The HfO2 ferroelectric–metal heterojunction and its emergent electrostatic potential: comparison with ZrO2 and SiO2

Abstract

Transistors have been protagonists in the electronic device world since 1948. As miniaturization occurred, new materials, architectures, and fabrication strategies advanced. However, the choice of materials relative to getting the best chance to minimize Boltzmann's tyranny does not yet rely on predicting how the materials work together in heterojunctions. Herein, we show how conductors, Al and Cu, and insulators, ZrO₂ and HfO₂, in a 2D horizontal contact cell, such as Cu/HfO₂/Al, align their surface potentials and, consequently, their chemical potentials besides their electrochemical potentials or Fermi levels, either at the interface or at the individual surfaces away from the interface, depending on the impedance at the interface. The materials show that they are connected and responsive as a system within a cm range. HfO₂ may behave as a ferroelectric at nanoparticle sizes or when doped with Zr⁴⁺ in HfO₂–ZrO₂ mixtures. Herein, we show that the μm-sized loose particles of HfO₂ with their stable crystalline structure can equalize their surface potentials and, consequently, their chemical potentials with the metals’ counterparts at the heterojunctions, at OCV, or in a closed circuit with a 1 kΩ resistor load, which has only been demonstrated before with ferroionics and ferroelectric glasses. The ability to propagate surface plasmon polaritons (SPPs) at THz-frequencies was also observed, superimposing the equalization of the surface potentials along the materials’ interfacial cross-sections. The μm-sized HfO₂ shows a high capacity for polarizing, increasing its dielectric constant to >10⁵, while characterized in Cu/HfO₂/Al and Cu/HfO₂/Cu cells by scanning Kelvin probe (SKP) with the probe at different heights, cyclic voltammetry (CV or I–V), and electrical impedance spectroscopy (EIS). Using ab initio simulations, the optimized crystalline structure and electrical, electrostatic, and thermal properties of HfO₂ were determined: electron localization function (ELF), band structure, Fermi surface, thermal conductivity, and chemical potential vs. the number of charge carriers. We highlight that emergent ferroelectric and topologic plasmonic transport was distinctly observed for HfO₂ in a horizontal-like cell containing two metal/HfO₂ heterojunctions without electromagnetic pump application.