CrN electronic structure and vibrational modes: An optical analysis
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 were determined from transmission and reflection spectra of -thick epitaxial CrN(001) layers that were grown on MgO(001) by ultrahigh vacuum magnetron sputtering at and were found to be pure cubic single crystals by x-ray diffraction , , and scan analyses. The imaginary part of the dielectric function exhibits a steep onset at as well as peaks at 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 . This is attributed to local magnetic moments that cause splitting of the bands and the formation of an indirect band gap of , 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 , and values of and 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 and ), respectively, and a Born effective charge of . The vibrational frequencies are confirmed by Raman spectroscopy peaks at 800, 1170, and 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.