Growth of gold monolayers on polycrystalline tantalum

0038-1098/88 $3.00 + .00 Pergamon Press plc

Solid State Communications, Vol. 65, No. 12, pp. 1605-1608, 1988. Printed in Great Britain.

G R O W T H O F G O L D M O N O L A Y E R S ON P O L Y C R Y S T A L L I N E T A N T A L U M S. Raaen and O.-M. Nes Physics Department, University of Trondheim-NTH, N-7034 Trondheim-NTH, Norway

(Received 10 November 1987 by L Balslev) Ultraviolet photoemission, work function and electron energy loss measurements have been performed on thin gold overlayers on a polycrystalline tantalum substrate. The evolution of the Au 5d band has been correlated with changes in collective excitations of the conduction electrons as well as with variations in the work function.

M E T A L overlayer systems have in recent years attracted considerable interest [1]. This is partly due to the fact that electronic structure as well as chemical reactivity can be dramatically altered with overlayers down to one monolayer [2]. The evolution of the electronic structure of the thick overlayer as well as the initial modifications at low coverages, is of importance for the understanding of adatom-substrate and adatom-adatom interactions. This present work considers Au overlayers on polycrystalline substrates of Ta and C. Graphite is maybe the most convenient choice of a weakly interacting substrate where the adatom-adatom interactions will dominate over the adatom-substrate interactions, whereas in the case of e.g. Ta the substrate interactions cannot be neglected. Studies on graphite have shown that metal atoms form a few large clusters on a crystalline surface, and many small ones on amorphous C [3-5]. Thus, Au on a crystalline graphite substrate may serve as a reference system for the case of rapid clustering, to which the Au on Ta system can be compared. The separation of the 5d-bands in the two elements Ta and Au facilitates the study of the evolution of the Au photoemission upon evaporation onto the substrate. In addition to photoelectron spectroscopy, Auger electron, and electron energy loss measurements have been performed on these systems. The photoelectron and electron energy loss spectra (EELS) were recorded in an angle-integrated mode using a VSW (Vacuum Science Workshop) HA50 "bolt on" analyzer. The exciting radiation was the Hel- (21.2eV) and the Hen- (40.8eV) lines from a discharge lamp, and a total resolution of about 0.2 eV was obtained in the photoemission spectra. The electron energy loss spectra were recorded in the reflection mode using a primary beam energy of 280 eV. Here the total resolution was about 1 eV. The electron gun was a VSW EG5. Work function measurements were

carried out by biasing the sample at - 5 V and subtracting the width of the photoelectron spectrum from the photon energy, using the He~ line at 21.2eV. Au was evaporated from a thoroughly outgassed tungsten basket. The Ta and C substrates were thin foils (Ta, 99.997% from Alfa Products; C, 99.8% from Goodfellow Metals) that were cleaned by flash heating to above 2000 C. The substrates were polycrystalline as evidenced by the absence of LEED spots; however, I

'

I

'

I

I

I

/~l

Au/Ta

I

l

~ / ~ HeII

4,J

v

oq 4-J

I

8

,

I

,

,

,

I

6 4 2 0 Binding energy (eV)

Fig. I. Photoemission spectra of gold on tantalum taken at 40.8eV photon energy. Curves a-h show increasing A u coverage: a-clean Ta, b-0.3 M L , c0.7 ML, d-l.1 ML, e-l.7 ML, f-2.3 ML, g-3.4 ML, and h-5.8 ML.

1605

G O L D M O N O L A Y E R S ON P O L Y C R Y S T A L L I N E T A N T A L U M

1606 I '

I '1

'

r 'I

'l

'I

'

I

AulC

HEll j~ /

P_

v<

.~I~IL 12

10

fJll

B

6

llll

4

2

I,ll

O

BINDING ENERGY (EV)

Fig. 2. Photoemission spectra, at 40.8 eV photon energy, of gold on graphite. Curves A - G represent increasing gold coverages; 0.1, 0.2, 0.3, 0.9, 2.4, 3.9 and 5.4 ML, respectively. thin Ta films are known to recyrstallize in the (110)-direction under certain conditions, and we cannot rule out the possibility of some tendency of preferred orientation. The photoemission from C showed a strong dependence on the emission angle. We attribute this to the layered structure of graphite; the crystallites being rotated in the surface plane so that the aximuthal angle is averaged out, and the angle to the surface normal being well defined. One particular angle was chosen for these experiments on C, and the substrate contribution to the photoemission was subsequently subtracted out. The Au coverage was calibrated by Auger electron spectroscopy, using a model where the assumption was made that Au forms an even layer on Ta. Gold was evaporated at a rate so that 2 minutes correspond to 2.2ML (monolayers). The pressure in the chamber was in the low 10-to torr range while taking measurements, and during evaporations rose to the high 10-~°torr range. Surface cleanliness was verified by Auger electron and valence

Vol. 65, No. 12

band photoelectron spectroscopy. Our electron analyzer was confined to an energy range of 0-300 eV, and therefore Auger and EELS measurements were limited to this range. Photoemission spectra (taken at a photon energy of 40.8 eV) for various gold coverages on tantalum and graphite are shown in Figs. 1 and 2, respectively. The carbon background has been subtracted out in Fig. 2. At low coverages of Au on Ta, two peaks appear at binding energies 5.0 and 6.3 eV. The positions of these features do not change appreciably until a coverage of slightly more than one monolayer has been reached. At this point a dramatic increase in the photoemission intensity nearer the fermi level is taking place, and the formation of a band with two well resolved features, centered at binding energies near 3.0 and 6.5 eV, is observed. The energy band shows relatively small changes from a gold coverage of a little less than 2 M L to the thick overlayer. In an early work, Plummer and Rhodin [6] concluded that the electronic configuration of Au was different on the surface of tungsten than in the ground state. Considering the proximity of the 5d and 6s levels in atomic gold [7] and the effect of a small perturbation, it is conceivable that the large shifts in the d-states in Fig. 1 are caused by a d 9---~ d l° configurational change as gold grows on tantalum. A higher d-electron count will increase the localization of the conduction electrons and hence lead to a better screening in the final state of the photoemission process. In addition to this effect, the development of a band structure will tend to shift states towards the fermi level, due to improved screening. The photoemission spectra of Au on C show very small variations in going from low coverages of a fraction of a monolayer to the thick overlayer regime. Here, the two well resolved components are located near the same energies as in the case of a thick Au layer on Ta. These data can be understood in terms of clustering of gold atoms on the graphite substrate [3]. The lateral interactions are much larger than the coupling to the substrate, and the atoms coalesce. The coordination numbers in these clusters are large enough so that a relatively complete band structure is formed, with a non-vanishing density-of-states at the fermi level even for relatively small depositions of Au. Auger electron measurements were done to obtain information on the morphology of the evaporated overlayer. The data on Ta could be modeled by a simple even layer growth, whereas on C the attenuation of the substrate was less than expected from such a model. These results indicate that clustering are taking place to a large extent on the polycrystalline graphite substrate, whereas that does not seem to

Vol. 65, No. 12

GOLD MONOLAYERS ON POLYCRYSTALLINE TANTALUM i

5.5

i

i

i

i

I

I

I

I

2

3

4

5

1607

Au/TA

~5.0

2 4.5

£ t~

Au COVERAGE (ML)

Fig. 3. Variations in the work function with gold coverage. happen in the case of Ta. Still, however, there might be a small tendency of clustering or island formation on the metallic substrate that is not resolved in these measurements. The dependence of the work function on gold coverage is shown in Fig. 3 for the Ta substrate. The work function of the bare substrate was measured to be 4.6eV [8]. At very low coverages we observed a small increase, then a decrease in the work function up to a gold coverage of about 1 ML. The value subsequently increased, and by about 6 M L it had reached 5.3 eV, where it remained for heavier depositions of Au on Ta. An initial increase in the work function would be expected due to the strong electron affinity of gold; a negative charge transfer to the adatoms will result in a negative double layer at the surface. The observed low initial increase of the work function is believed to be caused by the Smoluchowski effect; evaporation of atoms on a substrate will lead to a smoothening of the charge density, and a decrease in the work function due to the formation of electric dipoles directed outwards [9]. This effect will weaken for coverages in excess of about one monolayer. Eventually, near 6 ML, the work function approaches the value of the thick gold sample [10]. Electron energy loss spectra were recorded with a primary beam energy of 280 eV for the same gold coverages on tantalum as in the photoemission data. Curve a of Fig. 4 is obtained on the clean Ta substrate, showing a plasmon structure near 20 eV loss energy, and the lowered plasmon losses at about 11 eV energy [11 ]. Both of these structures contains a surface contribution at the low energy side of main losses [11]; the separation being prohibitively small for the features to be resolved. Small modifications in the spectra are observed at low coverages, 0-1.1 M L of gold; the main plasmon feature shifts from 20 eV (curve a) to about

30

20 i0 Loss energy (eV)

0

Fig. 4. Electron energy loss spectra, at 280 eV primary beam energy. Curves a-h represents the same Au coverages on Ta as in Fig. 1. 21.5 eV (curve d). We observe a significant change in the EELS in the region of 1.1-1.7 M L gold coverage. A new peak emerges at a loss energy o f 27 eV (curve e). This alteration of the loss signal takes place at the same coverage where the photoemission intensity showed a substantial increase towards the fermi level, see Fig. 1. The newly appeared peak weakens and finally disappears for heavier gold coverages, e.g. 6 M L . In addition to these changes, a strong loss feature due to plasmon excitations in gold, develops near 6-7 eV. The top spectrum in Fig. 4 resembles that of a solid gold sample [12], the 25eV peak has been assigned to a d ~ f i n t e r b a n d transition [13, 14], and a feature at 33 eV, barely discernible in our data, has been attributed to an EXAFS like mechanism [15]. The most significant observation in the electron loss spectra may be the apparent splitting of the main Ta plasmon peak in (curve e) of Fig. 4, which occurs at coverages above one monolayer. Plasmons in these d-electron metals cannot be described by the simple free-electron model, but nevertheless, the peak that appears at 27eV is a reflection of changes in the conduction electron density. The detailed nature of this feature is not understood, at least not by us, at present. However, it appears at the same coverage where the d-emission of Au shows significant alterations that are indicative of the formation of a conduc-

1608

G O L D MONOLAYERS ON POLYCRYSTALLINE T A N T A L U M

tion band, and may thus be associated with collective electron excitations in a low-dimensional band in the gold layer, or even in a Ta-Au compound at the interface. In conclusion, we have observed changes in both the photoemission and energy loss spectra at coverages of gold near one monolayer on a polycrystalline Ta substrate. The work function is at a minimum near this coverage, which is where the Au d-band is being developed, as evidenced by shifts of 5d-emission. The large shifts in the gold d-states may be partly caused by a d 9 --o d I° configurational change as the Au layer evolves. Extensive clustering is observed on a graphite substrate, but does not happen on tantalum. Further studies on single crystal surfaces, including LEED and core level measurements that could elucidate the role of possible interface reactions, would be of interest. REFERENCES 1.

M.W. Ruckman & M. Strongin, Phys. Rev. B29, 7105 (1984); references cited therein.

Vol. 65, No. 12

X. Pan, M.W. Ruckman & M. Strongin, Phys. Rev. B35, 3734 (1987). 3. S.B. Dicenzo & G.K. Wertheim, Comments Solid State Phys. 11, 203 (1985). 4. J.R. Arthur & A.Y. Cho, Surf Sci. 36, 641 (1973). 5. W.F. Egelhoff & G.G. Tibbetts, Solid State Commun. 29, 53 (1979). 6. E.W. Plummer & T.N. Rhodin, J. Chem. Phys. 49, 3479 (1968). 7. S. Raaen, J.W. Davenport & Myron Strongin, Phys. Rev. B33, 4360 (1986). 8. R.G. Wilson, J. Appl. Phys. 37, 3170 (1966). 9. R. Smoluchowski, Phys. Rev. 60, 661 (1941). 10. H.C. Potter & J.M. Blakely, J. Vac. Sci. Technol. 12, 635 (1975). 11. W.K. Schubert & E.L. Wolf, Phys. Rev. B20, 1855 (1979). 12. M. Schluter, Z. Physik. 250, 87 (1972). 13. A. Otto & E. Petri, Sol. State Commun. 20, 823 (1976). 14. J. Hermanson, J. Anderson & G. Lapeyre, Phys. Rev. B12, 5410 (1975). 15. C. Kunz, Optical Properties of Solids, New Developments (Edited by B.O. Seraphin), North Holland, Amsterdam (1976). 2.