Abstract
Homologous series are layered phases that can have a range of stoichiometries depending on an index n. Examples of perovskite-related homologous series include (ABO3)nAO Ruddlesden–Popper phases and (Bi2O2) (An−1BnO3n+1) Aurivillius phases. It is challenging to precisely control n because other members of the homologous series have similar stoichiometry and a phase with the desired n is degenerate in energy with syntactic intergrowths among similar n values; this challenge is amplified as n increases. To improve the ability to synthesize a targeted phase with precise control of the atomic layering, we apply the x-ray diffraction (XRD) approach developed for superlattices of III–V semiconductors to measure minute deviations from the ideal structure so that they can be quantitatively eradicated in subsequent films. We demonstrate the precision of this approach by improving the growth of known Ruddlesden–Popper phases and ultimately, by synthesizing an unprecedented n = 20 Ruddlesden–Popper phase, (ATiO3)20AO where the A-site occupancy is Ba0.6Sr0.4. We demonstrate the generality of this method by applying it to Aurivillius phases and the Bi2Sr2Can–1CunO2n+4 series of high-temperature superconducting phases.
Highlights
The growth of high quality superlattices and homologous series requires precise calibration
The RuO2 layers are on average 10% closer together than in Fig. 1(a), and each layer is populated by 10% less RuO2, resulting in identical stoichiometry
Since this structure was grown with a single target, correction is not as straightforward as shutter-controlled molecular-beam epitaxy (MBE), but it has been demonstrated that the periodicity of Ruddlesden–Popper or Aurivillius-phase films can be controlled with single-target pulsed-laser deposition (PLD) by changing the growth rate,50 or film deposition temperature,86–89 suggesting that calibration is possible, albeit challenging
Summary
The growth of high quality superlattices and homologous series requires precise calibration. When grown by molecular-beam epitaxy (MBE), an ion gauge (beam flux monitor) or quartz-crystal microbalance can be used to get a rough measure of source fluxes, and this approximation can be improved upon by observing reflection high-energy electron diffraction (RHEED) oscillations during codeposition or shuttered deposition on a calibration sample These methods are often insufficient to consistently grow oxide superlattices and homologous series with sharp interfaces because they require fluxes in the right stoichiometric ratio and the delivery of precise doses of these stoichiometric fluxes to build up the desired superlattice layering. This control is most readily achieved by adjusting deposition times, but it has been demonstrated by pulsed-laser deposition (PLD) from a single target.
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