XPS, UPS AND XAES studies of oxygen adsorption on polycrystalline Mg at ∼100 and ∼300 K

Surface Science 69 (1977) 581-608 0 North-Ho~and Pub~shing Company

XI’S, UPS AND XAES STUDIES OF OXYGEN ADSORPTION POLY~RYSTALLINE

Mg AT -100

AND -300

ON

K

J.C. FUGGLE * Insfitut fiir Festkbrperphysik,

Technische Universitd Mtinchen, D-8046 Garching, W. Germany

Received 9 May 1977; manuscrjpt received in final form 24 June 1977

Our results show that at both -300 and -100 K the adsorption of oxygen on evaporated Mg slows dramatically when oxide layers -7 A thick have formed on the surface. At 100 K the initial sticking coefficient is approximately 0.1. The oxidation goes through two phases at both temperatures; initial adsorption of up to approximately one monolayer, followed by formation of a layer with many characteristics similar to those of bulk MgO. During initial adsorption at 300 K the bulk of the oxygen adsorbed must sit below the metal surface, at 100 R its position is unclear. At both 100 and 300 K the photoelectron spectra from the surface layer produced by initial adsorption of oxygen are unlike those from either MgO or Mg. Even at “saturation” coverage, XAES (X-ray excited Auger electron spectroscopy) provides evidence for a layer unlike MgO, or Mg, at the MgO/Mg interface. It is found that shifts of 0.5 eV in the photoelectron peaks from the oxide layer can be induced by only -5 X 1013 oxygen atoms cmw2. 1. Introduction

It is now clear that to explain all the results of investigations of adsorption of simple molecules on clean transition metals we have to postulate a bewildering multiplicity of adsorption states, often even for adsorption of a gas on one single crystal face. Adsorption on simple metals has been less studied, perhaps because simple metals are less important, economically and technologically, as catalysts in the chemical industry. However, oxidation of all metals is of direct importance and adsorption on simple metals is of additional academic importance because on these metals theoretical studies using free-electron approximations may be able to yield sensible results and a basis for interpretation of experiment. Further, studies of adsorption on simple metals may give a new perspective to studies of transition metals; for instance it would be interesting to know if the complexity found in adsorption on transition metals is just another of their peculiarities, or a universal phenomena in adsorption processes. The main purposes of these investigations were: (1) to extend photoelectron spectroscopic investigations of oxygen adsorption on Mg to adsorption at low temperatures; * Present address: lnstitut fur Festkorperforschung, 581

KFA Jiilich, D-5170 Jiilich, West Germany.

582

J.C. Fuggle / Oxygen adsorption on Mg

(2) to obtain firmer estimates of the quantity of oxygen adsorbed on Mg at oxygen pressures up to 1Ow4 Pa; (3) to seek evidence to decide whether oxygen can form true “chemisorbed” layers on Mg, or whether small islands of oxides form even at lower coverages; (4) to observe shifts of Mg and 0 XPS, UPS, and Auger peaks which are found to give some indication of the complexity of oxygen adsorption on evaporated Mg surfaces. Some work in the literature acted as a useful basis for our study. The Mg KLL Auger spectra of Mg metal and MgO have been extensively investigated [l-7]. The assignment of the peaks is not in doubt; here we will report changes in the Mg KLa,sLa,s (lDa and ‘S) region of the spectrum, as a function of oxygen coverage. In previous XPS studies of the 0 1s peak during oxygen adsorption on Mg at room temperature [8] “saturation coverage” was defined as the coverage at which the rate of growth of the 0 is peak with increasing oxygen coverage dropped very strongly, implying a drop in sticking coefficient by a factor of >lOO. The investigation also showed that the sticking coefficient was greatest at -l/2 this “saturation coverage” and that adsorption did not completely stop at the “saturation coverage” but continued much more slowly. Investigations of the Mg XPS peaks [4,8,9] suggested that at “saturation” the oxide film was still thinner than the mean inelastic escape depth of -1400 eV electrons (i.e. <15-20 A) [lo]. UPS investigations of oxygen adsorption at room temperature showed a strong peak -6 eV below Ef [ 1 l] (similar in energy to that caused by oxygen adsorption on many transition metals [ 121 which is attributed to 0 2p-like levels). Theoretical studies [ 13,141 using free-electron approximations predicted a resonance -2 eV below Ef due to oxygen chemisorbed on top of Mg. Lang and Williams [ 131 suggested that the peak could be deeper if oxygen were nearer the surface than is normal in chemisorption but it would then be significantly broader than the observed peak. It has been suggested that the discrepancy between calculation and experiment indicates that adsorbed oxygen penetrates below the surfaces of polycrystalline Al and Mg at -300 K [ 11 ,151. At room temperature the photoelectric work function of polycrystalline Mg decreases during initial oxygen adsorption from -3.3 eV for the clean metal to a minimum of -1.8 eV after exposure to 1 S-2 L 02 [ 161 (1 L = 1 Langmuir = lo@ Torr set z 1.3 X low4 N m-* set). No pressure measurement was possible during the impressive early experiments of Cashman and Huxford [ 171 but these authors also found a decrease in the photoelectric work function of Mg during initial exposure to oxygen (similar to that found in ref. [ 161) followed by an increase to about 5 eV at high exposures. The initial decrease in work function is strong evidence that much of the first oxygen adsorbed at room temperature does not sit on top of the magnesium, but is incorporated below the surface. Thus again the calculations of refs. [13] and [14] may not be relevant to oxygen adsorption by polycrystalline Mg at room temperature. We will present the results of investigations using uv photoelectron, X-ray photoelectron, and X-ray excited Auger electron spectroscopies (UPS, XPS, and XAES

J. C. Fuggle / Oxygen adsorption on Mg

583

respectively). Previous investigations have shown that the shifts of substrate XI’S peaks during chemisorption of gases on metals are much smaller than those which occur on oxidation (see e.g. refs. [18-201). This immediately suggests possible diagnostic tests for chemisorption and oxidation of metal surfaces. AES shifts are known to be generally larger than those found in XPS [21] and we investigate the Mg KLL Auger spectra during adsorption of O2 on Mg. In addition we will show that small shifts of XPS and UPS peaks provide some information on the mechanism of oxygen adsorption. The intensities of adsorbate XPS peaks have frequently been used to estimate the quantities of gas adsorbed on a given surface (see e.g. refs. [22-243). The most useful procedure is to define the coverage relative to a gas coverage which may be easily reproduced (e.g. monolayer coverage). I-Iere we will use the “saturation” COVerage, which for Mg is considerably more than a monolayer, as our fixed point, and make rough (+ -40%) estimate of the absolute coverage using the attenuation of the XPS peak from the metal as a result of oxidation and an estimated inelastic electron mean free path for MgO.

2. Experimental The spectrometer used for this work has been described elsewhere 1251. The experimental resolution (Mg Kol linewidth + instrumental broadening) was -1 .O eV for studies of the Mg Lr and Las XPS peaks and -1.3 eV for studies of the 0 IS XPS peak. For the Mg K XPS peak Al I& and experimental resolution of -1 .O eV were used. The instrumental broadening for XAES was -0.5 eV and for UPS either 0.1 or 0.2 eV. “Clean” magnesium surfaces were prepared by evaporation, from a W filament, onto a stainless steel plate which had been polished to 0.05 pm. The fnament was surrounded by a stainless steel shield. The pressure in the equipment rose from 3-6 X 1O-9 to 7-13 X 10e8 Pa during evaporation with this unit. For most low temperature experiments the probe was cooled -100 K by passing liquid nitrogen through the manipulator before evaporation commenced. Cooling the manipulator from 300 to -100 K caused it to contract by 2-3 mm. The bellowssealed mechanism which allows for movement of the probe [2S] can be used to correct for the contraction but the correction is not perfect and the intensity of XPS peaks is quite sensitive to sample position. This means we can make only approximate (*IO%) direct comparison of peak intensities at 300 and 100 K but we can compare the relative intensity of different peaks at the two temperatures to k-S%. Data was collected with the help of a Nuclear Data 4410 computer. Peak positions, given with respect to the measured Fermi level, were not located as described in ref. [20], but by drawing the vertical line which best bisected the top part of the observed peak, as this latter procedure required less computer programming. Peak intensities were estimated by using the computer to integrate the peak (minus linearly extrapolated background) between two representative points either side of the

584

J.C. Fuggle / Oxygen adsorption on Mg

peak; this procedure is consistent with those outlined in [26]. Changing the position of the two points either side of the peaks by up to 0.3 eV caused the intensities to change by <2% and we may use this figure as a measure of the error in estimation of relative peak intensities. However we stress that we cannot accurately estimate, or correct for, possible errors due to intensity in shake-up, shake-off, and energy loss peaks (see e.g. refs. [24,27]). Our earlier studies of Mg oxidation [5,8,9] were made with a Vacuum Generators ESCA III spectrometer [28] fitted with a modified sample preparation chamber [8,9]. In the present work the absence of a sample preparation precluded introduction of large quantities of oxygen without danger of changing the detector effciency. Thus, in this paper, we only report studies with oxygen exposures <5 X 1016 molecules cm-*. In this work exposures have been converted directly into the number of molecules striking the surface, but we stress that this does not eliminate inaccuracies inherent in the pressure gauge readings. When samples are prepared by evaporation in a sample preparation chamber one may position the filament a long way from any other metal which could be heated and degas material during evaporation. One may also cool the whole sample preparation chamber to minimise degassing. In this work it was necessary to shield the filament so that magnesium was only evaporated onto the sample substrate. With this latter arrangement it was found more difficult to prepare a clean Mg surface. It was necessary to evaporate -3000 A Mg onto the substrate between each experiment to obtain a surface contaminated by only -5% of a monolayer of oxygen, whereas in previous experiments [8,9] -500 A reduced the detected oxygen signal to below the level produced by 5% of a monolayer. As we discuss later we believe this difference is important in interpretation of some results presented here. Because we had to use such large quantities of magnesium for each experiment it was necessary to evolve procedures in which the Mg did not have to be evaporated too often. Here, after evaporating Mg to prepare a clean surface and checking with XPS that no impurities were present, we made short scans (8 min) of the peak of interest at up to 20 different oxygen coverages before starting a new experiment. As the base vacuum during the experiments was 3-6 X 10w9 Pa no excessive contamination could have built up during these experiments.

3. Results 3.1. 0 Is peak intensities and their use for calibration of coverages Fig. 1 shows the growth of the observed 0 1s XPS peak as a function of exposure of a cooled, freshly evaporated, Mg surface to oxygen. The signal: noise ratio is sufficiently good to allow the 0 1s peak intensity to be used to estimate the oxygen coverage in terms of .$, the fraction of “saturation” coverage. g is used instead of 0 to emphasize that the saturation coverage is much more than one monolayer of

J. C. Fuggle f Oxygen adsorption on Mg 0 1s Peak of oxygen

during on

585

adsorption

Mg at

-100 K

Oxygen

+-

7.7 -

3.6 Clean

535

530

535 -

R E

530

(eV)

Fig. 1. The 0 1s XPS peak observed after various exposures of a clean Mg surface to oxygen. The exposures are given in molecules 02 X 1014 cm-* striking surface. Note the small, but definate, shift of the 0 is peak as a function of coverage.

oxygen. We define g = 1 after an exposure of 5 X 1016 molecules 02 cmP2 when oxygen adsorption virtually ceases, both at -300 and -100 K. We thus normalize the peak intensity at any coverage to the peak intensity after exposure to 5 X 1016 molecules 02 cm-* and plot a calibration curve of ,$ as a function of oxygen exposure, as shown in fig. 2a which may then be used to estimate the exposure required to produce a particular coverage. It is stressed that the “saturation” coverage in this case is more than a monolayer; investigation of the metal peaks described later in this paper suggest that the oxide layer thickness (-7 A) is comparable to the inelastic attenuation length of -100 eV electrons (15-20 A). This thickness of the oxide film can lead to inaccuracy in estimation of coverage as the oxygen atoms at the bottom of the “saturated” oxide layer contribute less to the spectrum than those at the surface. No correction can be made for this effect unless the morphology of initial oxide formation is known, i.e. does the oxide grow in islands? (in which case no correction is needed), or layer by layer? The points in fig. 2a are taken from three separate experiments and it is seen that the reproducibility is very good for oxygen adsorption on a cooled Mg surface. The results shown in fig. 2b do not show the same reproducibility for adsorption at -300 K. In previous experiments [8] they were sufficiently reproducible for kinetic information to be derived. The differences between successive points in the individual curves of figure 2b follow sensible trends suggesting that the reason for the non-reproducibility at 300 K lies in the surface, not in the measurement of peak intensity, or the gas pressure. The only explanation we can give for this is that there are large variations in surface structure of different films deposited at -300 K. This

J. C. Fuggle / Oxygen adsorption on Mg

586

1.0

.

’ .

I

075,

*

.*

w

:

. .. .

? l

yn 0.5 i

& e ? 0 0.25. 0

5

E i:

s’ ‘= J

*

l

.

ADSORPTION

OF

Mg

K

AT

-100

100 a

Exposure

OXYGEN

BY

200

crnm2 x 10’~)

(molecules

1.0 -

b

Exposure

ADSORPTION

OF

Mg

K

AT

-300

(molecules

cm-a

OXYGEN

BY

x 1014)

Fig. 2. (a) Plot of oxygen coverage as a function of oxygen exposure for a Mg film at 100 K. The points are results from three separate runs. Here, as throughout this paper, the oxygen coverage is determined from the integrated 0 is peak intensity relative to that at saturation coverage. As explained in the text no correction has been made for the possible effects of oxide thickness on the relationship between oxygen concentration and integrated 0 1s peak intensity. (b) As for (a) but with the Mg surface at 300 K. (+), (g), and (0) are the results from three separate runs. () approximate results of previous experiments [8].

is feasible as deposition rates and film thicknesses used here (-3000 @experiment) were large. We must conclude from data in fig. 2b that we cannot prepare a reliable calibration curve of oxygen coverage plotted against oxygen exposure at room temperature. Note, however, that the ratio of Mg:O peak intensity at “saturation” at 300 K (0 1s : MgLr : MgLa,s = 3.42 : 1.00 : 0.64 f 0.1) at 300 K was constant within experimental error, from which it may be concluded that the thickness of oxide formed was the same in each case.

J. C. Fuggie / Oxygen

adsorption

587

on Mg

531.0 I . -300 K

5308.

;

. ’.

b b ;” b

-99

b 0

b

5306

’

I 5304

5: Y

Q

.

; 530.2 * +u ..” 53oci;

q

0.2 -

04 COVERAGE,

06 5 -

BE

PO

Fig. 3. Plot of the 0 1s peak binding energy with respect to EF as a function of coverage for Mg surfaces oxidized at 100 and 300 K. Three rtms were made at each temperature.

Comparison of the ratios of 0 1s and Mg 2s or 2p peak intensities at -300 and -100 K suggested that approximately equal (k-S%) quantities of oxygen were adsorbed at both temperatures. In some runs at -300 K a shoulder on the high BE side of the 0 1s peak was more marked than at 100 K. It may be seen in fig. 1 that the position of the peaks depends on the coverage. This is shown more graphically in fig. 3 where the results of 3 experiments each at -100 and 300 K are plotted. It was also observed that if a Mg film, saturated with oxygen at 100 K, was warmed to -300 K the 0 1s peak shifted -0.5 eV to higher BE (lower KE) with no apparent loss of oxygen. If a film saturated with oxygen at -300 K was cooled to -100 K no change occurred. If, however, this film was then exposed to an extra 5 X 1Or6 molecules cmv2 O2 the peak intensity increased only 1.4% + 2% but the whole peak shifted -0.5 eV to lower BE. If the sample was then warmed again the peak moved back to near its original position. 3.2. Studies of Mg XPS peaks Fig. 4 shows the changes in shape of the Mg La,3 peaks during reaction with oxygen. On the left hand side the coverages at -100 K have been estimated using the calibration curve in fig. 2a. On the right hand side results for substrate at -300 K are shown, and here only oxygen exposures are given. The following points are worthy of note (a) At both 100 and 300 K the peak apparently changes shape very little at low oxygen coverage (t < -0.25) but the signal: noise ratio is not very good. At high coverage shoulders are clearly seen on the high BE side of the peaks. (b) For the spectra from Mg surfaces “saturated” with oxygen a curve resolution was attempted using the peak from clean Mg as one of the “basis” functions. The

588

J. C. Fuggle / Oxygen

* Fig. 4. Mg L2,3 XPS peaks from Mg surface (exposure). For the results at 300 K the striking the surface.

adsorption

on Mg

B E (~‘4) at 100 K (300 K) as a function of oxygen coverage exposure is given in molecules 02 X 1014 cmh2

results, shows as full lines in fig. 4, suggest that the contribution from the clean metal is attenuated -45% by the oxide overlayer. The shoulders caused by oxygen adsorption are apparently due to second peaks which are broader than the clean Mg peaks and may be composites. (c) At high oxygen coverages the Mg L 2,s peaks appear broader at 300 K because the “shoulders” on the Mg L2,s peaks are generally shifted less to higher BE than at 100 K. Fig. 5a shows the change in shape of the Mg K XPS peaks as a function of oxygen coverage for a magnesium film at -100 K. As at 300 K, the intensity of the Mg K peak increases by -60% as a result of saturation of the surface with oxygen (see also fig. 12). As in the case of the Mg L,J peak, no dramatic change in the Mg K peak shape occurs at low coverage (g < -0.25). However here the signal: noise ratio is sufficiently good to attempt curve resolution of the peaks into two components even at low coverages. Fig. Sb shows the results of such curve resolutions in which the Mg K XPS peak has been used as one of the “basis” functions and subtracted away from the observed spectra until the peak shape for the second component looks sensible. Such curve resolutions are, of course, not totally unambiguous (see e.g. fig. 4 in ref. [20] and the discussion there). However it is clear that the contribution to the Mg K peaks from the metal below the oxide layer at saturation is attenuated -85% whereas for the Mg L peaks attenuation was -45% (see also fig. 12). It is also clear from fig. 5b that the position of the second component, induced

J. C. Fuggle / Oxygen adsorption on Mg Mg K peak surface

./-,A

from

exposed

a to

cooled

589

Mg

oxygen

-A 1306

1303

1306

-

a

.63

1303

B E (A’) Contributions

from

overlayers T

I

b

.

1306

.

.

1303

.

1306

1303

-

B E (eV) Fig. 5. (a) Mg XPS peak from a Mg film at 100 K at various oxygen coverages (b) Contributions to the Mg K XPS peak thought to arise from the Mg atoms most affected by the adsorbed oxygen. These “difference” spectra are obtained by scaling down the spectrum from clean Mg by an appropriate amount and subtracting this contribution from the “covered” spectrum.

by oxygen adsorption, is a strong function of coverage. This is shown graphically in figure 6 together with results for 300 K. The salient features of the curves in fig. 6 are the BE maxima of the oxide ‘contributions to the Mg K peak at [ - 0.4 and the shift of the curve at 100 K to lower BE. In additional experiments it, was found that if a Mg surface was oxidized at K and then warmed to 300 K the component of the Mg K peak due to oxide shifted -0.5 eV to higher BE. If a Mg specimen oxidized at 300 K was cooled no change in the Mg K peak was observed, but if the sample was then exposed to 5 X 1016 mol-

J. C. Fuggle / Oxygen adsorption on Mg

590

13OZQ

c_--

1301.2~

/ /

/

/

,’

t 13034,

0

+

0 0 +++ d

K

\

+ oo+ 0

1304.0

-----. -300

H4

+

o

+

+

0 $100 %is

K l

0”+ 8 8

+ 13036 5: ss 1303L W al

13032 t ,303.0r .,,..,...‘.

0

.

02 -

Mg K peak..I.. metal .I.......... ... . . . DL

COVERAGE,

06

0.8

. 1.0

fs -

Fig. 6. The

position of the contribution to the Mg K XPS peaks thought to arise from the Mg atoms most affected by oxygen adsorption at 100 and at 300 K. The contributions are obtained as described for fig. 5b.

ecules cm-’ BE.

O2 the contribution

to the peak due to oxide shifted -0.5 eV to lower

3.3. Studies crf Mg KLL Auger peaks

In fig. 7a we show the effects of oxygen adsorption at 100 K on the region of the Al Ka excited Mg KLL Auger spectrum between 1178 and 1188 eV. The Mg KL~LJ (‘D) and KLzLz C’S) peaks from clean Mg are found at 1185.6 and 1180.3 eV respectively. At first glance it might seem that no important change in the spectrum occurs until f is nearly OS, when a new K&L3 (‘D) peak forms at -1181 eV. (The shifted Kbb (‘S) peak would be at -1176 eV but is hidden by a plasmon loss). However even at low coverages the (‘D) peak intensity decreases noticeably with respect to that from clean Mg. The “curve resolutions” shown in fig. 7b were obtained by first scaling down the clean spectrum, so that the differences in intensity at the KbL3 (‘D) peak on the high ICE side of the peak are the same as those in the “covered” spectra, and then subtracting the scaled down “clean” spectra from the covered spectra. Thus the “difference spectra” in fig. 7b represent approximately the contributions to the spectra from those Mg atoms which have been most strongly affected by oxygen adsorption.

J.C. FugglejOxygenadsorption

Mg

KG-&,3

Auger

on

Mg

591

at -100 K

:;*:

I180

1185 K E

a

1180

II85

(~‘4) ___c -100 K

I180

b

1165

1160 KE

1185

(eV1 -

Pig. 7. (a) Mg KL2,3L2,3 (‘S and ID) Auger peaks from Mg surfaces at 100 K at various oxygen coverages. (b) Contributions to the Mg KL 2,3L2,3 Auger spectra thought to arise from the “oxidized” overlayer in the spectra from Mg surfaces at 100 K and various oxygen coverages.

The results in fig. 7b indicate that at low coverages (g < -0.4) a very broad shoulder on the low KE side of the KLaLs (‘D) peak grows as oxygen is adsorbed and this shoulder persists even at high coverages where a new (‘D) peak at -118 1 eV becomes important. The intensity in this shoulder is too great to be due to “metal” electrons which have been inelastically scattered in the oxide layer. The new peak at -1181 eV is seen to shift to higher KE with increasing coverage. Note also that there is a small negative peak in the difference spectra to the high KE side of the clean Mg KL2Ls (‘D) peak. This is reminiscent of the behaviour of difference spectra of XPS peaks from transition metals when a very small shift of the peak is observed 1201.

592

J. C. Fuggle / Oxygen

1180

a

adsorption

1185

on Mg

1185

1180

K E leV1 -

\ \ \ \ \ \ \

b

-

0.5

coverage>

5

--a

1.0

Fig. 8. (a) Contributions to the Mg KL2,3L2,3 Auger spectra thought to arise from the “oxidized” overlayer in the spectra from Mg surfaces at 300 K and various oxygen exposures. (b) POSitiOnof the Mg KL2,3L2,3 (ID) peak from MgO plotted as a function of oxygen coverage.

Fig. 6a shows Auger difference spectra, for adsorption at -300 K, analogous to those shown for adsorption at -100 K. Fig. 8b shows the KE of the oxide Mg KLaLa (‘D) peak as a function of oxygen coverage, f. ~thou~ direct comparison of results at 100 and 300 K is difficult because we have no accurate estimate of the relative coverages at 300 K the following points should be noted: (a) The new (*D) peak is at lower KE at room temperature than at 100 K. (b) Many of the “difference” spectra observed at -300 K show a very broad (‘II)

593

J. C. Fuggle / Oxygen adsorption on Mg

peak, or group of peaks, which may be an indication of an ensemble of varying environments for Mg atoms within the oxide layer. (c) At low oxygen coverages, the broad shoulder on the low KE side of the (‘D) peak is found at both 100 and 300 K, but at 100 K its intensity relative to the (‘D) peak from the oxide is higher than at 300 K. It was found that if a Mg surface was oxidized at 100 K and then warmed to 300 K the Mg KLaLs (‘D) Auger peak due to oxide shifted typically -0.5 eV to lower KE (although a shift as large as -0.9 eV was observed in one experiment). If a Mg surface oxidized at 300 K was cooled to 100 K no change in the Mg KLL Auger spectrum was observed but if the surface was then exposed to -5 X 1016 (‘D) peak due to the oxide shifted -0.5 eV to molecules cm-* O2 the KbLs higher KE and the half width of the oxide peak was reduced from -2.6 to -2.4 eV. 3.4. UPS studies Fig. 9 shows the He I excited UPS spectra of Mg and oxygen adsorbed on Mg at 100 K at various coverages. Oxygen adsorption is seen to cause a peak at 6 eV BE to appear at low coverages whilst at coverages E greater than -0.25 a shoulder at -8.5 eV is clearly present. (This shoulder might be hidden in the background at lower coverages but this seems unlikely.) Both the shoulder and the main peak shift about 1 eV to lower BE with increasing coverage, and shift back to higher BE on warming to -300 K. The shift of the shoulder may not be identical to that of the main peak. The shifts of the main UPS (He I) peaks as a function of oxygen coverage are shown graphically in fig. 10a. Although it is difficult to compare results at 100 and 300 K because we lack accurate relative coverages at 300 K, the position of

10

5

Ef

5

Ef

B E Ce”,l’ Fig. 9. He I-excited UPS spectra from a Mg surface at iO0 K plotted as a function of oxygen coverage. l

594

J. C. Fug& / Oxygen adsorp tiotr ott Mg 60

P

-300

f--__----‘~

9

K

6

9

9

Cooled by liq. Nz

0

c

9 9

50

**

I 0

0.2 -

a

04 COVERAGE,

0.6

1.0

,i

02 / 0

0% e -

/ ‘z G?

DL

. @6

.

. 0.6

.

1.0

Covemge. 5 Fig, 10. (a) The position of the main peaks in the He I-excited UPS spectra of Mg surfaces at 100 and 300 K, ptotted as a function of oxygen coverage. (b) The intensity of the He I excited “0 2~” peaks from Mg at 100 K plotted as a function of coverage. b

the adsorbate UPS peaks is clearly less sensitive to coverage at 300 K than at 100 K. Fig. lob shows the growth of the adsorbate peak intensity in the He I excited spectra from Mg surfaces exposed to oxygen at 100 K. As shown, the intensity is not proportional to coverage as estimated from the XPS 0 1s peak intensity, particularly at $ < -0.25 where the intensity of the adsorbate peak is up to 6 times less than would be expected if UPS peak intensity were directly proportional to coverage. The He II excited spectrum from Mg was also studied. While here low signal: noise ratios prevented extensive observations, it was notable that in the He II spectra the peaks at -8.5 and -6 eV were approximately equal intensity whilst in He I spectra the 8.5 eV peak was a relatively inconspicuous shoulder.

J. C. Fuggle / Oxygen adsorption on Mg

595

3.5. Observations of surface plasmons It is well known from electron scattering experiments that the energies and intensities of surface plasmon losses are sensitive to surface conditions. The surface plasmon losses associated with the Mg 2p and 2s peaks weakened and shifted from -7.6 eV to -5.5 eV when the metal was oxidized [5] and similar effects were previously observed in XPS spectra of aluminium [29]. In the present studies the surface plasmon loss associated with the Mg K XPS peak was extremely sensitive to oxygen adsorption at either 100 or 300 K. At coverage 4 = 0.3 the surface plasmon loss was no longer observable and even at 5 = 0.06 (T= 100 K) it was broadened and only half as intense as the loss observed from the clean surface. Bradshaw et al. [30] have studied oxygen adsorption on single crystal Al and found a surface plasmon loss coupled to the 0 1s XPS peak. In the present studies a surface plasmon loss, but no bulk plasmon loss, was found to be associated with the 0 1s XPS peak from adsorbed oxygen. At coverage E = 0.3, the loss was -7.7 eV and the loss peak intensity was of the order of 10% of the main peak intensity. However no reliable, accurate, intensities for the loss peaks can be given because of uncertainties in determination of other contributions to the background.

4. Discussion 4.1. The quantity of oxygen adsorbed It was seen in figs. 4, 5, 7, and 8 that oxygen adsorption caused the metal La,3 and K XPS and KLL Auger peaks to be attenuated whilst peaks due to magnesium in the oxygen-containing layer on the surface grew on the low KE side of the metal peaks. The attenuation of the metal Mg KLL (‘D) peak as a function of coverage may be quite accurately estimated and is shown as a function of coverage at 100 K in fig. 11. For a uniform film thickness D, the attenuation A is given in terms of the

1.0,. 0.9.

. .

t 0.6.

.

Metal Mg KLa3L,, l

. l

gj 0.7. go.6

l

* l

. .

0.5. o-5

-

CD)

Coverage, 5

. .

.

00 1-o

-

Fig. 11. Intensity of the Mg KL2,3L2 3 (lD) peak from Mg metal plotted gen coverage for Mg surfaces reacted w&h 02 at 100 K.

as a function

of oxy-

J. C. Fuggle / Oxygen adsorption on Mg

596

inelastic mean free path length h, and the take-off, angle 0 measured with respect to the surface plane, by 1 - A = exp(-D/h

sin 0).

In these experiments, 0 is nominally 30’ although surface roughness produces some inaccuracy; at full coverage A = 0.51. As we describe below, at large coverages the oxygen-containing overlayer is MgO. We know from arguments based on the total intensity in the Mg XPS peaks (described in 4.5) that the inelastic mean free paths for -1200 eV electrons in Mg and MgO are approximately equal. The inelastic mean free path in Mg can reasonably be estimated at -20 f 5 A for -1200 eV electrons by comparison with measurements on other metals [ 10,3 1] thus 0.49 = exp(-D/20

sin 30”) = exp(-D/10)

,

D=7A.

Possible errors in this treatment are those due to false estimation of h (-25%) sin 0 (-10%) and in A (-3% in A, ~6% in D) giving a total error of -4O%, i.e. D = 7 f 3 A. However despite this large possible error it is certain that at saturation coverage an oxide layer, containing several times as much oxygen as a normal chemisorbed monolayer, covers the surface. In these arguments it has been assumed that the MgO film has uniform thickness. As only a small Mg metal contribution is visible in the Mg K XPS peak from Mg saturated with Oa, very little Mg metal can be in the top -4 A which contributes significantly to XPS spectra when the KE of the outgoing electrons is only -200 eV and 0 = 30”. If the oxide film is seldom thinner than 4 A and has an effective thickness of -7 A for our experiments it cannot have very non-uniform thickness. None of our experiments are sensitive to oxygen diffusion into the bulk and this is not discussed here. We can check the value for oxide film thickness using the relative intensity of the observed Mg and 0 peaks and the calculated photoionization cross section of the Mg Lr, La3 and 0 is peaks [32]. Such a calculation was successfully used, by Madey et al, to estimate the oxygen coverage on W(100) surfaces [2]. The calculation is rather tedious, and thus not described in detail, but yields a calculated 0 Mg Li : Mg La3 peak intensity ratio of 3.13 : 1.00 : 0.69 for Mg covered with 7 8, MgO and the experimental conditions used. (The intensities were corrected for the spectrometer transmission factors.) The ratios found experimentaly are 3.42 : 1 .OO : 0.64 at 100 K and 3.22 : 1 .OO : 0.62 at 300 K. The agreement between calculation and experiment is gratifying but we should not forget that we had to make exactly the same assumptions about inelastic mean free paths in MgO and Mg here as in the previous calculations where the oxide thickness was calculated from attenuation of the metal XPS and Auger peaks. The molar volume of MgO is 11.25 cme3. Thus a 7 + 3 A thickness of MgO contains 3.7 * 1.5 X 1015 oxygen atoms cmp2. This may be compared with a value of 10’ 5 atoms cmw2 normally accepted as typical of monolayer coverage for a film. The increase in 0 K: Mg L peak intensity ratio at 100 K (-6%, cited in last para-

J. C. Fuggle / Oxygen adsorption on Mg

591

graph but one) does not greatly exceed the estimated experimental error (5.5% for peak ratios). The increase in 0 1s peak intensity when a Mg surface previously saturated with oxygen at 300 K and cooled to 100 K is exposed to 5 X 1016 molecules cm-* 02 is also comparable to the estimated error. (1.4% is the average result of three experiments which could be caused by an increase of -5 X 10r3 oxygen atoms cm-* in the surface region; no statement can be made about the quantity of oxygen which diffuses into the bulk.) 4.2. 0 1s peak intensities and adsorption kinetics As discussed in section 3.1 we do not know details of the morphology and stoichiometry during the growth of the magnesium oxide film on Mg and this could lead to underestimates of the rate of growth of the oxide layer at high coverage. If the oxide grows in uniform layers the contribution to the spectrum of an oxygen atom at the metal-oxygen interface is only 1 - exp(-D/X sin 0) (D is the oxide thickness, X is the mean electron attenuation length and f3 is the electron take-off angle) of the contribution of an oxygen atom at the oxide surface. This could lead to errors which are never large in these experiments but should be remembered when considering the statements made in this sub-section. From the slope of the plot in fig. 2a and the thickness of the oxide layers at “saturation” we estimate for adsorption at 100 K an initial sticking coefficient of -0.1 at 100 K and a maximum of -0.3 at t - 0.4. Surface roughness could increase these estimates but at both 100 and 300 K the sticking coefficients are probably less than one. The slope of the coverage/exposure plots increased from its initial value to a maximum at about half coverage in all curves shown in figs. 2a and 2b. Such an increase is often interpreted as an indication of nucleation phenomena although this may not be the only explanation. 4.3. The nature of the processes involved in Mg oxidation The character of adsorption of up to at least one monolayer of oxygen on Mg are not known but at some stage it is probable that the adsorption leads to formation of a new “phase” on the surface with many properties to be expected of thin MgO layers. However even at coverages of several monolayers, possible small deviations from 1 : 1 stoichiometry, the interfaces between Mg and MgO, and between MgO and the vacuum, and disorder in the oxide layer may all affect the observed photoelectron spectra. We must first try to answer the question “are the initial stages of oxygen adsorption the same at 100 and 300 K?” Two differences were found between adsorption at 100 and 300 K. First, for equal coverages all the peaks in spectra from the “oxide” layer were at higher KE at 100 K than at 300 K. Second, at low coverages

MgO

Ill 19951 [41 [71

1304.3

1303.8-1304.1

1303.7

1304.2

_

1305.3

energies

530.7

530.1-530.4

530.2

530.5-532

_

W

89.6

_ 90.4 _

_

88.5 88.55 89.0 88.5 88.6

M&l

531.0

-

530.5-532

_ _ _ _

0 1s

50.4b

W

51.6 _ 50.9 50.2

_

49.4 49.6

49.6 49.4 49.9

M&z ,3

oxygen,

1180.5

1179.7 1180.0 1180.0 1180.5-1179.8 b sh 1185 -1181 1181.2 and b sh 1185 -1181

1186.2 1185.6

1185.3 1185.5

1186.2

Auger

fault in the equipment

1174.8 1175.4 1177.0

1180.3

1179.8 1180.5 _

used

to EF

Auger

in eV, referred

MgKlZLZ(lS)

all energies

MgKL&(‘D)

for Auger peaks from Mg, MgO, and Mg + adsorbed

w = shoulders too weak to be reliably resolved. b -broad. sh = shoulder. a Many of these values are at 0.3 eV lower KE than in this work and it is thought there may have been an electronic for earlier work. b Curve resolution was used to remove portions of the spectra arising from Mg metal.

This work

This work

This work b

Mg

This work

MgO evap. Mg + 0 ads Mg + 0 ads. Mg + 0 sat. at 100 K Mg+Olow coverage at 100K Mg + 0 sat. at 300 K

1303.0 1303.0 1303.0 1303.4 1303.6

Mg Mg Mg Mg Mg

[41 [571 [61 [51 a [71

1303.0

MgK

Compound

Reference

Table 1 Binding energies for XPS peaks and kinetic

Ir n

J. C. Fuggle / Oxygen adsorption on Mg

599

the relative intensities in the Mg KL 2,3L 2,3 (‘D) Auger peak from Mg in MgO and the broad shoulder between the peaks from Mg and MgO was slightly different at 100 and at 300 K. These effects are not large but because they were observed we do not make the assumption that adsorption follows the same pathway at 100 and 300 K. Table 1 gives the binding energies for XPS peaks and kinetic energies for some KLL Auger peaks in Mg, MgO and oxidized Mg. At both 100 and 300 K the kinetic energies of the new Mg KLZ,JL2,3 Auger and XPS peaks found at “saturation” oxygen coverage are about one volt higher than those found in bulk MgO [l] and MgO formed by evaporation of Mg in an oxygen atmosphere [5]. We believe this shift is a manifestation of fields and slight non-stoichiometry in the oxide layer, as discussed in section 4.4. The new Mg peaks which appear on oxidation then support the idea that at high coverages an oxide similar to bulk MgO is formed on Mg. At high oxygen coverages the photoelectron spectra of the Mg surfaces show two

Table 2 Binding energies Spectra

He I UPS

of peaks found Sample

Mg

in valence

band spectra

of MgO Ref.

BE wrt EF a (eV) Peak 1

Peak 2

0 2s

5.9

8.5

This work

This work

oxidized at 300 K He II UPS

Mg oxidized at 300 K

6.1

8.6

XPS

Mg oxidized at 300 K

6

8

OK

MgO MgO MgO MgO

5.5 eV 5.5 eV 5.5 eV 7.1

8.5 eV 8.5 eV 9.6 b

MN MgO MN MN MgO M@

7.6 8.0 7 8.0 7.3

emission Mg I-2,3 emission Mg K emission

22.5

_

10.2 10.6 9 10.6 9.6

This work

[331 [341 ;::;

24

23.6 22.1

1361 1371 1381 [391 I401 [4Il

a X-ray emission spectra gives the energy differences between the core and valence levels. In order to obtain binding energies with respect to !?F from the emission spectra the core level binding energies (-0.3 eV) derived from ref. [5] have been used. b Another peak at greater BE has been assigned to the LtL2,3 transition.

600

J. C. Fuggle / Oxygen adsorption on Mg

peaks at approximately 6 and 9.5 eV. At higher photon energies the peak at -8.5 is intense relative to the peak at -6 eV than in the He I spectra. We know of no reported photoelectron studies of the valence bands of MgO but we can compare our results with the X-ray emission spectra of MgO. It is notable that in 0 K, Mg K and Mg La,3 emission spectra (table 2) and in calculations [42,43] the occupied valence bands of MgO are split, so that we do not have to invoke the presence of two oxygen species to explain the two peaks in the 0 2p region found by UPS and XPS. It is not entirely clear to us why the peaks in Mg K and L emission spectra are found at higher BE than in XPS, UPS and 0 K emission spectra. The shoulder found in the He I and He II excited UPS spectra of oxidized Mg at -8.5 eV (at both 100 and 300 K) is either not present at low coverages or is shifted very strongly towards the peak at -6 eV. (The same result has been found in He I spectra by other workers [ll]. If we accept that the UPS spectra from MgO contains two peaks, then the oxygen present at low coverage cannot be arranged into islands with the characteristics of MgO. At 300 K we know from work function measurements [16,17] that the oxygen is below the metal surface. Our results suggest, that at least at coverages, t < 0.15, the 0 2p levels do not form the same band structure as in MgO, regardless of whether the Mg surface is at 100 or 300 K. Fig. lob shows the intensity of the 0 2p-like peak in He I excited UPS spectra from the Mg surface, plotted as a function of coverage. The curve is not at all linear. The flattening of the curve at high coverages can be attributed to the low escape depth of the electrons; at high coverages the oxide film is so thick that the intensity has practically reached the value expected from bulk MgO. At low coverages we note, however, that the observed intensity is much less than expected on a linear increase model. If we allow for the low escape depth of the electrons the relative intensity should increase even more steeply at low coverages and the discrepancy is even greater. Lang and Williams [13] have calculated the position of the adsorbate resonance in UPS caused by oxygen chemisorbed on jellium with the electron density of Al and Mg to be approximately 2 eV below Ef. Yu et al. [ 151 and Flodstrom et al. [ 1 l] argued that the position of the observed resonance 5-6 eV below Ef for oxygen on these metals shows that the resonance is due to oxygen incorporated below the surface, or in oxide. We suggest the same interpretation may be used here. However, the ability to use XPS to calibrate the coverage allows us to note that at low coverages the peak at 5-6 eV grows much more slowly than linearly with increasing coverage. One possible explanation is that at 100 K and low coverage much of the oxygen is adsorbed in a form which has little effect on the UPS spectrum [44]. This second form of oxygen might be chemisorbed on the surface. A shoulder is found in the KLL Auger “difference spectra” of Mg at all non-zero oxygen coverages (figs. 7b and 8a). At high coverages this broad shoulder is most easily explained by interface effects; metal atoms at the metal surface will be “bonded” to both metal and oxygen atoms and core holes created in the lower regions of the metal oxide will be marginally more stabilized by relaxation (or the “image charge potential”) within the metal (see e.g. ref. [45]). At lower coverages, more

J. C. Fuggle / Oxygen adsorption on Mg

601

especially at 100 K, the shoulder is more intense than the peak at -1181 eV due to the “oxide”. From this we again deduce that the adsorption does not proceed through growth of large islands, within which the oxide structure is independent of coverage. If this were the case, the shape of the “difference spectra” should not change so drastically with coverage, only the intensity should change. Chemisorption of oxygen at the Mg surface would also result in shifts much smaller than those found in pure oxide, and the presence of a broad shoulder at low coverages is consistent with, but certainly not proof of, chemisorption at low coverages. One last piece of evidence can be related to the nature of oxygen adsorption at low coverages. As mentioned in section 3.5, a surface plasmon was observed on the low KE side of the 0 1s XPS peak from a Mg surface at 100 K and at ,$= 0.3. The observation of surface plasmons, but no bulk plasmons, suggests that the oxygen was not more than -5 A below the surface at this stage of the adsorption process, but interpretation of such losses is not a trivial problem [46]. 4.4. Shifts of peaks from the “oxide" layer as a function of coverage The shifts of 0 1s and Mg K XPS peaks, Mg KL2,31.,a3 (‘D) Auger peaks and UPS 0 2p peaks from the oxide layer are shown as a function of coverage in figs. 3, 6, 8, and 10a respectively. We should remember that the shifts given are shifts of the whole peaks and it is usually not possible to say a priori whether or not the observed peaks contain many unresolved contributions whose relative weighting changes with coverage. The XPS and Auger peak shifts show certain features in common. Apart from the 0 “2~” UPS peaks all peaks from the “oxide” layer studied decrease in KE (z increase in BE), up to a coverage of -0.4. These shifts may be due to transformation of chemisorbed oxygen to oxide. Alternatively, if layer growth is involved, decrease in stablization of the final state by relaxation in the metal as the oxide thickens (and the average distance between atoms in the oxide and the metal increases) would lead to a similar shift. A further explanation is that the shifts are a manifestation of changes in oxide structure as the film thickness increases. The absence of a BE increase in the 0 2p-like UPS peaks does not necessarily contradict any of these explanations because valence band effects need not parallel core level effects. The shape of the plots of Mg K peak shift against coverage differ slightly from those of the 0 1s peaks. This might be due to errors in curve resolution of the Mg K peak or to differences in the electron escape depths at the two energies. All the observed XPS, UPS and XAES peaks from the oxide layer shift typically 0.5 eV to higher KE when an extra -1.4% of oxygen is adsorbed in the surface region at 100 K on a Mg surface previously saturated with oxygen at 300 K: i.e. a small proportion of the oxygen atoms cause the large shift and we should not think in terms of changes in the weighting of the different contributions to all the observed peaks, but rather in terms of effects which could cause the majority of the

602

J. C. Fuggle / Oxygen adsorption

on Mg

contributions to the peaks to shift. The absence of a shift on cooling alone shows that we are not dealing with a phase transformation or anomalous effects of the spectrometer construction. Using information given in 4.1 we note that 1.4% of the saturation oxygen coverage of Mg is -5 X 1013 atoms cm-*. The thickness of the oxide film was thought to be -7 8. Let us assume that the shifts are caused by a dipole layer with a magnitude of 0.5 eV which its negative charges situated on the extra oxygen atoms and its positive changes located at the metal surface. The charge q/atom is then given by [471 0.5 = Nq d/e = 4nc* Nq d/10’

,

where N is the number of adsorbed atoms m-* and E is the permitivity of the layer which we set equal to that of vacuum for our order of magnitude calculation. c is the velocity of light and SI units are used. Thus q 2: 0.3 electrons. Obviously this treatment contains several approximations and the estimate of charge is very rough. However it does show it is feasible that charges on the extra adsorbed atoms cause the shift of the observed peaks on cooling the saturated Mg films and exposing them to oxygen, or on warming films saturated at 100 K. It is not at all clear how the potential created by such dipoles would vary within such thin oxide films and how they would effect peak shapes: (additional complications arise because although MgO is a good insulator slight enrichment with Mg causes it to become an n-type semiconductor [48]) studies at a variety of electron take-off angles might provide information on this point. The presence of the dipoles could be confirmed by studies of the Mg work function during the adsorption processes at -100 and -300 K. It should be noted that the fields produced in the “oxide” film by the dipoles suggested above would be very high (0.5 V/7 A = 7 X lo6 V cm-‘) and thus comparible with those important in field assisted diffusion and other aspects of oxidation when oxide layers are -1 O-l 0,000 a thick [49,50]. 4.5. On electron escape depths The Mg K peak from the oxidized Mg surfaces was approximately 1.6 times as intense as that from Mg metal and -15% of the peak observed from the surfaces saturated with oxygen was thought to arise from the metal underneath the oxide (see fig. 12). Thus an infinitely thick layer of oxide would give a Mg K peak -1.45/ 0.85 2: 1.7 times as intense as that from Mg metal. This increase in intensity must arise because the intrinsic and extrinsic losses [51] are less effective in MgO than in Mg. The intrinsic losses in these systems amount to at very most 30% of the total losses [30,52] and we make the approximation that they are similar for Mg and MgO. The mean inelastic escape depth of the Mg K electrons in MgO, relative to that in Mg, may then be estimated using the fact that for a given spectrometer con-

603

J. C. Fuggle / Oxygen adsorption on Mg

&

06 04

o-‘.

. .*

. . . *Mg

. 05 ‘. -

Coverage, 5

K from

.

‘.

Metal

M l

-

Fig. 12. Plot of the total integrated Mg K XPS peak intensity and the intensity of the contribution from Mg metal, as a function of oxygen coverage, for Mg surfaces reacted with 02 at 100 K.

figuration Z= k&p

and electron energy, the intensity ,

of a given peak, I, is [53] (1)

where k is a constant, u is the partial ionization cross-section for the level, h is the mean inelastic electron escape depth and p is the density in atoms cmm3 of the element in question. Using (l), the observed increase in Mg K peak intensity and the value 1.23 for pr,,&pr+s, we find that at 180 eV KE the inelastic escape depth in MgO is approximately 1.4 (*-lo%) times that in Mg itself. A similar treatment of the Mg Ls,s peak suggested that at -1200 eV the inelastic escape depths in Mg and MgO were about equal (+20% if the same approximations are made). Penn [31] has published a method for calculating inelastic escape depths for compounds based on the assumption that the compounds can be treated as free electron metals with all the valence electrons contributing to the free electron density. Using his method we calculate that at -200 eV the inelastic escape depth in MgO should be -40% less than in Mg and at 1200 eV -50% less. These results are in unambiguous disagreement with our experimental results at both energies. The main error in the interpretation of the experiment arises through possible differ-

604

J. C. Fuggle 1 OxJjgen adsorption on Mg

ences in intrinsic losses in Mg and MgO and is not large enough to explain the discrepancy. Penn stated [3 l] that his assumptions could “not be readily evaluated at the time of publication due to lack of sufficient experimental measurements”. Our results suggest that this assumptions are not valid for materials with large band gaps and that his calculations for Al203 certainly have limited validity. 4.6. Comparison with other oxidatiorl

experiments

A work function decrease during the initial stages of oxygen adsorption on Mg at -300 K was observed both by Cashman and Huxford [ 171 and by Gesell and Arakawa [ 161. The latter observed a decrease of -1.5 eV after exposure to -7 X 1014 molecules O2 cm-*. Our results suggest this exposure corresponds to a coverage of less than half a monolayer although caution is advisable here because it is notoriously difficult to compare gas exposures made in different laboratories. The large decrease in work function during initial exposure of Mg surfaces at -300 K to oxygen indicates that most of the oxygen is incorporated below the Mg “surface” so that an inverted double layer of charge is formed. It would be interesting to know if a similar decrease occurs at 100 K where we have no evidence to rule out chemisorption of oxygen on the metal surface and some evidence that adsorption does not follow quite the same path as at 300 K. Cashman and Huxford [17] suggested that the increase in work function, to above the clean metal value found by them, at high oxygen coverages was an increase to the work function of bulk MgO. The shifts we observe in photoelectron peaks from the oxide layer are consistent with the idea that the work function of Mg surfaces at high oxygen coverages is increased by small quantities of oxygen adsorbed on the oxide film. Measurements of work function changes during adsorption at low temperatures would be helpful here. Aluminium is the next element in the periodic table to magnesium and it also forms a thin protective film of oxide on exposure to oxygen. Roberts and Wells [.54] have investigated the work function changes during adsorption of oxygen on evaporated aluminium films. They found that at low temperature (77-90 K) the work function of an aluminium film saturated with oxygen was -0.8 eV higher than that of a film saturated at room temperature. The work functions of the surfaces saturated at low temperatures sunk to near the room temperature value when the film was warmed. It was also found that if an aluminium film was saturated at room temperature and then cooled no change in work function occurred until the surface was exposed to more oxygen. On further exposure of this film to oxygen a small uptake of gas and a large increase in the work function (
J. C. Fuggle / Oxygen adsorption on Mg

605

face and create a large dipole. At higher temperatures this oxygen on the oxide surface is unstable, either because of diffusion of oxygen and metal atoms through the oxide layer, or because the oxygen desorbs from the surface. It is notable that shifts of photoelectron peaks as a function of coverage have also been reported for oxide layers on Al [8]. Using SIMS Benninghoven and Wiedman [56] found that the yield of O$, MgO-, and MgOz ions first became significant after exposures of Mg at 300 K to 200-500 X 1014 molecules Oa cm-*. Our results indicate that at 300 K some oxide is formed at very low exposures and the observation of these ions in SIMS is not only dependent on the concentration of oxide. We note that on our evaporated films the shift of the photoelectron peaks to higher KE did not start until exposures of 7.5-100 X 1014 O2 molecules cm-*. It is possible that the yields of 05, MgO-, and MgO? in SIMS are linked to the presence of the species which cause the shifts to higher KE in photoelectron spectroscopy. Observation of the structure of SIMS spectra during oxidation of Mg at low temperature and on warming Mg surfaces, saturated at -100 K, to -300 K might provide conclusive evidence on this point.

5. Conclusions (1) Oxygen at low pressures adsorbs on clean Mg at both 100 and 300 K until an oxide layer -7 f 3 A thick is formed. Such a layer of MgO contains 3.7 X 10” oxygen atoms cm-‘. (2) At 100 K the sticking coefficient of oxygen is initially approximately 0.1 and increases to a maximum of -0.3 at approximately 0.4 of the saturation coverage. (3) Oxidation of Mg at both 100 and 300 K causes two new features to arise near the Mg KL2,sL2,a (lD> peak in the XAES spectrum from Mg: a peak shifted -5 eV to lower KE, which is attributed to the Mg Kb,sL2,s (‘D) transition of Mg in MgO, and a very broad shoulder attributed to the transition in Mg atoms near the Mg/MgO interface. (4) Peaks in the XPS, UPS and XAES spectra from the oxide film on Mg shift in a complex way as a function of oxygen coverage. The shifts can be as large as -1 eV. (5) All peaks from the oxide layer at “saturation” are at lower BE at 100 K than at 300 K. If a Mg surface is oxidized to “saturation” at 300 K, cooled to 100 K and exposed to more oxygen, then the oxygen near the surface increases by only -1.4%, but the peaks from the oxide layer (XPS, UPS and XAES) all shift -0.5 eV to higher KE If the sample is then warmed the peaks all shift back. The shifts as a function of coverage and temperature found here suggest caution is appropriate in interpretation of chemical shifts in oxide layers, particularly when using chemical shifts to identify metal species with different oxidation numbers. (6) The observed shifts may be explained by a combination of relaxation and/or

606

J. C. Fuggle / Oxygen adsorption on Mg

chemical effects at low coverages and dipoles introduced by changes in the concentration of a small number of oxygen atoms, with at least partial negative charges, at or near the oxide/vacuum interface. More experimental evidence is required to check the validity of these explanations. (7) XPS, UPS and XAES results obtained during oxidation of Mg at 100 K all indicate that there are large differences in the products of initial oxidation of Mg (UP to -1015 oxygen atoms cm-*) and oxides formed at higher coverages. At 100 K the results are all consistent with, but certainly do not prove, formation of a chemisorbed layer of oxygen on Mg before formation of MgO. (8) There is some evidence, from the XAES studies and the shifts of peaks from the oxide layers, that adsorption of oxygen does not follow quite the same pathway at 100 and 300 K. It is thus possible that chemisorption of oxygen on the surface of Mg occurs at 100 K. Such chemisorption can be ruled out at 300 K on the basis of work function measurements [ 16,171 and previous UPS measurements [ 11 ,151. (9) The mean inelastic electron escape depth of 180 eV electrons in “MgO” is -1.4 times that in Mg. At 1200 eV the escape depths are about the same in Mg and MgO.

Acknowledgements I thank A.M. Bradshaw, D. Menzel, H. Neddermeyer, J.C. RiviBre, M. $njiC, E. Umbach and A.R. Williams for stimulating discussions on this work and D. Menzel and E. Umbach for reading the manuscript. I am also grateful to W. Bgck for technical assistance and for writing the programs used to run the N.D. 4410 computer. This work was supported financially by the Deutsche Forschungsgemeinschaft through S.F.B. 128.

References [l] A. Fahlman, R. Nordberg, C. Nordling and K. Siegbahn, 2. Physik 192 (1966) 476. [2] P.W. Palmberg, in: Electron Spectroscopy, Ed. D.A. Shirley (North-Holland, Amsterdam, 1972) p. 835. [3] P.J. Bassett, T.E. Gallon, M. Prutton and J.A.D. Matthew, Surface Sci. 33 (1972) 213. [4] C.D. Wagner and P. Biloen, Surface Sci. 35 (1973) 82. [5] J.C. Fuggle, L.M. Watson, D.J. Fabian and S. Affrossman, J. Phys. F5 (1975) 375. [6] L. Ley, F.R. MC Feely, S.P. Kowalczyk, J.G. Jenkin and D.A. Shirley, Phys. Rev. Bll (1975) 600. [7] N.C. Halder, J. Alonso and W.E. Swartz, Z. Naturforsch. 30a (1975) 490; Phys. Rev. B13 (1976) 2428. [8] J.C. Fuggle, L.M. Watson, D.J. Fabian and S. Affrossman, Surface Sci. 49 (1975) 61. [9] J.C. Fuggle, unpublished results. [lo] See, e.g. I. Lindau and W.E. Spicer, J. Electron Spectr. 3 (1974) 409, and references therein; C.J. Powell, Surface Sci. 44 (1974) 29, and references therein.

J. C. Fuggle f Oxygen adsorption on Mg [ll]

601

S.A. Flodstrom, L.C. Petersson and S.B.M. Hagstrom, J. Vacuum Sci. Technol. 13 (1976) 280. [12] D. Menzel, J. Vacuum Sci. Technol. 12 (1975), 313 and references therein: A.M. Bradshaw, L.S. Cederbaum and W. Domcke, in: Structure and Bonding, Vol. 24, Ed. C.K. Jdrgensen (Springer, Heidelberg, 1975) p. 133, and references therein. [13] N.D. Lang and A.R. Williams, Phys Rev. Letters 34 (1975) 531. [ 141 J. Harris and G.S. Painter (unpublished, cited in ref. [ 111). [15] K.Y. Yu, J.N. Miller, P. Chye, W.E. Spicer, N.D. Lang and A.R. Williams, Phys. Rev. B14 (1976) 1446. [16] T.F. Gesell and E.T. Arakawa, Surface Sci. 33 (1972) 419. [ 171 R.I. Cashman and W.S. Huxford, Phys. Rev. 48 (1935) 734. [18] J.C. Fuggle and D. Menzel, Chem. Phys Letters 33 (1975) 37. [19] C.R. Brundle and A.F. Carley, Chem. Phys Letters 33 (1975) 41. [20] J.C. Fuggle and D. Menzel, Surface Sci. 53 (1975) 21. [21] see e.g C.D. Wagner, Faraday Disc. Chem. Sot. 60 (1975) and references therein. [22] T.E. Madey and N.E. Erickson, Chem. Phys. Letters 19 (1973) 487. [23] S.J. Atkinson, C.R. Brundle and M.W. Roberts, Faraday Disc. Chem. Sot. 58 (1974) 62. [24] E. Umbach, J.C. Fuggle and D. Menzel, J. Electron Spectr. 10 (1976) 15. [25] A.M. Bradshaw and D. Menzel, Vakuum Technik 24 (1975) 15, and references therein. 1261 J.C. Fuggle, T.E. Madey, M. Steinkilberg and D. Menzel, Surface Sci. 52 (1975) 521. [27] J.C. Fuggle, T.E. Madey, M. Steinkilberg and D. Menzel, Chem. Phys. Letters 33 (1975) 233. [28] C.R. Brundle, M.W. Roberts, D. Latham and K. Yates, J. Electron Spectr. 3 (1974) 241. [29] A. Barrie, Chem. Phys Letters 19 (1973) 109. [30] A.M. Bradshaw, W. Domcke and L.S. Cederbaum, Phys. Rev. B (1977), in press. [31] D.R. Penn, J. Electron Spectr. 9 (1976) 29. [32] J.H. Scofield, J. Electron Spectr. 8 (1976) 129. [33] D.W. Fischer, J. Chem. Phys. 42 (1965) 3814. [34] H.U. Chun and D. Hendel, Z. Naturforsch. 22a (1967) 1401. [35] V.A. Fomichev, T.M. Zimkina and 1.1. Zhukova, Soviet Phys.-Solid State 10 (1969) 2421. [36] D.W. Fischer and W.L. Baun, Spectrochim. Acta 21 (1965) 443. [37] C.G. Dodd and G.L. Glen, J. Appl. Phys. 39 (1968) 5376. [38] P.Gallon,Compt. Rend. (Paris) 248 (1959) 1985. [ 391 F. Freund, Phys. Status Sohdi (b) b66 (1974) 271. [40] H. Neddermeyer, Dissertation, Munich (1969). [41] H. Karlsson and M. Siegbahn, Z. Physik 88 (1934) 76. [42] J. Yamashita, Phys. Rev. 111 (1958) 733; M.L. Cohen, P.J. Lin D.M. Roessier and W.C. Walker, Phys. Rev. 155 (1967) 992; E.V. Zarochentsev and E. Ya Fain, Soviet Phys.-Solid State 17 (1976) 1344. [43] S.T. Pantelides, D.J. Mickish and A.B. Kunz, Phys. Rev. BlO (1974) 5203. [44] The intensity of adsorbate resonances in UPS is not necessarily proportional to coverage as wiU be discussed elsewhere (J.C. Fuggle and D. Menzel, in preparation). [45] J.W. Gadzuk, J. Vacuum Sci. Technol. 12 (1976) 289, and many references therein. [46] M. sunjii and D. SokEevii, Solid State Commun. 18 (1976) 373. [47] See e.g. R.V. Culver and F.C. Tompkins, Advan. Catalysis 11 (1959) 67. [48] B.E. Hopkins and D. Kubachewski, Oxidation of Metals and Alloys (Butterworths, London, 1967). [49] N. Cabrera and N. Mott, Rept. Prog. Phys. 12 (1948-49) 163. [50] See, e.g., A.T. Fromhold, Theory of Metal Oxidation, in: Defects of Crystalline Solids, Vol. 9, Eds. S. Amelinckx et al. (North-Holland, Amsterdam, 1976) and references therein.

608

J. C. Fuggle / Oxygen adsorption

on Mg

[Sl] M. Sunjid (see, e.g., ref. [48] and Surface Sci. 68 (1977) 479) rightly dislikes the separation of “intrinsic” and “extrinsic” losses in XPS because they are coupled and interference terms are important. The whole concept of a “mean electron escape depth” begins to break down when these interference terms are included. However the concept remains useful as long as we recognize that it is an approximation. [52] See, e.g., J.C. Fuggle, L.M. Watson and D.J. Fabian, J. Electron Spectr. 9 (1976) 99, and references therein; D. Penn, Phys. Rev. B (1977) in press. [53] P. Cadman, S. Evans, J.D. Scott and J.M. Thomas, J.C.S. Faraday II 71 (1975) 1777. [54] M.W. Roberts and B.R. Wells, Surface Sci. 8 (1967) 453; 15 (1969) 325. [55) R.L. Wells and T. Fort, Jr., Surface Sci. 33 (1972) 172. [56] A. Benninghoven and L. Wiedman, Surface Sci. 41 (1974) 483. [57] J. Tejeda, M. Cardona, N.J. Schevchik, D.W. Langer and E. Schonherr, Phys. Status Solidi (b) 58 (1973) 189.