The discovery of the HfO2‐based ferroelectric films has opened new opportunities for using this silicon‐compatible ferroelectric material to realize low‐power logic circuits and high‐density non‐volatile memories. The functional performances of ferroelectrics are intimately related to their dynamic response to external stimuli, such as electric fields at finite temperatures. In the case of HfO2‐based films, the time‐dependent imprint and wake‐up effect, which distinguish them from conventional ferroelectrics, play important roles in understanding the remaining reliability issues, such as insufficient endurance. In this study, the time‐dependent imprint process is carefully investigated using Hf0.5Zr0.5O2 (HZO) films with different ferroelectric properties and defect density. The amount of redistributed charge, which causes imprint during polarization retention, is affected by the remanent polarization of the ferroelectric layer, suggesting that the depolarization field corresponding to the remanent polarization generates and works as a driving force of charge redistribution. The time‐dependent measurement of the imprint distinguishes the origins of charge redistribution processes, which have different time constants. In addition, the correlation between the amount of redistributed charge and the dielectric relaxation of the HZO films is discussed. Correlations are identified between the redistributed charge and the dielectric relaxation, indicating that the mobile charge contributes to the time‐dependent imprint.
To investigate the effects of small amounts of oxygen on the crystal growth process and structural changes in HfO2-based films, film was deposited on a Si substrate using HfO2 and ZrO2 targets via RF magnetron co-sputtering with small amounts of added O2. Even when the deposition was carried out without heating, the most stable monoclinic phase mainly formed at O2 partial pressures above 1 mPa, where the sputtering maintained the oxide mode in the as-deposited state. With decreasing O2 partial pressure, the amorphous component increased. During the annealing process, the metastable tetragonal or orthorhombic phase crystallized when the amorphous film was deposited at a lower O2 partial pressure of 1 mPa. The volume fraction of the metastable phase decreased abruptly at an O2 partial pressure at which the sputtering mode changed from metal mode to oxide mode. These results indicate that the O2 partial pressure during deposition have an effect on the crystal growth process and causes structural changes in the film even after the annealing process.
Ferroelectric HfO2-based thin films, which have been attracting a great deal attention because of their potential use in various applications, are known for their unique properties, such as a large time-dependent imprint and wake-up effect, which differentiate them from conventional ferroelectric materials. In this study, direct piezoelectric measurement was employed to investigate the state of polarization during the retention and wake-up process without applying an electric field. The polarization-electric field hysteresis loop of a sputtered Hf0.5Zr0.5O2 (HZO) film with a thickness of 10 nm showed a time-dependent imprint at room temperature during polarization retention, and the internal electric field that generated the imprint gradually increased from 0.05 to 0.6 MV/cm. While a space charge density of more than 1 μC/cm2 is required to form such an internal electric field, it was found that the magnitude of the direct piezoelectric response did not change at all during polarization retention. On the other hand, both the remanent polarization and direct piezoelectric response increased during the wake-up process. Based on the difference in the variation over time of these two characteristics, 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.
Recently, a number of papers have demonstrated sub-60 mV/decade switching by using the negative capacitance (NC) effect in ferroelectric-gate FETs. However, the physical picture is not yet understood. In this paper, an alternative physical picture for emerging NC is proposed and the development of the NC stage at the ferroelectric/semiconductor hetero-junction is described. Proposed physical picture is based on two factors, 1. “decrease in an additional voltage originated from the depolarization field by surface potential of semiconductor” and 2. “Change in the distribution ratio of gate voltage (VG) to voltage applied to the ferroelectric layer (VF) and surface potential of the semiconductor (ψS) due to the capacitance change of semiconductor.” With considering these two essential phenomena, time-resolved simulations of the NC stage emerging at the ferroelectric/semiconductor hetero-junction were carried out. This NC phenomena expressed by the negative differential of the DF for the VF, i.e. (∂DF/∂VF<0), emerging in the MFS (metal/ferroelectric/semiconductor) capacitor without inserting dielectric layer, are dynamically simulated to discuss the proposed NC process. The simulation results clearly reveal that the NC stage is originated from the existence of additional voltage caused by the depolarization field by surface potential of semiconductor originated from the existence of remanent polarization of ferroelectric layer, and change in the capacitance of the semiconductor during polarization switching. The different physical picture from steady-state NC and transient NC can be clearly shown.
Printing technique was applied to preparing pyroelectric elements of infrared detectors. Completely sintered ceramic films were obtainable using only pastes that were made from raw powders as fine as 1.0 µm. The films indicated ferroelectricity. The measured pyroelectric coefficient, resistivity and relative permitivity were nearly same as those obtainable on the bulk ceramics formed by conventional sintering technique.
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