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With the development of microelectronics technology, hafnium oxide (HfO2) has become the research focus of new ferroelectric materials because of its compatibility with Si-based semiconductor technology, suitable relative dielectric constant, good thermal and chemical stabilities, and a large band gap. HfO2 is a typical material that “phase structure determines properties, and properties determine applications.” The ferroelectric property is confirmed by the fact that the phase structure of thin films is stable in a noncentrosymmetric Pca21 orthorhombic phase. Therefore, the prerequisite for stabilizing and improving the ferroelectric properties of HfO2 thin films is to regulate HfO2 in the metastable orthorhombic phase. In general, HfO2 films can be composed of multiple phases, such as the monoclinic, orthorhombic, and tetragonal (or cubic) phases. Among these, the tetragonal phase is the parent phase of the orthogonal and monoclinic phases, and there is no possibility of phase transformation between the orthogonal and monoclinic phases. Therefore, a consensus is reached in the actual phase structure regulation of HfO2 thin films, that is, inhibiting the monoclinic phase formation can also improve the ferroelectric properties of thin films. Considering the orthogonal phase regulation mechanism, the stability factors of the orthogonal phase in HfO2 thin films are reviewed and summarized based on aspects such as film thickness, doping elements, grain orientation, annealing process, and electrode materials. For instance, the orthogonal phase content of HfO2 thin films decreases with the increase of film thickness; appropriate content of element doping can stabilize the orthogonal phase of HfO2 thin films. Both the high heating rate and short annealing time during the heat treatment are important factors in ensuring the orthogonal stability of HfO2 films. Especially, strain also affects the phase structure regulation. First, top electrode clamping can stabilize thin HfO2 films in the orthogonal phase by applying force on the films. Second, thin HfO2 films with a single orientation can be obtained through epitaxial growth, and it can be well observed how the strain regulates the thin film phase structure; that is, applying the tensile strain to the thin film is an effective means to stabilize the orthogonal phase. Simultaneously, with successful HfO2 film preparation with several atomic layers, it is revealed that the rhombohedral phase has out-of-plane spontaneous polarization and ferroelectric properties with compressive strain generation. Finally, the future prospects of thin HfO2 film development are discussed in this paper. |
