Wake-Up Free Ultrathin Ferroelectric Hf_0.5Zr_0.5O₂ Films

The development of the new generation of non-volatile high-density ferroelectric memory requires the utilization of ultrathin ferroelectric films. The most promising candidates are polycrystalline-doped HfO₂ films because of their perfect compatibility with silicon technology and excellent ferroelectric properties. However, the remanent polarization of HfO₂ films is known to degrade when their thickness is reduced to a few nanometers. One of the reasons for this phenomenon is the wake-up effect, which is more pronounced in the thinner the film. For the ultrathin HfO₂ films, it can be so long-lasting that degradation occurs even before the wake-up procedure is completed. In this work, an approach to suppress the wake-up in ultrathin Hf_0.5Zr_0.5O₂ films is elucidated. By engineering internal built-in fields in an as-prepared structure, a stable ferroelectricity without a wake-up effect is induced in 4.5 nm thick Hf_0.5Zr_0.5O₂ film. By analysis of the functional characteristics of ferroelectric structures with a different pattern of internal built-in fields and their comparison with the results of in situ piezoresponse force microscopy and synchrotron X-ray micro-diffraction, the important role of built-in fields in ferroelectricity of ultrathin Hf_0.5Zr_0.5O₂ films as well as the origin of stable ferroelectric properties is revealed.