Growth behaviors of epitaxial barium molybdate (BaMoO₃, BaMoO₄) film

Abstract

Transition metal oxides have been a research hotspot for basic scientific research and frontier applications. Owing to the presence of d<italic/> electrons and strong electron correlation, a wealth of physical phenomena emerges in the transition metal oxide family. In particular, extremely fruitful research progress is achieved in a 3d orbital elemental system. In comparison, the 4d transition metal oxides need more attention. Molybdate has excellent optical and electrical properties. Among <i>A</i>MoO₃ (<i>A</i> = Ca, Sr, Ba), only BaMoO₃ has not been reported for epitaxial films to date. In this work, high-quality epitaxial films of BaMoO₃ and BaMoO₄ are prepared by using the pulsed laser deposition. We conduct the oxygen partial pressure modulation experiments and the results show that the growth of BaMoO₃ is sensitive to oxygen partial pressure. Also, BaMoO₃ has a geometrically similar lattice structure to BaMoO₄, and there exists epitaxial competition between BaMoO₃ and BaMoO₄. These two points make the preparation of epitaxial BaMoO₃ films more challenging. The key to the preparation of epitaxial BaMoO₃ thin films is the reduced laser target material, high vacuum environment, and ultra-low oxygen partial pressure. The epitaxy competition can be avoided by using the SrTiO₃ (111) substrate. We conduct oxygen partial pressure modulation experiments on a narrow scale and reveal a self-assembled superlattice of epitaxial BaMoO₃ film on a SrTiO₃(111) substrate. Both the satellite peaks in the XRD pattern and the HRTEM results indicate the superlattice period of about 7.04 Å. The oxygen partial pressure is the only parameter that regulates this phenomenon, so we presume that the essence of the self-assembled superlattice is periodic oxygen-induced lattice defects. Finally, electrical transport characterization experiments are conducted on representative BaMoO₃ films. The <inline-formula><tex-math id="M1">\begin{document}$\rho \text{-} T$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220736_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220736_M1.png"/></alternatives></inline-formula> curve measurements and fitting results show that the epitaxial BaMoO₃ films on SrTiO₃(001) substrates have better conductivities. The electrical transport properties of BaMoO₃ films grown on SrTiO₃(111) substrates are dominated by electron-phonon scattering, and BaMoO₃ films grown on SrTiO₃(001) substrate have stronger electron-electron scattering interactions. The resistivity of the self-assembled superlattice BaMoO₃ films is relatively high and electron-electron scattering plays an important role in determining the electrical transport property.