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Ferrell and Berreman modes are absorption resonances in thin metal films and polar-dielectric media that arise from radiative bulk plasmon-polariton and phonon-polariton excitations. Compared to surface polaritons, Ferrell and Berreman modes occur due to volume charge oscillations across the medium and provide a unique pathway for light–matter interactions. Though the resonances are studied individually, stringent polarization and material requirements have prevented their observation in one host medium. Here, we show simultaneous excitation of Ferrell and Berreman absorption resonances in refractory epitaxial TiN/Al0.72Sc0.28N plasmonic metal/polar-dielectric hyperbolic metamaterials in the visible and far-infrared spectral ranges. The nanoscale periodicity of the superlattices enables the coupling of bulk plasmons (and longitudinal optical phonons) across different TiN (and Al0.72Sc0.28N) layers and allows polarization matching with free-space light that results in Ferrell (and Berreman) mode excitations. Ferrell and Berreman absorption resonances can be used for strong light confinement in radiative cooling, thermophotovoltaics, and other dual-band applications. [ABSTRACT FROM AUTHOR]

Scandium nitride (ScN) is an emerging rock salt indirect bandgap semiconductor and has attracted significant interest in recent years for thermoelectric energy conversion, as a substrate for defect-free GaN growth, as a semiconducting component in single-crystalline metal/semiconductor superlattices for thermionic energy conversion, as well as for Al1−xScxN-based bulk and surface acoustic devices for 5G technologies. Most ScN film growth traditionally utilizes physical vapor deposition techniques such as magnetron sputtering and molecular beam epitaxy, which results in stoichiometric films but with varying crystal quality, orientations, microstructures, and physical properties. As epitaxial single-crystalline ScN films with smooth surfaces are essential for device applications, it is important to understand the ScN growth modes and parameters that impact and control their microstructure. In this Letter, we demonstrate that large adatom mobility is essential to overcome the Ehrlich–Schwoebel (E–S) and grain boundary migration barriers and achieve defect (voids, dislocations, stacking faults, etc.)-free single-crystalline ScN films. Using the substrate temperature to tune adatom mobility, we show that nominally single-crystalline ScN films are achieved when the homologous temperature is higher than ∼0.3. For homologous temperatures ranging from 0.23 to 0.30, ScN films are found to exhibit significant structural voids in between pyramidal growth regions with multiple in-plane orientations resulting from additional lateral growth off the facets of the pyramids and broken epitaxy after ∼80 nm of growth. The in-depth discussion of the growth modes of ScN presented here explains its varying electrical and optical properties and will help achieve high-quality ScN for device applications. [ABSTRACT FROM AUTHOR]

Scandium nitride (ScN) is an emerging rock salt III-nitride semiconductor and has attracted significant interest in recent years for its potential thermoelectric applications as a substrate for high-quality epitaxial GaN growth and as a semiconducting component for epitaxial singlecrystalline metal/semiconductor superlattices for thermionic energy conversion. Solid-solution alloys of ScN with traditional III-nitrides such as AlxSc1xN have demonstrated piezoelectric and ferroelectric properties and are actively researched for device applications. While most of these exciting developments in ScN research have employed films deposited using low-vacuum methods such as magnetron sputtering and physical and chemical vapor depositions for thermoelectric applications and Schottky barrier-based thermionic energy conversion, it is necessary and important to avoid impurities, tune the carrier concentrations, and achieve high-mobility in epitaxial films. Here, we report the high-mobility and high-thermoelectric power factor in epitaxial ScN thin films deposited on MgO substrates by plasma-assisted molecular beam epitaxy. Microstructural characterization shows epitaxial 002 oriented ScN film growth on MgO (001) substrates. Electrical measurements demonstrated a high room-temperature mobility of 127 cm2/V s and temperature-dependent mobility in the temperature range of 50-400K that is dominated by dislocation and grain boundary scattering. High mobility in ScN films leads to large Seebeck coefficients (175 lV/K at 950 K) and, along with a moderately high electrical conductivity, a large thermoelectric power factor (2.3103 W/m-K2 at 500 K) was achieved, which makes ScN a promising candidate for thermoelectric applications. The thermal conductivity of the films, however, was found to be a bit large, which resulted in a maximum figure-of-merit of 0.17 at 500 K. [ABSTRACT FROM AUTHOR]

Chromium nitride (CrN) is a well-known hard coating material that has found applications in abrasion and wear-resistant cutting tools, bearings, and tribology applications due to its high hardness, high-temperature stability, and corrosion-resistant properties. In recent years, CrN has also attracted significant interest due to its high thermoelectric power factor, and for its unique and intriguing metal-insulator phase transition. While CrN bulk single-crystals exhibit the characteristic metal-insulator transition accompanied with structural (orthorhombic-to-rocksalt) and magnetic (antiferromagnetic-to-paramagnetic) transition at ∼260–280 K, observation of such phase transition in thin-film CrN has been scarce, and the exact cause of the absence of such transitions in several studies is not well-understood. In this work, the formation of the secondary metallic Cr 2 N phase during the growth is demonstrated to inhibit the observation of metal-insulator phase transition in CrN thin films. When the Cr-flux during deposition is reduced below a critical limit, epitaxial and stoichiometric CrN thin film is obtained that reproducibly exhibits the phase transition. Annealing of the mixed-phase film inside reducing NH 3 environment converts the Cr 2 N into CrN, and a discontinuity in the electrical resistivity at ∼277 K appears, which supports the underlying hypothesis. Demonstration of the inhibited metal-semiconductor phase transition in CrN due to the presence of secondary Cr 2 N phase is similar to the previous finding of the substantial change in its mechanical hardness and reduction in thermoelectric properties. A clear demonstration of the origin behind the controversy of the metal-insulator transition in CrN thin films marks significant progress and would enable its nanoscale device realization. [Display omitted] [ABSTRACT FROM AUTHOR]

• Demonstration of lattice-matched (111) oriented ScN growth on (0001) GaN epilayer • High-resolution STEM images of the interface reveal the absence of misfit dislocations • Formation of rotational twin domains during the growth of (111) oriented ScN • Real space imaging of rotational twin domains in (111) ScN thin films • Superior hall mobility was obtained in comparison to (001) oriented ScN Scandium nitride (ScN) has attracted significant interest in recent years for diverse electronic and thermoelectric applications. Most ScN growth utilizes MgO and Al 2 O 3 as substrates that lead to extended defects such as dislocations and grain boundaries. (0001) GaN exhibits less than 0.1% lattice mismatch with (111) ScN, and therefore, should result in single-crystalline ScN film growth. However, not much attention is devoted to understanding the microstructure of ScN on GaN substrates. In this work, we present (111) oriented lattice matched ScN film growth on (0001) GaN substrates. X-ray pole figure exhibited six spots for the (002) ScN superimposed on that of (101) GaN separated by 60° corresponding to twin domains of threefold symmetry for (111) ScN, which are furthermore directly imaged by lattice resolved microscopy images. The presence of twins is attributed to vertical dislocations originating at the GaN/Al 2 O 3 interface and extending throughout the GaN film to its interface with ScN. [Display omitted] [ABSTRACT FROM AUTHOR]