Takeshi Yoshimura

EPITAXIALLY GROWN YMNO3 FILM : NEW CANDIDATE FOR NONVOLATILE MEMORY DEVICES

We have proposed ReMnO3 (Re:rare earth) thin films as a new candidate for nonvolatile memory devices. In this letter, we report on fabrication of (0001) YMnO3 films on (111)MgO, (0001)ZnO:Al/(0001) sapphire, and (111)Pt/(111)MgO using rf magnetron sputtering. We succeeded in obtaining (0001) epitaxial YMnO3 films on (111) MgO and (0001)ZnO:Al/(0001) sapphire substrate, and polycrystalline films on (111)Pt/(111)MgO. The dielectric properties of the epitaxial and polycrystalline YMnO3 films are almost the same. The dielectric permittivities of both films are smaller than those reported for YMnO3 single crystal.

Growth mechanism of YMnO3 film as a new candidate for nonvolatile memory devices

We have proposed ReMnO3 (Re: rare earth) thin films for nonvolatile memory devices. We examine the growth mechanism of YMnO3 films on (0001)ZnO:Al/(0001) sapphire substrate using rf magnetron sputtering and pulsed laser deposition methods with oxide compound target. We have succeeded in obtaining (0001) epitaxial YMnO3 films on (111) MgO and (0001)ZnO:Al/(0001) sapphire substrate, and polycrystalline films on (111)Pt/(111)MgO. For an optimal structure, the film needed much less oxygen from the gas phase compared to other oxide films. The composition (Y/Mn ratio) of the YMnO3 films changed drastically by varying the partial oxygen pressure in the sputtering gas. In addition the Y/Mn ratio slinfled with sputter time due to target surface modification, probably caused by Y segregation. An extremely small amount of oxygen is required to form the YMnO3 crystal. This was confirmed by pulsed laser deposition experiments.

Ferromagnetic and ferroelectric behaviors of A-site substituted YMnO3-based epitaxial thin films

Dielectric and magnetic properties of A-site substituted YMnO3 epitaxial films were studied. Stoichiometric epitaxial YMnO3 films clearly show ferroelectric behavior in P–E and C–V measurements. The I–V property is well explained by the Pool–Frenkel-type carrier emission with p-type conduction due to the existence of Mn4+, and the activation energy is calculated to be 0.58 eV. The leakage current decreases by substituting Y for Zr and increases by Li or Mg substitution. Although YMnO3 films exhibit antiferromagnetic magnetization behavior regardless of the crystallographic orientation and the carrier concentration, Li-doped sample displays parasitic ferromagnetic behavior (weak ferromagnetism). Substituting Y for Yb enhances the ferromagnetic interaction.

Single-Wall Carbon Nanotube Field Effect Transistors with Non-Volatile Memory Operation

We describe the fabrication and electrical characteristics of single-wall carbon-nanotubes field-effect transistors (CNT-FETs) with a non-volatile memory function using ferroelectric thin films as gate insulators. The ferroelectric-gate CNT-FETs were fabricated using single-wall CNTs synthesized from alcohol by catalytic chemical vapor deposition and sol–gel derived PbZr0.5Ti0.5O3 thin films. The ferroelectric-gate CNT-FETs showed modulation of the drain current with the gate voltage and the threshold voltage shift (memory window) on the drain current–gate voltage characteristics. Moreover, the memory window was saturated around 1.1 V as the gate voltage sweeping range increased. These results indicate that carriers in CNTs are controlled by spontaneous polarization of the ferroelectric films. Because ferroelectrics exhibit complex couplings between their electrical, structural, mechanical, thermal, and optical properties, and because CNTs have unique mechanical and electrical properties, ferroelectric-gate CNT-FETs offer promise as potentially useful nanoelectronics devices not only for non-volatile memory elements but also for high-sensitivity sensors.

Direct Piezoelectric Properties of Mn-Doped ZnO Epitaxial Films

The direct piezoelectric properties of Mn-doped ZnO epitaxial films prepared by pulse laser deposition was investigated. The resistivity of the ZnO films was markedly decreased by Mn doping. The effective transverse piezoelectric coefficient (e31,f) of the ZnO films was measured by substrate bending and collecting developed charges. The dependence of induced charge density on applied strain indicates that the obtained piezoelectric response originates from the spontaneous polarization of the Mn-doped ZnO films. The e31,f values of 3 and 5 at. % Mn-doped ZnO films were determined to be -1.09 and -1.10 C/m2, respectively, which are comparable to the theoretical value.

Detailed structural analysis and dielectric properties of silicon nitride film fabricated using pure nitrogen plasma generated near atmospheric pressure

A detailed structural analysis and dielectric property measurements of silicon nitride films fabricated using atmospheric pressure (AP) plasma were carried out, and the results were compared to those of nitride films fabricated using a radio frequency plasma. Using AP plasma, 1.8-nm-thick silicon nitride films composed of Si3N3.5O0.7 were obtained at nitridation temperatures ranging from 25to500°C. X-ray photoelectron spectroscopy using a monochromatic AlKα source at 1486.6eV and high-resolution Rutherford backscattering spectrometry revealed approximately 10% more nitrogen atoms corresponding to the N–Si3 bond in the film using AP plasma than those using rf plasma. In the range of 25–500°C, the nitridation temperature did not affect the leakage current densities of the films fabricated using AP plasma. Films fabricated at 25°C showed leakage current density of as low as 7×10−2A∕cm2 at 5MV∕cm. This value was one order of magnitude lower than that using rf plasma. The direct-tunneling current simulation an...

Analysis of nitrogen plasma generated by a pulsed plasma system near atmospheric pressure

Stable discharging of pure nitrogen can be maintained even at atmospheric pressure when alternative pulsed voltage is applied between two parallel plate electrodes. We evaluated the nitrogen plasma generated by an alternative pulsed voltage system. The excited nitrogen species in a pure nitrogen plasma was evaluated using optical emission spectroscopy. In the discharging space, the largest peak detected corresponded to the N2 second positive system. Additionally, the emission peaks from the excited nitrogen atom were detected at a pressure of as high as 45Torr. In the downstream space, we detected emission peaks from the N2 Herman’s infrared system as well as the N2 second positive system. The N2 (AΣu+3) state is considered to be the origin of the N2 Herman’s infrared system. The emission intensities from the N2 second positive system and the N2 Herman’s infrared system increase with increasing nitrogen gas pressure, whereas the emission intensity from the N2+ first negative system decreases.

Polarization Hysteresis Loops of Ferroelectric Gate Capacitors Measured by Sawyer-Tower Circuit

Polarization hysteresis properties of ferroelectric gate capacitors were investigated using a quasi metal-ferroelectric-metal-insulator-semiconductor (MFMIS) capacitor which was constructed by the series connection of a metal-ferroelectric-metal (MFM) capacitor and a metal-insulator-semiconductor (MIS) capacitor. By the polarization-voltage measurement using the MIS capacitor, it was revealed that the difficulty of the polarization hysteresis measurement for ferroelectric gate capacitors is caused by the low response frequency of the inversion region formation. The result indicates that the ferroelectric film in ferroelectric gate capacitors is not fully polarized when the inversion layer is not formed completely. Since the response frequency of the inversion region formation can be increased by illumination, symmetric polarization hysteresis loops could be obtained even in the ferroelectric gate capacitors. The polarization hysteresis loops measured using the MFMIS capacitor agreed well with those calculated by the series connection of MFM and MIS capacitors.

Piezoelectric Vibrational Energy Harvester Using Lead-Free Ferroelectric BiFeO3 Films

We have proposed that BiFeO3 films are suitable for piezoelectric vibrational energy harvester (VEH) applications, because BiFeO3 has high spontaneous polarization and low dielectric permittivity. We demonstrated that energy can be harvested by a micromachined VEH using a BiFeO3 film deposited using a sol–gel process. A VEH with a resonant frequency of ~98 Hz produced an output voltage of 1.5 VG-1 and electrical power of 2.8 µWmm-3G-2 (G=9.8 m/s2) at a load resistance of 1 MΩ. Using the analytical model for VEH, the generalized electromechanical coupling factor was estimated to be 0.41%. These results were comparable to those of the best-performing VEHs using other piezoelectric films.

Magnetic Properties of Uniformly Ce-Doped Si Thin Films with n-Type Conduction

Magnetic properties and the chemical bonding state of Ce for a diluted magnetic semiconductor (DMS), n-type Si:Ce thin films with a Ce concentration below 8.0 at. % grown by low-temperature molecular beam epitaxy (LT-MBE) are investigated. LT growth enables the films to have a uniform distribution of Ce in epitaxial Si films with a Ce concentration up to 8.0 at. %. The precipitation of the second phase, such as cerium silicide, is not recognized in the film with a Ce concentration of 4.0 at. % by electron diffraction using transmission electron microscopy (TEM) analysis. All the films exhibit n-type conduction. The electron density increases with increasing Ce concentration up to 0.07 at. % because of the generation of electrons by point defects. Then, the density decreases with increasing Ce concentration up to 1.1 at. % because of the compensation by the hole generated by substitutionally dissolved Ce3+. A positive magnetization due to a Ce3+ ion is observed in all the films. Unlike p-type Si:Ce, however, ferromagnetic or super-paramagnetic behavior is not observed in uniformly Ce-doped Si films with n-type conduction, suggesting that the hole plays an important role in the anomaly observed in magnetotransport behaviors in Ce-doped Si films.