OPTICAL STUDY OF THE STOICHIOMETRY-DEPENDENT ELECTRONIC-STRUCTURE OF TICX, VCX, AND NBCX

The stoichiometry-dependent bulk electronic structure has been studied by optical spectroscopy for single crystals of TiCx with x=0.95 and 0.70, VCx with x=0.86 and 0.76, and NbCx with x=0.93, 0.84, and 0.71. The reflectance was measured in the energy range 0.540 eV for all of the samples and up to 100 eV for TiC0.95, VC0.86, and NbC0.93. By correcting for the surface-roughness effect using the measured roughness values, the data were Kramers-Kronig analyzed to obtain the dielectric function and related functions. The observed interband transitions have been interpreted on the basis of existing calculations for the energy-band structure and partial density of states. The main peaks above 4 eV were assigned to transitions at and around the X and Q portions in the Brillouin zone. Their shifts to lower energy and reduced intensities with decreasing carbon concentration are discussed in terms of p-d hybridization. Features arising from transitions involving the initial or final states near the Fermi level were observed in the low-energy region. Their dramatic stoichiometry-dependent behavior provides clear experimental evidence that the Fermi level moves downward in VCx and NbCx with lowering carbon content. A structure that develops remarkably in intensity as x decreases appears around 2 eV in VCx and NbCx, while no counterpart is discern- ible in TiCx. This feature can be attributed to a transition between vacancy-induced states with a1g and t1u symmetries. The contribution of the metal p state to the optical spectra is discussed in relation to both the intrinsic peaks and the vacancy-induced feature. The electron-energy-loss functions show distinct plasmon structures that weaken and shift to lower energy with decreasing x. The main volume plasmon peak is close to the free-electron-gas plasmon energy. A small Drude plasma structure has been observed at 2.7 eV in NbC0.93, being rapidly attenuated with increasing carbon vacancies. © 1990 The American Physical Society.