We discuss dopant effects on the ferroelectricity of HfO2. We prepared many kinds of doped HfO2 with both cations (Y, Sc, Al, Si, Ge, Zr, Nb) and anion (N). We found not only dopant-dependent but also dopant-independent contributions to ferroelectric properties. The former one obviously comes from dopant atomic nature such as ionic charge or size, while the latter one is the point we are interested in.
Total polarization PSW as a function of the dopant concentration X, in which X and PSW denote the dopant ratio in total atoms including anion and the absolute value of polarization at V=0, respectively. A small amount of N doping enables to achieve the ferroelectric HfO2, while Zr slowly and Ge moderately help the ferroelectric phase formation of HfO2. More interestingly, the sensitivity curve seems to be similar in shape irrespective of dopant species. This fact suggests that HfO2 ferroelectricity is achieved by HfO2 intrinsic properties. In HfO2 and ZrO2, the structural phase transitions from the tetragonal to monoclinic phases are understandable from the martensitic transition viewpoint. In the doped HfO2 case, it is inferred that the ferroelectric orthorhombic phase may be along this phase transition. In that sense, the dopant may help to increase the kinetic barrier against stabilizing the monoclinic phase irrespective of dopant species.
This work was supported by JST-CREST Grant Numbers JPMJCR14F2, Japan.
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