Artificially Designed Compositionally Graded Sr-Doped NaTaO₃ Single-Crystalline Thin Films and the Dynamics of Their Photoexcited Electron–Hole Pairs

Sr-doped NaTaO3 (NTO) single-crystalline films with different Sr compositions of up to 20 mol %, including an NTO film, were first fabricated on LaAlO3(001) substrates by pulsed laser deposition. A gradual simultaneous increase of the lattice constants and the optical absorption edge energy was found as the doped Sr composition increased, seemingly exhibiting a structural discontinuity at a critical doping level of around 10 mol %. Extended X-ray absorption fine structure (EXAFS) analysis revealed that the doped Sr atoms were substituted for both the A and B sites, with their stoichiometric number ratio close to that previously reported for bulk Sr-doped NTO samples, resulting in the Sr-doped NTO films being still highly insulative comparable to the NTO film. Second, in spite of the well-known intrinsic nature of the high diffusivity of Sr ions in oxides, the compositionally graded Sr-doped NTO films were successfully fabricated almost as designed to have the upward and downward Sr composition gradients with a maximum doping level of 10 mol % from the substrate along the growth direction within the films. The preliminary time-resolved microwave conductivity measurement demonstrated that the lifetime of the photoexcited carriers in the compositionally graded Sr-doped NTO films was longer than in any of the NTO and homogeneous Sr-doped NTO films.