Metalorganic chemical vapor deposition of Sr x TiyO z thin films by using mixed metal precursorsO z thin films by using mixed metal precursors

Strontium titanate (Sr _x Ti _y O _z ) thin films were prepared by a chemical vapor deposition method using gaseous compounds, obtained by vaporizing a solid mixture of Sr(dpm)₂ and Ti(O-iPr)₂(dpm)₂ in one step, as the metal sources. The compositions of the films changed in proportion to the ratio of the precursors in the solid mixture, which is contrary to the case of films obtained from a mixture of individual precursor vapors. In the latter case, the film composition was not proportional to the mixing ratio of the precursors. The strontium titanate concentration in the film was changed by the deposition temperature even when the Sr/Ti ratio of the feed was fixed, i.e., the Sr/Ti ratio in the film decreased at high temperatures. An SrTiO₃ film, with an Sr/Ti ratio of 1/1, was obtained at 450 ‡C by using vapors from a solid mixture containing the metal precursors at a Sr/Ti of 1/1. The temperature, 450 ‡C in this case, was lower than that for producing the same film composition by a liquid injection method, 550 ‡C. The decomposition of the Ti and Sr precursors included in the solid mixture and possible reactions between them at elevated temperatures were investigated by thermogravimetry, differential scanning calorimetry, and mass spectrometry. When the solid mixture was heated, the Sr-O bond, that connected Sr to the dpm ligand, was dissociated at temperatures lower than 100 ‡C and the isopropoxide ligand of the Ti precursor was dissociated from the Ti atom at temperatures below 150 ‡C. At 162 ‡C, Ti(O-iPr)₂(dpm)₂ melted, forming an oligomer by reaction with surrounding Ti and Sr precursors. This reaction was confirmed by the presence of a mass peak at m/e=585, corresponding to a hetero-metallic compound containing Sr and Ti. The hetero-metallic compound vaporized at temperatures below 200 ‡C and eventually participated in the formation of a SrTiO₃ film.