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An ISS-XPS study on the oxidation of Al(111); identification of stoichiometric and reduced oxide surfaces
233
Surface Science 157 (1985) 233-243 North-Holland, Amsterdam
AN ISS-XPS STUDY ON THE OXIDATION OF Al(lll); IDENTIFICATION OF STOICHIOMETRIC AND REDUCED SURFACES C. OCAL,
B. BASURCO
OXIDE
and S. FERRER
Departamento de Rsica Fundamental e Instituio de Fkicadel Estado Sblido, Universidad Auth~onoma de Madrid, Cantoblanco, 28049 - Madrid, Spain Received 31 August 1984; accepted for publication 27 December 1984
We have studied by ISS and XPS the initial stages of the 3D oxidation of Al(lll). We found that the incorporation of oxygen into the crystal bulk is well described by a second order process with an activation energy of 0.50*0.05 eV. XPS indicates that the chemical shift of the 2p emission upon oxidation varies from 2.6 to 3.7 eV depending on growth procedure and oxide thickness. The growth procedure also affects noticeably the surface composition. We have been able to prepare oxide surfaces with an important concentration of oxygen vacancies. The rate of growth, at low temperature of these defective oxides is several times larger than the rate exhibited by more perfect oxides. These results are discussed in the light of the Cabrera-Mott theory of oxidation of metals.
1. Introduction The early stages of the interaction of oxygen with Al(lll) have been extensively investigated since the photoemission study of Flodstrom et al. in 1978 [l]. This study showed the existence of an ordered (1 X 1) oxygen overlayer for the initial interaction of oxygen with Al(111). There is general agreement in the literature that this chemisorbed oxygen layer exists on Al(111) occupying threefold hollow sites. There is also agreement on the existence of another oxide-like state that increases in abundance with increasing oxygen coverage. This state is thought to correspond to an oxygen underlayer [2]. This oxide-like state is characterized by a chemical shift of 2.7 eV of the Al 2p level in contrast with the shift of 1.4 eV corresponding to the chemisorbed state. Both states may exist simultaneously although in general the chemisorbed state dominates at low oxygen exposures. Experiments also show that for heavy oxygen exposures the oxide layer closely resembles amorphous Al,O, [3]. The formation of this 3D oxide is accompanied by the disappearence of the LEED pattern. It is clear from the literature that most of the common surface techniques have been used to characterize the early stages of the oxidation of Al(111) but 0039-6028/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
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of AI(I I I)
surprisingly enough we have not found any ion scattering (1%) study. In addition, the studies on the 3D oxide formation are scarce [4,5]. In this paper we present a combined 1% and X-ray photoelectron spectroscopy (XPS) study concentrated mainly in investigating the formation and structure of the 3D oxide. As low energy ISS is an extremely surface sensitive technique, we have used it to study the penetration to the interior of the Al crystal of the adsorbed oxygen atoms, initially on the Al(111) surface, when the crystal temperature is increased from 80 K to higher temperatures. We have found an activation energy of 0.50 + 0.05 eV for oxygen incorporation into the bulk of the crystal. We have also studied the surface composition and reactivity of 3D oxide layers of thickness ranging from 3 to 18 atomic layers. For an intermediate oxide thickness (4-5 oxide layers) we have found that the surface composition of the oxide monitored by ISS may vary noticeably depending on the growth procedure. Schematically we have observed that the oxygen-aluminum concentration ratio at the surface can vary from values close to those corresponding to the stoichiometry of the Al,O,, to values more than two times smaller that are indicative of an important concentration of oxygen vacancies at the surface of the oxide. These two types of oxide, so different when viewed with ISS, showed identical XPS spectra. The ulterior growth of these two limiting cases of surface oxides is clearly different. The defective oxides grow faster than the stoichiometric ones. These results are discussed in the view of the Cabrera-Mott theory of oxidation of metals at low temperatures.
2. Experimental The experiments were performed in a standard ion pumped ultra-highvacuum system (Leybold-Heraeus) equipped with a Mg X-ray source, an hemispherical analyzer, a 3 kV ion gun and a residual gas analyzer. The base as pressure was 3 X lo- lo Torr . The Al crystal face was of (111) orientation determined by the Laue technique. The crystal temperature was measured with a Chromel-Alumel thermocouple in mechanical contact with the crystal. A liquid nitrogen loop allowed to cool the crystal down to 80 K, and a tungsten filament was used for radiative heating. The crystal was cleaned by cycles of Ar bombardment and annealing to 800 K. Surface cleanliness was checked with XPS. Oxygen was introduced in the system through a variable leak valve. The photoemission spectra shown in this paper were recorded at the constant resolution mode of the analyzer. Ion scattering spectra were performed with 500 eV He ions. The purity of the gas was controlled with the mass spectro-meter. The scattering angle was 130”. The ion dose on the crystal was measured to be 6 X 10” ions cm-2 s- ‘. While recording ISS spectra the analyzer was kept in the AE/E constant mode. We found some sputtering of adsorbed oxygen due to the impinging He ions. Its effect on the value of the activation energy of
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the penetration process (see section 3.1) was checked to be negligible. On the kinetic data, the sputtering was negligible for the experiments at high temperatures but a small correction was introduced in the low temperature data to account for it.
3. Results and discussion 3.1. Initial incorporation
of the adsorbed oxygen into the bulk of the crystal
After cleaning and annealing at 800 K the Al(111) surface, the ISS spectra consisted of a single peak corresponding to the surface aluminum atoms as depicted in fig. 1. When the crystal was cooled at 80 K and exposed to 500 L of oxygen, a new peak due to 0 appeared as shown in the upper trace of fig. 1. The total scattered ion signal is clearly diminished and a peak at 0.48 which corresponds to oxygen atoms on the surface is visible. For the low ion energies utilized, the first layer scattering accounts for about 95% of the intensity of the spectrum as has been shown by Davis and Carver [6]. Therefore the spectra of fig. 1 are fairly indicative of the surface composition and structure. If the Al crystal is heavily oxidized up to 15-20 layers (see below), the corresponding O/Al peak intensity ratio in the ISS spectrum is 1.5. This result is in
E,=SOO eV 8 =130
I
0
Al I
I
-;Jk
500L
0,
80K
3
m
x2.5
x .Z F 0) E
.l 04
0.6
0.8
E/E,
Fig. 1. Low energy ion scattering spectra of clean Al(111) (bottom) and clean Al(111) exposed to 500 L of oxygen at 80 K (top).
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agreement with a previous study in a polycrystalline Al sample oxidized by exposure to the air [7]. The fact that the value of the O/Al ISS ratio coincides with the O/Al ratio of the stoichiometric formula unit of Al,O, is fortuitous since the ISS intensities for a fixed experimental set up depend both on chemical composition and surface structure. In the discussion that follows we will assume for simplicity that there are no important changes in the atomic structure of the different oxide surfaces and we will utilize the O/Al peak ratio to monitor the chemical composition of the oxide surfaces. When the Al surface, exposed at 500 L of oxygen at 80 K, is warmed for a certain time at 400-700 K and then cooled rapidly below room temperature, the corresponding ISS spectrum shows an increase of the Al peak intensity and a decrease of the oxygen signal. This is indicative of oxygen incorporation into the bulk of the crystal since it is known that no desorption to the gas phase takes place. As the intensity of the oxygen peak in the spectrum is proportional to the oxygen concentration at the surface, by measuring the decrease of the oxygen peak intensity as a function of time for a fixed annealing temperature we can obtain the activation energy for penetration of oxygen into the crystal bulk. We performed experiments at 539 and 683 K for annealing times t ranging from 10 to 1500 s. If the incorporation of oxygen into the bulk of the crystal were the result of a random-walk diffusion process, the intensity Z, of the oxygen peak in the ISS spectrum after keeping the crystal at a pre-selected temperature during t seconds, should decrease as tr’/* [8,9]. Our data did not obey this behaviour at all, indicating that the incorporation of oxygen was not the result of a random-walk process. Good agreement with the experiments was obtained by considering the incorporation of oxygen as the result of the following elemental chemical reaction: 0,” + where jumps atom state above
i
+
(0
-
i)incOrporatd,
i represents an interstitial site, i.e., an oxygen atom on the surface, O,,, to an interstitial non-occupied site in the second layer resulting in an incorporated at the subsurface. This atomic structure of the oxide-like has been previously proposed by Stohr et al. [2]. The rate law of the reaction is
d[%,]/dt
=
-k[Q,,l [iI,
where as usually the brackets denote concentration and k is the velocity constant. We will assume that [i], = [O,,], for t = 0 and t + 0 which gives upon integration [%I,’
-[Go,],::
= kt,
which corresponds to a linear dependence with time of the reciprocal of the intensity of the oxygen peak in the ISS spectrum. Fig. 2 displays the experimental data which exhibit a good linear behaviour for two independent
l!J
50
:
:
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C. Oral et al. / ISS- XPS s&y
:
1q0
:
:
:
:
lS0
:
t
I
E,= 0~50~0-05 eV
-
0.04
t; 100
500
1000
1500
Time (SC.) Fig. 2. Reciprocal of the intensity of the oxygen signal in the ion scattering spectrum as a function of time for two different temperatures: 685 K (time scale at the top) and 539 K (time scale at the bottom).
experiments at 685 and 539 K. The straight lines are the best fit to the data. From their slopes we obtain k. By writing k = v exp( - E,/kT) where v is the pre-exponential that we take as lOI s-’ and E, is the activation energy barrier for oxygen penetration, we obtain E, = 0.50 + 0.05 eV per atom. 3.2. Growth of the 30 oxide Fig. 3 shows a series of photoemission spectra after various oxygen treatments of the Al(lll) surface. Curve a corresponds to the Al 2p emission (at 72.65 eV) for the clean surface. Curves b and c display the resulting spectra after oxygen exposures of 500 L at room temperature and lo5 L and 80 K respectively. In spectrum b a weak, oxygen-induced emission at the high BE side of the metal peak is visible. Its BE, relative to the metal peak, is 2.4-2.8 eV and corresponds to the Al 2p oxide emission previously reported [l]. In curve c this oxide emission is clearly visible at 2.6 eV. Now, we will describe the procedure that we used to grow relatively thick oxides. In order to grow 3D oxides, the clean Al(lll) crystal was annealed to 700 K, kept at this temperature and subsequently was exposed to an oxygen partial pressure of 2 X 10m6 Torr during periods of time ranging from 5 min to 1 h. After this treatment the oxygen gas was pumped down and after obtaining a UHV environment, the crystal was cooled to room temperature. By increas-
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study on oxidation ofAi(ll
I)
90’ -18 layers L2’ -9 layers 35’
-8 layers
16’ -L ‘I? t
12’ - 3 314t
.a i
--+I--’
b
’
80
70
76
Bmding Energy
9’
-31
L’ -1 l/2 1
,0rL4
8OK
500 L Ot r.t.
;
clean
72
( cV)
Fig. 3. Photoemission spectra of the Al 2p electronic level: (a) clean surface; (b) surface exposed to 500 L of oxygen at room temperature; (c) surface exposed to lo5 L of oxygen at 80 K. Spectra from (d) to Cj) show the evolution of both, intensity and chemical shift of the higher binding energy emission due to the growing of the oxide film.
ing the oxygen exposure we obtained oxide films of increasing thickness. Curves d to j in fig. 3 show the corresponding photoemission spectra for a series of oxide films. The duration of the oxygen treatment is also indicated in the figure. As can be observed the intensity of the emission of the Al 2p electronic level in the metal decreases while the corresponding one in the oxide increases. We may obtain an approximate value of the thickness of the oxide films by measuring the intensities of the oxide emissions relative to the metal ones and by using an experimentally determined mean free path of the photoelectrons in the solid [lo]. This results in an attenuation factor of 20% for the decrease in intensity of the Al 2p emission due to one atomic layer of oxide. The calculated oxide thickness in monolayers are indiated in the figure. Its precision is estimated to be a 20%.
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As it is clearly noticeable in fig. 3, the chemical shift of the 3D oxide is not constant but varies from 2.6 to 3.7 eV in going from curves c to j. That is, increasing oxide thickness results in a larger chemical shift of the oxide. Previously published data by Barre [ll] indicate a chemical shift for the Al 2p emission of 2.7 eV for an aluminum film evaporated in vacuum and oxidized at room temperature in 1 atm of oxygen during 30 min. The metal and oxide relative intensities in the corresponding XPS spectrum are similar to those of curve g in fig. 3 where the oxide emission appears at 3.2 eV from the metallic one. The agreement with our data is rather poor. We believe that this is due to the preparation procedure of the oxide as will be discussed later. In addition, one sees frequently in the literature that the value of the chemical shift of Al in a-alumina is taken from the data of Barre. Our data show that the situation is not so simple since the shift depends on oxide thickness and preparation procedure. 3.3. The “stoichiometric”
and “reduced” oxides
We are going to show now, how two slightly different oxidation schemes lead to important differences in the surface composition of the resulting oxide. The oxides grown as mentioned in section 3.2 (curves d to j in fig. 3), displayed an oxygen-aluminum peak intensities ratio in the ISS spectrum which varied continuously from 0.5 (for the oxide d) to 1.5 (for the oxide j). This indicates a tendency to the stoichiometry of the formula unit of Al,O, for increasing oxide thickness. If, instead of the oxidation scheme described in section 3.2, the clean Al(111) surface was exposed to 500 L of oxygen at room temperature and subsequently (after pumping the 0, gas) the oxidation procedure mentioned in section 3.2 was reproduced, i.e., exposed to 2 x 10m6 Torr of oxygen during several minutes at 700 K, then pumped down the oxygen gas and cooled to room temperature, the resulting oxide surface was always much richer in oxygen (viewed with ISS) that an oxide of the same thickness (monitored by XPS) grown as mentioned in section 3.2. Fig. 4 illustrates this result. The bottom curves are the photoemission spectra which are virtually identical and correspond to oxides - 4 layers thick. The top curves are the ISS spectra for an oxide grown as mentioned in section 3.2 (right) and for an oxide grown as mentioned above. The oxygen-aluminum peak intensity ratios in the ISS spectra are 0.5 (right) and 1.1 (left). For the sake of clarity we will design both types of oxide surfaces as reduced and stoichiometric respectively. Another important point that we want to mention concerns the time interval involved to grow the oxides characterized by the spectra in fig. 4. Whereas for the reduced oxide the treatment at 2 x lop6 Torr of 0, was only of 12 min, for the stoichiometric oxide it was 3 times longer. This result shows that the annealing of a room-temperature adsorbed oxygen layer, inhibits the ulterior
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study an oxidationof A&ill)
stoichiomotric
04
0.6
r,r
o-4
E/E,
0.8
0%
Al 2p .
,,.‘.
-it
74
76 B
E {eV)
72
70
t
76
74 B
Al 2p :. :
72
70
E (ev)
Fig. 4. XPS and tSS spectra for an stoichiometric and a reduced (see text) oxide films of a thickness of - 4 layers. Note the difference in the KS spectra for virtually identical XPS spectra.
oxide growth. Probably this occurs since the annealing of the adsorbed layer creates a sort of surface network of oxide as has been previously suggested [15]. This network acts as a passivation layer for the ulterior oxide growth by preventing the penetration of oxygen ad-atoms into the crystal bulk. In fact we think that this oxide network is not the result of a random-walk diffusion of oxygen into the crystal bulk but that rather corresponds to the picture presented in section 3.1 where a solid state surface reaction is taking place producing a layer of subsurface oxide. Ulterior oxidation results in a 3D oxide surface which is richer in oxygen that the reduced oxide where the network is not formed. The difference in oxidation rates at low temperature of the reduced and stoichiometric oxides is also illustrated in the results of the following experiments. If the oxide surfaces characterized by the spectra of fig. 4 are cooled down to 80 K and subsequently they are exposed to 10’ L of oxygen, the resulting increase in the intensity of the oxide emission in the XPS spectra correspond to 0.75 atomic layers for the reduced oxide and to - 0.1 layers for the stoichiometric. The same experiment repeated in a more stoichiometric oxide of similar thickness (O/Al ratio of 1.3 instead of 1.1) gives an increase of the oxide photoemission peak corresponding to - 0.06 layers. For comparison, on a clean Al(lll) surface at 80 K, TO5 L of oxygen exposure result in an oxide XPS emission corresponding to 1 monolayer.
C. Ucaf et al. / IS.%XPS study on oxidationof A&III)
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We considered the possibility that the surfaces of the reduced oxides prepared as mentioned in section 3.2 were not microscopically homogeneous but consisted of islands of oxide with the stoichiometry of the alumina and patches of clean metal. If this were the case the oxygen uptake at 80 K would be essentially due to the adsorption of oxygen in the metal patches since as we have seen, the uptake in oxides with the surface stoichiometry of the alumina is very small. We checked the validity of this model and we are going to show that is not compatible with our data. Let us imagine that 6 and 1 - 8 are the surface coverages of the stoichiometric oxide islands and metal patches respectively for the surface of a reduced oxide, 4 atomic layers thick, with an O-Al ratio of 0.5 in the ISS spectrum. Let ZA, be the intensity in the ISS spectrum of the Al peak for the clean Al(ll1) surface. When the crystal is heavily oxidized and exhibits, in the ISS spectrum the stoichiometry of the alumina, the intensity of the aluminum peak, I$ is 5.9 times smaller than the corresponding one for the clean crystal, i.e., Z,;l/Zi; = 5.9. For the reduced oxide we have Z$ZL, = 0.5 where the superscript r states for reduced. The following relations should hold
z;, = (1 - @)I;
+ ezj; = (5.9 - 4.90)Z,!,
then Z&‘ZLi = (e/5.9 - 4.98) (Z$/Zi;). The last term in this expression is 1.5 according to the experiments. By substitution we get 6Z= 0.75 which means that the ISS data for the reduced oxide are compatible with a surface consisting of a 25% of metal patches. If this were the case, the experiments of lo5 L of exposure at 80 K on the reduced oxide should be interpreted essentially as the result of the adsorption on the metal patches. As the oxygen uptake in the clean metal surface is 1 monolayer, the uptake on the reduced oxide surface should be 1 X 0.25 monolayers plus the uptake in the oxide islands which should amount, at the most, 0.1 X 0.75 monolayers. The resulting total uptake should be then less than 0.3 monolayers that should be equal to the experimental value of 0.75 obtained for the oxygen uptake in the reduced oxide. The above simple arguments show that the experimental data are not compatible with the model of the inhomogeneous surface at least in a first appro~mation. A discussion on the validity of this type of analysis in ion scattering experiments can be seen in the paper by Bertolini et al. [13]. Our interpretation is therefore that the surface is microscopically homogeneous and contains an important concentration of anion vacancies. ISS indicated that after the oxygen exposure at 80 K, most of these surface vacancies were filled up with oxygen atoms. SEXAFS investigations by Norman et al. [3] indicate that the O-Al nearest neighbour distance in the surface aluminum oxide resulting of oxidizing Al(111) surfaces or polycrystalline samples, corresponds to an average atomic
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IIj
coordination characteristic of amorphous oxides which have a number of oxygen nearest neighbours to the Al atoms between 3 (initial oxide) to 6 (corundum). Also, studies on the structure of thin, anodic, alumina films investigated by the X-ray radial distribution function [12] show that the observed scattering intensities of the films (which are amorphous) are quantitatively well described by a conformation involving oxygen vacancies. Previous studies on the oxidation of Al(111) [S] show that the growth of thin oxide layers follows a logarithmic law according to the predictions of the Cabrera-Mott theory of oxidation of metals at low temperatures [14]. This theory states that the growth of very thin films of oxides on its metal support occurs via an ionic diffusion assisted by an electric field in the oxide layer. This field is originated by an anionic layer on the surface of the oxide and a cationic counterpart in the metal-oxide interface. These charged layers result from the establishment of the electronic equilibrium between the substrate metal and the adsorbed oxygen. Our data indicate that defective oxides grow more readily than non-defective ones. This means that surface defects lower the activation energy for ion diffusion through the oxide layer. Finally we want to make a comment on the discrepancy on the value of the chemical shift of the Al 2p emission reported by Barre [11J and by us (see section 3.2). We performed experiments which showed that a large oxygen exposure of the stoichiometric oxide (4 layers thick) at 80 K, caused an additional shift of 0.4 eV in the oxide photoemission signal towards smaller BE. The resulting value of the chemical shift of the Al 2p emission is then 2.9 eV which is in rather good agreement with the value of 2.7 eV reported by Barre. We think that in this experiment the high exposure and low temperature utilized, caused the stabilization of an anionic oxygen layer on the surface according to the Cabrera-Mott theory. The electrostatic potential difference originated by this charged layer and its cationic counterpart in the metal, caused a shift to lower BE of the oxide emission relative to the metal one in the photoemission spectrum [16]. If, on the contrary, the oxide is grown in a high vacuum environment and at high temperature, the anionic layer on the oxide is not stabilized and the mentioned shift does not occur. These experiments are now in progress and will be reported in detail in the near future. 4. Conciusions The main conclusions of this paper are: (i) The incorporation into the crystal bulk of oxygen adsorbed on the Al(lll) surface, is well described by a second order process involving empty sites in the subsurface. The resulting activation energy for incorporation is 0.50 _t 0.05 eV. (ii) The chemical shift of the 2p photoemission line upon oxidation of the Al(111) surface varies from 2.6 to 3.7 eV depending on thickness and growth procedure.
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243
(iii) The growth procedure of the 3D oxides may affect noticeably the surface composition. Schematically two types of thin (4 atomic layers) oxides have been identified. One of them (stoichiometric) exhibits a surface composition (monitored by ISS) close to the one represented by the formula unit Al ,O, and the other (reduced) has an important concentration of oxygen vacancies at the surface. The rate of oxide growth, at low temperatures, is several times larger for the reduced than for the stoichiometric oxides.
Acknowledgements Special thanks to Dr. N. Garcia for enlighting discussions thanks also to Dr. R. Miranda and to Dr. M. Salmeron for the critical reading of the manuscript. This work has been supported by the CAICYT through grant No. 1154-83.
References [I] S.A. Flodstrom, C.W.B. Martinsson, R.Z. Bachrach, S.B. Hagstrom and R.S. Bauer, Phys. Rev. Letters 40 (1978) 907. [2] J. Stohr, L.I. Johansson, S. Brennan, M. Hecht and N. Miller, Phys. Rev. B22 (1980) 4052. [3] D. Norman, S. Brennan, R. Jaeger, and J. Stohr, Surface Sci. 105 (1981) L297. [4] B.E. Hayden, W. Wyrobisch, W. Oppermann, S. Hachicha, P. Hofman and A.M. Bradshaw, Surface Sci. 109 (1981) 207. [5] P. Hofmann, W. Wyrobisch and A.M. Bradshaw, Surface Sci. 80 (1979) 344. [6] S.M. Davis and J.C. Carver, Surface Sci. 124 (1983) L12. [7] D.P. Smith, Surface Sci. 25 (1971) 171. [8] E.S.R. Gopal, Statistical Mechanics and Properties of Matter (Wiley, New York, 1976) p. 104. [9] P.M. Hall and J.M. Morabito, Surface Sci. 59 (1976) 624. [lo] M. Salmeron, S. Ferrer, M. Jazzar and G.A. Somorjai, Phys. Rev. B28 (1983) 1158. [ll] A. Barre, Chem. Phys. Letters 19 (1973) 109. [12] Y. Oka, T. Takahashi, K. Okada and S. Iwai, J. Non-Crystalline Solids 30 (1979) 349. [13] J.C. Bertolini, J. Massardier, P. Delichere, B. Tardy, B. Imelik, Y. Jugnet, T.M. Due, L.D. Temmerman, C. Creemers, H. Van Hove and A. Neyens, Surface Sci. 119 (1982) 95. [14] N. Cabrera and N.F. Mott, Rept. Progr. Phys. 12 (1949) 163. [15] F.P. Fehlner and N.F. Mott, Oxidation Metals 2 (1970) 59. [16] The same shift was observed in the 0 1s oxide emission.
Surface Science 157 (1985) 233-243 North-Holland, Amsterdam
AN ISS-XPS STUDY ON THE OXIDATION OF Al(lll); IDENTIFICATION OF STOICHIOMETRIC AND REDUCED SURFACES C. OCAL,
B. BASURCO
OXIDE
and S. FERRER
Departamento de Rsica Fundamental e Instituio de Fkicadel Estado Sblido, Universidad Auth~onoma de Madrid, Cantoblanco, 28049 - Madrid, Spain Received 31 August 1984; accepted for publication 27 December 1984
We have studied by ISS and XPS the initial stages of the 3D oxidation of Al(lll). We found that the incorporation of oxygen into the crystal bulk is well described by a second order process with an activation energy of 0.50*0.05 eV. XPS indicates that the chemical shift of the 2p emission upon oxidation varies from 2.6 to 3.7 eV depending on growth procedure and oxide thickness. The growth procedure also affects noticeably the surface composition. We have been able to prepare oxide surfaces with an important concentration of oxygen vacancies. The rate of growth, at low temperature of these defective oxides is several times larger than the rate exhibited by more perfect oxides. These results are discussed in the light of the Cabrera-Mott theory of oxidation of metals.
1. Introduction The early stages of the interaction of oxygen with Al(lll) have been extensively investigated since the photoemission study of Flodstrom et al. in 1978 [l]. This study showed the existence of an ordered (1 X 1) oxygen overlayer for the initial interaction of oxygen with Al(111). There is general agreement in the literature that this chemisorbed oxygen layer exists on Al(111) occupying threefold hollow sites. There is also agreement on the existence of another oxide-like state that increases in abundance with increasing oxygen coverage. This state is thought to correspond to an oxygen underlayer [2]. This oxide-like state is characterized by a chemical shift of 2.7 eV of the Al 2p level in contrast with the shift of 1.4 eV corresponding to the chemisorbed state. Both states may exist simultaneously although in general the chemisorbed state dominates at low oxygen exposures. Experiments also show that for heavy oxygen exposures the oxide layer closely resembles amorphous Al,O, [3]. The formation of this 3D oxide is accompanied by the disappearence of the LEED pattern. It is clear from the literature that most of the common surface techniques have been used to characterize the early stages of the oxidation of Al(111) but 0039-6028/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
234
C. Oral et al. / ISS-XPS
study on oxrdatron
of AI(I I I)
surprisingly enough we have not found any ion scattering (1%) study. In addition, the studies on the 3D oxide formation are scarce [4,5]. In this paper we present a combined 1% and X-ray photoelectron spectroscopy (XPS) study concentrated mainly in investigating the formation and structure of the 3D oxide. As low energy ISS is an extremely surface sensitive technique, we have used it to study the penetration to the interior of the Al crystal of the adsorbed oxygen atoms, initially on the Al(111) surface, when the crystal temperature is increased from 80 K to higher temperatures. We have found an activation energy of 0.50 + 0.05 eV for oxygen incorporation into the bulk of the crystal. We have also studied the surface composition and reactivity of 3D oxide layers of thickness ranging from 3 to 18 atomic layers. For an intermediate oxide thickness (4-5 oxide layers) we have found that the surface composition of the oxide monitored by ISS may vary noticeably depending on the growth procedure. Schematically we have observed that the oxygen-aluminum concentration ratio at the surface can vary from values close to those corresponding to the stoichiometry of the Al,O,, to values more than two times smaller that are indicative of an important concentration of oxygen vacancies at the surface of the oxide. These two types of oxide, so different when viewed with ISS, showed identical XPS spectra. The ulterior growth of these two limiting cases of surface oxides is clearly different. The defective oxides grow faster than the stoichiometric ones. These results are discussed in the view of the Cabrera-Mott theory of oxidation of metals at low temperatures.
2. Experimental The experiments were performed in a standard ion pumped ultra-highvacuum system (Leybold-Heraeus) equipped with a Mg X-ray source, an hemispherical analyzer, a 3 kV ion gun and a residual gas analyzer. The base as pressure was 3 X lo- lo Torr . The Al crystal face was of (111) orientation determined by the Laue technique. The crystal temperature was measured with a Chromel-Alumel thermocouple in mechanical contact with the crystal. A liquid nitrogen loop allowed to cool the crystal down to 80 K, and a tungsten filament was used for radiative heating. The crystal was cleaned by cycles of Ar bombardment and annealing to 800 K. Surface cleanliness was checked with XPS. Oxygen was introduced in the system through a variable leak valve. The photoemission spectra shown in this paper were recorded at the constant resolution mode of the analyzer. Ion scattering spectra were performed with 500 eV He ions. The purity of the gas was controlled with the mass spectro-meter. The scattering angle was 130”. The ion dose on the crystal was measured to be 6 X 10” ions cm-2 s- ‘. While recording ISS spectra the analyzer was kept in the AE/E constant mode. We found some sputtering of adsorbed oxygen due to the impinging He ions. Its effect on the value of the activation energy of
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the penetration process (see section 3.1) was checked to be negligible. On the kinetic data, the sputtering was negligible for the experiments at high temperatures but a small correction was introduced in the low temperature data to account for it.
3. Results and discussion 3.1. Initial incorporation
of the adsorbed oxygen into the bulk of the crystal
After cleaning and annealing at 800 K the Al(111) surface, the ISS spectra consisted of a single peak corresponding to the surface aluminum atoms as depicted in fig. 1. When the crystal was cooled at 80 K and exposed to 500 L of oxygen, a new peak due to 0 appeared as shown in the upper trace of fig. 1. The total scattered ion signal is clearly diminished and a peak at 0.48 which corresponds to oxygen atoms on the surface is visible. For the low ion energies utilized, the first layer scattering accounts for about 95% of the intensity of the spectrum as has been shown by Davis and Carver [6]. Therefore the spectra of fig. 1 are fairly indicative of the surface composition and structure. If the Al crystal is heavily oxidized up to 15-20 layers (see below), the corresponding O/Al peak intensity ratio in the ISS spectrum is 1.5. This result is in
E,=SOO eV 8 =130
I
0
Al I
I
-;Jk
500L
0,
80K
3
m
x2.5
x .Z F 0) E
.l 04
0.6
0.8
E/E,
Fig. 1. Low energy ion scattering spectra of clean Al(111) (bottom) and clean Al(111) exposed to 500 L of oxygen at 80 K (top).
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agreement with a previous study in a polycrystalline Al sample oxidized by exposure to the air [7]. The fact that the value of the O/Al ISS ratio coincides with the O/Al ratio of the stoichiometric formula unit of Al,O, is fortuitous since the ISS intensities for a fixed experimental set up depend both on chemical composition and surface structure. In the discussion that follows we will assume for simplicity that there are no important changes in the atomic structure of the different oxide surfaces and we will utilize the O/Al peak ratio to monitor the chemical composition of the oxide surfaces. When the Al surface, exposed at 500 L of oxygen at 80 K, is warmed for a certain time at 400-700 K and then cooled rapidly below room temperature, the corresponding ISS spectrum shows an increase of the Al peak intensity and a decrease of the oxygen signal. This is indicative of oxygen incorporation into the bulk of the crystal since it is known that no desorption to the gas phase takes place. As the intensity of the oxygen peak in the spectrum is proportional to the oxygen concentration at the surface, by measuring the decrease of the oxygen peak intensity as a function of time for a fixed annealing temperature we can obtain the activation energy for penetration of oxygen into the crystal bulk. We performed experiments at 539 and 683 K for annealing times t ranging from 10 to 1500 s. If the incorporation of oxygen into the bulk of the crystal were the result of a random-walk diffusion process, the intensity Z, of the oxygen peak in the ISS spectrum after keeping the crystal at a pre-selected temperature during t seconds, should decrease as tr’/* [8,9]. Our data did not obey this behaviour at all, indicating that the incorporation of oxygen was not the result of a random-walk process. Good agreement with the experiments was obtained by considering the incorporation of oxygen as the result of the following elemental chemical reaction: 0,” + where jumps atom state above
i
+
(0
-
i)incOrporatd,
i represents an interstitial site, i.e., an oxygen atom on the surface, O,,, to an interstitial non-occupied site in the second layer resulting in an incorporated at the subsurface. This atomic structure of the oxide-like has been previously proposed by Stohr et al. [2]. The rate law of the reaction is
d[%,]/dt
=
-k[Q,,l [iI,
where as usually the brackets denote concentration and k is the velocity constant. We will assume that [i], = [O,,], for t = 0 and t + 0 which gives upon integration [%I,’
-[Go,],::
= kt,
which corresponds to a linear dependence with time of the reciprocal of the intensity of the oxygen peak in the ISS spectrum. Fig. 2 displays the experimental data which exhibit a good linear behaviour for two independent
l!J
50
:
:
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C. Oral et al. / ISS- XPS s&y
:
1q0
:
:
:
:
lS0
:
t
I
E,= 0~50~0-05 eV
-
0.04
t; 100
500
1000
1500
Time (SC.) Fig. 2. Reciprocal of the intensity of the oxygen signal in the ion scattering spectrum as a function of time for two different temperatures: 685 K (time scale at the top) and 539 K (time scale at the bottom).
experiments at 685 and 539 K. The straight lines are the best fit to the data. From their slopes we obtain k. By writing k = v exp( - E,/kT) where v is the pre-exponential that we take as lOI s-’ and E, is the activation energy barrier for oxygen penetration, we obtain E, = 0.50 + 0.05 eV per atom. 3.2. Growth of the 30 oxide Fig. 3 shows a series of photoemission spectra after various oxygen treatments of the Al(lll) surface. Curve a corresponds to the Al 2p emission (at 72.65 eV) for the clean surface. Curves b and c display the resulting spectra after oxygen exposures of 500 L at room temperature and lo5 L and 80 K respectively. In spectrum b a weak, oxygen-induced emission at the high BE side of the metal peak is visible. Its BE, relative to the metal peak, is 2.4-2.8 eV and corresponds to the Al 2p oxide emission previously reported [l]. In curve c this oxide emission is clearly visible at 2.6 eV. Now, we will describe the procedure that we used to grow relatively thick oxides. In order to grow 3D oxides, the clean Al(lll) crystal was annealed to 700 K, kept at this temperature and subsequently was exposed to an oxygen partial pressure of 2 X 10m6 Torr during periods of time ranging from 5 min to 1 h. After this treatment the oxygen gas was pumped down and after obtaining a UHV environment, the crystal was cooled to room temperature. By increas-
C. Oral et al. / IS-XPS
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study on oxidation ofAi(ll
I)
90’ -18 layers L2’ -9 layers 35’
-8 layers
16’ -L ‘I? t
12’ - 3 314t
.a i
--+I--’
b
’
80
70
76
Bmding Energy
9’
-31
L’ -1 l/2 1
,0rL4
8OK
500 L Ot r.t.
;
clean
72
( cV)
Fig. 3. Photoemission spectra of the Al 2p electronic level: (a) clean surface; (b) surface exposed to 500 L of oxygen at room temperature; (c) surface exposed to lo5 L of oxygen at 80 K. Spectra from (d) to Cj) show the evolution of both, intensity and chemical shift of the higher binding energy emission due to the growing of the oxide film.
ing the oxygen exposure we obtained oxide films of increasing thickness. Curves d to j in fig. 3 show the corresponding photoemission spectra for a series of oxide films. The duration of the oxygen treatment is also indicated in the figure. As can be observed the intensity of the emission of the Al 2p electronic level in the metal decreases while the corresponding one in the oxide increases. We may obtain an approximate value of the thickness of the oxide films by measuring the intensities of the oxide emissions relative to the metal ones and by using an experimentally determined mean free path of the photoelectrons in the solid [lo]. This results in an attenuation factor of 20% for the decrease in intensity of the Al 2p emission due to one atomic layer of oxide. The calculated oxide thickness in monolayers are indiated in the figure. Its precision is estimated to be a 20%.
C. Oral et al. / ISS-XPS
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239
As it is clearly noticeable in fig. 3, the chemical shift of the 3D oxide is not constant but varies from 2.6 to 3.7 eV in going from curves c to j. That is, increasing oxide thickness results in a larger chemical shift of the oxide. Previously published data by Barre [ll] indicate a chemical shift for the Al 2p emission of 2.7 eV for an aluminum film evaporated in vacuum and oxidized at room temperature in 1 atm of oxygen during 30 min. The metal and oxide relative intensities in the corresponding XPS spectrum are similar to those of curve g in fig. 3 where the oxide emission appears at 3.2 eV from the metallic one. The agreement with our data is rather poor. We believe that this is due to the preparation procedure of the oxide as will be discussed later. In addition, one sees frequently in the literature that the value of the chemical shift of Al in a-alumina is taken from the data of Barre. Our data show that the situation is not so simple since the shift depends on oxide thickness and preparation procedure. 3.3. The “stoichiometric”
and “reduced” oxides
We are going to show now, how two slightly different oxidation schemes lead to important differences in the surface composition of the resulting oxide. The oxides grown as mentioned in section 3.2 (curves d to j in fig. 3), displayed an oxygen-aluminum peak intensities ratio in the ISS spectrum which varied continuously from 0.5 (for the oxide d) to 1.5 (for the oxide j). This indicates a tendency to the stoichiometry of the formula unit of Al,O, for increasing oxide thickness. If, instead of the oxidation scheme described in section 3.2, the clean Al(111) surface was exposed to 500 L of oxygen at room temperature and subsequently (after pumping the 0, gas) the oxidation procedure mentioned in section 3.2 was reproduced, i.e., exposed to 2 x 10m6 Torr of oxygen during several minutes at 700 K, then pumped down the oxygen gas and cooled to room temperature, the resulting oxide surface was always much richer in oxygen (viewed with ISS) that an oxide of the same thickness (monitored by XPS) grown as mentioned in section 3.2. Fig. 4 illustrates this result. The bottom curves are the photoemission spectra which are virtually identical and correspond to oxides - 4 layers thick. The top curves are the ISS spectra for an oxide grown as mentioned in section 3.2 (right) and for an oxide grown as mentioned above. The oxygen-aluminum peak intensity ratios in the ISS spectra are 0.5 (right) and 1.1 (left). For the sake of clarity we will design both types of oxide surfaces as reduced and stoichiometric respectively. Another important point that we want to mention concerns the time interval involved to grow the oxides characterized by the spectra in fig. 4. Whereas for the reduced oxide the treatment at 2 x lop6 Torr of 0, was only of 12 min, for the stoichiometric oxide it was 3 times longer. This result shows that the annealing of a room-temperature adsorbed oxygen layer, inhibits the ulterior
C. Oral et nf. / ISS-XPS
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study an oxidationof A&ill)
stoichiomotric
04
0.6
r,r
o-4
E/E,
0.8
0%
Al 2p .
,,.‘.
-it
74
76 B
E {eV)
72
70
t
76
74 B
Al 2p :. :
72
70
E (ev)
Fig. 4. XPS and tSS spectra for an stoichiometric and a reduced (see text) oxide films of a thickness of - 4 layers. Note the difference in the KS spectra for virtually identical XPS spectra.
oxide growth. Probably this occurs since the annealing of the adsorbed layer creates a sort of surface network of oxide as has been previously suggested [15]. This network acts as a passivation layer for the ulterior oxide growth by preventing the penetration of oxygen ad-atoms into the crystal bulk. In fact we think that this oxide network is not the result of a random-walk diffusion of oxygen into the crystal bulk but that rather corresponds to the picture presented in section 3.1 where a solid state surface reaction is taking place producing a layer of subsurface oxide. Ulterior oxidation results in a 3D oxide surface which is richer in oxygen that the reduced oxide where the network is not formed. The difference in oxidation rates at low temperature of the reduced and stoichiometric oxides is also illustrated in the results of the following experiments. If the oxide surfaces characterized by the spectra of fig. 4 are cooled down to 80 K and subsequently they are exposed to 10’ L of oxygen, the resulting increase in the intensity of the oxide emission in the XPS spectra correspond to 0.75 atomic layers for the reduced oxide and to - 0.1 layers for the stoichiometric. The same experiment repeated in a more stoichiometric oxide of similar thickness (O/Al ratio of 1.3 instead of 1.1) gives an increase of the oxide photoemission peak corresponding to - 0.06 layers. For comparison, on a clean Al(lll) surface at 80 K, TO5 L of oxygen exposure result in an oxide XPS emission corresponding to 1 monolayer.
C. Ucaf et al. / IS.%XPS study on oxidationof A&III)
241
We considered the possibility that the surfaces of the reduced oxides prepared as mentioned in section 3.2 were not microscopically homogeneous but consisted of islands of oxide with the stoichiometry of the alumina and patches of clean metal. If this were the case the oxygen uptake at 80 K would be essentially due to the adsorption of oxygen in the metal patches since as we have seen, the uptake in oxides with the surface stoichiometry of the alumina is very small. We checked the validity of this model and we are going to show that is not compatible with our data. Let us imagine that 6 and 1 - 8 are the surface coverages of the stoichiometric oxide islands and metal patches respectively for the surface of a reduced oxide, 4 atomic layers thick, with an O-Al ratio of 0.5 in the ISS spectrum. Let ZA, be the intensity in the ISS spectrum of the Al peak for the clean Al(ll1) surface. When the crystal is heavily oxidized and exhibits, in the ISS spectrum the stoichiometry of the alumina, the intensity of the aluminum peak, I$ is 5.9 times smaller than the corresponding one for the clean crystal, i.e., Z,;l/Zi; = 5.9. For the reduced oxide we have Z$ZL, = 0.5 where the superscript r states for reduced. The following relations should hold
z;, = (1 - @)I;
+ ezj; = (5.9 - 4.90)Z,!,
then Z&‘ZLi = (e/5.9 - 4.98) (Z$/Zi;). The last term in this expression is 1.5 according to the experiments. By substitution we get 6Z= 0.75 which means that the ISS data for the reduced oxide are compatible with a surface consisting of a 25% of metal patches. If this were the case, the experiments of lo5 L of exposure at 80 K on the reduced oxide should be interpreted essentially as the result of the adsorption on the metal patches. As the oxygen uptake in the clean metal surface is 1 monolayer, the uptake on the reduced oxide surface should be 1 X 0.25 monolayers plus the uptake in the oxide islands which should amount, at the most, 0.1 X 0.75 monolayers. The resulting total uptake should be then less than 0.3 monolayers that should be equal to the experimental value of 0.75 obtained for the oxygen uptake in the reduced oxide. The above simple arguments show that the experimental data are not compatible with the model of the inhomogeneous surface at least in a first appro~mation. A discussion on the validity of this type of analysis in ion scattering experiments can be seen in the paper by Bertolini et al. [13]. Our interpretation is therefore that the surface is microscopically homogeneous and contains an important concentration of anion vacancies. ISS indicated that after the oxygen exposure at 80 K, most of these surface vacancies were filled up with oxygen atoms. SEXAFS investigations by Norman et al. [3] indicate that the O-Al nearest neighbour distance in the surface aluminum oxide resulting of oxidizing Al(111) surfaces or polycrystalline samples, corresponds to an average atomic
242
c. OcaI et al.
/ rss-XPS
silt+
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oxidationofA&l
IIj
coordination characteristic of amorphous oxides which have a number of oxygen nearest neighbours to the Al atoms between 3 (initial oxide) to 6 (corundum). Also, studies on the structure of thin, anodic, alumina films investigated by the X-ray radial distribution function [12] show that the observed scattering intensities of the films (which are amorphous) are quantitatively well described by a conformation involving oxygen vacancies. Previous studies on the oxidation of Al(111) [S] show that the growth of thin oxide layers follows a logarithmic law according to the predictions of the Cabrera-Mott theory of oxidation of metals at low temperatures [14]. This theory states that the growth of very thin films of oxides on its metal support occurs via an ionic diffusion assisted by an electric field in the oxide layer. This field is originated by an anionic layer on the surface of the oxide and a cationic counterpart in the metal-oxide interface. These charged layers result from the establishment of the electronic equilibrium between the substrate metal and the adsorbed oxygen. Our data indicate that defective oxides grow more readily than non-defective ones. This means that surface defects lower the activation energy for ion diffusion through the oxide layer. Finally we want to make a comment on the discrepancy on the value of the chemical shift of the Al 2p emission reported by Barre [11J and by us (see section 3.2). We performed experiments which showed that a large oxygen exposure of the stoichiometric oxide (4 layers thick) at 80 K, caused an additional shift of 0.4 eV in the oxide photoemission signal towards smaller BE. The resulting value of the chemical shift of the Al 2p emission is then 2.9 eV which is in rather good agreement with the value of 2.7 eV reported by Barre. We think that in this experiment the high exposure and low temperature utilized, caused the stabilization of an anionic oxygen layer on the surface according to the Cabrera-Mott theory. The electrostatic potential difference originated by this charged layer and its cationic counterpart in the metal, caused a shift to lower BE of the oxide emission relative to the metal one in the photoemission spectrum [16]. If, on the contrary, the oxide is grown in a high vacuum environment and at high temperature, the anionic layer on the oxide is not stabilized and the mentioned shift does not occur. These experiments are now in progress and will be reported in detail in the near future. 4. Conciusions The main conclusions of this paper are: (i) The incorporation into the crystal bulk of oxygen adsorbed on the Al(lll) surface, is well described by a second order process involving empty sites in the subsurface. The resulting activation energy for incorporation is 0.50 _t 0.05 eV. (ii) The chemical shift of the 2p photoemission line upon oxidation of the Al(111) surface varies from 2.6 to 3.7 eV depending on thickness and growth procedure.
C. Oral et al. / ISS- XPS study on oxidation of AI(I I I)
243
(iii) The growth procedure of the 3D oxides may affect noticeably the surface composition. Schematically two types of thin (4 atomic layers) oxides have been identified. One of them (stoichiometric) exhibits a surface composition (monitored by ISS) close to the one represented by the formula unit Al ,O, and the other (reduced) has an important concentration of oxygen vacancies at the surface. The rate of oxide growth, at low temperatures, is several times larger for the reduced than for the stoichiometric oxides.
Acknowledgements Special thanks to Dr. N. Garcia for enlighting discussions thanks also to Dr. R. Miranda and to Dr. M. Salmeron for the critical reading of the manuscript. This work has been supported by the CAICYT through grant No. 1154-83.
References [I] S.A. Flodstrom, C.W.B. Martinsson, R.Z. Bachrach, S.B. Hagstrom and R.S. Bauer, Phys. Rev. Letters 40 (1978) 907. [2] J. Stohr, L.I. Johansson, S. Brennan, M. Hecht and N. Miller, Phys. Rev. B22 (1980) 4052. [3] D. Norman, S. Brennan, R. Jaeger, and J. Stohr, Surface Sci. 105 (1981) L297. [4] B.E. Hayden, W. Wyrobisch, W. Oppermann, S. Hachicha, P. Hofman and A.M. Bradshaw, Surface Sci. 109 (1981) 207. [5] P. Hofmann, W. Wyrobisch and A.M. Bradshaw, Surface Sci. 80 (1979) 344. [6] S.M. Davis and J.C. Carver, Surface Sci. 124 (1983) L12. [7] D.P. Smith, Surface Sci. 25 (1971) 171. [8] E.S.R. Gopal, Statistical Mechanics and Properties of Matter (Wiley, New York, 1976) p. 104. [9] P.M. Hall and J.M. Morabito, Surface Sci. 59 (1976) 624. [lo] M. Salmeron, S. Ferrer, M. Jazzar and G.A. Somorjai, Phys. Rev. B28 (1983) 1158. [ll] A. Barre, Chem. Phys. Letters 19 (1973) 109. [12] Y. Oka, T. Takahashi, K. Okada and S. Iwai, J. Non-Crystalline Solids 30 (1979) 349. [13] J.C. Bertolini, J. Massardier, P. Delichere, B. Tardy, B. Imelik, Y. Jugnet, T.M. Due, L.D. Temmerman, C. Creemers, H. Van Hove and A. Neyens, Surface Sci. 119 (1982) 95. [14] N. Cabrera and N.F. Mott, Rept. Progr. Phys. 12 (1949) 163. [15] F.P. Fehlner and N.F. Mott, Oxidation Metals 2 (1970) 59. [16] The same shift was observed in the 0 1s oxide emission.