High-temperature stability of Ca3TaGa3Si2O14 in various atmospheres up to 1000 °C

Abstract

The effect of high-temperature annealing under various conditions on piezoelectric Ca${}_3$TaGa${}_3$Si${}_2$O₁₄ (CTGS) substrates was investigated. The substrates were exposed to Ar, N${}_2$, air, O${}_2$, high vacuum (HV) or H${}_2$ atmosphere at 800 °C and 1000 °C. A comprehensive analysis of the annealed substrates was performed by X-ray diffraction, scanning electron microscopy of the sample surface and scanning transmission electron microscopy in combination with energy dispersive X-ray spectroscopy of cross sections of the substrate surface. In addition, changes of the chemical composition and the electronic properties of the substrate surface were investigated by X-ray photoelectron spectroscopy. The analyses demonstrated a minor degradation of the surface of the CTGS during annealing at 800 °C in HV or in H${}_2$. This was strongly increased after annealing at 1000 °C. No significant deviation of the CTGS surface was observed for the other annealing atmospheres. A possible change of the chemical reactivity of the annealed substrates in combination with an oxidable material was tested using RuAl thin films deposited on the substrate, which were then annealed at 800 °C in HV. The chemical reaction between film and substrate was not avoided or reduced by the high-temperature pre-annealing. It is concluded that during high-temperature application under certain conditions, especially in HV or H${}_2$, uncovered surfaces of the CTGS degrade and lead to a reduction of the piezoelectric properties of the substrate and with this of the functionality of the device. Barrier layers are required to prevent the chemical reaction between the substrate and reactive metallic layers on top.