Preparation of Functional Oxide Perovskites: A Study on the Growth-Microstructure-Property Relationships of(N a,K)NbOx Thin Films

Abstract: Sodium-potassium niobate is a material with ferroelectric and piezoelectric properties that has been known since 1954. Recently, perovskite structured ferroelectric thin films have gained a lot of attention due to their wide area of potential applications, including non-volatile memories, infrared sensors, optical switches, and ultrasonic transducers. Perovskite-structured thin films of (Na,K)NbOx (hereafter NKN) show high piezoelectric and dielectric constants, low losses (tan δ), and tunable properties, which makes them particularly interesting for device applications.The possibility of fabricating high electrical performance NKN films at low growth temperatures is of high technological importance. In this thesis, we investigated the feasibility of using reactive rf magnetron sputtering to obtain high-quality dielectric NKN films at low growth temperatures.Specifically, the aims of this work were (1) to obtain a better understanding of the relation between the process conditions and the resulting chemical compositions and microstructures of NKN films, (2) to investigate how their physical properties depend on the composition and the microstructure, and (3) to optimize the synthesis for obtaining films with improved electrical and mechanical properties.Our results can be summarized as follows: High oxygen partial pressures avoid oxygen deficient films. On the other hand, energetic oxygen ions or neutrals cause an etching of the film surface, changing the metal stoichiometry of the films. Changing the substrate temperature has no effect on the film stoichiometry in the temperature range investigated here. However, increasing the substrate temperature results in larger grain sizes and the precipitation of an additional crystal phase. Substituting Si/SiO2 by Pt80Ir20 substrates allows to obtain crystalline NKN films already at temperatures as low as 300°C.Furthermore, it was found that the mechanical and electrical performance of the films area strong function of their microstructure and chemical composition: The mechanical properties such as the Young's modulus and the hardness improve by improving the crystalline quality of the NKN films, and the highest dielectric constants are obtained for stoichiometric films.

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