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Stoichiometric control of oxide thin films

In perovskites (ABO3) and its oxygen deficient brownmillerite phase (ABO2.5), a large freedom in composition allows various ferroic orders to exist, such as ferromagnetism and ferroelectricity. To take advantage of the tunability of physical properties in perovskites, Molecular Beam Epitaxy (MBE) is an excellent thin film growth method for such systems, as each element is evaporated separately, allowing for a precise control of elemental ratios in the film. Reflection High Energy Electron Diffraction (RHEED) enables determination of the stoichiometry for the A and B sites in SrCoO2.5 (see figure).

Alongside the reflexes from the SrCoO2.5 brownmillerite phase, Co excess causes small “speckles” arising from transmission of the electron beam through CoO islands, whereas Co deficiency leads to amorphous films. Thus, the stoichiometry of SrCoO2.5 thin films can be determined in-situ to a high degree of precision.

RHEED patterns of SrCoO2.5 thin films RHEED patterns of SrCoO2.5 thin films grown on SrTiO3(001) with a) Co/Sr ratio of 1:1 and b) 12 % Co excess. Corresponding AFM measurements of same samples with c) stoichiometric and 12% Co excess e) and d) comparison of the corresponding XRD peaks. RHEED at 15keV. [1]
Copyright: Forschungszentrum Jülich

References:

  • P. Schöffmann, S. Pütter, J. Schubert, W. Zander, J. Barthel, P. Zakalek, M. Waschk, R. Heller, and T. Brückel, Tuning the Co/Sr stoichiometry of SrCoO2.5 thin films by RHEED assisted MBEgrowth, Mater. Res. Express 7, 116404 (2020)