Takeshi Kawae

Improved leakage and ferroelectric properties of Mn and Ti codoped BiFeO3 thin films

Polycrystalline BiFeO3 (BFO), Ti-doped BFO, Mn-doped BFO, and (Mn, Ti)-codoped BFO (BFMT) thin films were fabricated on Pt/SrTiO3 (100) substrate by pulsed laser deposition. Observed leakage current behavior in those ion-doped BFO films indicated the dominance of space-charge-limited current in the high electric field region. The leakage current of the BFMT film was much reduced in relation to the other films due to the formation of deep traps. In the BFMT film, well saturated P-E hysteresis curves were observed. Remanent polarization and coercive field for maximum electric field of 2100 kV/cm were 75 μC/cm2 and 310 kV/cm, respectively.

Composition Dependence in BiFeO3 Film Capacitor with Suppressed Leakage Current by Nd and Mn Cosubstitution and Their Ferroelectric Properties

(Nd,Mn)-cosubstituted BiFeO3 (BFO) films with various Nd and Mn compositions were fabricated on a Pt/SrTiO3(100) substrate by pulsed laser deposition. X-ray diffraction patterns and atomic force microscopy images indicated that the suppression of impurity phases and a smooth surface morphology were realized by Nd substitution in the BFO films. Furthermore, by combining with Nd substitution, a small amount of Mn substitution in BFO films is effective for reducing the leakage current density by three orders of magnitude compared with that of a BFO film capacitor. The polarization vs electric field (P–E) curves showed a strong dependence of measurement frequency in the range of 0.1–2 kHz, and well-saturated P–E hysteresis loops were observed at 20 kHz at room temperature. The remanent polarization and coercive field at a maximum electric field of 1.9 MV/cm were approximately 70 µC/cm2 and 0.32 MV/cm, respectively.

Reduced Leakage Current and Ferroelectric Properties in Nd and Mn Codoped BiFeO3 Thin Films

Polycrystalline BiFeO3 (BFO), Nd-doped (BNF), Mn-doped (BFM), and (Nd,Mn)-codoped BiFeO3 (BNFM) thin films were fabricated on Pt/SrTiO3(100) substrate by pulsed laser deposition. According to the X-ray diffraction analysis and atomic force microscope observation, suppression of impurity phases and improvement of surface morphology were realized in the BNFM film. In the BNFM film, the leakage current was reduced compared with BFO, BNF, and BFM films, and P–E hysteresis curves were observed with measurement frequency of 100 Hz at room temperature. The remanent polarization and coercive field were approximately 100 µC/cm2 and 250 kV/cm, respectively.

Polarization-Induced Photovoltaic Effects in Nd-Doped BiFeO3 Ferroelectric Thin Films

Nd-doped BiFeO3 (BNF) thin films were fabricated on SrRuO3 (SRO)-coated (100) Nb-doped SrTiO3 substrates by pulsed laser deposition, and nondoped BiFeO3 (BFO) thin films were also fabricated similarly for comparison. Then, Nd-doping effects on ferroelectric and photovoltaic properties were evaluated. Polarization-induced photovoltaic effects were observed in both the BFO and BNF solar cell structures with top and bottom electrodes under intense laser illumination. Using Au top electrodes, enhanced photovoltaic properties were observed in the BNF cell compared with the BFO cell. To improve the photovoltaic properties of the BNF cell, instead of the Au top electrodes, In–Sn–O (ITO) top electrodes were employed for the BNF cell. As a result, the photovoltaic properties were found to be markedly improved, resulting in an open circuit voltage of 0.81 V and a short circuit current density of 12.1 mA/cm2.

Electrical characteristics of MoS2 field-effect transistor with ferroelectric vinylidene fluoride-trifluoroethylene copolymer gate structure

Ferroelectric field-effect transistors (FeFET) based on MoS2 have recently been shown to exhibit considerable potential for use in nano sized non-volatile memory devices. Here, we demonstrated fabrication and characterization of FeFET based on MoS2 channel with vinylidene fluoride (VDF)-trifluoroethylene (TrFE) copolymer as back-gate insulator. In this device, counterclockwise hysteresis behavior was observed in the drain current–gate voltage curve, which is indicative of interaction between MoS2 carrier modulation and ferroelectric polarization switching. Furthermore, our VDF-TrFE/MoS2 FeFET exhibited only n-type behavior, a maximum linear mobility of 625 cm2/V s, a large memory window width of 16 V, and a high on/off current ratio of 8 × 105.

Optical Properties of BiFeO3-System Multiferroic Thin Films

The optical properties of the chemical solution deposition (CSD)-derived BiFeO3 (BFO) film and the pulsed laser deposition (PLD)-derived (Bi,Nd)(Fe,Mn)O3 (BNFM) film were evaluated by spectroscopic ellipsometry, and their optical constants (n, k) and band gaps were determined. At a wavelength of 600 nm, the refractive indexes of 3.22 and 2.87 were estimated for the BFO and BNFM films, respectively, although the existence of a refractive index gradient was suggested in the BFO film. In addition, at a wavelength of 1550 nm, which is generally used for optical communication, the refractive indexes of 2.91 and 2.59 were estimated for the BFO and BNFM films, respectively. The band gaps of the BFO and BNFM films were estimated to be 2.79 and 2.72 eV, respectively, and it was confirmed that the extinction coefficients of both films were almost zero at wavelengths larger than 600 nm. These results suggest that the BFO-system multiferroic films have a high potential as an optical material with a high refractive index.

Sub-micron sized intrinsic Josephson junctions in YBa2Cu3O7-X whiskers

To investigate the detailed junction properties of intrinsic Josephson junctions (IJJs) in YBa2Cu3O7−X (YBCO) without shunted grain boundaries and with good uniformity, we fabricated sub-micron sized IJJs in a YBCO whisker by using a focused-ion-beam etching. The sub-micron sized junctions showed clear multiple branches with a hysteresis structure in the current–voltage curve, like IJJs in Bi2Sr2CaCu2O8+X. The temperature dependence of the critical current demonstrates that the junction behaviour drastically changes from that of an SNS type to an SIS type when the critical temperature is decreased. The critical current density of the junctions, in the order of 10–100 kA cm−2, corresponds to Josephson plasma frequencies from a few hundred GHz to 1–2 THz.

Influence of SrRuO3 Bottom Electrode Thickness on Electric Properties of (Bi,Pr)(Fe,Mn)O3 Ultra-Thin Film Capacitor

(Pr,Mn)-codoped BiFeO3 (BPFM) ultra-thin films of 85 nm thickness were deposited on a SrRuO3 (SRO)-coated (100) Nb-doped SrTiO3 substrate by pulsed laser deposition. Regardless of the SRO film thickness, the SRO and BPFM thin films showed perfectly (h00)-orientation on the substrate. On the other hand, the surface morphology and leakage current of the BPFM films strongly depended on the SRO film thickness. The BPFM ultra-thin film on a 115 nm thick SRO electrode shows a hysteresis loop without any influences of leakage current. The remnant polarization 2Pr and the coercive voltage 2Vc at the maximum applied voltage of 10 V are 82 µC/cm2 and 7.2 V, respectively.

Etching rate, optical transmittance, and charge trapping characteristics of Al-rich Al2O3 thin film fabricated by rf magnetron cosputtering

Radio-frequency magnetron cosputtering by setting an Al metal plate on an Al2O3 target is an effective method for fabricating Al-rich Al2O3 thin film, which is used as a charge storage layer for nonvolatile Al2O3 memory. The wet etching rate of the fabricated AlO film increases with Al content. The optical transmittance decreases when the Al content in AlO increases. Using rf magnetron cosputtering, the authors fabricated an Al2O3 film with an embedded Al-rich Al2O3 and investigated the charge trapping characteristics after a 30 min annealing at 550 °C. C-V characteristics indicate that the Al-rich Al2O3 structure is very stable after annealing. The charge trap density in Al-rich Al2O3 after annealing is much larger than that in stoichiometric Al2O3.

Influences of low-temperature postdeposition annealing on memory properties of Al/Al2O3/Al-rich Al-O/SiO2/p-Si charge trapping flash memory structures

Charge trapping flash (CTF) memory structure was fabricated on a SiO2/p-Si substrate using Al-rich Al-O as a charge trapping layer. Capacitance–voltage curves of the specimen indicated a large memory window width of 4.8 V with a maximum operation voltage of 7 V due to the charge trapping in the Al-rich Al-O layer. With N2 postdeposition annealing treatment at 350 °C, charge retention characteristics of the CTF memory structure were drastically improved without any serious loss in charge trap density. Furthermore, the proposed structure showed excellent memory characteristics, such as endurance and stable data retention even at 100 °C.