The impact of rapid thermal annealing for the ferroelectricity of undoped sputtered HfO2 and its wake-up effect

Abstract

Fundamental aspects of ferroelectric HfO[Formula: see text], a fluorite-type oxide, are not understood yet. This is evident by different theories regarding, e.g., the wake-up effect or the antiferroelectric-like behavior of HfO[Formula: see text] manufactured with different doping or deposition techniques. Therefore, we focus on sputtered and undoped HfO[Formula: see text] to gain deeper understanding of the ferroelectric properties of pure HfO[Formula: see text]. A temperature gradient on a [Formula: see text] substrate during rapid thermal annealing led to different ferroelectric device performances in terms of remnant polarization and the wake-up effect. The results from the electrical characterization are compared to observations by transmission electron microscopy, performed on pristine and trained samples in plan-view as well as in cross section. We observed that different temperature treatments caused effects at the interfaces of the TiN electrodes and also affected the microstructure and defect concentration of the HfO[Formula: see text] itself. Devices from the hot corner showed wake-up free ferroelectricity with a remnant polarization below 10 [Formula: see text], whereas devices from the cold corner showed a strong wake-up effect with remnant polarization starting from 0 to above 20 [Formula: see text] after [Formula: see text] cycles. After observing a small structural transformation in trained devices, we attributed this strong wake-up effect to gradual ferroelastic switching of pristine [110] oriented grains with in-plane polarization to partially out-of-plane polarization, while the predominantly [Formula: see text] oriented grains in the hot corner can suppress the wake-up effect.