A semiconductor structure with a metal gate structure includes a first type field-effect transistor having a first gate including: a high k dielectric material on a substrate, a first metal layer on the high k dielectric material layer and having a first work function, and a first aluminum layer on the first metal layer. The first aluminum layer includes an interfacial layer including aluminum, nitrogen and oxygen. The device also includes a second type field-effect transistor having a second gate including: the high k dielectric material on the substrate, a second metal layer on the high k dielectric material layer and having a second work function different from the first work function, and a second aluminum layer on the second metal layer.