A.J. Melmed

On the atomic structure of {001}W

Abstract Results of a field ion microscope study of the clean surface structure of the {001} surface plane of tungsten are presented. The major conclusions are that {001}W is reconstructed over the temperature range 15–580 K, and that the reconstructed surface contains an alternating vertical component to the displacements of the W surface atoms. There are two magnitudes of verticaldisplacement, each arrayed with p(2 × 2) symmetry, and if the differences in the magnitude are neglected, the vertical displacements are arrayed in the well known (√2 × √2) R 45° configuration. Details of this newly developed experimental approach for the study of surface reconstruction are reported, along with a number of control experiments which exclude the possibility that these results are artifacts due to the experimental technique. The discussion includes a comparison of the present results with those drawn from other experimental techniques, primarily low energy electron diffraction.

Ellipsometry—LEED study of the adsorption of oxygen on (011) tungsten☆

Abstract The direct combination of ellipsometry and low-energy-electron diffraction (LEED) techniques has been extended to a study of room temperature oxygen adsorption on (011) tungsten. It is shown that, by use of a special continuous data-taking method, ellipsometry is easily capable of detecting small fractions of a monolayer of adsorbed oxygen on this metal crystal face. From the partial correlation of LEED and ellipsometry data, average sticking probability for oxygen adsorption in the first 1 2 monolayer of atoms is deduced to be 0.2 or 0.4, depending on the choice of adsorption model. The experimental ellipsometry results are not understandable in detail in terms of existing ellipsometry theory. However, in view of the LEED results, it is tentatively concluded that the present results support a previously published complicated adsorption model involving oxygen atom-tungsten atom rearrangements.

Surface Self‐Diffusion of Nickel and Platinum

The surface rearrangement of nickel and platinum due to (1) an electric field (build‐up) and (2) surface tension (annealing) as the net driving force, has been studied by field‐electron emission microscopy over the temperature range 510°–750°K for Ni and 550°–850°K for Pt. An Arrhenius‐type relationship between temperature and time was found in both cases, so that an activation energy could be associated with each process. It is assumed that this is an average activation energy for surface diffusion.The results of the field build‐up experiments gave an activation energy of Qf=19.1±1.9 kcal/mole for Ni over the negative field range (28–38)×106 V/cm, and Qf=26.3±2.6 kcal/mole for Pt over the negative field range (27–39)×106 V/cm. It was possible to measure the effect of the electric field using a method devised earlier by Bettler and Charbonnier. This gave calculated zero‐field activation energies of 20.6±2.1 kcal/mole for Ni and 29.7±3.0 kcal/mole for Pt. The results of the annealing experiments (no electr...

Interactions of sulfur with nickel surfaces: Adsorption, diffusion and desorption

The kinetics of adsorption, surface diffusion and thermal desorption of sulfur on Ni surfaces have been studied using field electron emission microscopy methods. The sticking probability for elemental sulfur sublimed onto a Ni specimen is approximately unity for Ni substrate temperatures from 77 to 530 K. The maximum average work function for ~ monolayer coverage of S ranged from 4.7 to 5.05 eV in different experiments; for fractional monolayer doses of S, surface diffusion was rapid at T 400 K, with activation energies ranging from 15 to 28 kcalmol. For multilayer adsorption of sulfur, diffusion occurs without motion of a sharp boundary, and there is evidence of extensive surface reaction between S and Ni (emission from small “crystallites” is evident in the field emission patterns). Sulfur desorbs from Ni at temperatures above 1500 K.

Analysis of high resistivity semiconductor specimens in an energy‐compensated time‐of‐flight atom probe

It is shown that high resistivity semiconductor specimens (at least up to 8.6×103, and probably up to 2.4×104 Ω cm) can be analyzed in a conventional energy‐compensated time‐of‐flight atom probe by using pulses of longer than usual duration and that the necessary pulse width increases with specimen resistance.

The structure of field-evaporated hexagonal close-packed metal surfaces: rhenium and ruthenium

Abstract Field-ion micrographs of field-evaporated ruthenium are analyzed and compared with similar micrographs of rhenium. Certain surface topological features appearing in Ru at 21 °K and 77 °K do not appear in Re at these same temperatures, although both metals have the hexagonal close-packed structure. The Ru field-evaporated surface clearly develops an exposed A, B, A, B, … sequence of net planes in regions where this is expected due to the hcp structure, whereas the Re field-evaporated structure develops an AB, AB,… sequence, giving half the number of distinguishable net plane edges, in the same crystallographic regions. The difference is discussed in terms of geometric and electronic factors which influence the field evaporation process.

Epitaxial crystal growth of hcp metals on bcc metals: Dysprosium on tungsten

Abstract Surface diffusion of dysprosium on tungsten is discussed and activation energies for multilayer diffusion over various substrate planes are measured. Nucleation and epitaxial crystal growth are investigated and it is shown that single crystal or polycrystal layers can be grown under controlled conditions in the field emission microscope. The epitaxial relationships found were: (0001) Dy | (011) W with either [11 2 0] Dy | [001] W or [11 2 0] Dy | [111] W.

Helium field-ion microscopy of hexagonal close-packed metals

Abstract The atomic arrangement of curved hexagonal close-packed metal surfaces is more complex than curved surfaces of body-centered and face-centered cubic metals. He field-ion microscopy discloses apparent differences in the surface structures of the h.c.p.metals Ru and Re field-evaporated under similar experimental conditions. The effect of temperature and field strength on the structure developed by field-evaporation is analyzed, and the structures of the (100) or (1010 and (001) or (0001) plane regions in particular are discussed in relation to their respective field-ion images.

Feasibility of ToF atom-probe analysis of silicon

Abstract Data are presented which establish that silicon with electrical resistivity up to at least 10 Ω cm can be field-evaporated and analyzed in a conventional energy-compensated ToF atom-probe instrument.

Optical Constants of (011) Tungsten in the Visible Region

Optical constants for tungsten measured ellipsometrically at room temperature in the visible spectrum are reported. Results are given for (011)-oriented tungsten characterized as clean by low-energy-electron diffraction, for the same surface saturated with oxygen, and for the surface after exposure to air at atmospheric pressure for several hours. Results are compared with earlier work and differences are attributed primarily to differences of surface contamination.