Reversible transition between the polar and antipolar phases and its implications for wake-up and fatigue in HfO2-based ferroelectric thin film

Abstract

Atomic-resolution Cs-corrected scanning transmission electron microscopy revealed local shifting of two oxygen positions (O_I and O_II) within the unit cells of a ferroelectric (Hf_0.5Zr_0.5)O₂ thin film. A reversible transition between the polar Pbc2₁ and antipolar Pbca phases, where the crystal structures of the 180° domain wall of the Pbc2₁ phase and the unit cell structure of the Pbca phase were identical, was induced by applying appropriate cycling voltages. The critical field strength that determined whether the film would be woken up or fatigued was ~0.8 MV/cm, above or below which wake-up or fatigue was observed, respectively. Repeated cycling with sufficiently high voltages led to development of the interfacial nonpolar P4₂/nmc phase, which induced fatigue through the depolarizing field effect. The fatigued film could be rejuvenated by applying a slightly higher voltage, indicating that these transitions were reversible. These mechanisms are radically different from those of conventional ferroelectrics.

Fig. 1. The observation of oxygen atoms of single orthorhombic (O-) phase grain in pristine TiN/Hf_0.5Zr_0.5O₂ (HZO, 15 nm)/TiN device.

a Scanning transmission electron microscopy (STEM) high-angle annular dark-field (HAADF) image of the TiN/HZO/TiN cross-section. b Annular bright-field (ABF) image extracted from the green square area in a, in which the position of the O atomic columns can be clearly detected with different offset and direction. According to the O atomic columns deviated from the center of the four nearest Hf/Zr columns, it can be divided into O_I type (center) and O_II type (off-center). Some of the O_II-site columns shift along the [001], while others shift to the opposite direction. c The atomic models of the Pbc2₁ and Pbca phases along [010] direction. The purple, navy blue, and cyan-colored solid balls correspond to the Hf/Zr, O_I (center), and O_II (off-center) ions, respectively.

Fig. 2. Electrical measurement results of different TiN/HZO/TiN devices: 15-nm-thick Hf_0.5Zr_0.5O₂, 10-nm-thick Hf_0.5Zr_0.5O₂, 10-nm-thick Hf_0.4Zr_0.6O₂, 10-nm-thick Hf_0.6Zr_0.4O₂, and the nanolaminated 10-nm-thick Hf_0.5Zr_0.5O₂ films.

a The 2P_r of 15-nm-thick Hf_0.5Zr_0.5O₂ device measured by ±3 V P–V loop and the corresponding leakage currents measured at 2 V, as a function of switching cycles, which was performed by applying bipolar voltage pulses of ±5 V at f = 500 kHz. b The trend of 2P_r changes of different devices with the switching cycles at a low field of ±1.8 MV/cm. c The corresponding leakage current changes with the cycles. d The rejuvenation process of different devices by applying field pulses of ±3 MV/cm. e Ten cycles of fatigue/rejuvenation process on the 15-nm-thick Hf_0.5Zr_0.5O₂ device under ±2.2 V, 5 × 10⁶ bipolar pulses for fatigue and ±4.0 V, 1 × 10⁴ bipolar pulses for recovery.

Fig. 3. The observation of O_AFEPbca phase returning to O_FEPbc2₁ phase after wake-up process.

a Cross-section STEM-HAADF image of the TiN/HZO/TiN device after wake-up under the bipolar triangle pulses of ±4 V, 10⁴ cycles with f =500 kHz, in which a [010] oriented HZO O-phase grain is examined. b ABF image acquired from the green square area in a, the majority of the off-center O_II atomic columns shifted along the [001] direction (green), demonstrating a Pbc2₁-dominated phase structure after the wake-up. The inset magnified ABF image shows the shifting direction of O_II atomic columns deviating from the center of the four nearest Hf/Zr columns, fitting well with Pbc2₁ structure.

Fig. 4. Phase transition from O_FEPbc2₁ to O_AFEPbca phase after fatigue process.

a Cross-section HAADF image of the TiN/HZO/TiN device after fatigue at V_a = ±2.5 V, f = 500 kHz, for 10⁷ times. The image shows an O-phase grain projected along [010] zone axis. b The corresponding ABF image of a, demonstrating clearly the off-center O_II atomic columns shifted along the [001] direction (green) and [00$\overline{1}$] direction (purple) alternately, showing a Pbca dominated structure. c The magnified ABF image acquires from the green square area in b, and the arrows show the deviating direction of O_II atomic columns from the center of the four nearest Hf/Zr columns.

Fig. 5. Energy landscape and DFT calculations.

a schematic diagrams of the energy landscape for the Pbca (center global minimum) and Pbc2₁ (two local minima at finite P) under zero E_eff and non-zero E_eff. b calculated energies of the Pbca, Pbc2₁(–P_s), and Pbc2₁(+P_s) as a function of E_eff.