A SrRuO3 thin film has been widely used as a metal electrode in electronic devices based on transition metal oxides, and hence it is important to understand its thermal transport properties to minimize a thermal degradation problem during the device operation. Using the time-domain thermoreflectance measurement technique, we investigate the cross-plane thermal conductivity of the SrRuO3 thin films with a thickness variation from 1 μm to 8 nm. We find that the thermal conductivity is reduced from about 6 W m−1 K−1 for the 1 μm thick film to about 1.2 W m−1 K−1 for the 8 nm thick film, and attribute this behavior to the boundary scattering of thermal carriers which originally have the mean free path of about 20 nm in a bulk state. Also, we observe a clear dip behavior of the thermal conductivity in the intermediate thickness around 30 nm which suggests an existence of a strong scattering source other than the film boundary. We explain this result by considering an additional interfacial scattering at the tetragonal-orthorhombic phase boundary which is formed during the strain relaxation with an increase of the film thickness.
KEYWORDS.ferroelectric polarization rotation, LiNbO3 thin films, vacancy engineering, spatial inversion symmetry breaking, second harmonic generation.
ABSTRACT.The direction of ferroelectric polarization is prescribed by the symmetry of the crystal structure.Therefore, rotation of the polarization direction is largely limited, despite the opportunity it offers in understanding important dielectric phenomena such as piezoelectric response near the morphotropic phase boundaries and practical applications such as ferroelectric memory. In this study, we report the observation of continuous rotation of ferroelectric polarization in orderdisorder type LiNbO3 thin films. The spontaneous polarization could be tilted from an out-of-plane to an in-plane direction in the thin film by controlling the Li vacancy concentration within the hexagonal lattice framework. Partial inclusion of monoclinic-like phase is attributed to the breaking of macroscopic inversion symmetry along different directions and the emergence of ferroelectric polarization along the in-plane direction.
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