Thin Film Preparation And Characterization Of Perovskite Manganese Oxides With Three Oxidation States

During the last few years, transition metal perovskite oxides have attracted considerable scientific and technological attention for the high temperature superconductive and colossal magnetoresistance. Among these oxides, perovskite manganites of the type R1?xAxMnO3 (R: La, Pr, Nd etc and A: Ca, Sr, Ba, Pb etc) hold special interest. By the partial substitution of alkaline-earth metals for rare earth elements, the manganese ion presents the mixed valence of Mn3+ and Mn4+. The electronic phase diagram of this strongly correlated class of materials is very rich. Manganites, depending on the value of x and temperature, exhibit a variety of ordering and transitions, such as paramagnetic (PM) to ferromagnetic (FM) transition, insulator to metal (IM) transition, charge-orbital ordering (COO), and antiferromagnetic (AFM) ordering, which often result in phase coexistence, i.e., phase separation. The FM double exchange (FM-DE) is invoked to explain the magnetic and transport properties of the manganites in the CMR regime. However, it has now been accepted in general that FM-DE, due to the restricted validity in the vicinity of TC/TIM, is not sufficient to explain the various physical properties of manganites. Consequently, other interactions, which compete with the FM-DE mechanism, such as electron-lattice interaction, spin-spin interaction and spin-lattice interaction, are also known to play a key role.It is well known that ceramic oxides are usually prepared by traditional high temperature solid-state methods. The oxide carbonate nitrate precursors were calcined at the temperature in excess of 1000℃with grinding repeatedly, but for the high temperature the cost was increased and the chemical impurities could be introduced into the ceramic product by milling. Hydrothermal technique has been proved to be a good method to the synthesis of metastable phases and microporous solids. In the past several years, a series of perovskite manganites have been synthesized with hydrothermal method by our group, such as ErMnO3, YMnO3, HoMnO3, DyMnO3 and SmMnO3. By the partial substitution of alkaline-earth metals for rare earth elements, we get the Pr1-xCaxMnO3 and Nd1-xCaxMnO3. In our further studies we obtained a family of perovskite manganese oxides, La1-x-yCaxKyMnO3 (x = 0.74-0.18, y = 0.01-0.14) through the partial substitution of Ca2+ and K+ for La3+ under the condition of strong alkali media. Just for the Ca2+ and K+ co-doped, the Mn ion in LaMnO3 presents the marvelous triolet mixed valence states of Mn3+, Mn4+ and Mn5+.We measured the I-V curves of the single crystal sample by conductive atom force microscopy (C-AFM) (Seiko, SPA 400). The I-V curve of the triplet valence manganites perovskite oxide is the same as that of the molecular rectifier theoretically calculated by Aviram and Ratner at 1973. Thus we asserted that there are atomic scale p-n junctions with Mn3+-Mn4+-Mn5+ type in La1-x-yCaxKyMnO3. The current density of the single crystal could be 105 A/cm2. The grinding sample has shown transient state photovoltaic response. And then all of these properties of La1-x-yCaxKyMnO3 may lead to the potential applications as new atomic-scale devices, such as photoelectric conversion and quantum information memory. But for the limit of hydrothermal synthesis, the size of the single crystal could be 10-50μm, and this went against for the further development and using.The aim of this study is to prepare the La1-x-yCaxKyMnO3 thin films, whose properties should be same to the single crystal sample. Using the advance deposition method, we probe into the synthesis condition and set up the foundation for the further development and effective utilization.Firstly, we prepared the film upwards directly by the familiar hydrothermal method in the Teflon-lined stainless-steel autoclave. The SrTiO3 single crystal substrates were hanged in the reaction solution by the Teflon mould, and then the thin epitaxial films were deposited on the substrate along with the synthesis of the single crystal. The prepared films were attested to be the La1-x-yCaxKyMnO3 epitaxial films by X-ray and SEM. So the hydrothermal on-site method for the La1-x-yCaxKyMnO3 film preparing is easy and exercisable.Secondarily, to avoid re-sputtering (due to oxygen ions generated in the plasma), we adopted the off-axis radio frequency magnetron sputtering to deposit large scale films. To measure the electrical properties of the film individually, gold (99.99%) was thermally evaporated onto the cleanly Indium Tin Oxide Conductive Glass (ITO) as the bottom electrode. We tested that the work function of our sample was 4.9 eV that is much closer to that of the reference gold (5.1 eV), which could ensure the ohmic electrical contact and avoid the Schottky effect. The composition of films was determined by Atom Absorption Spectrometers (PE), Inductively Coupled Plasma (ICP), X-ray Photoelectron Spectrometer (XPS) and Energy Dispersive X-ray Analysis (EDAX). The XRD analysis of the thin films manifested that the deposition results in polycrystalline films for the no oriented gold films grow on the ITO and there are no trace of the parasitic phases except for the peaks of the ITO glass and gold film. In our compound, the modulated phase exhibits the stripe structure with the doubled a-axis and b-axis lattice period, paralleling to c-axis direction, which was observed in both the bulk and the film at room temperature. The fast Fourier transform (FFT) images, represents not only a rectangular pattern of the basic Bragg spots, which are typical for a regular pseudo-cubic crystal lattice, but also the additional superlattice reflections with a wave vector q=a*/2. The current-voltage (I-V) measurements on the thin films by C-AFM display the ideal p-n junctions rectifying characteristics as that of single crystal. So we can say the two-dimensional thin films whose properties are similar to the single crystal sample have been prepared successfully.Thirdly, the La1-x-yCaxKyMnO3 epitaxial films were deposited on SrTiO3, LaAlO3 and 0.7 wt % Nb–SrTiO3 by the off-axis radio frequency magnetron sputtering. It is attested that the films have the same epitaxy with the single crystal substrates by x-ray diffraction (Rigaku 18 kW X-ray Diffractometer). The surface morphology was tested by AFM and the undulation is not beyond 10nm. The current densities in xy plane of the La1-x-yCaxKyMnO3 on 0.7 wt % Nb–SrTiO3 are similar to that of the single crystal.