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Secondary electron emission changes due to metal monolayer adsorption
181
Surface Science 138 (1984) 181-190 North-Holland, Amsterdam
SECONDARY ELECTRON EMISSION CHANGES MONOLAYER ADSORPTION C. ARGILE,
M.-G. BARTHES-LABROUSSE
DUE TO METAL
and G.E. RHEAD
Laboratoire de Physic0 -Chimie des Surfaces (ERA 313). ENSCP, II Rue Pierre et Marie Curie, F- 75005 Paris, France
UniversitP Pierre et Marie Curie,
Received 14 July 1983; accepted for publication 3 November 1983
Changes in secondary emission during the adsorption of lead on copper (111) and (100) surfaces, followed by monitoring the crystal current (SEECC technique), are compared with changes in the work function as well as with variations in the Auger signals. The work function variation for the (100) face exhibits a sudden change that may be due to a transition of the adsorbate from a 1D (atomic chain) type of structure to a 2D structure. The secondary emission changes are not explained entirely by the work function variations; there are also effects that are correlated with changes in adsorbate structure and which are probably due to diffraction at the surface.
1. Introduction In two recent papers [1,2] we have reported a simple method for following changes in surface and thin film composition and structure based on measurements of the net current flowing into a crystal specimen under electron beam bombardment. The technique, for which the acronym SEECC was proposed (secondary electron emission crystal current), depends on the sensitivity of secondary emission to surface conditions. The crystal current i, is in fact equal to (S - l)i,, where 6 is the total secondary emission coefficient (including reflected primaries) and i, is the primary electron current. The principle of this type of measurement is well known and it is used qualitatively in scanning secondary electron emission microscopy. However, detailed quantitative comparisons with surface techniques such as Auger electron spectroscopy (AES) had not previously been made. Results obtained on different single crystal substrates showed that the adsorption of monolayer quantities of metals could be readily followed by the crystal current measurements and that the precision was higher than that attained by AES. It was also found possible to detect changes in adsorbed layer structures that were much less apparent from the AES data. 0039-6028/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
At first sight the most obvious explanation for the observed variations of the crystal current would be that adsorption or structural changes have a marked effect on the work function: changes in the surface barrier would influence especially the low-energy electrons in the secondary emission spectrum. We suggested [1.2], however. that other factors could also be important: for example, possible enhanced surface ionization associated with special scattering and diffraction events in the adsorbate and the near-surface layer. The experiments reported here were made to check the influence of work function changes, not measured previously, and to see to what extent the crystal current variations can be correlated with, and explained by. the lowering of the surface barrier. The results show that the lowering of the barrier does not entirely explain the observations and that there ure additional effects that can bc correlated with changes in ad-layer crystallinity. We have also found that measurements of the crystal current can actually help in the interpretations of the LEED patterns from a series of different adsorbed layer structures.
2. Experimental
The experimental equipment and conditions were the same as those used in the earlier work [1,2] but with the addition of a Kelvin vibrating capacitor probe for work function (A+) measurements. (This technique was not available for the earlier work and in presenting the new data there is some unavoidable overlap with the content of the previous publications.) For the SEECC measurements we used the electron gun incorporated in an Auger cylindrical mirror analyser (CMA) with the electron beam incident normally on the specimen surface. Two copper specimens with orientations (111) and (100) were mounted side-by-side on a rotatable manipulator and were electrically connected to ground outside the vacuum chamber via a microammeter. The manipulator was turned so that the specimens faced in succession the positions for lead metal vapour deposition (from a simple crucible source with a rotatable shutter), A+ measurements, AES and SEECC measurements. Depositions were made with the specimens at room temperature. The Kelvin probe was piezoelectrically driven (Delta-Phi-Elektronik. Julich) and had a molybdenum reference electrode. This probe was mounted on a second rotatable manipulator and shielded from the metal vapour source by a stainless steel sheet. Since we were interested only in relative changes in the work function the reference electrode was used as received, without any attempt at cleaning. The reproducibility of the measurements was better than * 10 mV. In making the A+ measurements certain difficulties arose from a non-uniformity of the metal deposit across the specimen. The Kelvin probe apparently
C. Argrle
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183
sampled an area of about 10 mm2, whereas the electron beam used for AES and SEECC measurements had a cross section about 10 times smaller. As a result, and because of a certain inhomogeneity of the deposit, some inconsistencies were found in comparing the A+ and AES or SEECC data. To overcome this difficulty, measurements of the changes in the work function were also made by recording the beginning of the secondary emission spectrum with the CMA and following the displacement of the spectrum along the energy scale during the adsorption process. (A constant potential of - 18 V was applied to the specimen in order to offset effects of stray fields between the specimen and the analyser that would otherwise have made observations of the beginning of the spectrum difficult.) This technique was less sensitive than the Kelvin probe technique, but it enabled us to correlate accurately features of the plot of A+ against deposition time with the AES and SEECC data while the Kelvin probe gave more accurate measurements of the amplitude of the A# changes.
3. Results The results obtained with the different techniques are presented in figs. 1 and 2. To make the comparison easier the work function data have been inverted (the decrease in the work function is plotted as a positive quantity) and the origins of the plots have been displaced vertically. The ranges over which various LEED patterns are observed have been well established in earlier work [3,4]. These ranges are also shown in the figures. It is obvious that although the Auger plots are very similar for the two substrates both the work function changes and the crystal current variations are different for the two crystalline orientations. For the (111) face a sharp maximum is observed in the crystal current at the monolayer coverage 8 = 1, as reported earlier [l]. The break in the crystal current i, at about l/3 of a monolayer (P) is less evident than in the earlier work (differences may occur due to differences in the deposition rate [2]) but a different break is apparent at about B = 5/9 (Q); a change in slope near this point (within 10% of a monolayer) is also visible in the plot of the lead Auger signal (more easily observed obliquely). The plots for the work function changes (A$B) show no such features: A+ decreases linearly up to a coverage of about 0 = 0.2 and then at a progressively slower rate. When the monolayer coverage is reached the work function attains a plateau, or is very slowly decreasing. This is an especially significant result since beyond the monolayer coverage the crystal current decreases sharply. The results for the (100) face are more complicated. The crystal current measurements were first reported in ref. [2]. In subsequent work [5] these data were obtained with more precision using a more stable electron gun. Identical
184
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results were obtained in the present work. the Auger signals coincide with the onset ments and the crystal current (T). It is These can be placed at positions which close to 5/12 of a monolayer (P) there
P(lxl)
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Fig. 1. Data obtained for the deposition of lead on Cu(ll1). Upper curves: Auger signals for the substrate and adsorbate; (0) signal for copper (63 eV), (0) signal for lead (93 ev) (above: ranges of observed LEED patterns). (IJ) Decrease in the work function (Kelvin probe measurements), (W) decrease in the work function (electron beam measurements), (A) crystal current measurements.
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adsorbate Auger signals and also a more rounded break in the crystal current; at 8 = l/2 (Q) breaks in the A+ measurements; at B = 2/3 (R) a sharp maximum in the crystal current followed by a drop which can not be
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Fig. 2. Data obtained for the deposition of lead on Cu(100). Upper curves: Auger signals for the substrate and adsorbate; (0) signal for copper (63 ev), (0) signal for lead (93 eV) (above: ranges of observed LEED patterns). (0) Decrease in the work function (Kelvin probe measurements). (m) decrease in the work function (electron beam measurements). (A, v) crystal current measurements (two sets of results).
correlated with any variations in the other measurements, there appears a plateau in the crystal current.
and at 8 = 5/6
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4. Discussion Although the general increase in the crystal current during adsorption (corresponding to an increasing number of secondary electrons being emitted into the vacuum) can be explained by the general decrease in the work function, which will affect especially the very low-energy secondaries. it is clear that the detailed variations of A+ and i, are not correlated. Moreover. a plateau value for A+ corresponds to a plateau in i, for the (100) face but not for the (Ill) face. Neither is there any simple connection between i, and the variations of the Auger signals. There are. however, connections between the I, variations and the changes of structure of the adsorbed layer. For the (111) face it has been shown [6] that the growth mode for lead deposits at room temperature is Frank-van der Merwc (layer-by-layer) while for the (100) face [4] the mode is Stranski -Krastanov (monolayer followed by crystallites). On the (111) face the second layer apparently grows slowly because of a reduced sticking probability and the diffuse LEED patterns observed [3] show that it is poorly crystallised. The existence of this second layer is apparently indicated by the decrease in i, after the monolayer. No such effect occurs for the (100) orientation. Overlayers of lead on copper frequently lead to four-fold periodicities in the coincidence lattice. This has been attributed [3,4,6] to the fact that in dense atomic chains of lead the length of a segment of 3 atoms (with the bulk crystalline parameter) is within 3% of four diameters of the copper substrate atoms. The p(4 x 4) LEED pattern for the monolayer structure on Cu(ll1) has been interpreted in terms of a pseudo-hexagonal close-packed lead layer with 9 atoms in the unit coincidence mesh [l]. The breaks near l/3 and 5/9 of a monolayer may therefore be related to distinct stages in the formation of this dense structure. For the (100) substrate the various breaks in fig. 2 can be understood in terms of the structural models shown in fig. 3. (For a presentation of the patterns for this system and a discussion of the structures see refs. [3.4,7].) The interpreted by monolayer LEED pattern, ~(542 X 42 )R45”. was originally Henrion and Rhead [3] (HR) in terms of a pseudo-hexagonal overlayer with 6 lead atoms for 10 copper atoms in the unit coincidence mesh. Hoesler and Moritz [7] (HM) have recently proposed a different interpretation (but with the same concentration of lead). This interpretation, shown on the extreme right of fig. 3, is based on LEED intensity analyses; it places all the adsorbate atoms at or close to hollow (four-fold coordination) substrate sites. This type of struc-
C. Arple
er al. / Secondary
electron emrssvron charlxes
187
ture was discussed by Biberian and Huber on the basis of crystalline symmetry [8]. The model of fig. 3d is derived from a (42 x a)R45’ structure (fig. 3c) and in their work HM observed the pattern for such a structure at a coverage 0 = 5/6. Surprisingly, this pattern was not reported by HR nor in the later study by Sepulveda and Rhead [4] (SR). However, a reexamination of the original photographs obtained by HR (the Polaroid pictures have kept well since 1969!) does show some of the patterns for the c(5v’2 X fi)R45’ structure with the extra l/2 l/2 spots corresponding to the (42 x fi)R45” structure. At the time Auger spectroscopy was not available and it was thought that the extra spots were due to an impurity. Moreover, the later work of SR, which did use Auger spectroscopy, showed that the ~(50 x Ji )R45” structure existed at lower coverages than that necessary for a completed (fi X &)R45” structure (see the top band of fig. 2). This apparent contradiction as regards the existence of the (0 x fi)R45” structure can be resolved if we suppose that a ~(542 x fi)R45” pattern can also be obtained from a second structure at a lower coverage as shown in fig. 3b. At a coverage of 8 = 2/3 it is possible to fill the coincidence mesh with 4 atoms, all in hollow sites. and this too is a structure derived from the o/2 x fi)R45’ arrangement. Thus the (42 X {2)R45” pattern would tend to occur only for a very narrow range of coverages preceded and followed by the same pattern (with possible differences in spot intensities). This would explain why the (42 X 42 )R45’ pattern was
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Fig. 3. Models for the structures of lead on Cu(100) at different coverages (0: fraction of a complete monolayer; Pb/Cu: ratio of adsorbate to first-layer substrate atomic densities). The structures (c) and (d) were obtained by Hoesler and Moritz [7]. The intersections of the net represent the hollow four-fold coordination substrate sites.
188
C. Arxde
er al. / Secondary
electron em,.r~,on changes
overlooked in the earlier work and why the existence of the (542 x 42 )R45” pattern was reported for relatively low coverages by SR. At low and medium coverages a (242 X 2fi)R45’ pattern is observed (previously labelled c(4 x 4) in the alternative nomenclature by HR and SR). The Auger study by SR concluded that there were only 3 atoms in this unit mesh and so corrected an earlier interpretation of HR which assumed a denser arrangement. A very simple interpretation, as proposed by SR, is to place a chain of lead atoms across the diagonal of the unit mesh (3d,, = 4d,,), but this is unlikely to be correct if the structures that follow are due to adsorption in hollow sites. We therefore propose the model shown in fig. 3a which comprises the same (11) oriented chains that appear in the later structures. plus vacant sites. Effectively this is a (fi X fi)R45” arrangement with vacant sites that are separated. The models shown are of course ideal structures: a more random distribution of vacant sites but with the tendency of avoiding adjacent vacancies would be more realistic. We return now to possible explanations of the various breaks in fig. 2. The range S to T corresponds to the growth of the final monolayer arrangement. fig. 3d. Over this range the i, plot apparently exhibits a dip below the final plateau level. S represents the unique coverage for which the (42. x fi )R45” structure can exist alone. The break at R corresponds to the beginning of formation of the (a x fi)R45” structure. For lower coverages a succession of structures is less obvious: for example there is no feature in the curves that would correspond to the completion of the (2& x 2fi)R45’ structure (Pb/Cu = 3/8). At these low coverages there may occur mixtures of the structures of figs. 3a and 3b. (A precise correlation with the times for the first (subjective) observations of a particular LEED pattern is difficult because a certain minimum diffracted intensity is always required.) A further difficulty is that since the substrate is at ambient temperature we may form metastable structures. The break in the A+ plot at Q (half monolayer coverage) corresponds to the first point at which the c(5fi X &!)R45” structure of fig. 3b can start to form. At lower coverages we may expect the (2{2 X 2J2 )R45” structure to begin to dominate once the adsorbate density of Pb/Cu = 2/8 has been passed. In fact just before this point (P) there are breaks in i, and in the Auger signals. Breaks in the Auger signals are to be expected if there are changes in the sticking probability [6]. The slight break in the lead Auger signal near P indicates a small decrease in sticking but the break in the copper signal is anomalous in that it apparently indicates an increase in sticking. Since it is in this coverage region that the first crystalline adsorbed layers are observed (only the p(1 x 1) pattern is seen at lower coverages) a possible explanation would be that the attenuation by the lead layer is slightly structure-dependent. Sharp breaks in plots of A+ as a function of coverage have been observed for a number of systems. In the case of metallic adsorption, observations for lead deposited onto silver (111) [9] showed a sharp change in the A+ variation
C. Argile et al. / Secondary electron emission changes
189
at the half-monolayer coverage. This coverage corresponded to a structural change from a dispersed p(fi X 6) arrangement to the beginning of formation of a close-packed hexagonal structure. Similarly, for thallium on copper (100) [lo] a sharp discontinuity was observed at about 0.6 monolayers and this change was attributed, on the basis of LEED observations, to a transition from a one-dimensional type structure (adsorbed atoms in chains) to a compact, two-dimensional, structure. Photoemission experiments confirmed, from the point of view of electronic structure, the 1D to 2D character of the transition. The break observed for Pb/Cu(lOO) at precisely one-half monolayer coverage can be ascribed perhaps to a similar change in adsorbed layer dimensionality. In fact, as noted, the half-monolayer coverage corresponds, according to the proposed models, to the beginning of formation of the first c(5fi x &)R45O structure (fig. 3b). In this structure, the closest distance between chains of atoms becomes for the first time equal to the interatomic distance along the chains. (In the (2fi X 2fi)R45’ structure, the closest distance between chains is twice the atomic diameter.) Incidentally, the break in A+ for the adsorbate concentration Pb/Cu = 3/10 confirms the existence of a c(5fi X fi)R45” structure at this half-monolayer coverage since this atomic ratio cannot be obtained with any other plausible unit mesh. For the (111) face the variation of A+ is monotonic (fig. 1). This behaviour is compatible with the structural variations: there are no sharp changes as the monolayer structure, p(4 X 4) arrangement, develops. The above discussion shows that the sudden variations in the crystal current (secondary electron emission yield) can be related to changes in the adsorbate crystalline structure and that these effects are superimposed on the general increase in yield with coverage which is expected due to the decrease in the work function. We cannot at present give a detailed explanation of the variations of the crystal current. Theories of secondary emission treat a number of processes that occur between the arrival of the primary electrons and the emission of secondaries [ll-131. In general, however, theoretical treatments have been “one-dimensional” and have neglected possible effects in the plane of the surface (an approximation justifiable for most of the early literature, for which the targets were usually not single crystals). But the crystallinity of the surface layers may have an influence through electron scattering and diffraction at different stages: the partial reflection of the primary beam, the penetration of the adsorbed primary beam, and the creation and emergence of the secondaries. We suggested earlier, for example, that enhanced surface ionization could occur due to scattering of the primary beam along the surface. This suggestion was based on observations of “resonance” effects in Auger emission [14] (enhancement of the emission from the surface for certain energies of the incident beam). Other possible effects of surface scattering and diffraction
cannot be excluded and further experiments can be envisaged different mechanisms by varying the temperature, by changing incidence and energies of the primary beam and by examining energy distributions of the secondaries.
to check for the angle of angular and
5. Conclusion There is a renewed interest in secondary emission studies of surfaces and thin films as shown, for example. by the recent work of Park and collaborators [ 15,161 and Bargeron and collaborators [17.18] with an emphasis on the physics of the scattering mechanisms. We have shown here that in an adsorption experiment simple yield measurements can be correlated with structural changes in the ad-layer and that changes in yield are not necessarily related to work function changes.
References [l] [2] [3] [4] [5] 161 [7] [8] [9] [lo] [ 1 l] [12] (131 [14] [IS] [16] [17] 1181
M.G. Barth&-Labrousse and G.F.. Rhead. Surface Sci.. 116 (1982) 217. C. Argile and G.E. Rhead. J. Phys. C (Solid State Phys.) 30 (1982) L193. J. Henrion and G.E. Rhead, Surface Sci. 29 (1972) 20. A. Sepulveda and G.E. Rhead. Surface Sci. 66 (1977) 436. C. Argile and G.E. Rhead, Surface Sci. 135 (1983) 18. M.G. Barth&s and G.E. Rhead, Surface Sci. 80 (1979) 421. W. Hoesler and W. Moritz. Surface Sci. 117 (1982) 196. J.P. Biberian and M. Huber. Surface Sci. 55 (1976) 259. K. Takayanagi. D.M. Kolb, K. Kambe and G. Lehmpfuhl, Surface Sci. 100 (1980) 407. C. Binns and C. Norris. Surface Sci. 115 (1982) 395. A.J. Dekker, in: Solid State Physics. Vol. 6. Eds. F. Seirr and D. Turnbull (Academic Press. New York, 1958) p. 251. 0. Hachenberg and W. Brauer. Advan. Electron. Electron Phys. 11 (1959) 413. P. Sigmund and S. Tongaard, in: Inelastic Particle-Surface Collisions. Eds. E. Taglauer and W. Heiland (Springer, Berlin, 1982) p. 2. M.G. Barthes and G.E. Rhead. J. Phys. D (Appl. Phys.) 13 (1980) 747. R.L. Park, Appl. Surface Sci. 4 (1980) 250. B.T. Jonker. N.C. Bartelt and R.L. Park. Surface Sci. 127 (1983) 183. C.B. Bargeron. B.H. Nail and A.N. Jette, Surface Sci. 120 (1982) L483. A.N. Jette. B.H. Nail and C.B. Bargeron. Phys. Rev. 827 (1983) 708.
Surface Science 138 (1984) 181-190 North-Holland, Amsterdam
SECONDARY ELECTRON EMISSION CHANGES MONOLAYER ADSORPTION C. ARGILE,
M.-G. BARTHES-LABROUSSE
DUE TO METAL
and G.E. RHEAD
Laboratoire de Physic0 -Chimie des Surfaces (ERA 313). ENSCP, II Rue Pierre et Marie Curie, F- 75005 Paris, France
UniversitP Pierre et Marie Curie,
Received 14 July 1983; accepted for publication 3 November 1983
Changes in secondary emission during the adsorption of lead on copper (111) and (100) surfaces, followed by monitoring the crystal current (SEECC technique), are compared with changes in the work function as well as with variations in the Auger signals. The work function variation for the (100) face exhibits a sudden change that may be due to a transition of the adsorbate from a 1D (atomic chain) type of structure to a 2D structure. The secondary emission changes are not explained entirely by the work function variations; there are also effects that are correlated with changes in adsorbate structure and which are probably due to diffraction at the surface.
1. Introduction In two recent papers [1,2] we have reported a simple method for following changes in surface and thin film composition and structure based on measurements of the net current flowing into a crystal specimen under electron beam bombardment. The technique, for which the acronym SEECC was proposed (secondary electron emission crystal current), depends on the sensitivity of secondary emission to surface conditions. The crystal current i, is in fact equal to (S - l)i,, where 6 is the total secondary emission coefficient (including reflected primaries) and i, is the primary electron current. The principle of this type of measurement is well known and it is used qualitatively in scanning secondary electron emission microscopy. However, detailed quantitative comparisons with surface techniques such as Auger electron spectroscopy (AES) had not previously been made. Results obtained on different single crystal substrates showed that the adsorption of monolayer quantities of metals could be readily followed by the crystal current measurements and that the precision was higher than that attained by AES. It was also found possible to detect changes in adsorbed layer structures that were much less apparent from the AES data. 0039-6028/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
At first sight the most obvious explanation for the observed variations of the crystal current would be that adsorption or structural changes have a marked effect on the work function: changes in the surface barrier would influence especially the low-energy electrons in the secondary emission spectrum. We suggested [1.2], however. that other factors could also be important: for example, possible enhanced surface ionization associated with special scattering and diffraction events in the adsorbate and the near-surface layer. The experiments reported here were made to check the influence of work function changes, not measured previously, and to see to what extent the crystal current variations can be correlated with, and explained by. the lowering of the surface barrier. The results show that the lowering of the barrier does not entirely explain the observations and that there ure additional effects that can bc correlated with changes in ad-layer crystallinity. We have also found that measurements of the crystal current can actually help in the interpretations of the LEED patterns from a series of different adsorbed layer structures.
2. Experimental
The experimental equipment and conditions were the same as those used in the earlier work [1,2] but with the addition of a Kelvin vibrating capacitor probe for work function (A+) measurements. (This technique was not available for the earlier work and in presenting the new data there is some unavoidable overlap with the content of the previous publications.) For the SEECC measurements we used the electron gun incorporated in an Auger cylindrical mirror analyser (CMA) with the electron beam incident normally on the specimen surface. Two copper specimens with orientations (111) and (100) were mounted side-by-side on a rotatable manipulator and were electrically connected to ground outside the vacuum chamber via a microammeter. The manipulator was turned so that the specimens faced in succession the positions for lead metal vapour deposition (from a simple crucible source with a rotatable shutter), A+ measurements, AES and SEECC measurements. Depositions were made with the specimens at room temperature. The Kelvin probe was piezoelectrically driven (Delta-Phi-Elektronik. Julich) and had a molybdenum reference electrode. This probe was mounted on a second rotatable manipulator and shielded from the metal vapour source by a stainless steel sheet. Since we were interested only in relative changes in the work function the reference electrode was used as received, without any attempt at cleaning. The reproducibility of the measurements was better than * 10 mV. In making the A+ measurements certain difficulties arose from a non-uniformity of the metal deposit across the specimen. The Kelvin probe apparently
C. Argrle
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changes
183
sampled an area of about 10 mm2, whereas the electron beam used for AES and SEECC measurements had a cross section about 10 times smaller. As a result, and because of a certain inhomogeneity of the deposit, some inconsistencies were found in comparing the A+ and AES or SEECC data. To overcome this difficulty, measurements of the changes in the work function were also made by recording the beginning of the secondary emission spectrum with the CMA and following the displacement of the spectrum along the energy scale during the adsorption process. (A constant potential of - 18 V was applied to the specimen in order to offset effects of stray fields between the specimen and the analyser that would otherwise have made observations of the beginning of the spectrum difficult.) This technique was less sensitive than the Kelvin probe technique, but it enabled us to correlate accurately features of the plot of A+ against deposition time with the AES and SEECC data while the Kelvin probe gave more accurate measurements of the amplitude of the A# changes.
3. Results The results obtained with the different techniques are presented in figs. 1 and 2. To make the comparison easier the work function data have been inverted (the decrease in the work function is plotted as a positive quantity) and the origins of the plots have been displaced vertically. The ranges over which various LEED patterns are observed have been well established in earlier work [3,4]. These ranges are also shown in the figures. It is obvious that although the Auger plots are very similar for the two substrates both the work function changes and the crystal current variations are different for the two crystalline orientations. For the (111) face a sharp maximum is observed in the crystal current at the monolayer coverage 8 = 1, as reported earlier [l]. The break in the crystal current i, at about l/3 of a monolayer (P) is less evident than in the earlier work (differences may occur due to differences in the deposition rate [2]) but a different break is apparent at about B = 5/9 (Q); a change in slope near this point (within 10% of a monolayer) is also visible in the plot of the lead Auger signal (more easily observed obliquely). The plots for the work function changes (A$B) show no such features: A+ decreases linearly up to a coverage of about 0 = 0.2 and then at a progressively slower rate. When the monolayer coverage is reached the work function attains a plateau, or is very slowly decreasing. This is an especially significant result since beyond the monolayer coverage the crystal current decreases sharply. The results for the (100) face are more complicated. The crystal current measurements were first reported in ref. [2]. In subsequent work [5] these data were obtained with more precision using a more stable electron gun. Identical
184
C. Argile et al. / Secondary electron emission changes
results were obtained in the present work. the Auger signals coincide with the onset ments and the crystal current (T). It is These can be placed at positions which close to 5/12 of a monolayer (P) there
P(lxl)
P(4x4)
The monolayer breaks in the plots of of plateaux in both the A+ measurepossible to locate four other breaks. will be justified in the discussion: at are breaks in both the substrate and
diffuse ___-_.______
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Fig. 1. Data obtained for the deposition of lead on Cu(ll1). Upper curves: Auger signals for the substrate and adsorbate; (0) signal for copper (63 eV), (0) signal for lead (93 ev) (above: ranges of observed LEED patterns). (IJ) Decrease in the work function (Kelvin probe measurements), (W) decrease in the work function (electron beam measurements), (A) crystal current measurements.
185
C. Argile et al. / Secondary electron emission changes
adsorbate Auger signals and also a more rounded break in the crystal current; at 8 = l/2 (Q) breaks in the A+ measurements; at B = 2/3 (R) a sharp maximum in the crystal current followed by a drop which can not be
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Fig. 2. Data obtained for the deposition of lead on Cu(100). Upper curves: Auger signals for the substrate and adsorbate; (0) signal for copper (63 ev), (0) signal for lead (93 eV) (above: ranges of observed LEED patterns). (0) Decrease in the work function (Kelvin probe measurements). (m) decrease in the work function (electron beam measurements). (A, v) crystal current measurements (two sets of results).
correlated with any variations in the other measurements, there appears a plateau in the crystal current.
and at 8 = 5/6
(S)
4. Discussion Although the general increase in the crystal current during adsorption (corresponding to an increasing number of secondary electrons being emitted into the vacuum) can be explained by the general decrease in the work function, which will affect especially the very low-energy secondaries. it is clear that the detailed variations of A+ and i, are not correlated. Moreover. a plateau value for A+ corresponds to a plateau in i, for the (100) face but not for the (Ill) face. Neither is there any simple connection between i, and the variations of the Auger signals. There are. however, connections between the I, variations and the changes of structure of the adsorbed layer. For the (111) face it has been shown [6] that the growth mode for lead deposits at room temperature is Frank-van der Merwc (layer-by-layer) while for the (100) face [4] the mode is Stranski -Krastanov (monolayer followed by crystallites). On the (111) face the second layer apparently grows slowly because of a reduced sticking probability and the diffuse LEED patterns observed [3] show that it is poorly crystallised. The existence of this second layer is apparently indicated by the decrease in i, after the monolayer. No such effect occurs for the (100) orientation. Overlayers of lead on copper frequently lead to four-fold periodicities in the coincidence lattice. This has been attributed [3,4,6] to the fact that in dense atomic chains of lead the length of a segment of 3 atoms (with the bulk crystalline parameter) is within 3% of four diameters of the copper substrate atoms. The p(4 x 4) LEED pattern for the monolayer structure on Cu(ll1) has been interpreted in terms of a pseudo-hexagonal close-packed lead layer with 9 atoms in the unit coincidence mesh [l]. The breaks near l/3 and 5/9 of a monolayer may therefore be related to distinct stages in the formation of this dense structure. For the (100) substrate the various breaks in fig. 2 can be understood in terms of the structural models shown in fig. 3. (For a presentation of the patterns for this system and a discussion of the structures see refs. [3.4,7].) The interpreted by monolayer LEED pattern, ~(542 X 42 )R45”. was originally Henrion and Rhead [3] (HR) in terms of a pseudo-hexagonal overlayer with 6 lead atoms for 10 copper atoms in the unit coincidence mesh. Hoesler and Moritz [7] (HM) have recently proposed a different interpretation (but with the same concentration of lead). This interpretation, shown on the extreme right of fig. 3, is based on LEED intensity analyses; it places all the adsorbate atoms at or close to hollow (four-fold coordination) substrate sites. This type of struc-
C. Arple
er al. / Secondary
electron emrssvron charlxes
187
ture was discussed by Biberian and Huber on the basis of crystalline symmetry [8]. The model of fig. 3d is derived from a (42 x a)R45’ structure (fig. 3c) and in their work HM observed the pattern for such a structure at a coverage 0 = 5/6. Surprisingly, this pattern was not reported by HR nor in the later study by Sepulveda and Rhead [4] (SR). However, a reexamination of the original photographs obtained by HR (the Polaroid pictures have kept well since 1969!) does show some of the patterns for the c(5v’2 X fi)R45’ structure with the extra l/2 l/2 spots corresponding to the (42 x fi)R45” structure. At the time Auger spectroscopy was not available and it was thought that the extra spots were due to an impurity. Moreover, the later work of SR, which did use Auger spectroscopy, showed that the ~(50 x Ji )R45” structure existed at lower coverages than that necessary for a completed (fi X &)R45” structure (see the top band of fig. 2). This apparent contradiction as regards the existence of the (0 x fi)R45” structure can be resolved if we suppose that a ~(542 x fi)R45” pattern can also be obtained from a second structure at a lower coverage as shown in fig. 3b. At a coverage of 8 = 2/3 it is possible to fill the coincidence mesh with 4 atoms, all in hollow sites. and this too is a structure derived from the o/2 x fi)R45’ arrangement. Thus the (42 X {2)R45” pattern would tend to occur only for a very narrow range of coverages preceded and followed by the same pattern (with possible differences in spot intensities). This would explain why the (42 X 42 )R45’ pattern was
Pb/C 8
u
3/8=0.375
4/l 0
5110
6110
0.625
0.66
0.83
1.0
a
b
c
d
Fig. 3. Models for the structures of lead on Cu(100) at different coverages (0: fraction of a complete monolayer; Pb/Cu: ratio of adsorbate to first-layer substrate atomic densities). The structures (c) and (d) were obtained by Hoesler and Moritz [7]. The intersections of the net represent the hollow four-fold coordination substrate sites.
188
C. Arxde
er al. / Secondary
electron em,.r~,on changes
overlooked in the earlier work and why the existence of the (542 x 42 )R45” pattern was reported for relatively low coverages by SR. At low and medium coverages a (242 X 2fi)R45’ pattern is observed (previously labelled c(4 x 4) in the alternative nomenclature by HR and SR). The Auger study by SR concluded that there were only 3 atoms in this unit mesh and so corrected an earlier interpretation of HR which assumed a denser arrangement. A very simple interpretation, as proposed by SR, is to place a chain of lead atoms across the diagonal of the unit mesh (3d,, = 4d,,), but this is unlikely to be correct if the structures that follow are due to adsorption in hollow sites. We therefore propose the model shown in fig. 3a which comprises the same (11) oriented chains that appear in the later structures. plus vacant sites. Effectively this is a (fi X fi)R45” arrangement with vacant sites that are separated. The models shown are of course ideal structures: a more random distribution of vacant sites but with the tendency of avoiding adjacent vacancies would be more realistic. We return now to possible explanations of the various breaks in fig. 2. The range S to T corresponds to the growth of the final monolayer arrangement. fig. 3d. Over this range the i, plot apparently exhibits a dip below the final plateau level. S represents the unique coverage for which the (42. x fi )R45” structure can exist alone. The break at R corresponds to the beginning of formation of the (a x fi)R45” structure. For lower coverages a succession of structures is less obvious: for example there is no feature in the curves that would correspond to the completion of the (2& x 2fi)R45’ structure (Pb/Cu = 3/8). At these low coverages there may occur mixtures of the structures of figs. 3a and 3b. (A precise correlation with the times for the first (subjective) observations of a particular LEED pattern is difficult because a certain minimum diffracted intensity is always required.) A further difficulty is that since the substrate is at ambient temperature we may form metastable structures. The break in the A+ plot at Q (half monolayer coverage) corresponds to the first point at which the c(5fi X &!)R45” structure of fig. 3b can start to form. At lower coverages we may expect the (2{2 X 2J2 )R45” structure to begin to dominate once the adsorbate density of Pb/Cu = 2/8 has been passed. In fact just before this point (P) there are breaks in i, and in the Auger signals. Breaks in the Auger signals are to be expected if there are changes in the sticking probability [6]. The slight break in the lead Auger signal near P indicates a small decrease in sticking but the break in the copper signal is anomalous in that it apparently indicates an increase in sticking. Since it is in this coverage region that the first crystalline adsorbed layers are observed (only the p(1 x 1) pattern is seen at lower coverages) a possible explanation would be that the attenuation by the lead layer is slightly structure-dependent. Sharp breaks in plots of A+ as a function of coverage have been observed for a number of systems. In the case of metallic adsorption, observations for lead deposited onto silver (111) [9] showed a sharp change in the A+ variation
C. Argile et al. / Secondary electron emission changes
189
at the half-monolayer coverage. This coverage corresponded to a structural change from a dispersed p(fi X 6) arrangement to the beginning of formation of a close-packed hexagonal structure. Similarly, for thallium on copper (100) [lo] a sharp discontinuity was observed at about 0.6 monolayers and this change was attributed, on the basis of LEED observations, to a transition from a one-dimensional type structure (adsorbed atoms in chains) to a compact, two-dimensional, structure. Photoemission experiments confirmed, from the point of view of electronic structure, the 1D to 2D character of the transition. The break observed for Pb/Cu(lOO) at precisely one-half monolayer coverage can be ascribed perhaps to a similar change in adsorbed layer dimensionality. In fact, as noted, the half-monolayer coverage corresponds, according to the proposed models, to the beginning of formation of the first c(5fi x &)R45O structure (fig. 3b). In this structure, the closest distance between chains of atoms becomes for the first time equal to the interatomic distance along the chains. (In the (2fi X 2fi)R45’ structure, the closest distance between chains is twice the atomic diameter.) Incidentally, the break in A+ for the adsorbate concentration Pb/Cu = 3/10 confirms the existence of a c(5fi X fi)R45” structure at this half-monolayer coverage since this atomic ratio cannot be obtained with any other plausible unit mesh. For the (111) face the variation of A+ is monotonic (fig. 1). This behaviour is compatible with the structural variations: there are no sharp changes as the monolayer structure, p(4 X 4) arrangement, develops. The above discussion shows that the sudden variations in the crystal current (secondary electron emission yield) can be related to changes in the adsorbate crystalline structure and that these effects are superimposed on the general increase in yield with coverage which is expected due to the decrease in the work function. We cannot at present give a detailed explanation of the variations of the crystal current. Theories of secondary emission treat a number of processes that occur between the arrival of the primary electrons and the emission of secondaries [ll-131. In general, however, theoretical treatments have been “one-dimensional” and have neglected possible effects in the plane of the surface (an approximation justifiable for most of the early literature, for which the targets were usually not single crystals). But the crystallinity of the surface layers may have an influence through electron scattering and diffraction at different stages: the partial reflection of the primary beam, the penetration of the adsorbed primary beam, and the creation and emergence of the secondaries. We suggested earlier, for example, that enhanced surface ionization could occur due to scattering of the primary beam along the surface. This suggestion was based on observations of “resonance” effects in Auger emission [14] (enhancement of the emission from the surface for certain energies of the incident beam). Other possible effects of surface scattering and diffraction
cannot be excluded and further experiments can be envisaged different mechanisms by varying the temperature, by changing incidence and energies of the primary beam and by examining energy distributions of the secondaries.
to check for the angle of angular and
5. Conclusion There is a renewed interest in secondary emission studies of surfaces and thin films as shown, for example. by the recent work of Park and collaborators [ 15,161 and Bargeron and collaborators [17.18] with an emphasis on the physics of the scattering mechanisms. We have shown here that in an adsorption experiment simple yield measurements can be correlated with structural changes in the ad-layer and that changes in yield are not necessarily related to work function changes.
References [l] [2] [3] [4] [5] 161 [7] [8] [9] [lo] [ 1 l] [12] (131 [14] [IS] [16] [17] 1181
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