Dysprosium-doped ​cadmium oxide as a gateway material for mid-infrared plasmonics

Journal name:
Nature Materials
Volume:
14,
Pages:
414–420
Year published:
DOI:
doi:10.1038/nmat4203
Received
Accepted
Published online

Abstract

The interest in plasmonic technologies surrounds many emergent optoelectronic applications, such as plasmon lasers, transistors, sensors and information storage. Although plasmonic materials for ultraviolet–visible and near-infrared wavelengths have been found, the mid-infrared range remains a challenge to address: few known systems can achieve subwavelength optical confinement with low loss in this range. With a combination of experiments and ab initio modelling, here we demonstrate an extreme peak of electron mobility in Dy-doped ​CdO that is achieved through accurate ‘defect equilibrium engineering’. In so doing, we create a tunable plasmon host that satisfies the criteria for mid-infrared spectrum plasmonics, and overcomes the losses seen in conventional plasmonic materials. In particular, extrinsic doping pins the ​CdO Fermi level above the conduction band minimum and it increases the formation energy of native oxygen vacancies, thus reducing their populations by several orders of magnitude. The substitutional lattice strain induced by Dy doping is sufficiently small, allowing mobility values around 500 cm2 V−1 s−1 for carrier densities above 1020 cm−3. Our work shows that ​CdO:Dy is a model system for intrinsic and extrinsic manipulation of defects affecting electrical, optical and thermal properties, that oxide conductors are ideal candidates for plasmonic devices and that the defect engineering approach for property optimization is generally applicable to other conducting metal oxides.

At a glance

Figures

  1. Doping- and temperature-dependent properties of CdO:Dy.
    Figure 1: Doping- and temperature-dependent properties of ​CdO:Dy.

    a, Transport data for ​CdO:Dy grown on ​MgO(100) substrates summarizing carrier concentration (cm−3), carrier mobility (μ) and conductivity (σ) as a function of dysprosium concentration. b, Temperature-dependent sheet carrier concentration (ns) as a function of [Dy] for ​CdO:Dy grown on ​MgO(100) substrates. c, Temperature-dependent resistivity (Ω cm) as a function of [Dy] grown on ​MgO(100) substrates. d, Residual resistivity ratio, measured thermal conductivity (κt) and theoretical values for the electron contribution to thermal conductivity (κe) calculated using the Wiedemann–Franz law for ​CdO:Dy grown on ​MgO(100) substrates as a function of Dy concentration.

  2. DFT calculations and mechanistic zone model for lattice defects in CdO:Dy.
    Figure 2: DFT calculations and mechanistic zone model for lattice defects in ​CdO:Dy.

    ac, Defect formation energies as a function of Fermi level for oxygen vacancies (VO⋅⋅), cadmium vacancies (VCd′′), and Dy substituents on the Cd site (DyCd) in the O-poor (a), O-mid (b) and O-rich (c) limits. The slope of each line at a given Fermi level corresponds to the charge of the defect at that Fermi level. d, VO⋅⋅ concentration (solid lines) and carrier concentration n (dashed lines) as a function of DyCd concentration. Plots are shown for a set of given oxygen chemical potentials, reported as ΔμO relative to the O-rich extreme with more negative values being more O-poor. e, Idealized schematic summarizing the individual contributions of defects to the observed electronic transport in ​CdO:Dy. [VO⋅⋅], [DyCd], mobility (μe) and conductivity (σ) are plotted versus [Dy]. Lattice thermal conductivity (κl), total thermal conductivity (κt), electronic thermal conductivity (κe) and the theoretical values for κe are also plotted versus [Dy]. The Dy-doping space is divided into 5 different zones, where within each zone the observed transport is regulated by a different mechanism.

  3. Reflectivity maps for CdO:Dy thin films recorded in the Kretschmann configuration.
    Figure 3: Reflectivity maps for ​CdO:Dy thin films recorded in the Kretschmann configuration.

    ad, Simulated reflectivity data using the tabulated properties as input (a,c), and experimental data (b,d) for ​CdO:Dy with 1.3 × 1020 (a,b) and 3.3 × 1020 cm−3 (c,d). In each map, the SPP dispersion can be seen as an angle-dependent dip in the reflected light intensity (darker shades). The tunability of the SPP with changing carrier concentration can be seen as the shift of the reflectance dips towards higher energies. The high carrier mobility in ​CdO:Dy (low loss) results in the sharp absorption bands seen both in experimental and simulated data.

  4. Calculated quality factors for four selected CdO:Dy alloys as a function of energy.
    Figure 4: Calculated quality factors for four selected ​CdO:Dy alloys as a function of energy.

    a, Calculated SPP propagation distances. b, Calculated electric field confinement for the SPP. c, Calculated M11D figures of merit. d, Calculated QLSPR figures of merit. The values of the dysprosium concentrations of the different alloys and the corresponding carrier mobility μ is shown in the legend.

  5. Surface quality of CdO:Dy on MgO(100) substrates.
    Figure 5: Surface quality of ​CdO:Dy on ​MgO(100) substrates.

    Atomic force microscope topography scan of as-grown ​CdO:Dy demonstrating the atomically flat growth habit of ​CdO:Dy on ​MgO(100) substrates. The observed r.m.s. roughness is <150 pm.

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Author information

Affiliations

  1. Department of Materials Science, North Carolina State University, Raleigh, North Carolina 27695, USA

    • Edward Sachet,
    • Christopher T. Shelton,
    • Joshua S. Harris,
    • Benjamin E. Gaddy,
    • Douglas L. Irving &
    • Jon-Paul Maria
  2. Department of Mechanical Engineering and Materials Science and Department of Electrical Engineering, Duke University, Durham, North Carolina 27708, USA

    • Stefano Curtarolo
  3. Department of Materials Science and Engineering, University of Virginia, Charlottesville, Virginia 22904, USA

    • Brian F. Donovan
  4. Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, Virginia 22904, USA

    • Patrick E. Hopkins
  5. Sandia National Laboratories, Albuquerque, New Mexico 87185, USA

    • Peter A. Sharma,
    • Ana Lima Sharma &
    • Jon Ihlefeld
  6. Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695, USA

    • Stefan Franzen

Contributions

E.S., S.F. and J-P.M. proposed the concept and experiments with support by S.C. C.T.S. and E.S. developed the MBE deposition and doping technique for the growth of ​CdO:Dy. E.S. led the experimental/analytical efforts with support from C.T.S., B.F.D., P.E.H., P.A.S., A.L.S. and J.I. The DFT simulations and analysis of theoretical results were performed by D.L.I., B.E.G. and J.S.H. All authors mentioned above discussed and contributed to the paper.

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The authors declare no competing financial interests.

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