Diamond film on Langasite substrate for surface acoustic wave devices operating in high frequency and high temperature

The new piezoelectric crystal langasite (La3Ga5SiO14; abbreviated as LGS) that presents a high chemical stability at high temperature is investigated as substrate for the new layered structure Diamond/LGS surface acoustic wave (SAW) devices. Theoretical study was performed in order to calculate the evolution, as a function of diamond film thickness, of phase velocity and electromechanical coupling coefficient (K2) of the Rayleigh mode and its higher modes. The calculation results show that phase velocity up to 12 000 m/s and up to 9500 m/s are obtained respectively for the mode 2 and mode 1, while the mode 0 exhibits high K2 values up to 2.9%. Experimental study was focused on the deposition of nanocrystalline diamond films on LGS substrate. First attempts show that good nanocrystalline properties may be obtained in Ar/H2/CH4 microwave discharges.