Impact of film composition on crystal structure and ferroelectricity in (Al1−x−yGaxScy)N ternary wurtzite thin films

The wurtzite-structure ternary alloys of (Al,Ga,Sc)N films with a high Sc/(Al+Ga+Sc) ratio are successfully deposited by a reactive magnetron sputtering method. The breakthrough in ScN solid-solution limit in AlN with wurtzite structure [up to Sc/(Al + Ga + Sc) = 0.47] is achieved, and its wurtzite-phase diagram region is expanded by adding a relatively small amount of Ga [Ga/(Al+Ga+Sc) < 0.1] into the (Al,Sc)N film. The c- and a-axes lattice constants depend strongly on Ga/(Al+Ga+Sc) and Sc/(Al+Ga+Sc) ratios, respectively, and the Sc/(Al+Ga+Sc) ratio primarily determines the internal structure showing a crystal anisotropy parameter, u parameter. The dielectric constant increases with the increase of Sc/(Al+Ga+Sc) ratio. In addition, the coercive field significantly decreases from 5.8 to 1.8 MV/cm with an increasing Sc/(Al+Ga+Sc) ratio from 0.33 to 0.47, while polarization slightly reduces with the increased u parameter. This result provides a new way to expand the content of the ScN solid solution in the wurtzite film to further decrease the coercive field for the memory device applications.