Auger electron spectroscopy of adsorbed metal monolayers

Surface Science 66 (1977) 436-448 0 North-Holland Publishing Company

AUGER ELECTRON SPECTROSCOPY OF ADSORBED METAL MONOLAYERS Lead on low-index and stepped copper surfaces A. SEPULVEDA and G.E. MEAD Laboratoire de Physics-Chi~ie

des Surfaces, ENSCP, Universit& Pierre et Marie Curie, 1 I, rue Pierre et Marie Curie 75005 Paris France Received 23 March 1977

Lead was deposited onto single crystals of copper (orientations (llO), (loo), (711) and (5 11) in ultrahigh vacuum. For all substrate orientations there form dense monolayers characteristic of the Stranski-Krastanov growth mode. During growth of the monolayer (at constant vapour flux) the Auger signals of the adsorbate and the substrate do not vary with exact linearity as a function of deposition time. The signal-versus-time plots show changes of slope that are associated with changes in the adsorbed layer structures as observed by LEED. More gradual variations occur near the compietion of the dense monolayer. All these changes are ascribed to variations in the sticking probab~ty. Adsorbed layers on the stepped surfaces tend to be disordered, possibly due to high mobility. The results are discussed in terms of the possibilities and difficulties for quantitative Auger electron spectroscopy.

1. Introduction The need to make Auger electron spectroscopy (AES) of surfaces more quantitative is a current preoccupation in many laboratories. One approach to this problem is to calibrate the spectrometer by means of slowly deposited metal overlayers [ 11. In the first stages of deposition metals tend to form monolayers with dense closepacked arrangements which may thus provide well defined and readily characterized standard “test” surfaces [2]. In this paper we report the results of expe~ents set up to examine critically certain aspects of this method: the linearity of the response as a function of the adsorbed quantity, the possible effect of surface defects such as steps, the growth mode of the adsorbed metal and the reproducibility of the AES data. Several papers [l-4] discuss the ideal behaviour of the substrate and adsorbate (condensate) Auger signals during the formation of ultrathin films by various growth mechanisms. ideally during the formation of a monolayer the adsorbate signal increases linearly as the coverage increases while there is a linear decrease in the substrate signal. After the completion of the first monolayer both signals vary more slowly so that “knees” appear that serve to identify the completed monolayer stage. Such knees appear both for the Stanski-Kranstanov (mono436

A. Sepulveda, GE. R/wad / AES ofadsorbed metal monokqvers; Pb on Cu

431

layer followed by small crystallites) and for the Frank-Van der Merwe (layerby-layer) growth modes. In the first case the completion of the monotayer is followed by constant - or very slowly varying - Auger signals while for the second case the growth of each successive layer is characterized by linear variations which become progressively slower as the thickness increases f3]. In a typical experiment the clean substrate is exposed to a constant vapour flux and the Auger signals are plotted as a function of time (duration of deposition). We refer to such plots as AS-T plots. In most work reported so far only a small number of points (typically less than ten) has been used to establish the “monolayer knees” and the observations appear to confirm the ideal linear variations. In the work reported here we have found by making a large number of measurements that the variations up to the monolayer knee are not strictly linear. Changes occur that can be related to variations of the sticking probability associated with changes in the adsorbate structure. The results can provide useful confirmations of LEED interpretations. 2. Exper~entai The general experimental arrangement and procedures have been described elsewhere [ 11. The single crystal metal substrate was cleaned by argon ion bombardment followed by annealing. With the background pressure in the vacuum chamber in the lOma Pa range lead was evaporated onto the substrate at a rate of the order 1 monolayer per hour from a crucible maintained at a stabilized temperature of about 600°C. A series of successive short depositions was made (each equivalent to about l/SOth of a monolayer), the vapour beam being interrupted by a rotated shutter. After each deposition the specimen was rotated to the appropriate position for Auger analysis. The AES measurements were made with a 120” retarding grid analyser: moduiating voltage 2.7 V peak-to-peak, primary beam energy (lancing incidence) 2.3 keV. During each experimental run the Auger spectrum was checked for the presence of contaminants, especially carbon, oxygen and sulphur. The run was continued until the first sharp knees occurred in the plots of signal against time. The surface was then recleaned and the experiment repeated. During some runs the adsorbate structure was monitored by observation of the LEED pattern. 3. Results Fig. 1 shows results obtained for deposition onto the (110) substrate. Peak-topeak heights in the differentiated Auger spectra are recorded. Three sets of AS-T plots are shown to illustrate the reproducib~ity. The general form of the plots, ending in flat plateaux, corresponds to the Stranski-Krastanov growth mechanism. The p(5 X 1) LEED pattern, already ascribed in a previous study [5] to a dense monolayer arrangement, is first seen just before the plateau signals are reached.

438

A. Sepulvcda, G.E. R&ad f AES of adsorbed metal monolayers; Pb on Cu

pb/Cu(llo)

Cu(63eY)

*p(l .l Lds2ao~

op(5.1)----pbQ*l)

C-

B4 Fig. 1. Deposition of Pb on Cu (110). Three sets of AS-T plots, i, Auger signal (origin displaced) normalized to the initial substrate signal. r, deposition time normalized to tr , the intersection of the plateau and the initial linear rise of the Pb signal. The straight lines represent ideal behaviour. LEED patterns observed for the ranges indicated.

The plots depart slightly from the ideal behaviour in two ways. First, the linear rise of the lead signal does not pass exactly through the origin. This is an artefact of the experimental procedure and it can be understood from the form of the recorded spectra (fig. 2). The background of that part of the spectrum on which the lead signal appears has a small, but non-zero, slope. As F&i&e [6] has pointed out, this leads to a small systematic error (io) in the peak-to-peak measurements. Because of this effect it was difficult to make measurements of the lead signal over about the first tenth of a monolayer and few signals are plotted over this range. We note that although the lead signal is initially not easy to measure, adsorption is detected by the decrease in the substrate signal. Secondly, there is a “rounding off’

A. Sepuiveda, GE. Rhead / AITSof adsorbed

metal

monolayers; Pb on Cu

439

t/t, .0.X?

Fig. 2. Auger spectra for the clean substrate (t = 0) and with about l/3 monolayer of lead. Peaks: (a) 63 eV (Cu), (b) 93 eV (Pb), (cf 104 eV (Cu), (df 85 eV (unidentified). - io is the systematic error in the lead signal.

of the knees - for both the lead and the copper signal - that begins at about the coverage required to form the (5 X 1) structure. Typical plots obtained for deposition onto the (100) face are shown in fig. 3. Again the general form of the AS-T plots indicates the Stranski-Krastanov mechanism and the ~(242 x42) 45’ pattern, ascribed to a dense hexagonal monolayer [S], is the final pattern to be observed and it appears just before the start of the plateau signals. However, rather than a “rounding off’ of the knees there occur sharp changes of slope at about the point where the c(4 X 4) structure is completed and the final structure starts. Thus, as for the (110) face, there is a change in the plot associated with a change in adsorbate structure. This is discussed later in terms of the structures illustrated in fig. 6. Results for the (7 11) and (5 11) substrates are shown in figs. 4 and 5. Significant departures from a straight line are observed before the monolayer plateau is reached. These will be discussed in terms of the structures shown in fig. 7.

440

Pb/Cu (100)

Cu(63eV)

Fig. 3. Deposiiion

of Pb on

Gu(tOl3). Pb/Cu fill)

Fig. 4. Depasitiarr of Pb on Cu(7 11). The arrows indicate probable positians for changes of siope.

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it

Pt&u

(511)

PC1 -1)

t

L

t1

t

Fig. 5. Deposition of Pb on Cu(5 11).

4. Interpretation

of results

For the Stranski-Krastanov mechanism - which, because of the constant plateaux signals appears to be the growth mode for each substrate we have examined - the AS-T plots tiould give, ideally, simple straight lines with sharp knees. Experimentally we find small changes before the formation of the complete monolayer. There occur changes of slope, especially noticeable in the adsorbate signal, that correlate with changes in the adlayer structure as observed by LEED. In fact each change in slope marks the completion of one structure and the start of another more dense arrangement (see table 1). The start of a new arrangement is not easy to detect accurately by LEED alone because two structures can coexist over a range of coverage and it can be difficult to detect small fractions of a structure. The departures from linearity would seem at first sight to be due to changes in the sticking probability - i.e. changes in the significance (scale) of the t-axis. But

0.27

1.00 1.00

9.69

8.88

x 1)

x 1)

(711)-p(4

(511))p(4

0.28

0.30 0.26 0.21

1 .oo 0.85 0.66

x 1) x 2)

0.20

0.64

-P(4 -c(2

0.31

1.00

9.23(1.00 ) 3.84(0.416) 5.77(0.625) 9.61(1.04 ) 8.70(1.00 ) 8.16(0.937) 5.44(0.625)

(100)-c (5J2 x J2) 45” -c(4 x 4) 2 atoms 3 atoms 5 atoms (llO)-p(5 x 1)

(3) Pb Auger signal normalized to clean copper signal

(2) Pb Auger signal normalized to dense monolayer signal

(1) Ideal density (lead atoms/nm2) (normalized density in brackets)

of LEED and AES data

Substrate orientation, adlayer coincidence mesh

Table 1 Comparison

0.58

0.58

0.48 0.43 0.34

0.31

0.52

(4) Fractional attenuation in copper signal (signal lost)

1.00

1.00

1.00 0.74 0.54

0.48

1.00

(5) Time to complete structure (normalized to beginning of plateau)

A. Sepulveda, GE. Rhead /A&T of adsorbed metal monolayers; Pb on CU

443

we have to examine another explanation: possible changes in the Auger emission per adsorbed atom - changes in the scale of the i-axis. Theoretical discussions of quantitative Auger spectroscopy have not until recently examined in detail the process of Auger emissiurz - what happens to the Auger electron once created. It is now clear from attempts to calculate the angular distributions of Auger electrons [7] that there can occur many scattering events with neighbouring atoms before the emitted electron reaches the analyser. It is therefore not trivial to ask whether, for example, an adsorbed atom inserted in a dense layer might contribute less to the Auger emission than an isolated adsorbed atom. This might happen if in the dense layer the emission is partially attenuated by nei~bouring adsorbate atoms. A similar effect might occur with atoms adsorbed at defects in the substrate - surface vacancies and steps. In other words: since we allow for attenuation by atoms that are obviously in the path between the Auger emitter and the analyser (e.g. an overlayer covering a substrate) should we in some way allow for attenuation by atoms in the same plane as the emitter since these too can intercept the Auger electron? The changes in slope for the adsorbate signal that we observe as the ad-layer structures change to more compact arrangements might perhaps be evidence for such changes in the effective Auger sensitivity. It is more difficult to assess the possible effect of changes of adsorbate structure on the attenuation of the substrate signal but a similar effect might also be expected. It is conceivable, for example, that adatoms inserted into the remaining vacancies of an ad-layer could have less effect in attenuating the substrate signal than those previously adsorbed. Thus changes in slope could occur shnult~eously in both adsorbate and substrate signals due either to changes in sticking probability or to changes in Auger sensitivity or to both. There is a way of distinguishing between these alternatives. First, with the LEED data one can determine plausible structures for the various adsorbed layers. Then, if the Auger sensitivity is independent of structure (that is, if the effect described above is ne~igible) then the densities of the ideal structures should correlate with the measured Auger intensities and the changes in the plots can be ascribed to changes in sticking probability. On the other hand if the Auger sensitivity is structure-dependent and the sticking probability constant then the ad-layer densities should correlate with the times required to complete each structure. Table I summarizes the measurements that can be made from the AS-T plots and makes the comparison with the models obtained from the LEED data. We first discuss these models and show how the ad-layer densities in column (1) may be justified. In figs. 6 and 7 are presented “hard-sphere” models in the form of projections of planar monolayers without allowing for rumpling. (100)- c(.5t,ki? X t/2) 45”. Fig. 6e shows the structure proposed by Henrion and Rhead (H-R) [S]. Similar hexagonal or pseudo-hexagons compact arran~ents have been found for a large number of metal monolayers on various substrates [2]. The Pb-Pb interatomic spacing in this particular model is about 1% greater than in bulk lead.

444

A. Sepulwda,

G.6: Rhmd / AES of adsorbed metal monolayers; Pb on Cu

Fig. 6. Proposed structures on the (110) and (100) faces. (110): (a) c(2 X 2), (b) p(4 X l), (c) ~(5 X 1); (100): Cd)c(4 x 4), (e) ~(5 ,/2 x J2) 45”.

(100) - ~(4 X 4). The equivalent primitive lattice of this structure, p(2d2 X 242) 4.5”, is shown in fig. 6d. The lattice points are made to coincide with sites of four-fold coordination. Because the evidence suggested that this structure is dense and because the arrangement cannot place additional atoms at 4-fold sites H-R proposed an arrangement with five lead atoms in the unit mesh. Such a structure is difficult to understand without accepting a relatively high density of vacancies to relieve the stress [5]. The calculated densities for 2, 3 and 5 atoms per unit mesh are shown in column 1 expressed as fractions of the density of the hexagonal layer. Comparison with column 2 shows good agreement for 3 atoms per unit mesh. The relative time required to complete the structure (column 5) agrees with none of the model densities - from this result it would appear that the sticking probability changes and not the Auger sensitivity. A very plausible model can be made for a structure with 3 atoms per unit mesh, as shown in the fig. 6d. The arrangement of linear chains requires a Pb-Pb spacing only 3% less than that in the bulk crystal. Arrangements with a similar coincidence between rows of 3 lead atoms and 4 copper atoms have been found for the (111) substrate [5] and also for each of the substrates described below. We note that the arrangement of fig. 6d requires a halfatom shift between adjacent lead rows which suggests that there is a repulsive interaction. The tendency of lead atoms to form linear chains leading to “one-dimensional epitaxy” has already been pointed out by Drechsler [S] .

A. Sepulveda, G.E. Rhead f AL’S of adsorbed metal monolaycrs; Pb on Cu

445

(a)

Fig. 7. Proposed (511).

structures

for lead monolayers

on stepped copper surfaces: (a) (711, (b)

(110). All the structures observed on this face can be explained by adsorption in the atomic Ii0 grooves that form the structure of the substrate. The p(5 X 1) and c(2 X 2) arrangements have already been reported by H-R. In the present work we observed in addition a p(4 X 1) structure that exists over a short range of coverage. In this structure the Pb-Pb spacing is 3% less than in bulk lead whereas a difference of 9% is needed to explain the p(5 X 1) structure (there is room, however, for a 4% expansion in the direction perpendicular to the grooves). Comparison of columns 1, 2 and 5 of the table shows that for this substrate too there is no evidence that the Auger emission depends on the adsorbate structure. (711) - p(4 X I). Only this one ordered structure was observed for this face. In the first stages of deposition the LEED patterns exhibit only very small changes in the background intensity without the appearance of extra spots. The p(4 X 1) pattern appears quite suddenly at over half the time required to reach the plateau in the AS-T plot. The pattern is never very sharp. The absence of earlier ordered arrangements is puzzling and suggests that the ad-layer is either highly disordered or

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in rapid diffusive motion - perhaps facilitated by fast diffusion along the step edges. A final pseudo-hexagonal monolayer (fig. 7a) with three atomic rows just fitting the terrace width would readily explain the p(4 X 1) coincidence mesh. (.SZl) - p/4 X 2). The results for this face are very similar to those for the (711) substrate. No changes are observed in the LEED pattern over the first stages of deposition. Then at about 3/4 of the time required to reach the plateau in the AS-T plot a p(4 X 1) pattern appears, very similar to that observed for (711). The slightly later development of the p(4 X 1) pattern may be related to the higher density of steps on this surface and again the possibility of high diffusivities may be invoked. The p(4 X 1) arrangment can also be attributed to a pseudo-hexagonal arrangement (fig. 5a) - this time with two atomic rows fitting onto the terrace width. In this layer the departure from a true hexagonal arrangement is greater than for the arrangement on the (711) face.

5. Discussion In the absence of contradictory evidence it seems reasonable to interpret the changes in slope occurring before the monolayer plateau as variations in sticking probability rather than changes in Auger sensitivity. Strictly we should speak of the apparent sticking probability since the method does not detect any deposited metal that goes into the early growth of small crystallites because of their relatively small surface area. Thus the “rounding-off’ of the knees that occurs for the (100) and (110) faces can be ascribed either to the start of crystallite growth or to decreased sticking due to the lower probability of finding a vacant site in the ad-layer. The earlier changes of slope that occur for (711) and (511) would appear to be due to a different phenomenon. In both cases there is an apparent decrease in sticking just before the ordered p(4 X 1) structure becomes visible in LEED. This point is reached earlier for (7 11) than for (5 11). It is likely that sticking is higher at step edges and that adsorption occurs there first. The steps have a lower density on (711) so that they would become saturated earlier on this surface. In fact it is possible to divide the plots into distinct regions - 3 for (711) and 2 for (5 11) (figs. 4 and 5) - each of which would correspond to successive rows of lead atoms. In a recent study of oxygen adsorption on W (100) Bauer et al. [9] have also found evidence, from changes in AS-T plots, of sudden changes of sticking coefficients. The first change was correlated with the full development of the first ordered adsorbate structure as observed by LEED. Our results confirm these authors conclusions that step-wise changes of the sticking coefficient can occur at particular coverages. The stepped surfaces exhibit p(1 X 1) patterns, i.e. with only substrate reflexions, up to very high coverages and therefore they cannot be ascribed to ordered adsorbed layers. Disordered adsorbate layers on various stepped surfaces have been reported by several authors [ 10,111. The reason for the disorder is not fully under-

A. Scpulvedn, GE. Rhead / Ah5 of adsorbed metal monolayers; Pb on Cu

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stood. It would be interesting to devise experiments to test whether the steps do influence the mobility of the adsorbate: changes in mobility could influence the sticking coefficient as well as the structure. In this connection Matysik [12] has recently made some interesting observations on possible dynamic effects in the nucleation of an adsorbed layer. The tentative interpretations of the tinal p(4 X 1) patterns on the stepped surfaces as pseudo-hexagonal close-packed overlayers are supported by observations on several systems that have shown that steps do not necessarily disturb the formation of such close-packed arrangements [13-l 51. We have considered the Auger data for each face separately. Although the AS-T plots show that for each particular specimen the data is reproducible usually to within at least I%, much larger variations occur if a specimen is replaced by another (of the same orientation). The signals at the knees (normalised to the clean substrate signal) cannot then be reproduced to better than 10% between specimens. We provisionally ascribe this lack of reproducibility to a dependence on the exact position of the specimen and especially on the surface inclination with respect to the primary electron beam which we cannot control to within better than *5’ with our present arrangement. We believe that a lack of precise definition of this angle may limit reproducibility in many AES experiments. Although our results for the stepped surfaces are not sufficiently precise to be very conclusive they do appear to show that steps can be decorated by a metallic adsorbate and subsequently detected by AES. This was one of the original aims of these experiments. However, for detecting the presence of accidental steps on a low-index surface this decoration method is less promising than originally hoped [21-

6. Conclu~ons Correlated LEED-AES experiments on adsorbed metal overlayers can be made with sufficient precision to distinguish between different models for observed coincidence lattices. On this basis a new model is proposed for the Cu(100) - c(4 X 4)Pb structure in which lead is adsorbed in linear chains. In plots of Auger signals as a function of deposition time at constant vapour flux changes can be observed that can be related to changes in the apparent sticking probability. On low-index surfaces these changes occur near the completion of the dense monolayer and can be related to the lower density of ad-layer vacancies. On stepped surfaces the changes occur at lower coverages and can be related to preferential adsorption at steps. The absence of ordered structures of lead up to quite high coverages on stepped surfaces suggests that the adsorbed atoms are in diffusive motion and that the steps provide high diffusivity paths.

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A. Sepuivedu, GE. Rhead / AES of adsorbed metal monolayers; Pb on Cu

Acknowiedgements This work was carried out with the support of the Centre National de la Recherthe Scientifique (ERA 3 13 and Contrat ATP IUO8). A.S. acknowledges receipt of a French government grant.

References [l] C. Argile and G.E. Rhead, Surface Sci. 53 (1975) 659. [2] G.E. Rhead, 3. Vacuum Sci. Technol. 13 (1976) 603. [ 31 T.E. GalIon, Surface Sci. 17 (1969) 486. [4] M.P. Seah, Surface Sci. 32 (1972) 703. [S] J. He&on and G.E. Rhead, Surface Sci. 29 (1972) 20. [6] J.C. Riviere, Contemp. Phys. 14 (1973) 513. [7] D. Aberdam, R. Baudoing, E. Blanc and C. Gaubert, Surface Sci. 57 (1976) 306. [8] M. Drechsler, in: Basic Problems in Thin Film Physics, Eds. R. Niedermayer and H. Mayer fiandenhoeck and Ruprecht, Gottingen, 1966) p. 18. [9] E. Bauer, H. Poppa and Y. Viswanath, Surface Sci. 58 (1976) 517. [lo] B. Lang, W. W. Joyner and G.A. Somorjai, Surface Sci. 30 (1972) 454. [ 111 B.E. Nieuwenhuys, D.I. Hagen, G. Rovida and G.A. Somorjai, Surface Sci. 59 (1976) 155. [ 121 K.J. Matysik, J. Appl. Phys. 47 (1976) 3826, 3833,4359. [13] J. Perdereau and G.E. Rhead, Surface Sci. 24 (1971) 555. [14] H. Papp and J. Pritchard, Surface Sci. 53 (1975) 371. [15 ] R.H. Roberts and J. Pritchard, Surface Sci. 54 (1976) 687.