Abstract
Segregation is a crucial phenomenon, which has to be considered in functional material design. Segregation processes in perovskite oxides have been the subject of ongoing scientific interest, since they can lead to a modification of properties and a loss of functionality. Many studies in oxide thin films have focused on segregation toward the surface using a variety of surface-sensitive analysis techniques. In contrast, here we report a Ca segregation toward an in-plane compressively strained heterostructure interface in a Ca- and Mn-codoped bismuth ferrite film. We are using advanced transmission electron microscopy techniques, X-ray photoelectron spectroscopy, and density functional theory (DFT) calculations. Ca segregation is found to trigger atomic and electronic structure changes at the interface. This includes the reduction of the interface strain according to the Ca concentration gradient, interplanar spacing variations, and oxygen vacancies at the interface. The experimental results are supported by DFT calculations, which explore two segregation scenarios, i.e., one without oxygen vacancies and Fe oxidation from 3+ to 4+ and one with vacancies for charge compensation. Comparison with electron energy loss spectroscopy (EELS) measurements confirms the second segregation scenario with vacancy formation. The findings contribute to the understanding of segregation and indicate promising effects of a Ca-rich buffer layer in this heterostructure system.
Highlights
Perovskite materials have a structure of 2 or more cations and oxygen in the arrangement ABO3 and provide a huge variety of physical phenomena and functional properties.[1]
We demonstrate the segregation of the Ca dopant in a bismuth ferrite thin film at the interface with a strontium titanate substrate, which was formed via diffusion during and shortly after the thin-film deposition, while still at high deposition temperatures of 700 °C, using various transmission electron microscopy (TEM) techniques as well as density functional theory (DFT) calculations
The interface structure and composition were investigated in the [010]c zone axis orientation regarding the cubic STO substrate, which corresponds to the [010]pc axis of the bismuth ferrite thin films
Summary
Perovskite materials have a structure of 2 or more cations and oxygen in the arrangement ABO3 and provide a huge variety of physical phenomena and functional properties.[1]. Mn doping on the B-site has been shown to restore the weak ferromagnetic behavior received through Ca doping to the antiferromagnetic behavior of pure BiFeO3.7 For such doping-related phenomena, segregation processes of cations are a critical aspect since they can lead to strong agglomeration at surfaces, grain boundaries, or interfaces and a depletion of the bulk. They have a significant influence on the behavior and function.[8] a deeper understanding of the processes underlying the segregation process is required to prevent property degradation. This is the first observation of A-site dopant diffusion toward interfaces in BiFeO3, and the understanding of this process is crucial to prevent device failure in future applications
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