Elsevier

Applied Surface Science

Volume 493, 1 November 2019, Pages 982-993
Applied Surface Science

Full length article
Growth process and grain boundary defects in Er doped BaTiO3 processed by EB-PVD: A study by XRD, FTIR, SEM and AFM

https://doi.org/10.1016/j.apsusc.2019.07.003Get rights and content

Highlights

  • Er:BaTiO3 thin films were successfully deposited by EB-PVD at room temperature.

  • XRD of Er:BaTiO3 thin films exhibited tetragonal structure.

  • FTIR revealed presence of adsorption of hydroxyl group on the surface of thin films.

  • Grain size and roughness (RMS) values of Er:BaTiO3 thin films on Si(100) and Si(100)-Au depend on annealing temperature.

  • Use of Si probes provides AFM contrast on the surface of grain and cavities formed between the grain boundaries.

Abstract

The study of density of grain boundary defects and grain shape was conducted for Erbium doped Barium Titanate (BTE) thin films, which were deposited on Si(100) and Si(100)-Au substrates by electron beam physical vapor deposition (EB-PVD). The crystallite as well as grain size show strong dependence on thermal annealing. Analysis of surface morphologies revealed variations of growth, after thermal annealing process, in films deposited on Si(100) and Si(100)-Au. In films deposited on Si(100) and Si(100)-Au, the root mean square (RMS) roughness value increased from 1.95 to 6.33 nm and 1.73 to 12.9 nm, respectively. These values correspond to 3.2% and 3.7% of the deposited thickness, which is quite low in comparison to the film thickness. Diameters of cavities are observed more significant in films deposited on Si(100)-Au, which increased from 35 to 250 nm. Moreover, the evolution of growth also revealed changes in the surface morphology via surface energy analysis. The results also revealed that chemisorption and physisorption by grain boundary in Si(100)-Au substrate, makes the cavities hydrophilic. These observations are fundamentally important for understating the mechanism of formations of hydrophilic surfaces and changes in their properties owing to the interdiffusion and diffusion effects by the grain boundary. Furthermore, shape of the grains on the surface and the effects of density of grain boundary defects are discussed.

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 × 106 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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