Title: Study of the metal-insulator transition in thin and ultrathin films of vanadium sesquioxide grown with molecular beam epitaxy
Other Titles: Studie van de metaal-isolator transitie in dunne en ultra-dunne lagen van vanadium gegroeid met moleculaire bundel epitaxie
Authors: Dillemans, Leander
Issue Date: 27-Mar-2014
Abstract: V2O3 (vanadium sesquioxide) is widely considered to be a model system for strongly correlated electron systems. At ambient pressure, bulk V2O3 undergoes a metal-insulator transition (MIT) at a temperature of 150-160K from a strongly correlated rhombohedral paramagnetic metallic phase (PM) to a monoclinic anti-ferromagnetic insulating phase (AFI), accompanied by an abrupt increase in resistivity of up to 7 decades. Upon application of pressure or doping with Ti, the MIT temperature between the PM and AFI phases decreases until eventually the PM phase is stabilised at all temperatures down to 0K. Doping with e.g. Cr causes a transition to a rhombohedral paramagnetic insulating phase (PI), without any change in longe-range order. This dissertation is concerned with the growth and properties of thin (< 100 nm) and ultrathin (< 10 nm) layers of V2O3. The layers were grown by means of oxygen-assisted molecular beam epitaxy on (0001)-Al2O3 and on buffer layers which were grown on (0001)-Al2O3. The layers have been studied with high-resolution X-ray diffraction (XRD), reciprocal space mapping (RSM), X-ray reflectivity (XRR), and transport and optical measurements at cryogenic temperatures. The V2O3 phase was confirmed from the structural symmetry, lattice spacings, density, transport properties and optical properties.The first goal of this work was to be able to tune V2O3 to any point on the bulk phase diagram by means of temperature, Cr doping, stoichiometry and epitaxial strain. The second goal was to study the influence of the proximity of surfaces and interfaces on the properties of V2O3 layers. We have observed that oxygen deficient layers and layers under in-plane tensile strain grown on (0001)-Al2O3 exhibit transport properties which correspond qualitatively to those of Cr-doped V2O3. This suggests the equivalence between oxygen deficiency, tensile in-plane strain and Cr doping in the context of the generalised phase diagram. Buffer layers of Cr2O3 and (V1-xCrx)2O3 have been deposited epitaxially on (0001)-Al2O3, with the intention of using these buffers as template layers for the growth of V2O3 with a controlled strain state. It was found from RSM that V2O3 layers grown directly on (0001)-Al2O3 are almost relaxed with bulk-like transport properties for thicknesses of 17 nm and more. Ultrathin layers of 6 and 4 nm thick display a coherent component, which is correlated with a significant attenuation of the PM-AFI MIT. We suggest that electronic effects, possibly in combination with clamping of the layer, could be responsible for this. In contrast, thin and ultrathin V2O3 layers grown on Cr2O3 buffer layers display bulk-like lattice parameters and we conclude that the Cr2O3 acts to structurally decouple the V2O3 layer from the Al2O3 substrate. A pronounced MIT can still be observed in a V2O3 layer of only 3.5 nm grown on Cr2O3, which puts a new lower limit on the thickness above which an MIT can be observed in V2O3 layers.A dead layer, where the quasiparticle weight isstrongly reduced with respect to bulk V2O3, has been theoretically predicted and experimentally reported in literature. Our data indicate that the thickness of this layer should be smaller than 1 nm. This finding is relevant for the possible integration of ultrathin layers of correlated electron systems in novel device concepts. Growth of (Cr1-xAlx)2O3 buffer layers requires further optimisation before it can be used effectively as a template for V2O3 growth. We have also grown (V1-xCrx)2O3 on (0001)-Al2O3, with the goal of stabilising the PI phase. Cr concentration values extracted from fits of particle-induced X-ray emission correspond well with the nominal values, confirming good control over the concentration. XRD scans suggest that Cr substitutes for V in the lattice. No PM-PI transition could be observed in the temperature-dependent transport, which we ascribe to a combination of phase coexistence in a large temperature interval and a reduced resistivity ratio between both phases in thin films. Finally, we have synthesised V2O3/Cr2O3 superlattices with small modulation length on (0001)-Al2O3. Characteristic superlattice reflections could be observed in XRD and XRR scans, and the modulation lengths extracted from both techniques corresponds well with the nominal values. The transport properties depend strongly on modulation length. Possible mechanisms for this dependence were discussed. We suggest that superlattice structures could be fertile ground for the realisation of interface effects in V2O3.
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Solid State Physics and Magnetism Section

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