The structures and methods disclosed herein include changing composition of a metal alloy layer in an epitaxial electrode material to achieve tunable work functions for the electrode. In one example, the tunable work function is achieved using a layered structure, in which a crystalline rare earth oxide (REO) layer is epitaxially over a substrate or semiconductor, and a metal layer is over the crystalline REO layer. A semiconductor layer is thus in turn epitaxially grown over the metal layer, with a metal alloy layer over the semiconductor layer such that the ratio of constituents in the metal alloy is used to tune the work function of the metal layer.