Phase stability and ferroelectricity in HfO2 - Y2O3 nanolaminate films

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.