Optical properties of black NiO and CoO single crystals studied with spectroscopic ellipsometry

Using spectroscopic ellipsometry, we measure the dielectric functions of NiO and CoO single crystals, which are grown by using the Verneuil (flame fusion) process. Taking the 2nd derivative of the dielectric function, we identify several optical structures below and near the band gap. We fit the 2nd derivative of the dielectric function by using an exciton lineshape and the standard critical point (SCP) model. The band gap energies of NiO and CoO are estimated to be 3.87 eV and 5.43 eV, respectively. The optical structures below and near the band gaps are attributed to intra-atomic d-d transitions and are estimated to be 1.2, 1.3, 1.8, 2.6, 2.9, 3.2, 3.4, 3.7, and 4.2 eV for NiO and 1.61, 2.26, 2.72, and 3.6 eV for CoO. Both the band gap and the d-d transition energies of NiO and CoO are compared to the literature values. The derivative spectra show unambiguously the d-d transitions near and above the band gap and those below the band gap. The lineshape fitting using the SCP model provides accurate values for the band gap energy and the d-d transitions. In addition, the SCP analysis verifies the exciton nature of the band gap and the d-d transitions. We attribute this to the formation of Frenkel excitons in NiO and CoO transition metal oxides.