Control of nonlinear dielectric property of oxide artificial superlattices by the lattice strain for application to devices

本研究旨在借由晶格应变来控制钙钛矿型人工超晶格的介电性质，以应用于设计上。在以前的研究中发现，具有10个钙钛矿电池的堆叠周期的人工超晶格显示出极高的介电常数，但其温度依赖性尚不清楚，尽管对于实际器件的应用具有重要意义。在这项研究中，一种技术来测量微波区的超薄膜的介电常数的温度依赖性，并将其应用于由分子束外延方法制成的人工超晶格。发现人工超晶格的高介电常数随温度的变化是稳定的，这对人工超晶格的应用具有重要意义。用常规的RIP溅射法制备了人工超晶格，并测量了其介电性能。结果表明，BaTiO_3/SrTiO_3的超晶格结构虽然在X射线衍射谱中没有观察到超衍射，但仍表现出介电常数的提高。研究了电极效应对BaTiO_3-SrTiO_3薄膜非线性介电性能的影响。采用RIP溅射法在SrTiO_3单晶衬底上制备了SrRuO_3底电极。在薄膜上沉积In、Au、Ag和Pt的顶电极，并测量介电性能。发现介电常数明显依赖于电极金属，并利用肖特基势垒理论对结果进行了分析。

This study was aiming at controlling dielectric properties of perovskite type artificial superlattice by the strain induced in the crystal lattice for applications to devises. It was revealed in the previous studies that the artificial superlattices with the stacking period of 10 perovskite cells showed an extremely high dielectric permittivity, but its temperature dependence was not known in spite of the importance for the applications for practical devices. In this study, a technique to measure temperature dependence of the dielectric permittivity of ultra thin films in microwave region was developed first and it was applied to the artificial superlattices made by a molecular beam epitaxy method. It was found the high dielectric permittivity of artificial superlattice was stable with the change of temperature, which was an important result for the application of artificial superlattices. The artificial superlattices were formed by a conventional RIP sputtering and their dielectric properties ware measured. It was found from the measurements that the superlattices of BaTiO3/SrTiO3 indicated the enhancement of permittivity even thought the super diffractions could not be observed in the XRD profiles. The electrode effects on the nonlinear dielectric properties of BaTiO_3-SrTiO_3 films were investigated. The films were deposited on SrRuO_3 bottom electrodes formed on single crystal SrTiO_3 substrate by RIP sputtering process. Top electrodes of In, Au, Ag and Pt were deposited on the films and the dielectric properties were measured. it was found that the dielectric permittivity markedly depended on the electrode metals and the results were analyzed using the theory of Schottky barriers.