Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation S. A. Chambers, H. W. Chen, T. J. Wagener, J. H. Weaver; Summary Abstract: Nucleation and growth of ultrathin metallic overlayers on single‐crystal transition‐metal surfaces. J. Vac. Sci. Technol. A 1 May 1988; 6 (3): 1994–1995. https://doi.org/10.1116/1.575223 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAVS: Science & Technology of Materials Interfaces and ProcessingJournal of Vacuum Science & Technology A Search Advanced Search |Citation Search

We have used high-energy Auger-electron diffraction and associated kinematical scattering calculations to determine that Co, which is hcp at room temperature and atmospheric pressure, can be grown as a stable, unstrained fcc film on Ni(001). Studies of the early stages of growth show that the first monolayer equivalent does not cover the surface uniformly but rather forms two-dimensional patches with limited amounts of a second layer. The second monolayer equivalent adds to the second layer of each patch and provides a limited number of third-layer atoms. When the total Co coverage exceeds two monolayer equivalents, the overlayer coalesces into a continuous fcc film with no detectable strain up to 30 monolayers (the highest coverage studied). For comparison, we have measured angular distributions for Cu/Ni(001) with 0.5 and 1.0 monolayer of Cu. From surface thermodynamic considerations, Cu is expected to wet the surface uniformly. However, Cu also tends to form two-layer-deep patches at a coverage of one monolayer equivalent. These results demonstrate that limited surface mobility is significant in determining the morphology of monolayer metal films, in agreement with theoretical models of overlayer growth behavior.

High-energy Auger and medium-energy backscattered electron diffraction are shown to be useful structural probes for ultra-thin epitaxial films. Coherent scattering of such electrons in the energy regime of 500 to 1000 eV leads to intensity modulations of the order of 50% in an angle-resolved measurement. These modulations are well accounted for by a straightforward kinematical scattering formalism in which trial geometries are employed and varied. Optimal agreement with experiment consistently results from a single geometry, leading to a unique solution to the structural problem. Atomic coordinates and elastic strain associated with lattice mismatch at the interface can be routinely determined with sub-Angstrom precision. Moreover, first layer morphology and structural rearrangement associated with intermixing can be monitored. We present results from the CuNi(001), CoNi(001), FeCu(001), Cu/Fe/Cu(001) and FeGaAs(001) systems which illustrate these conclusions.

We have combined high-angular-resolution Auger-electron diffraction, kinematical scattering calculations, low-energy-electron diffraction (done in a pulse-counting mode), and high-energy-resolution x-ray photoelectron spectroscopy to examine the formation of the Fe/GaAs(001)-c(8\ifmmode\times\else\texttimes\fi{}2) interface. We find that clusters of bcc Fe at least three atomic layers deep grow in registry with the substrate for coverages up to \ensuremath{\sim}4 monolayer equivalents. These clusters contain Ga and As atoms which have been liberated from the GaAs substrate. Above this coverage, the clusters coalesce into a continuous bcc Fe matrix with a lattice constant equal to half that of GaAs and with principal crystallographic axes parallel to those of the substrate. This epitaxial Fe overlayer contains Ga and As in solution in the bcc lattice with the impurity atoms occupying interstitial face-center sites. The concentration of Ga and As decreases with distance from the GaAs substrate. At the same time, we find clear evidence for surface segregation of As and enrichment of the near-surface region.

We have investigated the atomic structure of annealed Co/Si(111) interfaces prepared by the evaporation of one-, two-, and thirty-monolayer equivalent coverages of Co. Experimental and theoretical angular distributions of the Co LMM Auger intensity reveal that for all coverages investigated a ${\mathrm{CoSi}}_{2}$ phase forms which is rotated 180\ifmmode^\circ\else\textdegree\fi{} about the surface normal relative to the substrate. At low coverages, the ${\mathrm{CoSi}}_{2}$ phase is in the form of clusters which appear to be two to three ${\mathrm{CoSi}}_{2}$ layers in thickness and are terminated by a Si(111) bilayer. The Si(111) overlayer, which is in addition to the terminal plane of Si atoms associated with the outermost ${\mathrm{CoSi}}_{2}$ layer, also appears to be rotated 180\ifmmode^\circ\else\textdegree\fi{} about the surface normal relative to the Si substrate.

We have used Auger electron diffraction with high angular resolution to measure elastic strain at a pseudomorphic metal-metal interface. Shifts in the position of the $\mathrm{Cu} {L}_{3}{M}_{4,5}{M}_{4,5}$ Auger intensity maximum along the [101] direction betray expansion of the Cu lattice normal to the Cu/Ni(001) interface resulting from the Cu-Ni lattice mismatch. In the pseudomorphic regime (up to 14 \AA{} of Cu), the Cu lattice constant perpendicular to the interface has been determined to be 3.71 \ifmmode\pm\else\textpm\fi{} 0.03 \AA{} while the lattice constant is 3.52 \AA{} in the plane of the interface (the lattice constant of Ni). Thus, the unit-cell volume of Cu is 46.0 \ifmmode\pm\else\textpm\fi{} 0.04 ${\mathrm{\AA{}}}^{3}$ in the pseudomorphic overlayer, compared to a bulk value of 47.0 ${\mathrm{\AA{}}}^{3}$. Above 14 \AA{}, the lattice constant perpendicular to the interface drops as a result of dislocation generation and the relief of elastic strain. The critical coverage at which strain relief begins and the dependence of strain on coverage are in good agreement with simple classical models.

We have investigated the role of the angle of electron beam incidence in producing anisotropies in angle-resolved Auger electron emission from single crystals. By performing angle-dependent measurements of Pd MNN Auger emission from Pd(001) with two different angles of incidence, we show that the Auger electron angular distributions are independent of incident beam direction. Rather, the observed intensity modulation is the result of Auger electron scattering and interference, as demonstrated by the comparison of experimental angular distributions with those calculated using kinematical scattering theory. In the calculation, it is assumed that atoms in each layer below the surface receive equal incident beam flux for all angular orientations (i.e., no incident beam channeling). Agreement between theory and experiment is good, indicating that although incident beam channeling surely occurs, the enhanced depth of penetration is ineffective at producing Auger electrons which can reach the surface without suffering inelastic scattering.