Thin-film interface engineering of low-voltage tunable ferroelectric varactors with oxide electrodes

具有可调谐介电材料(Ba，Sr)TiO_3和金属电极的铁电变容二极管被认为是在千兆赫频率可调谐电子器件中的应用。其高度可调的介电常数，以及耐用性、快速的调谐速度和低功耗，使各种集成微波元件应运而生。在我们上一个项目的工作中，基于低阻的SrMoO_3电极薄膜和(Ba，Sr)TiO_3可调介质的全氧化物薄膜外延铁电变容管的概念已经被证明。这一概念有两个优点，只有使用极薄的薄膜作为可调电介质才能实现：低(电池)电压下的可调性和高频下的操作。为了实现全氧化物外延铁电变容二极管的全部潜能，必须解决涉及的氧化物界面缺陷化学的几个基本科学问题，这些问题是变容二极管功能的关键。本后续项目的目标是在铁电变容二极管中钙钛矿型外延氧化物之间热力学和动力学稳定的界面的原子水平上的工程，以允许在热力学相图中具有不相容稳定区域的功能材料的生长。界面将使用薄膜中间层作为氧扩散屏障和材料生长动力学的微调来稳定。工程多层结构的材料参数(晶体和电子结构、介电常数、化学计量比、形貌)将与变容二极管的电学性能参数如可调谐性、漏电流和微波损耗相关。变容二极管在千兆赫兹频率下的电学表征不仅是一种器件表征方法，同时也是对材料性质的一种高度灵敏的测量，并证明了这种多层结构在模型器件中的适用性。薄膜变容二极管异质结的介电模型允许提取界面的介电参数，而这些介电参数在电学测量中是无法直接获得的。材料科学和电气工程之间的交叉学科方法对该项目至关重要，因为铁电变容二极管的高性能是使用新型高导电氧化物电极材料SrMoO_3的薄膜电极实现的。

Ferroelectric varactors with tunable dielectric (Ba,Sr)TiO3 and metallic electrodes are considered for applications in tunable electric devices at gigahertz frequencies. Their highly tunable permittivity, as well as endurance, fast tuning speed, and low power consumption have innovated various integrated microwave components. In our work on the previous project, the concept of all-oxide thin-film epitaxial ferroelectric varactors has been proven based on thin films of low-resistive SrMoO3 electrodes and (Ba,Sr)TiO3 tunable dielectric. The concept has two advantages that can only be achieved using extremely thin films as tunable dielectric: tunability at low (battery) voltages and operation at high frequencies. Several fundamental scientific questions regarding the defect chemistry of the involved oxide interfaces, which are the key to varactor functionality, must be addressed to be able to realize full potential of the all-oxide epitaxial ferroelectric varactors.This follow-up project aims at an engineering on the atomic level of the thermodynamically and kinetically stable interfaces between epitaxial perovskite oxides in ferroelectric varactors, to allow the growth of functional materials with incompatible stability regions of their thermodynamic phase diagrams. The interfaces will be stabilized using thin-film interlayers as oxygen diffusion barriers and fine adjustments of the materials growth kinetics. The materials parameters of the engineered multilayer structure (crystal and electronic structure, permittivity, stoichiometry, morphology) will be correlated with the varactor electrical performance parameters such as tunability, leakage current and microwave losses. The varactor electric characterization at gigahertz frequencies serves not only as a device characterization method but at the same time as a highly sensitive measure of the materials properties, and demonstrates the applicability of the multilayer structure in a model device. Dielectric modelling of thin-film varactor heterostructures allows extraction of the dielectric parameters of the interfaces, which are not directly accessible in the electric measurements. The interdisciplinary approach between Materials Science and Electrical Engineering is essential for the project as high performance of the ferroelectric varactors is achieved using thin film electrodes of the novel highly conducting oxide electrode material SrMoO3.