An advanced bilayer gate dielectric stack consisting of Sc2 O3 La2 O3 Si Ox annealed in nitrogen at 300 °C was studied by scanning tunneling microscopy using bias dependent imaging. By changing the sample bias, electrical properties of different layers of the dielectric stack can be studied. At a sample bias of +3.5 V, the conduction band of the La2 O3 layer is probed revealing a polycrystalline film with an average grain size of about 27 nm, in good agreement with that determined from planar transmission electron microscopy. High conductivity at grain boundaries, due possibly to dangling bonds, can be observed in this layer, as also observed in grain boundary assisted current conduction in metal-oxide-silicon structures. Imaging at a sample bias of -4 V probes the interfacial Si Ox layer and an amorphouslike image of the interfacial layer is obtained.