Artificially Designed Compositionally Graded Sr-Doped NaTaO<sub>3</sub> Single-Crystalline Thin Films and the Dynamics of Their Photoexcited Electron–Hole Pairs
Rio Konno, Shingo Maruyama, Takumu Kosaka, Ryuzi Katoh, Ryota Takahashi, Hiroshi Kumigashira, Nobuyuki Ichikuni, Hiroshi Ōnishi, Yuji Matsumoto
Abstract
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.