Ferroelectricity in a quasiamorphous ultrathin BaTiO₃ film

Until now, the quasiamorphous (QA) phase in BaTiO₃ (BTO), SrTiO₃ (STO), and BaZrO₃ was achieved by pulling a thick film through a steep temperature gradient. Here, we show that a room-temperature deposited ultrathin film, subsequently annealed in O₂ can also produce a QA phase. The atomic, electronic, and ferroelectric (FE) structure of a QA, ultrathin BTO grown on STO were studied by x-ray diffraction (XRD), x-ray photoelectron diffraction (XPD), x-ray photoelectron spectroscopy (XPS), and piezoforce microscopy (PFM). The absence of long-range order is confirmed by in- and out-of-plane XRD as well as Ti 2p XPD. FE polarized domains with good retention have been successfully written into the QA film and exhibit a clear P-E hysteresis loop. Substrate clamping frustrates volume expansion during annealing leading to a QA film. Photoelectron spectroscopy confirms a similar overall electronic structure as for thicker films but with some significant differences. Simple charge-transfer arguments are not sufficient to explain the high-resolution core-level spectra. Ba, Ti, and O all show components associated with a surface region. We suggest that the observation of such a component in the Ti 2p spectrum is linked with the high dynamic charge tensor induced by the large off-center displacement of the Ti ion.