Synthesis and properties of oxynitrides (La,Sr)Ti(O,N)3 thin films

We report on the synthesis, optical and electrical properties of thin films of La_xSr_(1−x)Ti(O,N)₃, x = 0, 0.25, 0.50, 0.75 and 1. The films were grown by a soft chemistry process from polymeric precursor solutions, which were deposited by spin coating on 100-oriented SrTiO₃ substrates. After drying, the organic matrix was burned at 400 °C. These steps were repeated six times to obtain a reasonable thickness of the films. Afterwards, the samples were heated in a tube furnace at 950 °C in flowing ammonia. The oxynitride films showed different colours from red-orange for LaTiO₂N to greenish-blue for SrTiO₃:N. The films were characterised by X-ray diffraction, scanning electron microscopy, atomic force microscopy, UV–vis spectroscopy, secondary ion mass spectrometry and electrical measurements. The results show that the films are polycrystalline and have the perovskite structure. Their thickness is about 440 nm and the average roughness value is 7.5 nm. UV–vis transmittance measurements showed a strong decrease in the band gap values for the oxynitrides compared to the respective oxides. The values change from approximately 3.2 eV for the oxides to an average value of 2.4 eV for the oxynitrides. The electrical measurements indicate a change in the electrical behaviour from insulator for LaTiO₂N to semiconductor for SrTiO₃:N.