Growth, structure, and high temperature stability of zirconium diboride thin films

Abstract

Morphologically stable, electrically conductive thin films are required for emerging harsh environment sensors that can operate at temperatures above 1000 °C. Zirconium diboride (ZrB2) is an ultrahigh temperature ceramic with a melting temperature greater than 3200 °C and in bulk form has an electrical conductivity of ∼107 S/m. Thin films of varying Zr:B ratio have been deposited on sapphire substrates by electron beam coevaporation from elemental sources. An appropriate ratio of the elemental fluxes was determined to produce nearly stoichiometric ZrB2 thin films. Films deposited at ambient substrate temperatures are amorphous as measured by x-ray diffraction, while films grown at 600 °C show textured ZrB2 nanocrystallites in an amorphous matrix, regardless of composition. When annealed in ultrahigh vacuum at 800 and 1000 °C for 1 h, nanocrystalline films show further grain growth with a 〈101¯0〉 ZrB2 preferred orientation normal to the substrate, while the low nucleation rate in the amorphous films inhibits the formation of any substantial grains. Film conductivity ranges between 0.13 and 6.3 × 105 S/m, increasing with zirconium content and crystallite grain size. Besides grain growth, no micron-scale structural or morphological changes were observed with annealing, suggesting that ZrB2 films can act as stable electrodes in high temperature environments.