CrN electronic structure and vibrational modes: An optical analysis

The optical properties of paramagnetic CrN over the wavelength range 250 nm–30 m were determined from transmission and reflection spectra of 44–11 000-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 -2 , , and scan analyses. The imaginary part of the dielectric function exhibits a steep onset at =0.64 eV as well as peaks at =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 1019 cm−3. 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 0.46 eV, 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 0=48.7 0.2 meV, and values of dc=53 5 and =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 =48.7 0.2 and 75.6 6.8 meV , respectively, and a Born effective charge of 4.4 0.9. The vibrational frequencies are confirmed by Raman spectroscopy peaks at 800, 1170, and 1330 cm−1 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.