Surface structure of ultra-thin Al2O3 films on metal substrates

Article abstract of DOI:10.1016/S0039-6028(00)00041-8

The surface structure of ultra-thin Al₂O₃ films on NiAl(110) and Ru(0001) was investigated by using high-resolution electron energy loss spectroscopy (HREELS). On a semi-empirical basis, the three phonon loss features, v₁ (380-430 cm^-1), v₂ (620-660 cm^-1), and v₃ (850-900 cm^-1), of crystalline Al₂O₃ films were assigned to collective excitations of the microscopic vertical stretching motion of in-phase O-Al layers and to the stretching motions of the tetrahedrally and octahedrally coordinated Al-O species, respectively. To a good approximation, the distinct observation of the v₁ mode in the HREEL spectra of Al₂O₃ films can be a sign for the synthesis of a vertically well-defined Al₂O₃ phase. The relative intensity between the v₂ and the v₃ loss features has been used to determine the most probable oxide structure, amorphous, transition, or α-like, by using the relative site occupation ratio of Al³⁺ cations to hexagonal closely packed O^2- anions.

Full text of DOI:10.1016/S0039-6028(00)00041-8

Surface Science 448 (2000) L207–L212
www.elsevier.nl/locate/susc
Surface Science Letters
Surface structure of ultra-thin A1 O films on metal substrates
2
3
Myoung Bok Lee a,*, Jung Hee Lee a, Brian G. Frederick b,
Neville V. Richardson c
a School of Electronic and Electrical Engineering, Kyungpook National University, Taegu, 702-701 South Korea
b Department of Chemistry, University of Maine, Orono, ME 04469, USA
c School of Chemistry, University of St. Andrews, St. Andrews KY16 9ST, UK
Received 23 August 1999; accepted for publication 29 December 1999
Abstract
The surface structure of ultra-thin Al O films on NiAl(110) and Ru(0001) was investigated by using high-
2
3
resolution electron energy loss spectroscopy (HREELS). On a semi-empirical basis, the three phonon loss features,
n (380~430 cm−1), n (620~660 cm−1), and n (850~900 cm−1), of crystalline Al O films were assigned to collective
1
2
3
2 3
excitations of the microscopic vertical stretching motion of in-phase O–Al layers and to the stretching motions of the
tetrahedrally and octahedrally coordinated Al–O species, respectively. To a good approximation, the distinct
observation of the n mode in the HREEL spectra of Al O films can be a sign for the synthesis of a vertically well-
1
2 3
defined Al O phase. The relative intensity between the n and the n loss features has been used to determine the
2
3
2
3
most probable oxide structure, amorphous, transition, or a-like, by using the relative site occupation ratio of
Al3+cations to hexagonal closely packed O2− anions. © 2000 Published by Elsevier Science B.V. All rights reserved.
Keywords: Aluminum oxide; Crystallization; Electron energy loss spectroscopy (EELS); Oxidation; Ruthenium; Surface structure,
morphology, roughness and topography
1
. Introduction
various phases mainly result from the alternative
arrangements of Al3+cations in the tetrahedral
and the octahedral vacancies of the cubic closely
packed and/or hexagonal closely packed oxygen
arrays. In most transition bulk phases [1], the
Al3+cations are known to occupy both octahedral
and tetrahedral sites, but the relative occupation
ratios are largely unknown. It was an even more
complicated job to correctly assign the phase of
ultra-thin aluminium oxide films due in part to the
structural differences between thin-film and known
bulk-crystal structures [2] and in part to the unre-
leased strain in the thin films [3].
Heat treatment of boehmite (n-AlOOH) in air
has produced a series of transition aluminas, such
as n-, d- and h-, before final conversion to the
most stable, anisotropic a-phase Al O [1]: tetrahe-
drally coordinated Al3+cations in the amorphous
phase, both tetrahedrally and octahedrally coordi-
nated Al3+cations in the transition phase, and
octahedrally coordinated Al3+cations in the
a-phase. The structural differences between the
2
3
*
Corresponding author. Fax: +82-53-950-5505.
E-mail address: mblee@ee.kyungpook.ac.kr (M.B. Lee)
High-resolution electron energy loss (HREEL)
0
039-6028/00/$ - see front matter © 2000 Published by Elsevier Science B.V. All rights reserved.
PII: S0039-6028 ( 00 ) 00041-8

LETTERS
SURFACE SCIENCE
L208
M.B. Lee et al. / Surface Science 448 (2000) L207–L212
spectra of thin alumina films on metal substrates
show commonly three distinct phonon features
tion (LEED), Auger electron spectroscopy (AES),
X-ray photoelectron spectroscopy ( XPS), temper-
ature-programmed desorption (TPD), and
HREELS. A rotatable, single-stage hemispherical
analyzer (VSW HA50) was equipped for the angu-
lar-dependent AES, XPS, and HREELS experi-
ments with an electron-gun source (VSW EG5), a
twin anode, a non-monochromatized X-ray source
(PSP TA10), and a fixed, single-stage electron
monochromator (VSW monochromator), respec-
tively. Most vibrational EEL spectra were collected
in a specular direction at a scattering angle of 45°
with respect to the surface normal and at incident
beam energies of 4–30 eV. Most of the measure-
ments were made near 300 K at a background
pressure in the 1–3×10−10 mbar range. The typical
resolutions (full width at half maximum) of the
elastic peak in the HREEL spectrum were
60–70 cm−1 for a well-ordered alumina surface.
near the loss energies of 380~430 cm−1 (n ),
1
6
20~660 cm−1 (n ), and 850~900 cm−1 (n ) with
2
3
slightly different relative loss intensities between
the modes [4–9]. From the very initial oxidation
of single-crystal Al(111) and parameterized lattice
dynamical calculations, Erskine and Strong [10]
and Strong et al. [4] first proposed that the n , n ,
1
2
and n modes originated from the out-of-phase
motion between the in-phase surface and subsur-
3
face Al–O layers (n ), the out-of-phase motion of
1
the surface Al–O species (n ), and the subsurface
2
Al–O species (n ). However, Chen et al. [6] ques-
tioned this structural assignment for the n mode,
indicating that it was relevant to incomplete oxida-
3
1
tion of the ultra-thin Al films deposited on
Mo(110). Furthermore, Frederick et al. [7]
reported that rather than being due to incomplete
oxidation of Al species, the mode was due to an
extraordinary dipole-activity enhancement of the
dipole inactive FK modes near the point, which
was expected, especially in strained ultra-thin films
on metal substrates [3]. Thus, the preparation of
long-range-ordered alumina films seems a prere-
quisite for deriving a reliable vibrational model
and for identifying defective oxide surfaces in
many practical applications.
3. Results and discussion
Fig. 1 shows HREEL spectra obtained from
NiAl(110) samples with 200 and 2000 L O expo-
2
sures near room temperature and from NiAl(110)
samples exposed to 2000 L of oxygen and subse-
quently annealed at 600 and 1300 K in UHV. With
increasing oxygen exposure, several new phonon
bands appeared in the low-frequency regime, and
a very similar vibrational spectrum was also
reported for low doses of oxygen on Al(111)
single-crystals [4,5,10]. These phonon bands can
be properly assigned to the deformative, symmet-
ric, and asymmetric stretching motions of surface
AlO clusters rather than to a structured Al O
In this study, we have prepared a series of fully
oxidized, crystalline Al O film, in the thickness
2
3
˚
range of 5–30 A on NiAl(110) substrate. In order
to test the effects of the metallic Al-species, we
also prepared ultra-thin crystalline Al O films of
2
3
˚
about 5–30 A on Ru(0001) and further annealed
them in an ultra-high vacuum (UHV) for complete
oxidation and better crystallinity. We propose an
idea for the reliable determination of an oxide
phase using the relative intensity ratio between the
three typical phonon features of the ultra-thin
Al O films.
x
2 3
film [11]. On this basis, it would be unreliable to
find a close relationship between the phonon bands
of surface AlO clusters and those of structured
x
Al O thin films. Final annealing at 1300 K in
2
3
2 3
UHV resulted in a long-range, well-ordered alu-
minium-oxide thin film with a thickness of about
˚
2. Experimental
5 A and the corresponding HREEL spectrum
illustrated three, symmetric phonon bands, n , n ,
1
2
This investigation was carried out in a two-
and n at 400, 630, and 870 cm−1, respectively,
about 1270, 1485, and 1730 cm−1.
3
level, stainless-steel, UHV apparatus with surface
preparation facilities and many in-situ character-
ization tools, such as low-energy electron diffrac-
accompanied by three high-frequency modes at
Thicker overlayers on NiAl(110) with thick-

SURFACE SCIENCE
LETTERS
M.B. Lee et al. / Surface Science 448 (2000) L207–L212
L209
Fig. 1. HREEL spectra from the NiAl(110) surfaces: (a) 200 L and (b) 2000 L O exposure at 300 K; annealed at (c) 600 K and at
2
(
d) 1300 K after a 2000 L O exposure. E =5 eV, and h =45°.
2
p
i
˚
nesses of 18, 25 and 30 A also resulted in three
tures, at 430, 640, and 870 cm−1, resembling the
similar phonon bands at loss energies of 390 (n ),
loss energies and the relative intensity ratios of the
1
˚
(
n ), and 890 (n ) cm−1. The relative intensities of
well-ordered 5 A Al O film on NiAl(110). Heavily
2
3
2 3
˚
the three phonon modes with respect to the elastic
oxidized 20–30 A Al O films deposited on
Ru(0001) in O or in an O /H O mixture also
2
3
˚
peak were clearly different from those of the 5 A
2
2
2
film and increased quasi-linearly as the thickness of
the oxide on NiAl(110) increased. In fact, the
relative intensity of the highest-frequency mode at
demonstrated the corresponding three phonon loss
features at 380, 630, and 900 cm−1, and further
annealing resulted in only a slight enhancement of
the signal-to-noise ratio. Details of the loss frequen-
cies and the relative intensities are given in Table 1.
In a semi-empirical sense, a new origin can be
suggested for each of the three main loss features
˚ ˚
0 eV was ~0.025 for 5 A, ~0.111 for 18 A,
1
~
˚ ˚
0.133 for 25 A, and 0.196 for 30 A. Almost the
same phonon structures were obtained from the
HREELS measurements on ultra-thin crystalline
Al O films on Ru(0001) substrates and with a
in transition Al O films consisting of both octahe-
2
3
2 3
˚
thickness of about 5–30 A, and the results are
illustrated in Figs. 2 and 3. As shown, a fully
dral and tetrahedral Al3+occupation. The n loss
3
feature at 820~900 cm−1 is due to a group of
˚
oxidized Al O film of about 5 A on Ru(0001)
reveals three well-resolved symmetric phonon fea-
microscopic stretching motions between the octa-
2
3
hedrally coordinated Al3+cation and its six near-

LETTERS
SURFACE SCIENCE
L210
M.B. Lee et al. / Surface Science 448 (2000) L207–L212
˚
Fig. 2. HREEL spectra of thicker oxide films on NiAl(110) recorded at E =10 eV, h =45°: (a) 18, (b) 25, and (c) 30 A.
p
i
est-neighbour O2− anions, the n loss feature at
motions between the tetrahedrally coordinated
spectra of ultra-thin Al O films, one can propose
2 3
2
6
20~650 cm−1 is due to a group of stretching
the feasibility of vertically well-layered Al O films
2
3
on Al(111) [4,5,10], Mo(110) [6], NiAl [8,9] and
Ru(0001) [7]. Near-surface Al–O layers may
determine the strengths of the dipole moments of
Al3+cation and its four nearest-neighbour O2−
anions, and the n loss feature at 380~430 cm−1
1
is due to a vertical stretching motion of the subse-
the n mode so that the resultant intensity may be
1
quent in-phase O–Al planes. Therefore, a clear
less sensitive to sample thickness changes from 5
˚
observation of the n loss feature may be a sign
to 24 A on Ru(0001), as indicated in Fig. 3.
1
for the synthesis of vertically well-defined crystal-
Instead, long-range, well-ordered films on
NiAl(110) show a systematic increase in relative
intensity with increasing oxide thickness from 2 to
10 Al–O layers.
line Al O films on NiAl(110) and Ru(0001), as
2
3
shown in Figs. 1 and 3. The n loss mode may also
1
indicate highly near-surface localized characteris-
tics, such as less dispersive energy loss and rela-
tively lower intensity, depending largely on the
thin-film processing parameters of oxidation/
annealing temperature and the metal substrates
used. From both previous and present HREEL
The population of the tetrahedral Al3+ sites
in-phase alumina with a spinel structure is twice
that of the octahedral Al3+ sites, and three slightly
different sets of O–Al bonds exist in the octahe-
dron AlO9− cluster configuration and two values
6

SURFACE SCIENCE
LETTERS
M.B. Lee et al. / Surface Science 448 (2000) L207–L212
L211
˚
Fig. 3. HREEL spectra of crystalline 5 and 24 A Al O films on Ru(0001) recorded at E =10 eV and h =45°.
2
3
p
i
in the tetrahedron geometry [12]. Since the relative
hedron and the octahedron geometries to the
oscillator strength was totally unknown for octahe-
dral and tetrahedral coordination, it was not easy
to estimate the probable occupation ratio between
the tetrahedral and the octahedral sites in most
transition phases. However, now, using an occupa-
tion ratio of 1:2 between the tetrahedral and octahe-
dral sites, especially for the n-phase (most of
tetrahedral sites will be unoccupied), one can
roughly correlate the ratio 1(1×4):3(2×6) of the
total microscopic Al–O oscillators between the tetra-
observed relative intensity ratio between the n and
2
the n modes. Thus, this expected ratio of 0.33
3
between the tetrahedron and the octahedron geome-
tries seems to be in good agreement with the
observed intensity ratio of ~0.33 between the n
2
and the n modes in thicker Al O films on
3
2 3
NiAl(110). The broadness of the n and the n loss
2
3
features may be relevant to the existence of multi-
oscillators with slightly different O–Al bond
strengths in each cluster geometry. On this basis,
Table 1
Energies and relative intensities of the n , n , n single and n +n , n +n , 2n multiple loss events observed from ultra-thin Al O
1
2
3
1
3
2
3
3
2 3
films on metal substrates (mode unit: cm−1)
n mode
n mode
n mode
n
mode
n
mode
n
mode
1
2
3
13
/I
23
/I
33
/I
I /I
I /I
I /I
I
I
I
1
0
2
0
3
0
13
0
23
0
33
0
Al O on Al(111)
430
430
645
640
0.008
639
0.017
620
0.0055
653
865
880
0.32
900
0.063
870
0.027
887
1290
1250
1510
1460
1730
1690
2
3
5
2
5
3
A Al O on Ru(0001)
2
3
0
.008
4A Al O on Ru(0001)
374
1270
1485
1730
2
3
0
.008
A Al O on NiAl(110)
400
1270
0.000509
1258
1485
0.000412
1540
1730
0.00057
1758
2
3
0
.0058
0A Al O NiAl(110)
387
2
3
0
.0336
0.065
0.196
0.0194
0.0266
0.0334

LETTERS
SURFACE SCIENCE
L212
M.B. Lee et al. / Surface Science 448 (2000) L207–L212
˚
the most probable structure of the 18–30 A Al O
the tetrahedral sites may reflect electronic, vibra-
tional, and structural characteristics of an a-like
Al O phase, but from the presence of the n and
2
3
films prepared on NiAl(110) would be a n-like
transition phase with an Al3+occupation ratio of
2
3
2
1
:2 between the tetrahedral and the octahedral sites
the n modes in HREEL spectra, most of the
3
of the cubic/hexagonal closely packed oxygen arrays.
previously reported oxide films on metal substrates
˚
can be suggested as having transition phases/
structures with slightly different Al3+ occupation
ratios between the tetrahedral and the octahedral
sites of the cubic/hexagonal closely packed oxygen
arrays. Reliable lattice-dynamical modeling based
on the proposed microscopic oxide structures
would be a great help for further detailed explana-
tions of both the behaviors of the three loss
features and the relevant electronic, optical, and
structural characteristics of ultra-thin crystalline
The 5 A film on NiAl(110) can be proposed to
have a specific structure with a 3.3 times higher
Al3+ occupation of octahedral sites with respect
to the tetrahedral sites since the observed intensity
ratio was 0.20 between the n and the n modes.
A similar discussion is possible for samples on
Ru(0001), and the analysis results in an occupa-
tion ratio of 1:2.7 for the 5 A film and 1:2.5 for
the 24 A film since their observed intensity ratios
between the n and the n modes were 0.25 and
.27, respectively. Therefore, the considerably
2
3
˚
˚
2
3
Al O films on metal substrates.
0
2 3
enhanced Al3+ occupation of the octahedral sites
with respect to the tetrahedral sites may reflect
electronic, vibrational, and structural characteris-
tics of an a-like Al O phase, but from the presence
Acknowledgements
Dr. M. B. Lee wishes to acknowledge the invalu-
able technical support and advice by reseach staff
during his study at the surface Science Research
Centre of Liverpool University in the United
Kingdom.
2
3
of the n and the n modes in HREEL spectra,
2
3
most of the previously reported oxide films on
metal substrates can be suggested to have trans-
ition phases/structures with slightly different
Al3+ occupation ratios between the tetrahedral
and the octahedral sites of the cubic/hexagonal
closely packed oxygen arrays.
References
[
[
1] S.J. Wilson, J.D.C. McConnel, J. Solid State Chem. 34
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Rev. B 26 (1982) 3483.
(
4
. Conclusions
The HREEL spectra were newly analyzed to
[
[
determine the most probable structure by using
the three typical phonon loss features of the crystal-
line Al O films, n (380~430 cm−1), n (620~
2
3
1
2
6
60 cm−1), and n (850~900 cm−1). New origins
[5] J.E. Crowell, J.G. Chen, J.J.T. Yates, Surf. Sci. 165
1986) 37.
[6] P.J. Chen, M.L. Colaianni, J.J.T. Yates, Phys. Rev. B 41
1990) 8025.
3
2
(
for the n , the n , and the n loss features were
1
3
proposed as being collective excitations of a verti-
cal stretching motion of the subsequent in-phase
O–Al planes and a group of microscopic stretching
motions between the octahedrally and tetrahe-
drally coordinated Al–O species in transition
Al O film, respectively. Thus, the observation of
an intense n loss feature can be a sign for the
synthesis of vertically well-defined crystalline
Al O films on metal substrates.
(
[
7] B.G. Frederick, G. Apai, T.N. Rhodin, Phys. Rev. B 44
(1991) 1880.
8] R.M. Jaeger, H. Kuhlenbeck, H.-J. Freund, M. Wuttig,
W. Hoffman, R. Franchy, H. Ibach, Surf. Sci. 259
[
(
1991) 235.
[
9] P. Gassmann, R. Franchy, H. Ibach, Surf. Sci. 319
(1994) 95.
2
3
1
[
[
10] J.L. Erskine, R.L. Strong, Phys. Rev. B 25 (1982) 5547.
11] B.G. Frederick, M.B. Lee, N.V. Richardson, Surf. Sci. 348
(1996) L71.
12] S. Thomas, P.M.A. Sherwood, Anal. Chem. 64 (1992)
2488.
2
3
In conclusion, the considerably enhanced Al3+
[
occupation of the octahedral sites with respect to