藤沢 浩訓

Abstract Flash lamp annealing (FLA) and rapid thermal annealing (RTA) were comparatively investigated to clarify their effects on the crystallinity and ferroelectric properties of Al-doped HfO ₂ (HAO) thin films. HAO films with an Al concentration of 0%–12% were deposited by atomic layer deposition and crystallized using either FLA or RTA. Grazing-incidence X-ray diffraction revealed that FLA induces stronger in-plane tensile stress in the HAO layer than RTA by suppressing stress relaxation in the TiN electrode during millisecond-scale ultrafast heating. This stress promotes the ferroelectric orthorhombic phase while suppressing the monoclinic phase. Electrical measurements showed that FLA-treated films exhibited higher remanent polarization than RTA-treated films. Under FLA, both crystallinity and polarization peaked at an Al concentration of 2%, whereas higher Al concentrations suppressed crystallization and ferroelectricity. These results indicate that FLA is an effective low-thermal-budget annealing method for high-performance HAO films with optimized Al concentration.

We identify two key strategies for enhancing polarization properties: the engineering of delta-doped interfacial layers and the application of millisecond-scale flashlamp annealing (FLA), using TiN/Al:HfO2 (HAO)/TiN ferroelectric capacitors. The delta-doped Al2O3 layer remains stably localized at the interfaces even after high-temperature annealing at 1000 °C for 5 ms. This interfacial localization between the electrodes and HAO films significantly influences both the polarization behavior and the crystallization characteristics. Delta doping at the top electrode interface yields a remanent polarization (2Pr) value approximately 10% higher than that observed by doping at the bottom interface. This enhancement is attributed to the position-dependent formation of oxygen vacancies (Vo), which vary with the location of the delta-doped layer. Grazing incidence x-ray diffraction analysis reveals that delta doping at the top interface promotes the formation of ferroelectric orthorhombic phases more effectively than in undoped structures. Furthermore, a peak angle analysis of HAO films indicates that capacitors with top interface delta doping exhibit higher diffraction angles compared to those without doping, suggesting an increased tensile stress exists within the HAO layer. FLA treatment further amplifies this tensile stress relative to rapid thermal annealing, thereby contributing to improved ferroelectric properties. These findings underscore the importance of both interfacial delta doping and low-thermal-budget FLA treatment in achieving enhanced ferroelectric performance and highlight their potential for the fabrication of high-performance ferroelectric devices.

Abstract Graphene transfer onto a ferroelectric Mn-doped BiFeO₃ (BFMO)/SrRuO₃ (SRO)/SrTiO₃ (001) (STO) heterostructure via a self-assembled monolayer (SAM) was systematically investigated. 3-aminopropyltriethoxysilane (APTES), a widely recognized silane coupling agent, was employed as the SAM, and the BFMO surface was chemically modified with APTES. A polarization-dependent enhancement in graphene transfer was observed with the application of APTES, and high-quality graphene was obtained only on BFMO with a spontaneous polarization ( P _s ) oriented along [ ]_pc. Furthermore, the photovoltaic characteristics of the graphene/BFMO/SRO heterostructure were evaluated. The graphene acted as a transparent electrode, and the open-circuit voltage exhibited a clear increase in the SAM-modified sample compared with the sample without SAM modification.

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.