Full length articleGrowth process and grain boundary defects in Er doped BaTiO3 processed by EB-PVD: A study by XRD, FTIR, SEM and AFM
Graphical abstract
Introduction
Ferroelectric materials with Erbium doped Barium Titanate (BTE) perovskite type structure have attracted considerable attention due to their practical applications as photonic devices and next generation flat panel displays [[1], [2], [3]]. Dielectric, electric and mechanical properties of large-area and high-quality thin films have been intensively studied and significant advances have been made towards practical device applications of thin BTE films [[1], [2], [3], [4]]. Chemical vapor deposition (CVD), pulsed laser deposition (PLD), spin coating, sputtering deposition and electron beam physical vapor deposition (EB-PVD), among other techniques, can yield high quality films on substrates such as Si, Pt, TiO [[5], [6], [7], [8], [9], [10]]. EB-PVD method yields polycrystalline structures with uniform grain size in the order of a few hundred nanometers [[10], [11], [12]]. However, their stability in the grain boundary remains poorly studied. One of the main causes of instability in grain boundary is the chemical defects, since they affect the electric, dielectric, mechanical and chemical properties. These properties are critically affected by the small grain sizes, which potentially reduces the electronic mobility [7,13,14], lowers the thermal conductivity [15] and reduces the ultimate mechanical strength [16]. The grain boundaries are formed during the growth process through simultaneous nucleation of adjacent islands [[15], [16], [17], [18]]. In addition to the grain boundary formation, chemical and point defects can also be observed. However, defects such as grain boundaries in thin films, that are yet poorly exploited, might be controlled with the increase in temperature [16]. Thus, the study of grain boundaries in thin BTE films is important for the characterization of BTE, in order to understand structural and microstructural effects induced by thermal annealing and also for the optimization of the growth conditions.
A number of reports demonstrate methods for characterizing thin films by using transmission electron microscopy, scanning tunneling microscopy or atomic force microscopy (AFM) [4,19,20]. Although these techniques can obtain nanoscale resolution for thin film grain boundaries, they are mostly used to obtain the surface morphology rather than to obtain the distribution of the grain boundaries. In the case of BaTiO3, it was found that the grain growth and increase in roughness decrease with source-drain current [[21], [22], [23]]. However, there are other parameters as well that influence the grain growth and roughness aside from the drain current, like temperature treatments after the deposition of thin film. An interesting point about these defects is the possibility that the density of grain boundary defect decreases with thermal annealing. In previous studies, Foster et al. [24] and Clabel et al. [7], have indicated an increase in defect density after the onset of crystallization.
In this work, the effects of annealing of BTE films are investigated, mainly on the surface morphology and on the grain boundary. A mechanism for the rise of grain boundary defects in thin films is proposed, which is based on the AFM, SEM and FTIR results. Finally, it is shown that these complementary techniques provide valuable information about the surface structure of thin BTE films.
Section snippets
Experimental section
Thin films of BTE were grown on Si(100) and Si(100)-Au, using EB-PVD. Erbium doped Barium Titanate (BTE) target was prepared by the solid-state reaction (SSR), incorporating highly pure starting materials like BaCO3 (99.9%), TiO2 (99%) and Er2O3 (99.5%) [7]. The films were produced in an evaporation chamber at a pressure of 4 × 10−6 mbar, using an electron beam gun (model 231, Telemark), operated at 5 kV and 50 mA. Tantalum crucibles were used in order to withstand high temperature during the
Results and discussion
The X-ray diffraction (XRD) patterns, obtained from the thin films with thickness between 35 to 280 nm, grown on Si(100) and Si(100)-Au substrate before and after the thermal treatment are shown in the Fig. 1. For thin BTE films on Si(100) and on Si(100)-Au, all the diffraction peaks, corresponding to the (111), (220) and (111), (200), (220) crystallographic planes, respectively, can be indexed to the tetragonal structure of BaTiO3, which is in agreement with the Inorganic Crystal Structure
Conclusion
Erbium doped BaTiO3 films were prepared on Si(100) and Si(100)-Au substrates, using EB-PVD as a substrate at room temperature. Effects of thermal treatments on structural and microstructural properties of Erbium doped BaTiO3 thin films grown on Si(100) and Si(100)-Au were investigated. The increase in the crystallite size and decrease in the strain lead to the growth of particle size, for BTE deposited on both substrates, Si(100) and Si(100)-Au. The grain size, roughness and thickness values
Acknowledgments
The authors are thankful for the financial support by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) (project number 158934/2018-0) and by the Centro de Pesquisa em Óptica e Fotônica (CePOF), São Paulo, Brazil (project 2013/07276-1).
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