Infrared Plasmon and Phonon-Polaritons in Polar Semiconducting Scandium Nitride (ScN)

Interaction of light with collective free electron oscillations termed plasmon-polariton and with polar lattice vibrations termed phonon-polariton, are essential to confine the light in subwavelength dimensions and for strong optical 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 plasmon- and phonon-polariton in one host material with low loss is challenging due to the mutually conflicting physical property requirements.<br/><br/>In this work, we present high-quality plasmon- and phonon-polaritons in epitaxial polar ScN thin films deposited on (001) MgO substrates using dc-magnetron sputtering in the ultra-high vacuum (1 × 10^-9 Torr). As-deposited ScN thin films exhibit an <i>n</i>-type carrier concentration of (2-4) × 10²⁰ cm^-3 primarily due to the presence of oxygen impurities. Due to such high carrier concentrations, the Fermi level in ScN resides inside the conduction band, about 0.2-0.3 eV above the band edge. Mg-hole doping is used to reduce the high carrier concentration, and<i> p</i>-type ScN is <i>achieved</i> along with high hole mobility. Scanning transmission electron microscopy (STEM) is used to characterize the microstructure of the film. Both oxygen and magnesium-doped ScN films deposited in this work on (001) MgO substrates at high-temperatures are epitaxial and nominally single-crystalline with [001](001) ScN || [001](001) MgO relationships.<br/><br/>Spectroscopic ellipsometry and Fourier-transform infrared spectroscopy (FTIR) analysis indicate that the highly <i>n</i>-type doped ScN thin films exhibit low-loss short-wavelength IR plasmon resonance in the 1500-2500 nm spectral range. Due to the polar semiconducting nature, Mg-doped low carrier concentration ScN also exhibits phonon-polaritons between the 340 cm^-1- 677 cm^-1 spectral range. Excitation of the surface phonon-polariton modes is also demonstrated by polarization-dependent reflectivity measurement in the attenuated-total-reflection (ATR) mode using the Kretschmann configuration. A clear dip in the <i>p</i>-/<i>s</i>-polarized reflection spectrum at ~ 1.95 µm and at ~ 16 µm correspond to the surface plasmon-polariton (SPP) and surface phonon-polariton (SPhP) resonances respectively.<br/><br/>The plasmonic properties of ScN film are comparable to other IR plasmonic materials such as transparent conductive oxides and doped semiconductors. Also, ScN exhibit a very high figure-of-merit for SPhP propagation compared to its peer materials. Demonstration of plasmon- and phonon-polariton in one host material, ScN through carrier concentration control make it attractive for applications in epsilon-near-zero metamaterials, optical communication, solar-energy harvesting and other nanophotonic applications.<br/><br/><br/>References:<br/><br/>K. C. Maurya, D. Rao, S. Acharya, P. Rao, A. I. K. Pillai, S. K. Selvaraja, M. Garbrecht and B. Saha, "Polar Semiconducting Scandium Nitride as an Infrared Plasmon and Phonon-Polaritonic Material" <i>Nano letters (In-press, 2022).</i><br/><br/>K. C. Maurya, A. I. K. Pillai, M. Garbrecht and B. Saha, "Simultaneous Light-Harvesting at Visible and Mid-Infrared Frequencies with Epitaxial TiN/Al0.72Sc0.28N/TiN Metal/Polar-dielectric/Metal Metamaterials" <i>(In-review, 2022).</i><br/><br/><i>K. C. Maurya, V. M. Shalaev, A. Boltasseva and B. Saha, </i><i>"</i><i>Reduced Optical Losses in Refractory Plasmonic Titanium Nitride (TiN) Thin Films Deposited With Molecular Beam Epitaxy</i><i>" </i><b><i>Opt. Mater. Express. 10, 2679 (2020).</i></b>