Study On Surface Strueture Of SrTiO₃ And Related Films

A number of intriguing properties emerge upon the formation of the epitaxial interface between the insulating SrTiO3 and related filns. These properties, which include a quasi two-dimensional conducting electron gas, low temperature superconductivity, and magnetism, are not present in the bulk materials. The observation of this interface phenomenon is a display of recent advances in thin film deposition and characterization techniques. In addition, many research groups have been devoted in resolving the fundamental physics of the origin of those transport properties. It is generally accepted that the relation between the structure and the properties of the SrTiO3/LaAlO3 interface can be explained by taking into account the relative contribution of three structural aspects:electronic interface reconstruction, oxygen vacancies, and structural deformations (including cation disorder). The emerging phase diagram is much richer than for related bulk oxides. But the complex interplay between these effects remains unclear. It is also not clear whether the whole phases diagram has yet been identified, or whether some or all of these three mechanisms are adequate to account for the experimental observations. In all, the observation of other interfaces related to SrTiO3 also provides an extension to the range of exceptional electronic properties of complex oxides.In this thesis, we introduce related work about STO based on our experiment result. We also mainly introduce the problems which the first principle can work out on the relationship between its microstructure and related properties. A series of interesting results has been obtained from our study. (1) Surface Structure of Strontium TitanatePerfect and defective surface structures for (001) SrTiO3 are determined in considering a supercell with eleven atomic layers using the first-principle calculations. The amplitude of the surface rumpling for the SrO-terminated surface is much larger than that for TiO2-terminated surface, although both of SrO -and TiO2-terminated surface are stable for a comparable range of the TiO2 chemical potential. The distance between the first and second planes compresses while that of the second and third planes expands due to the relaxation of the slab. The top sites of the oxygen atoms of SrO-termined surface and the fourfold symmetry hollow sites of TiO2-termined surface are favorable for Ti or Sr adsorbate. The relative stability of the defect species or reactions vary with the equilibrium conditions. The dominant surface defect is Ti substitutional defect and a TiO-TiO2 double layers may form at the surface, which has been confirmed by experiments.(2) La2/3Ca1/3MnO3 ultrathin films grow on SrTiO3 SurfaceUltrathin La2/3Ca1/3MnO3 films 3-24nm thick grown on top of (001) SrTiO3 (STO), (001) yttrium stabilized ZrO2 (YSZ) and (001) LaAlO3 (LAO) single crystal substrates have been studied by means of synchrotron X-ray radiation and atomic force microscopy (AFM), allowing us to analyze the biaxial strain effect on the orientation and growth dynamics of films. (OkO) oriented LCMO films are formed on STO and LAO substrates compared to only (001) reflections that is present on YSZ substrate. Rocking curve width for symmetric scan reveals that more inhomogeneous state is produced in LCMO/YSZ system. With increasing thickness, both sets of films undergo dramatic surface evolution:starting with relative flat surface separated by lots of pinholes and grooves, followed by improved crystallite quality but 2D-island morphology in LCMO/STO; from typical layer-by-layer growth to hatched morphology which could be seen as the onset of 3D island structure in LCMO/LAO; in contrast, in the most mismatched LCMO/YSZ system, initially quasi-1D rod feature is dominant and ends with growing island-like structure in thicker films. More importantly, the driving forces for morphology instabilities have been discussed in the paper.