Atomic scale confirmation of ferroelectric polarization inversion in wurtzite-type AlScN
Niklas Wolff, Simon Fichtner, Benedikt Haas, Md Redwanul Islam, Florian Niekiel, Maximilian Kessel, O. Ambacher, Christoph T. Koch, Bernhard Wagner, Fabian Lofink, Lorenz Kienle
Abstract
This work presents the first atomic scale evidence for ferroelectric polarization inversion on the unit cell level in a wurtzite-type material based on epitaxial Al0.75Sc0.25N thin films. The electric field induced formation of Al-polar inversion domains in the originally N-polar film is unambiguously determined by atomic resolution imaging using aberration-corrected scanning transmission electron microscopy (STEM). Anisotropic etching supports STEM results confirming a complete and homogenous polarization inversion at the film surface for the switched regions and the virtual absence of previous inversion domains in as-deposited regions. Local evidence of residual N-polar domains at the bottom electrode interface is observed and can be explained by both stress gradients and electric field deflection. The epitaxial relationship of the sapphire/AlN/Mo/AlScN multilayer stack is discussed in detail. Selected-area electron diffraction experiments and XRD pole figures reveal a Pitsch–Schrader type orientation relation between the Mo electrode and the AlScN film.