Topotactical growth of thick perovskite oxynitride layers by nitridation of single crystalline oxides

摘要

Thick films of the perovskite-related oxynitrides LaTiO_2N, NdTiO_2N, SrNbO_2N and SrTaO_2N were synthesised by nitridation of single crystals of the corresponding oxides with general composition ASO_(3.5). The oxide crystals were obtained by optical floating zone growth. They correspond to n = 4 member of the A_nB_nO_(3n+2) family of layered perovskites and were reacted at temperatures between 900 deg C and 1050 deg C to form the oxynitrides. Electron probe microanalysis proved the presence of nitrogen in a surface layer of a few micrometer thickness. Cross-section SEM revealed additional thin stripes of oxynitride within the bulk of the crystals, indicating that nitrogen is incorporated preferably parallel to the perovskite-type layers, which in turn are connected in a zipper-type mechanism. The formation of the desired perovskite-type oxynitrides was confirmed by X-ray diffraction. Pole figure measurements proved an epitaxial orientation ASO_2N (110)[001] || ABO_(3.5) (001)[100]. The mosaicity of the oxynitrides both in polar and azimuthal direction was very small (<2 deg) indicating a nearly single crystalline quality of the surface layer. The nitridation of the crystals results in a dramatic change in colour. Optical spectroscopy revealed shifts of the absorption edge by more than 200 nm to longer wavelengths with respect to the parent oxides, corresponding to a reduction of the band gap energies by 1.4-1.8 eV.