Vibrational Spectrum and Thermal Conductivity of Rare‐Earth Semiconducting Erbium Nitride Thin Films

Rare-earth semiconducting mononitrides (RENs) are an emerging class of materials due to their unique electronic and magnetic properties originating from strongly localized 4f orbitals. Erbium nitride (ErN) is one of the most promising REN and attracts significant interest for spintronics, thermoelectric, and Gifford-McMahon cryo-cooler applications. However, despite such progress, growth and characterization of the physical properties of ErN are rather challenging due to its propensity for oxidation, and no report on its thermal transport properties exists to date. Recently, high-quality ErN thin films are deposited and are stabilized in ambient with thin capping layers. Herein this letter, first-principles density functional perturbation theory to model the vibrational spectrum of ErN is utilized and the calculations with the phonon frequency measurements with inelastic Raman spectroscopy are verified. Consistent with its polar dielectric nature, ErN exhibits a longitudinal-optical transverse-optical phonon mode splitting at the Gamma-point with a separation of 333 cm(-1). Time-domain thermoreflectance is used to measure the low-temperature (80 -300 K) thermal conductivity of ErN films. At room temperature, ErN films exhibit a low thermal conductivity of 1.16 +/- 0.15 and 2 +/- 0.2 W mK(-1) on (001) MgO and (0001) Al2O3 substrates, respectively, making them attractive for thermoelectrics and thermal barrier coating applications.