Direct and modulated reflectance spectra of MnO,CoO, and NiO

Direct reflectance, thermoreflectance, and electroreflectance have been measured for MnO, CoO, and NiO above the fundamental edge. Spectra of all three materials support a model containing both localized and one-electron band states. In MnO peaks with temperature coefficients of ～ 10^?3 eV/K were observed at 5.7 and 6.9 eV, temperature-independent structure occurred at 5.4, 6.3, and 7.2 eV, and spectral features with indeterminate temperature dependence were seen at 4.6 and 5.5 eV. The temperature-dependent structure was assigned to one-electron interband transitions associated with anion 2p and cation 4s states; the temperature independent structure was assigned to crystal field split localized interionic transitions between the 3d-states of neighboring Mn ions. Thermoreflectance spectra for CoO exhibited temperature dependent structure (9.5 × × 10^?4 eV/K at 5.0, 6.0, and 7.2 eV. A strong, temperature dependent electroreflectance oscillation was seen in NiO near 6.2 eV. On the basis of these data the interband gap between the anion 2p and cation 4s bands was determined to be 5.7 eV in MnO, 6.0 eV in CoO, and 6.2 eV in NiO.