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An Angle Resolved Sims and Auger Electron Spectroscopy Study of the Oxidation Of Al(100)
J.W. Ward (Editor), Catalysis 1987 © 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
845
AN ANGLE RESOLVED SIMS AND AUGER ELECTRON SPECTROSCOPY STUDY OF THE OXIDATION OF Al(lOO)
L.L. LAUDERBACK and S.A. LARSON Department of Chemical Engineering, The University of Colorado, Colorado 80309-0424
Boulder,
ABSTRACT Angle resol ved SIMS (ARSIMS) and Auger electron spectroscopy (AES) have been used to study the interaction of oxygen with Al( 100) at 300 K. ARSIMS measurements of the Al+ azimuthal angle distribution and AES measurements show that oxygen chemisorbs on Al(lOO) without reconstructing the Al surface for oxygen exposure up to roughly 120L. As the exposure exceeds 120L an amorphous oxide begins to grow which completely disorders the Al surface at an exposure of about 1200L. ARSIMS measurements of the 0- polar angle distribution are shown to be sensitive to the occupation of surface versus subsurface oxygen bonding sites. These measurements indicate that while surface sites are preferentially populated upon initial exposure to oxygen, both surface and subsurface sites become populated prior to oxide formation at "-'l20L. INTRODUCTION Although a number of surface science studies of the interaction of oxygen with Al(lOO) have been reported (1-7), much uncertainty still exists about the nature of the oxidation process on this face. While some studies suggest that the Al(lOO) surface begins to oxidize immediately upon initial exposure to oxygen, other studies indicate a two step oxidation process is involved in which oxygen chemisorbs prior to oxide formation. For example, Flodstrom et al (1,2) concluded, on the basis of A12p core level photoemission studies, that while a chemisorbed oxygen state precedes oxide formation on Al(lll), an amorphous oxide forms on Al( 100) upon initial exposure to oxygen and grows according to the island growth model. A12p core level spectra of Eberhardt and Kunz (3) were also interpreted as indicating direct penetration of oxygen atoms beneath the surface and oxide formation upon initial exposure of Al(lOO) to oxygen. In contrast to these studies however, Michel et al (5,6) concluded, on the basis of AES measurements, that formation of a chemisorbed state does occur on Al (100) prior to the formation of an amorphous oxide. Their results show that while the Al(68 ev) and 0(510 ev) AES signals generally decrease and increase respectively with increasing oxygen exposure from the outset, the Al(54 ev) signal, characteristic of Al oxide, does not
846 begin to grow until the oxygen coverage exceeds roughly .5ML. This was interpreted as indicating that only a chemisorbed oxygen phase existed for coverages below ~.5ML. LEED measurements (4,6,7) generally show that diffraction spots characteristic of the Al(lOO) plane gradually fade away from the outset of oxygen exposure and completely disappear for oxygen coverages above .5ML. While the complete disappearance of the LEED pattern is consistent with the formation of an amorphous oxide at high coverages, interpretation of the initial fading is ambiguous since this could be caused either by random chemisorption of oxygen on the ordered Al( 100) surface or by disordering of the surface by direct formation of an amorphous oxide. No diffraction spots due to adsorbed oxygen have been observed and the oxygen binding sites are unknown. The present work is in part aimed at clarifying whether or not a chemisorbed phase precedes the formation of an amorphous oxide on Al(lOO) by correlating AES measurements with angle resolved secondary ion mass + spectrometry (ARSIMS) measurements of the Al azimuthal angle distribution as . + a f unc t ron of oxygen exposure. We show that the Al ARSIMS measurements directly reflect the local geometric arrangement of the Al substrate atoms and thereby directly indicate the disordering of the Al surface caused by amorphous oxide formation. we also report ARSIMS measurements of the 0- polar angle distribution (PAD) which are shown to be sensitive to the occupation of surface versus subsurface oxygen binding sites. From the measurements we show how the population of surface and subsurface bind sites relates to the level of oxygen exposure and to the formation of chemisorbed and amorphous oxide phases on Al(lOO).
EXPERIMENTAL The ARSIMS and AES measurements were performed in a two-level tur-10 bomolecular pumped stainless steel UHV chamber (base pressure = 5 x 10 torr) that is also equipped with low energy electron diffraction (LEED) and thermal desorption spectroscopy techniques. AES measurements were carried out using a PHI, single pass, cylindrical mirror analyzer and a co-axial electron gun. The primary electron energy was 5.0 Kev. A low current (.5 wA) de focused electron beam was used in order to minimize the increase in sample temperature and reduction of the surface by the electron beam. A schematic of the ARSIMS experiment is shown in Figure lAo The primary + Ar ion beam was generated by a Colutron model G-2 ion gun and was mass analyzed by a Wien type velocity filter. All experiments were performed with + 4.5 Kev Ar ions imping in on the sample at normal inc idence and with a 2. current density of 5 x 10- amps/cm The primary ion beam was collimated to a diameter of 2.4 mm by a grounded aperture located 3 cm in front of the sample as shown in Figure LA, Secondary ions emitted from the surface were accepted into a bessel box energy analyzer through an entrance aperture that
g
00
847
A
CRYSTAL
0.24 em
' '~
~--I._-.....;:::.;...-
1 - 3 . 0 em
DEFLECTION PLATES
ell1 -/;1 T
+
ENERGY ANALVZER AND QUADRUPOLE MASS SPECTROMETER
B
-
rp=O
o
( 100) Fig. 1. Schematic of the ARSIMS apparatus (A) and azimuthal orientation of the Al(lOO) plane (B). is located in a grounded shield placed around the end of the energy analyzer. After passing through the energy analyzer the secondary ions were subsequently mass analyzed by an Extranuclear Laboratories quadrapole mass spectrometer and detected using pulse counting techniques. The azimuthal angle (¢) of the detected secondary ions was variable from 0 to 180 degrees by rotating the crystal about its surface normal. The azimuthal angle is defined relative to the crystal orientation, as determined by LEED, in Figure lB. The polar angle (8) of the detected secondary ions could be varied from 33 to 90 degrees, as measured from the surface normal, by translating the crystal directly away from or directly towards the primary ion beam. As seen from Figure lA, variation of the polar angle in this manner also causes variation of the angle at which the secondary ions pass through the entrance aperture to the energy analyzer. The trajectory of the secondary ions is therefore realigned after passing through the entrance aperture by a set of electrostatic deflection plates (see Figure lA) to achieve maximum transmission through the energy analyzer for a given pass energy and polar
848 angle. We note that the energy band width associated with the deflection plates is much greater than the +1.5 ev band width of the bessel box analyzer, so the bessel box controls the energy resol.ut ion as well as the energy selection. The pass energy used in all experiments was 10.0 ev. + All reported Al azimuthal angle distributions were obtained at a fixed o + polar angle of 45 by measuring the Al signal intensity a discrete 10 degree 0 0 intervals from cjJ = 0 to cjJ = 180 • All polar angle distributions were obtained by measuring the 0 signal intensity at discrete intervals from 0 = 0 0 0• 33 to 0= 65 with the azimuthal angle fixed at cjJ = 45 A small intensity correction has been included in the reported 0 polar angle distributions to 0 0 correct for changes in the angular resolution, which varies from 6 to 9 as the polar angle increases from 33 to 65 degrees. This correction normalizes o all 0 intensities to that corresponding to an angle resolution of 7 which is the actual resolution at a polar angle of 45 degrees. The Al(lOO) crystal was obtained precut and oriented from the Monocrystals Company (99.999% purity) and was subsequently polished using diamond paste ( .05 u m). The crystal was mounted on a sample holder via two stainless steel clips. A chromel-alumel thermocouple was attached to one mounting clip for monitoring the sample temperature. Heating was accomplished by electron bombardment from behind the sample. The surface cleaning procedure consisted of many cycles of an annealing to ~4500C and ion bombardment. Surface cleanliness and orientation was verified by AES and LEED respectively.
RESULTS AND DISCUSSION AES of Oxygen Adsorption on Al(lOO) The peak-to-peak intensities of the 0(510 ev), Al(68 ev) and Al(54 ev) AES signals are plotted in Figure 2 as a function of oxygen exposure. While the 0(510 ev) and Al(68 ev) signals increase and decrease respectively with increasing oxygen exposure from the outset, the Al (54 ev) signal, characteristic of Al oxide, does not begin to grow until the exposure exceeds roughly ~ 120L. This suggests that adsorption of oxygen, for exposures up to ~ 120L, may produce a chemisorbed oxygen phase instead of a surface oxide. Growth of the Al(54 ev) feature does, however, indicate growth of a surface oxide for increasing exposures above -v 120L. The 0(510 ev) signal increases almost linearly with increasing exposures up to the point where the Al(54 ev) signal begins to grow. The slope of the O(510 ev) curve then begins to decrease until the O(510 ev) signal nearly saturates at roughly ~1200L. The linear increase in the 0(510 ev) signal is consistent with earlier AES studies of oxygen uptake on Al(lOO) (1) and suggests that new adsorption sites may be created as the uptake proceeds. This has been interpreted previously by Martinson et al (1) as indicating that oxygen may initially diffuse into the bulk upon adsorption, thereby maintaining a full complement of surface adsorption sites and/or that oxygen
849
o
0.2
0.4
0.6
0.8
OXYGEN EXPOSURE (L Xl0- Z )
1
1.2
Fig. 2. AES peak-to-peak intensities of the 0(510 ev), Al(68 ev) and Al(54 ev) signals as a function of oxygen exposure at 300 K. adsorption may proceed according to the island growth mechanism in which new adsorption sites are generated at the perimeter of growing oxygen islands until the island coalesce (1). In the latter case, defect sites are assumed to act as nucleation centers for island growth. Both of these interpretations are consistent with results to be discussed below which indicate diffusion of oxygen into the subsurface region for exposures >30L and the presence of defect oxygen adsorption sites. we note, lx>wever, that the present AES results, which also suggest that oxygen chemisorbs for exposures within the region of the linear uptake (D-120L), are inconsistent with the work of Martinson et al, (1) who concluded that a surface oxide grows from the onset of oxygen exposure. The correspondence of the initial decrease in slope of the 0(510 ev) curve with the initial growth of the Al(54 ev) signal indicates that formation of the oxide reduces the rate of oxygen uptake. This could reflect a lower oxygen sticking coefficient and/or oxygen uptake limited by diffusion of oxygen into the bulk. We examine next the local geometric arrangement of the surface Al atoms as a function of oxygen exposure using ARSIMS. Since oxide formation on Al(lOO) is widely believed to disorder the surface (1-6), these measurements provide additional independent insight into the oxidation process which helps to
850 construct a consistent model of oxygen adsorption and oxidation on Al(lOO). + Al ARSIMS - Azimuthal angle distributions of Al+ ejected from the Al (100) surface -7 following increasing exposures to oxygen at 300 K (P = 5 x 10 torr) are 02 shown in Figure 3A, while Figure 3B presents a plot of the anistropy of the Al+ angle distribution as a function of oxygen exposure. Here, we define the angular anistropy as the ratio of the maximum to minimum intensity of each angle distribution. In the case of the clean surface, the fourfold symmetry of the Al(lOO) plane is clearly reflected by the angle distribution which 0 0 exhibits intensity maxima at ep = 45 and 135 and minima at ep = 0 and 180 • Comparison of the angle distribution for the clean surface with the crystal orientation, illustrated in Figure IB, shows that the directions of the intensity maxima correspond to ejection in the directions of the open spaces between surrounding nearest neighbor atoms while the intensity minima corresponds to ejection directly towards the surrounding nearest neighbor atoms. A preference for substrate ions to eject from ordered surfaces in the directions between nearest neighbor atoms has been observed previously (8-10) and is in agreement with the predictions of molecular dynamics simulations of ion-surface collisions carried out for ordered copper and nickel surfaces (810) and recently by us for the Al(lOO) surface (11). The computer simulations show that the angular anistropy results from a tendency for ejecting atoms to channel between nearest neighbor atoms where the repulsive potential is a minimum. Ejection directly towards nearest neighbor atoms is often blocked by strong scattering of the ejecting atom by the nearest neighbor atom. The observed angle distributions thus serve as a direct probe of the local geometric arrangement of nearest neighbor atoms on the original substrate surface. + Examination of the Al angle distribution as a function of oxygen exposure (Figure 3) shows that exposures up to ~120L have little effect on the angle distribution as the peak widths, positions and the angular anistropy all remain essentially constant. However, the anistropy is seen to decrease with increasing exposure above 120L until the angle distribution becomes isotropic at an exposure of ~ 1200L. we also note that while the angular anistropy decreases with increasing exposure above ~120L, the peak positions and widths remain nearly constant. The invariance of the Al+ angle distribution for exposures up to ~ 120L indicates that the geometric arrangement of the Al atoms remains the same as that for the clean Al(lOO) surface for exposures up to this level. This clearly demonstrates that an amorphous oxide does not form on Al(lOO) upon initial exposure to oxygen. This result, in combination with the observation that the AES Al(54 ev) signal, characteristic of Al oxide, does not begin to
851
A
1500L
B
1.4
400L
~1.3
o
0::
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"
!!!
D
a a
z
"
"
<1.1
D
D
1 10 50 90 130 1 70
AZIMUTHAL ANGLE
o
0.2 0.4 0.6 0.8 1 2 OXYGEN EXPOSURE (L xt 0-
)
(DEGREES)
Fig. 3. Azimuthal angle distributions of Al+ ejected from Al(lOO) following oxygen exposures of OL, 30L, 10~L, 400L and 1500L at 300 K (A), and azimuthal angular anistropy of ejected Al as a function of oxygen exposure (8). grow until
the exposure exceeds 'V120L furthermore strongly suggests that
oxygen chemisorbs on Al( 100) without reconstructing the Al( 100) surface for exposures up to 'V120L.
we
thus interpret the results of previous studies
(4,6,7) which indicate fading of LEED spots characteristic of the Al( 100) plane upon initial exposure to oxygen as being caused by random chemisorption of oxygen atoms as opposed to formation of an amorphous oxide. The observation that the Al+ angular anistropy decreases for increasing exposures above 'V120L, while the peak positions and widths remain characteristic of the Al( 100)
surface shows that the Al( 100) surface gradually
reconstructs to form disordered surface regions that coexist with unreconstructed regions of the surface for increasing exposures above 120L until the surface becomes completely disordered at 'V1200L. The initial decrease in the angular anistropy that occurs at 'V 120L is also well correlated to the initial appearance of the AES Al(54 ev) signal and the initial reduction in slope of the 0(510 ev) curve as shown in Figure 2. This provides strong evidence that an amorphous oxide phase first begins to form on Al(lOO) at an exposure of 'V 120L and continues to grow until the entire surface becomes amorphous at an exposure of l200L. The AES and Al+ARSIMS results discussed so far consistently indicate that oxygen chemisorbs on Al(lOO) without reconstructing the surface for exposures up to 'V 120L, while increasing exposures above 'V l20L causes growth of an amorphous oxide which completely disorders the surface at an exposure of '\J.200L. We examine next ARSIMS measurements of the polar angle distribution
852
of ejected 0
ions as a function of oxygen exposure which are shown to be
sensitive
the
to
population of surface versus subsurface oxygen binding
sites.
o
ARSIMS A series of polar angle distributions (PADS) of 0
Al (100)
ions ejected from the
surface recorded
=
oxygen at 300 K (P
following various exposures of the surface to 7 5 x 10- torr) are shown in Figure 4A. All polar angle
o=
distributions exhigit an 0- intensity minimum near maximum near
e = 50
0
45
0
and an intensity
ions tend to eject in at least o two distinct preferred polar angles: one occurring at e = 50 and the other 0 at or below e = 33 • To our knowledge, this is the first example of measured •
This indicates that 0
polar angle distributions that exhibit two distinctly resolvable preferred polar angles of ejection, as previous PAD measurements have typically exhibited only a simple intensity maximum.
we
do not believe the structure
observed in the present 0-PADS is caused by an artifact of the measurement + + method since our PAD measurements of Al and Al ions ejected from clean 2 0 0
Al(lOO) exhibit only simple intensity maxima at e = 47
and O = 43
respec-
tively. In considering the origin of the structure observed in the 0 PADS we first note that recent computer simulations of ion surface collisions appear to indicate a general tendency for atoms located in subsurface positions on the original surface to eject at much lower polar angles than surface atoms (12,13). For example, Coudray and Slodzian (12) have recently shown, using the program MARLOWE,
that While Cu atoms sputtered from a clean Cu(OOl)
surface exhibit a PAD with a simple maximum at
e = 50
0
,
in the presence of a
C(2 x 2) cesiumoverlayer, the maximum in the Cu PAD decreased dramatically to
e '" 0 0 •
Our own recent molecular dynamics simulations of Ar + bombardment
of clean Al(lOO) similarly show that while Al atoms ejected from the surface exhibit a PAD having a simple maximum at O = 50
0
, Al atoms ejected from the o second layer exhibit a PAD with a maximum at only e = 15-20 (13). The computer simulations predict that subsurface atoms tend to eject at lower polar
angles than surface atoms because ejection of subsurface atoms at high polar angles is usually blocked by collisions with atoms in the surface layer. Subsurface atoms that initially tend to eject at low polar angles, on the other hand, are more likely to be channeled upward between escape
the
surface.
These
computer
preferred polar angles of ejection at
simulations,
e
=
50
o
and
e
thus =
33
0
su~face
suggest
atoms and that
-
the
in the 0 PADS may
reflect 0 ions ejected from surface and subsurface sites respectively. In order to experimentally substantiate the link between the structure observed in the O-PADS and surface versus subsurface oxygen binding sites, we have measured the 0 PAD after exposing the Al( 100) surface to 120L at 300 K and heating to 525 K. The result is shown by the dashed curve in Figure 4A where it is compared with the 0-PAD for the corresponding case where the
853
B
8,33°
I:"
-,-:if"
8=50·
o
40
80
1 20 1 500
OXYGEN EXPOSURE (L)
30
40
50
60
70
POLAR ANGLE (DEGREES) Fig 4. Polar angle distributions of 0- ejected from Al(lOO) following oxygen exposures of 15~, 30L, 60L, 120L gnd_1500L at 300 K. (A), ang r~lative intensities of the 0 signal at 0 = 33 (0 (0 = 33» and at 0 = 50 (0 (0 = 50» and the 0-( 0 = 50)/0-( 0 = 33) ratio as a function of oxygen exposure (B). The dashed curve shows the 0- polar angle distribution obtained after heating to 525 K.
surface was not heated following a 120L oxygen exposure at 300 K. This comparison shows that heating to 525 K substantially reduces the 0- intensity at 0 0 o = 50 relative to that at 0 = 33 • Since our AES measurements show a decrease in the 0(510 ev)/Al(68 ev) ratio upon heating to 525 K, indicating diffusion of oxygen into the bulk, we interpret the observed decrease in the 0-( 0= 50)/0-( 0= 33) ratio as a direct reflection of a decrease in the ratio of occupied surface to subsurface oxygen sites which strongly supports the interpretation, based on computer simulations, that the preferred polar o 0 angles of ejection at 0 = 50 and 0 = 33 reflect the occupation of surface
854 and subsurface sites respectively. Figure 4A shows that the 0 intensities corresponding to the preferred = 50 o (0- ( 0 = 50)) and 0 = 33 0 (0- (0 = 33)) also char:!ge
angles of ejection at 0
significantly with increasing exposure. This is seen more clearly in Figure 4B which presents a plot of the 0-( 0 = 50) and 0-( 0 = 33) intensities as well as the 0-( 0 = 50) /0-( 0 = 33) ratio as a function of oxygen exposure. The 0-(0
= 50 0 )
and 0-(0
o ( 0 = 50)
= 33)
signals exhibit substantially different behavior: the
intensity increases in a smooth continuous fashion from the outset
of oxygen exposure while the 0 (0
=
33)
signal
initially increases very
rapidly with increasing exposure between 0 and 15L, then levels off between 15 and 30L and subsequently increases again as the exposure increases above 30L. The 0-( 0 = 50)/0-( 0 = 33)
ratio is seen to increase by a factor of
nearly 2 as the exposure increases from 15 to 6OL, then decreases as the exposure increases between 60 and 120L and becomes essentially independent of oxygen exposure above 120L. Before interpreting the 0
intensity variations plotted in Figure 4B we
note that changes in the electronic structure of the surface due to adsorption of electronegative oxygen atoms are, in general, expected to substantially influence the 0- ion yields at high oxygen coverages where oxygen atoms are adsorbed in close proximity to each other. However, in the exposure range between 0 and roughly
~30L,
we expect the influence of such electronic
matrix effects to be small since AES measurements (see Figure 2) show that the oxygen coverage remains very low «.05ML) within this exposure range. We therefore expect the relative 0 intensities to be indicative of the relative oxygen coverage for exposures up to roughly ~30L. At higher oxygen exposures, where the influence of electronic matrix effects makes interpretation of the
o
intensity variations more difficult, we focus principally on the 0-(0 =
50)/0 ( 0 = 33) intensity ratio, which is expected to be relatively insensitive to matrix effects since the influence of such effects common to both 0-( 0 = 50) and 0-( 0 = 33) divide out. Thus, in view of the sensitivity of the 0 (0 adsorbed in surface sites, as described above, initial increase in the 0-( 0
=
50) signal to oxygen atoms we interpret the observed
50) signal with increasing oxygen exposure as
reflecting the population of surface sites which begins essentially from the outset of oxygen exposure. on the other hand, the rapid initial increase in the 0-( 0 = 33) signal that occurs for increasing exposures up to 15L, its leveling off between 15 and 30L and its subsequent increase again as the exposure increases above ~30L
suggests that the 0-( 0 = 33) signal has con-
tributions from two different types of adsorbed oxygen which populate at two different levels of oxygen exposure. In particular, the initial rapid increase and subsequent leveling off of the 0-(0 = 33) signal is indicative of the rapid population of a type of adsorption site that becomes essentially completely filled at an exposure of only
~
15L. We note that the number of
such sites is very small since the AES measurements (see Figure 2) show that
855
at 15L the 0(510 ev)/Al(68ev) peak-to-peak intensity ratio is only .08 indicating an oxygen coverage of roughly 'V .04ML. We tentatively suggest that the
initial rapid population of such a
small number of
sites
probably
reflects oxygen adsorption into a small number of active defect sites. Additional studies of oxygen adsorption within the 0-15 L region are presently being conducted to confirm this assignment and will be reported separately. In view of the sensitivity of the 0-(0 = 33) signal to subsurface oxygen, as discussed above, we interpret the second distinct increase in the 0-( 0 = 33) signal that occurs for increasing exposures above 'V 30L as indicating the onset of penetration of oxygen beneath the surface and population of subsurface sites. Thus, assuming changes in the 0-( 0= 50)/0-( 0= 33) ratio reflect changes in the relative population of surface to subsurface sites for exposures above 15L (where defect sites become saturated) the rapid increase in the 0-( 0 = 50)/0-(0 = 33) ratio as the exposure increases above 'V15L simply reflects the fact that surface sites populate from
the outset of oxygen
exposure while subsurface sites do not begin to populate until the exposure exceeds 30L. As the exposure exceeds 'V 30L and subsurface sites begin to populate, the slope of the 0-( 0 = 50)/0-(0 = 33) curve gradually decreases reflecting a decrease in the relative rate of population of surface to subsurface sites. Eventually the slope of the 0-( 0 = 50)/0-(0 = 33) curve becomes negative as the exposure increases between 60 and 120L which suggests that subsurface sites populate more rapidly than surface sites in this ex-
+
posure region. In light of the AES and Al ARSIMS results discussed above, which indicate that oxide formation and associated reconstruction of the Al(lOO) surface does not begin until the exposure exceeds'V120L, we conclude from the present observations that both surface and subsurface sites become populated before oxide formation and surface reconstruction begins.
we
note
that this conclusion is also consistent with the work of Crowell et al (14) and Chen et al (15) who similarly concluded on the basis of HREELS measurements that both surface and subsurface oxygen sites become populated before oxidation begins on Al(lll). The invariance of the 0-( 0 = 50) /0-( 0 = 33) ratio as well as the invariance of the relative shape of the 0-PAD (see Figure 4A) for oxygen exposures above 'V120L suggests that population of surface and subsurface sites saturates at 'V 120L. Since the AES and Al +ARSIMS results show that the surface first begins to oxidize and disorder as the exposure increases above 'V120L, this suggests that oxidation is probably caused by penetration of oxygen atoms beneath the oxygen saturated surface and subsurface layers and that the O-PAD is insensitive to the occupation of these deeper sites. This interpretation is reasonable on the basis of computer simulations which indicate a
very low probability for ejection of atoms below the second layer. we believe this interpretation is also consistent with the continued parallel increase in the 0-( 0 = 50) and 0-( 0 = 33) signals with increasing exposure above 'V120L since penetration of electronegative oxygen beneath the surface reduces the
856
surface work function (6) which typically increases the yield of negative ions. CONCLUSIONS The interaction of oxygen with Al( 100) at 300 K has been studied using + AES, Al ARSIMS and 0 ARSIMS. The results of this study suggest the following model for the interaction of oxygen with Al(lOO): as the oxygen exposure increases from 0 to ce 30L, oxygen atoms chemisorb at surface and defect sites with the defect sites becoming saturated at ce 15L. As the oxygen exposure increases further between 30 and 60L, subsurface sites begin to populate and at ce120L both surface and subsurface sites become saturated. For exposures up to ce120L the Al(lOO) surface does not oxidize or reconstruct. However, as the exposure increases above ce120L an amorphous oxide first begins to grow which completely disorders the surface at an exposure of ce1200L. 0 ARSIMS measurements suggest that oxidation is probably associated with penetration of oxygen atoms beneath the top two oxygen saturated layers. This study also demonstrates the potential of ARSIMS for probing the local geometric structure of surfaces and for identifying adsorbate binding sites. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
C.W.B. Martinson and S.A. Flodstrom, Surface Sci., 80 (1979) 306. S.A. Flodstrom, C.W.B. Martinson, R.Z. Bachr ack , S.B.M. Hagstrom and R.S. Bauer, Phys. Rev. Lett., 40 (1978) 907. W. Eberhardt and C. Kunz, Surface Sci., 75 (1978) 709. I.P. Batra and L. Kleinman, J. of Elec. Spec. and Related Phenomena, 33 (1984) 175. R. Michel, C. Jourdan, J. Gastaldi and J. Denien, Surface Sci., 84 (1979) L509. R. Michel, J. Gastalki, C. Allasia and C. Jourdan, Surface SCi., 95 (1980) 309. M.L. den Boer, T.L. Einstein, W.T. Elam, R.L. Park, L.D. Roelofs and G.E. Laramore, Phys. Rev. Lett., 44 (1980) 496. N. Winograd and B.J. Garrison, Arc. Chern. Res., 13 (1980) 406. S.P. Holland, B.J. Garrison and N. Winograd, Phys. Rev. Lett., 43 (1979) 220. R.A. Gibbs, S.P. Holland, R.E. Foley, B.J. Garrison and N. Winograd, J. Chern. Phys., 76 (1982) 684. L.L. Lauderback and A. Lynn, to be published. C. Coudray and G. Slodzian, Nucl. Inst. and Methods in Phys. Res. B15 (1986) 29. L.L. Lauderback and A. Lynn, to be published. J •E. Crowell, J.G. Chen and J. T. Yates, Jr., Surface Sc i , , 165 (1986) 37. J.G. Chen, J.E. Crowell and J.T. Yates, Jr., Phys. Rev. B, 33 (1986) 1436.
845
AN ANGLE RESOLVED SIMS AND AUGER ELECTRON SPECTROSCOPY STUDY OF THE OXIDATION OF Al(lOO)
L.L. LAUDERBACK and S.A. LARSON Department of Chemical Engineering, The University of Colorado, Colorado 80309-0424
Boulder,
ABSTRACT Angle resol ved SIMS (ARSIMS) and Auger electron spectroscopy (AES) have been used to study the interaction of oxygen with Al( 100) at 300 K. ARSIMS measurements of the Al+ azimuthal angle distribution and AES measurements show that oxygen chemisorbs on Al(lOO) without reconstructing the Al surface for oxygen exposure up to roughly 120L. As the exposure exceeds 120L an amorphous oxide begins to grow which completely disorders the Al surface at an exposure of about 1200L. ARSIMS measurements of the 0- polar angle distribution are shown to be sensitive to the occupation of surface versus subsurface oxygen bonding sites. These measurements indicate that while surface sites are preferentially populated upon initial exposure to oxygen, both surface and subsurface sites become populated prior to oxide formation at "-'l20L. INTRODUCTION Although a number of surface science studies of the interaction of oxygen with Al(lOO) have been reported (1-7), much uncertainty still exists about the nature of the oxidation process on this face. While some studies suggest that the Al(lOO) surface begins to oxidize immediately upon initial exposure to oxygen, other studies indicate a two step oxidation process is involved in which oxygen chemisorbs prior to oxide formation. For example, Flodstrom et al (1,2) concluded, on the basis of A12p core level photoemission studies, that while a chemisorbed oxygen state precedes oxide formation on Al(lll), an amorphous oxide forms on Al( 100) upon initial exposure to oxygen and grows according to the island growth model. A12p core level spectra of Eberhardt and Kunz (3) were also interpreted as indicating direct penetration of oxygen atoms beneath the surface and oxide formation upon initial exposure of Al(lOO) to oxygen. In contrast to these studies however, Michel et al (5,6) concluded, on the basis of AES measurements, that formation of a chemisorbed state does occur on Al (100) prior to the formation of an amorphous oxide. Their results show that while the Al(68 ev) and 0(510 ev) AES signals generally decrease and increase respectively with increasing oxygen exposure from the outset, the Al(54 ev) signal, characteristic of Al oxide, does not
846 begin to grow until the oxygen coverage exceeds roughly .5ML. This was interpreted as indicating that only a chemisorbed oxygen phase existed for coverages below ~.5ML. LEED measurements (4,6,7) generally show that diffraction spots characteristic of the Al(lOO) plane gradually fade away from the outset of oxygen exposure and completely disappear for oxygen coverages above .5ML. While the complete disappearance of the LEED pattern is consistent with the formation of an amorphous oxide at high coverages, interpretation of the initial fading is ambiguous since this could be caused either by random chemisorption of oxygen on the ordered Al( 100) surface or by disordering of the surface by direct formation of an amorphous oxide. No diffraction spots due to adsorbed oxygen have been observed and the oxygen binding sites are unknown. The present work is in part aimed at clarifying whether or not a chemisorbed phase precedes the formation of an amorphous oxide on Al(lOO) by correlating AES measurements with angle resolved secondary ion mass + spectrometry (ARSIMS) measurements of the Al azimuthal angle distribution as . + a f unc t ron of oxygen exposure. We show that the Al ARSIMS measurements directly reflect the local geometric arrangement of the Al substrate atoms and thereby directly indicate the disordering of the Al surface caused by amorphous oxide formation. we also report ARSIMS measurements of the 0- polar angle distribution (PAD) which are shown to be sensitive to the occupation of surface versus subsurface oxygen binding sites. From the measurements we show how the population of surface and subsurface bind sites relates to the level of oxygen exposure and to the formation of chemisorbed and amorphous oxide phases on Al(lOO).
EXPERIMENTAL The ARSIMS and AES measurements were performed in a two-level tur-10 bomolecular pumped stainless steel UHV chamber (base pressure = 5 x 10 torr) that is also equipped with low energy electron diffraction (LEED) and thermal desorption spectroscopy techniques. AES measurements were carried out using a PHI, single pass, cylindrical mirror analyzer and a co-axial electron gun. The primary electron energy was 5.0 Kev. A low current (.5 wA) de focused electron beam was used in order to minimize the increase in sample temperature and reduction of the surface by the electron beam. A schematic of the ARSIMS experiment is shown in Figure lAo The primary + Ar ion beam was generated by a Colutron model G-2 ion gun and was mass analyzed by a Wien type velocity filter. All experiments were performed with + 4.5 Kev Ar ions imping in on the sample at normal inc idence and with a 2. current density of 5 x 10- amps/cm The primary ion beam was collimated to a diameter of 2.4 mm by a grounded aperture located 3 cm in front of the sample as shown in Figure LA, Secondary ions emitted from the surface were accepted into a bessel box energy analyzer through an entrance aperture that
g
00
847
A
CRYSTAL
0.24 em
' '~
~--I._-.....;:::.;...-
1 - 3 . 0 em
DEFLECTION PLATES
ell1 -/;1 T
+
ENERGY ANALVZER AND QUADRUPOLE MASS SPECTROMETER
B
-
rp=O
o
( 100) Fig. 1. Schematic of the ARSIMS apparatus (A) and azimuthal orientation of the Al(lOO) plane (B). is located in a grounded shield placed around the end of the energy analyzer. After passing through the energy analyzer the secondary ions were subsequently mass analyzed by an Extranuclear Laboratories quadrapole mass spectrometer and detected using pulse counting techniques. The azimuthal angle (¢) of the detected secondary ions was variable from 0 to 180 degrees by rotating the crystal about its surface normal. The azimuthal angle is defined relative to the crystal orientation, as determined by LEED, in Figure lB. The polar angle (8) of the detected secondary ions could be varied from 33 to 90 degrees, as measured from the surface normal, by translating the crystal directly away from or directly towards the primary ion beam. As seen from Figure lA, variation of the polar angle in this manner also causes variation of the angle at which the secondary ions pass through the entrance aperture to the energy analyzer. The trajectory of the secondary ions is therefore realigned after passing through the entrance aperture by a set of electrostatic deflection plates (see Figure lA) to achieve maximum transmission through the energy analyzer for a given pass energy and polar
848 angle. We note that the energy band width associated with the deflection plates is much greater than the +1.5 ev band width of the bessel box analyzer, so the bessel box controls the energy resol.ut ion as well as the energy selection. The pass energy used in all experiments was 10.0 ev. + All reported Al azimuthal angle distributions were obtained at a fixed o + polar angle of 45 by measuring the Al signal intensity a discrete 10 degree 0 0 intervals from cjJ = 0 to cjJ = 180 • All polar angle distributions were obtained by measuring the 0 signal intensity at discrete intervals from 0 = 0 0 0• 33 to 0= 65 with the azimuthal angle fixed at cjJ = 45 A small intensity correction has been included in the reported 0 polar angle distributions to 0 0 correct for changes in the angular resolution, which varies from 6 to 9 as the polar angle increases from 33 to 65 degrees. This correction normalizes o all 0 intensities to that corresponding to an angle resolution of 7 which is the actual resolution at a polar angle of 45 degrees. The Al(lOO) crystal was obtained precut and oriented from the Monocrystals Company (99.999% purity) and was subsequently polished using diamond paste ( .05 u m). The crystal was mounted on a sample holder via two stainless steel clips. A chromel-alumel thermocouple was attached to one mounting clip for monitoring the sample temperature. Heating was accomplished by electron bombardment from behind the sample. The surface cleaning procedure consisted of many cycles of an annealing to ~4500C and ion bombardment. Surface cleanliness and orientation was verified by AES and LEED respectively.
RESULTS AND DISCUSSION AES of Oxygen Adsorption on Al(lOO) The peak-to-peak intensities of the 0(510 ev), Al(68 ev) and Al(54 ev) AES signals are plotted in Figure 2 as a function of oxygen exposure. While the 0(510 ev) and Al(68 ev) signals increase and decrease respectively with increasing oxygen exposure from the outset, the Al (54 ev) signal, characteristic of Al oxide, does not begin to grow until the exposure exceeds roughly ~ 120L. This suggests that adsorption of oxygen, for exposures up to ~ 120L, may produce a chemisorbed oxygen phase instead of a surface oxide. Growth of the Al(54 ev) feature does, however, indicate growth of a surface oxide for increasing exposures above -v 120L. The 0(510 ev) signal increases almost linearly with increasing exposures up to the point where the Al(54 ev) signal begins to grow. The slope of the O(510 ev) curve then begins to decrease until the O(510 ev) signal nearly saturates at roughly ~1200L. The linear increase in the 0(510 ev) signal is consistent with earlier AES studies of oxygen uptake on Al(lOO) (1) and suggests that new adsorption sites may be created as the uptake proceeds. This has been interpreted previously by Martinson et al (1) as indicating that oxygen may initially diffuse into the bulk upon adsorption, thereby maintaining a full complement of surface adsorption sites and/or that oxygen
849
o
0.2
0.4
0.6
0.8
OXYGEN EXPOSURE (L Xl0- Z )
1
1.2
Fig. 2. AES peak-to-peak intensities of the 0(510 ev), Al(68 ev) and Al(54 ev) signals as a function of oxygen exposure at 300 K. adsorption may proceed according to the island growth mechanism in which new adsorption sites are generated at the perimeter of growing oxygen islands until the island coalesce (1). In the latter case, defect sites are assumed to act as nucleation centers for island growth. Both of these interpretations are consistent with results to be discussed below which indicate diffusion of oxygen into the subsurface region for exposures >30L and the presence of defect oxygen adsorption sites. we note, lx>wever, that the present AES results, which also suggest that oxygen chemisorbs for exposures within the region of the linear uptake (D-120L), are inconsistent with the work of Martinson et al, (1) who concluded that a surface oxide grows from the onset of oxygen exposure. The correspondence of the initial decrease in slope of the 0(510 ev) curve with the initial growth of the Al(54 ev) signal indicates that formation of the oxide reduces the rate of oxygen uptake. This could reflect a lower oxygen sticking coefficient and/or oxygen uptake limited by diffusion of oxygen into the bulk. We examine next the local geometric arrangement of the surface Al atoms as a function of oxygen exposure using ARSIMS. Since oxide formation on Al(lOO) is widely believed to disorder the surface (1-6), these measurements provide additional independent insight into the oxidation process which helps to
850 construct a consistent model of oxygen adsorption and oxidation on Al(lOO). + Al ARSIMS - Azimuthal angle distributions of Al+ ejected from the Al (100) surface -7 following increasing exposures to oxygen at 300 K (P = 5 x 10 torr) are 02 shown in Figure 3A, while Figure 3B presents a plot of the anistropy of the Al+ angle distribution as a function of oxygen exposure. Here, we define the angular anistropy as the ratio of the maximum to minimum intensity of each angle distribution. In the case of the clean surface, the fourfold symmetry of the Al(lOO) plane is clearly reflected by the angle distribution which 0 0 exhibits intensity maxima at ep = 45 and 135 and minima at ep = 0 and 180 • Comparison of the angle distribution for the clean surface with the crystal orientation, illustrated in Figure IB, shows that the directions of the intensity maxima correspond to ejection in the directions of the open spaces between surrounding nearest neighbor atoms while the intensity minima corresponds to ejection directly towards the surrounding nearest neighbor atoms. A preference for substrate ions to eject from ordered surfaces in the directions between nearest neighbor atoms has been observed previously (8-10) and is in agreement with the predictions of molecular dynamics simulations of ion-surface collisions carried out for ordered copper and nickel surfaces (810) and recently by us for the Al(lOO) surface (11). The computer simulations show that the angular anistropy results from a tendency for ejecting atoms to channel between nearest neighbor atoms where the repulsive potential is a minimum. Ejection directly towards nearest neighbor atoms is often blocked by strong scattering of the ejecting atom by the nearest neighbor atom. The observed angle distributions thus serve as a direct probe of the local geometric arrangement of nearest neighbor atoms on the original substrate surface. + Examination of the Al angle distribution as a function of oxygen exposure (Figure 3) shows that exposures up to ~120L have little effect on the angle distribution as the peak widths, positions and the angular anistropy all remain essentially constant. However, the anistropy is seen to decrease with increasing exposure above 120L until the angle distribution becomes isotropic at an exposure of ~ 1200L. we also note that while the angular anistropy decreases with increasing exposure above ~120L, the peak positions and widths remain nearly constant. The invariance of the Al+ angle distribution for exposures up to ~ 120L indicates that the geometric arrangement of the Al atoms remains the same as that for the clean Al(lOO) surface for exposures up to this level. This clearly demonstrates that an amorphous oxide does not form on Al(lOO) upon initial exposure to oxygen. This result, in combination with the observation that the AES Al(54 ev) signal, characteristic of Al oxide, does not begin to
851
A
1500L
B
1.4
400L
~1.3
o
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!!!
D
a a
z
"
"
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D
D
1 10 50 90 130 1 70
AZIMUTHAL ANGLE
o
0.2 0.4 0.6 0.8 1 2 OXYGEN EXPOSURE (L xt 0-
)
(DEGREES)
Fig. 3. Azimuthal angle distributions of Al+ ejected from Al(lOO) following oxygen exposures of OL, 30L, 10~L, 400L and 1500L at 300 K (A), and azimuthal angular anistropy of ejected Al as a function of oxygen exposure (8). grow until
the exposure exceeds 'V120L furthermore strongly suggests that
oxygen chemisorbs on Al( 100) without reconstructing the Al( 100) surface for exposures up to 'V120L.
we
thus interpret the results of previous studies
(4,6,7) which indicate fading of LEED spots characteristic of the Al( 100) plane upon initial exposure to oxygen as being caused by random chemisorption of oxygen atoms as opposed to formation of an amorphous oxide. The observation that the Al+ angular anistropy decreases for increasing exposures above 'V120L, while the peak positions and widths remain characteristic of the Al( 100)
surface shows that the Al( 100) surface gradually
reconstructs to form disordered surface regions that coexist with unreconstructed regions of the surface for increasing exposures above 120L until the surface becomes completely disordered at 'V1200L. The initial decrease in the angular anistropy that occurs at 'V 120L is also well correlated to the initial appearance of the AES Al(54 ev) signal and the initial reduction in slope of the 0(510 ev) curve as shown in Figure 2. This provides strong evidence that an amorphous oxide phase first begins to form on Al(lOO) at an exposure of 'V 120L and continues to grow until the entire surface becomes amorphous at an exposure of l200L. The AES and Al+ARSIMS results discussed so far consistently indicate that oxygen chemisorbs on Al(lOO) without reconstructing the surface for exposures up to 'V 120L, while increasing exposures above 'V l20L causes growth of an amorphous oxide which completely disorders the surface at an exposure of '\J.200L. We examine next ARSIMS measurements of the polar angle distribution
852
of ejected 0
ions as a function of oxygen exposure which are shown to be
sensitive
the
to
population of surface versus subsurface oxygen binding
sites.
o
ARSIMS A series of polar angle distributions (PADS) of 0
Al (100)
ions ejected from the
surface recorded
=
oxygen at 300 K (P
following various exposures of the surface to 7 5 x 10- torr) are shown in Figure 4A. All polar angle
o=
distributions exhigit an 0- intensity minimum near maximum near
e = 50
0
45
0
and an intensity
ions tend to eject in at least o two distinct preferred polar angles: one occurring at e = 50 and the other 0 at or below e = 33 • To our knowledge, this is the first example of measured •
This indicates that 0
polar angle distributions that exhibit two distinctly resolvable preferred polar angles of ejection, as previous PAD measurements have typically exhibited only a simple intensity maximum.
we
do not believe the structure
observed in the present 0-PADS is caused by an artifact of the measurement + + method since our PAD measurements of Al and Al ions ejected from clean 2 0 0
Al(lOO) exhibit only simple intensity maxima at e = 47
and O = 43
respec-
tively. In considering the origin of the structure observed in the 0 PADS we first note that recent computer simulations of ion surface collisions appear to indicate a general tendency for atoms located in subsurface positions on the original surface to eject at much lower polar angles than surface atoms (12,13). For example, Coudray and Slodzian (12) have recently shown, using the program MARLOWE,
that While Cu atoms sputtered from a clean Cu(OOl)
surface exhibit a PAD with a simple maximum at
e = 50
0
,
in the presence of a
C(2 x 2) cesiumoverlayer, the maximum in the Cu PAD decreased dramatically to
e '" 0 0 •
Our own recent molecular dynamics simulations of Ar + bombardment
of clean Al(lOO) similarly show that while Al atoms ejected from the surface exhibit a PAD having a simple maximum at O = 50
0
, Al atoms ejected from the o second layer exhibit a PAD with a maximum at only e = 15-20 (13). The computer simulations predict that subsurface atoms tend to eject at lower polar
angles than surface atoms because ejection of subsurface atoms at high polar angles is usually blocked by collisions with atoms in the surface layer. Subsurface atoms that initially tend to eject at low polar angles, on the other hand, are more likely to be channeled upward between escape
the
surface.
These
computer
preferred polar angles of ejection at
simulations,
e
=
50
o
and
e
thus =
33
0
su~face
suggest
atoms and that
-
the
in the 0 PADS may
reflect 0 ions ejected from surface and subsurface sites respectively. In order to experimentally substantiate the link between the structure observed in the O-PADS and surface versus subsurface oxygen binding sites, we have measured the 0 PAD after exposing the Al( 100) surface to 120L at 300 K and heating to 525 K. The result is shown by the dashed curve in Figure 4A where it is compared with the 0-PAD for the corresponding case where the
853
B
8,33°
I:"
-,-:if"
8=50·
o
40
80
1 20 1 500
OXYGEN EXPOSURE (L)
30
40
50
60
70
POLAR ANGLE (DEGREES) Fig 4. Polar angle distributions of 0- ejected from Al(lOO) following oxygen exposures of 15~, 30L, 60L, 120L gnd_1500L at 300 K. (A), ang r~lative intensities of the 0 signal at 0 = 33 (0 (0 = 33» and at 0 = 50 (0 (0 = 50» and the 0-( 0 = 50)/0-( 0 = 33) ratio as a function of oxygen exposure (B). The dashed curve shows the 0- polar angle distribution obtained after heating to 525 K.
surface was not heated following a 120L oxygen exposure at 300 K. This comparison shows that heating to 525 K substantially reduces the 0- intensity at 0 0 o = 50 relative to that at 0 = 33 • Since our AES measurements show a decrease in the 0(510 ev)/Al(68 ev) ratio upon heating to 525 K, indicating diffusion of oxygen into the bulk, we interpret the observed decrease in the 0-( 0= 50)/0-( 0= 33) ratio as a direct reflection of a decrease in the ratio of occupied surface to subsurface oxygen sites which strongly supports the interpretation, based on computer simulations, that the preferred polar o 0 angles of ejection at 0 = 50 and 0 = 33 reflect the occupation of surface
854 and subsurface sites respectively. Figure 4A shows that the 0 intensities corresponding to the preferred = 50 o (0- ( 0 = 50)) and 0 = 33 0 (0- (0 = 33)) also char:!ge
angles of ejection at 0
significantly with increasing exposure. This is seen more clearly in Figure 4B which presents a plot of the 0-( 0 = 50) and 0-( 0 = 33) intensities as well as the 0-( 0 = 50) /0-( 0 = 33) ratio as a function of oxygen exposure. The 0-(0
= 50 0 )
and 0-(0
o ( 0 = 50)
= 33)
signals exhibit substantially different behavior: the
intensity increases in a smooth continuous fashion from the outset
of oxygen exposure while the 0 (0
=
33)
signal
initially increases very
rapidly with increasing exposure between 0 and 15L, then levels off between 15 and 30L and subsequently increases again as the exposure increases above 30L. The 0-( 0 = 50)/0-( 0 = 33)
ratio is seen to increase by a factor of
nearly 2 as the exposure increases from 15 to 6OL, then decreases as the exposure increases between 60 and 120L and becomes essentially independent of oxygen exposure above 120L. Before interpreting the 0
intensity variations plotted in Figure 4B we
note that changes in the electronic structure of the surface due to adsorption of electronegative oxygen atoms are, in general, expected to substantially influence the 0- ion yields at high oxygen coverages where oxygen atoms are adsorbed in close proximity to each other. However, in the exposure range between 0 and roughly
~30L,
we expect the influence of such electronic
matrix effects to be small since AES measurements (see Figure 2) show that the oxygen coverage remains very low «.05ML) within this exposure range. We therefore expect the relative 0 intensities to be indicative of the relative oxygen coverage for exposures up to roughly ~30L. At higher oxygen exposures, where the influence of electronic matrix effects makes interpretation of the
o
intensity variations more difficult, we focus principally on the 0-(0 =
50)/0 ( 0 = 33) intensity ratio, which is expected to be relatively insensitive to matrix effects since the influence of such effects common to both 0-( 0 = 50) and 0-( 0 = 33) divide out. Thus, in view of the sensitivity of the 0 (0 adsorbed in surface sites, as described above, initial increase in the 0-( 0
=
50) signal to oxygen atoms we interpret the observed
50) signal with increasing oxygen exposure as
reflecting the population of surface sites which begins essentially from the outset of oxygen exposure. on the other hand, the rapid initial increase in the 0-( 0 = 33) signal that occurs for increasing exposures up to 15L, its leveling off between 15 and 30L and its subsequent increase again as the exposure increases above ~30L
suggests that the 0-( 0 = 33) signal has con-
tributions from two different types of adsorbed oxygen which populate at two different levels of oxygen exposure. In particular, the initial rapid increase and subsequent leveling off of the 0-(0 = 33) signal is indicative of the rapid population of a type of adsorption site that becomes essentially completely filled at an exposure of only
~
15L. We note that the number of
such sites is very small since the AES measurements (see Figure 2) show that
855
at 15L the 0(510 ev)/Al(68ev) peak-to-peak intensity ratio is only .08 indicating an oxygen coverage of roughly 'V .04ML. We tentatively suggest that the
initial rapid population of such a
small number of
sites
probably
reflects oxygen adsorption into a small number of active defect sites. Additional studies of oxygen adsorption within the 0-15 L region are presently being conducted to confirm this assignment and will be reported separately. In view of the sensitivity of the 0-(0 = 33) signal to subsurface oxygen, as discussed above, we interpret the second distinct increase in the 0-( 0 = 33) signal that occurs for increasing exposures above 'V 30L as indicating the onset of penetration of oxygen beneath the surface and population of subsurface sites. Thus, assuming changes in the 0-( 0= 50)/0-( 0= 33) ratio reflect changes in the relative population of surface to subsurface sites for exposures above 15L (where defect sites become saturated) the rapid increase in the 0-( 0 = 50)/0-(0 = 33) ratio as the exposure increases above 'V15L simply reflects the fact that surface sites populate from
the outset of oxygen
exposure while subsurface sites do not begin to populate until the exposure exceeds 30L. As the exposure exceeds 'V 30L and subsurface sites begin to populate, the slope of the 0-( 0 = 50)/0-(0 = 33) curve gradually decreases reflecting a decrease in the relative rate of population of surface to subsurface sites. Eventually the slope of the 0-( 0 = 50)/0-(0 = 33) curve becomes negative as the exposure increases between 60 and 120L which suggests that subsurface sites populate more rapidly than surface sites in this ex-
+
posure region. In light of the AES and Al ARSIMS results discussed above, which indicate that oxide formation and associated reconstruction of the Al(lOO) surface does not begin until the exposure exceeds'V120L, we conclude from the present observations that both surface and subsurface sites become populated before oxide formation and surface reconstruction begins.
we
note
that this conclusion is also consistent with the work of Crowell et al (14) and Chen et al (15) who similarly concluded on the basis of HREELS measurements that both surface and subsurface oxygen sites become populated before oxidation begins on Al(lll). The invariance of the 0-( 0 = 50) /0-( 0 = 33) ratio as well as the invariance of the relative shape of the 0-PAD (see Figure 4A) for oxygen exposures above 'V120L suggests that population of surface and subsurface sites saturates at 'V 120L. Since the AES and Al +ARSIMS results show that the surface first begins to oxidize and disorder as the exposure increases above 'V120L, this suggests that oxidation is probably caused by penetration of oxygen atoms beneath the oxygen saturated surface and subsurface layers and that the O-PAD is insensitive to the occupation of these deeper sites. This interpretation is reasonable on the basis of computer simulations which indicate a
very low probability for ejection of atoms below the second layer. we believe this interpretation is also consistent with the continued parallel increase in the 0-( 0 = 50) and 0-( 0 = 33) signals with increasing exposure above 'V120L since penetration of electronegative oxygen beneath the surface reduces the
856
surface work function (6) which typically increases the yield of negative ions. CONCLUSIONS The interaction of oxygen with Al( 100) at 300 K has been studied using + AES, Al ARSIMS and 0 ARSIMS. The results of this study suggest the following model for the interaction of oxygen with Al(lOO): as the oxygen exposure increases from 0 to ce 30L, oxygen atoms chemisorb at surface and defect sites with the defect sites becoming saturated at ce 15L. As the oxygen exposure increases further between 30 and 60L, subsurface sites begin to populate and at ce120L both surface and subsurface sites become saturated. For exposures up to ce120L the Al(lOO) surface does not oxidize or reconstruct. However, as the exposure increases above ce120L an amorphous oxide first begins to grow which completely disorders the surface at an exposure of ce1200L. 0 ARSIMS measurements suggest that oxidation is probably associated with penetration of oxygen atoms beneath the top two oxygen saturated layers. This study also demonstrates the potential of ARSIMS for probing the local geometric structure of surfaces and for identifying adsorbate binding sites. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
C.W.B. Martinson and S.A. Flodstrom, Surface Sci., 80 (1979) 306. S.A. Flodstrom, C.W.B. Martinson, R.Z. Bachr ack , S.B.M. Hagstrom and R.S. Bauer, Phys. Rev. Lett., 40 (1978) 907. W. Eberhardt and C. Kunz, Surface Sci., 75 (1978) 709. I.P. Batra and L. Kleinman, J. of Elec. Spec. and Related Phenomena, 33 (1984) 175. R. Michel, C. Jourdan, J. Gastaldi and J. Denien, Surface Sci., 84 (1979) L509. R. Michel, J. Gastalki, C. Allasia and C. Jourdan, Surface SCi., 95 (1980) 309. M.L. den Boer, T.L. Einstein, W.T. Elam, R.L. Park, L.D. Roelofs and G.E. Laramore, Phys. Rev. Lett., 44 (1980) 496. N. Winograd and B.J. Garrison, Arc. Chern. Res., 13 (1980) 406. S.P. Holland, B.J. Garrison and N. Winograd, Phys. Rev. Lett., 43 (1979) 220. R.A. Gibbs, S.P. Holland, R.E. Foley, B.J. Garrison and N. Winograd, J. Chern. Phys., 76 (1982) 684. L.L. Lauderback and A. Lynn, to be published. C. Coudray and G. Slodzian, Nucl. Inst. and Methods in Phys. Res. B15 (1986) 29. L.L. Lauderback and A. Lynn, to be published. J •E. Crowell, J.G. Chen and J. T. Yates, Jr., Surface Sc i , , 165 (1986) 37. J.G. Chen, J.E. Crowell and J.T. Yates, Jr., Phys. Rev. B, 33 (1986) 1436.