On the exceptional temperature stability of ferroelectric AlScN thin films

TL;DR

This study demonstrates that ferroelectric AlScN thin films maintain their structure and polarization stability up to 1100°C, surpassing the thermal stability of most other ferroelectric materials, making them highly suitable for harsh environment applications.

Contribution

The paper provides conclusive evidence that AlScN ferroelectric films are structurally stable and retain polarization at temperatures above 1100°C, which is unprecedented among thin film ferroelectrics.

Findings

AlScN maintains wurtzite structure up to 1100°C

Permittivity remains stable up to 1000°C with onset of divergence only at high temperatures

Structural and polarization stability confirmed by multiple in situ and ex situ techniques

Abstract

Through its dependence on low symmetry crystal phases, ferroelectricity is inherently a property tied to the lower temperature ranges of the phase diagram for a given material. This paper presents conclusive evidence that in the case of ferroelectric AlScN, low temperature has to be seen as a purely relative term, since its ferroelectric-to-paraelectric transition temperature is confirmed to surpass 1100{\deg}C and thus the transition temperature of virtually any other thin film ferroelectric. We arrived at this conclusion through investigating the structural stability of 0.4 - 2 ${\mu}$m thick Al$_{0.73}$Sc$_{0.27}$N films grown on Mo bottom electrodes via in situ high-temperature X-ray diffraction and permittivity measurements. Our studies reveal the wurtzite-type structure of Al$_{0.73}$Sc$_{0.27}$N is conserved during the entire 1100{\deg}C annealing cycle, apparent through a constant c over a lattice parameter ratio. In situ permittivity measurements performed up to 1000{\deg}C strongly support this conclusion and include what could be the onset of a diverging permittivity only at the very upper end of the measurement interval. Our in situ measurements are well-supported by ex situ (scanning) transmission electron microscopy and polarization and capacity hysteresis measurements. These results confirm the structural stability on the sub-${\mu}$m scale next to the stability of the inscribed polarization during the complete 1100{\deg}C annealing treatment. Thus, AlScN is the first readily available thin film ferroelectric with a temperature stability that surpasses virtually all thermal budgets occurring in microtechnology, be it during fabrication or the lifetime of a device - even in harshest environments.