Atomic scale confirmation of ferroelectric polarization inversion in wurtzite-type AlScN

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.

From Fully Strained to Relaxed: Epitaxial Ferroelectric Al1‐xScxN for III‐N Technology

“…Clear current peaks during positive and negative voltage sweeps are visible, meaning there are M-polar as well as N-polar regions inside the same, asdeposited film-unlike in the exclusively N-polar films deposited by similar processes on metal electrodes. [24] Measurements of the piezoelectric coefficient d 33,f on asdeposited pads in comparison to d 33,f measured on pads with prior switching to full N-polarity by applying a field above the positive coercive field confirm the as-deposited multipolar state: The as-deposited d 33,f for a 200 nm thick Al 0.72 Sc 0.28 N capacitor is 19.4% smaller than after switching the same capacitor to full N-polarity. Furthermore, by looking at the full loop of the 100 nm film depicted in Figure 9a, it is apparent that the switching dynamics are completely different for the first unipolar excitation signal.…”

“…Clear current peaks during positive and negative voltage sweeps are visible, meaning there are M‐polar as well as N‐polar regions inside the same, as‐deposited film—unlike in the exclusively N‐polar films deposited by similar processes on metal electrodes. [ 24 ]…”

“…Registered STEM datasets were recorded with an aberration corrected electron probe allowing for the unambiguous determination of Al 0.72 Sc 0.28 N unit cell polarity at the atomic scale. [ 24 ] Although growing epitaxially, the nanoscale grain sizes and in‐plane mosaicity of the investigated film implicated a challenging task for high‐resolution STEM investigation. The small grain size causes overlapping orientations and irregularities in the field of view.…”

Wake‐Up in Al1−xBxN Ferroelectric Films

“…Films that were woken up to different levels were etched in 45% KOH solution; the two different polarities have different etch rates. [3,21,22] The etched films were then characterized using an optical microscope, a profilometer, and field emission scanning electron microscope (FESEM, Zeiss Merlin) images.…”

Piezoelectric effect and polarization switching in Al$_{1-x}$Sc$_x$N

“…It had already been found that ScAlN alloys exhibit enhanced piezoelectricity, 1 with a piezoelectric constant e 33 larger than piezoelectric materials that are presently in use. 2 Excitingly, Fichtner et al 3 demonstrated ferroelectric switching and reported a high paraelectric transition temperature of 600 • C for Al 0.64 Sc 0.36 N. Very recently, the electric-fieldinduced formation of Al-polar domains in originally Npolar ScAlN thin films has been directly observed using scanning transmission electron microscopy, 4 providing direct evidence for ferroelectric switching in ScAlN alloys at the atomic scale.…”