Microstructure and piezoelectric properties of c-axis ScAlN films on the Y-128° LiNbO3 substrate

• This work successfully deposited highly c-axis orientated ScAlN films on Y-128o LiNbO3 substrates by a co-sputtering technique.
• Sc was doped into AlN lattice to promote the piezoelectricity of thin films. The phase transition transformed from hexagonal (AlN) to cubical (ScN) structure with increased Sc concentration. At the Sc content of 14.7 % at., the d33 increase to 13.28 pC/N, which is nearly two times larger than pure AlN.
• This study discuss the effect of RTA process on the crystallinity and the piezoelectricity of ScAlN films. After the RTA treatment, the d33 achieved highest value (18.26 pC/N) which is 37.5 % higher than that without RTA process.

Highly c-axis preferred orientation ScAlN films were deposited on Y-128° lithium niobate using the co-sputtering technique. The substitution of Sc atoms at Al positions causes a lattice distortion and induces a large piezoelectric response. X-ray diffraction patterns indicated that most of the films showed (002) peaks and the best crystallinity appeared at the Sc content of 9.0 at.%. The cross-section of TEM results showed that the higher c-axis orientation columnar structure formed as the film thickness increased, and the structure transforms from wurtzite to cubical structure from the nano-beam diffraction analysis. Then the piezoelectric coefficient would be enhanced obviously at the Sc content of 18.1 at.% and the value is nearly two times larger than pure AlN. After rapid thermal annealing treatment, the crystal quality could be improved and the highest value of piezoelectric coefficient was achieved at 18.26 pC/N.