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

Abstract

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.