Kazuhiro Maeda

Electrical Characteristics of Controlled-Polarization-Type Ferroelectric-Gate Field-Effect Transistor

We propose a novel ferroelectric-gate field-effect transistor (FET) with a polar semiconductor channel, which is called a controlled-polarization-type ferroelectric-gate FET. The essential factor for this device is the stabilization of the spontaneous polarization of the ferroelectric film by the spontaneous polarization of a polar semiconductor, such as ZnO or GaN. We demonstrate two types of controlled-polarization-type ferroelectric-gate thin film transistors (TFTs), which are the ZnO/YMnO3/Pt bottom-gate-type structure and Pt/PZT/ZnO top-gate-type structure. Both structures show n-channel-type transistor operation with the memory effect originating from ferroelectricity. From the electrical characteristics of ferroelectric-gate TFTs, the effects of ferroelectricity, gate leakage current, space charge, and charge injection on the device operation are discussed.

Polarization Switching Behavior of YMnO3 Thin Film at around Magnetic Phase Transition Temperature

The effects of antiferromagnetic ordering on the ferroelectric polarization switching behavior of an YMnO3 epitaxial thin film were investigated. The effects of magnetic ordering on ferroelectric polarization switching and dielectric permittivity under bias electric field at around the Neel temperature of YMnO3 (80 K) were studied. From temperature dependences of the dielectric permittivity and spontaneous polarization, an increase in the domain size with decreasing temperature was suggested. From high-speed capacitance measurement, it was also determined that domain wall motion increased with decreasing temperature. Moreover, it was determined that the suppression of polarization switching started at around 120–130 K, which is higher than Neel temperature of YMnO3.

Influence of antiferromagnetic ordering on ferroelectric polarization switching of YMnO/sub 3/ epitaxial thin films

Hexagonal YMnO3 has antiferromagnetism and ferroelectricity simultaneously below 70 K. In this study, the domain switching behavior of ferroelectric polarization at around Neel temperature is investigated. From the experimental results of the frequency and temperature dependences of coercive electric filed (Ec) obtained using polarization-electric field (P-E) hysteresis loop, the cross-correlation phenomena between magnetics and ferroelectrics are discussed in detail. The P-E hysteresis loops of the films were measured in the frequency range from 1 Hz to 10 kHz and the measurement temperature was varied from 10 to 150 K. Frequency dependence of Ec was analyzed using Ishibashi-Oriharas theory. Change in the Ec was measured as a function of the temperature. It gradually increased with decreasing the temperature. In this regime, the change was well fitted to Devonshires classical theory. Below 120 K, however, it drastically increased and appart from the theory.

Impedance Analysis of Controlled-Polarization-Type Ferroelectric-Gate Thin Film Transistor Using Resistor–Capacitor Lumped Constant Circuit

We propose a novel ferroelectric-gate field-effect transistor using a polar oxide semiconductor channel, which is called a controlled-polarization (CP)-type ferroelectric-gate thin film transistor (TFT). Although the CP-type ferroelectric-gate TFTs with a ZnO/YMnO3 structure shows nonvolatile memory operation, the relationship between the electrical characteristics of the TFTs and the direction of spontaneous polarization of the ferroelectric layer (PSFe) below the channel has not been revealed. In this study, the direction of PSFe is analyzed by the impedance spectra of the channel conductance because it can be expected that the channel conductance depends on the direction of the PSFe. The five conditions of the channel conductance are assumed and the impedances between the source electrode and the gate electrode of each condition are calculated by SPICE. The direction of PSFe at various gate voltages is determined by the comparison of the calculated results and experimental results. It was found that the channel conductance of the ferroelectric-gate TFT has steep change by the change of the direction of PSFe.

Influence of Antiferromagnetic Ordering on Ferroelectric Polarization Switching of YMnO 3 Epitaxial Thin Films

In this study, the switching behavior of ferroelectric polarization of (0001) YMnO3 epitaxial films at around Neel temperature was investigated. From the experimental results of the frequency and temperature dependences of coercive electric filed (Ec) obtained from polarization-electric field (P-E) hysteresis loop, the cross-correlation phenomena between magnetics and ferro-electrics are discussed in detail. The P-E hysteresis loops of the films were measured in the frequency range from 1 Hz to 10 kHz, and the temperature was varied from 10 to 150 K. Frequency dependence of Ec accorded with Ishibashi-Oriharas theory at the measured temperature range. However, temperature dependence of Ec disagreed with Devonshires theory below 120 K, which is close to the Neel temperature of the YMnO3 epitaxial film.

Correlation between the intra-atomic Mn3+ photoluminescence and antiferromagnetic transition in an YMnO3 epitaxial film

We have investigated the temperature dependence of the intra-atomic Mn3+ photoluminescence (PL) in a multiferroic YMnO3 epitaxial film grown by pulsed laser deposition. The most significant result is that the temperature dependence of the Mn3+ PL intensities is highly correlated with that of the magnetic spin order originating from the antiferromagnetic transition. In contrast, the decay times of the Mn3+ PL bands are independent of temperature; namely, the transition probabilities are not affected by the antiferromagnetic transition. The above results suggest that the electron transfer process to the Mn3+ excited state from the conduction band is dominated by the magnetic spin order.

Ferroelectric Properties of Magnetoferroelectric YMnO3 Epitaxial Films at around the Neel Temperature

The switching behavior of the ferroelectric polarization of magnetoferroelectric YMnO3 epitaxial films at around the Néel temperature was investigated to discuss the effect of the antiferromagnetic ordering on the ferroelectric domain wall motion. From neutron diffractions to investigate the antiferromagnetic ordering and measurements of polarization-electric field, dielectric constant, and the time constant of the domain wall motion, the change of the domain wall motion is suggested at around 120 K, which is higher than the Néel temperature (~80 K).

Dielectric Behavior of YMnO3 Epitaxial Thin Film at around Magnetic Phase Transition Temperature

Relationship between magnetic ordering and ferroelectric polarization switching in YMnO3 epitaxial thin film was investigated. It was found that Néel point of the YMnO3 film is below 80 K, which is consistent with that of YMnO3 single crystal by neutron diffraction. From temperature dependence of the polarization-electric field hysteresis loops and the dielectric permittivity-voltage characteristics, ferroelectric polarization switching behavior was investigated from 300 to 10 K in detail. The ferroelectric polarization switching behavior accords with Ishibashi-Orihara’s theory. Moreover, it was found that the dielectric permittivity under bias electric field have a anomaly below 80 K and the nucleation density for the ferroelectric polarization switching decreases below 130 K, which is higher than 80K.