Effect of scandium content on structure and piezoelectric properties of AlScN films deposited by reactive pulse magnetron sputtering

AlxSc1-xN films were deposited by reactive pulse magnetron co-sputtering from aluminum and scandium targets without additional substrate heating at deposition rates between 100 and 150 nm/min. With increasing incorporation of scandium into the hexagonal wurtzite structure, the piezoelectric properties are drastically improved. The piezoelectric charge coefficient d(33) is increased from 8.4 pC/N for AlN up to 23.6 pC/N for AlxSc1-xN with 33% scandium. Between 35 and 43% scandium content a relative broad maximum with high piezoelectric coefficients between 26.9 and 273 pC/N was detected. A further increase of scandium concentration above 50% results in the formation of the cubic and centrosymmetric rock salt structure and therefore the complete loss of piezoelectric properties. By FE-SEM, XRD and TEM investigations it was shown that up to up to 43% scandium concentration the wurtzite structure becomes more and more disordered and the c/a ratio is decreased from 1.6 to 1.27. Nevertheless, no aluminum or scandium segregation could be detected by high resolution TEM investigations. The Young's modulus of the wurtzite phase is reduced with increasing scandium concentration from 340 GPa to 185 GPa. The drastic improvement of the piezoelectric properties can be explained by weakening of the chemical bonding and by high distortion of the wurtzite structure. (C) 2016 Elsevier B.V. All rights reserved.