Flux-mediated epitaxy for ferroelectric Bi₄Ti₃O₁₂ single crystal film growth

Abstract

We propose the “Flux-mediated epitaxy” as a novel concept for the growth of single crystalline films of incongruent, volatile, and high-temperature-melting compounds. In flux-mediated eptitaxy, by supplying materials precursors from the gas phase through the liquid flux films pre-deposited on the substrate, a quasi-thermodynamic equilibrium condition is obtained at the interface between the growing films and the flux films. This process has been demonstrated in this paper by fabricating ferroelectric Bi₄Ti₃O₁₂ films, which has volatile Bi oxide.

The most important step in this process is the selection of the right flux material, which is hard to predict due to the lack of an appropriate phase diagram. In order to overcome this problem, we have selected the combinatorial approach. A series of ternary flux libraries composed of two self-fluxes (Bi₂O₃ and Bi₄Ti₃O₁₂) and a third impurity flux were fabricated on SrTiO₃ (001) substrates. After that, stoichiometric Bi₄Ti₃O₁₂ films were grown on each of these flux libraries at a temperature presumed to melt the flux. High-throughput characterization with the concurrent X-ray diffraction method resulted in the identification of CuO containing Bi₂O₃ as the flux material for the growth of single crystalline Bi₄Ti₃O₁₂ films. Stoichiometric Bi₄Ti₃O₁₂ films fabricated by using a novel CuO containing Bi₂O₃ are qualified to be single crystalline judging from their large grain size and the electrical properties equivalent to bulk single crystal’s.