Optical transparency, tunable conducting properties and easy processability make metal oxides key materials for advanced optoelectronic devices. This Review discusses recent advances in the synthesis of these materials and their use in applications. Metal oxides (MOs) are the most abundant materials in the Earth's crust and are ingredients in traditional ceramics. MO semiconductors are strikingly different from conventional inorganic semiconductors such as silicon and III–V compounds with respect to materials design concepts, electronic structure, charge transport mechanisms, defect states, thin-film processing and optoelectronic properties, thereby enabling both conventional and completely new functions. Recently, remarkable advances in MO semiconductors for electronics have been achieved, including the discovery and characterization of new transparent conducting oxides, realization of p-type along with traditional n-type MO semiconductors for transistors, p–n junctions and complementary circuits, formulations for printing MO electronics and, most importantly, commercialization of amorphous oxide semiconductors for flat panel displays. This Review surveys the uniqueness and universality of MOs versus other unconventional electronic materials in terms of materials chemistry and physics, electronic characteristics, thin-film fabrication strategies and selected applications in thin-film transistors, solar cells, diodes and memories.

Metal oxides for optoelectronic applications

Heterostructured materials are an emerging class of materials with superior performances that are unattainable by their conventional homogeneous counterparts. They consist of heterogeneous zones wi...

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Heterostructured materials: superior properties from hetero-zone interaction

Borophene (a two-dimensional boron sheet) is a new type of two-dimensional material, which was recently grown successfully on single crystal Ag substrates. In this paper, we investigate the electronic structure and bonding characteristics of borophene by first-principles calculations. The band structure of borophene shows highly anisotropic metallic behaviour. The obtained optical properties of borophene exhibit strong anisotropy as well. Finally, the thermodynamic properties are investigated based on the phonon properties.

The electronic, optical, and thermodynamic properties of borophene from first-principles calculations

Transparent conducting oxides constitute a unique class of materials combining properties of electrical conductivity and optical transparency in a single material. They are needed for a wide range of applications including solar cells, flat panel displays, touch screens, light emitting diodes and transparent electronics. Most of the commercially available TCOs are n-type, such as Sn doped In2O3, Al doped ZnO, and F doped SnO2. However, the development of efficient p-type TCOs remains an outstanding challenge. This challenge is thought to be due to the localized nature of the O 2p derived valence band which leads to difficulty in introducing shallow acceptors and large hole effective masses. In 1997 Hosono and co-workers (1997 Nature 389 939) proposed the concept of 'chemical modulation of the valence band' to mitigate this problem using hybridization of O 2p orbitals with close-shell Cu 3d (10) orbitals. This work has sparked tremendous interest in designing p-TCO materials together with deep understanding the underlying materials physics. In this article, we will provide a comprehensive review on traditional and recently emergent p-TCOs, including Cu(+)-based delafossites, layered oxychalcogenides, nd (6) spinel oxides, Cr(3+)-based oxides (3d (3)) and post-transition metal oxides with lone pair state (ns (2)). We will focus our discussions on the basic materials physics of these materials in terms of electronic structures, doping and defect properties for p-type conductivity and optical properties. Device applications based on p-TCOs for transparent p-n junctions will also be briefly discussed.

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P-type transparent conducting oxides.

Efficient nonlinear optical interactions are essential for many applications in modern photonics. However, they typically require intense laser sources and long interaction lengths, requirements that often render nonlinear optics incompatible with new nanophotonic architectures in integrated optics and metasurface devices. Obtaining materials with stronger nonlinear properties is a crucial step towards applications that require lower powers and smaller footprints. Recently, a new class of materials with a vanishing permittivity, known as epsilon-near-zero (ENZ) materials, has been reported to exhibit unprecedented ultrafast nonlinear efficiencies within sub-wavelength propagation lengths. In this Review, we survey the work that has been performed on ENZ materials and the related near-zero-index materials, focusing on the observation of various nonlinear phenomena (such as intensity-dependent refraction, four-wave mixing and harmonic generation), the identification of unique field-enhancement mechanisms and the study of non-equilibrium dynamics. Degenerately doped semiconductors (such as tin-doped indium oxide and aluminium-doped zinc oxide) are particularly promising candidates for ENZ-enhanced nonlinear optical applications. We conclude by pointing towards possible future research directions, such as the search for ENZ materials with low optical losses and the elucidation of the mechanisms underlying nonlinear enhancements. Materials with vanishingly small dielectric permittivity, known as epsilon-near-zero materials, enable strong ultrafast optical nonlinear responses within a sub-wavelength propagation length. This Review surveys the various observations of nonlinear phenomena in this class of materials.

Nonlinear optical effects in epsilon-near-zero media

The fundamental challenge for designing transparent conductors used in photovoltaics, displays and solid-state lighting is the ideal combination of high optical transparency and high electrical conductivity. Satisfying these competing demands is commonly achieved by increasing carrier concentration in a wide-bandgap semiconductor with low effective carrier mass through heavy doping, as in the case of tin-doped indium oxide (ITO). Here, an alternative design strategy for identifying high-conductivity, high-transparency metals is proposed, which relies on strong electron-electron interactions resulting in an enhancement in the carrier effective mass. This approach is experimentally verified using the correlated metals SrVO3 and CaVO3, which, despite their high carrier concentration (>2.2 × 10(22) cm(-3)), have low screened plasma energies (<1.33 eV), and demonstrate excellent performance when benchmarked against ITO. A method is outlined to rapidly identify other candidates among correlated metals, and strategies are proposed to further enhance their performance, thereby opening up new avenues to develop transparent conductors.

Correlated metals as transparent conductors

We present the optical function spectra of Cu2SnSe3 determined from 0.30 to 6.45 eV by spectroscopic ellipsometry (SE) at room temperature. We analyze the SE data using the Tauc-Lorentz model and obtain the direct-bandgap energy of 0.49 ± 0.02 eV, which is much smaller than the previously known value of 0.84 eV for the monoclinic-phase Cu2SnSe3. We also perform density-functional theory calculations to obtain the complex dielectric function data, and the results show good agreement with the experimental spectrum. Finally, we discuss the electronic origin of the main optical structures.

Optical function spectra and bandgap energy of Cu2SnSe3

Optical properties of commercially available gadolinium gallium garnet (Gd3Ga5O12) single crystals have been studied over a spectral range from 0.034 to 5.887 eV using spectroscopic ellipsometry and transmittance measurements via a near infrared to ultraviolet spectral range multichannel ellipsometer and a Fourier transform infrared (FTIR) ellipsometer. The complex dielectric function (ϵ = ϵ1 + iϵ2) spectra from 0.034 to 5.887 eV have been determined. Transmission measurements have been performed to more accurately extract the absorption coefficient near the band edge and identify sub band gap features. Analysis of ϵ for Gd3Ga5O12 indicates a direct band gap at 5.66 ± 0.01 eV, sub band gap absorption features from 3.95 to 5.06 eV, and nine transverse optical phonon modes at 288, 313, 329, 361, 370, 467, 577, 609, and 670 cm−1 and longitudinal optical phonon modes at 288, 327, 338, 370, 461, 490, 592, 663, and 707 cm−1.

Optical properties of single‐crystal Gd3Ga5O12 from the infrared to ultraviolet

Approach for extracting complex dielectric function spectra in weakly-absorbing regions

Optical properties in the form of the complex dielectric function spectra (e = e1 + ie2) of commercially produced single crystal bismuth germanate (Bi4Ge3O12) have been studied using spectroscopic ellipsometry from 0.033 to 6.478 eV. Transmission measurements have also been performed in order to more accurately obtain low values of the absorption coefficient close to the band edge. Critical point parameters have been determined by analyzing the interband transitions. Infrared extended measurements yield vibrational modes corresponding to chemical bonding environments and the lattice structure. The overall analysis yields e from 0.033 to 6.478 eV and provides information about the crystal such as the energy of the band gap at 4.16 ± 0.01 eV; interband transitions at 4.49, 4.75, 4.81, 5.08, 5.59, and 6.08 eV; transverse optical phonon modes at 283, 364, 395, 447, 702, 729, and 778 cm−1 and longitudinal optical phonon modes at 292, 386, 445, 458, 710, 779, and 817 cm−1.

Hamna F. Haneef

Papers

Correlated metals as transparent conductors

Optical function spectra and bandgap energy of Cu2SnSe3

Optical properties of single‐crystal Gd3Ga5O12 from the infrared to ultraviolet

Approach for extracting complex dielectric function spectra in weakly-absorbing regions

Optical properties of single crystal Bi4Ge3O12 from the infrared to ultraviolet