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ELSEVIER Surface Science 306 (1994) 269-278

XPS characterization of ultra-thin MgO films on a Mo( 100) surface

Jason S. Corneille, Jian-Wei He, D. Wayne Goodman * Department of Chemistry, Texas A&M University, College Station, TX 77843-3255, USA

(Received 7 July 1993; accepted for p&cation 12 November 1993)

Abstract

The oxidation of ultra-thin Mg films supported on a Mo(100) surface has been studied using X-ray photoelectron spectroscopy (XPS) in the 90-1300 K sample temperature range. Upon adsorption of oxygen onto Mg thin films or deposition of Mg in the presence of oxygen, a Mg(2p) XPS feature at N 50.5-50.8 eV is observed. The binding energy of this peak is higher than that of metallic Mg(2p) (at 49.6 eV) and is assigned to oxidized magnesium. The associated O(ls) XPS spectra exhibit two peaks which can be attributed to a dioxygen species concluded to be magnesium peroxide and the lattice oxygen in MgO. Upon annealing the peroxide containing film to _ 700 K, the magnesium peroxide is reduced to MgO through the loss of oxygen and metallic magnesium existing within the film is subsequently oxidized to MgO. Mg deposition in an oxygen background (- 1O-6 Torr) onto the Mo(100) surface at 300 K produces essentially stoichiometric MgO films.

1. Introduction

Due to the important applications of metal oxides in catalysis, ceramics and microelectronics, studies of ultra-thin metal oxide films grown on single-crystal substrates have recently become the focus of many efforts within the surface science community 111. In a typical study, metal oxide films have been prepared by either oxidation of a metal surface or deposition of the metal onto a substrate in ambient oxygen. Compared to conventional studies on bulk polycrystalline or single-crystal oxides, the use of thin films to study the microscopic chemical processes on insulating oxide surfaces has several advantages: (a) A thin

* Corresponding author.

oxide film often has good electric conductivity due to the dissipation of electrical charge via the metallic substrate. This consequently simplifies the use of electron spectroscopic techniques by reducing the charge build-up on oxides. (b) Un- der ultra-high vacuum conditions, it is generally easy to prepare a clean oxide film. (c) The use of a metal as a support eliminates difficulties often associated with the studies of bulk oxides such as heating or mounting a sample.

We have recently performed studies of the oxidation of ultra-thin Mg films supported on a Mo(100) surface using X-ray photoelectron spectroscopy (XPS). The results are reported and discussed in this paper. The ultra-thin MgO films were prepared by either post oxidation of Mg films or deposition of Mg in a background of 0,. The objective of this study is to develop a better

0039-6028/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0039-6028(93)E1022-R

270 J.S. Corneille et al. /Surface Science 306 (1994) 269-278

understanding of the microscopic chemistry involved in the interaction of oxygen with Mg thin films and to prepare a high quality MgO film as a model catalyst for mechanistic studies of reactions. Magnesium oxide is used as a catalyst for isotope exchange reactions, dehydrogenation reactions and the oxidative coupling of methane [2-51. A Mo(100) crystal was used as the support- ing substrate because both Mo(100) and MgO have a fourfold symmetry with a lattice mismatch of only 5.4%. MgO has been shown to grow epitaxially on a Mo(100) surface with a (100) orientation [6]. Also, MO is a refractory metal of which a clean surface is easily prepared.

Over the last decades, there have been numer- ous investigations of the interaction of oxygen with metallic Mg surfaces [6-121 using surface sensitive spectroscopies. In a recent study, Peng and Barteau reported that upon an exposure of - 5 L 0, at room temperature, a continuous MgO film formed on a Mg(0001) surface with a thickness of - 20 A [7]. The X-ray photoelectron spectra showed the Mg(2p) peak to have a binding energy of 50.8 eV for MgO, whereas the binding energy of metallic Mg was 49.5 eV. Previ- ous studies have proposed a three-step mecha- nism for the formation of magnesium oxide: oxygen chemisorption on the surface and incorporation below the surface, oxide formation and oxide thickening [lo- 121.

2. Experimental

The present experiments were performed in a conventional ultra-high vacuum chamber (base pressure 54 X lo- ” Torr) equipped with an X-ray photoelectron spectrometer, LEED optics, and a mass spectrometer for temperature-programmed desorption (TPD) spectra. The details of this chamber have been given previously [13].

For the calibration of the XPS spectrometer, N 200 ML of Mg was dosed onto the Mo(100) surface at room temperature. This Mg film showed no 0, C or S impurities within the AES or XPS detection limit (< 1 at%). The linearity (dispersion) of the spectrometer was calibrated

using the Mg(ls) (1309.0 eV) and Mg(2p) (49.6 eV) core level binding energies that were reported previously by several laboratories (see Ref. [lo] and references therein). The work function of the XPS spectrometer was then determined by referencing to the Mo(3d,,,) XPS peak which has a binding energy of 227.7 eV [14]. In the study of the Mg/Mo(lOO) and the O/Mg/ Mo(100) surfaces, the Mg core level binding energies were all measured with reference to the binding energy of the MoOd,,,) peak. For the MgO(100) single crystal experiment, the reference level of the insulating sample is not simply the Fermi level of the spectrometer. Since the Fermi level of the sample and the spectrometer are not in equilibrium and the magnitude of the surface potential during data acquisition was not measured, the Mg(2p) core level was arbitrarily set to be 50.5 eV. (This is in good agreement with previously observed values for magnesium oxide films [7,10].) The pass energy of the spectrometer was set at 50 eV for this work, and the substrate Mo(3d,,,) peak exhibited a full width at half- maximum (FWHM) of 1.50 eV which can be taken as the instrumental resolution. The core level shifts are reported with an experimental error of + 0.03 eV. The XPS spectra in this paper were acquired using AlKa X-rays (1486.6 eV). Curve fitting utilized a nonlinear least squares routine using mixed Gaussian-Lorentzian peak shape-and a linear baseline.

A 0.5 mm Ta wire was spot-welded onto the back face of the Mo(100) crystal and connected to two Cu feedthroughs that were immersed in a liquid nitrogen reservoir. This method of sample mounting allowed cooling the crystal to 90 K and resistively heating to 1400 K. In addition, the sample could be heated to 2300 K using e--beam heating. The sample temperature was measured using a W-5%Re/W-26%Re thermocouple spot-welded to the sample’s back face. The Mo(100) surface was cleaned using the procedure described in Ref. [15]. The MgO(100) crystal was mounted using a Ta wire cage connected to the probe via two copper feedthroughs. Preparation of the crystal involved Ar bombardment and subsequent annealing to 973 K in 1 X 10m6 Torr of

0,.

IS. Corneilk et al. /Surface Science 306 (1994) 269-218 271

Mg deposition was performed by thermal evaporation of a high-Purim Mg ribbon that was wrapped around a tungsten filament. The Mg source was degassed extensively prior to each dosage. The Mg flux was monitored using the mass spectrometer in the chamber, and the deposition rate was calibrated using Mg TPD area analysis where the saturation of the desorption peaks at T > 600 K is taken to correspond to 1 ML [6,9]. A deposition rate of 1 ML/min was used in this work. Oxygen exposure was carried out using a leak valve through which research- grade (2 99.998%) oxygen was admitted.

3. Results

3.1. Mg overlayers on Mo(100)

Fig. 1 shows the Mg(2p) and Mg(ls) XPS spectra of Mg/Mo(lOO) surfaces at several Mg coverages (0,). The Mg deposition and spectral collection were carried out at a sample temperature of 100 K. For a coverage of 1 ML, the spectra exhibit binding energies of 49.7 eV for M~2p) and 1303.3 eV for Mg(ls) core levels. As the Mg coverage is increased, both the Mg(2p) and Mg(ls) peaks shift to lower binding energies. At 8,, = 40 ML, the binding energies are 49.6 eV for Mg(2p) and 1303.0 eV for Mg(ls) which are close to the values for single-crystal Mg surfaces. Table 1 lists the Mg(2p) and Mg(ls) binding energies for the Mg thin films for several Mg coverages and the corresponding values reported previously for a Mg@OOl) surface [7] and a thick Mg thin film ElOl.

3.2. Oxygen adoption on Mg(7 hf..) /~o~lOO~

Fig. 2 shows the 00s) and Mg(2p) XPS spectra for 7 ML of Mg on a Mo(100) surface with O2 exposures of 3 L (a), 10 L (b) and 30 L (c). The Mg deposition and subsequent adsorption of 0, were carried out at a sample temperature of 370 K. For an oxygen exposure of 3 L, a shoulder is evident on the Mg(2p) spectrum. As the 0, exposure increases, the metallic Mg(2p) peak at 49.6 eV decreases in intensity, and the oxidized Mg(2p)

81 - 41

-7

1

~

0.

0.

1304 1300 64 50 46

44 -

IL

IL 1 BINDING ENERGY (eV)

Fig. 1. Mg(ls) and Mg(2p) XFS spectra of Mg/MdlOO) surfaces. The Mg was dosed onto the MdlOO) surface at the indicated coverages. The sample temperature during big deposition and spectral collection was 1Mt K.

peak at w 50.8 eV increases in intensity. The 00s) spectra show a peak at 530.3-530.6 eV and a shoulder at N 532.8 eV at all three 0, exposures. As will be discussed later, the lower binding energy peak is attributed to MgO, while the higher binding energy peak corresponds to Mg peroxide.

Table 1 Mg(2p) and Mg(ls) binding energies for uftra-thin Mg films on a Mo(100) surface at Mg coverages @,,J of 1, 7 and 40 ML; the binding energies for Mg film (3CiIO A) and Mg@OOl) single crystal [10,7] are also listed

eMg (MU Mg(2p) (eV) MgW (eV)

1 49.7 1303.3 7 49.7 1303.0 40 49.6 1303.0 Mg(OOO1) 49.5

3000_& 49.6 1303.0

272 J.S. Corneilfe et al. /Surface Science 306 Cl9941269-278

Fig. 3 presents the ratio of the integrated intensities of the O(ls) and Mg(2p) peaks as a function of 0, exposure. The integrated intensity ratios have been normalized to the O/Mg integrated intensity ratio of a single-crystal MgO(100) surface. It is clear that by an 0, exposure of N 10 L, the film demonstrates a 1: 1 stoichiometry. Above 10 L, the film shows a ratio higher than 1, indicating an excess of oxygen in the film.

As the oxidized fiIm was annealed above 700 K, the binding energies of Mg(2p), Mg(ls) and O(ls) exhibit simult~eous shifts in binding energy. This is more ctearly demonstrated in Fig. 4. To acquire the data for this figure, 7 ML of Mg

Mg2p

534 532 530 54 52 50 48

BINDING ENERGY (eV)

Pig. 2. o(ls) and MgfZp) XPS spectra of O/Mg(7 ML)/ MoflCO). Seven ML of Mg was dosed onto the MO surface, followed by 0, exposures of (a) 3 L, (b) 10 L and (c) 30 L. The sample temperature during the spectral collection and 0, exposure was - 370 K.

I

/ OlMg

I -- 10 20 30

0, EXPOSURE (L)

Fig. 3. Integrated intensity ratio of 0 to Mg for the O/M&7 ML)/Mo(lOO) surface as a function of 0, exposure. The sample temperature during 0, exposure was 371 K. The ratio has been normalized to that of single-crystal MgO(100).

was dosed onto the Mo(100) surface, exposed to 30 L of 0, at 370 K, and the surface was then annealed to elevated temperatures for the spectral collection.

Fig. 5 shows the annealing effect on the O(ls) and Mg(2p) core levels for the oxygen adsorbed Mg(7 ML)/Mo(l~) surfaces. Seven ML of Mg were deposited onto the MO substrate at 100 K, foIlowed by an oxygen exposure of 400 L. The oxygen adsorbed Mg/Mdl~~ surface was then annealed to 309 K (a>, 698 K (b) and 904 K (cl. Upon 0, exposure at 100 K, two peaks are evident in the O(ls) spectrum at 530.7 and 532.9 eV. There is a corresponding shift in the Mg(2p) feature toward higher binding energy with respect to that of metallic magnesium in the Mg(2p) spectrum. After annealing to 698 K, the intensity of the O(ls) peak at 532.9 eV is greatly reduced with a concurrent increase in the intensity of the 530.7 eV peak. With annealing, the Mg(2p) peak shifts toward a higher binding energy at 698 K and then to a lower binding energy at 904 K with little change in the peak intensity.

J.S. Corneille et al. /Surface Science 306 (1994) 269-278 273

z Mg2p O/Mg(7ML)IMo(l00) @)

[z ~:“--------,

z 50.0 -

s O/Mg(7ML)/Mo(lOO) @

[zii; \

m 1303.6 -

5i 530.40 -

8 I

700 800 900 1000 1100 1200 1300

ANNEAL TEMPERATURE (K)

Fig. 4. Mg(2p) and O(ls) binding energies for an O/Mg(7

ML)/Mo(lOO) surface as a function of annealing temperature.

The 7 ML Mg film was dosed with 30 L of 0, at 370 K and

then annealed to elevated temperatures.

3.3. Deposition of Mg in 0, ambient

We have also prepared MgO films by evapo- rating Mg in an oxygen background. Fig. 6 presents the O(ls) and Mg(2p) spectra after dosing 7 ML of Mg onto a Mo(100) substrate at 300 K with 0, pressures of 2.5 x lo-’ (a), 5 x lo-’ (b) and 1 X lo-’ Torr Cc). At an 0, pressure of 2.5 X lop8 Torr, the O(ls) exhibits two peaks and the Mg(2p) region consists of a peak at - 49.6 eV with a high binding energy shoulder. As the oxygen background pressure increases, the intensity of the high binding energy O(ls) peak decreases, and the Mg(2p) peak of MgO at N 50.8 eV increases with a concurrent decrease in the intensity of metallic Mg. At 1 X lo-’ Torr, both the 00s) and Mg(2p) spectra exhibited single peaks, in essentially stoichiometric proportions and with no evidence of metallic Mg or dioxygen species.

Fig. 7 shows the 00s) and Mg(2p) spectra from a MgO/Mo(lOO) surface which was prepared in the following sequence: (a) 7 ML of Mg was evaporated onto the MO substrate at an oxygen background pressure of 1 X lop6 Torr at a sample temperature of 100 K; (b) the film was annealed to 299, 727 and 917 K for spectra a, b and c, respectively. After the Mg deposition at 100 K and annealing to 299 K, the O(ls) spectrum shows two peaks at 529.7 and 532.4 eV. Annealing to 727 K causes a significant decrease in the intensity of the higher binding energy peak. The Mg(2p) peak exhibited a shift towards higher binding energy upon annealing.

Fig. 8 shows the effects of annealing on the O(ls) and Mg(2p) spectra. The spectra in a were

Mg2p r50.4

534 532 530 52 50 48

BINDING ENERGY (eV) Fig. 5. O(k) and Mg(2p) XPS spectra of O-adsorbed Mg(7 ML)/Mo(lOO) surface. The 7 ML Mg film was dosed with 400

L of 0, at 100 K and then annealed to (a) 309 K, (b) 698 K

and (c) 904 K.

J.S. Comeille et al. /Surface Science 306 (1994) 269-278

534 532 530 528

M@p

. . I.-L ‘ I I

52 50 48

BINDING ENERGY (eV) Fig. 6. 00s) and MgCLp) XPS spectra of MgO/M~l~}. Seven ML of Mg was dosed at a sample temperature of 300 K with 0, pressures of (a) 2.5~10-~, (b) 5X 10-s and Cc> 1 x IO-’ Torr.

obtained after dosing Mg in an 0, background pressure of 1 X lOA Torr at a sample temperature of 300 K. The film was then annealed at 900 K for 10 min and spectra b were recorded. The spectra of c are from a single-crystal MgOClOO) and serve as references.

4. Discussion

4.1. The core level shifts of Mg thin films

Fig. 1 shows a decrease in the binding energy of w 0.1 eV for the Mg(2p) level and 0.3 eV for the M&s) level, as the Mg coverage increases from 1 to 40 ML. These core level shifts with a change of Mg coverage can be explained either by

changes in the metal coordination number [16,17] or by charge transfer from the Mg to the substrate (see Ref. [lS] and references therein). The coordination number explanation is based on the fact that the core level binding energy of an atom in a solid correlates closely with the effective coordination number of this atom. The electron charge density on the atom decreases as the effective coordination number decreases, and this consequently leads to a core level shift to higher binding energy. This explanation has been extensively applied to explain the core level shift of a metal surface and a metal thin film when compared to the corresponding bulk values and has demonstrated excellent agreement with experimental results [l&17].

36 532 528 54 52 50 48

BINDING ENERGY (eV) Fig. 7. O(k) and Mg(2p) XPS spectra of Mg0(7 ML)/ Mo(100). Seven ML of Mg was dosed onto a Mo(100) surface at an 0, pressure of 1 x 10m6 Torr at a sample temperature of 100 K. The film was then annealed to (a) 299 K, (b) 727 K and Cc> 917 K.

JS. Comeille et al. /Surface Science 306 (1994) 269-278 275

534 532 530 528 54 52 50 48

BINDING ENERGY (eV)

Fig. 8. O(ls) and Mg(2p) XPS spectra of MgO(7 ML)/ Mo(100). Seven ML of Mg was dosed onto a Mo(100) surface at a sample temperature of 300 K with an 0, pressure of 1 x 10m6 Torr (spectra a). The film was then annealed to 900 K for 10 min (spectra b). The O(k) and Mg(2p) XPS spectra for single-crystal MgO(100) are also given as references (spectra cl.

Previous studies have shown that monolayer Mg forms a pseudomorphic structure on Mo(lOO), i.e. the Mg atoms adopt the symmetry and lattice constants of the Mo(100) surface [6,9]. The surface density of the pseudomorphic layer is thus 1.01 X 1015 atoms/cm2, the atomic density of the Mo(100) surface. For multilayers of Mg, the Mg atoms form the hexagonal close-packed structure with the basal plane parallel to the Mo(100) surface [6,9]. The surface density of Mg(0001) is 1.94 x 1015 atoms/cm2, 1.9 times that of the Mo(100) surface. Therefore, as the Mg coverage changes from 1 to 40 ML, the effective coordination number increases as a result of the surface structural change (from pseudomorphic to the

Mg(0001) structure) with the development of the three-dimensional overlayer. This increase in the effective coordination number consequently leads to the binding energy decrease with an increase in the Mg coverage, as shown in Fig. 1. It is also possible that electron charge transfer may occur from Mg adatoms of the first monolayer film to the Mo(100) surface, yielding a higher binding energy for 1 ML of Mg compared to 40 ML. Charge transfer has been proposed to explain the core level shift for Cu overlayers on Ta(llO), Re(OOOl), Rh(100) and Pt(ll1) surfaces [181.

Previous studies have reported Mg(2p) binding energies in the range 49.4-49.6 eV, as summa- rized in Table 1 in Ref. [lo]. Recent work on single-crystal Mg(0001) has found the Mg(2p) binding energy to be 49.5 eV [7]. In the present work, a Mg film of 200 ML on Mo(100) also showed a Mg(2p) binding energy of 49.6 eV referenced to the MoOd,,,) peak at 227.7 eV. For the metallic Mg(ls) spectrum, the value of 1303.0 eV was used and measured from 200 ML of Mg on Mo(100).

4.2. Core level shifts and stoichiometry of the post-oxidized, oxide thin films

In order to accurately determine binding energies in photoelectron spectroscopies a reference level must be established. Knowledge of this value is critical to the interpretation of core level shifts and chemical states. For metallic samples, the reference level is simply the Fermi level of the sample which is in equilibrium with that of the spectrometer. This is not the case for semicon- ducting and insulating samples. With insulating samples, photoionization creates a surface with a positive potential with respect to that of the spectrometer. This effectively increases the observed core level binding energy when the measurement is referenced to the Fermi level of the spectrometer [19]. By growing a thin layer of the insulating species on a metallic substrate by which electrical charge can be dissipated, this effect can be reduced.

Another property that is intrinsic to the sample and will cause a shift in the binding energy, is a change in the Fermi level of an insulator or

276 J.S. Comeille et al. /Surface Science 306 (1994) 269-278

semiconductor when referenced to the valence and conduction bands. The Fermi level can be presumed to be halfway between the valence band and conduction band. In reality, the position is not well known, and may shift as a result of a small concentration of defects without a specific change in the chemical state of the ma- jority of the surface atoms, In addition, the alignment of bands between a metal and an insulator will depend crucially on the electronic character of the interface. Any change at the interface during post-growth treatment (i.e. annealing) will affect the alignment of the bands, which in turn will contribute to the observed shift in the core levels.

It is difficult to determine definitively the cause of variations in the surface chemical potential since these variations are due to a combination of effects such as initial/final state effects, surface charging, interface properties, and defects and chemical modifications. In the present study, we have considered each of these effects, but it should be stated that a definitive interpretation for the core level shifts is generally not possible for closed-shell insulators such as a MgO thin film since these contributing effects are highly interrelated. For example, the presence of defects will not only change the chemical potential or “Fermi level” of the surface by altering the electronic structure but may be large enough to facili- tate the tunneling of electrons to relieve surface charging. In addition, the defect may consist of some variation in stoichiometry such as a peroxide in MgO or metallic magnesium occurring in the oxide (slight enrichment of MgO with Mg causes the sample to become an n-type semiconductor [ZO]) which may act as an electron donor or acceptor species. In view of these anomalies, it is therefore assumed that each effect may contribute to various extent. These effects are interrelated to give rise to the overall shift.

Previous experiments have shown evidence of the presence of Mg peroxide in the growth of MgO thin films and on MgO single-crystal surfaces. Using electron microscopy techniques of imaging, diffraction and spectroscopy, Wang et al. have found that Mg peroxide can be prepared on the surface of MgO single crystals 1211. Upon

studying the valence band spectra of thin Mg films exposed to oxygen, Malik et al. [22] found that several new peaks at higher oxygen exposures indicated the presence of a molecular dioxygen species. They proposed that this species was probably magnesium peroxide. The authors also point out that the peaks that they observed do not arise from surface hydroxides. The O;, Oz-, and 0; oxide species occurring in Li, K and Cs have been characterized using their valence band and core level spectra [23,24]. These studies showed that the O(ls) XPS of the peroxide species, Oz-, is centered - 2-3 eV higher in binding energy than that of the oxide (02-I. The superoxide, 0; was observed at - 4-5 eV higher than the oxide. In all of the spectra recorded in this study, either one or two states were observed in the 00s) spectra. The oxide state was observed at N 530.5 eV, in good agreement with previous results [7,25]. Depending on the preparation sequence, a second peak was sometimes observed at - 532.5-533.5 eV and was inter- preted to be indicative of the peroxide species. It should be noted that the hydroxy species can exist in this range [7]; however, no evidence of water or hydrogen desorption was observed in temperature-programmed desorption experiments. Also, the partial pressure of these contaminants, measured with a mass spectrometer, was I 4 X 10-r’ Torr under film growth conditions.

In Fig. 2, post-oxidation at room temperature of a Mg film yields a film containing both the oxide and peroxide. Both species grow with increasing exposures along with a concurrent decrease in the metallic species. It is reasonable to expect that post oxidation of metallic Mg films produces a gradient of decreasing oxygen content deeper within the film due to restricted oxygen diffusion (the surface region would be expected to be rich in oxygen). With increasing 0, exposure there is a slight shift in the O(ls) peaks to lower binding energy. This shift is accompanied by the Mg(2p) peak occurring at a relatively low binding energy for the nearly completely oxidized species. This may be as a result of interfacial effects or dipolar characteristics between the oxidized and unoxidized portions of the sample.

By annealing a 7 ML film exposed to 30 L of

J.S. Comeille et al. /Surface Science 306 (1994) 269-278 211

oxygen up to N 700 K, the binding energy of the Mg and 0 peaks increased. (This effect will be discussed in the following paragraph in connec- tion with Fig. 5.) A shift towards lower-binding was observed upon annealing the film to temperatures above 700 K, as shown in Fig. 4. This effect may be a result of changes in the oxide, the interface structure or defect generation. The sharp drop in binding energy at 1250 K may be correlated with TPD results [6] which indicate some desorption (this probably increases structural and/or point defects) of the film in the 1100-1300 K range. This is accompanied by a decrease in the XPS intensity.

Fig. 5 shows the changes in the XPS spectra of a post-oxidized film as a function of annealing temperature. It is clear that annealing reduces the intensity of the higher binding energy 00s) and increases the intensity of the lower binding energy peak. These changes in the 00s) and Mg(2p) spectra indicate a decomposition of the peroxide species to the oxide and oxidation of some remaining metallic magnesium. The shift of the 0 and Mg peaks to a higher binding energy upon annealing the film from 309 to 698 K may result, in part, from the transformation of the peroxide to oxide. Alternatively, a decrease in the stability of the final state (due to relaxation of the metallic species) with further oxidation/ desorption may also give rise to this shift. (Desorption is not likely as the Mg peak intensity remains essentially constant.) The same effects discussed earlier and demonstrated in Fig. 4 can be used to explain the drop in binding energy upon annealing from 698 to 904 K in Fig. 5. It should be noted that the small high binding energy peaks in the 00s) spectra of Figs. 5(b) and (c) are decon- volution artifacts (which are also seen in Figs. 6 and 8) arising from peak asymmetry due to the inelastic scattering of photoemitted electrons.

4.3. Core level shifts and stoichiometry of the oxide film grown in a background of oxygen

The in-situ oxidation at increasing pressures of 0, at 300 K in Fig. 6 shows an increase in the oxidized state and a concurrent decrease in

metallic Mg. A film grown in a background of oxygen is anticipated to be much more homoge- neous than one grown by post-oxidation. The broadened oxygen peak observed when the film was grown at 2.5 X lo-’ Torr of oxygen may contain a small contribution from the peroxide. However, because of the large metallic Mg com- ponent, this broadening is more likely due to variation in coordination environments along with associated final state effects within the film. This effect would not be expected for the post-oxidized film in Fig. 1 because the oxide/metal interface is relatively abrupt in comparison with that in the in-situ oxidized film. In the case of the post- oxidized film, most of the oxygen atoms are associated with a fully coordinated oxide or peroxide and are not homogeneously distributed among the unoxidized metallic species.

After annealing the film in Fig. 7 to 299 K, the film consists of the peroxide, oxide and possibly some coordinatively unsaturated magnesium. In Fig 7(d), the strong peroxide O(ls) feature indi- cates extensive peroxide formation at 100 K. At very low temperature, oxygen is strongly chemi- sorbed, however, dissociation of adsorbed 0, is not favored. Therefore, the in-situ oxidation pro- cess exhibits a “burying” effect of the 0, by Mg during the film growth, resulting in greater oxygen incorporation. After annealing to 727 K, the spectra show some decomposition of the peroxide into the oxide; however, this behavior is different from the analogous annealing sequence in the post-oxidized film in that some peroxide still ex- ists. The O(ls) peak is shifted to higher binding energy along with a slight shift and asymmetric broadening towards higher binding energy for the Mg peak. This is consistent with shifts observed for peroxide decomposition in the post-oxidized film (Fig. 5). Upon annealing to 917 K, the peaks continue to shift to slightly higher binding energy. This trend sharply contrasts with the observations shown in Fig. 4 but is reasonable if one considers that further decomposition of the peroxide has been shown to shift the core levels towards higher binding energy. The shift towards lower binding energy in Fig. 4 likely also plays a role, but the overall shift is offset by the peroxide decomposition.

278 J.S. Corneille et al. /Surface Science 306 (1994) 269-278

In Fig. 8(a), in-situ oxidation at 1 x lop6 Torr of oxygen and 300 K produced a stoichiometric film of MgO that compares well with the XPS of single crystal MgO (shown in Fig. 8~). Upon annealing the film to 900 K in Fig. 8(b), the peak positions shifted towards lower binding energies. As discussed earlier, reasonable causes of this shift are changes in the oxide structure, the interface region, or defect generation.

5. Conclusions

The post-oxidation of Mg thin films and the deposition of Mg in the presence of an oxygen background were studied with XPS. The presence of an oxide and a peroxide state was found in MgO films grown using each synthesis procedure. The peroxide state was identified by an O(ls) peak which was - 2-3 eV higher in binding energy than that of the stoichiometric oxide. An- nealing the films caused a reduction of the peroxide species and further oxidation of residual metallic Mg. The core level shifts followed a complex pattern upon annealing. These shifts are dependent on the relative stoichiometry and on a combination of effects including surface charging, interface properties, defects, initial/ final state effects, and related changes in the reference level. MgO films that are highly suitable for material studies or as model catalysts, can be grown stoi- chiometrically by the evaporation of metallic Mg at a rate of 1 ML/min (as discussed in Section 2) in the presence of 1 X lop6 Torr of oxygen onto a Mo(100) sample at 300 K.

6. Acknowledgments

The authors acknowledge with pleasure the partial support of this work by the US Depart- ment of Energy, Office of Basic Energy Science, Division of Chemical Sciences.

7. References

111

121

131

141

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161

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1121

[131

1141

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[171 [I81

1191

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El1

B.21

D31

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