Structural characterization of niobium oxide thin films grown on SrTiO_3 (111) and (La,Sr)(Al,Ta)O_3 (111) substrates

摘要

Niobium oxide thin films have been grown by molecular beam epitaxy on SrTiO_3 (STO) (111) and (Lao_(0.18)Sr_(0.82))(Al_(0.59)Ta_(0.41))O_3 (LSAT) (111) substrates. Transmission electron microscopy (TEM) confirmed the formation of high quality films with coherent interfaces. Films grown with higher oxygen pressure on STO (111) resulted in a (110)-oriented NbO_2 phase with a distorted rutile structure, which can be described as body-centered tetragonal. The a lattice parameter of NbO_2 was determined to be ～ 13.8 A in good agreement with neutron diffraction results published in the literature. Films grown on LSAT (111) at lower oxygen pressure produced the NbO phase with a defective rock salt cubic structure. The NbO lattice parameter was determined to be a ≈ 4.26 Å. The film phase/structure identification from TEM was in good agreement with in situ x-ray photoelectron spectroscopy measurements that confirmed the dioxide and monoxide phases, respectively. The atomic structure of the NbO_2/STO and NbO/LSAT interfaces was determined based on comparisons between high-resolution electron micrographs and image simulations.