H. Roux

Évolutions morphologiques des pointes métalliques par traitement thermique (vérification de la théorie de nichols et mullins)

Abstract The morphological evolution of conical tungsten tips of cone angles between 1 and 15° at temperatures between 1800° and 3000°K is studied inside a scanning electron microscope. The experimental results confirm the theory of Nichols and Mullins: existence of a critical cone angle α = 3°, formation of solid drops for α 3°. For a temperature in the order of 2900 °K (clean surface) the curvature radius at the tip apex increases with time according to the t 1 4 law. For temperatures below 2700°K discrepancies are found, which are due to an evaporation of tungsten compounds produced at the surface by a chemical reaction with the residual gas (10 −5 torr). The measurements at 2900°K have enabled the determination of the surface diffusion coefficient of tungsten: D = 2.4 ×10 6 cm 2 /sec.

Une détermination du coefficient d'auto-diffusion de surface avec des pointes à émission de champ (tungstène)

Abstract To measure the surface self-diffusion coefficient, a method is described which is based on the increase of the curvature radius of a tip (Nichols and Mullins). This radius increase depending on temperature and time is measured under ultra high vacuum condition. The cleanliness of the surface is controlled by field electron microscopy. The radius variation is measured in situ by Fowler-Nordheim characteristics, and the initial and final radius by scanning electron microscopy. Measured are, for tungsten between 2100 and 2850°K, the activation energy ( 3.1 eV atome ) and the diffusivity ( 0.9 cm 2 s ) .

Adsorption, diffusion and self-diffusion on tungsten surfaces with adsorbed palladium

The influence of a Pd adsorption layer on the surface self-diffusion of tungsten has been measured using a field emitter technique. By adsorption of 1.2 × 1015 Pd atoms/cm2 — measured by high energy ion backscattering — the surface self-diffusion energy decreases from 70 to 54 kcal/mole, and the diffusivity from 0.95 to about 0.2 cm2/s, which corresponds to an increase of the diffusion coefficient by a factor 40 at 1600 K and 80 at 1400 K. Furthermore are determined (1) adsorption lifetimes in adsorption-desorption equilibria, (2) the isosteric adsorption heat (103 kcal/mole), and (3) the coefficient of Pd surface diffusion on W resulting in diffusion energies of 32 kcal/mole around (001) and 17 kcal/mole around (111).

Une détermination du coefficient d'autodiffusion de surface en présence d'une couche d'adsorption à l'aide de pointes àémission de champ (nickel sur tungsténe)

The tip blunting technique to measure the surface self-diffusion of clean metals (A. Piquet, Vu Thien Binh, H. Roux, R. Uzan and M. Drechsler) is extended to study the influence of an adsorption layer on diffusion. The system studied is nickel on tungsten. The increase of the apex radius is measured by means of FEM characteristics. In the temperature range used (1200–1500 K), the nickel monolayer (1.16 × 1015 atoms/cm2) is maintained by compensation of desorbed Ni atoms with a continual flux from an evaporation source. The adsorption life time between 1350 and 1500 K decreases from 850 to 16 s. The conservation of the degree of coverage leads to a method to determine the desorption activation energy of nickel (Ed = 4.56 eV/atom). The surface self-diffusion data of tungsten with a nickel monolayer are found to be D0 = 3 × 10−3cm/2s and Qs = 1.9 eV/atom, compared to the clean tungsten data D0 = 1 cm2/s and Qs = 3.1 eV/atom. The Ni monolayer increases the surface self-diffusion coefficient by a factor 160 at 1200 K and 20 at 1500 K. The results are discussed with respect to nickel activated sintering of tungsten powders.

Processus simultane d'évaporation libre et de diffusion de surface☆

Abstract If a conical metal tip is annealed in vacuum, the radius at the apex increases with time by surface diffusion. This was calculated by Nichols and Mullins and recently verified by the authors. This phenomenon is now calculated in case of a simultaneous action of evaporation and surface diffusion. By evaporation the blunting rate should be lowered until it becomes zero for a critical value of the curvature radius. Numerical data for different temperatures and cone angles are calculated for some metals (W, Mo, Pt, Ni, Cu). Measured are profile changes of Mo tips during annealing. The lowering of the blunting rate and the existence of a limit radius are confirmed. An evaporation can also occur as a consequence of a surface reaction. The measured radius changes of tungsten tips annealled in 1.5 × 10 −5 torr and 2.5 × 10 −5 torr of oxygen agree fairly well with the predictions. Surface self-diffusion measurements at high temperatures may be erroneous if evaporation is not considered, an example for Mo at 2150°K is given.

A measurement of surface diffusion across steps on a curved surface (Pd on W)

Abstract The surface diffusion of palladium on the curved part of a tungsten crystal is studied by field electron microscopy. The variation of the local coverage distribution is measured by a probe-hole device on the stepped surface region around (001). The measured data allow a determination of the mass transport surface diffusion coefficient D of Pd on W across atomic steps as a function of temperature, coverage and step density. D has been found (1) to be constant for a given step density and for coverages lower than about 5 × 10 14 Pd adatoms/cm 2 , (2) to increase for higher coverages, and (3) to increase with increasing step density for a given coverage. The activation energy of the process is nearly constant (about 24 kcal/mol) for all coverages up to about 6 × 10 14 adatoms/cm 2 , while the pre-exponential factor of D increases with increasing step density. Interpretation of the results gives some information on the diffusion mechanism.

Ag mass transport on Si(111) in the 350–450°C temperature range

Abstract Ag mass transport on Si(111) has been studied by means of SEM and Auger analysis with spatial resolution, under ultra-high vacuum conditions. The spreading of Ag deposits is investigated in the 350–450 ° C temperature range where no desorption occurs. In order to avoid electromigration, the samples are heated by means of a halogen lamp. After the initial islands formation (Stranski-Krastanov growth mode) Ag begins to spread out of the initial deposit zone. The main features deduced from the concentration profiles analysis are: (i) for temperatures higher than ~400° C, the profiles show a rather constant concentration ending in a very sharp front side which may be attributed to an unrolling carpet mechanism; (ii) for temperatures lower than ~400° C, the corresponding profiles show two gradient zones; (iii) for both cases, a t 3 4 kinetics law is found: it suggests that Ag/Si mass transport could be controlled by surface self diffusion of Ag atoms on the Ag 3D islands with an activation energy of ~2.4 eV/atom.

AES quantitative interpretation of the first stages of Pd2Si formation during Pd adsorption on Si(111) at room temperature

Abstract The quantitative analysis of the evolution of Pd and Si N(E) Auger peak amplitudes has been carried out as a function of Pd concentration on a Si(111) surface. Modelization of these evolutions has allowed us to conclude that the formation of the Pd2Si silicide occurs from the beginning of Pd adsorption even at room temperature.

Surface Diffusion: Recent Works (Abstracts)

The influence of interactions between adsorbed particles on their diffusion constant is investigated by kinetic Ising models with independent nearest neighbour hops. This leads to expressions for the ratio of the diffusion constant at arbitrary coverage θ to its value at θ = 0 as a function of the interaction energies relative to temperature. It is shown that under certain conditions this quantity obeys a particle-hole symmetry. Exact results in the whole range of densities are given in one dimension for nearest neighbour interaction. They already yield a qualitative agreement with experimental results and are also compared to corresponding numerical simulations. The introduction of a next nearest neighbour interaction is shown to produce drastic changes in the density dependence of the diffusion constant in some of the cases. A generalized quasichemical approximation and a virial expansion are made in two dimensions, leading to a better agreement with the measurements.

The Study of Surface Self-Diffusion by a Measuring Profile Changes of Field Emitter Tips (W)

If a conical metal tip is heated in ultra-high-vacuum, the curvature radius at the apex increases continuously. A measurement of this phenomenon permits to determine the surface self-diffusion coefficient using Herrings equation and Nichols and Mullins numerical data. The increase of the curvature radius at the apex is determined in situ by measuring the field electron current as a function of voltage and the final radius and profile are determined by scanning electron microscopy. The logarithm of the surface-self-diffusion coefficient of tungsten without adsorbed layer versus the reciprocal temperatures, varies linearly in the experimental region (D=3.2.10-8 cm2/sec at 2100 K and D=3.6.10-5 cm2/sec at 2900 K). As activation energy a value of 3.08 eV/atom is found.