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Adsorption of oxygen on clean single crystal faces of aluminium
Surface Scaence 62 (1977) 183-196 © North-Holland Pubhshmg Company
ADSORPTION OF OXYGEN ON CLEAN SINGLE CRYSTAL FACES OF ALUMINIUM P.O. GARTLAND
Department of Physws, The Norwegianlnstztute of Technology, N 7034 Trondhezm,Norway Received 31 May 1976;manuscript received in f'mal form 14 September 1976
Auger electron spectroscopy and work function measurements have been used to study the interaction of clean AI(lll) and AI(100) faces with oxygen at low pressure near room temperature The results for the two faces differ strongly Thus, the sucking probability of the (111) face decreases rapidly wRh coverage, while the work function increases shghfly, by 0 1 eV at 200 L In contrast, the sticking probabthty of the (100) faces goes through a maximum, whereas the work function decreases almost linearly with coverage, the total decrease at 200 L being 0.5-0.8 eV. The shape of the A1 L2,3VV spectrum from oxidized AI(100) Is independent of coverage, and it is m fact very smaflarto prewously reported spectra fzom oyad~ed polycrystalline alumimum. The corresponding spectrum from AI(lll) exhibits large changes with oxygen coverage and shows a previously unreported double peak at ~60 eV. The results are explained on the assumption that oxygen adsorbs randomly on the (111) face, and that thin (~5 A) islands of A120341ke oxide form on the (100) face
1. Introduction In the last decade a number ot studies have appeared concerning the mltlal oxadatlon of alummlum [ 1 - 7 ] . The techmques commonly apphed have been measurements of work function changes and mass uptake on polycrystalhne samples. The expenmental data presented so far have suffered from a large scatter. In particular, the work function has been observed to change m a comphcated and poorly reproduclble way. Thus, interpretations concerning the adsorption mechamsm, and the sholchiometric and electronic structure of the oxidized surfaces have been rather &fficult. As will be demonstrated in this paper, considerable progress is made towards understanding when single crystal surfaces are studied. In the course of a previous investigation of the photoelectric work function of single crystal faces of alummmm [8], adsorbed oxygen was observed to cause very amsotroplc changes in the work function. The present study was therefore undertaken m order to demonstrate unambiguously the tmportance of the surface orientation m the process of oxygen adsorption on alummium. The amsotropy Is demonstrated by stmultaneous recordings of the work functlons and the Auger electron spectra (AES) from the different faces of an alum1183
184
P 0 Gartland / Adsorption o f oxygen on alummtum
nIum single crystal The adsorption on the (111) and (100) faces, showing the greatest anisotropy, is discussed In some detad with particular emphasis on the adsorption kinetics, the mduced surface dipole, and the appearance of new structure in AES transitions lnvolvang the valence band.
2. Experimental The investigation was carried out in a UHV system with facilities for sample cleaning by ion bombardment and heatmg, AES, and photoelectric work function measurements. The Auger electron spectrometer was a standard cylindrical mirror analyzer (CMA) with an on-axis electron gun Spectra were recorded in the differentiated mode (dN[dE versus E), and the energy of a given Auger transition was taken at the minimum of a differentiated peak In order to minimize the heating of the sample the electron gun was operated at I kV and 25 ~tA. Possxble electron beana mduced desorption effects were momtored by observing the oxygen peak height while movang the sample at right angles to the electron beam. In similar experiments with CO on Mo and 02 on Cu usmg the same Auger spectrometer, such sample movements led to a sudden increase of the oxygen peak followed by an exponential decrease with time. No such effects were observed for 02 on A1. The work function measurements were carried out as described prexaously [8]. The photoelectnc threshold was determined through a straight line fit to the square root of the photoelectric yield in the range 0.1-0.4 eV above the threshold. The accuracy of the work function determination in the present experiment was -+0.02 eV. The sample was an alumlnlum single crystal with large flat faces cut parallel to the (100), (I 11), and (110) planes All faces were simultaneously cleaned by a few cycles of 1on bombardment and anneahng at 400°C. The surfaces were considered clean when the oxygen AES peak was well below 1% of the peak height after a 200 L exposure to oxygen gas Further detads concerning the experimental set-up and the cleaning procedure have been gwen elsewhere [8]. When the cleaned sample had been cooled down to room temperature, pure oxygen was let into the chamber via a bakeable leak valve. By adjustment of the valve the pressure could be kept constant within -+10%, while the ion pump and the liquid nitrogen cooled Ti subhmatlon pump were both running. Under these conditions a mass spectrometer analysis showed that the partial pressure of H20 was of the order of 1% of the O 2 partial pressure which was kept in the range of 1-3 × 10 -a Torr dunng all experimental runs. Lower oxygen pressures would raise the percentage of other active gases such as H20 , whale the photoelectric method of recording work function changes was too slow to be used at higher pressures. Thus, a study of the pressure dependence of the oxidation of A1 was not feasible in this situation In the actual pressure range the measurements showed saturation effects after I - 3 h. Thus the time scale was suitable for alternating recordings of work
P o Gartland
/Adsorption of oxygen
185
on alummmm
functions and Auger signals from the different crystal faces. These smaultaneous observations of the faces rule out any effects of pressure variations in the observed amsotropy. The temperature was kept as close to room temperature as possible. However, the electron beam from the Auger gun raised the temperature of the crystal to ~100°C durmg the measurements. Separate measurements of the work functions at room temperature showed no dramatic changes from the results at ~100°C, and the qualitative features of the AES measurements are assumed to be vahd also at room temperature.
3. Results The varying work function (~(hkl) measured as a function of oxygen exposure is shown in fig. 1. The qualitative features for the different faces were reproduced m several runs. All work function values remained rather constant for exposures up to 2 0 - 3 0 L (1 L = 1 Lan~umr = 10 -6 Torr see), then ~(111) mcreased and saturated at a value of ~0.1 eV above that of the clean face. For the (110)and (100) faces the work functions always decreased with increasing exposure. However, ~ 1 1 0 ) altogether changed by only about 0.06 eV, while ¢(100) saturated at 0 . 5 0.8 eV below the value for the clean face. Such a substantial reduction of the work functxon has prevtously [2] been attributed to adsorption of water vapour. In vaew of the small partial pressure of H20 in the vacuum vessel, and the different behavlour of the (111) and (110) faces, It seems more ltkely that the large reduction of the work function of the (100) face results from a reaction taking place between oxygen and alummium atoms on that particular crystal surface
~4
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i 100
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OXYGEN EXPOSURE ( L )
0
100 200 OXYGEN EXPOSURE ( L )
Fig. 1. Variation m the work function with oxygen exposure for chfferent single crystal faces of alummium. Fig. 2. Growth of oxygen AES peak (510 eV) with exposure on the (111) and (100) faces of an A1 single crystal.
P.O Gartland / Adsorptton o f oxygen on alummmm
186
The importance of the surface orientation m oxygen adsorption is confirmed by the AES measurements. Since the (111) and (100) faces showed the greatest differences in the work function measurements, the AES work was restricted mainly to these two faces. In ox]datmn studies by the AES techmque the peak to peak height of the oxygen KLL transitmn at 510 eV is frequently used as a measure of the oxygen concentratmn [9,10]. At coverages up to one monolayer, and with neghgible dlssolutmn into the bulk, the Auger signal is usually assumed to be stmply related to the number of adsorbed atoms. Despite the fact that these conditions may not always have been strictly fulfilled m the present experiment, the oxygen AES signal m fig. 2 is taken as a measure of the oxygen coverage. In order to discuss the possible reaction mechanisms the kmetm data presented m fig 2 were converted to a stroking probability as a functmn of the oxygen concentratmn. The sticking probability, s, may be defined as [11 ] (t)
s = N (dO/dQ),
where 0 is the oxygen concentration m monolayer units, N the number of molecules m one monolayer, and Q the total exposure. Apart from a factor of proportionality, dO/dQ is obtained by differentmtmg the curves m fig. 2. The results for s as a function of oxygen concentratmn (AES signal) are shown in fig. 3. The vemcal scale has been calibrated by setting an AES signal of 8 mV equal to one monolayer (to be justffied later). The total exposure Q has been converted from Langmmr umts into the number of impinged molecules by usmg the formula from kinet-
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5 10 OXYGEN AES PEAK (mV)
O~ygen AES peok (mv) o
tD < I
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100 200 OXYGEN EXPOSURE ( L )
Fig. 3. Oxygen slacking probablhty versus oxygen concentralaon on the (111) and (100) faces of an AI single crystal. Fig. 4. Reductaon of alummium 67.5 eV AES peak with oxygen exposure on the (100) and (111) faces. Insert. Slacking probabdity versus oxygen concentration as obtained from the rate of reducUon. Experimental parameters as given m fig. 2.
P 0 Gartland / Adsorpnon of oxygen on alummtum
187
lC gas theory, valid for constant pressure p, Q = pt/(2~rmkT)l/2.
(2)
Here t is the time of exposure, m the mass of a gas molecule, and T is the gas temperature. The accuracy of the absolute sticking probabdities thus obtained depends mainly upon the errors in the true pressure calibration and in the monolayer calibration, and it is probably not better than within a factor of two. However, the zero coverage sticking probabihtles, of 0.03 and 0.015 for the (11 l) and (100) faces respectively, compare reasonably well with the values obtained from oxidation studies of polycrystalhne f'flms [4,5]. Auger peaks from the substrate will also change upon gas adsorption. The peaks will be reduced by increased inelastic scattenng from the adsorbate, and for transitions involving valence levels, the transition probability may change considerably across the valence band due to chemlsorptlon bonding. The change of the main alumlnlum L 2 , a W peak at 67.5 eV, which IS shown in fig. 4, may therefore not be simply related to the oxygen coverage. However, as shown in the insert, sticking probability vanatlons obtained from the slopes of the curves m the figure quahtatwely exhibit tlSe same shapes as the ones derived from the oxygen AES-curves. This observation IS a further justification for using the curves m fig. 3 as a basis for a discussion of the kinetics of the adsorption process. In addition to the damping effect on the main alumlnIum peak, oxygen adsorption leads to changes in the fine structure associated w~th the A1 L2,3w transition. It has been shown that fine structure m this type of c o r e - b a n d - b a n d transition can be correlated with valence band density of states and resonance effects associated with surface molecular orbltals [12,13]. The line shape of a c o r e - b a n d - b a n d transition IS determined by the self-convolution of the density of states of the valence band, modulated by possible variations of the matrix element. This convolution produces fine structure around a given Auger feature within an energy range which is twice the energy width of the band. In addition, the experimentally observed hne shape is dependent on the escape probabihty, instrumental broadening, relaxation effects and inelastic scattering. Laclong a proper deconvolution procedure, it ~s therefore difficult to extract from the induced fine structure the molecular orbital energxes associated with the oxidation states. Nevertheless, oxygen induced structure in the LVV transition can be used as a "fingerprint" of the chemical enwronment of an adsorbed atom. Spectra obtained from AI(100) and A I ( I l l ) at vanous exposures are gtven m fig. 5. For the clean faces the spectra are identical, and in agreement with previous measurements [14-17]. A broad minimum at 52 eV may be Interpreted as a volume plasmon loss of the main peak at 67.5 eV. The sharper feature at 40.5 eV is probably due to an AI LIL2,aV transition. Upon exposure to oxygen the spectra from the two faces change differently. In the (100) spectra new peaks, labelled 1,2, and 3, are observed at 38.5, 47.0, and 54.0 eV. The spectrum obtained after 180 L exposure is nearly identical with the one from Al(411) exposed to 1 Torr sec and
188
P 0 Gartland /Adsorptton o f oxygen on alummtum AI(111) A1(111)
IgO 120 ~
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60
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E (eV) E (eV) F]g. 5. Oxygen reduced fine structure m the A1 L2 3w Auger transmons from AI(111) and AI(100). The spectra are recorded m the differential ~ode dN/dE, at various exposures to oxygen. Experimental parameters as gwen m fig 2, but with 17Mod reduced to 0.5 eV
Fig 6 Spectra of fig 5 after subtracuon of the contribution from the undisturbed substrate (see text) annealed, as reported by Jenkins and Chung [16] Spectra from the AI(111) face show features not observed on the (100) face, nor prewously reported Due to overlap between the oxygen reduced structures and the structure from the clean metal substrate, it Is hard to extract the accurate energy positions and the intensities of the new but rather weak peaks from the direct recordings in fig. 5, especially at low coverage. The situation is considerably improved by subtracting the contribution from the un&sturbed metal substrate This has been done by assuming the contribution from the metal substrate to be identical m shape with the bare metal spectrum, but reduced in Intensity In accordance with the reduction o f the main peak at 67.5 eV. The present subtraction procedure Is equivalent to the one frequently used m studies o f adsorbate effects in ultravmlet photoemlsslon spectra (UPS). As shown m fig. 6, the features from the "oxide layer" become more clearly resolved m the net spectra For the (100) face, the spectra at low coverage are identical to the ones at higher coverage except for a &fference in the intensity The (111) spectra of
P 0 Gartland/Adsorptton of oxygen on alummtum
189
fig. 6 show more comphcated changes with increasing exposure. At low exposures, the spectra are dominated by a double peak, no 4 and no. 5, at 5 9 - 6 2 eV. That this structure clearly decreases m intensity with increasing exposure, is an reformation not obtainable from the as-recorded spectra m fig. 5 Apart from th~s doublet the spectra from the (111) face at high exposures are identical with the ones from the (100) face, with peaks at 38.5, 47.0, and 54 0 eV. At tow coverage, however, peak no. 2 is found at ~2 eV higher energy. The upper curve (H) In fig. 6 shows the result of a 45 L exposure on the (111) face, followed by heating at 400°C for 15 mm.
4. Discussion The method of measuring work function changes has been applied frequently in studies of metal oxidation. In the case of alummmm the oxldat~on of polycrystalhne samples has led to work function results varying considerably from experiment to experiment. This poor reproducibility has been ascribed to various effects such as water vapour adsorption [2] or to the exastence of different kinds of surface oxides [3]. It is well known that the surface crystal structure of evaporated fdms depends critically upon the preparation conditions, e.g. the choice of substrate material and temperature. Thus, the relatwe amounts of (100), (111), and (110) oriented crystallltes could vary considerably from one sample to another. As evidenced by the present single crystal results flus would lead to a large scatter m the experimental data from polycrystalhne samples. The only way to solve this problem is to use single crystals. As will be discussed below, the reproducible data from each of the two faces (111) and (100) can be interpreted to gwe a consistent, although somewhat crude picture of the mltlal oxadatlon. 4.1. Kinencs
Information about the reaction mechanism for the mmal oxidation is obtained from the sticking probabdlty variations shown in figs 3 and 4, the work function measurements, and to some extent from the changes m the AES-spectra given m figs. 5 and 6. Although the sticking probability variations are very dependent upon the surface orientation, the initial numerical value is of the order of 10 -2 for all three faces. (The value for the (110) face was found to be intermediate between the (100) and (111) values.) This combination of structural and nonstructural dependence of the sticking probabihty can be explained by the assumption of a two-step process
-ld 02 ) 2%).
(3)
Here g, p and b denote gas phase, precursor state~ and a strongly bound state respec-
190
P 0 Gartland / Adsorptzon of oxygen on alummzum
tlvely, kl, k 2 , and k d being the rate factors. The rate equations governing the process can generally be written
dOp/dt= kl(1 - 0p)
- - kdO p - k2f(O ) Op ,
dO/dt = k2f(O ) Op ,
(4)
(5)
where 0p and 0 are the concentratmns m the precursor state and the strongly bound state. The functmn f(O) represents the fractmnal number of empty sites available for adsorption "seen" by the precursor molecule. These equatmns, w:th the specific form f(O) = (1 - 0) 2 have been the starting point for the analysis of different g a s metal systems [18,19]. The molecule precursor is assumed to reach an equthbrmm concentratmn m a very short time, and when th~s concentratmn is small, the sticklng probabdlty s(O) for the formation of the strongly bound state can be wntten
1 dO k2f(O) s(O) - kl dt - k2f(O ) + k d "
(6)
This result could also be obtamed from the more detailed model of Kohrt and Gomer [20]. From eq. (6) :t may be seen that a low mltial sticking probabdlty :s obtained by assuming k a >> k 2. In this case the variation of the stroking probablhty with coverage is &rectly proportional to f(O). Thus the structural msensmvlty of the initial sticking probabdlty can be explained if k2/k d ~ 10 -2 for all faces. With increasing coverage the adsorption may proceed differently on different faces, leading to face-dependent sticking probabdlty variations through the function f(O). Lacking an accurate cahbratlon in terms of coverage, f(O) cannot be obtained directly from fig. 3. For the (111) face, however, the square root of the stroking probabdlty decreases hnearly w:th oxygen concentration as shown In fig 7 Assuming a complete monolayer at the abscissa mtercept, this hneanty is equwalent to f(O) = (1 - 0) 2, wluch lmphes a dlssoclatwe adsorption with mobile and nonlnteractmg radmals. Another model, whmh can be fitted to the data within the expenmental error, is the one assuming bonding m lmmobtle pairs [21] ymldmg
f(O) = 1 - 1.75 (0 - 0.312502 - 0.083303 - 0.01750 s) .
(7)
A reasonable fit of a stmple model to the data does not necessarily Imply a smapie adsorption mechamsm. However, based upon the data of fig 7, we may conclude that, irrespective of the detads, the most general feature of the adsorptmn mechanism is that it obeys a dissoclatwe law based upon random occupatmn of surface sites Whde the stroking probabdlty data from (111) can be explained without consldenng reconstructmn or dissolutmn mto the bulk, the work function measurements are lndmatwe of such effects. From fig 8, m wh:ch the work functmn variatmns are shown as functmns of the oxygen concentration, we see that the work function remains constant within the experimental error up to approximately half the satura-
P 0 Gartland/Adsorpnon o f oxygen on alumzmum I
4.4
I
oeeeo (111)
z
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191
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z
m
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,
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I
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o-01
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5
10
OXYGEN AES P E A K l r n V }
2
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6
OXYGEN A E S PEAK (mY)
Fig. 7. Square root of the staclong probability versus concentration of oxygen adsorbed on AI(lll). Fsg. 8. Points- measured vanalaon m the work function of A1(111)and AI(100)with increasing concentration of adsorbed oxygen atoms. Sohd line expected variation of the photoelectncally determined work function assuming oxade growth in zslands with coverage-independent work function.
tlon coverage on the (111) face. Thus, when oxygen atoms adsorb on AI(111), the average dipole normal to the surface zs efficiently zero at low coverage. The difference m electronegativaty between A1 and O, and recent calculations of O on "jellium-alummmm" [22], in&cate that negatzve charge should be transferred from alummium to oxygen atoms. Work functaon measurements on other metal-oxygen systems have shown an initial linear increase with coverage [23], m&cating charge transfer to the oxygen atom sittmg outside the outmost substrate layer Thus, a neghglble charge transfer for O on A1 is not very hkely. A zero work functzon change could still result if the dzpoles are oriented parallel to the surface. For the (100) and (110) surfaces of alummmm this may be posszble without notzceable distortion 6f the lattice. However, even if the shortest known blndmg lengths of A1-O are used, oxygen sites cannot be co-planar wzth the outmost layer of (111) without apprecmble &stortzon of the lattzce. Thus, to explain the zero work funcUon change at low coverage the chemzsorbed oxygen atoms must be allowed to break up the alummmm lattice and form A I - O complexes, wtuch, on the average, show no net induced dipole moments normal to the surface. Only at higher coverages, where a description m terms of a "surface oxide" may be appropriate, a small net dzpole moment zs produced, possibly as a result of increasing mutual interaction between the imtially well separated complexes. That oxygen adsorption causes a serious dzstortlon of the aluminmm lattzce is also m agreement wzth LEED observations by Jona [1 ]. A relatively large number of adsorbed oxygen atoms may thus take posztzons below the surface. However, as evidenced by the present kinetic data, very few new adsorption sztes seem to be generated at the same time. The kinetics of the oxygen adsorption on the (100) face look very different from that on the (111) face. Still, some common features m the adsorptzon mecha-
192
P 0 Gartland / Adsorpnon of oxygen on alummtum
msm would be expected, especially since the initial stacking probability, s(O), is of the same order for all faces Random adsorption followed by a rapid dissolution Into the bulk would yield a nearly constant sticking probablhty over a large exposure range. To obtain a maximum, however, the diffusion constant has to depend upon position or coverage. Additional support for such a description is not very strong, while this is indeed the case for quite a &fferent mechanism based upon nucleation and growth [11 ]. In the island growth model the precursor molecule can dissociate and adsorb only at active sites, which differ from regular sites through a lower activation energy for adsorption. Initially, small oxide nuclei are formed at active sites on the clean surface, e.g. at crystallographic defects, whde the edges of the growing islands are assumed to prowde a number of new active sites roughly in proportion to the square root of the island areas. In a detailed description of an island growth model the surface &ffuslwty of the precursor molecules is an important parameter. However, If we still reqmre kcl >>k2, the Stlckln~ nrobablllty will have the same coverage dependence in the limits of high and low dlffl,SlVlty [9]. Here, the relative number of avadable empty sites, f(O), wall vary as x/0 following nucleation. When the islands start to grow together,f(0) will decrease and go to zero as the whole surface becomes covered. Considering the possibility that k d <~ k2, eq. (6) yields an initial constant sticking probabdlty of the order of unity, contrary to the observations. The initial rise m the sticking probability for the (100) face, as shown In fig. 3, IS not as fast as predicted by a x/0 law The main feature of the observed sticking probability variations, the maximum in each curve, was, however, reproducible in several expenmental runs The assumption of an island growth mechanism is strongly supported by the work function measurements The oxide islands may be considered to have a coverage-Independent low work function, ~)oxlde, while the areas lnbetween have the work function of the clean face, ~clean" According to the general theory for patchy surfaces, the average work functmn ~ is defined as [24] = f~ox,ae + (1 - f)~clean ,
(8)
where f is the fractional area of oxide on the surface. Now, i f ~ was identical to the the experimentally determined threshold, one would find a work function decreasing linearly with f, and thus with the oxygen AES signal. Such a hnearlty between and the oxygen concentration is not expected for random adsorption models, where the coverage dependence of the depolarization forces tends to reduce the rate of work function change at higher coverage. The work function change with oxygen concentration, as determined experimentally for AI(100) in fig. 8, shows no sign of such coverage dependent depolanzatlon effects The devlatmn from lmearlty at low coverage can be ascnbed to the photoelectnc method of determining the threshold. According to the theory [24], the total y M d in the first approximation
P 0 Gartland ~Adsorption of oxygen on alummtum
193
may be written y ec Box,de f(hco -- ~)2 + Bmetal(1 - f ) (hw - ¢clean) 2
(9)
Assuming the efficlencles of the oxide and metal areas to be about equal, I e. Boxtd e Bmetal, the fit of the square root of the yield to a straight hne m the actual frequency range results m a threshold determinat:on, ¢*, which Is shifted towards ~bclean at small values of f. This is demonstrated m fig. 8, where the sohd line represents the vanatlon of ¢* with oxygen concentration calculated from eq. (9), with ¢clean and Cox,de set equal to the experimental values for f = 0 and f = 1 respectively. Complete coverage, f = 1, is assumed at 9 mV AES signal As shown m the figure, the agreement between the measured work function change and what is expected from an Island growth model now become rather good for all oxygen concentrations The present model for oxide formation has been found to work for oxidation of other metals. For both Fe(100) [10], and NI(100) and (111) [9] the kinetics were interpreted by assuming island formation, and for the latter metal the formation of N10 was accompamed by a decrease m the work function of 0.6 eV below the value of the clean face. The present data provide no precise determination of the thickness of the oxide layers formed on the (111) and (100) faces of alummlum. However, a rough estimate may be obtamed from the decay of the A1 L2,3VV peaks at 67.5 eV as shown in fig. 4. The intensity of the 67 5 eV peak, I(d), after passmg through an oxide layer of thickness d can be expressed by
I(d)[I(O) = exp [-d(l cos o0].
(t 0)
Here c~ is the angle between the surface normal and the analyzer entrance sht. From fig 4 we get for I(d)/I(O) approxunately 0.25 for both faces at saturation. With this value, and c~ = 42.2 °, vahd for the present analyzer, we obtain at saturation d ~ l. Thus, the oxide thickness at an exposure of 200 L is equal to the mean free path I of 67.5 eV electrons m the oxide Experimental values of the mean free path m a large number of metals and a few non-metals mdlcate that there exist some universal relationship between ! and the electron energy E. The curve I(E) has a minimum at 5 0 - 1 0 0 eV The mmlmum values of l are typically 4 - 6 A for metals [25]. For AI203 the mean free path has been measured only at very high energies, where the values obtained coincide closely with the "universal curve". Assuming this to be true down to 67 5 eV, we obtain 5 -+ 1 A for l (67.5 eV) and thus for the oxide thickness as well. Irrespective of the finer de,ads of the models discussed so far, the main features may be summarized as follows. On the (111) face oxygen atoms are adsorbed randomly on the surface glvang rise to increasing mutual interaction with increasing concentration In the island formation model appropriate for the (100) face, the mutual Interaction between the oxygen species is not dependent upon coverage. The different new peaks observed in the AES spectra of figs. 5 and 6, although still
194
P o Gartland / Adsorption of oxygen on alummtum
umdentlfied, lend strong support to these general features of the models. In th (111) spectra a coverage dependence IS eastly observed for some of the peak posl tlons as well as for the peak shapes. Also, the intensity of the peaks change differ ently, only the 54 eV peak grows monotonically with increasing exposure. Fol the (100) face, the only coverage dependence observed is a monotonic increase m the peak intensities. 4.2 The Auger valence band spectrum
In the preceding section the kinetic data were shown to be strongly mdicatwe of different oxidation mechanisms on the (100) and (111)faces. As the Al L2,aVV spectra from the two surfaces in fgs. 5 and 6 are quahtatwely different at all exposures, the electromc structure associated with the adsorbed oxygen must be facedependent as well. AES spectra showing the same three peaks as nos. 1,2, and 3 here have been observed before by several groups [14-17,26]. These measurements have usually been performed on polycrystallme surfaces at some unspecified stage of oxidation, and interpretations of the results have mostly been restricted to the dominant peak, here no. 3. Qulnto and Robertson [14] used the X-ray data of Fomlchev [27] to identify this peak with a transmon mvolwng the valence band of A1203. Only one oxygen induced peak were resolved in their spectra, and they considered Auger transmons from only the upper peak in the oxade valence band. The same pomt of view was taken by Guennou, Dufour and Bonnelle [25], although all three peaks, nos. 1, 2, and 3, were well resolved In their spectra. They assigned the peak here labelled no. 2 and located 7 eV below peak no. 3 to a modified surface plasmon loss of peak no. 3. Although the energy shift is correct we observe the intensity ratm between peaks nos. 2 and 3 to be constant and equal to 0.7 throughout the whole exposure range on the (100) face. This is not in agreement with the expected behav lour of a loss peak. Prehmmary loss measurements made by use of the Auger analyzer in our laboratory show that for a primary energy of 70 eV the intensity of the 7 eV loss peak increases gradually and at " 2 0 0 L exposure it amounts to a max~mum of 10% of the primary peak intensity. Thus, peak no. 2, as well as peak no. 3, most hkely reflect structure m the oxade valence band through an L2,3VV transition. The oxide formed on AI(100) may have an electromc structure not slgmficantly different from that of amorphous Al203, as evidenced by the stmllanty between the Auger valence band spectra from that face and from polycrystalhne samples which have been subjected to atrrlosphenc oxadatlon [14,26]. Apart from minor differences m the energy posmons of the peaks nos. 1,2, and 3, the most strikmg difference between the (100) and (111) AES spectra is the double peak, labelled 4 and 5, which is unique for the (111) face. This structure may tentatively be assigned to a cross-transition A1 L2, 3 0 L2, 3 Al V, where O L2, 3 is an oxygen resonant level ~7 eV below the Fermi level. A peak at this energy has
P o Gartland ~Adsorption o f oxygen on alummtum
195
recently been observed m UPS from oxygen covered polycrystalline alummium [28,29]. However, until similar UPS experiments have been performed on single crystal faces, the origin of the ~60 eV structure in the AES spectra cannot be determined unambiguously. With increasing oxygen coverage the (111) spectrum approaches that of the (100) face, suggesting a description m terms of a surface oxide to become appropriate for the (111) face as well. The 5 9 - 6 2 eV doublet is, however, still observable m the (111)-spectra at 180 L of exposure. Thus, even at the highest exposures apphed in this work there are still slgmficant differences in the properties of the surface oxides formed on the (111) and (100) faces near room temperature. This is also evident from the work funchon measurements. The chemlsorptlon state on the (111) face is capable of transformation into the oxide state of the (100) face by a few minutes of heatmg at elevated temperatures The oxygen covered (111) face need not be saturated before heating as shown by the upper curve in fig 6. The effect of heating is probably to actwate migration of the randomly dlstnbuted oxygen atoms into A12Oa-llke islands as evidenced by the complete disappearance of the structure at ~60 eV.
5. Conclusion In this work AES and work function measurements have revealed a significant anlsotropy m the imtial stages of oxidation of AI(111)and AI(100). These observations show clearly that the surface orientation plays a key role m the process, a possibility which has gamed very little attention in the past. Data on the loneUcs, especially sticking probabihty variations, have been compared to current adsorption models. Models which &ffer only in some details cannot be separated within the present data. However, the anisotropy of the results does not depend upon the details of the models, but arises from a very fundamental difference in the adsorption mechanism. While adsorbmg randomly on the (111) surface, the oxygen atoms cluster mto islands of thin oxide on the (100) surface. The basis for these conclusions is not only the observed variations in the sticking probability, but also the behavlour of the work function and the Auger valence band spectra as the concentration of oxygen atoms increases. At all coverages the A1 L2,3VV spectra from the (111)and (100)faces differ qualitatively. The spectrum from (100) IS identical to spectra recorded from polycrystalhne samples oxidized m air, indicating an A1203-hke electronic structure of the oxide grown on AI(100). From the oxygen-covered (111) face the Auger valence band spectrum showed a double peak at ~60 eV, not prewously reported. The peak is tentatwely asstgned to a cross-transition involving electrons from a resonant O L2, 3 level and from the Fermi edge of the substrate metal. More insight mto the initial oxidation of alumimum will certainly be gained when similar experiments with smgle crystals are extended to a wider range of pres-
196
P 0 Gartland /Adsorptton o f oxygen on alumtmum
sures and t e m p e r a t u r e s , and using s u p p l e m e n t a r y t e c h m q u e s such as p h o t o e m l s s l o n spectroscopy and energy loss spectroscopy. Such e x p e n m e n t s are n o w in progress.
Acknowledgement The author wishes to express his gratitude to BJ. Slagsvold, J'.K. Grepstad and B. Kasemo for reading and commenting helpfully on drafts of this paper. Financial support from the Norwegian Research Councxl for Science and Humanities (NAVF) is also gratefully acknowledged.
References [1 ] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] ~21 ] [22] [23]
[24] [25] [26] [27] [28] [29]
F Jona, J. Phys. Chem. Sohds 28 (1967) 2155 E.E. Huber and C T. Kirk, Jr., Surface Scl. 5 (1966) 447. M.W. Roberts and B.R. Wells, Surface Scl. 15 (1969) 325. G. Dorey, Surface Scl. 27 (1971) 311. Wm.H. Kreuger and S.R Pollack, Surface Scl 30 (1972) 263. R L. Wells and T. Fort, Jr., Surface Scl. 33 (1972) 172. V K. Agarwala and T. Fort, Jr., Surface Scl 45 (1974) 470,48 (1975) 527. J.K. Grepstad, P.O. Gartland and B J. Slagsvold, Surface Scl. 57 (1976) 348. P.H HoUoway and J.B. Hudson, Surface Scl. 43 (1974) 123,141 G.W. Simmons and D J . Dwyer, Surface Scl 48 (1975) 373. J M Blakely, m Introductaon to the Properties of Crystal Surfaces (Pergamon, 1973). G.E. Beeker and H.D Hagstrum, Surface Scl. 30 (1972) 505. E.N. Slckafus, Phys. Rev. B7 (1973) 5105. D.T. Qumto and W.D. Robertson, Surface Scl 27 (1971) 649. M Suleman and E.B. Patlanson, J. Phys. F1 (1971) L21. L H. Jenkins and M.F. Chung, Surface Scl 28 (1971) 409. B D. Poweil and D.P. Woodruff, Surface Scl 33 (1972) 437. L.D. Schmldt, Catalysis Rev. 9 (1974) 115. C O. Steinbriichel, Surface Scl. 51 (1975) 539 C. Kohrt and R. Gomer, J. Chem. Phys. 52 (1970) 3283 J.K. Roberts, Proc. Cambridge Phd Soc. 34 (1938) 399 N.D. Lang and A R. Williams, Phys Rev. Letters 34 (1975) 531. O on Nffl00) and (111) see ref [9], O on C u ( l l l ) , (100) and (110) T.A. Delchar, Surface Scl. 27 (1971) 11, O on Ag(ll0) A. Engelhardt, Dr rer. nat Dlssertalaon, der Techmsche Umversltat Munthen (1975). J.C. Rlva~re, m Sohd State Surface Science, Ed. M. Green (Dekker, New York, 1969). C.R. Brundle, J Vacuum Sci. Technol 11 (1974) 212. H. Guennou, G. Dufour and C. BonneUe, Surface Scl 41 (1974) 547 V.A. Fomichev, Sowet Phys.-Solid State 8 (1967) 2312 K.Y. Yu, J.N. Miller, P. Chye,W.E. Spleer, N.D. Lang and A.R Wtlhams, to be pubhshed. S.A Flodstrbm, L.-G. Petterson and S.B.M. Hagstrom, J. Vacuum Scl. Teehnol., to be published.
ADSORPTION OF OXYGEN ON CLEAN SINGLE CRYSTAL FACES OF ALUMINIUM P.O. GARTLAND
Department of Physws, The Norwegianlnstztute of Technology, N 7034 Trondhezm,Norway Received 31 May 1976;manuscript received in f'mal form 14 September 1976
Auger electron spectroscopy and work function measurements have been used to study the interaction of clean AI(lll) and AI(100) faces with oxygen at low pressure near room temperature The results for the two faces differ strongly Thus, the sucking probability of the (111) face decreases rapidly wRh coverage, while the work function increases shghfly, by 0 1 eV at 200 L In contrast, the sticking probabthty of the (100) faces goes through a maximum, whereas the work function decreases almost linearly with coverage, the total decrease at 200 L being 0.5-0.8 eV. The shape of the A1 L2,3VV spectrum from oxidized AI(100) Is independent of coverage, and it is m fact very smaflarto prewously reported spectra fzom oyad~ed polycrystalline alumimum. The corresponding spectrum from AI(lll) exhibits large changes with oxygen coverage and shows a previously unreported double peak at ~60 eV. The results are explained on the assumption that oxygen adsorbs randomly on the (111) face, and that thin (~5 A) islands of A120341ke oxide form on the (100) face
1. Introduction In the last decade a number ot studies have appeared concerning the mltlal oxadatlon of alummlum [ 1 - 7 ] . The techmques commonly apphed have been measurements of work function changes and mass uptake on polycrystalhne samples. The expenmental data presented so far have suffered from a large scatter. In particular, the work function has been observed to change m a comphcated and poorly reproduclble way. Thus, interpretations concerning the adsorption mechamsm, and the sholchiometric and electronic structure of the oxidized surfaces have been rather &fficult. As will be demonstrated in this paper, considerable progress is made towards understanding when single crystal surfaces are studied. In the course of a previous investigation of the photoelectric work function of single crystal faces of alummmm [8], adsorbed oxygen was observed to cause very amsotroplc changes in the work function. The present study was therefore undertaken m order to demonstrate unambiguously the tmportance of the surface orientation m the process of oxygen adsorption on alummium. The amsotropy Is demonstrated by stmultaneous recordings of the work functlons and the Auger electron spectra (AES) from the different faces of an alum1183
184
P 0 Gartland / Adsorption o f oxygen on alummtum
nIum single crystal The adsorption on the (111) and (100) faces, showing the greatest anisotropy, is discussed In some detad with particular emphasis on the adsorption kinetics, the mduced surface dipole, and the appearance of new structure in AES transitions lnvolvang the valence band.
2. Experimental The investigation was carried out in a UHV system with facilities for sample cleaning by ion bombardment and heatmg, AES, and photoelectric work function measurements. The Auger electron spectrometer was a standard cylindrical mirror analyzer (CMA) with an on-axis electron gun Spectra were recorded in the differentiated mode (dN[dE versus E), and the energy of a given Auger transition was taken at the minimum of a differentiated peak In order to minimize the heating of the sample the electron gun was operated at I kV and 25 ~tA. Possxble electron beana mduced desorption effects were momtored by observing the oxygen peak height while movang the sample at right angles to the electron beam. In similar experiments with CO on Mo and 02 on Cu usmg the same Auger spectrometer, such sample movements led to a sudden increase of the oxygen peak followed by an exponential decrease with time. No such effects were observed for 02 on A1. The work function measurements were carried out as described prexaously [8]. The photoelectnc threshold was determined through a straight line fit to the square root of the photoelectric yield in the range 0.1-0.4 eV above the threshold. The accuracy of the work function determination in the present experiment was -+0.02 eV. The sample was an alumlnlum single crystal with large flat faces cut parallel to the (100), (I 11), and (110) planes All faces were simultaneously cleaned by a few cycles of 1on bombardment and anneahng at 400°C. The surfaces were considered clean when the oxygen AES peak was well below 1% of the peak height after a 200 L exposure to oxygen gas Further detads concerning the experimental set-up and the cleaning procedure have been gwen elsewhere [8]. When the cleaned sample had been cooled down to room temperature, pure oxygen was let into the chamber via a bakeable leak valve. By adjustment of the valve the pressure could be kept constant within -+10%, while the ion pump and the liquid nitrogen cooled Ti subhmatlon pump were both running. Under these conditions a mass spectrometer analysis showed that the partial pressure of H20 was of the order of 1% of the O 2 partial pressure which was kept in the range of 1-3 × 10 -a Torr dunng all experimental runs. Lower oxygen pressures would raise the percentage of other active gases such as H20 , whale the photoelectric method of recording work function changes was too slow to be used at higher pressures. Thus, a study of the pressure dependence of the oxidation of A1 was not feasible in this situation In the actual pressure range the measurements showed saturation effects after I - 3 h. Thus the time scale was suitable for alternating recordings of work
P o Gartland
/Adsorption of oxygen
185
on alummmm
functions and Auger signals from the different crystal faces. These smaultaneous observations of the faces rule out any effects of pressure variations in the observed amsotropy. The temperature was kept as close to room temperature as possible. However, the electron beam from the Auger gun raised the temperature of the crystal to ~100°C durmg the measurements. Separate measurements of the work functions at room temperature showed no dramatic changes from the results at ~100°C, and the qualitative features of the AES measurements are assumed to be vahd also at room temperature.
3. Results The varying work function (~(hkl) measured as a function of oxygen exposure is shown in fig. 1. The qualitative features for the different faces were reproduced m several runs. All work function values remained rather constant for exposures up to 2 0 - 3 0 L (1 L = 1 Lan~umr = 10 -6 Torr see), then ~(111) mcreased and saturated at a value of ~0.1 eV above that of the clean face. For the (110)and (100) faces the work functions always decreased with increasing exposure. However, ~ 1 1 0 ) altogether changed by only about 0.06 eV, while ¢(100) saturated at 0 . 5 0.8 eV below the value for the clean face. Such a substantial reduction of the work functxon has prevtously [2] been attributed to adsorption of water vapour. In vaew of the small partial pressure of H20 in the vacuum vessel, and the different behavlour of the (111) and (110) faces, It seems more ltkely that the large reduction of the work function of the (100) face results from a reaction taking place between oxygen and alummium atoms on that particular crystal surface
~4
t
At*02
(11~1)
I
I
P=I 5 I0 -e torr
I° I T =Ioo'c z
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ol. 2
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(110)
*
At +O2
z u_
o
P =1 5 l O ' S t o r r
T =25"C
/
.,//
I, ,zs#A
o (:OO o
0
0
i 100
i 200
OXYGEN EXPOSURE ( L )
0
100 200 OXYGEN EXPOSURE ( L )
Fig. 1. Variation m the work function with oxygen exposure for chfferent single crystal faces of alummium. Fig. 2. Growth of oxygen AES peak (510 eV) with exposure on the (111) and (100) faces of an A1 single crystal.
P.O Gartland / Adsorptton o f oxygen on alummmm
186
The importance of the surface orientation m oxygen adsorption is confirmed by the AES measurements. Since the (111) and (100) faces showed the greatest differences in the work function measurements, the AES work was restricted mainly to these two faces. In ox]datmn studies by the AES techmque the peak to peak height of the oxygen KLL transitmn at 510 eV is frequently used as a measure of the oxygen concentratmn [9,10]. At coverages up to one monolayer, and with neghgible dlssolutmn into the bulk, the Auger signal is usually assumed to be stmply related to the number of adsorbed atoms. Despite the fact that these conditions may not always have been strictly fulfilled m the present experiment, the oxygen AES signal m fig. 2 is taken as a measure of the oxygen coverage. In order to discuss the possible reaction mechanisms the kmetm data presented m fig 2 were converted to a stroking probability as a functmn of the oxygen concentratmn. The sticking probability, s, may be defined as [11 ] (t)
s = N (dO/dQ),
where 0 is the oxygen concentration m monolayer units, N the number of molecules m one monolayer, and Q the total exposure. Apart from a factor of proportionality, dO/dQ is obtained by differentmtmg the curves m fig. 2. The results for s as a function of oxygen concentratmn (AES signal) are shown in fig. 3. The vemcal scale has been calibrated by setting an AES signal of 8 mV equal to one monolayer (to be justffied later). The total exposure Q has been converted from Langmmr umts into the number of impinged molecules by usmg the formula from kinet-
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O~ygen AES peok (mv) o
tD < I
I
100 200 OXYGEN EXPOSURE ( L )
Fig. 3. Oxygen slacking probablhty versus oxygen concentralaon on the (111) and (100) faces of an AI single crystal. Fig. 4. Reductaon of alummium 67.5 eV AES peak with oxygen exposure on the (100) and (111) faces. Insert. Slacking probabdity versus oxygen concentration as obtained from the rate of reducUon. Experimental parameters as given m fig. 2.
P 0 Gartland / Adsorpnon of oxygen on alummtum
187
lC gas theory, valid for constant pressure p, Q = pt/(2~rmkT)l/2.
(2)
Here t is the time of exposure, m the mass of a gas molecule, and T is the gas temperature. The accuracy of the absolute sticking probabdities thus obtained depends mainly upon the errors in the true pressure calibration and in the monolayer calibration, and it is probably not better than within a factor of two. However, the zero coverage sticking probabihtles, of 0.03 and 0.015 for the (11 l) and (100) faces respectively, compare reasonably well with the values obtained from oxidation studies of polycrystalhne f'flms [4,5]. Auger peaks from the substrate will also change upon gas adsorption. The peaks will be reduced by increased inelastic scattenng from the adsorbate, and for transitions involving valence levels, the transition probability may change considerably across the valence band due to chemlsorptlon bonding. The change of the main alumlnlum L 2 , a W peak at 67.5 eV, which IS shown in fig. 4, may therefore not be simply related to the oxygen coverage. However, as shown in the insert, sticking probability vanatlons obtained from the slopes of the curves m the figure quahtatwely exhibit tlSe same shapes as the ones derived from the oxygen AES-curves. This observation IS a further justification for using the curves m fig. 3 as a basis for a discussion of the kinetics of the adsorption process. In addition to the damping effect on the main alumlnIum peak, oxygen adsorption leads to changes in the fine structure associated w~th the A1 L2,3w transition. It has been shown that fine structure m this type of c o r e - b a n d - b a n d transition can be correlated with valence band density of states and resonance effects associated with surface molecular orbltals [12,13]. The line shape of a c o r e - b a n d - b a n d transition IS determined by the self-convolution of the density of states of the valence band, modulated by possible variations of the matrix element. This convolution produces fine structure around a given Auger feature within an energy range which is twice the energy width of the band. In addition, the experimentally observed hne shape is dependent on the escape probabihty, instrumental broadening, relaxation effects and inelastic scattering. Laclong a proper deconvolution procedure, it ~s therefore difficult to extract from the induced fine structure the molecular orbital energxes associated with the oxidation states. Nevertheless, oxygen induced structure in the LVV transition can be used as a "fingerprint" of the chemical enwronment of an adsorbed atom. Spectra obtained from AI(100) and A I ( I l l ) at vanous exposures are gtven m fig. 5. For the clean faces the spectra are identical, and in agreement with previous measurements [14-17]. A broad minimum at 52 eV may be Interpreted as a volume plasmon loss of the main peak at 67.5 eV. The sharper feature at 40.5 eV is probably due to an AI LIL2,aV transition. Upon exposure to oxygen the spectra from the two faces change differently. In the (100) spectra new peaks, labelled 1,2, and 3, are observed at 38.5, 47.0, and 54.0 eV. The spectrum obtained after 180 L exposure is nearly identical with the one from Al(411) exposed to 1 Torr sec and
188
P 0 Gartland /Adsorptton o f oxygen on alummtum AI(111) A1(111)
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t
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50
60
70
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E (eV) E (eV) F]g. 5. Oxygen reduced fine structure m the A1 L2 3w Auger transmons from AI(111) and AI(100). The spectra are recorded m the differential ~ode dN/dE, at various exposures to oxygen. Experimental parameters as gwen m fig 2, but with 17Mod reduced to 0.5 eV
Fig 6 Spectra of fig 5 after subtracuon of the contribution from the undisturbed substrate (see text) annealed, as reported by Jenkins and Chung [16] Spectra from the AI(111) face show features not observed on the (100) face, nor prewously reported Due to overlap between the oxygen reduced structures and the structure from the clean metal substrate, it Is hard to extract the accurate energy positions and the intensities of the new but rather weak peaks from the direct recordings in fig. 5, especially at low coverage. The situation is considerably improved by subtracting the contribution from the un&sturbed metal substrate This has been done by assuming the contribution from the metal substrate to be identical m shape with the bare metal spectrum, but reduced in Intensity In accordance with the reduction o f the main peak at 67.5 eV. The present subtraction procedure Is equivalent to the one frequently used m studies o f adsorbate effects in ultravmlet photoemlsslon spectra (UPS). As shown m fig. 6, the features from the "oxide layer" become more clearly resolved m the net spectra For the (100) face, the spectra at low coverage are identical to the ones at higher coverage except for a &fference in the intensity The (111) spectra of
P 0 Gartland/Adsorptton of oxygen on alummtum
189
fig. 6 show more comphcated changes with increasing exposure. At low exposures, the spectra are dominated by a double peak, no 4 and no. 5, at 5 9 - 6 2 eV. That this structure clearly decreases m intensity with increasing exposure, is an reformation not obtainable from the as-recorded spectra m fig. 5 Apart from th~s doublet the spectra from the (111) face at high exposures are identical with the ones from the (100) face, with peaks at 38.5, 47.0, and 54 0 eV. At tow coverage, however, peak no. 2 is found at ~2 eV higher energy. The upper curve (H) In fig. 6 shows the result of a 45 L exposure on the (111) face, followed by heating at 400°C for 15 mm.
4. Discussion The method of measuring work function changes has been applied frequently in studies of metal oxidation. In the case of alummmm the oxldat~on of polycrystalhne samples has led to work function results varying considerably from experiment to experiment. This poor reproducibility has been ascribed to various effects such as water vapour adsorption [2] or to the exastence of different kinds of surface oxides [3]. It is well known that the surface crystal structure of evaporated fdms depends critically upon the preparation conditions, e.g. the choice of substrate material and temperature. Thus, the relatwe amounts of (100), (111), and (110) oriented crystallltes could vary considerably from one sample to another. As evidenced by the present single crystal results flus would lead to a large scatter m the experimental data from polycrystalhne samples. The only way to solve this problem is to use single crystals. As will be discussed below, the reproducible data from each of the two faces (111) and (100) can be interpreted to gwe a consistent, although somewhat crude picture of the mltlal oxadatlon. 4.1. Kinencs
Information about the reaction mechanism for the mmal oxidation is obtained from the sticking probabdlty variations shown in figs 3 and 4, the work function measurements, and to some extent from the changes m the AES-spectra given m figs. 5 and 6. Although the sticking probability variations are very dependent upon the surface orientation, the initial numerical value is of the order of 10 -2 for all three faces. (The value for the (110) face was found to be intermediate between the (100) and (111) values.) This combination of structural and nonstructural dependence of the sticking probabihty can be explained by the assumption of a two-step process
-ld 02 ) 2%).
(3)
Here g, p and b denote gas phase, precursor state~ and a strongly bound state respec-
190
P 0 Gartland / Adsorptzon of oxygen on alummzum
tlvely, kl, k 2 , and k d being the rate factors. The rate equations governing the process can generally be written
dOp/dt= kl(1 - 0p)
- - kdO p - k2f(O ) Op ,
dO/dt = k2f(O ) Op ,
(4)
(5)
where 0p and 0 are the concentratmns m the precursor state and the strongly bound state. The functmn f(O) represents the fractmnal number of empty sites available for adsorption "seen" by the precursor molecule. These equatmns, w:th the specific form f(O) = (1 - 0) 2 have been the starting point for the analysis of different g a s metal systems [18,19]. The molecule precursor is assumed to reach an equthbrmm concentratmn m a very short time, and when th~s concentratmn is small, the sticklng probabdlty s(O) for the formation of the strongly bound state can be wntten
1 dO k2f(O) s(O) - kl dt - k2f(O ) + k d "
(6)
This result could also be obtamed from the more detailed model of Kohrt and Gomer [20]. From eq. (6) :t may be seen that a low mltial sticking probabdlty :s obtained by assuming k a >> k 2. In this case the variation of the stroking probablhty with coverage is &rectly proportional to f(O). Thus the structural msensmvlty of the initial sticking probabdlty can be explained if k2/k d ~ 10 -2 for all faces. With increasing coverage the adsorption may proceed differently on different faces, leading to face-dependent sticking probabdlty variations through the function f(O). Lacking an accurate cahbratlon in terms of coverage, f(O) cannot be obtained directly from fig. 3. For the (111) face, however, the square root of the stroking probabdlty decreases hnearly w:th oxygen concentration as shown In fig 7 Assuming a complete monolayer at the abscissa mtercept, this hneanty is equwalent to f(O) = (1 - 0) 2, wluch lmphes a dlssoclatwe adsorption with mobile and nonlnteractmg radmals. Another model, whmh can be fitted to the data within the expenmental error, is the one assuming bonding m lmmobtle pairs [21] ymldmg
f(O) = 1 - 1.75 (0 - 0.312502 - 0.083303 - 0.01750 s) .
(7)
A reasonable fit of a stmple model to the data does not necessarily Imply a smapie adsorption mechamsm. However, based upon the data of fig 7, we may conclude that, irrespective of the detads, the most general feature of the adsorptmn mechanism is that it obeys a dissoclatwe law based upon random occupatmn of surface sites Whde the stroking probabdlty data from (111) can be explained without consldenng reconstructmn or dissolutmn mto the bulk, the work function measurements are lndmatwe of such effects. From fig 8, m wh:ch the work functmn variatmns are shown as functmns of the oxygen concentration, we see that the work function remains constant within the experimental error up to approximately half the satura-
P 0 Gartland/Adsorpnon o f oxygen on alumzmum I
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oeeeo (111)
z
==
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f
,
.
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i
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OXYGEN AES P E A K l r n V }
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~
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OXYGEN A E S PEAK (mY)
Fig. 7. Square root of the staclong probability versus concentration of oxygen adsorbed on AI(lll). Fsg. 8. Points- measured vanalaon m the work function of A1(111)and AI(100)with increasing concentration of adsorbed oxygen atoms. Sohd line expected variation of the photoelectncally determined work function assuming oxade growth in zslands with coverage-independent work function.
tlon coverage on the (111) face. Thus, when oxygen atoms adsorb on AI(111), the average dipole normal to the surface zs efficiently zero at low coverage. The difference m electronegativaty between A1 and O, and recent calculations of O on "jellium-alummmm" [22], in&cate that negatzve charge should be transferred from alummium to oxygen atoms. Work functaon measurements on other metal-oxygen systems have shown an initial linear increase with coverage [23], m&cating charge transfer to the oxygen atom sittmg outside the outmost substrate layer Thus, a neghglble charge transfer for O on A1 is not very hkely. A zero work functzon change could still result if the dzpoles are oriented parallel to the surface. For the (100) and (110) surfaces of alummmm this may be posszble without notzceable distortion 6f the lattice. However, even if the shortest known blndmg lengths of A1-O are used, oxygen sites cannot be co-planar wzth the outmost layer of (111) without apprecmble &stortzon of the lattzce. Thus, to explain the zero work funcUon change at low coverage the chemzsorbed oxygen atoms must be allowed to break up the alummmm lattice and form A I - O complexes, wtuch, on the average, show no net induced dipole moments normal to the surface. Only at higher coverages, where a description m terms of a "surface oxide" may be appropriate, a small net dzpole moment zs produced, possibly as a result of increasing mutual interaction between the imtially well separated complexes. That oxygen adsorption causes a serious dzstortlon of the aluminmm lattzce is also m agreement wzth LEED observations by Jona [1 ]. A relatively large number of adsorbed oxygen atoms may thus take posztzons below the surface. However, as evidenced by the present kinetic data, very few new adsorption sztes seem to be generated at the same time. The kinetics of the oxygen adsorption on the (100) face look very different from that on the (111) face. Still, some common features m the adsorptzon mecha-
192
P 0 Gartland / Adsorpnon of oxygen on alummtum
msm would be expected, especially since the initial stacking probability, s(O), is of the same order for all faces Random adsorption followed by a rapid dissolution Into the bulk would yield a nearly constant sticking probablhty over a large exposure range. To obtain a maximum, however, the diffusion constant has to depend upon position or coverage. Additional support for such a description is not very strong, while this is indeed the case for quite a &fferent mechanism based upon nucleation and growth [11 ]. In the island growth model the precursor molecule can dissociate and adsorb only at active sites, which differ from regular sites through a lower activation energy for adsorption. Initially, small oxide nuclei are formed at active sites on the clean surface, e.g. at crystallographic defects, whde the edges of the growing islands are assumed to prowde a number of new active sites roughly in proportion to the square root of the island areas. In a detailed description of an island growth model the surface &ffuslwty of the precursor molecules is an important parameter. However, If we still reqmre kcl >>k2, the Stlckln~ nrobablllty will have the same coverage dependence in the limits of high and low dlffl,SlVlty [9]. Here, the relative number of avadable empty sites, f(O), wall vary as x/0 following nucleation. When the islands start to grow together,f(0) will decrease and go to zero as the whole surface becomes covered. Considering the possibility that k d <~ k2, eq. (6) yields an initial constant sticking probabdlty of the order of unity, contrary to the observations. The initial rise m the sticking probability for the (100) face, as shown In fig. 3, IS not as fast as predicted by a x/0 law The main feature of the observed sticking probability variations, the maximum in each curve, was, however, reproducible in several expenmental runs The assumption of an island growth mechanism is strongly supported by the work function measurements The oxide islands may be considered to have a coverage-Independent low work function, ~)oxlde, while the areas lnbetween have the work function of the clean face, ~clean" According to the general theory for patchy surfaces, the average work functmn ~ is defined as [24] = f~ox,ae + (1 - f)~clean ,
(8)
where f is the fractional area of oxide on the surface. Now, i f ~ was identical to the the experimentally determined threshold, one would find a work function decreasing linearly with f, and thus with the oxygen AES signal. Such a hnearlty between and the oxygen concentration is not expected for random adsorption models, where the coverage dependence of the depolarization forces tends to reduce the rate of work function change at higher coverage. The work function change with oxygen concentration, as determined experimentally for AI(100) in fig. 8, shows no sign of such coverage dependent depolanzatlon effects The devlatmn from lmearlty at low coverage can be ascnbed to the photoelectnc method of determining the threshold. According to the theory [24], the total y M d in the first approximation
P 0 Gartland ~Adsorption of oxygen on alummtum
193
may be written y ec Box,de f(hco -- ~)2 + Bmetal(1 - f ) (hw - ¢clean) 2
(9)
Assuming the efficlencles of the oxide and metal areas to be about equal, I e. Boxtd e Bmetal, the fit of the square root of the yield to a straight hne m the actual frequency range results m a threshold determinat:on, ¢*, which Is shifted towards ~bclean at small values of f. This is demonstrated m fig. 8, where the sohd line represents the vanatlon of ¢* with oxygen concentration calculated from eq. (9), with ¢clean and Cox,de set equal to the experimental values for f = 0 and f = 1 respectively. Complete coverage, f = 1, is assumed at 9 mV AES signal As shown m the figure, the agreement between the measured work function change and what is expected from an Island growth model now become rather good for all oxygen concentrations The present model for oxide formation has been found to work for oxidation of other metals. For both Fe(100) [10], and NI(100) and (111) [9] the kinetics were interpreted by assuming island formation, and for the latter metal the formation of N10 was accompamed by a decrease m the work function of 0.6 eV below the value of the clean face. The present data provide no precise determination of the thickness of the oxide layers formed on the (111) and (100) faces of alummlum. However, a rough estimate may be obtamed from the decay of the A1 L2,3VV peaks at 67.5 eV as shown in fig. 4. The intensity of the 67 5 eV peak, I(d), after passmg through an oxide layer of thickness d can be expressed by
I(d)[I(O) = exp [-d(l cos o0].
(t 0)
Here c~ is the angle between the surface normal and the analyzer entrance sht. From fig 4 we get for I(d)/I(O) approxunately 0.25 for both faces at saturation. With this value, and c~ = 42.2 °, vahd for the present analyzer, we obtain at saturation d ~ l. Thus, the oxide thickness at an exposure of 200 L is equal to the mean free path I of 67.5 eV electrons m the oxide Experimental values of the mean free path m a large number of metals and a few non-metals mdlcate that there exist some universal relationship between ! and the electron energy E. The curve I(E) has a minimum at 5 0 - 1 0 0 eV The mmlmum values of l are typically 4 - 6 A for metals [25]. For AI203 the mean free path has been measured only at very high energies, where the values obtained coincide closely with the "universal curve". Assuming this to be true down to 67 5 eV, we obtain 5 -+ 1 A for l (67.5 eV) and thus for the oxide thickness as well. Irrespective of the finer de,ads of the models discussed so far, the main features may be summarized as follows. On the (111) face oxygen atoms are adsorbed randomly on the surface glvang rise to increasing mutual interaction with increasing concentration In the island formation model appropriate for the (100) face, the mutual Interaction between the oxygen species is not dependent upon coverage. The different new peaks observed in the AES spectra of figs. 5 and 6, although still
194
P o Gartland / Adsorption of oxygen on alummtum
umdentlfied, lend strong support to these general features of the models. In th (111) spectra a coverage dependence IS eastly observed for some of the peak posl tlons as well as for the peak shapes. Also, the intensity of the peaks change differ ently, only the 54 eV peak grows monotonically with increasing exposure. Fol the (100) face, the only coverage dependence observed is a monotonic increase m the peak intensities. 4.2 The Auger valence band spectrum
In the preceding section the kinetic data were shown to be strongly mdicatwe of different oxidation mechanisms on the (100) and (111)faces. As the Al L2,aVV spectra from the two surfaces in fgs. 5 and 6 are quahtatwely different at all exposures, the electromc structure associated with the adsorbed oxygen must be facedependent as well. AES spectra showing the same three peaks as nos. 1,2, and 3 here have been observed before by several groups [14-17,26]. These measurements have usually been performed on polycrystallme surfaces at some unspecified stage of oxidation, and interpretations of the results have mostly been restricted to the dominant peak, here no. 3. Qulnto and Robertson [14] used the X-ray data of Fomlchev [27] to identify this peak with a transmon mvolwng the valence band of A1203. Only one oxygen induced peak were resolved in their spectra, and they considered Auger transmons from only the upper peak in the oxade valence band. The same pomt of view was taken by Guennou, Dufour and Bonnelle [25], although all three peaks, nos. 1, 2, and 3, were well resolved In their spectra. They assigned the peak here labelled no. 2 and located 7 eV below peak no. 3 to a modified surface plasmon loss of peak no. 3. Although the energy shift is correct we observe the intensity ratm between peaks nos. 2 and 3 to be constant and equal to 0.7 throughout the whole exposure range on the (100) face. This is not in agreement with the expected behav lour of a loss peak. Prehmmary loss measurements made by use of the Auger analyzer in our laboratory show that for a primary energy of 70 eV the intensity of the 7 eV loss peak increases gradually and at " 2 0 0 L exposure it amounts to a max~mum of 10% of the primary peak intensity. Thus, peak no. 2, as well as peak no. 3, most hkely reflect structure m the oxade valence band through an L2,3VV transition. The oxide formed on AI(100) may have an electromc structure not slgmficantly different from that of amorphous Al203, as evidenced by the stmllanty between the Auger valence band spectra from that face and from polycrystalhne samples which have been subjected to atrrlosphenc oxadatlon [14,26]. Apart from minor differences m the energy posmons of the peaks nos. 1,2, and 3, the most strikmg difference between the (100) and (111) AES spectra is the double peak, labelled 4 and 5, which is unique for the (111) face. This structure may tentatively be assigned to a cross-transition A1 L2, 3 0 L2, 3 Al V, where O L2, 3 is an oxygen resonant level ~7 eV below the Fermi level. A peak at this energy has
P o Gartland ~Adsorption o f oxygen on alummtum
195
recently been observed m UPS from oxygen covered polycrystalline alummium [28,29]. However, until similar UPS experiments have been performed on single crystal faces, the origin of the ~60 eV structure in the AES spectra cannot be determined unambiguously. With increasing oxygen coverage the (111) spectrum approaches that of the (100) face, suggesting a description m terms of a surface oxide to become appropriate for the (111) face as well. The 5 9 - 6 2 eV doublet is, however, still observable m the (111)-spectra at 180 L of exposure. Thus, even at the highest exposures apphed in this work there are still slgmficant differences in the properties of the surface oxides formed on the (111) and (100) faces near room temperature. This is also evident from the work funchon measurements. The chemlsorptlon state on the (111) face is capable of transformation into the oxide state of the (100) face by a few minutes of heatmg at elevated temperatures The oxygen covered (111) face need not be saturated before heating as shown by the upper curve in fig 6. The effect of heating is probably to actwate migration of the randomly dlstnbuted oxygen atoms into A12Oa-llke islands as evidenced by the complete disappearance of the structure at ~60 eV.
5. Conclusion In this work AES and work function measurements have revealed a significant anlsotropy m the imtial stages of oxidation of AI(111)and AI(100). These observations show clearly that the surface orientation plays a key role m the process, a possibility which has gamed very little attention in the past. Data on the loneUcs, especially sticking probabihty variations, have been compared to current adsorption models. Models which &ffer only in some details cannot be separated within the present data. However, the anisotropy of the results does not depend upon the details of the models, but arises from a very fundamental difference in the adsorption mechanism. While adsorbmg randomly on the (111) surface, the oxygen atoms cluster mto islands of thin oxide on the (100) surface. The basis for these conclusions is not only the observed variations in the sticking probability, but also the behavlour of the work function and the Auger valence band spectra as the concentration of oxygen atoms increases. At all coverages the A1 L2,3VV spectra from the (111)and (100)faces differ qualitatively. The spectrum from (100) IS identical to spectra recorded from polycrystalhne samples oxidized m air, indicating an A1203-hke electronic structure of the oxide grown on AI(100). From the oxygen-covered (111) face the Auger valence band spectrum showed a double peak at ~60 eV, not prewously reported. The peak is tentatwely asstgned to a cross-transition involving electrons from a resonant O L2, 3 level and from the Fermi edge of the substrate metal. More insight mto the initial oxidation of alumimum will certainly be gained when similar experiments with smgle crystals are extended to a wider range of pres-
196
P 0 Gartland /Adsorptton o f oxygen on alumtmum
sures and t e m p e r a t u r e s , and using s u p p l e m e n t a r y t e c h m q u e s such as p h o t o e m l s s l o n spectroscopy and energy loss spectroscopy. Such e x p e n m e n t s are n o w in progress.
Acknowledgement The author wishes to express his gratitude to BJ. Slagsvold, J'.K. Grepstad and B. Kasemo for reading and commenting helpfully on drafts of this paper. Financial support from the Norwegian Research Councxl for Science and Humanities (NAVF) is also gratefully acknowledged.
References [1 ] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] ~21 ] [22] [23]
[24] [25] [26] [27] [28] [29]
F Jona, J. Phys. Chem. Sohds 28 (1967) 2155 E.E. Huber and C T. Kirk, Jr., Surface Scl. 5 (1966) 447. M.W. Roberts and B.R. Wells, Surface Scl. 15 (1969) 325. G. Dorey, Surface Scl. 27 (1971) 311. Wm.H. Kreuger and S.R Pollack, Surface Scl 30 (1972) 263. R L. Wells and T. Fort, Jr., Surface Scl. 33 (1972) 172. V K. Agarwala and T. Fort, Jr., Surface Scl 45 (1974) 470,48 (1975) 527. J.K. Grepstad, P.O. Gartland and B J. Slagsvold, Surface Scl. 57 (1976) 348. P.H HoUoway and J.B. Hudson, Surface Scl. 43 (1974) 123,141 G.W. Simmons and D J . Dwyer, Surface Scl 48 (1975) 373. J M Blakely, m Introductaon to the Properties of Crystal Surfaces (Pergamon, 1973). G.E. Beeker and H.D Hagstrum, Surface Scl. 30 (1972) 505. E.N. Slckafus, Phys. Rev. B7 (1973) 5105. D.T. Qumto and W.D. Robertson, Surface Scl 27 (1971) 649. M Suleman and E.B. Patlanson, J. Phys. F1 (1971) L21. L H. Jenkins and M.F. Chung, Surface Scl 28 (1971) 409. B D. Poweil and D.P. Woodruff, Surface Scl 33 (1972) 437. L.D. Schmldt, Catalysis Rev. 9 (1974) 115. C O. Steinbriichel, Surface Scl. 51 (1975) 539 C. Kohrt and R. Gomer, J. Chem. Phys. 52 (1970) 3283 J.K. Roberts, Proc. Cambridge Phd Soc. 34 (1938) 399 N.D. Lang and A R. Williams, Phys Rev. Letters 34 (1975) 531. O on Nffl00) and (111) see ref [9], O on C u ( l l l ) , (100) and (110) T.A. Delchar, Surface Scl. 27 (1971) 11, O on Ag(ll0) A. Engelhardt, Dr rer. nat Dlssertalaon, der Techmsche Umversltat Munthen (1975). J.C. Rlva~re, m Sohd State Surface Science, Ed. M. Green (Dekker, New York, 1969). C.R. Brundle, J Vacuum Sci. Technol 11 (1974) 212. H. Guennou, G. Dufour and C. BonneUe, Surface Scl 41 (1974) 547 V.A. Fomichev, Sowet Phys.-Solid State 8 (1967) 2312 K.Y. Yu, J.N. Miller, P. Chye,W.E. Spleer, N.D. Lang and A.R Wtlhams, to be pubhshed. S.A Flodstrbm, L.-G. Petterson and S.B.M. Hagstrom, J. Vacuum Scl. Teehnol., to be published.