On the exceptional temperature stability of ferroelectric Al_(1-x)SC_xN thin films

摘要

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 Ali_(1-x）Sc_xN, low temperature has to be seen as a purely relative term, since its ferroelectric-to-paraelectric transition temperature is confirmed to surpass 1100 °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 μm 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 that the wurtzite-type structure of Al_(0.73)Sc_(0.27)N is conserved during the entire 1100 °C annealing cycle, apparent through a constant c/a lattice parameter ratio. In situ permittivity measurements performed up to 1000 °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-μm scale next to the stability of the inscribed polarization during the complete 1100 °C annealing treatment. Thus, Al_(1-x)Sc_xN, 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 harsh- est environments.