ANALYSIS OF THE NBC(111)-(ROOT-3X-ROOT-3)R30-DEGREES-AL SURFACE-STRUCTURE BY IMPACT-COLLISION ION-SCATTERING SPECTROSCOPY

The structure of the NbC(111) clean surface and the aluminum adsorption system at room temperature have been studied by impact-collision ion-scattering spectroscopy, reflection high-energy electron diffraction (RHEED), low-energy electron diffraction, and computer simulation. It has been found that the clean surface exhibits a 1 X 1 RHEED pattern and no reconstruction occurs. The outermost layer consists of Nb atoms and relaxes inward by 0.20 +/- 0.05 angstrom, which is equal to 15.5 +/- 3.9% of the space between the first and second layers of the bulk. The carbon atoms in the second layer also shift inward by 0.05 +/- 0.05 angstrom. The Al-adsorbed surface has a (square-root 3 X square-root 3)R 30-degrees structure with slight modulation. The Al atoms occupy two-thirds of the threefold-hollow sites, under which a Nb atom of the third layer is situated. Adsorbed Al atoms make a honeycomb pattern. The distance between the Al atom and the surface has been determined to be 2.25 +/- 0.05 angstrom. The Al-Nb bond length is 2.90 +/- 0.05 angstrom.