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The initial stages of the oxidation of Al(111). I
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Surface Science North-Holland
surface science
296 (1993) 131-140
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The initial stages of the oxidation of Al( 111). I D.J. O’Connor a, E.R. Wouters b, A.W. Denier van der Gon b, J.F. van der Veen b, P.M. Zagwijn b and J.W.M. Frenken b a Department of Physics, Unioersity of Newcastle, Newcastle NSW 2308, Australia b FOM-Institute Received
for Atomic
15 December
and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam,
1992; accepted
for publication
25 June
Netherlands
1993
AK1 11) oxidation has been subjected to numerous studies using surface analysis techniques. Indeed it seems to have been used as a proving ground for techniques to establish the detail each can shed on the determination of the adsorption site of 0 on a clean Al(111) surface. In this first detailed ion scattering study of the oxidation of the Al(111) surface it has been established that at exposures of up to 100 L almost one monolayer of oxygen is adsorbed to the surface. This observation alone brings into question many models which conclude that the adsorption leads to a surface and subsurface layer of oxygen. Furthermore, by using shadowing and blocking associated with medium energy ion scattering (MEIS) it has been possible to exclude the existence of a significant subsurface site at exposures up to 100 L.
1. Introduction Aluminium oxidises readily in air to form an oxide layer which is stable and protects from further attack. The understanding of the initial stages of oxidation of a common simple metal would be expected to be complete with the wealth of surface analysis probes available today. This expectation has proven incorrect and in the most comprehensive review to date Batra and Kleinman [l] revealed that although up until 1984 there were over 200 papers published on the oxidation of Al, no clear consensus had been reached on the initial oxidation sites. A review of the more recent studies [21 has shown little real progress in the interim period. In an early low energy electron diffraction (LEED) study Jona [31 reported that on the (1001, (110) and (111) faces of Al no new diffraction spots were seen as a function of oxygen exposure from a clean surface. For the Al(111) surface the spots started to become diffuse after approximately 100 L exposure and all spots were lost at an exposure of 6000 L when the amorphous oxide state is reached. The lack of new diffraction spots 0039-6028/93/$06.00
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in the LEED images led to the conclusion that the oxygen adsorption site is a substitutional site or the configuration of sites retains the symmetry of the surface. There are two surface sites and four subsurface sites which have been considered as acceptable adsorption sites. Any one of the possible sites or a combination of one surface site and one subsurface site have been the initial adsorption states considered in this and most other studies. Al has the fee structure which in the (111) direction has ABCABC . . . stacking, and an interlayer spacing of 2.33 A. There are two potential surface adsorption sites referred to as B and C with the oxygen atom sited in threefold hollows directly above Al atoms of the second layer (B) and third layer (C) respectively. If any consensus can be found on these sites it is that the majority of siudies has placed the oxygen approximately 0.6 A above the surface Al layer and more recent experimentally based studies have favoured the C site. The four subsurface sites considered comprise two tetrahedral (tB, tC>, an octahedral (oC> and a displaced octahedral (doC) site (see fig. 1). The earliest indication that a subsurface site
B.V. All rights reserved
D.J. O’Connor et al. / The initial stages of the oxidation of AI(ll1).
132
l
0
o Al
Fig. 1. The proposed oxygen adsorption sites on and in an Al(111) surface. The surface is shown in side view, in a cut through the (ii01 plane. The 0 atoms are indicated by filled circles and the Al by open circles.
existed came from workfunction measurements. Many measurements (but not all) of the workfunction changes with oxidation found that the workfunction decreased [1,4-lo] upon oxidation implying that a double layer of oxygen was formed with one layer above the Al surface layer of atoms and a second between the first and second layers of Al. The existence of a stable subsurface site for oxygen also came from theoretical studies using slab and cluster calculations [1,2,11]. There were numerous experimental features explained by the existence of both surface and subsurface sites on Al(111); however, only two principal studies will be discussed here. The first is the Auger electron spectroscopy (AES) and LEED study [12] which identified a three stage adsorption process over the exposure ranges of O-55 L, 55-120 L and > 120 L. The most notable feature of that study was the clear changes to the AES signals for the AN68 eV>, AK55 eV> and O(506 eV) peaks. The first stage was interpreted to correspond to the filling of a surface monolayer of oxygen and the second stage to the development of a subsurface layer of oxygen. In a similar study also using LEED and AES [13] a four stage oxidation process was observed with coverage breaks at 30, 100 and 200 L exposure. The first conclusion drawn was that for exposures from O-30 L the oxygen was incorporated into the subsurface sites at a depth of O-O.5 A below the surface which is at variance to the previous AES study mentioned. For the 30-100 L range of exposure the workfunction increases and the
I
AN55 eV) peak becomes apparent. It was concluded that these features could be explained by the completion of a full oxygeOnchemisorbed overlayer at a distance of 0.73 A above the surface layer of Al. The strongest experimental evidence for a subsurface site came from electron energy loss spectroscopy (EELS) [14-161 in which three loss peaks at 40, 80 and 105 meV were observed. The 80 meV loss was attributed to a lattice mode in which the oxygen overlayer atoms vibrated 180 out of phase with the surface layer Al atoms. The 105 meV loss was associated with a similar motion between subsurface oxygen and the second layer Al atoms. The 40 meV loss was associated with a mode in which the surface 0 and Al move in phase with each other but 180” out of phase with the subsurface 0 and second layer Al which are moving in phase with each other. These conclusions were supported by subsequent studies [17-191. The assignment of the loss peaks in EELS to these modes of lattice vibration has been questioned by Bagus et al. [2,20]. In that study XPS measurements of the 00s) binding energies yielded two values. The angular behaviour of these peaks could be interpreted as either the lower binding energy peak being attributed to subsurface oxygen or to the existence of two binding energies experienced by surface oxygen atoms depending on whether they were within or on the perimeter of an island. The results of cluster calculations led to the latter conclusion. These findings were in agreement with a scanning tunnelling microscopy (STM) study [21l which identified 3 types of surface oxygen atom in the adsorption process. Bagus et al. [20] proposed that the EELS loss features be reassigned to vibrational features of oxygen atoms in surface islands. In a more recent STM study [22] it was found that when the oxygen adsorbs it dissociates and travels distances of 80 A before coming to rest. Once at rest no further displacements of the oxygen atoms were detected over an observation period of one hour. It was also shown that at small exposures, islands of oxide with less than 6 atoms predominate while at an exposure of 72 L
D.J. O’Connor et al. / The initial stages of the oxidation of Al(lII).
a range of sizes exist; however, most islands consisted of less than 12 atoms. In a recent normal incidence standing X-ray wavefield absorption study [23], the oxygen adsorption site was confirmed to adsorb 0.7 A above the surface in the C site. The authors critically reviewed previous studies and concluded that the more reliable measurements favoured a C site for the oxygen sitting 0.6-0.7 A above the surface Al layer.
2. Experiment The experiments were conducted in a multichamber UHV MEIS system [24] which achieved a base pressure of lo-” mbar. The system was equipped with LEED and AES and it was connected to a 200 kV accelerator which had a stability and ripple which was significantly less than the detector resolution. The sample was mounted on a high precision goniometer [251 which had three independent axes of rotation allowing alignment to within 0.1”. Vertical and horizontal translations were possible with an accuracy of 0.01 mm. The scattered ions were analysed by a toroidal electrostatic analyser [26] which could simultaneously detect ions scattered over an angular range of 20“ with an energy resolution of A E/E = 9 X lop4 [27]. The resolution function of the analyser was routinely measured during the experiment by directing a portion of the monoenergetic ion beam into the electrostatic analyser. The charged fraction of the backscattered projectiles was determined using a silicon surface barrier detector with deflection plates. The ion fraction is used to fix the relationship between measured yield and the number of visible surface atoms. In this study it was checked that the ion fraction is independent of the exit angle to the surface. The measurements undertaken on Al(111) were at room temperature and at temperatures as low as 190 K. For the low temperature measurements the sample was cooled by a continuous flow cryostat 1281 operating on liquid nitrogen. The cryostat was coupled with a Cu braid to a Be-Cu ring on the goniometer into which a MO
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ring of the sample holder fits. The MO ring is attached by sapphire rods to the MO holder onto which the sample was clamped. The temperature of the sample was measured with a Pt-100 resistor embedded in the MO holder. The Al crystal was spark cut from a single crystal of high purity and aligned to within 0.2” of the (111) orientation. The sample was mechanically polished to a high finish and cleaned in vacuum by Ar+ sputtering (2.6 PA/cm’, 650 eV) for one hour followed by 80 h annealing up to 830 K. The crystal surface quality was checked with LEED and minimum yield measurements with MEIS. Routine cleaning involved 20 min sputter cleaning followed by annealing at 760 K for 30 min. The cleanliness was monitored with AES and MEIS. The principal contaminants were C and 0 which after cleaning gave ratios Al (LMM)/C(KLL) and Al(LMM)/ O(KLL) in the undifferentiated spectra of 180 and 100. These were uncorrected for the transmission function of the CMA (AE/E = constant). It was estimated that the maximum contamination levels were less than 2% of a monolayer and no contaminant peaks were observed in the MEIS spectra. The oxygen contamination estimated by AES was calibrated against MEIS after adsorption. A number of studies identified the existence of adsorption pressure and target temperature dependence effects so at all times the adsorption was performed at room temperature and an oxygen partial pressure of (l-2) X lo-’ mbar. The damage caused by the ion beam was carefully monitored to ensure that it did not influence the measurements. The damage threshold was determined by establishing at what dose the measured scattering yield noticeably increased and then performing all further experiments at doses which were 25% of the critical dose. This corresponded to 10 PC/mm’. To determine the structure of the surface and the location of the adsorbed oxygen blocking and shadowing of medium energy (50-100 keV) H+ ions were used. The details of the applications of MEIS have previously been reviewed [29]. All experiments were performed in the (110) plane as in this geometry the oxygen atoms and the Al atoms are coplanar and hence full advantage can
D.J. O’Connor et al. / The initial stages of the oxidation of Al(lIl).
134
(4
M
GEOMETRY
GEOMETRY
fore scattering. This results in greater energy loss and broad peaks in the energy spectra leading to overlap between the 0 and Al peaks. To overcome this problem a large scattering angle (geometry I) was used for the room temperature measurements. At the reduced temperature the deeper layers are well shielded by the surface layers and the widths of the scattered ion peaks off Al and 0 are significantly reduced allowing both geometries I and II to be used.
I
3. Results
II [1101
,[llll
I
and discussion
3.1. Charged fraction
/ il 311 r11i1
Fig. 2. Definition of the two geometries used in this study. (a) Geometry I incident along the [ii01 direction in the (iTO) plane. (b) Geometry II incident along the [OOi] direction in the (ii0) plane.
be taken of shadowing and blocking techniques. Two scattering geometries were used in this study. The first (called geometry I), used in room temperature measurements, has the ion beam incident along the [ii01 direction at 54.7” to the surface [ii21 direction (see fig. 2). In this geometry the range of scattering angles is 60”-80” (which is 5.3”-25.3” to the surface in the exit direction). In geometry II the ion beam is incident along the [OO?] direction at 35.3” to the surface [ll?] direction and the range of scattering angles is 45”-65” (9.7”-29.7” to the surface). In geometry I the lower limit of the observed scattering angles is only 5” to the surface and hence the yield may be affected by surface roughness and angle dependent charge exchange. In all comparisons to simulation less weight is placed on the exit angle range of 5”-10” to the surface. At room temperature the surface atoms thermal vibrational amplitude is sufficiently large to allow the projectiles access to deeper layers be-
To relate the measured ion yield to a number of scattering centres it is essential that the charged fraction of the scattered projectiles be known. This was established to an accuracy of better than one percent by placing a surface barrier detector in a double alignment direction and measuring the scattered yield of all particles and then only of the neutrals, once the ions are deflected. These measurements were made for a range of conditions listed in table 1. It has been previously established that the charged fraction increases with increasing projectile energy [30,31]. From this study it is concluded that the charged fraction decreases with increasing oxygen coverage. 3.2. Clean Al(l11) The clean AI(111) has been studied to establish the initial structure and thermal vibration amplitudes. Comparison of the surface peak area (SPA) to simulations 1321 using the VEGAS code [33] revealed that a good agreement could be
Table 1 Measured charged fraction for hydrogen and 0 covered Al(111) surface Surface
condition
Clean Al(111) 50 L oxygen 100 L oxygen
scattered
off clean
E, = 50 keV
Eo = 100 keV
75% _
92% 86% 86%
63%
D.J. O’Connor et al. / The initial stages of the oxidation ofAl(Il1). Table 2 Thermal vibrational amplitudes determined for the clean surface of Al(111) from a comparison of the experimental results with the VEGAS simulation Temperature (K)
300 190
Vibration 01 Surface 0.15 0.095
amplitude
(&
Second
135
3.5
2.5
g2
layer
I
UB
layer
0.125 0.085
Bulk 0.10 0.075
2
1.5
I
[333]
found for an expansion of the first interlayer spacing by 1% [34]. The atoms of the bulk were assumed to vibrate with a one-dimensional rootmean-square amplitude of 0.106 A [35] and those in the first two layers to have a thermal vibration amplitude enhancement of 10% and 5%, respectively. In this simulation no account was taken of correlation of thermal vibrational displacements. In this study the surface blocking profile was fitted using the thermal parameters as variables. The shifts of the minima in the surface blocking profile, with respect to bulk crystal axes, are small and the best estimate from this study is that if there is a uniform relaxation then it is less than 1% of the interlayer spacing for the clean Al(111) surface. A similar behaviour was observed for the low temperature Al(111) measurements as revealed in table 2 and in fig. 3, where the simulated blocking profile (using VEGAS) is compared to an experimental result. As both geometry I and II were accessible at low temperatures the simulations were simultaneously fitted to both sets of results for the different geometries to obtain an optimal fit.
1 60
[iis)
65
I
I
[ii41
[ii61
70 Scattering Angle (degree)
75
60
Fig. 3. The comparison of the measured surface blocking profile (squares) and the VEGAS simulation (solid line) for 100 keV H scattered off clean Ahlll) in geometry I (incident beam parallel to the [ii01 direction at 54.7” to the surface [ii21 direction). The target was at room temperature and for the simulation the bulk thermal vibration amplitude was 0.10 A, while tht surface and second layer amplitudes were 0.15 and 0.125 A, respectively. No surface layer relaxation has been included in this simulation as no evidence for it has been found in this study.
proximately 100 L 0, (fig. 4). The increase is 0.35 monolayers with respect to the clean surface. Equally significantly the number of visible Al decreases for exposures greater than 100 L up to 200 L. It is important to remember that this effect is purely a measure of the number of
3.3. Exposure dependence of Al yield The LEED observations of the retention of the 1 x 1 spot pattern upon oxygen exposure up to 100 L led to the conclusion that the Al surface did not reconstruct. To check this finding the number of visible Al layers was determined at room temperature as a function of oxygen exposure to see if there is a significant change which would be indicative of a reconstruction. There is a linear increase in the number of Al atoms visible up to a maximum at an exposure of ap-
0
20
40
60
60 100 120 140 Oxygen Exposure(L)
160
160
i!d 10
Fig. 4. Change in the number of visible Al monolayers as a function of the oxygen exposure for 100 keV H scattered off Al(111) at 300 K using geometry I. The Al yield is taken as the average over the scattering angles 70”-75”. The straight line fit in the figure is included to guide the eye.
D.J. O’Connor et al. / The initial stages of the oxidation ofAl(III).
136
0.5
’ (3341
0 45
Fig. 5. keV H squares) incident etry II surface shift in
1
I [55Z]
/
50
/
liril 55 Scapsring Angle (degree)
60
65
Comparison of the surface blocking profiles for 100 at a sample temperature of 190 K for Al before (solid and after 100 L 0 exposure (open squares). The directions correspond to geometry I in (a) and geomin (b). The oxygen blocking is too weak to allow the site to be determined. There is no systematic or large the blocking minima indicating that oxidation does not induce a significant surface layer relaxation.
visible atoms as the charged fraction has been accounted for by the independent measurement of the charged fraction at exposures of up to 100 L. Some consideration has been given previously to the possibility that the outermost Al atomic layer relaxed outwards upon oxidation ill]. As there is no significant shift in the location of blocking minima for the Al signal upon oxidation (fig. 5) it can be concluded that any oxidation induced relaxation is less than 1%. To explain the increase in Al yield it is necessary to combine the previous LEED observations with those reported above and include consideration of the STM observations [22] in which it was shown that the 0 atoms form small islands of typically less than 12 atoms at 72 L exposure. At 100 L 0, exposure there is no significant shift in the position of the blocking minima, revealing that there is no simple reconstruction or relaxation of the surface layers upon oxidation. We are forced to interpret the increase in Al yield as “random” static displacements of Al, similar on average to those induced by strongly enhanced thermal vibrations. The initial exposure causes the Al yield to increase linearly to a maximum of
I
3.45 monolayers of Al at about 80 L. This coincides with the development of small oxide islands in which all atoms are at the edge of the island. These results suggest that the Al substrate near the island edges is more strained and contributes extra visible Al atoms. Once a critical coverage has been reached, at which the islands start to coalesce, the addition of further 0 atoms only decreases the number of edge 0 atoms (at about 80 L which corresponds to the 0.65 monolayer of 0). Then this enhanced yield component decreases. The rate of increase in the initial linear regime in fig. 4 is 0.0044 visible Al layers per langmuir exposure or 0.54 visible Al atoms per adsorbed 0 atom (see next section). The fact that the high-coverage Al yield does not decrease completely to the value for the clean surface, suggests that the final O-covered structure contains a considerable amount of disorder. This would explain the LEED observations that beyond 100 L exposure the spots gradually blur and fade. 3.4. Exposure dependence of 0 yield The absolute oxygen yield is essential to establish the credibility of some of the proposed models for adsorption sites. In some earlier studies it has been concluded that at an exposure of 100 L there are two complete atomic layers of 0 present; one surface and one subsurface. Using MEIS where the scattering cross section and charged fraction are well known, an accurate measurement of the coverage can be made (without the need to resort to standards). In the geometry used in this study there is no proposed oxygen site which could be hidden from the ion beam or analyser so the measured coverage accurately reflects the true coverage. The increase in oxygen coverage as a function of exposure (fig. 6) reveals that at 100 L the coverage is 72% and one monolayer is approached at an exposure of 200 L. There is a smooth increase in the oxygen coverage as a function of exposure and from this trend there is no evidence for a change in the rate of oxygen uptake at the breakpoints identified by AES measurements [12,13]. There is no need to consider a change resulting from populating sur-
D.J. O’Connor et al. / The initial stages of the oxidation of AI(III).
f
0.8
P e 2
0.6
5 8
0.4
04
0
50
150 100 200 Oxygen Exposure (L)
250
I
300
Fig. 6. The increase in the measured oxygen as a function of oxygen exposure for the Altlll) surface measured at 300 K using geometry I. The Al yield is taken as the average over the scattering angles 70”-75”. The solid curve is the fitted exponential dependence described in the text. All filled squares are the result of an average of several measurements as the exposure was increased. The two open squares are obtained from surface structure measurements in which the statistical counting uncertainty error was greatly reduced but were exposed for a considerable time to the background vacuum environment. The crosses are the Auger measurements of Soria et al. [13] scaled vertically to asymptotically approach unity and included for comparison. Our own AES measurements have not been included in this figure as, when they are scaled in the same way, they lie over the MEIS measurements.
face and subsurface sites. The coverages indicated in fig. 6 are the result of a number of different and repeated measurements. The solid points correspond to the averages of a series of measurements as the exposure was increased. These determinations were made within 15 min of exposure. The open squares are measurements taken from surface blocking profiles in which there is less statistical uncertainty; however, by the nature of the measurement it was taken over a period of up to 12 h after the adsorption. The solid curve in fig. 6 is a fit using the expression 0 = Y,[
1 - exp(
-E/E,)],
where 8 is the coverage of oxygen in monolayers (where one monolayer is defined as one oxygen atom for each surface Al atom), Y, is the limiting
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131
coverage, E is the exposure and E, is a characteristic exposure parameter. For the fitted curve in fig. 6 the constants take the values Y, = 1.02 monolayers of oxygen and E, = 79 L. This excellent fit of the coverage to an exponential curve is indicative of first order adsorption kinetics in which an oxygen molecule sticks if it encounters a clean Al surface region, but will desorb from the surface if that region is already covered. If the oxidation were a two stage process, leading to populating both surface and subsurface sites, then a more complicated oxygen uptake should be expected than the observed first order kinetics. Furthermore, the limiting value of one monolayer of oxygen at high exposures implies that one of the two potential classes sites (above or below the surface) is not populated. A serious consideration arises in the comparison between different studies as there is no reconstruction of the surface or other indication that a particular coverage or stage of the oxidation process has been achieved. The comparison between different experiments has always been on the basis of an exposure to an oxygen background pressure and the true exposure is then a function of the calibration of the ionisation gauge, the geometry of the chamber and other factors. To place our study in context with previously reported measurements a comparison is made (fig. 6) between the AES oxygen coverage measured by Soria et al. [131 and the MEIS determined coverage. The AES measurements were normalised to the MEIS measurement of one monolayer at 300 L exposure. Similar AES measurements were made in conjunction with the MEIS study but have not been included as they lie over the MEIS measurements when normalised to one monolayer at 300 L. If there had been a significant difference in exposures between our measurements and those of Soria et al. then the curves for AES and MEIS oxygen yield would be displaced horizontally at intermediate coverages. As there is no systematic disagreement between the oxygen coverage determined by Soria and our measurements we conclude that our exposures and coverages are close to those used in that study. This does not overcome the same difficulty in comparison to other studies.
138
D.J. O’Connor et al. / The initial stages of the oxidation of Al(111). I
3.5. Oxygen subsurface adsorption site The surface oxygen sites cannot be determined by blocking of the oxygen scattering yield as they sit above the surface Al layer and scattered ions will not experience the effects of any blocking centre. Thus if only surface sites are populated then the oxygen blocking profile will be constant with scattering angle. In theory the surface adsorption site may be determined by the identification of blocking of the Al signal by the abovesurface 0. In practice the oxygen acted as such a weak blocking centre that this approach could not be used. An alternative approach to the location of the surface oxygen site employing energy losses h!s located the oxygen in the surface C-site at 0.58 A above the surface [36,37]. The four subsurface sites are equally visible to the incident ion beam so will be uniformly illuminated. As they are situated below the Al surface layer, the projectiles scattered from subsurface oxygen will be blocked from particular directions by surface Al atoms. Each site has its own characteristic blocking pattern which is easily distinguished. In the event that both surface and subsurface sites are populated then the angular distribution will be a linear addition of each profile thus making the subsurface profile less distinct once added to the structureless surface site yield. The oxygen blocking profiles measured for geometry I (fig. 7a) and geometry II (fig. 7b) are compared to those predicted by the simulation for one monolayer of 0 in each of the subsurface sites at a target temperature of 190 K. The difference in the number of monolayers of oxygen measured experimentally between the two geometries is not significant as it is a measure of the reproducibility of the adsorption and exposure. Clearly the sensitivity of the angular distribution to the different sites is different as it depends on the prominence of the blocking features. There is no match between the simulated blocking profiles for the tA, OC and doC sites and the experimentally measured profile. The blocking feature for the tB site is not prominent in the simulation; however, any angular dependent feature that does exist in the results does not coincide with the shallow minima in the tB simulated
4(4
doC
[ii31
[ii41
I 65
0 60
I
I 70 Scattering Angle (degree)
75
clot
(b) -
o>--
.
lA
I
[iii]
[334] 0, 44
/ 46
I 46
50 52 54 Scattering Angle (degree)
56
56
Fig. 7. The angular blocking profile of the scattering yield of 100 keV H from oxygen compared to the simulation results for all proposed subsurface sites. The target temperature was 190 K and the simulation was for one monolayer of oxygen. For clarity the results of the simulations have been offset. The vertical bars on the right represent the O-l monolayer scale for tB, tA, OC and doC, respectively. The lack of a clear blocking dip reveals that the oxygen is not sitting in a subsurface site. Results from both (a) geometry I and (b) geometry II are shown.
results. Even in the possible event that oxidation led to a relaxation of the surface layer causing a shift in the position of the blocking minima no match can be found for any of the proposed subsurface sites. Apart from a weak angular trend there is no clear blocking feature which means that to within the limits of uncertainty there is no subsurface oxygen. This does not totally rule out the existence of a subsurface component. The sensitivity
D.J. O’Connor et al. / The initial stages of the oxidation of Al(ll1).
of MEIS to the subsurface sites under consideration depends on how pronounced the angular distribution is and it is evident from figure seven that some subsurface sites are easier to detect than others. Based on these sensitivities, we conclude that at 100 L exposure the subsurface component of the adsorbed oxygen is less than 10% if it is in the tB site and less than 5% if it is the tA, OC or doC sites. The weak angular dependence of scattered ion yield from oxygen may be caused by other processes than blocking. One possibility could be that it is indicative of a charged fraction which is angle dependent at small (< 20”) angles to the surface. This has been checked by measuring the charge fraction at 5” and 28” to the surface and no difference was found to within experimental uncertainty. The only other possibility is the use of an inappropriate scattering cross section which relates the scattered particle yield to surface atom concentration. If the true angle dependence of the scattering cross section differs from that of the Moliere potential then a gradual departure would be observed.
4. Conclusions The oxidation of Al(111) to an exposure of 100 L has been studied to test the claims made by other techniques. The first finding is that there is almost one monolayer of oxygen at this exposure. Furthermore, there is no observable shift in the Al blocking features which is evidence for the lack of relaxation of the outermost Al layer. A significant increase in the scattering yield from Al atoms is observed and is attributed to Al displacements induced by the oxygen adsorption. We propose that the Al substrate atoms at the perimeters on the 0 islands are strained away from their regular lattice positions. The full oxygen monolayer is still accompanied by a substantial difference in Al signal from the clean surface, which probably indicates that the final structure is not completely ordered. The measured coverage of oxygen at different exposures follows a simple first-order adsorption curve which is different to that found in previous
I
139
studies. The asymptotic oxygen coverage from the adsorption measurements is only one monolayer suggesting that one of the two possible locations for oxygen (above or below the surface) is not populated. This conclusion is further supported by oxygen blocking curve measurements which provide no evidence for a significant subsurface component to the initial oxygen coverage. This is in marked disagreement with some previous studies which rely on the existence of subsurface oxygen to explain experimental observations. The measured saturation coverage of one monolayer, the first-order adsorption kinetics and the lack of structure to the oxygen blocking curve all indicate an exclusive surface site. The limits of sensitivity to the measurement of a subsurface component is 5% for all sites except the tB site for which it is 10%.
Acknowledgements
This work is part of the research program of the Foundation for Fundamental Research on Matter (FOM) and was made possible by financial support from the Netherlands Organisation for the Advancement of Research (NWO). The valuable assistance of R. Vuthaluru is acknowledged in the performance of the computer simulations. Fruitful discussions with D.P. Woodruff are gratefully acknowledged.
References [l] I.P. Batra and L. Kleinman, J. Electron. Spectrosc. Relat. Phenom. 33 (1984) 175. [2] P.S. Bagus, C.R. Brundle, F. Illas, F. Parmigiani and G. Polzonetti, Phys. Rev. B 44 (1991) 9025. [3] F. Jona, J. Phys. Chem. Solids 28 (1967) 2155. [4] V.K. Agarwarla and T. Fort, Surf. Sci. 45 (1974) 470. [51 V.K. Agarwarla and T. Fort, Surf. Sci. 54 (1976) 60. 161 P.O. Gartland, Surf. Sci. 62 (1977) 183. 171 A.M. Bradshaw, P. Hofmann and W. Wyrobisch, Surf. Sci. 68 (1977) 269. [Sl P. Hofmann, W. Wyrobisch and A.M. Bradshaw, Surf. Sci. 80 (1979) 344. [91 R. Payling and J.R. Ramsey, J. Phys C. (Solid Phys.) 13 (1980) 505. [lOI J.A. Ramsey, Appl. Surf. Sci. 13 (1982) 159.
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D.J. O’Connor et al. / The initial stages of the oxidation of AI(IlI).
[ll] G. Pacchioni and P. Fantucci, Surf. Sci. 204 (1988) 587. [12] R. Michel, J. Gastaldi, C. Allasia, C. Jordan and J. Derrien, Surf. Sci. 95 (1980) 309. [13] F. Soria, V. Martinez, M.C. Muqoz and J.L. Saced&, Phys. Rev. B 24 (1981) 6926. [14] J.L. Erskine and R.L. Strong, Phys. Rev. B 25 (1982) 5547. [15] R.L. Strong, B. Firey, F.W. de Wette and J.L. Erskine, Phys. Rev. B 26 (1982) 3483. [16] R.L. Strong, B. Firey, F.W. de Wette and J.L. Erskine, J. Electron Spectrosc. Relat. Phenom. 29 (1983) 187. [17] C. Astaldi, P. Geng and K. Jacobi, J. Electron Spectrosc. Relat. Phenom. 44 (1987) 175. [18] J.E. Crowell, J.G. Chen and J.T. Yates, Jr., Surf. Sci. 165 (1986) 37. [19] J.G. Chen, J.E. Crowell and J.T. Yates, Jr., J. Chem. Phys. 34 (1986) 5906. [20] P.S. Bagus, F. Parmigiani, G. Polzonetti, F. Illas and C.R. Brundle, J. Vat. Sci. Technol. A 9 (1991) 1747. [21] J. Wintterlin, H. Brune, H. HGfer and R.J. Behm, Appl. Phys. A 47 (1988) 99. [22] H. Brune, J. Wintterlin, R.J. Behm and G. Ertl, Phys. Rev. Lett. 68 (1992) 624. [23] M. Kerkar, D. Fisher, D.P. Woodruff and B. Cowie, Surf. Sci. 271 (1992) 45. [24] P.M.J. Marte, A.P. de Jongh, J.W. Derks and J.F. van der Veen, Nucl. Instrum. Methods B 28 (1987) 76. [2.5] W.C. Turkenburg, E. de Haas, A.F. Neuteboom, J. Ladru and H.H. Kersten, Nucl. Instrum. Methods 126 (1975) 241.
I
[26] R.G. Smeenk, R.M. Tromp, H.H. Kersten, A.J.H. Boerboom and F.W. Saris, Nucl. Instrum. Methods 195 (1982) 581. [27] J. Vrijmoeth, P.M. Zagwijn, J.W.M. Frenken and J.F. van der Veen, Phys. Rev. L.&t. 67 (1991) 1134. [28] F. Huussen, J.W.M. Frenken and J.F. van de Veen, Vacuum 36 (1986) 259. [29] J.F. van der Veen, Surf. Sci. Rep. 5 (1985) 199. 1301 T.M. Buck G.H. Wheatley and L.C. Feldman, Surf. Sci. 35 (1973) 345. [31] W.F.S. Poehlman, R.S. Bhattacharya, D.A. Thompson and H.D. Barber, Surf. Sci. 100 (1980) 14. [32] A.W. Denier van der Gon, R.J. Smith, J.M. Gay, D.J. O’Connor and J.F. van der Veen, Surf. Sci. 227 (1990) 143. 1331 J.W.M. Frenken, R.M. Tromp and J.F. van der Veen, Nucl. Instrum. Methods B 17 (1986) 334. [34] H.B. Nielson and D.L. Adams, J. Phys. C 15 (1982) 615. [35] E.V. Zarochentsev, S.P. Kravchuk and T.M. Tarusina, Sov. Phys. Solid State 18 (1976) 239. [36] E.R. Wouters, D.J. O’Connor, J.F. van der Veen, P.M. Zagwijn, J. Vrijmoeth, W. Slijkerman and J.W.M. Frenken, Surf. Sci. 296 (1993) 141. [37] D.J. O’Connor, E.R. Wouters, A.W. Denier van der Gon, J. Vrijmoeth, P.M. Zagwijn, W.F.J. Slijkerman, J.W.M. Frenken and J.F. van der Veen, Surf. Sci. 287/288 (1993) 438.
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Surface Science North-Holland
surface science
296 (1993) 131-140
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The initial stages of the oxidation of Al( 111). I D.J. O’Connor a, E.R. Wouters b, A.W. Denier van der Gon b, J.F. van der Veen b, P.M. Zagwijn b and J.W.M. Frenken b a Department of Physics, Unioersity of Newcastle, Newcastle NSW 2308, Australia b FOM-Institute Received
for Atomic
15 December
and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam,
1992; accepted
for publication
25 June
Netherlands
1993
AK1 11) oxidation has been subjected to numerous studies using surface analysis techniques. Indeed it seems to have been used as a proving ground for techniques to establish the detail each can shed on the determination of the adsorption site of 0 on a clean Al(111) surface. In this first detailed ion scattering study of the oxidation of the Al(111) surface it has been established that at exposures of up to 100 L almost one monolayer of oxygen is adsorbed to the surface. This observation alone brings into question many models which conclude that the adsorption leads to a surface and subsurface layer of oxygen. Furthermore, by using shadowing and blocking associated with medium energy ion scattering (MEIS) it has been possible to exclude the existence of a significant subsurface site at exposures up to 100 L.
1. Introduction Aluminium oxidises readily in air to form an oxide layer which is stable and protects from further attack. The understanding of the initial stages of oxidation of a common simple metal would be expected to be complete with the wealth of surface analysis probes available today. This expectation has proven incorrect and in the most comprehensive review to date Batra and Kleinman [l] revealed that although up until 1984 there were over 200 papers published on the oxidation of Al, no clear consensus had been reached on the initial oxidation sites. A review of the more recent studies [21 has shown little real progress in the interim period. In an early low energy electron diffraction (LEED) study Jona [31 reported that on the (1001, (110) and (111) faces of Al no new diffraction spots were seen as a function of oxygen exposure from a clean surface. For the Al(111) surface the spots started to become diffuse after approximately 100 L exposure and all spots were lost at an exposure of 6000 L when the amorphous oxide state is reached. The lack of new diffraction spots 0039-6028/93/$06.00
0 1993 - Elsevier
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Publishers
in the LEED images led to the conclusion that the oxygen adsorption site is a substitutional site or the configuration of sites retains the symmetry of the surface. There are two surface sites and four subsurface sites which have been considered as acceptable adsorption sites. Any one of the possible sites or a combination of one surface site and one subsurface site have been the initial adsorption states considered in this and most other studies. Al has the fee structure which in the (111) direction has ABCABC . . . stacking, and an interlayer spacing of 2.33 A. There are two potential surface adsorption sites referred to as B and C with the oxygen atom sited in threefold hollows directly above Al atoms of the second layer (B) and third layer (C) respectively. If any consensus can be found on these sites it is that the majority of siudies has placed the oxygen approximately 0.6 A above the surface Al layer and more recent experimentally based studies have favoured the C site. The four subsurface sites considered comprise two tetrahedral (tB, tC>, an octahedral (oC> and a displaced octahedral (doC) site (see fig. 1). The earliest indication that a subsurface site
B.V. All rights reserved
D.J. O’Connor et al. / The initial stages of the oxidation of AI(ll1).
132
l
0
o Al
Fig. 1. The proposed oxygen adsorption sites on and in an Al(111) surface. The surface is shown in side view, in a cut through the (ii01 plane. The 0 atoms are indicated by filled circles and the Al by open circles.
existed came from workfunction measurements. Many measurements (but not all) of the workfunction changes with oxidation found that the workfunction decreased [1,4-lo] upon oxidation implying that a double layer of oxygen was formed with one layer above the Al surface layer of atoms and a second between the first and second layers of Al. The existence of a stable subsurface site for oxygen also came from theoretical studies using slab and cluster calculations [1,2,11]. There were numerous experimental features explained by the existence of both surface and subsurface sites on Al(111); however, only two principal studies will be discussed here. The first is the Auger electron spectroscopy (AES) and LEED study [12] which identified a three stage adsorption process over the exposure ranges of O-55 L, 55-120 L and > 120 L. The most notable feature of that study was the clear changes to the AES signals for the AN68 eV>, AK55 eV> and O(506 eV) peaks. The first stage was interpreted to correspond to the filling of a surface monolayer of oxygen and the second stage to the development of a subsurface layer of oxygen. In a similar study also using LEED and AES [13] a four stage oxidation process was observed with coverage breaks at 30, 100 and 200 L exposure. The first conclusion drawn was that for exposures from O-30 L the oxygen was incorporated into the subsurface sites at a depth of O-O.5 A below the surface which is at variance to the previous AES study mentioned. For the 30-100 L range of exposure the workfunction increases and the
I
AN55 eV) peak becomes apparent. It was concluded that these features could be explained by the completion of a full oxygeOnchemisorbed overlayer at a distance of 0.73 A above the surface layer of Al. The strongest experimental evidence for a subsurface site came from electron energy loss spectroscopy (EELS) [14-161 in which three loss peaks at 40, 80 and 105 meV were observed. The 80 meV loss was attributed to a lattice mode in which the oxygen overlayer atoms vibrated 180 out of phase with the surface layer Al atoms. The 105 meV loss was associated with a similar motion between subsurface oxygen and the second layer Al atoms. The 40 meV loss was associated with a mode in which the surface 0 and Al move in phase with each other but 180” out of phase with the subsurface 0 and second layer Al which are moving in phase with each other. These conclusions were supported by subsequent studies [17-191. The assignment of the loss peaks in EELS to these modes of lattice vibration has been questioned by Bagus et al. [2,20]. In that study XPS measurements of the 00s) binding energies yielded two values. The angular behaviour of these peaks could be interpreted as either the lower binding energy peak being attributed to subsurface oxygen or to the existence of two binding energies experienced by surface oxygen atoms depending on whether they were within or on the perimeter of an island. The results of cluster calculations led to the latter conclusion. These findings were in agreement with a scanning tunnelling microscopy (STM) study [21l which identified 3 types of surface oxygen atom in the adsorption process. Bagus et al. [20] proposed that the EELS loss features be reassigned to vibrational features of oxygen atoms in surface islands. In a more recent STM study [22] it was found that when the oxygen adsorbs it dissociates and travels distances of 80 A before coming to rest. Once at rest no further displacements of the oxygen atoms were detected over an observation period of one hour. It was also shown that at small exposures, islands of oxide with less than 6 atoms predominate while at an exposure of 72 L
D.J. O’Connor et al. / The initial stages of the oxidation of Al(lII).
a range of sizes exist; however, most islands consisted of less than 12 atoms. In a recent normal incidence standing X-ray wavefield absorption study [23], the oxygen adsorption site was confirmed to adsorb 0.7 A above the surface in the C site. The authors critically reviewed previous studies and concluded that the more reliable measurements favoured a C site for the oxygen sitting 0.6-0.7 A above the surface Al layer.
2. Experiment The experiments were conducted in a multichamber UHV MEIS system [24] which achieved a base pressure of lo-” mbar. The system was equipped with LEED and AES and it was connected to a 200 kV accelerator which had a stability and ripple which was significantly less than the detector resolution. The sample was mounted on a high precision goniometer [251 which had three independent axes of rotation allowing alignment to within 0.1”. Vertical and horizontal translations were possible with an accuracy of 0.01 mm. The scattered ions were analysed by a toroidal electrostatic analyser [26] which could simultaneously detect ions scattered over an angular range of 20“ with an energy resolution of A E/E = 9 X lop4 [27]. The resolution function of the analyser was routinely measured during the experiment by directing a portion of the monoenergetic ion beam into the electrostatic analyser. The charged fraction of the backscattered projectiles was determined using a silicon surface barrier detector with deflection plates. The ion fraction is used to fix the relationship between measured yield and the number of visible surface atoms. In this study it was checked that the ion fraction is independent of the exit angle to the surface. The measurements undertaken on Al(111) were at room temperature and at temperatures as low as 190 K. For the low temperature measurements the sample was cooled by a continuous flow cryostat 1281 operating on liquid nitrogen. The cryostat was coupled with a Cu braid to a Be-Cu ring on the goniometer into which a MO
I
133
ring of the sample holder fits. The MO ring is attached by sapphire rods to the MO holder onto which the sample was clamped. The temperature of the sample was measured with a Pt-100 resistor embedded in the MO holder. The Al crystal was spark cut from a single crystal of high purity and aligned to within 0.2” of the (111) orientation. The sample was mechanically polished to a high finish and cleaned in vacuum by Ar+ sputtering (2.6 PA/cm’, 650 eV) for one hour followed by 80 h annealing up to 830 K. The crystal surface quality was checked with LEED and minimum yield measurements with MEIS. Routine cleaning involved 20 min sputter cleaning followed by annealing at 760 K for 30 min. The cleanliness was monitored with AES and MEIS. The principal contaminants were C and 0 which after cleaning gave ratios Al (LMM)/C(KLL) and Al(LMM)/ O(KLL) in the undifferentiated spectra of 180 and 100. These were uncorrected for the transmission function of the CMA (AE/E = constant). It was estimated that the maximum contamination levels were less than 2% of a monolayer and no contaminant peaks were observed in the MEIS spectra. The oxygen contamination estimated by AES was calibrated against MEIS after adsorption. A number of studies identified the existence of adsorption pressure and target temperature dependence effects so at all times the adsorption was performed at room temperature and an oxygen partial pressure of (l-2) X lo-’ mbar. The damage caused by the ion beam was carefully monitored to ensure that it did not influence the measurements. The damage threshold was determined by establishing at what dose the measured scattering yield noticeably increased and then performing all further experiments at doses which were 25% of the critical dose. This corresponded to 10 PC/mm’. To determine the structure of the surface and the location of the adsorbed oxygen blocking and shadowing of medium energy (50-100 keV) H+ ions were used. The details of the applications of MEIS have previously been reviewed [29]. All experiments were performed in the (110) plane as in this geometry the oxygen atoms and the Al atoms are coplanar and hence full advantage can
D.J. O’Connor et al. / The initial stages of the oxidation of Al(lIl).
134
(4
M
GEOMETRY
GEOMETRY
fore scattering. This results in greater energy loss and broad peaks in the energy spectra leading to overlap between the 0 and Al peaks. To overcome this problem a large scattering angle (geometry I) was used for the room temperature measurements. At the reduced temperature the deeper layers are well shielded by the surface layers and the widths of the scattered ion peaks off Al and 0 are significantly reduced allowing both geometries I and II to be used.
I
3. Results
II [1101
,[llll
I
and discussion
3.1. Charged fraction
/ il 311 r11i1
Fig. 2. Definition of the two geometries used in this study. (a) Geometry I incident along the [ii01 direction in the (iTO) plane. (b) Geometry II incident along the [OOi] direction in the (ii0) plane.
be taken of shadowing and blocking techniques. Two scattering geometries were used in this study. The first (called geometry I), used in room temperature measurements, has the ion beam incident along the [ii01 direction at 54.7” to the surface [ii21 direction (see fig. 2). In this geometry the range of scattering angles is 60”-80” (which is 5.3”-25.3” to the surface in the exit direction). In geometry II the ion beam is incident along the [OO?] direction at 35.3” to the surface [ll?] direction and the range of scattering angles is 45”-65” (9.7”-29.7” to the surface). In geometry I the lower limit of the observed scattering angles is only 5” to the surface and hence the yield may be affected by surface roughness and angle dependent charge exchange. In all comparisons to simulation less weight is placed on the exit angle range of 5”-10” to the surface. At room temperature the surface atoms thermal vibrational amplitude is sufficiently large to allow the projectiles access to deeper layers be-
To relate the measured ion yield to a number of scattering centres it is essential that the charged fraction of the scattered projectiles be known. This was established to an accuracy of better than one percent by placing a surface barrier detector in a double alignment direction and measuring the scattered yield of all particles and then only of the neutrals, once the ions are deflected. These measurements were made for a range of conditions listed in table 1. It has been previously established that the charged fraction increases with increasing projectile energy [30,31]. From this study it is concluded that the charged fraction decreases with increasing oxygen coverage. 3.2. Clean Al(l11) The clean AI(111) has been studied to establish the initial structure and thermal vibration amplitudes. Comparison of the surface peak area (SPA) to simulations 1321 using the VEGAS code [33] revealed that a good agreement could be
Table 1 Measured charged fraction for hydrogen and 0 covered Al(111) surface Surface
condition
Clean Al(111) 50 L oxygen 100 L oxygen
scattered
off clean
E, = 50 keV
Eo = 100 keV
75% _
92% 86% 86%
63%
D.J. O’Connor et al. / The initial stages of the oxidation ofAl(Il1). Table 2 Thermal vibrational amplitudes determined for the clean surface of Al(111) from a comparison of the experimental results with the VEGAS simulation Temperature (K)
300 190
Vibration 01 Surface 0.15 0.095
amplitude
(&
Second
135
3.5
2.5
g2
layer
I
UB
layer
0.125 0.085
Bulk 0.10 0.075
2
1.5
I
[333]
found for an expansion of the first interlayer spacing by 1% [34]. The atoms of the bulk were assumed to vibrate with a one-dimensional rootmean-square amplitude of 0.106 A [35] and those in the first two layers to have a thermal vibration amplitude enhancement of 10% and 5%, respectively. In this simulation no account was taken of correlation of thermal vibrational displacements. In this study the surface blocking profile was fitted using the thermal parameters as variables. The shifts of the minima in the surface blocking profile, with respect to bulk crystal axes, are small and the best estimate from this study is that if there is a uniform relaxation then it is less than 1% of the interlayer spacing for the clean Al(111) surface. A similar behaviour was observed for the low temperature Al(111) measurements as revealed in table 2 and in fig. 3, where the simulated blocking profile (using VEGAS) is compared to an experimental result. As both geometry I and II were accessible at low temperatures the simulations were simultaneously fitted to both sets of results for the different geometries to obtain an optimal fit.
1 60
[iis)
65
I
I
[ii41
[ii61
70 Scattering Angle (degree)
75
60
Fig. 3. The comparison of the measured surface blocking profile (squares) and the VEGAS simulation (solid line) for 100 keV H scattered off clean Ahlll) in geometry I (incident beam parallel to the [ii01 direction at 54.7” to the surface [ii21 direction). The target was at room temperature and for the simulation the bulk thermal vibration amplitude was 0.10 A, while tht surface and second layer amplitudes were 0.15 and 0.125 A, respectively. No surface layer relaxation has been included in this simulation as no evidence for it has been found in this study.
proximately 100 L 0, (fig. 4). The increase is 0.35 monolayers with respect to the clean surface. Equally significantly the number of visible Al decreases for exposures greater than 100 L up to 200 L. It is important to remember that this effect is purely a measure of the number of
3.3. Exposure dependence of Al yield The LEED observations of the retention of the 1 x 1 spot pattern upon oxygen exposure up to 100 L led to the conclusion that the Al surface did not reconstruct. To check this finding the number of visible Al layers was determined at room temperature as a function of oxygen exposure to see if there is a significant change which would be indicative of a reconstruction. There is a linear increase in the number of Al atoms visible up to a maximum at an exposure of ap-
0
20
40
60
60 100 120 140 Oxygen Exposure(L)
160
160
i!d 10
Fig. 4. Change in the number of visible Al monolayers as a function of the oxygen exposure for 100 keV H scattered off Al(111) at 300 K using geometry I. The Al yield is taken as the average over the scattering angles 70”-75”. The straight line fit in the figure is included to guide the eye.
D.J. O’Connor et al. / The initial stages of the oxidation ofAl(III).
136
0.5
’ (3341
0 45
Fig. 5. keV H squares) incident etry II surface shift in
1
I [55Z]
/
50
/
liril 55 Scapsring Angle (degree)
60
65
Comparison of the surface blocking profiles for 100 at a sample temperature of 190 K for Al before (solid and after 100 L 0 exposure (open squares). The directions correspond to geometry I in (a) and geomin (b). The oxygen blocking is too weak to allow the site to be determined. There is no systematic or large the blocking minima indicating that oxidation does not induce a significant surface layer relaxation.
visible atoms as the charged fraction has been accounted for by the independent measurement of the charged fraction at exposures of up to 100 L. Some consideration has been given previously to the possibility that the outermost Al atomic layer relaxed outwards upon oxidation ill]. As there is no significant shift in the location of blocking minima for the Al signal upon oxidation (fig. 5) it can be concluded that any oxidation induced relaxation is less than 1%. To explain the increase in Al yield it is necessary to combine the previous LEED observations with those reported above and include consideration of the STM observations [22] in which it was shown that the 0 atoms form small islands of typically less than 12 atoms at 72 L exposure. At 100 L 0, exposure there is no significant shift in the position of the blocking minima, revealing that there is no simple reconstruction or relaxation of the surface layers upon oxidation. We are forced to interpret the increase in Al yield as “random” static displacements of Al, similar on average to those induced by strongly enhanced thermal vibrations. The initial exposure causes the Al yield to increase linearly to a maximum of
I
3.45 monolayers of Al at about 80 L. This coincides with the development of small oxide islands in which all atoms are at the edge of the island. These results suggest that the Al substrate near the island edges is more strained and contributes extra visible Al atoms. Once a critical coverage has been reached, at which the islands start to coalesce, the addition of further 0 atoms only decreases the number of edge 0 atoms (at about 80 L which corresponds to the 0.65 monolayer of 0). Then this enhanced yield component decreases. The rate of increase in the initial linear regime in fig. 4 is 0.0044 visible Al layers per langmuir exposure or 0.54 visible Al atoms per adsorbed 0 atom (see next section). The fact that the high-coverage Al yield does not decrease completely to the value for the clean surface, suggests that the final O-covered structure contains a considerable amount of disorder. This would explain the LEED observations that beyond 100 L exposure the spots gradually blur and fade. 3.4. Exposure dependence of 0 yield The absolute oxygen yield is essential to establish the credibility of some of the proposed models for adsorption sites. In some earlier studies it has been concluded that at an exposure of 100 L there are two complete atomic layers of 0 present; one surface and one subsurface. Using MEIS where the scattering cross section and charged fraction are well known, an accurate measurement of the coverage can be made (without the need to resort to standards). In the geometry used in this study there is no proposed oxygen site which could be hidden from the ion beam or analyser so the measured coverage accurately reflects the true coverage. The increase in oxygen coverage as a function of exposure (fig. 6) reveals that at 100 L the coverage is 72% and one monolayer is approached at an exposure of 200 L. There is a smooth increase in the oxygen coverage as a function of exposure and from this trend there is no evidence for a change in the rate of oxygen uptake at the breakpoints identified by AES measurements [12,13]. There is no need to consider a change resulting from populating sur-
D.J. O’Connor et al. / The initial stages of the oxidation of AI(III).
f
0.8
P e 2
0.6
5 8
0.4
04
0
50
150 100 200 Oxygen Exposure (L)
250
I
300
Fig. 6. The increase in the measured oxygen as a function of oxygen exposure for the Altlll) surface measured at 300 K using geometry I. The Al yield is taken as the average over the scattering angles 70”-75”. The solid curve is the fitted exponential dependence described in the text. All filled squares are the result of an average of several measurements as the exposure was increased. The two open squares are obtained from surface structure measurements in which the statistical counting uncertainty error was greatly reduced but were exposed for a considerable time to the background vacuum environment. The crosses are the Auger measurements of Soria et al. [13] scaled vertically to asymptotically approach unity and included for comparison. Our own AES measurements have not been included in this figure as, when they are scaled in the same way, they lie over the MEIS measurements.
face and subsurface sites. The coverages indicated in fig. 6 are the result of a number of different and repeated measurements. The solid points correspond to the averages of a series of measurements as the exposure was increased. These determinations were made within 15 min of exposure. The open squares are measurements taken from surface blocking profiles in which there is less statistical uncertainty; however, by the nature of the measurement it was taken over a period of up to 12 h after the adsorption. The solid curve in fig. 6 is a fit using the expression 0 = Y,[
1 - exp(
-E/E,)],
where 8 is the coverage of oxygen in monolayers (where one monolayer is defined as one oxygen atom for each surface Al atom), Y, is the limiting
I
131
coverage, E is the exposure and E, is a characteristic exposure parameter. For the fitted curve in fig. 6 the constants take the values Y, = 1.02 monolayers of oxygen and E, = 79 L. This excellent fit of the coverage to an exponential curve is indicative of first order adsorption kinetics in which an oxygen molecule sticks if it encounters a clean Al surface region, but will desorb from the surface if that region is already covered. If the oxidation were a two stage process, leading to populating both surface and subsurface sites, then a more complicated oxygen uptake should be expected than the observed first order kinetics. Furthermore, the limiting value of one monolayer of oxygen at high exposures implies that one of the two potential classes sites (above or below the surface) is not populated. A serious consideration arises in the comparison between different studies as there is no reconstruction of the surface or other indication that a particular coverage or stage of the oxidation process has been achieved. The comparison between different experiments has always been on the basis of an exposure to an oxygen background pressure and the true exposure is then a function of the calibration of the ionisation gauge, the geometry of the chamber and other factors. To place our study in context with previously reported measurements a comparison is made (fig. 6) between the AES oxygen coverage measured by Soria et al. [131 and the MEIS determined coverage. The AES measurements were normalised to the MEIS measurement of one monolayer at 300 L exposure. Similar AES measurements were made in conjunction with the MEIS study but have not been included as they lie over the MEIS measurements when normalised to one monolayer at 300 L. If there had been a significant difference in exposures between our measurements and those of Soria et al. then the curves for AES and MEIS oxygen yield would be displaced horizontally at intermediate coverages. As there is no systematic disagreement between the oxygen coverage determined by Soria and our measurements we conclude that our exposures and coverages are close to those used in that study. This does not overcome the same difficulty in comparison to other studies.
138
D.J. O’Connor et al. / The initial stages of the oxidation of Al(111). I
3.5. Oxygen subsurface adsorption site The surface oxygen sites cannot be determined by blocking of the oxygen scattering yield as they sit above the surface Al layer and scattered ions will not experience the effects of any blocking centre. Thus if only surface sites are populated then the oxygen blocking profile will be constant with scattering angle. In theory the surface adsorption site may be determined by the identification of blocking of the Al signal by the abovesurface 0. In practice the oxygen acted as such a weak blocking centre that this approach could not be used. An alternative approach to the location of the surface oxygen site employing energy losses h!s located the oxygen in the surface C-site at 0.58 A above the surface [36,37]. The four subsurface sites are equally visible to the incident ion beam so will be uniformly illuminated. As they are situated below the Al surface layer, the projectiles scattered from subsurface oxygen will be blocked from particular directions by surface Al atoms. Each site has its own characteristic blocking pattern which is easily distinguished. In the event that both surface and subsurface sites are populated then the angular distribution will be a linear addition of each profile thus making the subsurface profile less distinct once added to the structureless surface site yield. The oxygen blocking profiles measured for geometry I (fig. 7a) and geometry II (fig. 7b) are compared to those predicted by the simulation for one monolayer of 0 in each of the subsurface sites at a target temperature of 190 K. The difference in the number of monolayers of oxygen measured experimentally between the two geometries is not significant as it is a measure of the reproducibility of the adsorption and exposure. Clearly the sensitivity of the angular distribution to the different sites is different as it depends on the prominence of the blocking features. There is no match between the simulated blocking profiles for the tA, OC and doC sites and the experimentally measured profile. The blocking feature for the tB site is not prominent in the simulation; however, any angular dependent feature that does exist in the results does not coincide with the shallow minima in the tB simulated
4(4
doC
[ii31
[ii41
I 65
0 60
I
I 70 Scattering Angle (degree)
75
clot
(b) -
o>--
.
lA
I
[iii]
[334] 0, 44
/ 46
I 46
50 52 54 Scattering Angle (degree)
56
56
Fig. 7. The angular blocking profile of the scattering yield of 100 keV H from oxygen compared to the simulation results for all proposed subsurface sites. The target temperature was 190 K and the simulation was for one monolayer of oxygen. For clarity the results of the simulations have been offset. The vertical bars on the right represent the O-l monolayer scale for tB, tA, OC and doC, respectively. The lack of a clear blocking dip reveals that the oxygen is not sitting in a subsurface site. Results from both (a) geometry I and (b) geometry II are shown.
results. Even in the possible event that oxidation led to a relaxation of the surface layer causing a shift in the position of the blocking minima no match can be found for any of the proposed subsurface sites. Apart from a weak angular trend there is no clear blocking feature which means that to within the limits of uncertainty there is no subsurface oxygen. This does not totally rule out the existence of a subsurface component. The sensitivity
D.J. O’Connor et al. / The initial stages of the oxidation of Al(ll1).
of MEIS to the subsurface sites under consideration depends on how pronounced the angular distribution is and it is evident from figure seven that some subsurface sites are easier to detect than others. Based on these sensitivities, we conclude that at 100 L exposure the subsurface component of the adsorbed oxygen is less than 10% if it is in the tB site and less than 5% if it is the tA, OC or doC sites. The weak angular dependence of scattered ion yield from oxygen may be caused by other processes than blocking. One possibility could be that it is indicative of a charged fraction which is angle dependent at small (< 20”) angles to the surface. This has been checked by measuring the charge fraction at 5” and 28” to the surface and no difference was found to within experimental uncertainty. The only other possibility is the use of an inappropriate scattering cross section which relates the scattered particle yield to surface atom concentration. If the true angle dependence of the scattering cross section differs from that of the Moliere potential then a gradual departure would be observed.
4. Conclusions The oxidation of Al(111) to an exposure of 100 L has been studied to test the claims made by other techniques. The first finding is that there is almost one monolayer of oxygen at this exposure. Furthermore, there is no observable shift in the Al blocking features which is evidence for the lack of relaxation of the outermost Al layer. A significant increase in the scattering yield from Al atoms is observed and is attributed to Al displacements induced by the oxygen adsorption. We propose that the Al substrate atoms at the perimeters on the 0 islands are strained away from their regular lattice positions. The full oxygen monolayer is still accompanied by a substantial difference in Al signal from the clean surface, which probably indicates that the final structure is not completely ordered. The measured coverage of oxygen at different exposures follows a simple first-order adsorption curve which is different to that found in previous
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studies. The asymptotic oxygen coverage from the adsorption measurements is only one monolayer suggesting that one of the two possible locations for oxygen (above or below the surface) is not populated. This conclusion is further supported by oxygen blocking curve measurements which provide no evidence for a significant subsurface component to the initial oxygen coverage. This is in marked disagreement with some previous studies which rely on the existence of subsurface oxygen to explain experimental observations. The measured saturation coverage of one monolayer, the first-order adsorption kinetics and the lack of structure to the oxygen blocking curve all indicate an exclusive surface site. The limits of sensitivity to the measurement of a subsurface component is 5% for all sites except the tB site for which it is 10%.
Acknowledgements
This work is part of the research program of the Foundation for Fundamental Research on Matter (FOM) and was made possible by financial support from the Netherlands Organisation for the Advancement of Research (NWO). The valuable assistance of R. Vuthaluru is acknowledged in the performance of the computer simulations. Fruitful discussions with D.P. Woodruff are gratefully acknowledged.
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