F. Jona

A reliability factor for surface structure determinations by low-energy electron diffraction

Abstract Surface structure determination by LEED (low-energy electron diffraction) is usually based on visual evaluation of the correspondence between calculated and observed intensity spectra. After a critique of the visual-evaluation method and of several criteria that have been proposed to replace it, a definition is given for a reliability factor r for a single beam, which purports to have almost all advantages of the visual criterion but not its disadvantages. A convenient quantity is the reduced r -factor r r = r r random where

Determination of the c(2 × 2) structure of sulfur on Fe{001} by low-energy electron diffraction

r random is the average value of r for randomly chosen pair of curves. The reduced r factor is calculated for more than 100 beams from 7 different surface structures and a direct relationship is established between the language of visual evaluation and the numerical values of r ϵ . Next, a reliability factor R for a number of beams is defined and tested — the result being a scale of values with which to measure the credibility of a given model in surface structure analysis. Plots of R versus non-structural and structural parameters allow objective and reliable choice of the “correct” model, especially when a large number of beams are available for testing.

Determination of Surface Structure by LEED

In the early stages of reaction with sulfur, a clean Fe{001} surface develops a c(2 × 2) superstructure. A low-energy electron diffraction analysis of this structure leads to a model in which the S atoms lie in the four-fold symmetrical sites on the Fe{001} surface, the S-Fe interplanar spacing being 1.09 ± 0.05 A and corresponding to an effective radius of 1.06 A for the chemisorbed S atoms. In contrast to Fe{001} 1 × 1-O, the first interlayer spacing of the substrate here is not significantly expanded.

Atomic underlayer formation during the reaction of Ti{0001} with nitrogen

After a brief review of some noteworthy recent achievements of LEED crystallography we describe two important advances in methodology. One is experimental and concerns the rapid acquisition of LEED intensity data from display-type equipment with a computer-assisted television camera. The other is theoretical and concerns the calculation of LEED intensities with a new cluster approach that offers notable advantages over the schemes presently used, especially for structures involving large numbers of atoms and for high electron energies.

Trends in metal surface relaxation

Abstract Changes in the {0001} surface of Ti during exposure to nitrogen gas are monitored by low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Evidence is provided for the formation of a Ti{0001}1 × 1-N phase followed by a Ti {001} 3 × 3−30°-N structure. A LEED intensity analysis of the Ti{0001}1 × 1-N phase reveals that the N atoms penetrate into the octahedral interstitial holes underneath the first layer of Ti atoms. This is the first ordered monoatomic underlayer found in the earliest stages of any solid-gas interaction. The surface structure bears close resemblance to that of {111} planes in bulk TiN. We find a Ti-N bond length of 2.095 A to be compared with the value of 2.120 A in bulk TiN. The analysis of the Ti{0001}1 × 1-N structure indicates that Ti {0001} 3 × 3−30°-N is not a low-coverage phase. The importance of recognizing the existence of 1 × 1 phases prior to the formation of superstructures is emphasized, and some procedures for extracting the information from experimental observations are discussed.

Structural properties of epitaxial films of Fe on Cu and Cu-based surface and bulk alloys

Abstract New LEED (low-energy electron diffraction) results are reported for Fe{310} and Fe{210} surfaces, including multilayer relaxation of atomic planes both perpendicular and parallel to the surface. The structures of six different surfaces of iron are now known. A comparison of the results yields relaxation trends: top-layer relaxation is found to increase monotonically as the surfaces become more open; for the higher-index surfaces {211}, {310} and {210} “decay curves” of relaxation as functions of depth into the surface show a surface-independent decay length (the depth at which the crystal returns to a bulk-like arrangement) of approximately 2 A.

Epitaxial growth of body-centered-cubic nickel on iron

Thin films of Fe were grown epitaxially at room temperature on Cu{001}, on the ordered surface alloys Cu{001}c(2×2)-Au and Cu{001}c(2×2)-Pd, and on the bulk alloy Cu3Au{001}. The maximum thicknesses attained in well-crystallized Fe films were 18 layers on the first three surfaces but only 3 layers on Cu3Au {00}. Low-energy electron diffraction (LEED) intensity data from 1-, 2- and 3-layer films on Cu{001&} could not be fitted by models with complete layers of Fe, suggesting that the growth of these ultrathin films was not layer-by-layer. The 12-layer films were found to have a tetragonally distorted face-centered-cubic (fcc) structure with a= 3.61 A, c= 3.54 A and 4%-expanded first interlayer spacing. Analysis of the elastic strain gave an equilibrium lattice constant of 3.59 A for fcc Fe at room temperature. Comparison with lattice constants from total-energy band calculations shows that the Fe cannot be in the nonmagnetic phase, but could be in the ferromagnetic phase, or possibly in an antiferromagnetic phase with the same lattice constant. It is suggested that the first interlayer spacing is enlarged owing to the larger magnetic moment of the first layer.

The 2 × 1 structural reconstruction of Si{001}

Abstract Studies by low-energy electron diffraction (LEED) and Auger electron spectroscopy of nickel films grown in ultra-high vacuum onto a clean Fe{001} surface show that the films have the body-centered cubic structure with the same lattice constant and the same multilayer relaxation as the clean substrate, as long as they are thinner than about 6 layers. LEED intensity analyses show that the multilayer relaxation of both clean Fe{001} and 3-layer thick Ni films involves 5% contraction of the first and 5% expansion of the second interlayer spacing. These new values of the multilayer relaxation of clean Fe{001} represent an improvement over previous determinations. Thicker Ni films, up to 100 layers, have a complicated structure that is neither b.c.c. nor f.c.c. Short anneals at temperatures between 200 and 650°C cause rapid diffusion of Ni into the Fe substrate with little evidence for formation of the stable f.c.c. phase of Ni.

Growth of face-centred-cubic titanium on aluminium

Abstract New data and LEED (low-energy electron diffraction) intensity analysis have led to a new structure for Si{001} 2 × 1 which gives very satisfactory agreement with the data; the fit to experiment is substantially better than for any previous structure. The structure has a dimer bond length of 2.54 A, an average contraction of the first layer spacing of 8%, three kinds of asymmetric displacements of the two dimer atoms, and differs substantially from all other recently proposed structures.

Relaxation at clean metal surfaces

Ultrathin films of Ti are grown on Al{001} and Al{110} substrates and studied by quantitative low-energy electron diffraction (QLEED). On Al{001}, high-coverage films (thicker than about 10 A) exhibit a well-developed phase. QLEED finds the bulk of a 25 A film to have a body-centred tetragonal structure with a = 2.864 A and c = 4.28 A. Strain analysis shows that this structure cannot be strained body-centred cubic and identifies the equilibrium (i.e., the unstrained) phase of the film grown as face-centred cubic (fcc). This result is a direct proof of the existence of an epitaxially stabilized fcc modification of Ti (which is not encountered in nature at any temperature), whose lattice constant is found to be 4.15 A. On Al{110}, the films are found to grow to a thickness of only 5 to 6 A, after which the LEED pattern is almost obliterated. A 2 A film is shown to be a pseudomorphic overlayer at a distance of 1.58 A from the substrate. On Al{001}, low-coverage Ti films exhibit a weak c() LEED pattern, but the associated surface structure could not be determined; partial results indicate that the Ti atoms may reside in the second layer of the substrate.