Ferroelectricity Induced by Oxygen Vacancies in Rhombohedral ZrO₂ Thin Films

Rhombohedral phase Hf_xZr_1‐xO₂ (HZO, x from 0 to 1) films are promising for achieving robust ferroelectric polarization without the need for an initial wake‐up pre‐cycling, as is normally the case for the more commonly studied orthorhombic phase. However, a large spontaneous polarization observed in rhombohedral films is not fully understood, and there are also large discrepancies between experimental and theoretical predictions. In this work, in rhombohedral ZrO₂ thin films, we show that oxygen vacancies are not only a key factor for stabilizing the phase, but they are also a source of ferroelectric polarization in the films. This is shown experimentally through the investigation of the structural properties, chemical composition and the ferroelectric properties of the films before and after an annealing at moderate temperature (400 °C) in an oxygen environment to reduce the V_O concentration compared. The experimental work is supported by density functional theory (DFT) calculations which show that the rhombohedral phase is the most stable one in highly oxygen defective ZrO₂ films. The DFT calculations also show that V_O contribute to the ferroelectric polarization. Our findings reveal the importance of V_O for stabilizing rhombohedral ZrO₂ thin films with superior ferroelectric properties.