Study on Ca Segregation toward an Epitaxial Interface between Bismuth Ferrite and Strontium Titanate

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.

Ca Solubility in a BiFeO3-Based System with a Secondary Bi2O3 Phase on a Nanoscale

“…The formation of O vacancies has a lower expected Ca ratio but can be considered a more likely case due to our previous results. 61 Nonetheless, at the growth temperature of the film of 700 °C, according to the DFT simulations about 50 times more Ca is expected in BFO than in BO. With increasing temperature, the ratio becomes in general smaller.…”

“…The film structure was investigated in the [010] c cross-sectional orientation concerning the cubic STO substrate, which also complies with the [010] pc (pseudocubic) zone axis of bismuth ferrite. 61 The HAADF image in Figure 1 a shows a cross-section of the Ca- and Co-doped BiFeO 3 film (BCFCO) and the interface with the STO substrate in the bottom part. In the film, approximately 5 nm away from the interface, white stripes with a width of approximately one unit cell appear running in the [100] direction.…”

“…A more detailed explanation of these mechanisms can be found elsewhere. 46 , 61 The relative enthalpy of solution for the Fe-oxidation mechanism was calculated as Here, E (BFO,Ca) and E (BO,Ca), are the total energies of the respective supercell containing Ca atoms, while E (BFO) and E (BO) are the energies for the supercells without Ca. The relative enthalpy of solution for the case of oxygen vacancy (V O ) compensation was defined similarly Here, E (BFO, 2Ca,V O ) and E (BO, 2Ca,V O ), are the total energies of the respective supercell containing two Ca atoms and one V O .…”

Negatively Charged In-Plane and Out-Of-Plane Domain Walls with Oxygen-Vacancy Agglomerations in a Ca-Doped Bismuth-Ferrite Thin Film

“…These received cells are utilized to determine the absolute intensity from the unfiltered image by integrating over a defined area corresponding to each atomic column. Columns at the edge of the image that lie not entirely in it are excluded. , The integrated column intensity is a robust measurement, as it is insensitive to the probe shape and the defocus. , The second MatLab script was written to evaluate (i) the fitted atomic-column positions from the first script according to the spacings between the atomic sites, (ii) the polarization due to shifts of the central Fe atom, and (iii) to visualize the data. The EELS spectra were processed using the built-in functions of digital micrograph and additionally filtered in OriginPro 2016 using a Savitzky–Golay filter with a window of nine data points for denoising.…”

“…Ca doping in BiFeO 3 can induce oxygen vacancies because the replacement of Bi 3+ with Ca, which is an alkaline earth metal and therefore cannot have a higher oxidation state than 2+, has a hole doping effect. Stoichiometrically Ca doping in Bi 1– x Ca x FeO 3−δ would lead to δ = x /2 oxygen vacancies. − It has been reported that for doping ratios of x ≥ 0.2, the oxygen vacancies arrange in ordered superstructures. ,− Thereby, Ca is not uniformly distributed but can segregate to the energetically most favorable position. ,, When relatively thick films are under a compressive (tensile) strain, these superstructures are expected to be arranged parallel (perpendicular) to the interface, as shown in a study for LaCoO 3– x films . For thin, tensile-strained films where the surface energy becomes dominant compared to the bulk energy, the effect is seen, in order to minimize the surface energy, the perpendicular arranged oxygen defects become parallel .…”