Phase transitions and structure determination of ferroelectric hafnia-based thin films

The origin of ferroelectricity in hafnia is thought to be an orthorhombic polar (o-) phase stabilized by doping, stress or strain, all facilitated at the nanoscale. Therefore, mostly very thin films (below 10 nm) are available in the polar state, making it extremely challenging to determine the true nature of the polar phase. We recently reported the epitaxial growth of Hf0.5Zr0.5O2 on perovskite substrates under large compressive strain with large ferroelectric polarization. The films are (111) oriented and pole figures show 1:3 multiplicity indicating a rhombohedral (r-) phase. We aim to: (a) elucidate the exact structure of the (r-) phase and determine if the two reported polar phases are truly difelow ferent (b) characterise the high temperature transition recently observed in r-phase films and (c) explore the non-polar-to-polar phase transition, identifying the symmetry of the primary instability.

氧化铪（hafnia）中的铁电性（ferroelectricity）起源通常被认为是通过掺杂（doping）、应力（stress）或应变（strain）稳定得到的正交极性（o-）相（orthorhombic polar (o-) phase），上述稳定过程均在纳米尺度（nanoscale）下得以实现。因此，仅厚度低于10 nm的极薄薄膜可呈现极性态，这使得准确确定该极性相的真实本质极具挑战。我们近期报道了在大压应变（compressive strain）的钙钛矿衬底（perovskite substrates）上外延生长Hf₀.₅Zr₀.₅O₂薄膜的研究，该薄膜展现出较强的铁电极化（ferroelectric polarization）。该薄膜为（111）取向，极图（pole figures）显示出1:3的多重性（multiplicity），表明其晶相为菱方（r-）相（rhombohedral (r-) phase）。本研究旨在达成以下三项目标：(a) 阐明该菱方（r-）相的确切结构，并判定两种已报道的极性相是否确实存在差异；(b) 表征近期在菱方相薄膜中观测到的高温相变（high temperature transition）；(c) 探究非极性到极性的相变（non-polar-to-polar phase transition）过程，确定主要不稳定性（primary instability）的对称性。