On the exceptional temperature stability of ferroelectric Al1-xScxN thin films

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 Al1-xScxN, low temperature has to be seen as a purely relative term, since its ferroelectric-to-paraelectric transition temperature is confirmed to surpass 1100 degrees 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 Al0.73Sc0.27N films grown on Mo bottom electrodes via in situ high-temperature x-ray diffraction and permittivity measurements. Our studies reveal that the wurtzite-type structure of Al0.73Sc0.27N is conserved during the entire 1100 degrees C annealing cycle, apparent through a constant c/a lattice parameter ratio. In situ permittivity measurements performed up to 1000 degrees 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 degrees C annealing treatment. Thus, Al1-xScxN, there 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.