Hironori Fujisawa

Abstract Superior ferroelectric properties of Al-doped HfO₂ (HAO) thin films are demonstrated using flash lamp annealing (FLA). This annealing approach is a low-thermal-budget treatment that features short annealing times at the millisecond timescale. We first clarified the annealing conditions with respect to optimum ferroelectricity. The results show that 5-millisecond annealing at 1000 °C is sufficient for adequate crystallization, achieving a high 2P_r value of 17.3 μC cm⁻². By adjusting the cooling rate on the millisecond timescale during crystallization annealing, a high cooling rate of 182 °C ms⁻¹ exhibited a superior 2P_r value of 16.6 μC cm⁻², in contrast to a slow cooling rate of 12 °C ms⁻¹, which yielded a 2P_r value of 10.2 μC cm⁻². The results indicate that the control of the cooling rate is crucial for achieving an optimum 2P_r value, illustrating the potential of FLA for forming high-quality ferroelectric thin films.

Abstract We report the use of a low-thermal-budget annealing technique; flash lamp annealing (FLA), which provides an extremely short annealing time in the millisecond range, on the ferroelectric properties of Al-doped HfO₂ (HAO) films. HAO annealed at 1000 °C with 5 ms shows a higher remanent polarization value of 24.9 μC cm⁻² compared to rapid thermal annealing (RTA), without degradation of endurance. GIXRD shows a stronger peak intensity originating from the orthorhombic (o-) phase and is observed when using FLA, indicating the formation of a larger amount of the o-phase. We believe that this is a consequence of the low thermal budget of FLA, and that specifically FLA can minimize the relaxation of the compressive stress in the TiN electrodes, inducing a high tensile stress to the HAO films and therefore an enhancement of o-phase formation. These results indicate that FLA is a promising annealing method for HAO crystallization due to the enhancement of o-phase formation.

Recently, the bulk photovoltaic effect of BiFeO<inf>3</inf>(BFO) thin films has attracted much attention because of its above bandgap photovoltage for realizing novel photovoltaic devices. In this study, the epitaxial growth of 1-µm-thick Mn and Zn codoped BFO thin films has been demonstrated, and the effects of Mn and Zn codoping on the ferroelectric and bulk photovoltaic properties of the BFO thin films have been investigated. A 0.5% Mn and 0.5% Zn codoped BFO (BFMZO050) thin film on a SrRuO<inf>3</inf>-buffered vicinal-SrTiO<inf>3</inf>(001) substrate showed an atomically flat surface with a step-and-terrace structure, a low leakage current of 1.5 × 10⁻⁶A/cm²at 100 kV/cm, and well-saturated ferroelectric electric displacement–electric field (D–E) hysteresis loops. In addition, a Pt/BFMZO/Pt coplanar capacitor with an interelectrode distance of 260 µm illuminated by a violet laser (λ = 405 nm) showed an enhanced photovoltage of 145 V owing to the reduction in photoconductance by Mn and Zn codoping.

The structural evolution of high-quality 3.3–73.2-nm-thick tetragonal-like BiFeO<inf>3</inf>(T-BFO) thin films grown on LaAlO<inf>3</inf>(001) substrates and the bulk photovoltaic effect of the films were investigated. The T-BFO films were grown by rf magnetron sputtering, showing the Peudellösung fringes around the T-BFO (001) diffraction peak in X-ray diffraction θ–2θ patterns. These indicate the structural coherence between the surface and the interface in the surface normal direction of the films. High-resolution synchrotron X-ray diffraction analysis and transmission electron microscopy reveal that the lattice relaxation behavior from the M<inf>A</inf>monoclinic to M<inf>C</inf>monoclinic structure occurs as the film thickness increases. The domain structure was partly controlled by using a vicinal LAO (001) substrate along [100]. Regarding the current–voltage characteristics of the Pt/T-BFO/Pt coplanar capacitor under violet laser illumination, T-BFO films show an anomalous photovoltaic effect with an open-circuit voltage of 6.1 V and a short-circuit current of −290 pA along the [100]<inf>T-BFO</inf>direction.

Ferromagnetic magnetite (Fe<inf>3</inf>O<inf>4</inf>) thin films for magnetoelectric multiferroic applications were deposited on (200) (Bi<inf>3.25</inf>Nd<inf>0.65</inf>Eu<inf>0.10</inf>)Ti<inf>3</inf>O<inf>12</inf>(BNEuT)/(101) Nb:TiO<inf>2</inf>substrates by metalorganic chemical vapor deposition (MOCVD) using an iron(III) tris(2,2,6,6-tetramethyl-3,5-heptanedionato) precursor as the iron source. The BNEuT film utilized as a ferroelectric template material was in the form of freestanding nanoplates with narrow spaces between them. The effects of deposition conditions such as the deposition time and substrate temperature on the magnetic and structural characteristics of the Fe<inf>3</inf>O<inf>4</inf>/BNEuT composite films were investigated. All the films consisted of mostly single-phase Fe<inf>3</inf>O<inf>4</inf>with a cubic inverse-spinel structure. When deposition was carried out at temperatures of 400–420 °C, the filling rates of particles introduced into the narrow spaces between the BNEuT nanoplates exhibited high values of 76–89% including the amorphous phase. This suggested that the deposition in this temperature range made progress according to the growth mechanism of MOCVD in the surface reaction rate determining state. Room-temperature magnetic moment–magnetic field curves for Fe<inf>3</inf>O<inf>4</inf>thin films deposited at 400–500 °C for 60 min exhibited narrow rectangular hysteresis loops, indicating typical soft magnetic characteristics.

HfO<inf>2</inf>-based thin films are one of the key dielectric and ferroelectric materials in Si-CMOS LSIs as well as in oxide electronic nanodevices. In this study, we demonstrated the fabrication of a ZnO/(Hf,Zr)O<inf>2</inf>/ZnO-trilayered nanowire (NW) capacitor structure solely by metalorganic chemical vapor deposition (MOCVD). 15-nm-thick dielectric (Hf,Zr)O<inf>2</inf>and 40-nm-thick top ZnO electrode layers were uniformly grown by MOCVD on a ZnO NW template with average diameter, length, and aspect ratio of 110 nm, 10 µm, and ∼90, respectively. The diameter and aspect ratio of the resultant trilayerd NWs are 200–300 nm and above 30, respectively. The crystalline phase of HfO<inf>2</inf>and stacked the structure are also discussed.