The present disclosure describes a method for forming a barrier structure between liner-free conductive structures and underlying conductive structures. The method includes forming openings in a dielectric layer disposed on a contact layer, where the openings expose conductive structures in the contact layer. A first metal layer is deposited in the openings and is grown thicker on top surfaces of the conductive structures and thinner on sidewall surfaces of the openings. The method further includes exposing the first metal layer to ammonia to form a bilayer with the first metal layer and a nitride of the first metal layer, and subsequently exposing the nitride to an oxygen plasma to convert a portion of the nitride of the first metal layer to an oxide layer. The method also includes removing the oxide layer and forming a semiconductor-containing layer on the nitride of the first metal layer.