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Grown Strained Ca2fe2o5 Thin Films Synthesis and Cation Stoichiometry Effects of MBE- Grown Strained Ca2Fe2O5 Thin Films A Thesis Submitted to the Faculty Of Drexel University By Yizhou Yang In partial fulfillment of the Requirements for the degree Of Master of Science in Material Science and Engineering May 2018 ©Copyright 2018 Yizhou Yang. All Rights Reserved. Dedication This work is dedicated to the memories of my grandfather Huici Yang (1933-2018) i Acknowledgement Work on this thesis was supported by the National Science Foundation (grant number CMMI-1562223). Film synthesis utilized the RHEED instrument, which was acquired through an Army Research Office DURIP grant (W911NF-14- 1-0493). ii Table of Contents 2.1 Perovskite oxides...................................................................................... 5 2.2 Oxygen deficient perovskites ................................................................... 6 2.2.1 Effect of Epitaxial Strain on CaFeO2.5 ............................................ 11 2.2.2 Magnetic and Electronic Properties of Ca2Fe2O5 ........................... 15 2.2.3 Off-Stoichiometry of CaFexO2.5 ...................................................... 16 2.3 Oxyfluorides ........................................................................................... 18 2.3.1 Cation substitution .......................................................................... 19 2.3.2 Anion substitution ........................................................................... 20 3.1 Molecular Beam Epitaxy ........................................................................ 23 3.1.1 Substrate preparation and loading ................................................... 24 3.1.2 Effusion cells and QCM rate measurement .................................... 26 3.1.3 Reflection High-Energy Electron Diffraction (RHEED) ................ 28 3.1.4 Oxygen Mass Flow Controller ........................................................ 29 3.2 X-ray Diffraction and Reflectivity ......................................................... 30 3.2.1 X-ray Diffraction ............................................................................ 30 3.2.2 X-ray Reflectivity ........................................................................... 32 3.2.3 X-ray Photoelectron Spectroscopy (XPS) ...................................... 32 3.2.4 Rutherford Backscattering Spectrometry (RBS) ............................ 34 iii 3.3 Spectroscopic Ellipsometry .................................................................... 35 3.4 Fluorination ............................................................................................ 37 4.1 Synthesis of Ca2Fe2O5 films ................................................................... 40 4.2 The effect of cation off-stoichiometry ................................................... 47 4.3 Strain effects on Ca2Fe2O5 ..................................................................... 50 4.4 Fluorination of CFO films ..................................................................... 53 iv List of Tables Table 2.1 Strain induced by lattice mismatch between substrates and Ca2Fe2O5 films. ..................................................................................................................... 14 Table 4.1 List of samples ...................................................................................... 41 Table 4.2 The binding energies of Fe 2p3/2 of each bond involved in the fluorination process................................................................................................................... 58 v List of Figures Figure 1.1 Fuel cells and other electric generators. Reproduced from Ref. [7] ...... 2 Figure 1.2 Elements that can be accommodated within the perovskite structure. Reproduced from Ref. [8] ....................................................................................... 3 Figure 2.1 Schematic structure of a perovskite oxide. Reproduced from Ref. [14] 5 Figure 2.2 Schematic of the expansion of the SrTiO3- lattice upon incorporation of an oxygen vacancy (red circles: O atoms; blue diamonds: Ti atoms). [15] [16] Figure reproduced from Ref. [15] ........................................................................... 7 Figure 2.3 ADF STEM image of [110]c-oriented LSCO thin film grown on LSAT substrate. Figure reproduced from Ref. [18]. .......................................................... 8 Figure 2.4. In the brownmillerite unit cell, the left graph shows the b-axis direction is repeated every 4 layers. In the right graph, black solid lines mark the unit cell of the brownmillerite structure while white dashed lines mark the unit cell of the related cubic perovskite. ......................................................................................... 9 Figure 2.5 (a) Schematic of left and right tetrahedral chain. (b) Pnma symmetry that has L and R chain alternating between layers. (c) Pbcm symmetry that has L and R chain alternating within layers. Reproduced from Ref. [20]. ............................... 10 Figure 2.6 (a) Temperature dependent magnetization of SCO thin films. (b) magnetization hysteresis loops of SCO films at 10K. (c) Temperature dependent resistivity. Reproduced from Ref. [21] . ............................................................... 11 Figure 2.7 Blue dots represent the substrate, and red dots represent the film. (a) Film and substrates having matched in-plane lattice. (b) Film is compressed to align with the substrate. Reproduced from Ref. [16] The film is not uniform thus the out- of-plane lattice of the film changes. ...................................................................... 12 Figure 2.8 (a) X-ray diffraction pattern of Ca2Fe2O5 films on SrTiO3 (STO), (La0.3Sr0.7)(Al0.65Ta0.35)O3 (LSAT), LaAlO3 (LAO), and LaSrAlO4 (LSAO); the film peaks are shown in blue. (b) Schematic of film orientation on STO and LSAT. c) Schematic of film orientation on LAO and SLAO. Reproduced from Ref. [22]. ............................................................................................................................... 13 Figure 2.9. Schematics showing (a) oxygen vacancies parallel to the substrate and (b) oxygen vacancies perpendicular to the substrate. ........................................... 14 Figure 2.10 (a) Temperature dependence of resistivity measurement of Ca2Fe2O5 under applied magnetic fields. [27] (b) M(H) loops of La-doped Ca2-xFe2O5. Reproduced from Ref. [28] ................................................................................... 16 Figure 2.11 Off-stoichiometry impacts on the lattice parameter of LaFeO3 thin films. Reproduced from Ref. [30] ................................................................................... 17 vi Figure 2.12 X-ray diffraction patters of Ca2-xLaxFe2O5. [28] ................................ 18 Figure 2.13 (a) Lattice parameters for La1-xSrxMnO3. (b) Temperature dependence of resistivity for La1-xSrxMnO3; Tc represents the critical temperature for the ferromagnetic phase transition. Reproduced from Ref. [31] ................................. 20 Figure 2.14 (a) XRD plot for SrFeO3-δ (002) as-grown film, SrFeO3-δFδ (002) fluorinated film, and SrFeO3 (002) ozone annealed film. Reproduced from Ref. [33] 2 (b) Optical absorption spectra of SrFeO3-xFx, the inset shows (αhv) vs hv curves for SrFeO2F and LaFeO3 film. (c) Schematic showing density of states for SrFeO3- xFx. Reproduced from Ref. [34] ............................................................................ 21 Figure 2.15 (a) Spin-coat method used for fluorination of thin films: A polymer solution is used for spin coating, then the sample is annealed, then the polymer layer is removed. Fluorine atoms diffuse into the sample in step 3. (b) Vapor transport method: in a quartz tube, a fluoropolymer is placed at the upstream position relative to the sample. A carrier gas (Ar) carries the fluorine content through the quartz tube; fluorine reaches the sample and diffuses through it. Reproduced from Ref. [35] 22 Figure 3.1 (a) photo and (b) schematic example of the molecular beam epitaxy system used in this thesis. ..................................................................................... 23 Figure 3.2 Photo of sample holder from the load-lock. [36] ................................ 25 Figure 3.3 Metal sources heating up process for (a) calcium and (b) iron. .......... 26 Figure 3.4 Geometry of Staib Instraments RHEED. [37] ..................................... 29 Figure 3.5 The schematic diagram of diffraction of X-rays by a crystal (Bragg condition) [38]. ..................................................................................................... 30 Figure 3.6 Sample graph for X-ray reflectivity [40] ............................................. 32 Figure 3.7 Iron(III) 1s and 2p spectra. Due to the high-spin of Fe3+, there is a multiplet-split for Fe2p spectra. ............................................................................ 33 Figure 3.8 Example plot of RBS measured from a CaFeO2.5 film on MgO. ........ 35 Figure 3.9 (a) Photo of fluorination setup. (b) Schematic of fluorination process. Argon gas flows into the quartz tube, while inside the tube, there is a boat like container which containing fluoropolymer and film. ........................................... 37 Figure 4.1 (a) The substrate temperature and MBE main chamber pressure throughout the growth. (b) Shuttering sequence for both calcium and iron for the first two unit cells.................................................................................................
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