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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,
(Received
Texas A&M
University, College Station, TX 77843-3255, USA
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.
0039-6028/94/$07.00
0 1994 Elsevier
SSDI 0039-6028(93)E1022-R
Science
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) Under 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
B.V. All rights reserved
270
J.S. Corneille et al. /Surface
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 supporting 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 numerous 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. Previous studies have proposed a three-step mechanism 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
Science 306 (1994) 269-278
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 halfmaximum (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
271
Science 306 (1994) 269-218
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 researchgrade (2 99.998%) oxygen was admitted.
81
-
44
-
41
-7
1
IL
3. Results
0.
3.1. Mg overlayers on Mo(100)
0.
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)
~
IL
1304
1300
64
50
46
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 (3CiIOA) and Mg@OOl)
single crystal [10,7] are also listed
eMg (MU
Mg(2p) (eV)
MgW (eV)
1
7
40
Mg(OOO1)
49.7
49.7
49.6
49.5
1303.3
1303.0
1303.0
3000_&
49.6
1303.0
272
J.S. Corneilfe et al. /Surface
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
Science 306 Cl9941269-278
I
/
OlMg
--
I
10
Mg2p
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).
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.
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
z
Mg2p
[z
~:“--------,
z
50.0 -
s
[zii;
et al./Surface Science 306 (1994) 269-278
O/Mg(7ML)IMo(l00)
@)
O/Mg(7ML)/Mo(lOO)
@
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
\
m
1303.6 -
5i
530.40 -
273
Mg2p
r50.4
8
I
700
800
900
1000
1100
ANNEAL TEMPERATURE
1200
1300
(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 evaporating 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.
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
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].
M@p
. .
I.-L
534
532
530
528
‘
52
50
I
I
48
BINDING ENERGY (eV)
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.
Fig. 6. 00s)
and MgCLp) XPS spectra
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
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
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 summarized 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).
534
532
530
BINDING
528
54
ENERGY
52
50
48
(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
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 semiconducting 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
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 majority 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 facilitate 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
Science 306 (1994) 269-278
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 interpreted 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
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 connection 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 deconvolution 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
Science 306 (1994) 269-278
211
metallic Mg. A film grown in a background of
oxygen is anticipated to be much more homogeneous 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 component, 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 postoxidized 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 indicates extensive peroxide formation at 100 K. At
very low temperature, oxygen is strongly chemisorbed, however, dissociation of adsorbed 0, is
not favored. Therefore, the in-situ oxidation process 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 exists. 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. Annealing 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 stoichiometrically 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 Department of Energy, Office of Basic Energy Science,
Division of Chemical Sciences.
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