| AVS 55th International Symposium & Exhibition | |
| Electronic Materials and Processing | Tuesday Sessions |
| Session EM-TuA |
| Session: | Complex and Multifunctional Oxides |
| Presenter: | P. Reinke, University of Virginia |
| Authors: | W. Yin, University of Virginia K. West, University of Virginia J. Lu, University of Virginia Y. Sun, Stanford University S.A. Wolf, University of Virginia P. Reinke, University of Virginia |
| Correspondent: | Click to Email |
Vanadium dioxide is investigated as potential oxide barrier in spin switches, and undergoes a first order metal-to-insulator (MIT) transition at 340 K, which will provide an actively switchable interlayer as a critical element in the device. In order to incorporate VO2 layers in a complex multi-layer devices it is necessary to understand the relation between bulk and surface/interface properties in order to optimize device performance. Our study focuses on the comparison of the MIT in the bulk and at the surface of thin VO2 layers, and establishes an irreversible modification of the crystallite structure and surface for temperatures exceeding the MIT. The surface modification impacts on the strategies which are employed to build the magnetic contact on a VO2 layer. Highly oriented VO2 thin films were grown on (0001) sapphire single crystal substrates with a novel growth technique called reactive bias target ion beam deposition. In the analysis of the VO2 films we employed bulk-sensitive methods (x-ray diffraction, transport measurements) and surface sensitive techniques (photoelectron spectroscopy, STM). The samples were subjected to a heating cycle with repeated cycling through the MIT, and annealing to at least 100 K above the MIT. The VO2 films exhibit the the transition from the monoclinic to the tetragonal phase, with the concurrent change in conductivity by 10-3. The cycling across the MIT temperature and annealing have no impact on the abruptness and magnitude of the transition, and the bulk-dominated process exhibits therefore the requisite long-term stability. The stability of the surface with respect to annealing is dramatically lower, and electronic and structural changes occur. The observation of the valence band with PES as the film transits through the MIT temperature and the subsequent annealing clearly show that the surface partially retains its high-temperature metallic character. The onset of oxygen depletion at the surface is held responsible for this behavior, and a critical issue in tailoring the interface to the top contact. In addition to the changes on an atomic level, the annealing triggers a rearrangement in the orientation and shape of the crystallites, which is shown in a quantitative analysis of the STM images of the VO2 surface. The in-situ observation of the temperature-induced changes of the surface with STM will provide additional information on the oxygen-depletion and morphological changes in the VO2 surface.