Research ArticleAPPLIED PHYSICS

Electric dipole effect in PdCoO2/β-Ga2O3 Schottky diodes for high-temperature operation

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Science Advances  18 Oct 2019:
Vol. 5, no. 10, eaax5733
DOI: 10.1126/sciadv.aax5733
  • Fig. 1 Schottky junctions based on a layered PdCoO2 and β-Ga2O3 for high-temperature operation.

    (A) Energy band diagram of a metal-semiconductor interface with a Schottky barrier of ϕb. ϕm, work function of metal; χs, electron affinity of semiconductor; Evac, vacuum level; EF, Fermi energy; EC, conduction band minimum; V, bias voltage. The electron is depicted as a red circle. (B) Typical current-voltage characteristic of Schottky junction showing the current rectification behavior under forward (on-state) or reverse (off-state) bias voltage. The saturation current density Js is noted in red. (C) Temperature dependence of the saturation current density Js with Schottky barrier heights of 1.0 eV (blue), 1.4 eV (green), and 1.8 eV (red) calculated by the thermionic emission model using the Richardson constant of β-Ga2O3, A** = 41.1 A/cm2 (24). (D) Schematic image for the characterization of the PdCoO2/−β-Ga2O3 Schottky junction. (E) HAADF-STEM image of the PdCoO2/β-Ga2O3 interface. The crystal orientation is represented by arrows. (F) Enlarged image of the HAADF-STEM image of the PdCoO2/β-Ga2O3 interface. The anisotropic Ψd-s orbital proposed for the conduction band of PdCoO2 is schematically shown (20). Right: Corresponding crystal model. The alternating Pd+ and [CoO2] charged layers are shown based on the nominal ionic charges in the bulk PdCoO2. The actual charge state at the interface can be modified by electronic reconstruction with screening charges.

  • Fig. 2 High-temperature operation of the PdCoO2/β-Ga2O3 Schottky junctions with a large barrier height.

    (A) Current-voltage characteristics of the PdCoO2/β-Ga2O3 Schottky junction with a diameter of 200 μm at 227°C (blue) and 356°C (red). The gray lines are the linear fitting for the forward-bias region. The temperature dependence of the Schottky barrier height is plotted in the inset. (B) Temperature-dependent reverse current density under the bias voltage of −20 V |J−20V| plotted together with the reported values (2530). The ideal Js in Fig. 1C is also shown for ϕb = 1.0 eV (blue), 1.4 eV (green), and 1.8 eV (red). (C) Comparison of the Schottky barrier height with the reported values for elemental metal Schottky junctions (see the Supplementary Materials for the data and references used). Perpendicularly spread line data correspond to the range of the reported Schottky barrier height. The different colors correspond to the different surface orientations of the β-Ga2O3 layers. The linear trend from the reported values is shown as a broken line. The large red square corresponds to the data obtained for PdCoO2/β-Ga2O3.

  • Fig. 3 Energy band diagram of the PdCoO2/β-Ga2O3 interface.

    (A) 1/C2-V plots for the devices with D = 100 and 200 μm measured with an AC frequency of 1 kHz at room temperature. (B) Frequency dependence of the capacitance measured with 0.1-V excitation. (C) Top: Band diagram for the PdCoO2/β-Ga2O3 Schottky junction based on experimentally observed values. The energy difference of the conduction band bottom and the Fermi energy in β-Ga2O3 (ξ) is calculated using ξ = kBTln(NC/ND) and NC = 2(2πm*kBT/h2)3/2, where h is the Planck constant and m* = 0.342m0 is the effective mass of the electron in β-Ga2O3 (24). Bottom: Schematics of the PdCoO2/β-Ga2O3 interface. A conduction electron in the Pd+ layer is shown in red. The bulk-like charged layers of PdCoO2 are schematically depicted, neglecting possible charge reconstructions.

  • Fig. 4 Uniformity and reproducibility of the large barrier height in the PdCoO2/β-Ga2O3 Schottky junctions.

    (A) Optical microscopy image of the PdCoO2/β-Ga2O3 Schottky junction arrays. (B) Schottky barrier height obtained for the devices with different junction sizes. (C) Current-voltage characteristics of the 27 different devices. (D) Histogram of the Schottky barrier height for D = 100-μm devices. The Gaussian fitting (red curve) gives the central value ϕbμ = 1.76 eV and the width of distribution 2σ = 45 meV, where σ is the SD.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/10/eaax5733/DC1

    Fig. S1. Basic characterization of PdCoO2 thin films grown on β-Ga2O3 (−201).

    Fig. S2. Relationship between work function and lattice constant for typical metal electrodes with a (pseudo-)hexagonal lattice constant close to the representative hexagonal wide-gap semiconductors.

    Fig. S3. Temperature dependence of the Schottky barrier height and the ideality factor.

    Fig. S4. Determination of the work function for PdCoO2 using the ultraviolet photoelectron spectroscopy.

    Fig. S5. Stability of PdCoO2 thin films in a harsh environment.

    Table S1. Summary of the metal/β-Ga2O3 junctions reported in literature.

    Table S2. Summary of the partially oxidized metal/β-Ga2O3 junctions reported in literature.

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Basic characterization of PdCoO2 thin films grown on β-Ga2O3 (−201).
    • Fig. S2. Relationship between work function and lattice constant for typical metal electrodes with a (pseudo-)hexagonal lattice constant close to the representative hexagonal wide-gap semiconductors.
    • Fig. S3. Temperature dependence of the Schottky barrier height and the ideality factor.
    • Fig. S4. Determination of the work function for PdCoO2 using the ultraviolet photoelectron spectroscopy.
    • Fig. S5. Stability of PdCoO2 thin films in a harsh environment.
    • Table S1. Summary of the metal/β-Ga2O3 junctions reported in literature.
    • Table S2. Summary of the partially oxidized metal/β-Ga2O3 junctions reported in literature.

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