthanum manganite on MgO substrate. Deposition rates of each target on different substrate were calculated. ECG strontium doped lanthanum manganite thin films were prepared on the same substrates by using co-sputtering deposition with both targets deposited simultaneously. The as-deposit thin films using novel deposition procedure with different conditions were compared. The substrate temperature and power rate of each target are considered as the key factor for controlling the quality of the films. The surface morphology, microstructure and composition ratio of each element were characterized by AFM, XRD, HRTEM and XPS. A novel low angle XRD method was involved to examine the graded structure with the nano-scaled variation. It is found that polycrystalline structure with pseudo-cubic columnar domains was formed in the film on MgO substrate while epitaxial pseudo-cubic structure was obtained in the film on LAO substrate. The composition gradient of Sr doping was confirmed by EDS analysis along growth direction. The boundaries between each column were considered as a strain relaxation source in polycrystalline thin film. The ECG thin film, which is under the critical thickness, exhibits a novel strain relaxation which is considered as composition grade structure effect. Thus, the ECG buffer layer was confirmed as an effective method in strain controlling.To further complex the structure and develop for application, a triple-layer strontium doped lanthanum manganite thin films on LAO and MgO substrates were deposited by using co-sputtering deposition with lanthanum manganite (LMO) target and strontium manganite (SMO) target. The films were designed to have a LMO layer at bottom and SMO layer on surface with both La and Sr gradient layer in between. The microstructure of as-deposit films were identified as epitaxial structure. Two epitaxial layers were found in the film on LAO substrate where orthorhombic structure was obtained below about 20 nm and pseudo-cubic structure was formed on the top. Similar strain relaxation was also observed in the film. This may produce “new” materials with the properties that are not available in the individual component phases, which might also open up various new possibilities of designing new nanoscale structures with unusual cross coupled properties.