(Invited, Digital Presentation) Investigation of Polarization Behavior of HfO2 Thin Films Via Piezoelectric Characterization
Authors: Yoshimura Takeshi 
Published: November 2022

Nonvolatile memory based on field-effect transistors with ferroelectric gate dielectrics (FeFET) was first proposed in the 1960s[1]. Later, in the 1990s, FeFETs were proposed to operate as neuromorphic devices and were actively studied[2]. However, it has not been realized due to the difficulty of forming ferroelectric thin films on Si and obtaining a good Si/ferroelectric interface. This situation was greatly changed by the discovery of ferroelectricity in hafnium oxides [3]. Now, FeFETs have attracted attention again due to the high CMOS process compatibility of hafnium oxides as well as their good ferroelectricity even in ultra-thin films. FeFETs have many potential applications, including steep switching transistors and reservoir computing as well as nonvolatile memory. In addition, the understanding of the ferroelectric properties of HfO2-based thin films has been deepened. On the other hand, reliability issues such as robustness, switching endurance, fatigue characteristics, wake-up effects, and imprinting still remain to be addressed for practical applications. These are problems common to ferroelectric thin films, but because HfO2 is an ultra-thin film, they are not easy to analyze. Therefore, in this study, we focused on the evaluation of ferroelectricity by the direct piezoelectric effect. In contrast to the inverse piezoelectric response, the direct piezoelectric response is, in principle, independent of the film thickness, so we expect that the direct piezoelectric response could be analyzed even for HfO2-based thin films.

TaN/Hf0.5Zr0.5O2 (HZO)/TaN capacitor was prepared on a Si wafer. TaN electrodes were deposited by DC sputtering method. The thickness of the TaN films was 10 nm. The HZO film was then deposited by the RF magnetron sputtering method using HfO2 and ZrO2 targets. The fabricated capacitor was annealed in a rapid thermal annealing system at 700 °C. The details of the deposition are described in Ref. 4. The direct piezoelectric response of the HZO film was measured using the wafer flexure technique [5]. The strain was applied periodically to the sample, and the charge induced via the direct piezoelectric response was

measured using a lock-in amplifier. While the conventional ferroelectric characterization, including the P-E measurement, might be influenced by the space charge, leakage current so on, the direct piezoelectric response only depends on the remaanenrt polarization because there are no other physical phenomena similar to the direct piezoelectric response. We investigated the state of polarization during the retention and wake-up process. The film showed a time-dependent imprint at room temperature during polarization retention. The internal electric field that generated the imprint gradually increased from 0.05 to 0.6 MV/cm. In contrast, the magnitude of the direct piezoelectric response did not change during the polarization retention. Similar analyses were also carried out for the wake-up process. Based on the obtained results, we concluded that the non-ferroelectric layer exists at the interface between the HZO film and TaN electrode and gradually transitions to ferroelectric phases through the electric field cycle[6]. The presentation will also cover the domain observation of HZO films by the direct piezoeelectric response.

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[6] K. Takada, M. Murase, S. Migita, Y. Morita, H. Ota, N. Fujimura, and T. Yoshimura, Appl. Phys. Lett. 119, 032902 (2021)