Structures for reliability caps used in the manufacture of a field-effect transistor and methods for forming reliability caps used in the manufacture of a field-effect transistor. A layer comprised of a metal silicon nitride is deposited on a high-k dielectric material. The high-k dielectric material is thermally processed in an oxygen-containing ambient environment with the layer arranged as a cap between the high-k dielectric material and the ambient environment. Due at least in part to its composition, the layer blocks transport of oxygen from the ambient environment to the high-k dielectric material.