Thermal evolution of ferroelectric behavior in epitaxial Hf0.5Zr0.5O2

Abstract

Herein we report a cryogenic-temperature study on the evolution of ferroelectric properties of epitaxial Hf0.5Zr0.5O2 thin films on silicon. Wake-up, endurance, and fatigue of these films are found to be intricately correlated, strongly hysteretic, and dependent on available thermal energy. Field-dependent measurements reveal a decrease in polarization with temperature, which has been determined not to be an intrinsic change of the material property, rather, a demonstration of the increase in the coercive bias of the material. Our findings suggest that a deficiency in thermal energy suppresses the mobility of defects presumed to be oxygen vacancies during wake-up and trapped injected charge during fatigue, which is responsible for polarization evolution during cycling. This permits accelerated wake-up and fatigue effects at high temperatures where thermal energy is abundant but delays these effects at cryogenic temperatures.

Characterizing Ferroelectric Properties of Hf0.5Zr0.5O2 From Deep-Cryogenic Temperature (4 K) to 400 K

“…(Nfatig: number of cycles at Pfatig, Nwake: number of cycles at Pwake) which is fit to R(T) = Ae (-Ea/kT) (A: fitting constant, Ea: activation enrgy, k: Boltzmann constant) in order to obtain Ea. It is shown that the fatigued effect is accelerated with larger Vw (3.5 V) at elevated temperature from 300 K to 400 K. The extracted activation energy (Ea=42.97 meV) of Vw=3.5 V in the fatigue model [11] is approximately twice larger than of the smaller Vw (2.5 V) with Ea=23.67 meV, which is close to the previously reported Ea=23.4 meV in [11]. In Fig.…”

“…Note that the Pr is higher in the elevated temperature even in the initial state, possibly because that the PEALD HZO is already substantially woken-up (i.e., charges are evenly distributed) in the fresh cell, which is not the case in [12]. Similar trend was shown in [11] with epitaxially grown HZO which showed relatively less phase transition between ferroelectric phase and dielectric phase during the post-cycling process. In order to observe the accelerated wake-up/fatigue effects at deepcryogenic 4 K, the tp was increased from 2 µs to 100 µs in 2171 µm 2 capacitor in Fig.…”

An Overview of Ferroelectric Hafnia and Epitaxial Growth

“…In addition, thermally induced strain can also directly affect polarization rotation and wake‐up effects through complicated electromechanical coupling. [ 94 ]…”

“…[ 92,93 ] In addition, the thermal expansion coefficient for HZO is about 3.7 × 10 −6 K −1 , 0.8 × 10 −6 K −1 , and 7.5 × 10 −6 K −1 for the a, b and c axis, separately. [ 94 ] The much smaller thermal expansion coefficient of Si makes hafnia thin films under in‐plane tensile strain during cooling, and thus causing contraction of the out‐of‐plane lattice parameter of the orthorhombic (111) HZO. [ 38 ] The thermal expansion coefficient mismatch between the substrate and HZO layers provides an added biaxial strain on hafnia (111) layers, stabilizing the polar phase at room temperature.…”

Intrinsic ferroelectricity in Y-doped HfO2 thin films

“…It was reported that most of the polarization in the HZO(111) / LSMO(001) structures could be attributed to oxygen vacancy migration, 41 with the intrinsic ferroelectric polarization estimated to be less than 9 µC/cm 2 . In addition, it was reported that Pr in the 5.6-nm-thick HZO films grown on LSMO/LaNiO3/CeO2/YSZ/Si(100) decreases by a factor of 3 from 300 K to 20 K. 43 On the other hand, for the YHO(111) films studied in this work, Pr exhibits a large value at low temperature and stays nearly constant below 100 K. This suggests that the extrinsic contributions are minimal to the measured Pr at low temperature. The moderate overall change of Pr with temperature also indicates that the contribution from the intrinsic ferroelectricity dominates even at room temperature.…”