Spontaneous superlattice formation and electrical properties of Sr-excess SrTiO₃ thin film deposited on SrTiO₃(101) by dynamic aurora pulsed laser deposition

抄録

Epitaxial Sr-excess SrTiO₃ (ST) thin film (Sr/Ti = 1.41) was grown on ST(001) and ST(101) single-crystal substrates using dynamic aurora pulsed laser deposition (PLD) under a 200 mT magnetic field. The films spontaneously formed a superlattice structure comprising two layers having different concentrations of Ruddlesden–Popper (RP) planar faults. The superlattice periods of Sr-excess ST thin films deposited on ST(001) and ST(101) substrate were, respectively, 35 and 23 nm. For thin film deposited on ST(001), the in-plane lattice parameter coincided with the substrate, showing coherent growth. For thin film deposited on ST(101), coherent growth occurred along a direction ±45° declined against the substrate. The spontaneously formed superlattice was brought about by “up-hill diffusion” of spinodal decomposition. The direction of propagation of the composition wave of spinodal decomposition was regarded as perpendicular to the substrate and ±45° declined against the substrate for thin films deposited respectively on ST(001) and ST(101). The superlattice period of the thin film deposited on ST(101) (23 nm) is smaller by a factor of 1/√2 than that deposited on ST(001). This relation is explainable by the difference of the propagation direction of the composition wave. The thin film deposited on ST(001) is distorted tetragonally, whereas that on ST(101) is cubic. The Sr-excess ST thin film was also deposited on La-doped ST(101) (La-ST(101)) and La-ST(101) substrates to measure electrical properties. No change was found in the crystal structure and microstructure, irrespective of La-doping. The thin film deposited on La-ST(001) showed ferroelectricity. However, the film deposited on La-ST(101) shows no ferroelectricity. The difference of electrical properties is brought about by differences of crystal symmetry. The difference is also explainable from the perspective of thermodynamic phenomenology.