- Email: [email protected]
W(110)
Ultramicroscopy 18 (1985) 439-444 North-Holland, Amsterdam
439
OBSERVATION OF SURFACE DIFFUSION BY BIASSED SECONDARY ELECTRON IMAGING: THE CASE OF A g / W ( I I 0 ) G . W . JONES and J.A. VENABLES *
School of Mathematical and Physical Sciences, University of Sussex, Brighton BNI 9QH, UK Received 10 July 1985; presented at Symposium January 1985
The system A g / W ( l l 0 ) has been studied by SEM, AES, RHEED and a simple new technique, biassed secondary electron imaging. By depositing Ag past an edge and through a mask of holes, surface diffusion of Ag has been observed, in competition with the nucleation of crystals on top of adsorbed multilayers. The extent of surface diffusion, and of nucleation, is highly temperature dependent in the range 200-550°C. The biassed secondary electron technique is shown to have a much better signal-to-noise ratio than equivalent Auger electron line-scans or images; the contrast arises from changes in secondary electron production and detection, especially changes in work function and backscattering. The qualitative conclusions of the results for interactions between Ag atoms on W(ll0) are discussed.
1. Introduction
We have previously used the combination of scanning electron microscopy (SEM), Auger electron spectroscopy (AES) and reflection electron diffraction (RHEED) for studies of surface processes on a microscopic scale [1]. This combination, plus a digital data acquisition system [2], has been used to study nucleation and growth processes on surfaces in the case of A g / W ( l l 0 ) [3] and Ag/Si(100) and (111) [4]. These systems are examples of the Stranski-Krastanov growth mode, in which three-dimensional (3D) crystals form on top of, or in competition with, 2D adsorbed layers on the substrate surface. AES and RHEED have been used to characterise the layers, while SEM is very useful for observing, counting and sizing the crystals. More recently we have experimented with depositing Ag past a mask edge, and through a mask of holes, following the lead given several years ago by Butz and Wagner [5]. We have found that the deposited patches so formed can be seen readily by SEM, provided that the sample is biassed nega* Also at Department of Physics, Arizona State University, Tempe, Arizona 85287, USA.
tively to a few hundred volts. The systematics of the contrast as a function of incidence angle 00, Ag coverage, and bias voltage Vb were studied recently for Ag/W(ll0) and Ag/Si systems [6]. The sensitivity was shown to be < 0.1 monolayer for the case of Ag/Si(lll). One of the obvious applications of the technique is to study surface diffusion, both alone and in competition with nucleation a n d / o r reevaporation. By using finite-sized patches, which broaden, diffusion can be studied quantitatively; competition with nucleation can be studied by comparing the nucleation density as a function of temperature and deposition rate [7], both on the patches and on the unmasked area. Examples for A g / S i ( l l l ) are published elsewhere [4,6,8]; here we give examples for Ag/W(ll0). Silver on tungsten (110) is one of the most studied metal-metal crystal growth systems [3,9]. Analysis of the nucleation data [3,7,10], as well as recent data on work function changes as a function of temperature, AO(T ) [11], indicate that the lateral Ag pair bond E 2 = 0.3 eV, with the diffusion energy E d of Ag atoms over 2 ML Ag on the substrate = 0.1 eV. Measurements of patch widths and nucleation patterns offer the possibility of checking these implied values. In the present paper
0304-3991/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
440
G.W. Jones, J.A. Venables / Observation of surface diffusion by biassed SEI: A g / W ( l lO)
we show the microscopic features observed and give qualitative conclusions; we will give quantitative analyses elsewhere [12].
2. Experimental procedures The apparatus and experimental procedures used have been described in some detail previously [1-4,6]. Briefly, Ag is deposited onto cleaned W ( l l 0 ) crystals via a mask of holes which partly covers the 5 mm square sample. This mask is adjusted to within 0.1 mm of the sample during SEM observation, and Ag is deposited f r o m a Knudsen source [3,4] while the sample is held at the deposition temperature T, which is between room temperature and 550°C (823 K). The deposition rate R is = 0 . 5 M L / m i n , where 1 ML is taken as the atomic density of the (111) plane of Ag - 1.38 × 10 a5 atoms cm -2. The mask is removed after deposition and the sample is observed by SEM, AES (in the EN(E) mode), R H E E D and biassed imaging, typically at 30 kV beam voltage using 2 × 1 0 - 9 - 2 × 10 -8 A from the field emission SEM column, with the sample near room temperature. The initial cooling from the deposition temperature is sufficiently rapid (around 30 s to < 100°C) that we believe most of the diffusion takes place during and immediately after deposition at a well defined temperature. In this respect our results correspond to a different situation from those of Butz and Wagner [5], since they deposited at room temperature and subsequently annealed up to a given temperature. The geometry of the SEM detector, which undoubtedly affects the detailed contrast in the images, is described elsewhere [6].
3. Experimental results The range of results obtained is illustrated by figs. 1-3. Fig. 1 shows the effect of sample bias Vb on the secondary electron image of the mask edge region for a 5 M L Ag deposit at T = 400°C (673 K), observed at O0 = 20 °, expressed as a glancing angle to the surface plane. In fig. la, at zero bias, the islands appear dark, but the edge of the inter-
mediate layer can hardly be seen. Fig. l b shows bright layers and even brighter islands at Vb = - 2 0 0 V. Two edges, which we identify as steps at 1 and 2 ML, are clearly seen, and the width of the region denuded of islands can be measured. The width is = 17 # m in this case, several times the mean distance between islands, which is = 6/~m. It is also noticeable that the 1 and 2 ML edges are quite close together ( = 2 #m) on this scale. The temperature dependence of these processes is illustrated in fig. 2, which shows single patches, where 5 ML Ag has been deposited through holes 32 /~m wide ( × 100/xm long). Fig. 2a is for T = 300°C (573 K) viewed at 80 = 14 ° with Vb = - 500 V, whereas fig. 2b shows T = 460°C (733 K), viewed at 80 = 22 ° with Vb = - 200 V. The considerable difference in nucleation density and diffusion width (note the difference in scale between figs. 2a and 2b) over this relatively narrow temperature region is apparent. In fig. 2b, the islands only nucleate essentially along a line at the center of the patch; above T = 460°C, nucleation on the finite patches is suppressed entirely. However, on unmasked areas islands are still nucleated at these temperatures, and condensation is essentially complete (no reevaporation), up to T = 550°C (823K) [3]. At the lower temperature the width of the denuded region is - 4/~m with the 1-2 ML width - 1 /~m, and the islands - 1 . 8 /tm apart. The corresponding distances for the higher temperature illustrated in fig. 2b are - 38, 3 and 8/~m, if the last figure is taken as the island spacing in 1D along the line observed. Quantitative analysis of the layers can be done most readily using point AES analyses and Auger a n d biassed secondary electron line scans. This is illustrated in fig. 3 for the edge of fig. 1. The secondary electron scans are typically obtained with maximum zero suppression (trace a), as without it (trace b) the features are much more difficult to see. Auger line scans for Ag are taken using the digital system at energies A on the peak at 346 V and at equally spaced points B and C on the background at higher energies [2]. The righthand side of the trace corresponds to clean W, with the layers to the left and the blips corresponding to the islands (slightly different ones in case of a - b and A - B - C traces). Note that the
G.W. Jones, J.A. Venables / Observation of surface diffusion by biassed SEI: Ag/W(I IO)
441
Fig. 1. Biassed secondary electron images of 5 ML A g / W ( l l 0 ) , deposited past a mask edge at T = 673 K and R = 0.5 M L / m i n . ObservatiOns at 30 kV, with 00 (glancing angle) = 20°; the tilt is around the horizontal axis, so the vertical scale is compressed by cosec 00. (a) Zero bias, with Ag islands showing dark, and the layers essentially invisible; (b) Vb = -- 200 V, showing the islands bright and the 1 and 2 ML steps clearly visible.
442
G.W. Jones, J.A. Venables / Observation of surface diffusion by biassed SEI: A g~ W(110)
Fig. 2. Competition between surface diffusion and nucleation on an individual masked area. 5 ML Ag was deposited at R ffi 0.4 M L / m i n through a mask hole 32 p m wide by 100 p m long, observed at 30 kV: (a) T ffi 573 K, 00 ffi 140; (b) T ffi 733 K, 00 ffi 22 °. See text for discussion.
G.W. Jones, J.A. Venables / Obseroation of surface diffusion by biassed SEI: Ag/W(110)
B
j / ~ C
J
443
also trace (a), show evidence for a broadly diffused layer on top of these 2 ML. Whether the islands are growing on top of, or in competition with, a third layer, and whether this depends on substrate perfection is controversial [3,13] and will require further study. However, it is noticeable from fig. 3 that there is a very good correlation between the secondary electron and Auger line scans and that the secondary electron signal has a much better signal-to-noise ratio. The structure of these layers and islands have been investigated by diffraction techniques. RHEED patterns have been obtained corresponding to the W substrate, the 2 ML region, and the islands, under similar conditions to fig. 1, with 00 = 2 ° at 60 kV. The Ag islands show twinned (and streaked) (110) patterns parallel to (001) W corresponding to the Nishiyama-Wassermann orientation relationship, at least approximately, as found earlier using electron back-scattering patterns [15]. This orientation, and the closely related Kurdjumov-Sachs relationship, are generally found for metals condensed onto W(ll0) [9].
IOizm
Fig. 3. Biassed secondary electron and Auger line scans of the edge corresponding to fig. 1. Secondary electron scans (a) and (b) with and without zero suppression respectively. Energyselected scans: (A) 346 V, (B) 385 V and (C) 424 V. Auger line scans based on the algorithms shown. See text for discussion.
blips are strongly negative on the background traces B and C, and are even negative on the Ag peak trace A. This emphasises the need for data processing to obtain "true" representations of the Ag composition from EN(E) spectra. The ones we have used are shown in the bottom two traces of fig. 3. As discussed elsewhere [2], the (A - B)/(A + B) algorithm is the simplest digital signal corresponding to the analogue N'(E)/N(E) signal used previously; it is quasi-logarithmic and so de-emphasises the (now positive) island "blip". The signal (A - 2B + C)/(2B - C) is the simplest linear measure of the peak-to-background ratio. The island signal is now around twice that of the intermediate layer, which we interpret as corresponding to 2 ML Ag, as found previously [3]. This trace, and
4. Discussion
This paper and companion papers [6,8] show that biassed secondary electron imaging is a powerful technique for observing surface layers and studying diffusion in and on these layers. The technique is very simple, has a good signal-to-noise ratio, and can be quantified by correlation with micro-AES and RHEED. The contrast obtained is a function of several variables. In the case of Ag/W(ll0), where the effects of secondary electron production and work function change (A~b= -0.75 V for 2 ML Ag) go in opposite directions, various contrast levels have been observed as a function of 00 and amount deposited. What is surprising, however, is the large absolute value of the contrast. As can be seen from fig. 3 (trace b), this is - 10% for 2 ML Ag, and can be easily increased to > 20% by zero suppression (trace a); higher levels have been seen in some cases. Biassed secondary electron spectroscopy shows that the contrast arises largely from the low-energy secondaries which can escape from the
444
G. IV. Jones, J.A. Venables / Observation of surface diffusion by biassed S EI: Ag/W(11 O)
lower work-function Ag region under the influence of the bias field. These effects are discussed in more detail elsewhere [6]. But there are also large changes in the level of backscattered electrons which show up on the energy-selected line scans of fig. 3, which must also affect secondary electron production in a way which remains to be investigated in detail. The measurements relating to diffusion in competition with nucleation are analysed in detail elsewhere, and correlated with models to deduce adsorption, binding and diffusion energies [12] for comparison with values deduced from nucleation experiments [3,7,10]. However, several features are apparent from qualitative inspection of pictures such as figs. 1 and 2, given that condensation is essentially complete at the corresponding deposition temperatures. The diffusion width at low temperatures, on top of 2ML Ag, is a few times the mean distance between islands; this result is consistent with ideas of competitive capture as current in nucleation theory. The relatively small width of the 1 ML region, and the realtively sharp cutoff in the island distribution are, we believe, both due to the moderate attractive lateral interactions between the Ag atoms [3,10]. In the first case, the attractive interactions means that 3rd-layer atoms which fall over the edge into the 2nd layer tend to stick to this edge, so keeping the supply of 2nd-layer atoms low so that the first layer cannot advance much faster than the second. In the second case the relatively large critical nucleus size, i - 10, implies that the cut-off will be sharp, since the layer edge depresses the single-atom concentration n I linearly and the nucleation rate is proportional to the (i + 1)th power of n I [7].
5. Conclusion We have shown that biassed secondary electron imaging is a suitable technique for studying multilayer adsorption systems such as A g / W ( l l 0 ) , and that detailed measurements of diffusion in this system will lead to information about interactions between Ag atoms on the surface.
Acknowledgements We thank the SERC for continued support for this research, and for a studentship for G.W.J. under the CASE scheme in collaboration with CERL. We thank M. Hardiman and C.J. Harland for their support and criticism, J.M. Cowley for his hospitality, P.R. Buseck for his patience and Diane Stiles for her typing and assistance, during the writing of this paper.
References [1] J.A. Venables, G.D.T. Spiller, D.J. Fathers, C.J. Hadand and M. Hanbi~cken, Ultramicroscopy11 (1983) 149. [2] C.J. Harland and J.A. Venables, Ultramicroscopy17 (1985) 9; D.J. Fathers, C.J. Harland and J.A. Venables, in: Electron Microscopy and Analysis 1983, Guildford, 1983, Inst. Phys. Conf. Ser. 68, Ed. P. Doig (Inst. Phys., London-Bristol, 1983) p. 227. [3] G.D.T. Spiller, P. Akhter and J.A. Venables, Surface Sci. 131 (1983) 517. [4] M. Hanbi'~cken, M. Futamoto and J.A. Venables, Surface Sci. 147 (1984) 433. [5] R. Butz and H. Wagner, Surface Sci. 87 (1979) 69, 85; see also H. Wagner, in: Surface Mobilities on Solid Materials, Plenum NATO-ASI Series B86, Ed. Vu Thien Birth (Plenum, New York, 1982) p. 161. [6] M. Futamoto, M. Hanbi~cken, C.J. Harland, G.W. Jones and J.A. Venables, Surface Sci. 150 (1985) 430. [7] J.A. Venables, G.D.T. Spiller and M. Hanb~cken, Rept. Progr. Phys. 47 (1984) 399. [8] M. Hanbi'~cken,T. Doust, O. Osasona, (3. Le Lay and J.A. Venables, Surface Sci., in press. [9] E. Bauer, Appl. Surface Sci 11/12 (1982) 479; E. Bauer, in: Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. IIIA, Eds. D.A. King and D.P. Woodruff(Elsevier, Amsterdam, in press). [10] J.A. Venables, Vacuum 33 (1983) 701. [11] J. Kolaczkiewiczand E. Bauer, Surface Sci. 155 (1985) 700. [12] J.A. Venables, R. Kariotis, T.N. Doust and G.W. Jones, work in progress. [13] G. Gerth and M. Paunov, in: Proc. 9th Intern. Vacuum Congr. and 5th Intern. Conf. on Solid Surfaces, Madrid, 1983, Extended Abstracts, p. 148. [14] C.J. Harland, P. Akhter and J.A. Venables, J. Phys. E14 (1981) 175.
439
OBSERVATION OF SURFACE DIFFUSION BY BIASSED SECONDARY ELECTRON IMAGING: THE CASE OF A g / W ( I I 0 ) G . W . JONES and J.A. VENABLES *
School of Mathematical and Physical Sciences, University of Sussex, Brighton BNI 9QH, UK Received 10 July 1985; presented at Symposium January 1985
The system A g / W ( l l 0 ) has been studied by SEM, AES, RHEED and a simple new technique, biassed secondary electron imaging. By depositing Ag past an edge and through a mask of holes, surface diffusion of Ag has been observed, in competition with the nucleation of crystals on top of adsorbed multilayers. The extent of surface diffusion, and of nucleation, is highly temperature dependent in the range 200-550°C. The biassed secondary electron technique is shown to have a much better signal-to-noise ratio than equivalent Auger electron line-scans or images; the contrast arises from changes in secondary electron production and detection, especially changes in work function and backscattering. The qualitative conclusions of the results for interactions between Ag atoms on W(ll0) are discussed.
1. Introduction
We have previously used the combination of scanning electron microscopy (SEM), Auger electron spectroscopy (AES) and reflection electron diffraction (RHEED) for studies of surface processes on a microscopic scale [1]. This combination, plus a digital data acquisition system [2], has been used to study nucleation and growth processes on surfaces in the case of A g / W ( l l 0 ) [3] and Ag/Si(100) and (111) [4]. These systems are examples of the Stranski-Krastanov growth mode, in which three-dimensional (3D) crystals form on top of, or in competition with, 2D adsorbed layers on the substrate surface. AES and RHEED have been used to characterise the layers, while SEM is very useful for observing, counting and sizing the crystals. More recently we have experimented with depositing Ag past a mask edge, and through a mask of holes, following the lead given several years ago by Butz and Wagner [5]. We have found that the deposited patches so formed can be seen readily by SEM, provided that the sample is biassed nega* Also at Department of Physics, Arizona State University, Tempe, Arizona 85287, USA.
tively to a few hundred volts. The systematics of the contrast as a function of incidence angle 00, Ag coverage, and bias voltage Vb were studied recently for Ag/W(ll0) and Ag/Si systems [6]. The sensitivity was shown to be < 0.1 monolayer for the case of Ag/Si(lll). One of the obvious applications of the technique is to study surface diffusion, both alone and in competition with nucleation a n d / o r reevaporation. By using finite-sized patches, which broaden, diffusion can be studied quantitatively; competition with nucleation can be studied by comparing the nucleation density as a function of temperature and deposition rate [7], both on the patches and on the unmasked area. Examples for A g / S i ( l l l ) are published elsewhere [4,6,8]; here we give examples for Ag/W(ll0). Silver on tungsten (110) is one of the most studied metal-metal crystal growth systems [3,9]. Analysis of the nucleation data [3,7,10], as well as recent data on work function changes as a function of temperature, AO(T ) [11], indicate that the lateral Ag pair bond E 2 = 0.3 eV, with the diffusion energy E d of Ag atoms over 2 ML Ag on the substrate = 0.1 eV. Measurements of patch widths and nucleation patterns offer the possibility of checking these implied values. In the present paper
0304-3991/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
440
G.W. Jones, J.A. Venables / Observation of surface diffusion by biassed SEI: A g / W ( l lO)
we show the microscopic features observed and give qualitative conclusions; we will give quantitative analyses elsewhere [12].
2. Experimental procedures The apparatus and experimental procedures used have been described in some detail previously [1-4,6]. Briefly, Ag is deposited onto cleaned W ( l l 0 ) crystals via a mask of holes which partly covers the 5 mm square sample. This mask is adjusted to within 0.1 mm of the sample during SEM observation, and Ag is deposited f r o m a Knudsen source [3,4] while the sample is held at the deposition temperature T, which is between room temperature and 550°C (823 K). The deposition rate R is = 0 . 5 M L / m i n , where 1 ML is taken as the atomic density of the (111) plane of Ag - 1.38 × 10 a5 atoms cm -2. The mask is removed after deposition and the sample is observed by SEM, AES (in the EN(E) mode), R H E E D and biassed imaging, typically at 30 kV beam voltage using 2 × 1 0 - 9 - 2 × 10 -8 A from the field emission SEM column, with the sample near room temperature. The initial cooling from the deposition temperature is sufficiently rapid (around 30 s to < 100°C) that we believe most of the diffusion takes place during and immediately after deposition at a well defined temperature. In this respect our results correspond to a different situation from those of Butz and Wagner [5], since they deposited at room temperature and subsequently annealed up to a given temperature. The geometry of the SEM detector, which undoubtedly affects the detailed contrast in the images, is described elsewhere [6].
3. Experimental results The range of results obtained is illustrated by figs. 1-3. Fig. 1 shows the effect of sample bias Vb on the secondary electron image of the mask edge region for a 5 M L Ag deposit at T = 400°C (673 K), observed at O0 = 20 °, expressed as a glancing angle to the surface plane. In fig. la, at zero bias, the islands appear dark, but the edge of the inter-
mediate layer can hardly be seen. Fig. l b shows bright layers and even brighter islands at Vb = - 2 0 0 V. Two edges, which we identify as steps at 1 and 2 ML, are clearly seen, and the width of the region denuded of islands can be measured. The width is = 17 # m in this case, several times the mean distance between islands, which is = 6/~m. It is also noticeable that the 1 and 2 ML edges are quite close together ( = 2 #m) on this scale. The temperature dependence of these processes is illustrated in fig. 2, which shows single patches, where 5 ML Ag has been deposited through holes 32 /~m wide ( × 100/xm long). Fig. 2a is for T = 300°C (573 K) viewed at 80 = 14 ° with Vb = - 500 V, whereas fig. 2b shows T = 460°C (733 K), viewed at 80 = 22 ° with Vb = - 200 V. The considerable difference in nucleation density and diffusion width (note the difference in scale between figs. 2a and 2b) over this relatively narrow temperature region is apparent. In fig. 2b, the islands only nucleate essentially along a line at the center of the patch; above T = 460°C, nucleation on the finite patches is suppressed entirely. However, on unmasked areas islands are still nucleated at these temperatures, and condensation is essentially complete (no reevaporation), up to T = 550°C (823K) [3]. At the lower temperature the width of the denuded region is - 4/~m with the 1-2 ML width - 1 /~m, and the islands - 1 . 8 /tm apart. The corresponding distances for the higher temperature illustrated in fig. 2b are - 38, 3 and 8/~m, if the last figure is taken as the island spacing in 1D along the line observed. Quantitative analysis of the layers can be done most readily using point AES analyses and Auger a n d biassed secondary electron line scans. This is illustrated in fig. 3 for the edge of fig. 1. The secondary electron scans are typically obtained with maximum zero suppression (trace a), as without it (trace b) the features are much more difficult to see. Auger line scans for Ag are taken using the digital system at energies A on the peak at 346 V and at equally spaced points B and C on the background at higher energies [2]. The righthand side of the trace corresponds to clean W, with the layers to the left and the blips corresponding to the islands (slightly different ones in case of a - b and A - B - C traces). Note that the
G.W. Jones, J.A. Venables / Observation of surface diffusion by biassed SEI: Ag/W(I IO)
441
Fig. 1. Biassed secondary electron images of 5 ML A g / W ( l l 0 ) , deposited past a mask edge at T = 673 K and R = 0.5 M L / m i n . ObservatiOns at 30 kV, with 00 (glancing angle) = 20°; the tilt is around the horizontal axis, so the vertical scale is compressed by cosec 00. (a) Zero bias, with Ag islands showing dark, and the layers essentially invisible; (b) Vb = -- 200 V, showing the islands bright and the 1 and 2 ML steps clearly visible.
442
G.W. Jones, J.A. Venables / Observation of surface diffusion by biassed SEI: A g~ W(110)
Fig. 2. Competition between surface diffusion and nucleation on an individual masked area. 5 ML Ag was deposited at R ffi 0.4 M L / m i n through a mask hole 32 p m wide by 100 p m long, observed at 30 kV: (a) T ffi 573 K, 00 ffi 140; (b) T ffi 733 K, 00 ffi 22 °. See text for discussion.
G.W. Jones, J.A. Venables / Obseroation of surface diffusion by biassed SEI: Ag/W(110)
B
j / ~ C
J
443
also trace (a), show evidence for a broadly diffused layer on top of these 2 ML. Whether the islands are growing on top of, or in competition with, a third layer, and whether this depends on substrate perfection is controversial [3,13] and will require further study. However, it is noticeable from fig. 3 that there is a very good correlation between the secondary electron and Auger line scans and that the secondary electron signal has a much better signal-to-noise ratio. The structure of these layers and islands have been investigated by diffraction techniques. RHEED patterns have been obtained corresponding to the W substrate, the 2 ML region, and the islands, under similar conditions to fig. 1, with 00 = 2 ° at 60 kV. The Ag islands show twinned (and streaked) (110) patterns parallel to (001) W corresponding to the Nishiyama-Wassermann orientation relationship, at least approximately, as found earlier using electron back-scattering patterns [15]. This orientation, and the closely related Kurdjumov-Sachs relationship, are generally found for metals condensed onto W(ll0) [9].
IOizm
Fig. 3. Biassed secondary electron and Auger line scans of the edge corresponding to fig. 1. Secondary electron scans (a) and (b) with and without zero suppression respectively. Energyselected scans: (A) 346 V, (B) 385 V and (C) 424 V. Auger line scans based on the algorithms shown. See text for discussion.
blips are strongly negative on the background traces B and C, and are even negative on the Ag peak trace A. This emphasises the need for data processing to obtain "true" representations of the Ag composition from EN(E) spectra. The ones we have used are shown in the bottom two traces of fig. 3. As discussed elsewhere [2], the (A - B)/(A + B) algorithm is the simplest digital signal corresponding to the analogue N'(E)/N(E) signal used previously; it is quasi-logarithmic and so de-emphasises the (now positive) island "blip". The signal (A - 2B + C)/(2B - C) is the simplest linear measure of the peak-to-background ratio. The island signal is now around twice that of the intermediate layer, which we interpret as corresponding to 2 ML Ag, as found previously [3]. This trace, and
4. Discussion
This paper and companion papers [6,8] show that biassed secondary electron imaging is a powerful technique for observing surface layers and studying diffusion in and on these layers. The technique is very simple, has a good signal-to-noise ratio, and can be quantified by correlation with micro-AES and RHEED. The contrast obtained is a function of several variables. In the case of Ag/W(ll0), where the effects of secondary electron production and work function change (A~b= -0.75 V for 2 ML Ag) go in opposite directions, various contrast levels have been observed as a function of 00 and amount deposited. What is surprising, however, is the large absolute value of the contrast. As can be seen from fig. 3 (trace b), this is - 10% for 2 ML Ag, and can be easily increased to > 20% by zero suppression (trace a); higher levels have been seen in some cases. Biassed secondary electron spectroscopy shows that the contrast arises largely from the low-energy secondaries which can escape from the
444
G. IV. Jones, J.A. Venables / Observation of surface diffusion by biassed S EI: Ag/W(11 O)
lower work-function Ag region under the influence of the bias field. These effects are discussed in more detail elsewhere [6]. But there are also large changes in the level of backscattered electrons which show up on the energy-selected line scans of fig. 3, which must also affect secondary electron production in a way which remains to be investigated in detail. The measurements relating to diffusion in competition with nucleation are analysed in detail elsewhere, and correlated with models to deduce adsorption, binding and diffusion energies [12] for comparison with values deduced from nucleation experiments [3,7,10]. However, several features are apparent from qualitative inspection of pictures such as figs. 1 and 2, given that condensation is essentially complete at the corresponding deposition temperatures. The diffusion width at low temperatures, on top of 2ML Ag, is a few times the mean distance between islands; this result is consistent with ideas of competitive capture as current in nucleation theory. The relatively small width of the 1 ML region, and the realtively sharp cutoff in the island distribution are, we believe, both due to the moderate attractive lateral interactions between the Ag atoms [3,10]. In the first case, the attractive interactions means that 3rd-layer atoms which fall over the edge into the 2nd layer tend to stick to this edge, so keeping the supply of 2nd-layer atoms low so that the first layer cannot advance much faster than the second. In the second case the relatively large critical nucleus size, i - 10, implies that the cut-off will be sharp, since the layer edge depresses the single-atom concentration n I linearly and the nucleation rate is proportional to the (i + 1)th power of n I [7].
5. Conclusion We have shown that biassed secondary electron imaging is a suitable technique for studying multilayer adsorption systems such as A g / W ( l l 0 ) , and that detailed measurements of diffusion in this system will lead to information about interactions between Ag atoms on the surface.
Acknowledgements We thank the SERC for continued support for this research, and for a studentship for G.W.J. under the CASE scheme in collaboration with CERL. We thank M. Hardiman and C.J. Harland for their support and criticism, J.M. Cowley for his hospitality, P.R. Buseck for his patience and Diane Stiles for her typing and assistance, during the writing of this paper.
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