E.C. Viljoen

Sn bulk-to-surface diffusion in a Cu(111)(Sn) single crystal

Abstract The kinetics of the bulk-to-surface Sn diffusion in a dilute Cu(Sn)(111) single crystal had been investigated in the temperature range 678 to 794 K using Auger electron spectroscopy and low energy electron diffraction. It is shown that a sputter time correction is essential to obtain an accurate estimate of the diffusion coefficient D . After deriving the segregation equation for limited surface site accessibility, the modified Darken equations are used, with sputter time corrections, to determine the diffusion coefficient D and the segregation energy ΔG from the kinetic data. The data and fits are characterised by a high degree of accuracy and the fit parameters have the following values: D 0 = (7 ± 6) × 10 −6 m 2 /s, E = 168 ± 5 kJ/mol and ΔG = 76 ± 3 kJ/mol.

Linear programmed heating in surface segregation measurements

Abstract A new method of determining bulk diffusion parameters from surface segregation measurements is proposed whereby the sample temperature is increased linearly with time. The advantages of this method are discussed and a simple procedure is given which may be used to obtain the pre-exponential factor D 11 and the activation energy E from a single segregation run.

Orientation dependence of the surface segregation kinetics in single crystals

Experimental results are presented that support an earlier hypothesis that the bulk diffusion near surfaces is orientation dependent. From a literature study, the calculated surface energy of the (110) surface is higher than that of the (111) surface in fcc crystals, which could lead to different values of the bulk vacancy formation energy. This energy term determines the equilibrium bulk vacancy concentration and consequently also the bulk diffusion coefficient. Experimental surface segregation results were obtained for Cu(0.1%Sb) crystals for the said (110) and (111) surfaces using the linear heating method. The results, D 110 > D confirm the assumptions, and the order of magnitude experimental values of the activation energies are compatible with the calculated surface energies from the literature.

Sn segregation to the low index surfaces of a copper single crystal

Abstract The kinetics of the surface segregation to the low index surfaces of a Cu(111) 0.121 at% Sn, Cu(110) 0.072 at% Sn and a Cu(100) 0.099 at% Sn single crystal was investigated by means of Auger electron spectroscopy and low energy electron diffraction (LEED) in the temperature range 660 to 840 K. The data was fitted with a modified Darken model which describes the concentration-time dependence from time zero to equilibrium using equilibrium coverage values deduced from the LEED measurements. The diffusion coefficients for the bulk diffusion in the three directions are:D(111) = 9 × 10−5exp(−172/RT), D(110) = 5 × 10−5exp(−178/RT)andD(100) = 7 × 10−6exp(−168/RT)m2/s. The surface segregation energies are also found from the fits and areΔG(111) = 76 ± 5, ΔG(110) = 72 ± 5andΔG(100) = 64 ± 5, respectively.

Surface segregation measurements via the linear programmed heating method: Part A — Theory

Abstract Linear programmed heating (LPH) was developed to alleviate experimental and data analysis problems experienced in surface segregation measurements. In this article the technique of LPH, as well as the fit procedures used to extract the diffusion parameters D 0 and E and segregation energy Δ G from the experimental data, are described. In particular, a hierarchy of methods is proposed in order to determine a unique set of segregation parameters, starting with the simple Ficks model and progressing to the more advanced modified Darken model (MDA). Each of these models, its application and advantages are discussed in detail.

Effect of nitrogen and sulphur on the segregation of molybdenum on single crystalline Fe-3.5%Mo-N, S alloys

Abstract This work describes an experimental study of the effects of nitrogen and sulphur on the surface segregation of molybdenum in an Fe–3.5%Mo alloy. It is found that the presence of small concentrations of bulk dissolved nitrogen induce a strong synergetic segregation (cosegregation) of molybdenum and nitrogen. This effect is strongest for the (100)-oriented alloy, where an epitaxially stabilized MoN surface compound is formed. The synergetic effect of segregated sulphur on the segregation of molybdenum is found to be rather weak. This finding is consistent with the recent work of A. Rolland, M.M. Montagono, J. Cabane, Surf. Sci. 352–354 (1996) 206.

Measurement of the kinetics of surface segregation via the linear heating method: an experimental evaluation

Abstract Experimental results from a linear heating study of the bulk-to-surface diffusion of Sn to the surface of a Cu(100)–0.099 at.%Sn single crystal in the temperature range 600–750 K are presented. It is shown that a universal log-log plot of the surface enrichment versus temperature allows a direct determination of the diffusion parameters D0 and E, which are found to be D0 = 0.0063 m2/s and E = 198 kJ/mol. These values are compared with experimental results on the (111) surface and a fitting strategy, using methods with increasing sophistication, is proposed. The origin of the deviation and scatter of the diffusion coefficient values are discussed in terms of experimental constraints.

STABILITY OF TWO–DIMENSIONAL SURFACE NITRIDES ON TRANSITION METAL ALLOY SURFACES

The thermal stability of the two-dimensional surface nitrides CrN, MoN and WN has been studied on transition metal alloy surfaces. The surface nitrides are obtained by means of cosegregation of the constituents on Fe–15%Cr–N(100), Fe–3.5%Mo–N(100) and Fe–9%W–N(100) single crystals. The stability of the surface nitrides decreases in the sequence Cr>Mo>W, similar to the well-known three-dimensional bulk nitrides. All compounds exhibit 1×1 LEED structures, indicating epitaxial stabilization on the corresponding substrate surface. In a narrow temperature range T≈550°C the Fe–9%W–N(100) surface shows c(2×2) ordering of the segregated nitrogen atoms.

Electronic sample-and-hold circuit to minimize artifacts during high-temperature Auger electron spectroscopy measurements

The accurate control of sample temperature during Auger electron spectroscopy measurements was obtained in this laboratory using a pulsed heater current. This setup is favored instead of adjusting the dc level of the heater current not only because of its simplicity—it also provides the opportunity to measure the Auger signal during the “off” cycle, thus avoiding possible magnetic effects. The output of the lock-in-amplifier responds to this pulsed current as a differentiation circuit: It spikes on the positive and negative going edges. To eliminate this problem, a sample-and-hold circuit synchronized to the pulsed heating current frequency and a low-pass filter were inserted after the lock-in amplifier.