Charge-distribution changes accompanying the formation and changes in the composition of HfCx and TaCx.

Abstract

Core-level binding energies (BE's) and valence-band structures, determined with x-ray photoelectron spectroscopy, and C KVV Auger spectra were obtained for ${\mathrm{TaC}}_{\mathit{x}}$ (0.5\ensuremath{\lesssim}x\ensuremath{\lesssim}1.0) and ${\mathrm{HfC}}_{\mathit{x}}$ (0.6\ensuremath{\lesssim}x\ensuremath{\lesssim}1.0). In ${\mathrm{TaC}}_{\mathit{x}}$ the metal 4f BE, C 1s BE, and the BE of the main p-d valence-band peak decreased (moved toward the Fermi level) as x decreased; in ${\mathrm{HfC}}_{\mathit{x}}$ these BE's increased as x decreased. In ${\mathrm{TaC}}_{\mathit{x}}$ the largest BE shift with changing x was for the metal 4f BE; in ${\mathrm{HfC}}_{\mathit{x}}$ it was for the C 1s BE. For ${\mathrm{TaC}}_{\mathit{x}}$ the relative intensity in the valence-band spectra between 0 and 2 eV BE changed significantly as x decreased, eventually becoming the dominant intensity of the spectrum; for ${\mathrm{HfC}}_{\mathit{x}}$, deviations from x\ensuremath{\simeq}1 did not change the shape of the valence-band spectra appreciably. The data from both sets of materials are explained in terms of changes in the charge distributions accompanying changes in x. And although the two sets of spectroscopic behavior are quite different, the changes in the charge distributions we are proposing are nevertheless quite similar. In particular, it is proposed that, as x decreases for either ${\mathrm{HfC}}_{\mathit{x}}$ or ${\mathrm{TaC}}_{\mathit{x}}$, the electron charge increases in the vicinity of metal ions and decreases in the vicinity of carbon ions. An analysis of the C KVV Auger spectra is consistent with this interpretation in that the analysis suggests a reduction in C 2p-electron occupancy per carbon atom with decreasing x. The 4f BE's of Ta and Hf in the elemental metals also were measured and are used to discuss changes in the charge distributions that occur upon formation of ${\mathrm{HfC}}_{\mathit{x}}$ and ${\mathrm{TaC}}_{\mathit{x}}$.