X-RAY-SCATTERING DETERMINATION OF THE CU(110)-(2X3)N STRUCTURE

The structure of Cu(110)-(2X3)N has been investigated with x-ray reflectivity measurements of nine independent surface rods. Observed reflectivities ranging over five orders of magnitude were compared with calculations based on structural models. A pseudo-(100) reconstruction consisting of four [110BAR] copper rows in the outermost layer for every three of the bulk is favored both by symmetry and by an R-factor analysis. An excellent fit, with an R factor of 0.0051, is obtained for this model by including a large expansion of the interlayer spacing (57% greater than bulk) and large corrugation amplitude (50%, of the bulk interlayer spacing) in the outermost layer. These values can be understood by considering a hard-sphere model of the surface structure. The corrugation results in two raised and two lowered rows of copper atoms in the outermost layer and extends to several layers into the substrate (amplitudes from 9% to 22% of the bulk interlayer spacing). Vibrational amplitudes in the reconstructed layer are anisotropically enhanced by a factor of 2.8 normal to the surface and by 1.3 within the surface plane over bulk values. Nitrogen atoms are located 0.75 angstrom above the copper layer, and form a nearly (square-root 2 X square-root 2) R 45-degrees structure with respect to the reconstructed copper. This model is shown to be consistent with most previous experimental observations.