Phase stability and ferroelectricity in HfO₂–Y₂O₃ nanolaminate films

Abstract

HfO₂-based ferroelectrics have attracted much attention as key materials for silicon process compatible ferroelectric memories. The ferroelectric phase of HfO₂-based materials is a metastable orthorhombic phase whose potential for stabilization by chemical doping with various elements has been investigated. Recently, it was reported that a nanolaminate film composed of HfO₂ and ZrO₂ films have a ferroelectric phase. However, the stabilization mechanism of the ferroelectric phase is not completely understood. In this study, HfO₂–Y₂O₃ nanolaminate films were fabricated by pulsed laser deposition (PLD) and investigated the ferroelectricity of HfO₂–Y₂O₃ nanolaminate thin films. HfO₂–Y₂O₃ nanolaminate films, where Y₂O₃ is a paraelectric material, provide greater internal stress compared to HfO₂–ZrO₂ nanolaminate films. The crystal structure of HfO₂–Y₂O₃ nanolaminate films was evaluated by X-ray diffraction (XRD) and high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) measurements. The results of the HAADF-STEM observations revealed that the orthorhombic phase is stabilized by applying internal stress into HfO₂ in the nanolaminate film with Y₂O₃. The ferroelectricity of HfO₂–Y₂O₃ nanolaminate films was elucidated by carrying out piezoelectric response microscopy (PFM). The coercive electric field value shows a dependence on the thickness of each layer. We demonstrated the importance of the stress engineering in HfO₂ ferroelectricity, as it serves not only to stabilize the ferroelectric properties but also to regulate the coercive field.