Refractory plasmonic materials that have optical properties close to those of noble-metals and at the same time are environmentally friendly, commercially viable and CMOS-compatible could lead to novel devices for many thermo-photonic applications. Recently developed TiN thin films overcome some of the limitations of noble-metals, as their optical loss is larger than noble metals and conventional methods to deposit TiN films are not compatible for its integration with other semiconductors. In this work, high-quality epitaxial single-crystalline TiN thin films are deposited with plasma-assisted molecular beam epitaxy (MBE) that exhibit optical losses that are less than that of Au in most part of the visible (300 nm – 580 nm) and near-IR spectral ranges (1000 nm - 2500 nm). In addition, a large figure-of-merit for surface plasmon polariton (SPP) propagation length compared to the previously reported TiN films is achieved with the MBE-deposited films.
Harnessing solar energy by employing concentrated solar power (CSP) systems requires materials with high electrical conductivity and optical reflectivity. Silver, with its excellent optical reflectance, is traditionally used as a reflective layer in solar mirrors for CSP technologies. However, silver is soft and expensive, quickly tarnishes, and requires a protective layer of glass for practical applications. Moreover, supply-side constraints and high-temperature instability of silver have led to the search for alternative materials that exhibit high solar and infrared reflectance. Transition metal nitrides, such as titanium nitride, have emerged as alternative plasmonic materials to gold starting from a spectral range of ∼500 nm. However, to achieve high solar reflection (∼320–2500 nm), materials with epsilon-near-zero starting from the near-ultraviolet (UV) spectral region are required. Here, we show the development of refractory epitaxial hafnium nitride (HfN) and zirconium nitride (ZrN) thin films as excellent mirrors with a solar reflectivity of ∼90.3% and an infrared reflectivity of ∼95%. Low-loss and high-quality epsilon-near-zero resonance at near-UV (∼340–380 nm) spectral regions are achieved in HfN and ZrN by carefully controlling the stoichiometry, leading to a sharp increase in the reflection edge that is on par with silver. Temperature-dependent reflectivity and dielectric constants are further measured to demonstrate their high-temperature suitability. The development of refractory epitaxial HfN and ZrN thin films with high solar and infrared reflectance makes them excellent alternative plasmonic materials to silver and would pave their applications in CSP, daytime radiative cooling, and others.
The interaction of light with collective charge oscillations, called plasmon–polariton, and with polar lattice vibrations, called phonon–polariton, are essential for confining light at deep subwavelength dimensions and achieving strong resonances. Traditionally, doped-semiconductors and conducting metal oxides (CMO) are used to achieve plasmon–polaritons in the near-to-mid infrared (IR), while polar dielectrics are utilized for realizing phonon–polaritons in the long-wavelength IR (LWIR) spectral regions. However, demonstrating low-loss plasmon– and phonon–polaritons in one host material will make it attractive for practical applications. Here, we demonstrate high-quality tunable short-wavelength IR (SWIR) plasmon–polariton and LWIR phonon–polariton in complementary metal-oxide-semiconductor compatible group III–V polar semiconducting scandium nitride (ScN) thin films. We achieve both resonances by utilizing n-type (oxygen) and p-type (magnesium) doping in ScN that allows modulation of carrier concentration from 5 × 1018 to 1.6 × 1021 cm–3. Our work enables infrared nanophotonics with an epitaxial group III semiconducting nitride, opening the possibility for practical applications.
Hyperbolic metamaterials (HMMs) with extreme dielectric anisotropy have shown great promise in nanophotonic applications such as superlensing, enhancement of spontaneous emission, negative refraction, and the diverging photonic density of states. Noble metal-based metal/dielectric multilayers (e.g., Au/SiO2 and Ag/TiO2) and metallic (Au and Ag) nanowires embedded inside a dielectric matrix have been traditionally used to demonstrate HMM properties and for implementations into devices. Noble metals are, however, unstable at high temperatures, complementary metal oxide semiconductor incompatible, and difficult to deposit in thin-film form due to their high surface energies that limit their potential applications. TiN has emerged as an alternative plasmonic material to Au in recent years, and epitaxial TiN/Al0.72Sc0.28N metal/semiconductor superlattices were developed that exhibit excellent HMM properties. As TiN exhibits ε-near-zero (ENZ) at ∼500 nm, TiN/Al0.72Sc0.28N HMM also operates from ∼500 nm to long-wavelength regions. However, for several energy-conversion-related applications as well as for fundamental studies, it is desirable to achieve HMM wavelengths from the near-UV to the near-IR region of the spectrum. In this article, we demonstrate hyperbolic photonic dispersion in (Hf,Zr)N/ScN, a class of metal/semiconducting superlattice metamaterial that covers the near-UV to the near-IR spectral range. Epitaxial HfN/ScN, ZrN/ScN, and Hf0.5Zr0.5N/ScN superlattices are deposited on (001) MgO substrates and characterized with synchrotron-radiation X-ray diffraction as well as high-resolution electron microscopy techniques. Superlattices grow with cube-on-cube epitaxy and with sharp interfaces. Optical characterization reveals both type-I and type-II hyperbolic photonic dispersions as well as low losses and high figures-of-merit. Along with its high-temperature thermal stability, demonstration of HMM properties in (Hf,Zr)N/ScN metal/dielectric superlattices makes them potential candidates for HMM devices.
Raman scattering from coupled plasmon–longitudinal optical (LO) phonon modes in polar semiconductors is an effective tool to determine electronic properties, such as carrier concentration, mobility, carrier freeze‐out, relaxation times, etc., as well as to understand different types of electron (hole)–phonon interactions. The physics of such coupling mechanism traditionally utilizes the Drude dielectric permittivity that predicts an increase in coupled plasmon–LO phonon mode (LPP+) frequencies with an increase in carrier concentrations. Herein, it is demonstrated that for n‐type epitaxial scandium nitride (ScN) thin films, the frequencies of the coupled plasmon–LO phonon Raman modes exhibit red‐shift with increasing carrier concentrations, which is contrary to the predictions from the Drude theory. Utilizing the generalized Lindhard dielectric function that considers both the frequency and wave‐vector components of free‐electron plasma, it is demonstrated that such a decrease in the frequencies of the coupled plasmon–LO phonon mode in Raman spectra is related to the nonconserved wave vectors due to inelastic scattering from magnesium (Mg) impurities. Modeling of the experimental Raman line shape, intensity, and frequencies illustrates that the wave‐vector dependence of the coupled modes decreases with increasing electron concentrations. An asymmetric broadening of LO Raman modes is observed in films having large electron concentrations (>1020 cm−3) that are explained by Fano resonance.
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