An oxide or nitride semiconductor layer is formed over a substrate. A first conductive layer including a first element and a second element, and a second conductive layer including the second element are formed over the semiconductor layer. The first element is oxidized or nitrogenized near an interface region between the first conductive layer and the oxide or nitride semiconductor layer by heat treatment or laser irradiation. The Gibbs free energy of oxide formation of the first element is lower than those of the second element or any element in the oxide or nitride semiconductor layer.