The metal-insulator transition in vanadium dioxide: A view at bulk and surface contributions for thin films and the effect of annealing

Journal Article · Mon Jun 01 00:00:00 EDT 2009 · Journal of Applied Physics

DOI:https://doi.org/10.1063/1.3143787· OSTI ID:21352239

Yin, W; West, K G; Lu, J W; Pei, Y; Wolf, S A; Reinke, P ^[1]; Sun, Y ^[2]

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1. University of Virginia, Virginia 22904 (United States)
2. Stanford Synchrotron Radiation Lightsource, California 94025 (United States)

Vanadium dioxide is investigated as potential oxide barrier in spin switches, and in order to incorporate VO{sub 2} layers in complex multilayer devices, it is necessary to understand the relation between bulk and surface/interface properties. Highly oriented VO{sub 2} thin films were grown on (0001) sapphire single crystal substrates with reactive bias target ion beam deposition. In the analysis of the VO{sub 2} films, bulk-sensitive methods [x-ray diffraction (XRD) and transport measurements] and surface sensitive techniques [photoelectron spectroscopy (PES) and scanning tunneling microscopy and spectroscopy] were employed. The samples were subjected to heating cycles with annealing temperatures of up to 425 and 525 K. Prior to annealing the VO{sub 2} films exhibit the transition from the monoclinic to the tetragonal phase with the concurrent change in conductivity by more than a factor of 10{sup 3} and their phase purity is confirmed by XRD. Annealing to 425 K and thus cycling across the metal-insulator transition (MIT) temperature has no impact on the bulk properties of the VO{sub 2} film but the surface undergoes irreversible electronic changes. The observation of the valence band with PES during the annealing illustrates that the surface adopts a partially metallic character, which is retained after cooling. Annealing to a higher temperature (525 K) triggers a modification of the bulk, which is evidenced by a considerable reduction in the MIT characteristics, and a degradation in crystallite morphology. The local measurement of the conductivity with scanning tunneling spectroscopy shows the transition of the surface from predominantly semiconducting surface prior to annealing to a surface with an overwhelming contribution from metallic sections afterward. The spatial distribution of metallic regions cannot be linked in a unique manner to the crystallite size or location within the crystallites. The onset of oxygen depletion at the surface is held responsible for this behavior. The onset of bulk modification at higher temperatures is most likely linked to oxygen loss and effusion along the grain boundaries and concurrent onset of sintering. Our study focuses on the comparison of the MIT in the bulk and at the surface of thin VO{sub 2} 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 will be employed to build the metallic contacts to VO{sub 2} layers.

• OSTI ID: 21352239
• Journal Information: Journal of Applied Physics, Vol. 105, Issue 11; Other Information: DOI: 10.1063/1.3143787; (c) 2009 American Institute of Physics; ISSN 0021-8979
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
ANNEALING
DEPOSITION
ELECTRIC CONDUCTIVITY
GRAIN BOUNDARIES
ION BEAMS
MONOCLINIC LATTICES
MONOCRYSTALS
MORPHOLOGY
OXYGEN
PHASE TRANSFORMATIONS
PHOTOELECTRON SPECTROSCOPY
SAPPHIRE
SCANNING TUNNELING MICROSCOPY
SINTERING
SPATIAL DISTRIBUTION
THIN FILMS
TUNNEL EFFECT
VANADIUM OXIDES
X-RAY DIFFRACTION
BEAMS
CHALCOGENIDES
COHERENT SCATTERING
CORUNDUM
CRYSTAL LATTICES
CRYSTAL STRUCTURE
CRYSTALS
DIFFRACTION
DISTRIBUTION
ELECTRICAL PROPERTIES
ELECTRON SPECTROSCOPY
ELEMENTS
FABRICATION
FILMS
HEAT TREATMENTS
MICROSCOPY
MICROSTRUCTURE
MINERALS
NONMETALS
OXIDE MINERALS
OXIDES
OXYGEN COMPOUNDS
PHYSICAL PROPERTIES
SCATTERING
SPECTROSCOPY
TRANSITION ELEMENT COMPOUNDS
VANADIUM COMPOUNDS