Advanced Functional Materials

Cover image for Vol. 25 Issue 44

16_04b/2006Inside Front Cover: Perfect Bi4Ti3O12 Single-Crystal Films via Flux-Mediated Epitaxy (Adv. Funct. Mater. 4/2006)

Flux-mediated epitaxy has been developed for ferroelectric Bi4Ti3O12 single-crystal film growth, as shown on the inside cover. The key point is the selection of an appropriate flux material. A combinatorial high-throughput screening technique reported by Matsumoto and co-workers on p. 485 has led to the successful discovery of the novel flux composition, Bi–Cu–O, for Bi4Ti3O12 single-crystal film growth. This flux-mediated epitaxy is not limited to oxide epitaxy, but is also widely applicable to various promising materials for the realization of non-Si-based electronics, such as nitrides, carbides, and halides.

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.

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