Three-terminal field effect devices utilizing thin film vanadium oxide as the channel layer

Electrostatic control of the metal-insulator transition (MIT) in an oxide semiconductor could potentially impact the emerging field of oxide electronics. Vanadium dioxide (VO2) is of particular interest due to the fact that the MIT happens in the vicinity of room temperature and it is considered to exhibit the Mott transition. We present a detailed account of our experimental investigation into three-terminal field effect transistor-like devices using thin film VO2 as the channel layer. The gate is separated from the channel through an insulating gate oxide layer, enabling true probing of the field effect with minimal or no interference from large leakage currents flowing directly from the electrode. The influence of the fabrication of multiple components of the device, including the gate oxide deposition, on the VO2 film characteristics is discussed. Further, we discuss the effect of the gate voltage on the device response, point out some of the unusual characteristics including temporal dependence. A reversible unipolar modulation of the channel resistance upon the gate voltage is demonstrated for the first time in optimally engineered devices. The results presented in this work are of relevance toward interpreting gate voltage response in such oxides as well as addressing challenges in advancing gate stack processing for oxide semiconductors

Hall carrier density and magnetoresistance measurements in thin film vanadium dioxide across the metal-insulator transition

Temperature dependent magneto-transport measurements in magnetic fields of up to 12 Tesla were performed on thin film vanadium dioxide (VO2) across the metal-insulator transition (MIT). The Hall carrier density increases by 4 orders of magnitude at the MIT and accounts almost entirely for the resistance change. The Hall mobility varies little across the MIT and remains low, ~0.1cm2/V sec. Electrons are found to be the major carriers on both sides of the MIT. Small positive magnetoresistance in the semiconducting phase is measured

Compositional Tuning of Ultrathin Surface Oxides on Metal and Alloy Substrates Using Photons: Dynamic Simulations and Experiments

We report on the ability to modify the structure and composition of ultrathin oxides grown on Ni and Ni-Al alloy surfaces at room temperature utilizing photon illumination. We find that the nickel-oxide formation is enhanced in the case of oxidation under photo-excitation. The enhanced oxidation kinetics of nickel in 5% Ni-Al alloy is corroborated by experimental and simulation studies of natural and photon-assisted oxide growth on pure Ni(100) surfaces. In case of pure Ni substrates, combined x-ray photoelectron spectroscopy analysis, and atomic force microscope current mapping support the deterministic role of the structure of nickel passive-oxide films on their nanoscale corrosion resistance. Atomistic simulations involving dynamic charge transfer predict that the applied electric field overcomes the activation-energy barrier for ionic migration, leading to enhanced oxygen incorporation into the oxide, enabling us to tune the mixed-oxide composition at atomic length scales. Atomic scale control of ultrathin oxide structure and morphology in the case of pure substrates as well as compositional tuning of complex oxide in the case of alloys leads to excellent passivity as verified from potentiodynamic polarization experiments.Engineering and Applied SciencesPhysic

Anisotropic Magnetoresistance in Ga1−x_{1-x}Mnx_xAs

We have measured the magnetoresistance in a series of Ga$_{1-x}$Mn$_x$As samples with 0.033$\le x \le$ 0.053 for three mutually orthogonal orientations of the applied magnetic field. The spontaneous resistivity anisotropy (SRA) in these materials is negative (i.e. the sample resistance is higher when its magnetization is perpendicular to the measuring current than when the two are parallel) and has a magnitude on the order of 5% at temperatures near 10K and below. This stands in contrast to the results for most conventional magnetic materials where the SRA is considerably smaller in magnitude for those few cases in which a negative sign is observed. The magnitude of the SRA drops from its maximum at low temperatures to zero at T$_C$ in a manner that is consistent with mean field theory. These results should provide a significant test for emerging theories of transport in this new class of materials.Comment: 4 pages with 4 figures. Submitted to Physical Review

Diamond field-effect transistors with V2O5-induced transfer doping: scaling to 50-nm gate length

We report on the fabrication and measurement of hydrogen-terminated diamond field-effect transistors (FETs) incorporating V2O5 as a surface acceptor material to induce transfer doping. Comparing a range of gate lengths down to 50 nm, we observe inversely scaling peak output current and transconductance. Devices exhibited a peak drain current of ~700 mA/mm and a peak transconductance of ~150 mS/mm, some of the highest reported thus far for a diamond metal semiconductor FET (MESFET). Reduced sheet resistance of the diamond surface after V2O5 deposition was verified by four probe measurement. These results show great potential for improvement of diamond FET devices through scaling of critical dimensions and adoption of robust transition metal oxides such as V2O5

Structural and electronic properties of 2D (graphene, hBN)/H-terminated diamond (100) heterostructures

We report a first-principles study of the structural and electronic properties of two-dimensional (2D) layer/hydrogen-terminated diamond (100) heterostructures. Both the 2D layers exhibit weak van-der-Waals (vdW) interactions and develop rippled configurations with the H-diamond (100) substrate to compensate for the induced strain. The adhesion energy of the hexagonal boron nitride (hBN) layer is slightly higher, and it exhibits a higher degree of rippling compared to the graphene layer. A charge transfer analysis reveals a small amount of charge transfer from the H-diamond (100) surface to the 2D layers, and most of the transferred charge was found to be confined within the vdW gap. In the graphene/H-diamond (100) heterostructure, the semi-metallic characteristic of the graphene layer is preserved. On the other hand, the hBN/H-diamond (100) heterostructure shows semiconducting characteristics with an indirect bandgap of 3.55 eV, where the hBN layer forms a Type-II band alignment with the H-diamond (100) surface. The resultant conduction band offset and valence band offset are 0.10 eV and 1.38 eV, respectively. A thin layer of hBN offers a defect-free interface with the H-diamond (100) surface and provides a layer-dependent tunability of electronic properties and band alignment for surface-doped diamond field effect transistors

Structure-functional property relationships in rf-sputtered vanadium dioxide thin films

The study of metal-insulator transition (MIT) in VO2 thin films synthesized by means of rf sputtering from a VO2 target is presented. A comparison with conventional reactive sputtering from a V target is also given. Detailed x-ray diffraction analysis, electrical resistance switching, and infrared optical reflectance measurements confirm that our sputtering technique yields high-quality VO2 films. We discuss in depth how synthesis conditions affect MIT parameters derived from temperature dependence of electrical resistance. Sharp MIT is observed in films sputtered on technologically important Si substrates. The choice of Si (or sapphire) substrates results in the transition temperature above (below) the values obtained for single VO2 crystals. The MIT becomes narrower and stronger in thinner films. This is consistent with the assumption that the increased width of the MIT in thin films with respect to single crystals is the result of averaging of the transition parameters over a distribution of crystallites in the film. The measurements of MIT in VO2 patterned into devices do not reveal a noticeable lateral size effect down to 20 μ m devices, encouraging use of the phase transition in switching electronic devices. The effect of substrate temperature and ambient during the sputtering on MIT is discussed. While VO2 films are found to be stable in ambient environment with time, the additional exposure to UV radiation near room temperature is shown to enhance the oxidation kinetics and produce changes in film resistance. Using UV radiation as an additional tool to control the oxidation process during VO2 synthesis may allow the synthesis temperature to be lowered and an improvement in material quality. We anticipate these results may be of relevance to synthesizing functional oxide films with potential applications in electronics and sensors

Correlation between metal-insulator transition characteristics and electronic structure changes in vanadium oxide thin films

We correlate electron transport data directly with energy band structure measurements in vanadium oxide thin films with varying V-O stoichiometry across the VO2 metal-insulator transition. A set of vanadium oxide thin films were prepared by reactive dc sputtering from a V target at various oxygen partial pressures (O2 p.p.). Metal-insulator transition (MIT) characteristic to VO2 can be seen from the temperature dependence of electrical resistance of the films sputtered at optimal O2 p.p. Lower and higher O2 p.p. result in disappearance of the MIT. The results of the near edge x-ray absorption fine structure spectroscopy of the O K edge in identical VO films are presented. Redistribution of the spectral weight from σ* to Π* bands is found in the vanadium oxide films exhibiting stronger VO2 MIT. This is taken as evidence of the strengthening of the metal-metal ion interaction with respect to the metal-ligand and indirect V-O-V interaction in vanadium oxide films featuring sharp MIT. We also observe a clear correlation between MIT and the width and area of the lower Π* band, which is likely to be due to the emergence of the d|| band overlapping with Π*. The strengthening of this d|| band near the Fermi level only in the vanadium oxide compounds displaying the MIT points out the importance of the role of the d|| band and electron correlations in the phase transition

Electrical triggering of metal-insulator transition in nanoscale vanadium oxide junctions

200 nm diameter Au contacts were fabricated by e-beam lithography on sputtered thin film vanadium oxide grown on conducting substrates and current perpendicular to plane electron transport measurements were performed with a conducting tip atomic force microscope. Sharp jumps in electric current were observed in the I-V characteristics of the nano-VO2 junctions and were attributed to the manifestation of the metal-insulator transition. The critical field and dielectric constant were estimated from quantitative analysis of the current-voltage relationship and compared with reported values on micrometer and larger size scale devices. These results are of potential relevance to novel oxide electronics utilizing metal-insulator transitions