ADATOM DIFFUSION ON METALS : Ir ON W(110)

(1)

HAL Id: jpa-00229945

https://hal.archives-ouvertes.fr/jpa-00229945

Submitted on 1 Jan 1989

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

ADATOM DIFFUSION ON METALS : Ir ON W(110)

M. Lovisa, G. Ehrlich

To cite this version:

M. Lovisa, G. Ehrlich. ADATOM DIFFUSION ON METALS : Ir ON W(110). Journal de Physique

Colloques, 1989, 50 (C8), pp.C8-279-C8-284. �10.1051/jphyscol:1989847�. �jpa-00229945�

(2)

COLLOQUE DE PHYSIQUE

Colloque C8, suppl6ment au n o 11, Tome 50, novembre 1989

ADATOM DIFFUSION ON METALS : Ir ON w(IIo)(~)

M. LOVISA and G. EHRLICH

Materials Research Laboratory and Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, U.S.A.

Abstract

-

The diffusion of a single iridium atom on W(110) has been characterized by measurements of the mean square displacement at different temperatures, and of the distribution function governing displacements at a low temperature. Multiple jumps are found to make a sizable contribution to diffusive motion.

1

-

INTRODUCTION

Atomic diffusion is one of the important processes in the growth of crystals and overlayers, and as such we have been carrying out an extensive examination of how heavy metal atoms migrate on surfaces. In the first phase of this effort, S. C. Wang has made detailed studies of the diffusion characteristics as well as of the jump mechanisms for various metal atoms on W(211) /1,2/. Here we present a progress report on another aspect of our work, aimed at understanding atomic behavior on the most .densely packed plane of tungsten, W(110). Initial efforts have been concentrated on characterizing the diffusion mechanisms of iridium atoms, for which we have in the past explored the interactions between atoms leading to cluster formation 131.

2

-

EXPERIMENTAL DETAILS

The procedures and techniques for measuring atomic diffusion standard in our laboratory have already been set out elsewhere 111. This particular study relied on a field ion microscope somewhat different from the usual design. As is apparent from Fig. 1, the sample holder is mounted vertically, to facilitate the possible use of liquid nitrogen as a refrigerant. It must be emphasized, however, that in the present studies imaging has been done, as usual, with the coldfinger cooled to

=

20K by flowing a mixture of gaseous and liquid helium through it. Obser- vations ~f atomic motion have been started only after most thorough outgassing of the system and components, especially the micro-channel plate used for image intensification, usually lasting several weeks.

(')Supported by the Department of Energy under Contract DE-AC02-76ER01198

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989847

(3)

He Coolant

Phosphor Screen ,Window

Fig. 1 - Field ion microscope tube used in the present work.

This is part of an all-glass ultrahigh vacuum system, not shown.

Diffusivities D are obtained in the usual way from the mean square displacement < Ax2 > along one direction using the Einstein relation

where t denotes the length of the diffusion interval. Typically a hundred or so measurements are made at each temperature. These observations are used to map out the grid of binding sites on the surface, thus establishing an absolute distance scale for the quantitative estimation of the mean square displacement. The diffusion parameters are derived from the temperature dependence of the diffusivity, represented in terms of the activation energy E, and the prefactor Do by

where

Do = v12 exp ( S a ) ; (3)

v is an effective vibrational frequency, 1 the jump length, and S, the entropy of activation.

To estimate temperatures, the resistance of a small section of the support loop, defined by the potential leads, is measured with a double Kelvin bridge circuit 141, after calibration of the resistance at three setpoints.

3 - DIFFUSION O F IRIDIUM ADATOMS

Measurements of the diffusivity of a single iridium atom on the (1 10) plane are plotted at different temperatures, covering a range of 50 degrees, in Fig. 2. Roughly 200 observations are made at each temperature, but enlarged data samples were taken at 300 and 340K. From these observations we derive an activation energy for diffusion E, = .97 +.02eV, and a prefactor Do = 1 . 7 ( ~ 2 . 3 * ~ ) ~ 1 0 - ~ cm 2/s. A prefactor of 10-3cm2/s is considered normal

---

that is the value obtained if the jump length in Eq.(3) amounts to one surface spacing, and the entropy of

(4)

Fig. 2

-

Arrhenius plot for the diffusion of a single iridium atom on W(110). total number of observations = 2200.

activation is negligible. The prefactor for iridium is on the high side, but considering the uncertainty in E,, is nevertheless in the normal range. The activation energy itself is somewhat surprising in light of previous studies on W(l lo), summarized in Table 1 15-9/. Our activation energy is considerably higher than any of the previously reported values for iridium. In fact, it appears higher even than the a.ctivation energy for the diffusion of tungsten adatoms. In view of this disparity, we have made careful comparisons of the behavior of Ir and W adatoms on the same W(110) plane. In these studies, an iridium atom is placed on the (1 10) plane, and its displacements are recorded. The atom is then removed by field evaporation, to be replaced by a tungsten atom, whose motions are studied at extictly the same temperature. This sequence has been repeated at four temperatures, and yields the results in Fig. 3. Although these measurements have not been extended over a large enough range of temperatures to yield good diffusion characteristics, it is nevertheless clear that the mean square displacements of the two atoms do not differ much in the temperature range examined. Our limited experiments suggest a higher barrier to the diffusion of tungsten than of iridium, as previously reported by others 15-81. Where we differ from other reports is in the actual magnitude of the activation energy and the prefactor for diffusion

---

both are higher than previously reported. This disparity may arise from differences in the techniques for calibrating distance and

Table 1.

Diffusion on W(110)

D,(cm2/s) E,(eV) Ref

Present Work Ir 1.7x1e2 0.97

Bassett & Parsley Ir 8.9x1P5 0.78 6 W 2 . 1 ~ 1 0 - ~ 0.86 6 Tsong & Kellogg Ir ~.OXIO-~ 0.70 7 Cowan & Tsong W 4 . 5 ~ 1 0 - ~ 0.77 8 Tsong (1988) W 6 . 2 ~ 1 0 - ~ 0.90 9

(5)

Fig. 3

-

Comparative diffusion measurements for tungsten and iridium adatom on the same W(110) plane. The diffusion parameters are for internal comparisons only.

temperature scales. It will be considered again elsewhere, but here we will focus upon our primary concern, namely upon the mechanism of the diffusion process.

As already noted, the prefactor Do determined in our measurements is somewhat higher than normal, as it would be in accord with Eq. (3) if atomic jumps spanning more than a single surface spacing were making a significant contribution to the diffusion process. To pursue this possibility in greater depth, we havz made detailed measurements of the distance distribution function. For one-dimensional diffusion in which jumps to nearest neighbor sites occur at the rate

a,

and jumps to a second-nearest neighbor site take place at the rate P, the probability px(t) of finding a displacement x after a time t is given by 121

where I,(z) is the modified Bessel function of order x. These relations are readily adapted to describe the two- dimensional displacement distribution for iridium on W(110) on the assumption that jumps always occur along the close-packed directions.

Actual measurements of the displacement distribution, based on an initial sample of 824 determinations, are shown in Fig. 4. The best fit to the experiments is obtained with a double jump ratio

P/a

= 0.21, using the minimization routine Minuit /lo/. That this result is outside the range of statistical errors is clear on comparing the experiments in Fig. 4 with the values found if only single jumps are allowed. The latter is obviously a poor representation of the experiments. For iridium on W(110) it appears that diffusion at room temperature involves a significant contribution from long jumps.

(6)

Ir

^{on W(110)}

T = 300K, 20 sec

$1

<Ax2> = 0.302

11

B p/a = 0.21 I Experiments I

p/a=o

&a,

Fig. 4

-

Frequency of displacements observed for iridium atoms diffusing on W(110) at T = 300K. Best fit to the experiments is obtained with a double jump ratio pla = 0.21.

4

-

SUMMARY

For tungsten atoms on W(110) measurements of the displacement distribution during diffusion have previously been reported by Tsong and Casanova 11 I/, who concluded that single-jump processes were adequate to describe atomic motion. In view of their limited data base, however, there still remains doubt about the jump mechanism for tungsten atoms on W(110). On W(21 I), extensive studies of atomic jumping have been made for a variety of adatoms 121. It is of interest that on this plane, tungsten diffusion was found to occur exclusively by jumps between nearest-neighbor sites. In contrast, for adatoms of the platinum metals, a small contribution from longer jumps was detectable, which for iridium amounted to

=

4% of the total. In the diffusion of iridium on W(110), which on a hard-sphere model appears much smoother than W(21 I), the present measurements suggest that double jumps are much more important, amounting to roughly 20%. This is not surprising; on a smooth surface, deexcitation of an atom after a jump is expected to be difficult. It is too early to generalize from this very limited base of information

---

more quantitative measurements on different adatoms and substrates are clearly in order.

However, the rather significant contribution of double jumps for iridium on W(110) makes it likely that migra- tion by jumps covering more than a nearest-neighbor spacing may be a fairly common phenomenon.

ACKNOWLEDGMENTS

During the course of this study we have had unstinting help in experimentation from R. S. Chambers, and in building equipment from W. I. Lawrence.

(7)

REFERENCES

I l l Wang, S. C. and Ehrlich, G., Surf. Sci. 206 (1988) 451.

121 Wang, S. C., Wrigley, J. D., and Ehrlich, G., J. Chem. Phys. In Press.

131 Watanabe, F., and Ehrlich, G., Phys. Rev. Lett. 62 (1989) 1146.

141 Reed, D. A., and Ehrlich, G., Surf. Sci. 151 (1985) 143.

151 Bassett, D. W., in "Surface Mobilities on Solid Materials," edited by V. T. Binh (Plenum Press, New York 1983) p. 63.

161 Bassett, D. W., and Parsley, M. J., J. Phys. D3 (1970) 707.

171 Tsong, T. T. and Kellogg, G., Phys. Rev. B12 (1975).

181 Cowan, P. L., and Tsong, T. T., Phys Lett. 53A (1975) 383.

I91 Tsong, T. T., Rep. Prog. Phys. 51 (1988) 759.

I101 Dedoussis, Sp., Chardalas, M., and Charalarnbous, S., Comp. Phys. Comm. 31 (1984) 29.

I1 11 Tsong, T. T. and Casanova, R., Phys. Rev. B22 (1980) 1632.