Aluminum scandium nitride (AlScN) is highly expected to lead the new paradigm of nanoelectronic devices owing to its many desirable properties. However, current devices based on AlScN films have a small operating window compared to other ferroelectric materials because of the low breakdown field to coercive field (EBD/Ec) ratio. Recently, we have reported a method to increase the EBD/Ec ratio through a new design. We have used an interface engineering approach where we create multilayers of Al0.72Sc0.28N and Al0.64Sc0.36N films stacked in an alternating fashion. The Weibull analysis shows an increase in the characteristic breakdown field as high as 7.20 MV/cm for AlScN/7 (compared to 5.99 and 5.86 MV/cm for single layer Al0.72Sc0.28N and Al0.64Sc0.36N, respectively). Here, we have presented characterizations of the local atomic structure and nanoscale chemistry of multilayer ferroelectric AlScN films using transmission electron microscopy (TEM) techniques. Energy dispersive X-ray spectroscopy (EDS) maps show seemingly sharp interfaces where the chemical composition changes from Al0.64Sc0.36N to Al0.72Sc0.28N, and vice versa. The difference in Sc alloying level of layers generates significant interfacial misfit strain, which is expected to be the main reason for electrical breakdown strength enhancement. To test this hypothesis, we quantitatively examined the lattice spacing in AlScN multilayered film. The “four-dimensional scanning TEM” (4D-STEM) technique was employed to systematically investigate distribution of the strain. It was conclusively shown that a significant increase in the in-plane component of the lattice parameter at the interface. This work suggests that interfaces with intentionally designed lattice strain provide a new materials design strategy of AlScN for achieving an increase in the breakdown field to coercive field (EBD/Ec) ratios.

Characterization of the structure, composition and strain in multilayer ferroelectric aluminum scandium nitride films

ACS Spring 2022

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