Materials research plays a vital role in transforming breakthrough scientific ideas into next-generation technology. Similar to the way silicon revolutionized the microelectronics industry, the proper materials can greatly impact the field of plasmonics and metamaterials. Currently, research in plasmonics and metamaterials lacks good material building blocks in order to realize useful devices. Such devices suffer from many drawbacks arising from the undesirable properties of their material building blocks, especially metals. There are many materials, other than conventional metallic components such as gold and silver, that exhibit metallic properties and provide advantages in device performance, design flexibility, fabrication, integration, and tunability. This review explores different material classes for plasmonic and metamaterial applications, such as conventional semiconductors, transparent conducting oxides, perovskite oxides, metal nitrides, silicides, germanides, and 2D materials such as graphene. This review provides a summary of the recent developments in the search for better plasmonic materials and an outlook of further research directions.

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Alternative Plasmonic Materials: Beyond Gold and Silver

Theoretical models of Schottky-barrier height formation are reviewed. A particular emphasis is placed on the examination of how these models agree with general physical principles. New concepts on the relationship between interface dipole and chemical bond formation are analyzed, and shown to offer a coherent explanation of a wide range of experimental data.

Recent advances in Schottky barrier concepts

Instabilities in semiconductor heterostructure growth can be exploited for the self-organized formation of nanostructures, allowing for carrier confinement in all three spatial dimensions. Beside the description of various growth modes, the experimental characterization of structural properties, such as size and shape, chemical composition, and strain distribution is presented. The authors discuss the calculation of strain fields, which play an important role in the formation of such nanostructures and also influence their structural and optoelectronic properties. Several specific materials systems are surveyed together with important applications.

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Structural properties of self-organized semiconductor nanostructures

Real-time observations were made of the shape change from pyramids to domes during the growth of germanium-silicon islands on silicon (001). Small islands are pyramidal in shape, whereas larger islands are dome-shaped. During growth, the transition from pyramids to domes occurs through a series of asymmetric transition states with increasing numbers of highly inclined facets. Postgrowth annealing of pyramids results in a similar shape change process. The transition shapes are temperature dependent and transform reversibly to the final dome shape during cooling. These results are consistent with an anomalous coarsening model for island growth.

Transition states between pyramids and domes during Ge/Si island growth

Metal silicides have played an indispensable role in the rapid developments of microelectronics since PtSi was first used to improve the rectifying characteristics of diodes in early 1960s. This wo ...

Metal silicides in CMOS technology : Past, present, and future trends

We have performed a theoretical study of the electronic band structure, density of states, dielectric function and absorption coefficient of isostructural BaSi2, BaGe2 and SrGe2 compounds by means of different ab initio methods. All materials are found to be indirect band-gap semiconductors displaying almost equal dispersion of bands close to the gap region. The energy gaps of 0.83, 0.57 and 0.44 eV are estimated for BaSi2, BaGe2 and SrGe2, respectively. Analysis of the absorption coefficient of BaSi2 in comparison with data for other semiconducting silicides indicates its prospects for a solar cell application. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Isostructural BaSi2, BaGe2 and SrGe2: electronic and optical properties

In this part we present results of our ab initio calculations indicating that dispersion of the bands near the gap region for different phases of WO3 (namely, e-WO3, δ-WO3, γ-WO3, β-WO3, orth-WO3, α-WO3, and hex-WO3) is rather close. The rapid increase in the absorption coefficient starts at the lower energy range for α-WO3 and hex-WO3 than for the other phases in accordance with the calculated band gaps. An oxygen vacancy has turned out to decrease the gap by 0.50 eV and to shift the absorption coefficient to the lower energy range in the room temperature γ-WO3 phase. We have also traced changes caused by molybdenum and sulfur doping of γ-WO3. Only sulfur doped γ-WO3 has been revealed to display the formation of the impurity band along with a sizable reduction in the gap and the shift in the absorption coefficient to the lower energy range.

Tungsten oxides. I. Effects of oxygen vacancies and doping on electronic and optical properties of different phases of WO3

In the first part [D. B. Migas et al., J. Appl. Phys. 108, 093713 (2010)] electronic and optical properties of different phases of WO3 have been considered. In this part we present results of our ab initio calculations which clearly show that all Magneli phases of tungsten oxides WOx (namely, W32O84, W3O8, W18O49, W17O47, W5O14, W20O58, and W25O73) are characterized by metal-like properties. Their band structures display an energy gap in the valence band just below the Fermi level. We discuss how addition (removal) of oxygen atoms to (from) the unit cell of W18O49 affects the position of the Fermi level with respect to the energy gap and the charge carrier concentration. A possible mechanism has been suggested in order to switch from metallic to semiconducting properties for W18O49 and to explain experimental observations.

Tungsten oxides. II. The metallic nature of Magnéli phases

Band structure calculations for β‐FeSi2 have been performed by the linear muffin‐tin orbital method within the local density approximation scheme including exchange and correlation effects. A detailed analysis of the conduction and valence band structure around high‐symmetry points has shown the existence of a quasidirect band gap structure in the material. It is experimentally confirmed that between the threshold energy of optical interband transition of 0.73 eV and the first direct gap transition with appreciable oscillator strength at about 0.87 eV there is a region in which direct transition of low oscillator strength and indirect transitions overlap. That explains the tricky behavior of β‐FeSi2 in experimental investigations demonstrating it to be either a direct or indirect gap semiconductor.

Electronic and related properties of crystalline semiconducting iron disilicide

By means of first principles calculations, we have investigated the band structures of different phases of higher manganese silicides ($\mathrm{Mn}{\mathrm{Si}}_{x}$ with $x$ ranging from 1.73 to 1.75). In this family, ${\mathrm{Mn}}_{11}{\mathrm{Si}}_{19}$, ${\mathrm{Mn}}_{15}{\mathrm{Si}}_{26}$, and ${\mathrm{Mn}}_{27}{\mathrm{Si}}_{47}$ have been found to behave like degenerate semiconductors and, at the same time, like metals because the Fermi level stays partly in the energy gap and partly in the valence band close to its top. The spin-polarized calculations have revealed that these phases can be also treated as half-metals displaying 100% spin polarization of holes at the Fermi energy. On the contrary, ${\mathrm{Mn}}_{4}{\mathrm{Si}}_{7}$ is shown to be a semiconductor with the indirect band gap of $0.77\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Its dielectric function possesses some anisotropy effects with respect to different light polarizations. We have also discovered that the $\mathrm{Mn}{\mathrm{Si}}_{1.75}$ stoichiometry provides semiconductor properties without degeneracy. The role of stacking faults in the gap reduction of higher manganese silicides is discussed.

Dmitri B. Migas

Papers

Isostructural BaSi2, BaGe2 and SrGe2: electronic and optical properties

Tungsten oxides. I. Effects of oxygen vacancies and doping on electronic and optical properties of different phases of WO3

Tungsten oxides. II. The metallic nature of Magnéli phases

Electronic and related properties of crystalline semiconducting iron disilicide

Ab initio study of the band structures of different phases of higher manganese silicides