Structural Evolution of Nanoscale Ferroelectric Hf0.5Zr0.5O2 Layers as a Result of Their Cyclic Electrical Stimulation

Abstract

Despite the large number of already published articles on the ferroelectric properties of Hf_0.5Zr_0.5O₂ (HZO), this material still attracts considerable attention from the scientific community due to its potential for use in competitive nonvolatile HZO-based memory devices compatible with modern silicon technology. One of the main challenges in developing industrial-scale technology for such devices is the instability of the remanent polarization of the ferroelectric material under repeated switching by an external electric field. In particular, at the initial stages of such “cycling,” a significant increase in remanent polarization is typically observed (the so-called “wake-up” effect), followed by a decrease after a certain number of cycles (the so-called “fatigue” effect). The processes responsible for this instability remain a subject of debate. Using a previously developed methodology for analyzing the phase composition of ultrathin HZO layers via the NEXAFS synchrotron radiation technique, it is shown that in capacitors based on TiN/HZO/TiN structures, the “wake-up” effect observed during the first 10⁵ switching cycles is due to an increase in the relative content of the polar orthorhombic phase in HZO, resulting from a decrease in the content of the “parasitic” tetragonal phase. The results confirm that the electric field-stimulated structural phase transition in the films is one of the mechanisms explaining the evolution of the functional properties of HZO-based ferroelectric memory elements over their service life.