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    CrN electronic structure and vibrational modes: An optical analysis

    X. Y. Zhang and D. Gall

    • Rensselaer Polytechnic Institute, Troy, New York 12180, USA

    Phys. Rev. B 82, 045116 – Published 20 July, 2010

    DOI: https://doi.org/10.1103/PhysRevB.82.045116

    Abstract

    The optical properties of paramagnetic CrN over the wavelength range 250nm30μm were determined from transmission and reflection spectra of 4411000-nm-thick epitaxial CrN(001) layers that were grown on MgO(001) by ultrahigh vacuum magnetron sputtering at 700°C and were found to be pure cubic single crystals by x-ray diffraction ω2θ, ω, and φ scan analyses. The imaginary part of the dielectric function exhibits a steep onset at ω=0.64eV as well as peaks at ω=1.5 and 2.9 eV due to direct interband transitions and indicates a depletion in the density of states at the Fermi level with an upper limit for free carriers of 3×1019cm3. This is attributed to local magnetic moments that cause splitting of the t2g bands and the formation of an indirect band gap of 0.19±0.46eV, as estimated by comparing the optical transition energies with reported direct gap energies from calculations with different magnetic ordering and Coulomb interaction terms. The dielectric function shows a strong resonance at ω0=48.7±0.2meV, and values of εdc=53±5 and ε=22±2 below and above the resonance, respectively, providing values for transverse and longitudinal optical phonon frequencies at the zone center of 11.7 THz and 18.2 THz (corresponding to ω=48.7±0.2 and 75.6±6.8meV), respectively, and a Born effective charge of 4.4±0.9. The vibrational frequencies are confirmed by Raman spectroscopy peaks at 800, 1170, and 1330cm1 which are attributed to 2TO(X), 2LO(X), and 2LO(L) modes and correspond to single-phonon energies of 49.6 meV, 72.5 meV, and 82.5 meV, respectively. They are quantitatively comparable to those reported for ScN, a semiconductor with the same crystal structure as cubic CrN. In conclusion, both electronic interband transitions and optical phonon frequencies suggest that CrN is a Mott-Hubbard-type insulator with a small to negligible indirect band gap.

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