Chemical and spectroscopic studies of metal oxide surfaces
D. W. Goodman
Department of Chemistry, Texas A&M University, College Station, Texas 77843
~Received 11 November 1995; accepted 22 January 1996!
Thin oxide films ~e.g., 5–10 nm of SiO2 , Al2O3 , NiO, MgO! supported on a refractory metal
substrate ~e.g., Mo, W, Ta, Re! have been prepared by deposition of the oxide metal precursor in a
background of oxygen. The thin-film nature of these samples facilitates investigation by an array of
surface techniques, many of which cannot be effectively utilized on the corresponding bulk oxide.
The structural, electronic, and chemical properties of these films have been studied with temperature
programmed desorption, Auger electron spectroscopy, X-ray photoelectron spectroscopy, ion
scattering spectroscopy, high resolution electron energy loss spectroscopy, and infrared reflection
absorption spectroscopy. The results of these studies demonstrate the viability of using thin oxide
films as models for the corresponding bulk oxide. © 1996 American Vacuum Society.
I. INTRODUCTION
Decades of research on metal and semiconductor surfaces
have produced powerful spectroscopic techniques capable of
providing detailed information about surface structure and
composition.1–3 Although considerable work has been directed toward metal oxide surfaces4 – 8 ~and references
therein!, the extent of this work is significantly less compared to that carried out on metal and semiconductor surfaces. Oxides have received less attention because the techniques most often utilized to study semiconductor or metal
surfaces frequently are not suitable for insulators. For example, many of the interface characterization techniques require the use of charged particle probes, necessitating charge
transfer to or from the substrate. Therefore many of these
techniques cannot be applied conveniently to insulating materials. However, the use of thin films grown on conducting
substrates facilitates the use of surface spectroscopic probes
by stabilizing the surface potential of the insulator.9–23
The synthesis of thin oxide films can be achieved by a
wide variety of methods,24 –27 such as laser and arc deposition, sputtering deposition, sol–gel deposition, chemical vapor deposition ~CVD!, plasma-enhanced CVD, and laserinduced CVD. The preparation of ultrathin metal oxide films
for surface science studies has been carried out most frequently by oxidizing a metal substrate, forming a thin film of
the native oxide.28 –32 However, with this method difficulties
can arise in the control of film thickness and stoichiometry. A
further disadvantage of this method is that the accessible
temperatures of the oxide are limited by the thermal stability
of the parent metal substrate.
We have recently developed an approach to oxide sample
preparation for surface science studies that utilizes the
growth of ultrathin oxides films on refractory metal
substrates.33– 62 This approach offers the advantages mentioned above for thin films on a conducting substrate in addition to control, in many cases, of the oxide surface orientation. This methodology is especially appropriate for the
study of oxides that do not have natural cleavage planes,
such as Al2O3 .60 Furthermore, the use of a refractory substrate also facilitates sample temperature monitoring and
1526
J. Vac. Sci. Technol. A 14(3), May/Jun 1996
control up to very high temperatures. Compared to the alternative of directly oxidizing the native metal or metal alloys,
growing oxide thin films from combined atomic/molecular
beam sources on a dissimilar metal substrate offers several
advantages. In particular, this approach allows tailoring of
the substrate properties, such as orientation and lattice parameter, to best fit the oxide thin film, thus providing better
control of oxide thickness, stoichiometry, and crystallinity.63
The following briefly summarizes the work on thin oxide
films that has been carried out in our laboratories to date and
illustrates how this method of preparing these films can be
used to address a variety of surface chemical issues on oxide
surfaces.
II. EXPERIMENT
The experiments were carried out in an ultrahigh vacuum
~UHV! chamber ~base pressure of ;2310210 Torr! which
has been described in detail previously.15 Briefly, the UHV
chamber was equipped with high resolution electron energy
loss spectroscopy ~HREELS! ~LK2000!, Auger electron
spectroscopy ~AES!, ion scattering spectroscopy ~ISS!, low
energy electron diffraction ~LEED!, x-ray photoelectron
spectroscopy ~XPS!, infrared reflection adsorption spectroscopy ~IRAS!, and temperature programmed desorption
~TPD!. TPD measurements were carried out using a line-ofsight, quadrupole mass spectrometer ~QMS! and a linear
heating rate of ;5 K/s. It is well known that the electron
emission from a QMS can cause damage to a weakly bound
adsorbate, thus introducing spurious decomposition products
into the TPD data. To avoid such damage, the sample was
biased at 2100 V during the TPD experiments. The sample
could be either heated to 1500 K resistively or flashed to
2200 K using an electron-beam assembly. The temperature
was measured using a W–5%Re/W–26%Re thermocouple
spot welded to the sample edge.
Film deposition was achieved by evaporation of highpurity metal ribbons wrapped around a refractory metal filament in a controlled oxygen atmosphere.33–37 The absolute
rate of metal evaporation flux was determined by a combination of TPD and AES measurements.33,49 TPD typically
0734-2101/96/14(3)/1526/6/$10.00
©1996 American Vacuum Society
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D. W. Goodman: Studies of metal oxide surfaces
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FIG. 1. A ball model of epitaxial MgO overlayers grown on Mo~100!.
shows two features for the desorbing metal, a monolayer
~ML! feature and a multilayer feature. These TPD features
can be conveniently used to indicate the completion of the
first metal monolayer. The first break point in a plot of Auger
intensity versus deposition time also can be correlated with
the completion of the first monolayer. Combined, these
methods provide an excellent method for determining the
metal deposition rates. The thicknesses of the oxide films can
then be determined from the calibrated metal evaporation
rate and the deposition time.
In the TPD experiments, a directional gas doser was used
to introduce the molecules of interest to the crystal surface.
For the HREELS studies, the exposures were carried out via
backfilling the UHV chamber. The exposures are given in
langmuirs ~L5131026 Torr s! without correction for the local dose enhancement and gauge sensitivity.
III. RESULTS AND DISCUSSION
A. Magnesium oxide
The first example of our thin-film strategy is the recent
work on magnesium oxide films prepared on a Mo~100!
surface.33–39,41,45 Magnesium oxide has a rocksalt lattice, the
simplest metal–oxide crystal structure, and is thus relatively
easy to characterize. Accordingly, magnesium oxide has been
studied both experimentally64 – 66 and theoretically.67,68
Thin MgO films have been prepared by depositing Mg
onto a clean Mo~100! surface in a background of oxygen.
The growth of MgO films on Mo~100! was examined by
LEED in the 200– 600 K substrate temperature range.33 It
was found that at an optimum synthesis pressure of oxygen
~1.031027 Torr!, the MgO films grow epitaxially with the
^100& face of MgO oriented parallel to the Mo~100! face
~Fig. 1!.
These thin MgO films have been further characterized using AES and XPS.33,37 Auger spectra acquired following the
film synthesis as a function of the oxygen pressure show that
the metallic Mg0 (L 2,3VV) transition at 44.0 eV diminishes
and a new peak at 32.0 eV becomes more predominant as the
oxygen pressure is increased. The new peak is assigned to a
JVST A - Vacuum, Surfaces, and Films
FIG. 2. O(1s) and Mg(2 p) XPS spectra of MgO ~7 ML!/Mo~100!. Seven
monolayers of Mg were dosed onto a Mo~100! surface at a sample temperature of 300 K with an O2 pressure of 131026 Torr ~spectra a!. The film was
then annealed to 900 K for 10 min ~spectra b!. The O(1s) and Mg(2p) XPS
spectra for single-crystal MgO~100! are also given as references ~spectra c!.
Mg21 (L 2,3VV) Auger transition due to the formation of
MgO. The two distinct peaks, Mg0 (L 2,3VV) and Mg21
(L 2,3VV), coexist in the intermediate oxygen pressure range
and remain unchanged in their peak shape and position as the
background pressure of oxygen is varied. At 1.031027 Torr
oxygen, the Mg0 Auger transition feature completely disappears and only the Mg21 peak remains, implying that an
optimum synthesis pressure of oxygen has been reached.
Further XPS studies showed that the MgO films exhibit the
core-level features of Mg(2p) and O(1s) identical to those
of single-crystal MgO ~Ref. 62! ~Fig. 2!.
It was found that neither subsequent annealing in an oxygen atmosphere nor growth of the MgO films at oxygen pressures greater than 1.031027 Torr yielded visible improvements in the LEED patterns of epitaxial MgO films. The
oxygen-to-magnesium AES and XPS ratio also remained
constant upon further oxidation. The AES and XPS measurements, together with the LEED observations, suggest that the
MgO films have a one-to-one stoichiometry.62 There is no
evidence for the formation of intermediate suboxide precursors, e.g., Mg11.
A very important aspect of oxides is the nature of surface
defects. In this respect, recent results in our laboratories have
demonstrated the use of high resolution electron energy loss
spectroscopy to define in great detail the nature of color centers in Li-doped MgO and their importance in the catalytic
chemistry of this material for the oxidative coupling of
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D. W. Goodman: Studies of metal oxide surfaces
1528
FIG. 3. EELS spectra of Li/MgO/Mo~100!, taken after annealing to different
temperatures.
methane.41,45 The EELS spectra of Li/MgO/Mo~100! after
annealing at different temperatures are shown in Fig. 3. The
peaks at 1.6, 3.6, and 5.3 eV have been attributed have been
attributed to @Li1O2# centers, F aggregates, and F centers,
respectively. It was found that the rate of ethane formed via
the oxidative coupling of methane correlates well with the F
center concentration, consistent with these defects being the
active site for the crucial methane activation step.41,45
HREELS has also been used to study adsorbates on the
MgO~100! surface. In the application of HREELS to adsorbates on insulating substrates, a great difficulty encountered
is that the accompanying vibrational spectra are dominated
by losses due to excitation of surface optical phonons. Since
the intense multiple phonon losses generally extend over a
wide vibrational frequency range of the HREELS spectra, it
is not practical to directly observe adsorbate losses ~which
are several orders of magnitude smaller in intensity than the
phonon losses! in the 0– 4000 cm21 spectral range. In recent
studies,34,39,47,52,54,56,59 we have developed a new approach to
acquiring HREELS data in order to circumvent the difficulties associated with these phonon losses. By utilizing a highenergy incident electron beam, this new approach enables the
direct observation of weak loss features due to the excitation
of adsorbates without serious interference from intense multiple surface optical phonon losses. Interactions between
model MgO surfaces and various probe molecules, with acid
strengths ranging from those of carboxylic acids and alcohols to alkenes and alkanes, have been studied using
HREELS.35–39 HREELS data have shown that carboxylic acJ. Vac. Sci. Technol. A, Vol. 14, No. 3, May/Jun 1996
FIG. 4. HREELS spectra of methanol adsorbed on a ;30 ML MgO~100!
film at 90 K. ~a! 180 L CH3OH at 90 K; ~b! flash to 160 K; ~c! flash to 273
K; ~d! flash to 498 K; ~e! 5 L CH3OH at 90 K. One langmuir ~L!5131026
Torr s. The spectra were collected at E 0 .46 eV and a 8.5° off-specular
reflected beam direction.
ids, methanol ~see Fig. 4!, and water dissociate heterolytically on MgO surfaces, whereas the adsorption of ethylene
and ethane is associative. These results are in agreement with
results obtained for powdered MgO catalysts.35–39
B. Silica
Silicon oxides are very important materials and have
found wide applications ranging from catalyst supports to the
insulating materials in metal–oxide semiconductor ~MOS!
devices. Consequently there is a growing interest in the synthesis of thin silicon oxide films. The basic unit of silicon
dioxide has a SiO4 tetrahedral structure. A number of approaches, including chemical vapor deposition,25 plasma enhanced CVD,26 and laser induced CVD,27 have been used to
produce SiO2 films at low temperatures.
In our studies, thin silicon dioxide films were prepared on
Mo~110! by evaporating silicon in an oxygen
atmosphere.40,46 The stoichiometry of the films versus synthesis pressure of oxygen was then examined using AES,
IRAS, and XPS.40,46 Si and SiO2 species can be differentiated based on their characteristic Auger transition energies
and lineshapes.69,70 Both Si and Si2O Auger features are
present at oxygen pressures <1026 Torr ~see Fig. 5!. At oxygen pressures greater than 431026 Torr, the Si (LVV) Auger
feature at 91 eV, which is associated with elemental
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D. W. Goodman: Studies of metal oxide surfaces
FIG. 5. Auger electron spectra for thin films of silicon and silicon dioxide on
Mo~100! prepared by evaporating silicon at oxygen partial pressures of ~A!
0, ~B! 131026, and ~C! 431026 Torr.
silicon,69,70 disappears and Auger transitions characteristic of
silicon dioxide36,37 become fully developed. This behavior
indicates that the stoichiometric film of SiO2 has been obtained. There is no evidence of the presence of other species
such as SiO, Si2O, and Si2O3 .40,46
LEED, EELS, and IRAS studies40,46 show that the SiO2
films prepared at low substrate temperatures ~;300 K! are
disordered and likely consist primarily of short-ranged networks of @SiO4# tetrahedral units. Upon heating to higher
temperatures, the films become more ordered and transform
to a structure composed of long-range @SiO4# networks.40
C. Alumina
We have recently grown highly ordered ultrathin ~<100
Å! Al2O3 films on Ta~110! ~Ref. 60! and Mo~110! ~Ref. 71!
substrates between 800 and 1200 K. The resulting Al2O3
films exhibit LEED features characteristic of close-packed,
pseudohexagonal O22 planes. Depending on the degree of
lattice matching, the ultrathin epitaxial oxide overlayer
adopts an azimuthal orientation that is commensurate with
the substrate along one of the high symmetry directions.
Nonetheless, the symmetry and lattice mismatch between the
pseudohexagonal oxide overlayer and the bcc Mo, Ta substrates introduces regular interruptions in the twodimensional epitaxial oxide film. This feature appears as a
superstructure in LEED with a large periodicity and adds
considerable complexity to the diffraction patterns. The
physical origin of this superstructure is most likely regularly
spaced domain walls and/or one-dimensional dislocations.
The presence of such long-range ordered domain wall structures and/or dislocations in the oxide film is critically related
to the strain relief mechanism and the accommodation of
misfit in the oxide/substrate system.
The lattice vibrational features ~optical phonons! of the
epitaxial film also carry information regarding the coordinaJVST A - Vacuum, Surfaces, and Films
1529
FIG. 6. HREELS spectra of Al2O3 films grown on Mo~110!: ~a! d 203 Al
5 4.4 Å; ~b! d 203 Al 5 20.0 Å. The films were annealed to 1200 K in oxygen
ambient to improve their crystalline quality. The spectra were acquired at
E p 54.0 eV and at the specularly reflected beam direction.
tion of Al31, since both the local and crystal symmetry properties of the Al2O3 film are inherent in the observed vibrational modes.71 The growth of Al2O3 films has been
examined using HREELS, as shown in Fig. 6. The fundamental modes of the surface optical phonons ~below 1000
cm21 in frequency! and their multiples and combinations are
evident in the spectra. The surface optical phonon losses of
the thin film ~spectrum a! are characterized by a two-mode
pattern, whereas three modes of the phonon losses are typical
for the thick film ~spectrum b!.
D. Nickel oxide
The adsorption of formic acid, ammonia, formaldehyde,
and alcohols on NiO~100!/Mo~100! has been studied using
TPD and HREELS.47,52,56,72 Formic acid47 adsorbs associatively at 90 K and undergoes heterolytic dissociation upon
heating to .200 K to form a formate intermediate. The adsorbed formate species is proposed to bond to a Ni cation site
via one of the oxygen atoms of the formate in a monodentate
configuration. Upon further heating, formic acid desorbs
from the surface between 300 and 400 K via recombination
of surface formate and surface hydrogen. Methanol and
ethanol56 adsorb associatively at 90 K and desorb reversibly
upon heating. Based on the HREELS data, both alcohols are
proposed to bond to the Ni cation sites via their oxygen atom
with the methyl or ethyl group directed away from the surface. No g~OH! loss feature has been observed in HREELS,
indicating the formation of strong hydrogen bonds between
the hydroxyl hydrogen and the lattice oxygen.
Formaldehyde73 adsorbs molecularly and reversibly desorbs upon heating to 350 K with extensive polymerization.
Monomeric formaldehyde is chemisorbed onto Ni cationic
sites with an oxygen end-on h1~O! orientation, leading to a
shift of the g ~CO! feature to 1650 cm21.73 An h2~C,O! form
of adsorbed formaldehyde is indicated by a g~CO! loss feature at 1320 cm21 in the HREELS. Ammonia adsorbs molecularly via the nitrogen lone pair onto the Ni cation site.52
1530
D. W. Goodman: Studies of metal oxide surfaces
The HREELS data show that the umbrella mode of ammonia
exhibits great charge sensitivity and that the frequency of
this mode remains unchanged as a function of NH3 exposure.
E. Layered-oxide films
Layered oxides are an important class of environmental
relevant materials in that there are a wide range of clays that
are based on layers of oxides, particularly silicates.72,74 In
addition, thin layers of metal oxides may exist naturally as
overlayers, such as on the surfaces of clay particles,75 making thin layers of many other oxides also environmentally
relevant. Recently we have succeeded in growing epitaxial
films of MgO~100! on NiO~100! films, that, in turn, have
been grown on Mo~100!.61 The emphasis of these studies has
been on understanding how the interface between dissimilar
oxides influences the physical and chemical properties of
thin ~1–10 ML!, ordered oxide overlayers. The effects of the
lower oxide substrate on the electronic structure of the upper
layer have also been explored using a number of techniques
including XPS, UPS, and ELS. Our recent work with overlayers of MgO on NiO ~Ref. 61! and NiO on MgO ~Ref. 76!
has demonstrated the evolution of the electronic structure of
the overlayer oxide from two to three dimensions.61
Al2O3/SiO2 mixed oxide has been studied using TPD,
AES, XPS, and ISS.77 The mixed oxide was prepared by
evaporating metallic Al onto SiO2 supported on the Mo~100!
surface and then annealing. Depending on the annealing temperature, the following steps in the Al oxidation have been
observed:
SiO desorbs between 1000 and 1100 K, leaving a homogeneous Al2O3/SiO2 film. The important step for the formation
of Si–O–Al bonds is the concerted diffusion of aluminum
oxide into the bulk of the SiO2 film with the concomitant
desorption of volatile silicon monoxide as a result of the
solid state reaction of Si and SiO2 . It was also concluded
from the XPS core-level shifts that the electronic structure of
these films is very similar to that of bulk aluminosilicates.
IV. INFRARED REFLECTION ADSORPTION
SPECTROSCOPIES OF THIN OXIDE FILMS
Recent studies in our laboratories have demonstrated the
utility of using thin oxide films for IRAS
investigations.37,40,44,49,55,78 In contrast to bulk oxides, thin
films with thickness significantly less than the wavelength of
the probe can be used with an experimental configuration
identical to that employed for metal surfaces, namely, a near
grazing incidence and takeoff angle for the infrared beam.
This property of the thin film sample simplifies greatly the
task of applying the IRAS technique to oxides. Furthermore,
because the oxide is supported on a metal backing, signifiJ. Vac. Sci. Technol. A, Vol. 14, No. 3, May/Jun 1996
1530
cant gains are realized in the apparent cross sections of the
adsorbates due to electric field enhancements of the reflecting metal substrate.
The interaction of CO with MgO~100!/Mo~100! ~Ref. 35!
and NiO~100!/Mo~100! ~Ref. 78! has been investigated with
IRAS. Using isobars constructed from measurements in a
pressure range of 131028 –1 Torr and over a temperature
range of 93–280 K, the isosteric heat of CO adsorption has
been determined to be approximately 9.9 and 10.4 kcal/mol
for CO/MgO~100! ~Ref. 35! and CO/NiO~100!,78 respectively.
V. CONCLUDING REMARKS
These initial studies have shown that ultrathin films of
various oxides can be successfully grown on refractory substrates. The utility and effectiveness of this approach to the
study of oxide surfaces with an array of surface science techniques have been demonstrated. Clearly this approach offers
new opportunities and avenues for investigating a widerange of insulating materials in entirely new ways.
ACKNOWLEDGMENTS
The author acknowledges with pleasure the support of this
work by the Department of Energy, the Office of Basic Sciences, the Division of Chemical Sciences; by Laboratory Directed Research and Development ~LDRD! funding from Pacific Northwest Laboratories; and by the Robert A. Welch
Foundation.
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