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HREELS study of the oxidation of AL(111) between 300 and 20 K
Journal of Electron Spectroscopy and Related Phenomena, 44 (1987) 175-182
175
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
HREELS STUDYOF THE OXIDATIONOF AL(111) BETWEEN300 AND 20 K
C. ASTALDI, P. GENGand K. JACOBI Frltz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-IO00 Berlin 33, West Germany
SUMK~RY The oxidation of AI(111) was studied by HREELS for sample temperatures of 300, 120 and 20 K and for oxygen coverages below and above the monolayer coverage. The experimental results are explained within the following model: At 300 K oxygen is chemlsorbed both above and below the f i r s t Al layer, giving rise to losses between 65 and 80 meV. This oxygen double-layer grows In islands and stabilises the oxygen against diffusion into the bulk. At 120 K chemisorbed oxygen above the Al surface and a distribution of subsurface oxygen including oxidlc species with losses between 105 and 120 meV are found. At 20 K abovesurface oxygen is no longer stable enough to be measured by HREELS; sticking coefficient and rate of oxidation are increased.
INTRODUCTION A number of studies on the oxidation of A1 have recently been reviewed c r i t i c a l l y and in detail (ref. 1). In concentrating on the f i r s t oxidation step we address the open question of whether the oxygen chemisorbs in a single site above or in two different sites above and below the Al surface. This question has been raised by Strong et al. (ref. 2) through an HREELS experiment on A l ( l l l ) . For a sample temq~erature of 300 K and for small oxygen doses they found two losses at 80 and 105 meV. Later (ref. 3) they published spectra for which the 105 meV loss was only weak whereas the former 80 meV loss was resolved into two losses at 60 and 73 eV. Thus, disregarding the different loss energies for the moment, the important new result was the observation of two losses, l . e .
two
binding sites for the oxygen atom even in the f i r s t chemisorptlon step. Strong et al. (ref. 2) discussed their results in terms of high-symmetry binding sites as sketched In f l g .
I. This figure also indicates that the EXAFS measurements of
Norman et al. (ref. 4) could not c l a r i f y the situation since the sites C, tB and tA result in the same O-Al distance and could not be separated in t h e i r experiment. The oxygen Al adsorption system is also con~llcated with respect to tlme-dependent effects indicating the tendency of oxygen to diffuse into and to react wlth the bulk. Thus, In our earlier ARUPS study (ref. 5) we found the
0368-2048/87/$03.50
© 1987 Elsevier Science Publishers B.V.
176
~
P VIEW
BOp
?C
x I I I OtA J. . .
tB'kv
I I ,L . .
~ "1"
I ?oC
A-LAYER
-
I x I I I I
,, -
B-LAYER
Fig. 1. Scheme of the highsymmetry sites for chemisorbed oxygen at the AI(111) surface according to Strong et al. (ref. 2). Full dots stand for Al atoms. B and C give oxygen sites above the Al surface; tA and tB indicate tetrahedrally coordinated oxygen subsurface sites; oC is an octahedrally coordinated subsurface oxygen site.
C-LAYER S I D E VIEW
ordered chemisorbed layer to decompose within one hour. We found i t desirable to repeat the HREELSexperiment with variation of the sample temperature in order to eventually reduce the time-dependent effects or to stabilise the chemisorbed oxygen above the Al surface.
EXPERIMENT The experiment was performed in an UHV chamber with a base pressure below 1.10-10 mbar. The AI(111) sample was prepared by standard techniques including argon-ion sputtering and annealing at 700 K. Prior to insertion into the chamber i t was electropolished. The sample was cooled using liquid N2 and He. The temperature was measured with a Chromel-Constantan-thermocouple. The lowest temperatures which could be attained were estimated to be (120±5)K for N2 and (20±5)K for He cooling. The latter value was confirmed by the observation of physisorbed 02 multilayers. For sample temperatures of 300 and 120 K oxygen was introduced using a doser. The pressure in front of the doser was estimated to be five times higher than in the chamber. The HREEL spectrometer is described elsewhere (ref. 6). The spectra were measured with an energy resolution of 10 meV in the specular beam. Angle of incidence was 55 degree with respect to the sample normal.
177
RESULTS Spectra of different oxygen exposures at 300, 120 and 20 K are shown in f i g . 2. For the 300 K 60 L exposure the spectrum exhibits a loss peak at 65 meV with a shoulder at about 80 meV. At 120 L a feature at 105 meV develops which grows with dose. At 270 L the main peak is found at 73 meV. At 120 K the f i r s t oxygen dose induces a different loss spectrum. Again, there is a peak at 65 meV but no shoulder at 80 meV is found. The feature at 105 meV is visible already at this low dose and grows faster with dose than in the 300 K measurements. The feature at 65 meV changes similarly with dose as in the 300 K case. For a 20 K 2 L exposure very broad features around 70 and 100 meV are obtained. Already at 20 L the 105 meV loss is the dominant feature. There is a large increase of intensity for the higher doses at 120 and 20 K correlated with larger amounts of oxidic oxygen at 105 meV. A higher amount of oxidic species means a more irregular surface and therefore a higher efficiency in dissociating 02.
AI (111)-0
Ep=3eV
Oz(L) T=20K
20; \
// '~
i\ F\T= 120K
=
a
so,/ '~ f
T= 300 K
~"!
ts~o
3Xx iI
~120 \
l
~
0 20 ~0 60 80100120 ENERGYLOSS(meV)
Fig. 2. HREEL spectra for the clean AI(111) surface and after dosing with different amounts of oxygen in units of Langmulr (I L & 10-6 Torr.s). The sample temperature T is indicated. The energy of the incident electrons was 3 eV.
178
Fig. 3 shows how the HREEL spectra, which correspond to the lowest oxygen dose in f i g . 2, change with time at 300 K and at 120 K, respectively. At 300 K ( f i g . 3a) a weak structure at about 100 meV is grown after two hours. Within the same time the changes of the 120 K spectrum (fig. 3b) becomemore evident: the losses at 70 and 100 meV have become equally high. This indicates the surprising result that the chemisorption state is more stable at 300 than at 120 K. I t is interesting to note that for the 20 h curve in f i g . 3a a low-lying loss at 43 meV appears. This peak is observed also in f i g . 3b at 2 h and for the high oxygen doses in f i g . 2. This peak is always connected to a high intensity of the 105 meV-loss. l
l
l
l
l
l
l
l
AI(111)-O Ep=3~
l
l
l
l
I
~ L Oz T=~K
I
I
I
I
I
I
I
I
I
I
A1(111)-0 Ep= 3eV
I
I
I
I
30L Oz T= 120 K
b l:d
_< l-
m _z
I I I I I I I I I I I I
40 ~ ~ I ~ 120 ENERGY LOSS(m~)
I
0
I
I
I
I
I
I
I
I
I
I
I
I
I
I
20 40 60 80 100 120 140 ENERGY LOSS (meV)
Fig. 3. HREELspectra for oxygen doses equivalent to about I monolayerof oxygen. Parameter is the time after dosing, a) is for a sample temperature of T = 300 K and b) for T = 120 K.
DISCUSSION In f i g . 4 low-coverage spectra are reproduced together with our models for the chemisorption state at the three temperatures. The 300 K spectrum at 60 L is for
the
complete monolayer. At
higher coverages the
105 meV-loss becomes
perceptible, indicating oxldic bond formation as discussed below. Thls assignment of the menola.yer is in agreement wlth our earlier ARUPS measurements (ref. 5) where the ARUP spectra were smeared out between 30 and 60 L. There is a general tendency also reported In the literature that the sticking coefficients become smaller i f the surface becomes mere nearly perfect. At 120 K and 20 K an oxygen
179
monolayer cannot be defined since the sticking coefficient varies and diffusion into the bulk occurs from the very beginning.
0 CHEMISORBED OXYGEN ,,CK.OXI DIC OXYGEN
AI {111)-O Ep=3eV
T=20K
Oz-DOSE
///9//._)//////
Fig. 4. HREEL spectra for low oxygen doses as indicated for three different models to explain the spectra, as discussed in the text, are sketched schematically.
T=120K /////////////
50 Z
60
I 0 1
I
I
I
I
I
I
I
60 80 100 120 1}.0' 150 ENERGY LOSS (rneV)
From the 300 K spectra we conclude that the chemisoprtion gives rise to losses in
the 65 to 80 meV region.
temperatures or at
higher doses is
The loss at due to
105 meV found at
lower
an oxidic species. The l a t t e r
conclusion is in agreement with the measurements of Chen et al. (ref. 7) which Found a correlation between the 105 meV peak and the chemically shifted Auger LVV l i n e . This result found for a sample temperature of 135 K can certainly be generalized to 300 K and 20 K. Furthermore, from f i g . 2 i t becomes evident that for a l l temperatures studied there is more than one loss from the very beginning. At 300 K there is very l i k e l y more than one loss under the broad peak. Our data are compatible with three peaks which mostly are not resolved. During different oxidation stages their relative weight can change, thus giving rlse to quite different peak shapes and even peak shifts. At 300 K these different oxygen sites have to be assigned to overlayer oxygen at 65 meV and to underlayer oxygen at s l i g h t l y higher energies. According to f i g . I there are two underlayer sites (tB and OC) which are easy to reach for the oxygen and could explain two losses for the underlayer oxygen. I t is interesting to note that only the tB site would be compatible with the EXAFS result of Norman et al. (ref. 4) which found an O-Al-distance of 1.75 A.
180 This is exactly the value of the tetraeder site. I t should be mentioned, however, that they found 1.75 A for both the chemisorbed phase and the oxidic phase which were characterised by a chemical shift of 1.4 eV and 2.7 eV, respectively, for the Al 2p level. From LEED investigations (ref. 8) and from our earlier ARUPS study (ref. 5) we concluded that the 0 ( l x l ) layer grows in islands. In the ARUP spectra the twodimensional band structure appeared already at a coverage of 0.1 of a monolayer. Thus, at 300 K t i g h t l y packed islands b u i l t up having chemisorbed oxygen above and below the Al surface. The double layer islands are thought to be rather stable, preventing an easy Al-bond breaking to form oxidic species which give rise to the 105 meV loss. Also the calculations of Bylander and Kleinman (ref. 9) indicate that the oxygen double-layer is the energetically most favourable configuration at 300 K. Furthermore, i t
is interesting to note that in a He scattering experiment the
Rayleigh mode of the chemisorbed monolayer was shifted to higher energy with respect to the clean AI(111) surface, indicating a greater stiffness of the oxygen Al surface layer (ref. 10). I t could well be that this is a property of the proposed oxygen double-layer. The calculation of Strong et al.
(ref.
2),
which generally supports the double-layer island model, should be revised in the l i g h t of their own latest 300 K spectra (ref. 3) which are in very good agreement with ours. Further support of the oxygen double layer model for the chemisorbed state can be lent by a recent highly resolved synchrotron study of the chemical shifted Al 2p core-hole (ref.
11) The chemical shift of 1.4 eV known to be due to
chemisorbed oxygen (ref. 12) was resolved by these authors into three losses at 0.49, 0.97 and 1.46 eV. At 120 K the double-layer islands are not formed since only the above-surface oxygen peak at 65 meV is found together with a distribution of
subsurface
chemisorbed oxygen including oxidic species. The sticking coefficient is smaller by a factor of about 5. This indicates that the borderline of the double-layer islands may be quite favourable for the dissociation of the incoming 02 molecules or that the incorporation of the subsurface oxygen is somewhat hindered by the greater stiffness of the Al lattice at 120 K. The appearanceof the 105 meV loss also f i t s quite nicely into this picture. I f part of the oxygen is incorporated apart from the borderline of the doube-layerislands i t
can more easily break
bonds because the double-layer may cause an especially great stiffness of the surface layer. At 120 K the oxygen atoms are mobile enough to form islands of chemisorbed oxygen, as indicated by the well defined peak at 65 meV. These single-layerislands are less stable than the double-layer islands as found by our time-dependent measurements. The main argument for the island growth is drawn from the 20 K
181
experiment in which only weak intensity is found at 65 meV, and there is good reason
to
believe
that
the
surface
migration
of
oxygen is
prevented.
Interestingly, our lZO K results are basically in agreement with the results of Chen et al. (ref. 7). By changing the interpretation of these authors s l i g h t l y we think that t h e i r data are in complete agreement with our view that the 105-120 meV losses are correlated with the chemically shifted Al LVV Auger line, i . e . with A1203 formation. At 135 K their data indicate, as ours do, that the subsurface species is mainly oxldic and not the chemisorbed one found at 300 K. At 20 K the loss spectrum has completely changed again. About 90% of the oxygen is incorporated immediately. The sticking coefficient is even larger than at 300 K. At 20 K the incoming 02 molecules can be bound in a physisorption precursor
state.
From this
state they can migrate to
a site
suitable for
dissociation. During this process they get accommodated so that the dissociated oxygen atoms may no longer be able to diffuse to two-dimenslonal islands. Furthermore, the 02 precursor i t s e l f may be the main impediment for the oxygen atoms to accumulate into islands. For single atoms at the surface, not being bound into a special double-layer or single-layer island configuration, i t
is
much easier to diffuse into the bulk and also to break Al bonds. In order to explain our measurements at 20 K one may consider the following p o s s i b i l i t y . The diffusion of the oxidic species is certainly faster at 300 K than at 20 K. Therefore, one may ask whether at 20 K oxygen accumulates at the surface whereas at 300 K i t diffuses so quickly into the bulk that i t cannot be observed by HREELS. The experiment shown in f i g . 3a excludes such a p o s s i b i l i t y . At 300 K the chemisorbed oxygen is transferred into the oxidic one which does not diffuse deep into the bulk since i t is well observed by HREELS. For the high oxygen doses the main peak is at 105 meV. I t has occasional shoulders up to 120 meV and is accompanied by a weaker loss at about 40 meV. From HREELS this state is not very well defined. I t
follows also from the other
techniques that i t is a compiicated transition state to the f i n a l bulk oxide. In EXAFS the same O-Al-dlstance~of 1.75 A is found as for the chemisorption state. This value is different fromI the 1.88 A found for the heavily oxidized surface &
and also from the 1.915 A far ~-A1203 (ref. 4). In photoemission i t exhibits a chemical s h i f t of 2.7 eV of the Al 2p core level, but this peak is rather broad and may contain several different Al-O configurations. The increase of
the
sticking coefficient in our ~ experiment indicates that the surface breaks-up, becomes rough, and can no Ibnger be discussed in terms of atomic sites at a single crystalline surface. Finally, we comment on the layer of physisorbed 02. These results, for which we have not presented a figure here, are in very good agreement with those of others on Ag films (ref. 13). The only difference is that the peaks for dissociated oxygen are found before the moltllayer grows. Again overtones of the
182
02 stretching mode are observed. The Birge-Sponer plot gives values similar to the gas phase. CONCLUSION At
300 K oxygen chemisorbs on AI(111)
in
oxygen double-layer islands
containing oxygen atoms above and below the f i r s t Al layer. Two or three losses are found due to above-surface (65 meV) and subsurface (70-80 meV) oxygen. These double-layer islands are rather stable, slowing down the diffusion into bulk and the Al-bond breaking. At 120 K no double-layer islands but, rather, single-layer islands are formed and the subsurface oxygen is mainly oxidic (105-120 meV). At 20 K almost no above-surface oxygen is
found. The possib]ity of
a
physisorption precursor state at this temperature seems to be of great influence. It
increases greatly the sticking coefficient and seems to prevent the oxygen
island formation. For isolated oxygen atoms i t seems to be much easier to break Al bonds and diffuse into the bulk even at this low temperature than for oxygen incorporated in the r i g i d double-layer at 300 K. The small sticking coefficient for 02 at 300 K and the stable oxygen doublelayer islands renders the bulk oxidation more e f f i c i e n t at 120 and even at 20 K than at 300 K. REFERENCES 1 2 3 4 5 6 7
8 9 10 11 12 13
I . P . Batra and L. Kleinman, J. Electr. Spect. 33 (1984) 175. R. L. Strong, B. Firey, F. W. de Wette and J. L. Erskine, Phys. Rev. B 26 (1982) 3483. R. L. Strong, B. Firey, F. W. de Wette and O. L. Erskine, J. Electron. Spectr. 29 (1983) 187. D. Norman, S. Brennan, R. Jaeger and J. St6hr, Surf. Sci. 105 (1981) L 297. P. Hofmann, C. v. Muschwitz, K. Horn, K. Jacobi, A. M. Bradshaw, K. Kambe and M. Scheffler, Surf. Sci. 89 (1979) 327. R. Unwin, W. Stenzel, A. Garbout, H. Conrad and F. M. Hoffmann, Rev. Sci. Instrum. 55 (1984) 1809. O . A . Chen, J. E. Crowell and J. T. Yates, Jr., J. Chem. Phys. 84 (1986) 5906. C . W . B . Martinson, S. A. Flodstr6m, J. Rundgren and P. Westrin, Surf. Sci. 89 (1979) 102. D.M. Bylander and L. Kleinman, Phys. Rev. B 30 (1984) 2997. A. Lock, J. P. Toenntes and Ch. WOll, to be published. C. F. McConvtlle, D. L. Seymour, S. Bao and D. P. Woodruff, Surf. Sci. (ECOSS-9), to be published. S. A. Flodstr6m, C. W. B. Martinson, R. Z. Bachrach, S. B. Hagstr6m and R. S. Bauer, Phys. Rev. Lett. 40 (1978) 907. D. Schmetsser, O. E. Demuth and Ph. Avouris, Phys. Rev. B 26 (1982) 4857.
175
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
HREELS STUDYOF THE OXIDATIONOF AL(111) BETWEEN300 AND 20 K
C. ASTALDI, P. GENGand K. JACOBI Frltz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-IO00 Berlin 33, West Germany
SUMK~RY The oxidation of AI(111) was studied by HREELS for sample temperatures of 300, 120 and 20 K and for oxygen coverages below and above the monolayer coverage. The experimental results are explained within the following model: At 300 K oxygen is chemlsorbed both above and below the f i r s t Al layer, giving rise to losses between 65 and 80 meV. This oxygen double-layer grows In islands and stabilises the oxygen against diffusion into the bulk. At 120 K chemisorbed oxygen above the Al surface and a distribution of subsurface oxygen including oxidlc species with losses between 105 and 120 meV are found. At 20 K abovesurface oxygen is no longer stable enough to be measured by HREELS; sticking coefficient and rate of oxidation are increased.
INTRODUCTION A number of studies on the oxidation of A1 have recently been reviewed c r i t i c a l l y and in detail (ref. 1). In concentrating on the f i r s t oxidation step we address the open question of whether the oxygen chemisorbs in a single site above or in two different sites above and below the Al surface. This question has been raised by Strong et al. (ref. 2) through an HREELS experiment on A l ( l l l ) . For a sample temq~erature of 300 K and for small oxygen doses they found two losses at 80 and 105 meV. Later (ref. 3) they published spectra for which the 105 meV loss was only weak whereas the former 80 meV loss was resolved into two losses at 60 and 73 eV. Thus, disregarding the different loss energies for the moment, the important new result was the observation of two losses, l . e .
two
binding sites for the oxygen atom even in the f i r s t chemisorptlon step. Strong et al. (ref. 2) discussed their results in terms of high-symmetry binding sites as sketched In f l g .
I. This figure also indicates that the EXAFS measurements of
Norman et al. (ref. 4) could not c l a r i f y the situation since the sites C, tB and tA result in the same O-Al distance and could not be separated in t h e i r experiment. The oxygen Al adsorption system is also con~llcated with respect to tlme-dependent effects indicating the tendency of oxygen to diffuse into and to react wlth the bulk. Thus, In our earlier ARUPS study (ref. 5) we found the
0368-2048/87/$03.50
© 1987 Elsevier Science Publishers B.V.
176
~
P VIEW
BOp
?C
x I I I OtA J. . .
tB'kv
I I ,L . .
~ "1"
I ?oC
A-LAYER
-
I x I I I I
,, -
B-LAYER
Fig. 1. Scheme of the highsymmetry sites for chemisorbed oxygen at the AI(111) surface according to Strong et al. (ref. 2). Full dots stand for Al atoms. B and C give oxygen sites above the Al surface; tA and tB indicate tetrahedrally coordinated oxygen subsurface sites; oC is an octahedrally coordinated subsurface oxygen site.
C-LAYER S I D E VIEW
ordered chemisorbed layer to decompose within one hour. We found i t desirable to repeat the HREELSexperiment with variation of the sample temperature in order to eventually reduce the time-dependent effects or to stabilise the chemisorbed oxygen above the Al surface.
EXPERIMENT The experiment was performed in an UHV chamber with a base pressure below 1.10-10 mbar. The AI(111) sample was prepared by standard techniques including argon-ion sputtering and annealing at 700 K. Prior to insertion into the chamber i t was electropolished. The sample was cooled using liquid N2 and He. The temperature was measured with a Chromel-Constantan-thermocouple. The lowest temperatures which could be attained were estimated to be (120±5)K for N2 and (20±5)K for He cooling. The latter value was confirmed by the observation of physisorbed 02 multilayers. For sample temperatures of 300 and 120 K oxygen was introduced using a doser. The pressure in front of the doser was estimated to be five times higher than in the chamber. The HREEL spectrometer is described elsewhere (ref. 6). The spectra were measured with an energy resolution of 10 meV in the specular beam. Angle of incidence was 55 degree with respect to the sample normal.
177
RESULTS Spectra of different oxygen exposures at 300, 120 and 20 K are shown in f i g . 2. For the 300 K 60 L exposure the spectrum exhibits a loss peak at 65 meV with a shoulder at about 80 meV. At 120 L a feature at 105 meV develops which grows with dose. At 270 L the main peak is found at 73 meV. At 120 K the f i r s t oxygen dose induces a different loss spectrum. Again, there is a peak at 65 meV but no shoulder at 80 meV is found. The feature at 105 meV is visible already at this low dose and grows faster with dose than in the 300 K measurements. The feature at 65 meV changes similarly with dose as in the 300 K case. For a 20 K 2 L exposure very broad features around 70 and 100 meV are obtained. Already at 20 L the 105 meV loss is the dominant feature. There is a large increase of intensity for the higher doses at 120 and 20 K correlated with larger amounts of oxidic oxygen at 105 meV. A higher amount of oxidic species means a more irregular surface and therefore a higher efficiency in dissociating 02.
AI (111)-0
Ep=3eV
Oz(L) T=20K
20; \
// '~
i\ F\T= 120K
=
a
so,/ '~ f
T= 300 K
~"!
ts~o
3Xx iI
~120 \
l
~
0 20 ~0 60 80100120 ENERGYLOSS(meV)
Fig. 2. HREEL spectra for the clean AI(111) surface and after dosing with different amounts of oxygen in units of Langmulr (I L & 10-6 Torr.s). The sample temperature T is indicated. The energy of the incident electrons was 3 eV.
178
Fig. 3 shows how the HREEL spectra, which correspond to the lowest oxygen dose in f i g . 2, change with time at 300 K and at 120 K, respectively. At 300 K ( f i g . 3a) a weak structure at about 100 meV is grown after two hours. Within the same time the changes of the 120 K spectrum (fig. 3b) becomemore evident: the losses at 70 and 100 meV have become equally high. This indicates the surprising result that the chemisorption state is more stable at 300 than at 120 K. I t is interesting to note that for the 20 h curve in f i g . 3a a low-lying loss at 43 meV appears. This peak is observed also in f i g . 3b at 2 h and for the high oxygen doses in f i g . 2. This peak is always connected to a high intensity of the 105 meV-loss. l
l
l
l
l
l
l
l
AI(111)-O Ep=3~
l
l
l
l
I
~ L Oz T=~K
I
I
I
I
I
I
I
I
I
I
A1(111)-0 Ep= 3eV
I
I
I
I
30L Oz T= 120 K
b l:d
_< l-
m _z
I I I I I I I I I I I I
40 ~ ~ I ~ 120 ENERGY LOSS(m~)
I
0
I
I
I
I
I
I
I
I
I
I
I
I
I
I
20 40 60 80 100 120 140 ENERGY LOSS (meV)
Fig. 3. HREELspectra for oxygen doses equivalent to about I monolayerof oxygen. Parameter is the time after dosing, a) is for a sample temperature of T = 300 K and b) for T = 120 K.
DISCUSSION In f i g . 4 low-coverage spectra are reproduced together with our models for the chemisorption state at the three temperatures. The 300 K spectrum at 60 L is for
the
complete monolayer. At
higher coverages the
105 meV-loss becomes
perceptible, indicating oxldic bond formation as discussed below. Thls assignment of the menola.yer is in agreement wlth our earlier ARUPS measurements (ref. 5) where the ARUP spectra were smeared out between 30 and 60 L. There is a general tendency also reported In the literature that the sticking coefficients become smaller i f the surface becomes mere nearly perfect. At 120 K and 20 K an oxygen
179
monolayer cannot be defined since the sticking coefficient varies and diffusion into the bulk occurs from the very beginning.
0 CHEMISORBED OXYGEN ,,CK.OXI DIC OXYGEN
AI {111)-O Ep=3eV
T=20K
Oz-DOSE
///9//._)//////
Fig. 4. HREEL spectra for low oxygen doses as indicated for three different models to explain the spectra, as discussed in the text, are sketched schematically.
T=120K /////////////
50 Z
60
I 0 1
I
I
I
I
I
I
I
60 80 100 120 1}.0' 150 ENERGY LOSS (rneV)
From the 300 K spectra we conclude that the chemisoprtion gives rise to losses in
the 65 to 80 meV region.
temperatures or at
higher doses is
The loss at due to
105 meV found at
lower
an oxidic species. The l a t t e r
conclusion is in agreement with the measurements of Chen et al. (ref. 7) which Found a correlation between the 105 meV peak and the chemically shifted Auger LVV l i n e . This result found for a sample temperature of 135 K can certainly be generalized to 300 K and 20 K. Furthermore, from f i g . 2 i t becomes evident that for a l l temperatures studied there is more than one loss from the very beginning. At 300 K there is very l i k e l y more than one loss under the broad peak. Our data are compatible with three peaks which mostly are not resolved. During different oxidation stages their relative weight can change, thus giving rlse to quite different peak shapes and even peak shifts. At 300 K these different oxygen sites have to be assigned to overlayer oxygen at 65 meV and to underlayer oxygen at s l i g h t l y higher energies. According to f i g . I there are two underlayer sites (tB and OC) which are easy to reach for the oxygen and could explain two losses for the underlayer oxygen. I t is interesting to note that only the tB site would be compatible with the EXAFS result of Norman et al. (ref. 4) which found an O-Al-distance of 1.75 A.
180 This is exactly the value of the tetraeder site. I t should be mentioned, however, that they found 1.75 A for both the chemisorbed phase and the oxidic phase which were characterised by a chemical shift of 1.4 eV and 2.7 eV, respectively, for the Al 2p level. From LEED investigations (ref. 8) and from our earlier ARUPS study (ref. 5) we concluded that the 0 ( l x l ) layer grows in islands. In the ARUP spectra the twodimensional band structure appeared already at a coverage of 0.1 of a monolayer. Thus, at 300 K t i g h t l y packed islands b u i l t up having chemisorbed oxygen above and below the Al surface. The double layer islands are thought to be rather stable, preventing an easy Al-bond breaking to form oxidic species which give rise to the 105 meV loss. Also the calculations of Bylander and Kleinman (ref. 9) indicate that the oxygen double-layer is the energetically most favourable configuration at 300 K. Furthermore, i t
is interesting to note that in a He scattering experiment the
Rayleigh mode of the chemisorbed monolayer was shifted to higher energy with respect to the clean AI(111) surface, indicating a greater stiffness of the oxygen Al surface layer (ref. 10). I t could well be that this is a property of the proposed oxygen double-layer. The calculation of Strong et al.
(ref.
2),
which generally supports the double-layer island model, should be revised in the l i g h t of their own latest 300 K spectra (ref. 3) which are in very good agreement with ours. Further support of the oxygen double layer model for the chemisorbed state can be lent by a recent highly resolved synchrotron study of the chemical shifted Al 2p core-hole (ref.
11) The chemical shift of 1.4 eV known to be due to
chemisorbed oxygen (ref. 12) was resolved by these authors into three losses at 0.49, 0.97 and 1.46 eV. At 120 K the double-layer islands are not formed since only the above-surface oxygen peak at 65 meV is found together with a distribution of
subsurface
chemisorbed oxygen including oxidic species. The sticking coefficient is smaller by a factor of about 5. This indicates that the borderline of the double-layer islands may be quite favourable for the dissociation of the incoming 02 molecules or that the incorporation of the subsurface oxygen is somewhat hindered by the greater stiffness of the Al lattice at 120 K. The appearanceof the 105 meV loss also f i t s quite nicely into this picture. I f part of the oxygen is incorporated apart from the borderline of the doube-layerislands i t
can more easily break
bonds because the double-layer may cause an especially great stiffness of the surface layer. At 120 K the oxygen atoms are mobile enough to form islands of chemisorbed oxygen, as indicated by the well defined peak at 65 meV. These single-layerislands are less stable than the double-layer islands as found by our time-dependent measurements. The main argument for the island growth is drawn from the 20 K
181
experiment in which only weak intensity is found at 65 meV, and there is good reason
to
believe
that
the
surface
migration
of
oxygen is
prevented.
Interestingly, our lZO K results are basically in agreement with the results of Chen et al. (ref. 7). By changing the interpretation of these authors s l i g h t l y we think that t h e i r data are in complete agreement with our view that the 105-120 meV losses are correlated with the chemically shifted Al LVV Auger line, i . e . with A1203 formation. At 135 K their data indicate, as ours do, that the subsurface species is mainly oxldic and not the chemisorbed one found at 300 K. At 20 K the loss spectrum has completely changed again. About 90% of the oxygen is incorporated immediately. The sticking coefficient is even larger than at 300 K. At 20 K the incoming 02 molecules can be bound in a physisorption precursor
state.
From this
state they can migrate to
a site
suitable for
dissociation. During this process they get accommodated so that the dissociated oxygen atoms may no longer be able to diffuse to two-dimenslonal islands. Furthermore, the 02 precursor i t s e l f may be the main impediment for the oxygen atoms to accumulate into islands. For single atoms at the surface, not being bound into a special double-layer or single-layer island configuration, i t
is
much easier to diffuse into the bulk and also to break Al bonds. In order to explain our measurements at 20 K one may consider the following p o s s i b i l i t y . The diffusion of the oxidic species is certainly faster at 300 K than at 20 K. Therefore, one may ask whether at 20 K oxygen accumulates at the surface whereas at 300 K i t diffuses so quickly into the bulk that i t cannot be observed by HREELS. The experiment shown in f i g . 3a excludes such a p o s s i b i l i t y . At 300 K the chemisorbed oxygen is transferred into the oxidic one which does not diffuse deep into the bulk since i t is well observed by HREELS. For the high oxygen doses the main peak is at 105 meV. I t has occasional shoulders up to 120 meV and is accompanied by a weaker loss at about 40 meV. From HREELS this state is not very well defined. I t
follows also from the other
techniques that i t is a compiicated transition state to the f i n a l bulk oxide. In EXAFS the same O-Al-dlstance~of 1.75 A is found as for the chemisorption state. This value is different fromI the 1.88 A found for the heavily oxidized surface &
and also from the 1.915 A far ~-A1203 (ref. 4). In photoemission i t exhibits a chemical s h i f t of 2.7 eV of the Al 2p core level, but this peak is rather broad and may contain several different Al-O configurations. The increase of
the
sticking coefficient in our ~ experiment indicates that the surface breaks-up, becomes rough, and can no Ibnger be discussed in terms of atomic sites at a single crystalline surface. Finally, we comment on the layer of physisorbed 02. These results, for which we have not presented a figure here, are in very good agreement with those of others on Ag films (ref. 13). The only difference is that the peaks for dissociated oxygen are found before the moltllayer grows. Again overtones of the
182
02 stretching mode are observed. The Birge-Sponer plot gives values similar to the gas phase. CONCLUSION At
300 K oxygen chemisorbs on AI(111)
in
oxygen double-layer islands
containing oxygen atoms above and below the f i r s t Al layer. Two or three losses are found due to above-surface (65 meV) and subsurface (70-80 meV) oxygen. These double-layer islands are rather stable, slowing down the diffusion into bulk and the Al-bond breaking. At 120 K no double-layer islands but, rather, single-layer islands are formed and the subsurface oxygen is mainly oxidic (105-120 meV). At 20 K almost no above-surface oxygen is
found. The possib]ity of
a
physisorption precursor state at this temperature seems to be of great influence. It
increases greatly the sticking coefficient and seems to prevent the oxygen
island formation. For isolated oxygen atoms i t seems to be much easier to break Al bonds and diffuse into the bulk even at this low temperature than for oxygen incorporated in the r i g i d double-layer at 300 K. The small sticking coefficient for 02 at 300 K and the stable oxygen doublelayer islands renders the bulk oxidation more e f f i c i e n t at 120 and even at 20 K than at 300 K. REFERENCES 1 2 3 4 5 6 7
8 9 10 11 12 13
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