AlxSc1-xN films were deposited by reactive pulse magnetron co-sputtering from aluminum and scandium targets without additional substrate heating. With increasing incorporation of scandium on the aluminum sites of the hexagonal wurtzite structure the piezoelectric properties are drastically improved. The piezoelectric coefficient d33 is increased from about 8 pC/N for AlN without scandium up to 28 pC/N for AlxSc1-xN with about 40 % scandium.
The origins of this drastic enhancement of the piezoelectric properties are analyzed by XRD, FE-SEM and high resolution TEM investigations of the microstructure. Hardness and Young’s modulus of the layers are determined by nanoindentation measurements. It could be shown that with increasing scandium content up to 43 % the structure becomes more disordered and the c/a ratio of the wurtzite structure is decreased from 1.6 to 1.27. At scandium concentration of 43 % the wurtzite structure is completely disordered and the (001) texture is less pronounced compared to lower scandium concentrations. Simultaneously, the Young’s modulus of wurtzite phase is reduced with increasing scandium concentration from 340 GPa to 185 GPa, which can be explained by weakening of the chemical bonding. A higher polarity caused by the scandium incorporation into the wurtzite structure and the reduced Young’s modulus can explain the drastic improvement of the piezoelectric properties.
A further increase of scandium content above 50 % results in the formation of the cubic and centrosymmetric rock salt structure and therefore the complete loss of piezoelectric properties.