High temperature stability of electrically conductive Pt–Rh/ZrO₂ and Pt–Rh/HfO₂ nanocomposite thin film electrodes

Nanocomposite films made up of either Pt–Rh/ZrO₂ or Pt–Rh/HfO₂ materials were co-deposited using multiple e-beam evaporation sources onto langasite (La₃Ga₅SiO₁₄) substrates, both as blanket films and as patterned interdigital transducer electrodes for surface acoustic wave sensor devices. The films and devices were tested after different thermal treatments in a tube furnace up to 1,200 °C. X-ray diffraction and electron microscopy results indicate that Pt–Rh/HfO₂ films are stabilized by the formation of monoclinic HfO₂ precipitates after high temperature exposure, which act as pinning sites to retard grain growth and prevent agglomeration of the conductive cubic Pt–Rh phase. The Pt–Rh/ZrO₂ films were found to be slightly less stable, and contain both tetragonal and monoclinic ZrO₂ precipitates that also helps prevent Pt–Rh agglomeration. Film conductivities were measured versus temperature for Pt–Rh/HfO₂ films on a variety of substrates, and it was concluded that La and/or Ga diffusion from the langasite substrate into the nanocomposite films is detrimental to film stability. An Al₂O₃ diffusion barrier grown on langasite using atomic layer deposition was found to be more effective than a SiAlON barrier layer in minimizing interdiffusion between the nanocomposite film and the langasite crystal at temperatures above 1,000 °C.