An effective Landau-type model of Hf$_x$Zr$_{1-x}$O$_2$ thin film - graphene nanostructure

TL;DR

This paper develops an effective Landau-Ginzburg-Devonshire model to describe charge-polarization coupling in Hf$_x$Zr$_{1-x}$O$_2$ thin films with graphene, revealing how polarization states influence graphene conductivity and potential device applications.

Contribution

The study introduces a nonlinear, thickness- and composition-dependent Landau model for Hf$_x$Zr$_{1-x}$O$_2$ films, linking polarization behavior to graphene charge modulation.

Findings

Polarization affects graphene conductivity significantly.

Hf$_x$Zr$_{1-x}$O$_2$ polarization states control carrier concentration in graphene.

Model predicts tunable hysteresis and polarization states based on film composition and thickness.

Abstract

To describe the charge-polarization coupling in the nanostructure formed by a thin Hf$_x$Zr$_{1-x}$O$_2$ film with a single-layer graphene as a top electrode, we develop the "effective" Landau-Ginzburg-Devonshire model. This approach is based on the parametrization of the Landau expansion coefficients for the polar (FE) and antipolar (AFE) orderings in thin Hf$_x$Zr$_{1-x}$O$_2$ films from a limited number of polarization-field curves and hysteresis loops. The Landau expansion coefficients are nonlinearly dependent on the film thickness h and Zr/[Hf+Zr] ratio x, in contrast to h-independent and linearly x-dependent expansion coefficients of a classical Landau energy. We explain the dependence of the Landau expansion coefficients by the strong nonmonotonic dependence of the polar properties on the Hf$_x$Zr$_{1-x}$O$_2$ film thickness, grain size and surface energy. The proposed Landau free energy with five "effective" expansion coefficients, which are interpolation functions of x and h, describes the continuous transformation of polarization dependences on applied electric field and hysteresis loop shapes induced by the changes of x and h in the range 0 < x < 1 and 5 nm < h < 35 nm. Using the effective free energy, we demonstrated that the polarization of Hf$_x$Zr$_{1-x}$O$_2$ films influences strongly on the graphene conductivity, and the full correlation between the distribution of polarization and charge carriers in graphene is revealed. In accordance with our modeling, the polarization of the (5 - 25) nm thick Hf$_x$Zr$_{1-x}$O$_2$ films, which are in the ferroelectric-like or antiferroelectric-like states for the chemical compositions 0.35 < x < 0.95, determine the concentration of carriers in graphene and can control its field dependence. The result can be promising for creation of next generation Si-compatible nonvolatile memories and graphene-ferroelectric FETs.