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

Abstract

Fundamental aspects of ferroelectric HfO2, 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 HfO2 manufactured with different doping or deposition techniques. Therefore, we focus on sputtered and undoped HfO2 to gain deeper understanding of the ferroelectric properties of pure HfO2. A temperature gradient on a 10×10mm2 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 HfO2 itself. Devices from the hot corner showed wake-up free ferroelectricity with a remnant polarization below 10 μC/cm2, whereas devices from the cold corner showed a strong wake-up effect with remnant polarization starting from 0 to above 20 μC/cm2 after 106 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 ⟨111⟩ oriented grains in the hot corner can suppress the wake-up effect.