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

What is it about?

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 need still to be elucidated and new fabrication methods are of high interest. In this paper, the atomically resolved crystal structure of HfO2 in 3-layer structures of Ge-rich HfO2 intermediate layer capped by top (cap) HfO2 and cladded by bottom HfO2. There is a continuity of crystal growth from top and bottom HfO2 layers into intermediate layer. A spatial transition from monoclinic to orthorhombic phase was revealed within a region of a few atomic layers at the interface between cap and intermediate HfO2 layers. This result suggests the mechanism of orthorhombic and monoclinic phases formation by a martensitic-like transformation of the initially grown tetragonal phase. The samples fabrication method we used consists in 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 a 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.

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