Epitaxial RuO2 and IrO2 films by pulsed laser deposition on TiO2(110)

We present a systematic growth study of epitaxial RuO2(110) and IrO2(110) on TiO2(110) substrates by pulsed laser deposition. We describe the main challenges encountered in the growth process, such as a deteriorating material flux due to laser-induced target metallization or the delicate balance of under- vs over-oxidation of the “stubborn” Ru and Ir metals. We identify growth temperatures and oxygen partial pressures of 700 K, 1 × 10−3 mbar for RuO2 and 770 K, 5 × 10−4 mbar for IrO2 to optimally balance between metal oxidation and particle mobility during nucleation. In contrast to IrO2, RuO2 exhibits layer-by-layer growth up to 5 unit cells if grown at high deposition rates. At low deposition rates, the large lattice mismatch between film and substrate fosters initial 3D island growth and cluster formation. In analogy to reports for RuO2 based on physical vapor deposition [He et al., J. Phys. Chem. C 119, 2692 (2015)], we find these islands to eventually merge and grow to continue in a step flow mode, resulting in highly crystalline, flat, stoichiometric films of RuO2(110) (up to 30 nm thickness) and IrO2(110) (up to 13 nm thickness) with well-defined line defects.