Electrically conductive Pt-Rh/ZrO_2 and Pt-Rh/HfO_2 nanocomposite electrodes for high temperature harsh environment sensors

摘要

Nanocomposite films comprised of either Pt-Rh/ZrO_2 or Pt-Rh/HfO_2 materials were co-deposited using multiple e-beam evaporation sources onto langasite (La_3Ga_5SiO_(14)) substrates, both as blanket films and patterned interdigital transducer electrodes for surface acoustic wave (SAW) sensor devices. The films and devices were tested after different thermal treatments in a tube furnace up to 1200°C. X-ray diffraction and electron microscopy results indicate that Pt-Rh/HfO_2 films are stabilized by the formation of monoclinic HfO_2 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/ZrO_2 films were found to be slightly less stable, and contain both tetragonal and monoclinic ZrO_2 precipitates that also help prevent Pt-Rh agglomeration. Film conductivities were measured versus temperature for Pt-Rh/HfO_2 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 Al_2O_3 diffusion barrier grown on langasite using atomic layer deposition (ALD) was found to be effective in minimizing interdiffusion between the nanocomposite film and the langasite crystal.