Microstructure of ruthenium dioxide films grown on α-Al₂O₃ (0001), α-Al₂O₃ (1102), and SrTiO₃ (100) using reactive sputtering

A quantitative study was made of the composition and microstructure of RuO₂ films deposited on three different substrates using reactive sputtering. Most of the films had a composition within 2.5 wt. % of the correct stoichiometry; the only exceptions were films grown on Al₂O₃ (0001) at 150 °C, which had an oxygen-to-ruthenium ratio of 1 : 2.24. The excess oxygen was attributed to a thin oxygen-rich layer that encapsulated the grains. Hydrogen concentrations for the films deposited on Al₂O₃ (0001) were 14, 6, 6, and <0.5 at. % for room, 150, 300, and 450 °C growth temperatures respectively. The films deposited at room temperature were amorphous on Al₂O₃ (0001) and SrTiO₃ (100), but weakly crystalline on Al₂O₃ (1102). Highly oriented RuO₂ (100) films were produced on Al₂O₃ (0001) at deposition temperatures ≥150 °C. The in-plane alignment was [010]RuO₂//〈2110〉Al₂O₃ and a threefold mosaic microstructure was observed. The grain boundaries in these films were discontinuous until the substrate temperature was raised to 450 °C, where coherent grain boundaries were formed. The films grown on Al₂O₃ (1102) at 450 °C exhibited the epitaxial relationship: RuO₂(101)//Al₂O₃ (1102). The in-plane alignment was RuO₂〈101〉//Al₂O₃(1101), and the lattice parameters were the same as found in bulk RuO₂. Transmission electron microscopy indicated a large degree of imperfection in the region between coalescing grains. The RuO₂ films grown on SrTiO₃ (100) at room temperature were amorphous. The film grown at 450 °C showed a preferential orientation with RuO₂ (100)//SrTiO₃ (100), but without in-plane orientation.