藤沢 浩訓

A intermediate multidomain state and large crystallographic tilting of 1.78° for the (hh0)pc planes of a (001)pc-oriented single-domain Mn-doped BiFeO3 (BFMO) thin film were found when an electric field was applied along the [110]pc direction. The anomalous crystallographic tilting was caused by ferroelastic domain switching of the 109° domain switching. In addition, ferroelastic domain switching occurred via an intermediate multidomain state. To investigate these switching dynamics under an electric field, we used in situ fluorescent X-ray induced Kossel line pattern measurements with synchrotron radiation. In addition, in situ inverse X-ray fluorescence holography (XFH) experiments revealed that atomic displacement occurred under an applied electric field. We attributed the atomic displacement to crystallographic tilting induced by a converse piezoelectric effect. Our findings provide important insights for the design of piezoelectric and ferroelectric materials and devices.

HfO2-based ferroelectric materials do not necessarily require high-temperature annealing for crystallization, making them attractive for applications in transparent electronic devices on plastic or glass substrate. In this study, (Hf, Zr)O2 (HZO) films prepared via non-heating sputtering are investigated and their application to ferroelectric-gate thin-film transistors (TFTs) is demonstrated. The internal tensile stress induced by (In, Sn)O x (ITO) top-electrode deposition is found to promote the crystallization of HZO from the amorphous state to the ferroelectric phase. ITO/HZO (15-25 nm)/ITO capacitors prepared via the non-heating process exhibit ferroelectric hysteresis loops with remanent polarizations of 6-9 μC cm-2 and coercive fields of 0.6-1.1 MV cm-1. Ferroelectric-gate TFTs with a 10 nm thick ITO channel are also fabricated via the non-heating process. These TFTs show nonvolatile operation with an on/off ratio of ∼10. These findings demonstrate the potential of HZO for transparent devices on substrates with low thermal resistance prepared via the non-heating process.

Bismuth ferrite (BiFeO3: BFO) is a multiferroic material that exhibits ferroelectricity, antiferromagnetism, and ferroelasticity simultaneously at RT. BFO holds great promise as a ferroelectric semiconductor because of its ability to alter conductivity by reversing its spontaneous polarization. Moreover, BFO thin films doped with transition metals such as Mn or V can modulate their conductivity. Nevertheless, the mechanism of this conductivity change remains unclear because the effects of dopants on the local atomic structure of BFO are not fully understood. In this study, we investigated the local atomic structure around the Fe site in a V-doped BFO thin film by X-ray fluorescence holography. Reconstructed atomic structures from the Fe Kα hologram patterns revealed that the atomic structure stability of the V-doped BFO thin film differs from that of previously reported Mn-doped BFO thin films. The results provide important insights into the mechanism of controlling the conductivity of BFO thin films by dopants.

Recently, ferroelectric semiconductors has become a subject of interest with regard to potential applications in novel electronic and opto-electric devices. One of the most important aspects of employing these materials is band modulation based on spontaneous polarization to generate polarization charges acting as quasi-dopants at metal/ferroelectric and ferroelectric/ferroelectric interfaces. The present study fabricated graphene/Mn-doped BiFeO3 (BFMO)/SrRuO3/SrTiO3(001) capacitor structures with the BFMO having either upward or downward polarization. Band modulation at the graphene/BFMO interface as a result of polarization charges was evaluated using photoemission electron microscopy on the BL17SU beamline at the SPring-8 facility, Japan. The chemical shifts observed in Bi 4f and C 1s XPS spectra indicated that positive (negative) polarization charges acted as quasi-dopants for electron (hole) doping of the BFMO and graphene.

Ferroelectric gate-all-around (GAA) transistors with a nanowire (NW) channel standing vertically on the substrate would be a potential breakthrough to overcome limitations in the high-integration of ferroelectric memories. In the present study, we fabricated vertical ferroelectric GAA NW transistors (VFGAANWTs) with ZnO NWs (average diameters: 53-193 nm) as the channel, (Hf,Zr)O2 film (average thicknesses: 9.3-58 nm) as the gate ferroelectric, and Ti as the gate electrode. The channel length was 100-300 nm. The VFGAANWTs showed n-channel operation with on/off ratios of ∼10, and their on/off states were retained for at least 2 min by the ferroelectric polarization of (Hf,Zr)O2 after the gate bias was removed, demonstrating the nonvolatile electric field effect in the VFGAANWTs. The results confirm the strong potential of the VFGAANWTs in high-density ferroelectric memory applications.