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Initial interaction of oxygen with aluminium single crystal faces: A LEED, AES and work function study
Surface Science 95 (1980) 309-320 0 North-Holland Publishing Company
INITIAL INTERACTION OF OXYGEN WITH ALUMINIUM SINGLE CRYSTAL FACES: A LEED, AES AND WORK FUNCTION STUDY R. MICHEL, J. GASTALDI, C. ALLASIA and C. JOURDAN Centre de Recherche des Mkcanismesde la Croissance Cristalline,CNRS, Campus Luminy, Case 913, F-13288 MarseilleGdex 2, France
and J. DERRIEN D&partementde Physique, Facult6 des Sciences de Luminy, ERA CNRS 070899, F-13288 MarseiNeCt!dex 2, France Received 13 September 1979
Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and work function (Kelvin probe) measurements have been used to study the initial interaction of clean Al(11 I), (100) and (110) surfaces with oxygen at room temperature. The oxidation process was found to be surface orientation dependent, but a common feature has been always observed on the three low-index surfaces: they show two distinct phases, i.e. a chemisorbed phase followed then by an oxidized phase. From analysis of AES, LEED and Kelvin probe results, an adsorption mechanism of 0 on Al for each surface orientation is proposed.
1. Introduction The oxygen-aluminium interaction has been extensively investigated in the past either on polycrystalline surfaces [l-7] or on single crystals [8-131 by several surface techniques. Understanding this interface has also been of numerous theoretical interest [14-191. These studies have shown a large difference in the initial oxidation according to the surface orientation, particularly if the evidence of a chemisorbed phase on the (111) face was found and is now commonly admitted, there is little clear experimental result suggesting the existence of oxygen atoms chemisorbed on the (100) and (110) faces [20-211. Moreover, according to the available techniques, the coverage of these chemisorbed phases was not well determined and a large discrepancy remains in the literature. Recently, in a previous work [22], we have shown clearly, by means of Auger electron spectroscopy, the existence of two phases, chemisorbed and oxidized phases, taking place on the (111) and (100) faces. The measurements of the Auger peak intensity of the elemental Al (L23W transition, 68 ev), oxidized Al (cross 309
310
R. Michel et al. /Initial interaction of oxygen with Al
transition, 54 eV) and oxygen (KLL transition, 510 eV) allowed us to distinguish the chemisorbed phase from the oxidized phase and gave indication on their respective coverage range. Our Auger measurements were slightly different from previously published results [9,10]; although the general features of the Auger peaks were similar (increase of the 0 peak and oxidized Al peak, decrease of the elemental Al peak with oxygen exposure), we observed in fact several linear segments on the Auger curves, with well marked breaks which are generally displayed by a layer-by-layer growth or a surface compound formation [23]. These breaks allowed us to determine exactly the coverage of the chemisorbed and oxidized phases for the (111) and (100) surfaces. Such a discrepancy in the adsorption kinetics could be explained by two reasons: - On the one hand, Auger measurements were performed with a low current density (<5 PA/mm’). We have shown [24] that a current density >25 PA/mm’ gave rise to an important reduction of Al203 by the electron beam and a temperature increase of 80°C on the sample. - On the other hand, the aluminium surfaces were cleaned by heating to 600°C under UHV for several hours, while the other authors practised a cyclic treatment of ion bombardment and annealing. The ion sputter etching and annealing was more efficacious but the surfaces did not seem as perfect as expected [25]. In this paper, results obtained on the (111) (100) and (110) surfaces of Al, using AES, LEED and work function (A@) techniques, will be described. We will show clearly the existence of a chemisorbed phase and an oxidized phase for the all three low-index surfaces. We will also suggest, for each surface orientation, a model of 0 adsorption which is strongly supported by the well achieved correlation of our measurements from AES, LEED and AI#Itechniques.
2. Experimental All measurements of AES, LEED and A$ were carried out in a stainless-steel ultra high vacuum system (UHV) (basic pressure -lo-” Torr). A four-grid AES/LEED set up has been used. For AES measurements, a glancing incidence electron gun delivered a beam at 3 keV and 4 PA/mm’. In these conditions, we observed an increase of the sample temperature <5”C and we never observed a reduction of the oxide by the electron beam. Work function variations were measured with a Kelvin probe, using a gold reference electrode to avoid the reference contamination by oxygen. The accuracy in A@ measurements was +5 meV. All samples were obtained by the Czochralski growth method. They were oriented to within lo (Laue reflexion) of the (11 l), (100) and (110) planes, cut by
R. Michel et al. f Initial interaction of oxygen with Al
311
spark erosion and electropolished in a solution of acetic anhydride and perchloric acid at 8’C. The bright mirror finished surface was then mounted into the UHV vessel on a sample holder equipped with an electrical heater. A chromel-alumel thermocouple was attached directly at the sample surface. The sample was cleaned by heating to 600°C under UHV conditions for several hours [26]. We assume the sample to be clean when the ratio of the 0 Auger peak intensity (510 eV) on the Al Auger peak intensity (68 eV) is
3. Results 3.1. AES results Figs. 1,2 and 3 show the adso~tion kinetics of oxygen on Al{11 l), (100) and (110) surfaces at room temperature. All the three surfaces display a common feature on the Auger plots described by several linear segments with well marked breaks at SO-60 and 120 L (1 L = lO-‘j Torr X 1 s). We attribute these doses to the completion of the first and second layers of oxygen atoms as discussed below. The changes of slopes may be explained by a screening effect of the Auger electrons 1231. In the O-55 L range of oxygen exposure, the (111) face shows, fust, a rapid decrease of the Al (68 eV) peak and an increase of the 0 (5 10 ev> peak, fig. I. No evidence of the formation of the oxide is revealed as we do not observe the appearance of the oxidized Al peak (54 ev). In this first stage, oxygen atoms are only chemisorbed on the (111) face. In the 55-l 20 L exposure range, the 0 Auger peak intensity shows a second linear variation with a change of slope. This fact may be explained either by a decrease of the sticking coefficient or by the build up of a second overlayer which screens the Auger electrons. As we observe a second break at an exposure (120 L) nearly twice of that of the first break (55 L), we assume that the 0 atoms actually adsorb in a second layer. They react with the Al atoms to form a surface compound layer, as shown by the appearance and the increase of the oxidized Al (54 eV) peak. The (100) surface displays a different behaviour compared with the (11 I) surface, fig. 2. We observe first a rapid increase of the oxygen peak and a decrease of
R. Michel et crl./Initial ~nt~~ctjo~of oxygen with Al
312
1
A
elemental
r
oxidized
00
510
Al Al
6BeV 54eV
av
x 40
OXYGEN
EXPOSURE
(Langmuir = lO-4
Tow
x 0 )
Fig. 1. AES adsorption kinetics of oxygen on the (111) aluminium surface. Auger experiments were performed with 3 keV incident electron energy, and a beam current of 4 MA/mm*.
the elemental Al peak until an oxygen dose of ~20 L. This corresponds to the chemisorbed phase of oxygen on Al. But at the 20 L exposure, the AI and 0 Auger curves show already a changing slope, and at the same time the oxidized Al peak begins to increase. The formation of a surface oxidized layer seems to occur earlier on the more “open” (100) surface. We notice here that the chemisorbed phase (O-20 L) is saturated at nearly l/3 of the first monolayer (-57 L). At coverages higher than 57 L, the oxide layer formation continues to occur as for the (111) face. The oxidation mechanism seems to be the same as the 0 and Al Auger peaks display similar features. The Al(110) surface, fig. 3, is an intermediate case between the (111) and (100) surfaces. The chemisorbed phase is completed at nearly 36 L (-2/3 of the first layer). At higher exposure, evidence of the oxidized phase is demonstrated by the appearance of the oxidized Al peak (54 ev). After that dose, the oxidation mechanism displayed the same features as on the (111) and (100) surfaces.
313
OXYOEN ~xPoswE (Langmuir + ice6 forr r( $1 Fig. 2. AES ad~rption kinetics of oxygen on the (100) aluminium surface. Same conditions as in Tig. 1.
The clean (1 II) surface displayed a very bright 1 X 1 structure, characteristic of a well ordered surface. During the completion of the chem~orbed phase (O-60 L), the diffraction spot brigfrtness slightly faded away. No supplements spots have been observed with 0 adsorption. As the oxidized phase took place (260 L), all the spots began to loose their brightness and gradually disappeared. Probably the oxide formation causes a strong surface disorder. The same behaviour happens for the (100) and (110) surfaces with the spots ~~s~g now very rapidly, respectively at 220 L and 240 L, i.e. at the onset of the oxidized phase. Our LEED results are very similar to those observed recently by Martinson et al. f273.
R. Michel et al. j Initial interaction of oxygen with Al
(110) .
elemental
n
Oxodized
.
I
0
510
Al Al
66
‘3V
54 eV
ev
,
lb0
50
Fig. 3. AES adsorption in fig. 1.
kinetics
Xl
)L
1
150 OXYGEN
of oxygen
200 EXPOSURE
, (
250
bngmuir
on the (110) aluminium
q: 10-6
Torr
I
s)
surface. Same conditions
as
3.3. Ati measurements The work function changes, A#, of the three Al surfaces versus oxygen coverage are shown in fig. 4. The (111) and (100) surfaces display a common feature. During the chemisorbed phase (respectively >54 L and >18 L), the work function practically does not vary. When the oxidation stage (-54 L and -18 L) occurs, it decreases very rapidly, fig. 4. On the contrary, concerning the (110) surface, the work function decreases first very rapidly during the chemisorbed phase (-35 L) and then slightly slows down during the oxidized phase. These A$ results are in good agreement with previously published results [13], in the high exposure range. But at the initial stage of the chemisorption, we obtain different work function changes. This discrepancy may be explained by the use, in our case, of a lower oxygen pressure which allows us to well define the initial interaction of 0 on Al. We have confidence in our A4 measurements as they display drastic changes for chemisorbed-oxidized phase transition at nearly the same coverage range observed on the three faces with AES and LEED.
R. Michel et al. /Initial interaction of oxygen with AI
Aa, (eV
315
)
-1
I I
’
-1.6 0
.Y
_I
“7
OXYGEN
100 200 Fig. 4. Work function variations versus oxygen adsorption on (ill),
EXPOSURE
( L)
300 (110) and (100) surfaces.
4. Discussion We have performed all AES measurements under the same experimental conditions, in order to compare the peak intensities of the three surfaces. Figs. 1,2 and 3 are plotted with the same arbitrary units in y axis. For clean (111) and (100) surfaces the Al peak heights give nearly the same magnitude, as a comparable number of Auger electron emitting atoms is involved. In contrast the (110) surface has a much lower atomic density and therefore the Auger Al peak shows a smaller height. After 0 adsorption, the behaviour is different according to the surface orientation as discussed in detail below.
316
R. Michel et al. /Initial
interaction
of oxygen
with Al
(111)
0 0
ALUMINIUM
OXYGEN
ATOM
ATOM
0 atom m
0 atom in bridge posiiion
the fourfold centered site
TOP
SECTION
VIEW
VIEW
(110)
0 atom
in the
twofold centered site TOP
VIEW
SECTION
VIEW
Fig. 5. Different views of the adsorption models proposed in the monolayer range for the three low-index surfaces: (111) Surface: in the monolayer range, 0 atoms are adsorbed in the three-fold, three-fold centered sites (full circle). (100) Surface: in the l/3 monolayer range, 0 atoms are adsorbed in “bridge” positions (full circle). Note that several equivalent “bridge” sites are offered to 0 atoms. They occupy these sites in a random fashion, forming probably very small domains, compared with the coherence of the LEED beam. This fact may explain why we do not observe supplementary LEED spots. Additional 0 atoms are then adsorbed in “well” positions (dashed circle), to complete this fist monolayer. (110) Surface: in the 2/3 monolayer range, 0 atoms are first adsorbed in “well” positions (dashed circle). To complete the first monolayer, additional 0 atoms are situated in “bridge” positions. Again two possibilities are offered (full circle).
R. Michel et al. /Initial interaction of oxygen with Al
317
4.1. Adsorption of 0 on Al(ll1) In the O-5.5 L exposure range, the AES measurements seem to prove that there is a first chemisorbed 0 monolayer. This result is in good agreement with XPS experiments [ 11 ,12,28,29] ; the oxygen atoms are bonded to the Al atoms, staying outside of the plane of the Al surface [28]. This 0 layer should have a density ol one oxygen atom per substrate unit mesh, i.e. one atom per @z* (a is the cubic parameter of the Al crystal). The proposed model is drawn in fig. 5. The 0 atoms are situated in the three-fold, three-fold centered sites according to recent LEED intensities measurements [27]. In this case, no supplementary diffraction spot is expected to be seen as we observed. The out-of-plane oxygen atoms should give rise to dipole moments perpendicular to the surface and hence a work function increase which is not observed during this chemisorbed phase. This fact may be explained either by the A@ technique which actually probes a large area (-30 mm’) where there may be different defects and distorsion which then average dipole moments parallely to the surface, or simply by in-plane-adsorbed oxygen atoms which are indeed suggested by recent calculations on a locally expanded surface
1191. At higher exposure than that of the first chemisorbed
layer (>55 L), oxygen atoms react with the surface metallic atoms to form an amorphous oxide compound as observed by AES (appearance of the oxidized Al peak) by LEED (disappearance of the diffraction spots) and by A+ (a A$ decrease means that oxygen atoms are penetrating into the subsurface). We remark that the Auger 0 signal, at 120 L, does not change dramatically from that at 55 L (-10%). This fact suggests also that the 0 second layer is incorporated into the metallic subsurface, under the first Al layer in order to form the oxide layer. Consequently the Auger electrons, excited from these 0 atoms are screened by one layer of the Al atoms and by the outer-most layer of the chemisorbed 0 atoms. Their contribution to the Auger 0 signal then should be very small, as observed. For oxygen exposures higher than 120 L, the oxide layer formation continues to occur, as the oxidized Al (54 eV) peak continues to grow and consequently the elemental Al (68 eV) peak diminishes while the 0 peak increases very slowly, fig. 1. Nevertheless, these processes seem to saturate at exposures higher than 300 L. Probably the presence of a thin oxide film in the subsurface region reduces greatly the rate of the 0 atom incorporation into the bulk or the Al atom diffusion towards the surface through this oxide film. 4.2. Adsorption of 0 on Al(100) The oxygen peak intensity at 55 L (first monolayer) is equal to 10 units, fig. 1, for the (111) surface. This signal is provided by an atomic density of one 0 atom per (111) unit mesh (‘&/%zz). The oxygen peak intensity at the same coverage is equal to 13.5 units, fig. 2, for the (100) surface. Therefore it should come from an atomic density of 1.5 0 atoms per (100) unit mesh (u*/2). Taking into account this
318
R. Michel et al. 1 Initial interaction of oxygen with AI
atomic density, we then suggest the following adsorption model for the (100) surface: in the chemisorbed phase (O-20 L, -l/3 of the first layer), the 0 atoms are adsorbed in “bridge” position, i.e. one 0 atom bonded onto two Al atoms. The density of this chemisorbed phase is then 0.5 atom per unit mesh ($12). It corresponds also to l/3 of the first monolayer. Additional 0 atoms are adsorbed in “well” position, i.e. in the four-fold centered sites, on top of the Al atoms of the second subjacent substrate layer. The density of these “well” positioned 0 atoms is one atom per unit mesh (a”];?). This model is shown in fig. 5. We remark that after completion of the chemisorbed phase, additions 0 atoms adsorbed in “well” position could be easily involved in the formation of the oxide layer, provoking the appearance of the oxidized Al peak at -20 L, fig. 2. Moreover they are probably relaxed towards the bulk in order to favour the oxide bonds and then, are slightly screened by the chemisorbed atoms. This fact may explain the small changing slope on 0 and Al peaks, observed at 20 L. This model is also supported by A$ measurements. During the chemisorbed phase, as the 0 atoms are either outside the surface or on the surface, no noticeable A@has been measured. The onset of the oxidized phase (>20 L), which accompanies the adsorption of 0 atoms into four-fold centered sites and the probable incorporation of these atoms into the subsurface, gives rise to a rapid decrease of the work function, as observed in fig. 4. This oxidized phase is also con~rmed by the disappearance of the LEED pattern. The oxidation seems to be more important for the (100) surface than for the (11 I) surface, as the Auger peaks of 0 and oxidized Al are larger on the former surface. This fact may be explained at the more open (100) surface, by the easier penetration of 0 atoms into the matrix, enhancing the oxidation process. It is also confirmed by the larger A$ diminution shown in fig. 4. 4.3. Adsorption
of0 on Al(lI0)
Comparing the 0 intensity peak at 55 L for both the (111) and (110) surfaces in the same manner as in section 4.2, we find that the atomic density of the first 0 layer for the (110) surface is equal to 1.5 0 atoms per unit mesh (&?%r2). Therefore we suggest the fo~o~g adso~tion model: in the chemisorbed phase (O-36 L, -2/3 of the first layer), the 0 atoms are adsorbed in the two-fold centered sites on the top of the Al atoms of the second subjacent substrate layer. The density of this chemisorbed phase is 1 atom per unit mesh (i.e. 2f3 of the first layer). Additional 0 atoms are adsorbed in “bridge” position; the density of these “bridge” positioned 0 atoms is 0.5 atom per unit mesh. This model is shown in fig. 5. Again, the model is also strongly supported by the work function change. As the chemisorbed atoms are now in centered sites, fig. 5, slightly underneath the surface plane, they should provoke a rapid diminution of the work function as observed, fig. 4. At completion of this chemisorbed phase (-35 L), the additional oxygen atoms, adsorbed in “bridge” position, slightly outside the surface, tend to inverse the A+ variation, giving rise to the slowing down observed in the A# decrease. This model can also
R. Michel et al.
/Initial interaction ofoxygen with Al
319
explain the absence of a changing slope on the 0 Auger peak of -35 L, fig. 3, although we observed a break on the Al peak at the same coverage. According to this model, fig. 5, we notice first: there is not a large difference in height between the two types of adsorption sites and secondly the 0 atoms in these two adsorption sites do not overlap themselves. Therefore the screening effect of the Auger electron, excited from the first layer 0 atoms, is ne~i~ble.
5. Conclusion Results on the OfAl system, obtained by AES, LEED and A@techniques, are very complementary and a coherent correlation between them has been achieved. Our experiments confirm directly and clearly the existence of two distinct stages in Al oxidation mech~ism for all three low-index surfaces. At low coverage, oxygen atoms are chemisorbed on the surface and at higher coverage, they are incorporated into the bulk to form the oxide. The chemisorbed phase takes place during a larger coverage range at the denser surface. Consequently the penetration of 0 atoms into the bulk in order to form oxide compound seems faster on the more “‘open” surfaces. We have also proposed adsorption models for the O/Al system. These models have been produced in order to explain our AES, LEED and A@results. They seem to be the simplest adsorption mechanisms taking into account the experimental results in a clear manner for all the three surfaces. They are quite different from standard models, in particular for the (100) and (110) faces. Confnmation of these models will require much further experimentation. Therefore we hope that presentation of these models here will stimulate interest in performing both theoretical and experimental work to test it, in order to clarify the details of the chemisorption of oxygen on aluminium.
References [l] E.E. Huber and CT. Kirk, Surface Sci. 5 (1966) 447. [2] M.W. Roberts and B.R. Wells, Surface Sci. 15 (1969) 325. [ 31 R.L. WeBs and T. Fort Jr., Surface Sci. 33 (1972) 172. [4] V.K. Agarwala and T. Fort Jr., Surface Sci. 45 (1974) 470. [5] G. Dorey, Surface Sci. 27 (1971) 311. [6] W.H. Kruegger and S.R, Pollack, Surface Sci. 30 (1972) 263. [7] K.Y. Yu, J.N, Miller, W.E. Spicer, N.D. Lang and A.R. Williams, Phys, Rev. B15 (1976) 1446. [S] F. Jona, J. Phys. Chem. Solids 28 (1967) 2155. [9] P.O. Gartland, Surface Sci. 62 (1977) 183. lo] C.W.B. Martinson and S.A. Flodstrom, Surface Sci. 80 (1979) 306. 111 A.M. Bradshaw, P. Hofmann and W. Wyrobisch, Surface Sci. 68 (1977) 269.
320
R. Michel et al. /Initial interaction of oxygen with Al
[12] S.A. Flodstrom, R.Z. Bachrach, R.S. Bauer and S.B.M. Hagstrom, in: Proc. 7th Intern. Vacuum Congr. and 3rd Intern. Conf. on Solid Surfaces, Vienna, 1977, p. 869. [13] P. Hofmann, W. Wyrobisch and A.M. Bradshaw, Surface Sci. 80 (1979) 344. [14] N.D. Lang and A.R. Williams, Phys. Rev. Letters 34 (1975) 531; 37 (1976) 212. [15] J. Harris and G.S. Painter, Phys. Rev. Letters 36 (1976) 51. [16] I.P. Batra and S. Ciraci, Phys. Rev. Letters 39 (1977) 774. [17] R.P. Messmer and D.R. Salahub, Phys. Rev. B16 (1977) 3415. [ 181 G.S. Painter, Phys. Rev. B17 (1978) 662. [19] D.R. Salahub, M. Roche and R.P. Messmer, Phys. Rev. B18 (1978) 6495. [20] A. Bianconi, R.Z. Bachrach and S.A. Flodstrom, Phys. Rev. B19 (1979) 3879. [21] W. Eberhart and C. Kunz, Surface Sci. 75 (1978) 709. [22] R. Michel, C. Jourdan, J. Gastaldi and J. Derrien, Surface Sci. 84 (1979) 509. [ 231 G.E. Rhead, in: Proc. NATO Advanced Institute on the Electronic Structure and Reactivity of the Metal Surfaces, Namur, 1975 (Plenum, New York). [ 241 R. Michel, These de 3eme cycle, Aix-Marseille III (1979). [25] P. Marzo, These de 38me cycle, Aix-Marseille III (1979). [26j S.M. Bedair, F. Hofmann and M.P. Smith, J. Appl. Phys. 39 (1968) 4026. [27] C.W.B. Martinson, S.A. Flodstrom, J. Rundgren and P. Westrin, Surface Sci. 89 (1979) 102. [28] S.A. Flodstrom, C.W.B. Martinson, R.Z. Bachrach, S.B.M. Hagstrom and R.S. Bauer, Phys. Rev. Letters 40 (1978) 907. [29] A.M. Bradshaw, W. Domke and L.S. Cederbaum, Phys. Rev. B16 (1977) 480.
INITIAL INTERACTION OF OXYGEN WITH ALUMINIUM SINGLE CRYSTAL FACES: A LEED, AES AND WORK FUNCTION STUDY R. MICHEL, J. GASTALDI, C. ALLASIA and C. JOURDAN Centre de Recherche des Mkcanismesde la Croissance Cristalline,CNRS, Campus Luminy, Case 913, F-13288 MarseilleGdex 2, France
and J. DERRIEN D&partementde Physique, Facult6 des Sciences de Luminy, ERA CNRS 070899, F-13288 MarseiNeCt!dex 2, France Received 13 September 1979
Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and work function (Kelvin probe) measurements have been used to study the initial interaction of clean Al(11 I), (100) and (110) surfaces with oxygen at room temperature. The oxidation process was found to be surface orientation dependent, but a common feature has been always observed on the three low-index surfaces: they show two distinct phases, i.e. a chemisorbed phase followed then by an oxidized phase. From analysis of AES, LEED and Kelvin probe results, an adsorption mechanism of 0 on Al for each surface orientation is proposed.
1. Introduction The oxygen-aluminium interaction has been extensively investigated in the past either on polycrystalline surfaces [l-7] or on single crystals [8-131 by several surface techniques. Understanding this interface has also been of numerous theoretical interest [14-191. These studies have shown a large difference in the initial oxidation according to the surface orientation, particularly if the evidence of a chemisorbed phase on the (111) face was found and is now commonly admitted, there is little clear experimental result suggesting the existence of oxygen atoms chemisorbed on the (100) and (110) faces [20-211. Moreover, according to the available techniques, the coverage of these chemisorbed phases was not well determined and a large discrepancy remains in the literature. Recently, in a previous work [22], we have shown clearly, by means of Auger electron spectroscopy, the existence of two phases, chemisorbed and oxidized phases, taking place on the (111) and (100) faces. The measurements of the Auger peak intensity of the elemental Al (L23W transition, 68 ev), oxidized Al (cross 309
310
R. Michel et al. /Initial interaction of oxygen with Al
transition, 54 eV) and oxygen (KLL transition, 510 eV) allowed us to distinguish the chemisorbed phase from the oxidized phase and gave indication on their respective coverage range. Our Auger measurements were slightly different from previously published results [9,10]; although the general features of the Auger peaks were similar (increase of the 0 peak and oxidized Al peak, decrease of the elemental Al peak with oxygen exposure), we observed in fact several linear segments on the Auger curves, with well marked breaks which are generally displayed by a layer-by-layer growth or a surface compound formation [23]. These breaks allowed us to determine exactly the coverage of the chemisorbed and oxidized phases for the (111) and (100) surfaces. Such a discrepancy in the adsorption kinetics could be explained by two reasons: - On the one hand, Auger measurements were performed with a low current density (<5 PA/mm’). We have shown [24] that a current density >25 PA/mm’ gave rise to an important reduction of Al203 by the electron beam and a temperature increase of 80°C on the sample. - On the other hand, the aluminium surfaces were cleaned by heating to 600°C under UHV for several hours, while the other authors practised a cyclic treatment of ion bombardment and annealing. The ion sputter etching and annealing was more efficacious but the surfaces did not seem as perfect as expected [25]. In this paper, results obtained on the (111) (100) and (110) surfaces of Al, using AES, LEED and work function (A@) techniques, will be described. We will show clearly the existence of a chemisorbed phase and an oxidized phase for the all three low-index surfaces. We will also suggest, for each surface orientation, a model of 0 adsorption which is strongly supported by the well achieved correlation of our measurements from AES, LEED and AI#Itechniques.
2. Experimental All measurements of AES, LEED and A$ were carried out in a stainless-steel ultra high vacuum system (UHV) (basic pressure -lo-” Torr). A four-grid AES/LEED set up has been used. For AES measurements, a glancing incidence electron gun delivered a beam at 3 keV and 4 PA/mm’. In these conditions, we observed an increase of the sample temperature <5”C and we never observed a reduction of the oxide by the electron beam. Work function variations were measured with a Kelvin probe, using a gold reference electrode to avoid the reference contamination by oxygen. The accuracy in A@ measurements was +5 meV. All samples were obtained by the Czochralski growth method. They were oriented to within lo (Laue reflexion) of the (11 l), (100) and (110) planes, cut by
R. Michel et al. f Initial interaction of oxygen with Al
311
spark erosion and electropolished in a solution of acetic anhydride and perchloric acid at 8’C. The bright mirror finished surface was then mounted into the UHV vessel on a sample holder equipped with an electrical heater. A chromel-alumel thermocouple was attached directly at the sample surface. The sample was cleaned by heating to 600°C under UHV conditions for several hours [26]. We assume the sample to be clean when the ratio of the 0 Auger peak intensity (510 eV) on the Al Auger peak intensity (68 eV) is
3. Results 3.1. AES results Figs. 1,2 and 3 show the adso~tion kinetics of oxygen on Al{11 l), (100) and (110) surfaces at room temperature. All the three surfaces display a common feature on the Auger plots described by several linear segments with well marked breaks at SO-60 and 120 L (1 L = lO-‘j Torr X 1 s). We attribute these doses to the completion of the first and second layers of oxygen atoms as discussed below. The changes of slopes may be explained by a screening effect of the Auger electrons 1231. In the O-55 L range of oxygen exposure, the (111) face shows, fust, a rapid decrease of the Al (68 eV) peak and an increase of the 0 (5 10 ev> peak, fig. I. No evidence of the formation of the oxide is revealed as we do not observe the appearance of the oxidized Al peak (54 ev). In this first stage, oxygen atoms are only chemisorbed on the (111) face. In the 55-l 20 L exposure range, the 0 Auger peak intensity shows a second linear variation with a change of slope. This fact may be explained either by a decrease of the sticking coefficient or by the build up of a second overlayer which screens the Auger electrons. As we observe a second break at an exposure (120 L) nearly twice of that of the first break (55 L), we assume that the 0 atoms actually adsorb in a second layer. They react with the Al atoms to form a surface compound layer, as shown by the appearance and the increase of the oxidized Al (54 eV) peak. The (100) surface displays a different behaviour compared with the (11 I) surface, fig. 2. We observe first a rapid increase of the oxygen peak and a decrease of
R. Michel et crl./Initial ~nt~~ctjo~of oxygen with Al
312
1
A
elemental
r
oxidized
00
510
Al Al
6BeV 54eV
av
x 40
OXYGEN
EXPOSURE
(Langmuir = lO-4
Tow
x 0 )
Fig. 1. AES adsorption kinetics of oxygen on the (111) aluminium surface. Auger experiments were performed with 3 keV incident electron energy, and a beam current of 4 MA/mm*.
the elemental Al peak until an oxygen dose of ~20 L. This corresponds to the chemisorbed phase of oxygen on Al. But at the 20 L exposure, the AI and 0 Auger curves show already a changing slope, and at the same time the oxidized Al peak begins to increase. The formation of a surface oxidized layer seems to occur earlier on the more “open” (100) surface. We notice here that the chemisorbed phase (O-20 L) is saturated at nearly l/3 of the first monolayer (-57 L). At coverages higher than 57 L, the oxide layer formation continues to occur as for the (111) face. The oxidation mechanism seems to be the same as the 0 and Al Auger peaks display similar features. The Al(110) surface, fig. 3, is an intermediate case between the (111) and (100) surfaces. The chemisorbed phase is completed at nearly 36 L (-2/3 of the first layer). At higher exposure, evidence of the oxidized phase is demonstrated by the appearance of the oxidized Al peak (54 ev). After that dose, the oxidation mechanism displayed the same features as on the (111) and (100) surfaces.
313
OXYOEN ~xPoswE (Langmuir + ice6 forr r( $1 Fig. 2. AES ad~rption kinetics of oxygen on the (100) aluminium surface. Same conditions as in Tig. 1.
The clean (1 II) surface displayed a very bright 1 X 1 structure, characteristic of a well ordered surface. During the completion of the chem~orbed phase (O-60 L), the diffraction spot brigfrtness slightly faded away. No supplements spots have been observed with 0 adsorption. As the oxidized phase took place (260 L), all the spots began to loose their brightness and gradually disappeared. Probably the oxide formation causes a strong surface disorder. The same behaviour happens for the (100) and (110) surfaces with the spots ~~s~g now very rapidly, respectively at 220 L and 240 L, i.e. at the onset of the oxidized phase. Our LEED results are very similar to those observed recently by Martinson et al. f273.
R. Michel et al. j Initial interaction of oxygen with Al
(110) .
elemental
n
Oxodized
.
I
0
510
Al Al
66
‘3V
54 eV
ev
,
lb0
50
Fig. 3. AES adsorption in fig. 1.
kinetics
Xl
)L
1
150 OXYGEN
of oxygen
200 EXPOSURE
, (
250
bngmuir
on the (110) aluminium
q: 10-6
Torr
I
s)
surface. Same conditions
as
3.3. Ati measurements The work function changes, A#, of the three Al surfaces versus oxygen coverage are shown in fig. 4. The (111) and (100) surfaces display a common feature. During the chemisorbed phase (respectively >54 L and >18 L), the work function practically does not vary. When the oxidation stage (-54 L and -18 L) occurs, it decreases very rapidly, fig. 4. On the contrary, concerning the (110) surface, the work function decreases first very rapidly during the chemisorbed phase (-35 L) and then slightly slows down during the oxidized phase. These A$ results are in good agreement with previously published results [13], in the high exposure range. But at the initial stage of the chemisorption, we obtain different work function changes. This discrepancy may be explained by the use, in our case, of a lower oxygen pressure which allows us to well define the initial interaction of 0 on Al. We have confidence in our A4 measurements as they display drastic changes for chemisorbed-oxidized phase transition at nearly the same coverage range observed on the three faces with AES and LEED.
R. Michel et al. /Initial interaction of oxygen with AI
Aa, (eV
315
)
-1
I I
’
-1.6 0
.Y
_I
“7
OXYGEN
100 200 Fig. 4. Work function variations versus oxygen adsorption on (ill),
EXPOSURE
( L)
300 (110) and (100) surfaces.
4. Discussion We have performed all AES measurements under the same experimental conditions, in order to compare the peak intensities of the three surfaces. Figs. 1,2 and 3 are plotted with the same arbitrary units in y axis. For clean (111) and (100) surfaces the Al peak heights give nearly the same magnitude, as a comparable number of Auger electron emitting atoms is involved. In contrast the (110) surface has a much lower atomic density and therefore the Auger Al peak shows a smaller height. After 0 adsorption, the behaviour is different according to the surface orientation as discussed in detail below.
316
R. Michel et al. /Initial
interaction
of oxygen
with Al
(111)
0 0
ALUMINIUM
OXYGEN
ATOM
ATOM
0 atom m
0 atom in bridge posiiion
the fourfold centered site
TOP
SECTION
VIEW
VIEW
(110)
0 atom
in the
twofold centered site TOP
VIEW
SECTION
VIEW
Fig. 5. Different views of the adsorption models proposed in the monolayer range for the three low-index surfaces: (111) Surface: in the monolayer range, 0 atoms are adsorbed in the three-fold, three-fold centered sites (full circle). (100) Surface: in the l/3 monolayer range, 0 atoms are adsorbed in “bridge” positions (full circle). Note that several equivalent “bridge” sites are offered to 0 atoms. They occupy these sites in a random fashion, forming probably very small domains, compared with the coherence of the LEED beam. This fact may explain why we do not observe supplementary LEED spots. Additional 0 atoms are then adsorbed in “well” positions (dashed circle), to complete this fist monolayer. (110) Surface: in the 2/3 monolayer range, 0 atoms are first adsorbed in “well” positions (dashed circle). To complete the first monolayer, additional 0 atoms are situated in “bridge” positions. Again two possibilities are offered (full circle).
R. Michel et al. /Initial interaction of oxygen with Al
317
4.1. Adsorption of 0 on Al(ll1) In the O-5.5 L exposure range, the AES measurements seem to prove that there is a first chemisorbed 0 monolayer. This result is in good agreement with XPS experiments [ 11 ,12,28,29] ; the oxygen atoms are bonded to the Al atoms, staying outside of the plane of the Al surface [28]. This 0 layer should have a density ol one oxygen atom per substrate unit mesh, i.e. one atom per @z* (a is the cubic parameter of the Al crystal). The proposed model is drawn in fig. 5. The 0 atoms are situated in the three-fold, three-fold centered sites according to recent LEED intensities measurements [27]. In this case, no supplementary diffraction spot is expected to be seen as we observed. The out-of-plane oxygen atoms should give rise to dipole moments perpendicular to the surface and hence a work function increase which is not observed during this chemisorbed phase. This fact may be explained either by the A@ technique which actually probes a large area (-30 mm’) where there may be different defects and distorsion which then average dipole moments parallely to the surface, or simply by in-plane-adsorbed oxygen atoms which are indeed suggested by recent calculations on a locally expanded surface
1191. At higher exposure than that of the first chemisorbed
layer (>55 L), oxygen atoms react with the surface metallic atoms to form an amorphous oxide compound as observed by AES (appearance of the oxidized Al peak) by LEED (disappearance of the diffraction spots) and by A+ (a A$ decrease means that oxygen atoms are penetrating into the subsurface). We remark that the Auger 0 signal, at 120 L, does not change dramatically from that at 55 L (-10%). This fact suggests also that the 0 second layer is incorporated into the metallic subsurface, under the first Al layer in order to form the oxide layer. Consequently the Auger electrons, excited from these 0 atoms are screened by one layer of the Al atoms and by the outer-most layer of the chemisorbed 0 atoms. Their contribution to the Auger 0 signal then should be very small, as observed. For oxygen exposures higher than 120 L, the oxide layer formation continues to occur, as the oxidized Al (54 eV) peak continues to grow and consequently the elemental Al (68 eV) peak diminishes while the 0 peak increases very slowly, fig. 1. Nevertheless, these processes seem to saturate at exposures higher than 300 L. Probably the presence of a thin oxide film in the subsurface region reduces greatly the rate of the 0 atom incorporation into the bulk or the Al atom diffusion towards the surface through this oxide film. 4.2. Adsorption of 0 on Al(100) The oxygen peak intensity at 55 L (first monolayer) is equal to 10 units, fig. 1, for the (111) surface. This signal is provided by an atomic density of one 0 atom per (111) unit mesh (‘&/%zz). The oxygen peak intensity at the same coverage is equal to 13.5 units, fig. 2, for the (100) surface. Therefore it should come from an atomic density of 1.5 0 atoms per (100) unit mesh (u*/2). Taking into account this
318
R. Michel et al. 1 Initial interaction of oxygen with AI
atomic density, we then suggest the following adsorption model for the (100) surface: in the chemisorbed phase (O-20 L, -l/3 of the first layer), the 0 atoms are adsorbed in “bridge” position, i.e. one 0 atom bonded onto two Al atoms. The density of this chemisorbed phase is then 0.5 atom per unit mesh ($12). It corresponds also to l/3 of the first monolayer. Additional 0 atoms are adsorbed in “well” position, i.e. in the four-fold centered sites, on top of the Al atoms of the second subjacent substrate layer. The density of these “well” positioned 0 atoms is one atom per unit mesh (a”];?). This model is shown in fig. 5. We remark that after completion of the chemisorbed phase, additions 0 atoms adsorbed in “well” position could be easily involved in the formation of the oxide layer, provoking the appearance of the oxidized Al peak at -20 L, fig. 2. Moreover they are probably relaxed towards the bulk in order to favour the oxide bonds and then, are slightly screened by the chemisorbed atoms. This fact may explain the small changing slope on 0 and Al peaks, observed at 20 L. This model is also supported by A$ measurements. During the chemisorbed phase, as the 0 atoms are either outside the surface or on the surface, no noticeable A@has been measured. The onset of the oxidized phase (>20 L), which accompanies the adsorption of 0 atoms into four-fold centered sites and the probable incorporation of these atoms into the subsurface, gives rise to a rapid decrease of the work function, as observed in fig. 4. This oxidized phase is also con~rmed by the disappearance of the LEED pattern. The oxidation seems to be more important for the (100) surface than for the (11 I) surface, as the Auger peaks of 0 and oxidized Al are larger on the former surface. This fact may be explained at the more open (100) surface, by the easier penetration of 0 atoms into the matrix, enhancing the oxidation process. It is also confirmed by the larger A$ diminution shown in fig. 4. 4.3. Adsorption
of0 on Al(lI0)
Comparing the 0 intensity peak at 55 L for both the (111) and (110) surfaces in the same manner as in section 4.2, we find that the atomic density of the first 0 layer for the (110) surface is equal to 1.5 0 atoms per unit mesh (&?%r2). Therefore we suggest the fo~o~g adso~tion model: in the chemisorbed phase (O-36 L, -2/3 of the first layer), the 0 atoms are adsorbed in the two-fold centered sites on the top of the Al atoms of the second subjacent substrate layer. The density of this chemisorbed phase is 1 atom per unit mesh (i.e. 2f3 of the first layer). Additional 0 atoms are adsorbed in “bridge” position; the density of these “bridge” positioned 0 atoms is 0.5 atom per unit mesh. This model is shown in fig. 5. Again, the model is also strongly supported by the work function change. As the chemisorbed atoms are now in centered sites, fig. 5, slightly underneath the surface plane, they should provoke a rapid diminution of the work function as observed, fig. 4. At completion of this chemisorbed phase (-35 L), the additional oxygen atoms, adsorbed in “bridge” position, slightly outside the surface, tend to inverse the A+ variation, giving rise to the slowing down observed in the A# decrease. This model can also
R. Michel et al.
/Initial interaction ofoxygen with Al
319
explain the absence of a changing slope on the 0 Auger peak of -35 L, fig. 3, although we observed a break on the Al peak at the same coverage. According to this model, fig. 5, we notice first: there is not a large difference in height between the two types of adsorption sites and secondly the 0 atoms in these two adsorption sites do not overlap themselves. Therefore the screening effect of the Auger electron, excited from the first layer 0 atoms, is ne~i~ble.
5. Conclusion Results on the OfAl system, obtained by AES, LEED and A@techniques, are very complementary and a coherent correlation between them has been achieved. Our experiments confirm directly and clearly the existence of two distinct stages in Al oxidation mech~ism for all three low-index surfaces. At low coverage, oxygen atoms are chemisorbed on the surface and at higher coverage, they are incorporated into the bulk to form the oxide. The chemisorbed phase takes place during a larger coverage range at the denser surface. Consequently the penetration of 0 atoms into the bulk in order to form oxide compound seems faster on the more “‘open” surfaces. We have also proposed adsorption models for the O/Al system. These models have been produced in order to explain our AES, LEED and A@results. They seem to be the simplest adsorption mechanisms taking into account the experimental results in a clear manner for all the three surfaces. They are quite different from standard models, in particular for the (100) and (110) faces. Confnmation of these models will require much further experimentation. Therefore we hope that presentation of these models here will stimulate interest in performing both theoretical and experimental work to test it, in order to clarify the details of the chemisorption of oxygen on aluminium.
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R. Michel et al. /Initial interaction of oxygen with Al
[12] S.A. Flodstrom, R.Z. Bachrach, R.S. Bauer and S.B.M. Hagstrom, in: Proc. 7th Intern. Vacuum Congr. and 3rd Intern. Conf. on Solid Surfaces, Vienna, 1977, p. 869. [13] P. Hofmann, W. Wyrobisch and A.M. Bradshaw, Surface Sci. 80 (1979) 344. [14] N.D. Lang and A.R. Williams, Phys. Rev. Letters 34 (1975) 531; 37 (1976) 212. [15] J. Harris and G.S. Painter, Phys. Rev. Letters 36 (1976) 51. [16] I.P. Batra and S. Ciraci, Phys. Rev. Letters 39 (1977) 774. [17] R.P. Messmer and D.R. Salahub, Phys. Rev. B16 (1977) 3415. [ 181 G.S. Painter, Phys. Rev. B17 (1978) 662. [19] D.R. Salahub, M. Roche and R.P. Messmer, Phys. Rev. B18 (1978) 6495. [20] A. Bianconi, R.Z. Bachrach and S.A. Flodstrom, Phys. Rev. B19 (1979) 3879. [21] W. Eberhart and C. Kunz, Surface Sci. 75 (1978) 709. [22] R. Michel, C. Jourdan, J. Gastaldi and J. Derrien, Surface Sci. 84 (1979) 509. [ 231 G.E. Rhead, in: Proc. NATO Advanced Institute on the Electronic Structure and Reactivity of the Metal Surfaces, Namur, 1975 (Plenum, New York). [ 241 R. Michel, These de 3eme cycle, Aix-Marseille III (1979). [25] P. Marzo, These de 38me cycle, Aix-Marseille III (1979). [26j S.M. Bedair, F. Hofmann and M.P. Smith, J. Appl. Phys. 39 (1968) 4026. [27] C.W.B. Martinson, S.A. Flodstrom, J. Rundgren and P. Westrin, Surface Sci. 89 (1979) 102. [28] S.A. Flodstrom, C.W.B. Martinson, R.Z. Bachrach, S.B.M. Hagstrom and R.S. Bauer, Phys. Rev. Letters 40 (1978) 907. [29] A.M. Bradshaw, W. Domke and L.S. Cederbaum, Phys. Rev. B16 (1977) 480.