A nanoscale continuous transition from the monoclinic to ferroelectric orthorhombic phase inside HfO2 nanocrystals stabilized by HfO2 capping and self-controlled Ge doping

Abstract

Orthorhombic HfO₂ exhibits nanoscale ferroelectricity that opens the perspective of ultra-scalable CMOS integration of ferroelectric memories. However, many aspects of the metastable orthorhombic crystallization mechanisms still need to be elucidated and new fabrication methods are of high interest. In this paper, the atomically resolved crystal structure of HfO₂ is a 3-layer structure with a Ge-rich HfO₂ intermediate layer capped by a top (cap) HfO₂ layer and cladded by a bottom HfO₂ layer. There is a continuity of crystal growth from the top and bottom HfO₂ layers into the intermediate layer. A spatial transition from a monoclinic phase to an orthorhombic phase was revealed within a region of a few atomic layers at the interface between capped and intermediate HfO₂ layers. This result suggests the mechanism of orthorhombic and monoclinic phase formation by a martensitic-like transformation of the initially grown tetragonal phase. The sample fabrication method we used involved magnetron sputtering deposition of the 3-layer structures, i.e. a stack of top HfO₂/Ge-rich HfO₂ intermediate/bottom HfO₂ layers, followed by rapid thermal annealing. It results in self-optimized orthorhombic crystallization of HfO₂ by Ge nanoparticle segregation in the intermediate layer. The ferroelectric effects are revealed by polarization–voltage hysteresis loops and piezoresponse force microscopy measurements. The atomistic computations performed by using the density functional theory support the experimental results by showing that the Ge doping of HfO₂ leads to orthorhombic phase stabilization and increased Berry phase polarization.