Achieving the large remanent polarization of top heavily doped Al:HfO2 nanofilms embedded with Al-rich interlayers and revealing the underlying phase transition mechanism from atomic structure modelling

Abstract

Employing the plasma enhanced atomic layer deposition method, the top heavily doped Al:HfO₂ nanofilms embedded with Al-rich interlayers are fabricated with a thickness that varies between 12.8 nm and 13.6 nm, and a nominal dopant concentration of 7.7 mol%. The phase compositions and microstructures of Al:HfO₂ nanofilms are characterized by grazing incidence X-ray diffraction, transmission electron microscope and Time-of-Flight secondary ion mass spectrometry. The ferroelectric properties of top heavily doped Al:HfO₂ nanofilms are optimized by varying the annealing temperature and the distribution of Al-rich strips in HfO₂ matrix. The largest remanent polarization is found to be 60.68 μC/cm² (51.52 μC/cm² corrected by positive up-negative down test) for Si-11123 Al:HfO₂ nanofilm at the optimized annealing temperature of 750℃, and which is comparable to those of ferroelectric HfO₂ films prepared using epitaxial growth method. The large-scale density functional theory (DFT) calculation on a supercell model containing 2592 atoms for a 12 nm-thick Al:HfO₂ nanofilm elucidates that the top heavily doped interlayer mimics the role of capping layer that produces lattice distortions normal to film surface. Meanwhile, other Al-rich strips could create the shearing like atomic distortions in the lateral directions of the nanofilm. A synergistic interplay between those two types of mechanical confinement leads to the prominent ferroelectric polarization in top heavily doped Al:HfO₂ nanofilm. Additional ab-initio molecular dynamics simulations and dipole moment calculations with Berry phase method directly confirm the formation of ferroelectric o-phase in the mechanically confined region in both Al:HfO₂-(001) and Al:HfO₂-(101) nanofilms, and a low phase transition kinetic energy barrier height (∼7.0 kJ/mol) between t- and polar o-phase is predicted. It is also revealed that the resulting dipole moment in Al:HfO₂-(101) nanofilm could exhibit a titled alignment with respect to the surface normal, giving the detectable ferroelectric polarization in experiment.