Electrical Switching in Thin Film Structures Based on Molybdenum Oxides

We report on the experimental study of electrical instabilities in thin film structures on the basis of molybdenum oxides. Thin films of molybdenum oxide are obtained by thermal vacuum evaporation and anodic oxidation. The results of X-ray structural analysis, investigation of optical and electrical properties, are presented. It is shown that the initial vacuum-deposited oxide represents amorphous MoO3. In the MOM (metal-oxide-metal) structures with Mo oxide films obtained by the two methods, the effect of electrical switching with an S-shaped current-voltage characteristic is found. We put forward a hypothesis according to which the switching mechanism is associated with the development of electrical instability caused by the insulator-to-metal transition in Mo8O23. The switching channel, comprising this lower valence oxide, emerges in the initial film during the process of electrical forming of the MOM structure. The obtained results indicate the possibility of application of these structures in oxide micro- and nanoelectronics as electronic switches and other electronic devices. 1. Introduction Transition metal oxides (TMO) represent one of the most promising classes of substances with regard to their use in designing electronic components [1]. Molybdenum-oxygen system, like most TMOs, forms a number of oxides with variable valence of the cation, which are characterized by a variety of structural and physicochemical properties [1–3]. The basic binary molybdenum oxides are MoO2 and MoO3. Molybdenum dioxide crystallizes in the monoclinic structure which is formed by infinite chains of distorted octahedra MoO6, faces connected to each other due to van der Waals forces. Molybdenum trioxide MoO3 has a layered structure, whose main element is a double octahedron with a MoO6 link [2–5]. Molybdenum trioxide has several polymorphs, and the most thermodynamically stable of them is -MoO3 (space group Pnma). By lowering the ratio of oxygen to metal content down to 2.9 and below, one of the seven stable or metastable suboxides can form: Mo18O52, Mo9O26, Mo8O23, Mo5O14, and Mo17O47, as well as η- and γ-Mo4O11 [3]. Stoichiometric MoO3 is an insulator with a band gap of ~3？eV [1]. During reduction by means of either oxygen deficiency formation or introducing donor impurity atoms, additional donor levels near the conduction band bottom appear and the reduced oxide behaves therefore as a semiconductor. Polycrystalline MoO3, as well as a single crystal, has a gray-yellow color and the minimum value of the absorption edge is 440？nm which corresponds to the smallest