STRUCTURAL-ANALYSIS OF THE NITROGEN-ADSORBED NBC(111) SURFACE

The structure of the nitrogen-adsorbed NbC(111) surface has been investigated in ultrahigh vacuum by impact-collision ion-scattering spectroscopy (ICISS), reflection high-energy electron diffraction (RHEED), and by computer simulation. At room temperature, nitrogen is dissociatively adsorbed on the surface, which shows a diffuse RHEED pattern of (root 3 X root 3)R30 degrees structure at low coverage and (1X1) structure at saturation coverage. When the nitrogen-saturated surface is annealed at 950 degrees C for a few seconds, the RHEED pattern changes into a sharp (2X2) pattern. It has been found that, in both cases, a nitrogen atom goes into the threefold-hollow site on the second-layer carbon atom without changing the structure of the substrate surface. The height of a nitrogen atom from the first layer is 1.14 Angstrom and the N-Nb distance is 2.15 Angstrom, which is close to the sum of each of the ionic radii. The coverage of the (root 3 X root 3)R 30 degrees and (2X2) structure are estimated at 0.33 and 0.5 hit, respectively. Judging from the coverage, the apparent (2X2)structure is considered to be the (2X1)structure with three equivalent domains.