| AVS 60th International Symposium and Exhibition | |
| Spectroscopic Ellipsometry Focus Topic | Thursday Sessions |
| Session EL-ThP |
| Session: | Spectroscopic Ellipsometry Poster Session |
| Presenter: | C.M. Nelson, New Mexico State University |
| Authors: | C.M. Nelson, New Mexico State University T. Willett-Gies, New Mexico State University L.S. Abdallah, New Mexico State University S. Zollner, New Mexico State University |
| Correspondent: | Click to Email |
Nickel oxide (NiO) is an interesting material, because it is a Mott-Hubbard charge-transfer insulator and also displays antiferromagnetic ordering of electron spins [1]. Spectroscopic ellipsometry is able to investigate the electronic structure of NiO (from the visible and UV portions of the spectra) and also its lattice dynamics (using infrared ellipsometry). Our interest in the NiO optical constants is also of a practical nature, to model ellipsometry spectra of bulk Ni and Ni thin films with a native oxide of NiO.
We measured the ellipsometric angles ψ and Δ for single-side polished bulk NiO from 0.8 to 6.5 eV with angles of incidence from 65 to 75° to determine the dielectric function. A dispersion model for the optical constants was built using two Tauc Lorentz oscillators; one with a Lorentz oscillator resonance energy at 3.96 eV and a second one with a much smaller amplitude at 6.40 eV. These peaks are in agreement with reflectance data analyzed using Kramers-Kronig transforms [2]. Our model also included a surface roughness layer with 40 Å thickness. Atomic force microscopy measurements confirmed this layer, showing an RMS roughness of 42.5 Å. We will report accurate dielectric function data for NiO from 0.8 to 6.5 eV.
FTIR ellipsometry was also performed on bulk NiO from 290 to 1000 cm-1 to study the lattice vibrations. TO phonons were found at 392 cm-1 and 551 cm-1, with the corresponding LO modes at 592 cm-1 and 545 cm-1. The weak TO mode at 551 cm-1 results from the antiferromagnetic ordering of NiO, which doubles the unit cell and causes zone folding, making a zone-edge TO mode infrared-active. Previous FTIR absorption measurements of NiO [3] did not report the infrared-active zone-edge phonon. Usually, antiferromagnetic ordering is only observed using neutron scattering, not with FTIR optical methods.
[1] G.A. Sawatzky and J.W. Allen, Phys. Rev. Lett. 53, 2339 (1984).
[2] R.J. Powell and W.E. Spicer, Phys. Rev. B 2, 2182 (1970).
[3] R. Newman and R.M. Chrenko, Phys. Rev. 114, 1507 (1959).
* This work was supported by the National Science Foundation (DMR-1104934) and performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Sandia National Laboratory (Contract DE-AC04-94AL85000).