Electric field-induced domain structures in ferroelectric AlScN thin films

The analog switching properties of wurtzite-type ferroelectrics hold unforeseen potential for future-generation electronic devices, such as neuromorphic memory concepts based on memristive devices. However, investigative studies expanding our detailed knowledge on the physical properties of the ferroelectric domain walls and the modulation of large-scale domain patterns are still very limited. Up to date, the exact atomic configuration of the electric field-induced domain walls has not been identified due to its inclined and wedge-shaped three-dimensional nature. With this contribution, we provide direct experimental evidence on the atomic configuration of electric field-induced vertical inversion domain walls in ferroelectric Al0.85Sc0.15N thin films using advanced scanning transmission electron microscopy techniques. Despite their overall inclined character, the structure of vertical inversion domain walls can be atomically sharp and exhibit laterally facing metal(M)- and nitrogen(N)-polar dimers consistent with low-energy configurations predicted for the Al1−xScxN system. Although nanoscale regions with extended superposition structures are examined, this observation makes it rather unlikely that domain walls in the investigated system are necessarily stabilized by nonpolar supercells. Moreover, by the examination of electric field dependent domain patterns, we evidence the surprising stabilization of N-polar spike domains at the top electrode interface after electrical backswitching to the M-polar state and observed forward growth in the film volume from these residual domains. These results strengthen recent advancements on the realization of memristive devices given the possibility to modulate the density of charged domain walls enabling multi-bit memory operations.