Perfect Bi₄Ti₃O₁₂ Single‐Crystal Films via Flux‐Mediated Epitaxy

Abstract

Excellent crystallinity of material films and atomic control of their surface/interface, sufficient for the realization of their optimal physical properties, are technological premises for modern functional‐device applications. Bi4Ti3O12 and related compounds attract much interest as highly insulating, ferroelectric materials for use in ferroelectric random‐access memories. However, it has been difficult thus far for Bi4Ti3O12 films to satisfy such requirements when formed using vapor‐phase epitaxy, owing to the high volatility of Bi in a vacuum. Here, we demonstrate that flux‐mediated epitaxy is one of the most promising and widely applicable concepts to overcome this inevitable problem. The key point of this process is the appropriate selection of a multi‐component flux system. A combinatorial approach has led to the successful discovery of the novel flux composition of Bi–Cu–O for Bi4Ti3O12 single‐crystal film growth. The perfect single‐crystal nature of the stoichiometric Bi4Ti3O12 film formed has been verified through its giant grain size and electric properties, equivalent to those of bulk single crystals. This demonstration has broad implications, opening up the possibility of preparing stoichiometric single‐crystal oxide films via vapor‐phase epitaxy, even if volatile constituents are required.

Enhanced dielectric properties in single crystal-like BiFeO3 thin films grown by flux-mediated epitaxy

“…In the FME method, an oxide flux on the surface of the film plays a key role in improving the crystallinity of complex oxide films. 13 The vapor ablated from the target material by a pulsed laser meets the flux on the surface of the substrate, diffuses into the liquid intermediate phase and finally solidifies on a predeposited seed layer. The liquid phase ͑flux͒ prevents direct deposition of the vapor phase on the solid surface and improves crystallinity.…”

“…Compared to pulsed laser deposition ͑PLD͒, in the FME method, there is enhanced diffusion of the surface atoms in the flux above the quasimelting temperature ͑ϳ0.5T m ͒, which leads to film growth in equilibrium even at high growth rates. [13][14][15] In this work, BFO thin films were grown on SrRuO 3 ͑SRO͒ buffered ͑001͒ oriented SrTiO 3 ͑STO͒ substrates by standard PLD and by FME for comparison. We ablated a 10% Bi excess Bi 1.1 FeO 3 target with a KrF excimer laser ͑ = 248 nm͒ at 650°C and 20 mTorr oxygen partial pressure.…”

Flux-mediated epitaxy for ferroelectric Bi4Ti3O12 single crystal film growth

“…2(d). TEM results verified that this film had many out-of-phase boundaries [5]. Moreover, the secondary ion mass spectroscopy (SIMS) measurements showed that a high deficiency of Bi was observed, resulting in high leakage current densities, in the range of 10 −3 -10 −4 A/cm 2 .…”

“…One of the possible solutions is to replace the standard thin film fabrication process of these materials for a process using the flux materials as in the bulk fabrication process of single crystals. We have developed a method and named it as "flux-mediated epitaxy" [3][4][5]. This flux-mediated epitaxy is characterized by the dissolution of pulsed-laser ablated species into a liquid flux layer placed on the film/substrate surface and the crystallization of supersaturated species into the solid film under a quasi-equilibrium state between the growing film and the liquid flux.…”

In situ analysis of the thermal behavior in the Zr-based multi-component metallic thin film by pulsed laser deposition combined with UHV-laser microscope system

“…PLD is one of the well-known vapor phase deposition processes for fabricating ceramics and metallic thin films. We have enabled to fabricate multi-component metallic and ceramics thin films just by ablating a stoichiometric target placed in vacuum chamber with KrF excimer laser [9]. The prepared thin film sample can be trans- ferred to the LM system without exposing it to air so that serious oxidation of metallic thin films is evitable.…”