AlN films are employed in RF filters for wireless communication. We report on enhanced coupling factors k(t)(2) obtained by partial substitution of Al by Sc. Al0.88Sc0.12N films were deposited by reactive magnetron sputtering from an Al0.9Sc0.1 alloy target. They grew in the piezoelectric wurtzite phase with a similar microstructure as pure AlN films. The clamped d(33,f) increased considerably from 5.1 to 7.8 pm/V. The admittance measured at thin film bulk acoustic wave resonators was fitted to an equivalent circuit model and to 2-dimensional finite element simulation, yielding a k(t)(2) of 7.3% and a quality factor of 650. The material softens considerably. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3629773]