High temperature oxidation of CoAl(1 0 0)

Article abstract of DOI:10.1016/j.susc.2004.12.028

We have employed Auger electron spectroscopy (AES), high resolution electron energy loss spectroscopy (EELS), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) to investigate the growth of an Al ₂O₃ film on CoAl(1 0 0). While exposure to oxygen at room temperature leads to the formation of amorphous alumina, subsequent annealing at higher temperatures results in the growth of well-ordered θ-Al ₂O₃. Well-ordered Al₂O₃ films are also formed by oxidation at temperatures of 800 K and above. The oxide is characterized by Fuchs-Kliewer modes at around 430, 630, 780 and 920 cm ^-1. Oxide islands grow in two sets of domains perpendicular to each other. Under ultra-high vacuum conditions, self-limiting thickness of the oxide layer (9-10 A?) has been found. The band gap of the θ-Al ₂O₃ film on CoAl(1 0 0) is 4.3-4.5 eV.

Full text of DOI:10.1016/j.susc.2004.12.028

Surface Science 577 (2005) 139–150
www.elsevier.com/locate/susc
High temperature oxidation of CoAl(100)
z
*
Volker Rose , Vitali Podgursky, Ioan Costina, Ren e´ Franchy , Harald Ibach
Institut f u¨ r Schichten und Grenzfl a¨ chen (ISG 3) des Forschungszentrums J u¨ lich, D-52425 J u¨ lich, Germany
Received 29 September 2004; accepted for publication 22 December 2004
Available online 19 January 2005
Abstract
We have employed Auger electron spectroscopy (AES), high resolution electron energy loss spectroscopy (EELS),
low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) to investigate the growth of an
Al
2 3
O film on CoAl(100). While exposure to oxygen at room temperature leads to the formation of amorphous alu-
mina, subsequent annealing at higher temperatures results in the growth of well-ordered h-Al O . Well-ordered
2
3
Al O films are also formed by oxidation at temperatures of 800 K and above. The oxide is characterized by Fuchs–
2
3
ꢀ
1
Kliewer modes at around 430, 630, 780 and 920 cm . Oxide islands grow in two sets of domains perpendicular to each
˚
other. Under ultra-high vacuum conditions, self-limiting thickness of the oxide layer (9–10 A) has been found. The band
gap of the h-Al O film on CoAl(100) is 4.3–4.5 eV.
2
3
ꢀ
2005 Elsevier B.V. All rights reserved.
Keywords: AES; LEED; EELS; STM; Oxidation; Cobalt; Aluminum; Alumina
1
. Introduction
surfaces play a key role in the understanding of
metal oxide interfaces. In the last years the oxida-
tion of low index intermetallic alloy surfaces, such
The metal oxide interface is of considerable
intrinsic interest. Also it is of extreme importance
in many technological applications including het-
erogeneous catalysis [1] or magnetoelectronic de-
vices [2]. In particular, well-ordered oxide
as NiAl, Ni Al, CoGa [3], or FeAl [4], has been
3
intensively studied. Generally, the authors report
about the growth of well-ordered Al O films at
2
3
high oxidation temperatures. Due to thermody-
namic reasons the oxidation of this intermetallic
alloys is restricted to the Al atoms, while the Ni,
Co and Fe atoms remain unaffected. Furthermore,
it has been shown that different transient modifica-
tions of thin alumina films can be fabricated
at varying oxidation temperatures. CoAl is an
*
Corresponding author. Present address: Argonne National
Laboratory, Argonne, IL 60439, USA. Tel.: +49 2461 61 5765;
fax: +49 2461 61 3907.
E-mail address: v.rose@fz-juelich.de (V. Rose).
z
Deceased.
0
039-6028/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.susc.2004.12.028

140
V. Rose et al. / Surface Science 577 (2005) 139–150
intermetallic alloy with CsCl-type structure (B2)
˚
with a lattice constant of 2.862 A [5]. The (100)
of the Beetle type developed by Besocke [9] and
was operated in a constant current mode. The
sample was annealed by electron bombardment.
Temperature was measured by a WRe25%–
WRe3% thermocouple mounted directly to the
back of the sample. During the adsorption steps
the oxygen partial pressure was adjusted between
layers have an ABAB. . . stacking sequence. The
melting point is 1913 K. The clean CoAl(100) sur-
face shows a (1 · 1) LEED pattern with sharp
Bragg reflections [6]. The surface is terminated by
a nominal Al sublattice with approximately 30%
of Co atoms therein caused by antisite atom segre-
ꢀ
6
ꢀ8
1 · 10 and 1 · 10 mbar. The exposure is given
ꢀ
6
˚
gation [7], and exhibits 400–1000 A wide terraces
separated by double atomic steps.
in L units (1 L = 1.3 · 10 mbar s). During our
investigations presented in Section 3.2, oxidation
took place at room temperature and the sample
was subsequently annealed for 2 min at each step.
Our CoAl(100) substrate crystal had a diameter
of 10 mm and a thickness of 2.1 mm. The accuracy
of orientation was 0.1ꢁ after mechanical polishing.
The surface of the substrate was cleaned in
Our previous work [6] has shown that oxygen
exposure of CoAl(100) at room temperature leads
to the formation of amorphous alumina (a-Al O ).
2
3
After saturation with oxygen the thickness of the
˚
a-Al O film was estimated to be around 7 A.
2
3
Wide energy range electron energy loss spectro-
scopy (EELS) measurements showed that the
band gap of this oxide is 3.2 eV.
+
UHV by several cycles of Ar ion sputtering
(2 lA, 1 kV, 20 min) and by subsequent annealing
at 1470 K (10 min). Residual carbon contamina-
tion was removed by several cycles of heating to
The main purpose of this paper is the presenta-
tion of the oxidation of CoAl(100) in a wide tem-
perature range and to discuss the formation of
well-ordered alumina films. For that reason the
oxidation of CoAl(100) is studied by Auger elec-
tron spectroscopy (AES), high resolution electron
energy loss spectroscopy (EELS), low energy elec-
tron diffraction (LEED) and scanning tunneling
microscopy (STM).
ꢀ
6
1200 K in an O atmosphere at 1 · 10 mbar for
2
2 min followed by 2 min of annealing at 1470 K.
3. Results and discussion
3.1. The clean CoAl(100) surface
The dotted line in Fig. 1a represents the low en-
ergy region of an AES spectrum of the clean
CoAl(100) surface measured with an electron
gun in normal incidence geometry (NIAES). The
spectrum exhibits the transitions of Co (MNN at
53 and 95 eV) and Al (LMM at 68 eV). The same
transitions occur when the primary electrons hit
the surface under grazing incidence (GIAES) con-
ditions (Fig. 1b). A detailed analysis and compar-
ison of AES spectra for both geometries allows a
conclusion about the termination of the surface.
The peak-to-peak ratio CoMNN (53 eV)/AlLMM
(68 eV) amounts to 1.1 in NIAES, whereas in
GIAES this ratio decreases to 0.9. The latter mea-
surements prove that the Co/Al ratio varies within
the top atomic layers, e.g. the layers are enriched
with Al atoms. This corroborates our earlier inves-
tigations [6] and has been explained by antisite
atom segregation of Co atoms to the nominal Al
sublattice in the topmost surface plane [7].
2
. Experimental
The experiments were carried out in an ultra
high vacuum (UHV) system with a base pressure
ꢀ
11
typically of 8 · 10
equipped with a cylindrical mirror analyzer
CMA) for AES, an EEL spectrometer [8], a
mbar. The chamber was
(
three-grid LEED optics and a STM microscope.
A quadrupole mass spectrometer (QMS) was used
for the residual gas analysis. The surface sensitivity
of AES was improved by decreasing the incidence
angle (3ꢁ) of the electron gun in addition to a nor-
mal incidence using the same CMA. The EEL
spectra were taken in specular geometry at an inci-
dent angle of 57ꢁ with respect to the normal of the
CoAl(100) surface and at primary energies of
around 4 eV for vibrational studies and 16 eV for
wide energy scans required to determine the band
gap. The STM microscope was a modified version

V. Rose et al. / Surface Science 577 (2005) 139–150
141
ꢀ
1
(
DH = ꢀ1675 kJ mol ) is about twice the value
ꢀ
3 4
f
1
necessary for Co O formation (ꢀ891 kJ mol
)
and about seven times the value needed to form
ꢀ
1
CoO (ꢀ238 kJ mol ) [10]. GIAES conditions
(
Fig. 1b) yield a spectrum for 500 L of O on
2
CoAl(100) with nearly the same transitions but
with different relative intensities. The intensity of
Co LMM transitions decreases substantially with
respect to the OKLL (503 eV) peak. While the ratio
of CoLMM (775 eV)/OKLL (503 eV) amounts to
0.70 for NIAES, a decrease to 0.15 can be found
for GIAES. In addition, the AlLMM (68 eV) transi-
tion appears only as a weak shoulder. This is a
consequence of the improved surface sensitivity
of GIAES, e.g. screening of the CoAl transitions
through the oxide layer. The KLL transitions of
3
+
Al and Al are not resolved and produce a peak
at around 1376 eV. This shift of 7 eV in compari-
son to the NIAES measurement indicates the
3
+
strengthened influence of Al in the near surface
region.
Fig. 2 shows the peak-to-peak ratio of the
NIAES transitions of OKLL (503 eV) and CoLMM
(775 eV) as a function of O exposure. The inten-
2
sity of the OKLL (503 eV) transition is normalized
to the intensity of the oxygen-saturated surface,
whereas the intensity of the CoLMM (775 eV) tran-
sition is normalized to the intensity of the clean
CoAl surface. The curves reveal a high initial stick-
ing probability up to an exposure of around 30 L.
Fig. 1. AES spectra of the oxidation of CoAl(100) at 800 K (a)
investigated with a normal incidence primary electron gun
(NIAES) and (b) in grazing incidence conditions (GIAES). The
dotted line shows the low energy region of AES spectra
obtained for clean CoAl(100).
3
.2. Oxidation of CoAl(100) at 800 K
Fig. 1a shows an NIAES spectrum after the oxi-
dation of CoAl(100) with 500 L of O at 800 K. In
2
addition to the characteristic transitions of the
clean sample, i.e. Co (MNN at 53 and 95 eV,
LMM at 656, 716 and 775 eV) and Al (LMM at
6
8 and 84 eV, KLL at 1396), the transitions of O
+
3
(
KLL at 503) and Al (MNN at 35 and 53 eV,
KLL at 1378 eV) are observed. The Co
MNN
3
+
(53 eV) peak overlaps with the Al (51 eV). The
transition at around 1383 eV is generated by the
3
+
overlap of AlKLL (1396 eV) and Al (1378 eV).
It appears that an alumina layer is formed on the
surface. The formation of Al O is facilitated by
Fig. 2. Normalized peak-to-peak intensity of OKLL (503 eV)
and CoLMM (775 eV) NIAES transitions as a function of O2
exposure.
2
3
the fact that the heat of alumina formation

142
V. Rose et al. / Surface Science 577 (2005) 139–150
Already after 1.5 L O the intensity of the O
2
KLL
(
value. For exposures above 200 L, the O -uptake
503 eV) transition reaches half of its saturation
2
levels off, suggesting that the surface oxidation is
complete. The intensity of the CoLMM (775 eV)
transition decreases to 83% of its initial value after
1.5 L O , and the signal intensity saturation at
2
I = 0.52 can be found for exposures above 200 L
O . A logarithmic growth-rate of Al O and satu-
3
2
2
ration for high oxygen doses was also reported for
the NiAl and Ni Al surfaces (see Table 1) and can
3
be explained by the theory of Cabrera and Mott
[
11]. After reaching a certain thickness the electric
2
ꢀ
field between the O and the metal ions driving
the growth of the oxide, is insufficient to account
for ion movement. The attenuation of the Co
intensity is a consequence of the substrate signal
screening due to the growth of alumina on top of
the surface. We can estimate the oxide thickness
t of the overlayer using the equation
t ¼ ꢀk ꢁ cos x ꢁ lnðIÞ
where k = 18 A is the inelastic mean free path for
2 3
Fig. 3. Set of EEL spectra of the growth of Al O on
CoAl(100). In order to distinguish the different spectra, they
are vertically shifted. A calculated EEL spectrum is marked by
plus signs.
˚
7
75 eV electrons in alumina [12]. The entrance an-
gle of the CMA is x = 42ꢁ. Hence, the thickness of
the oxide film after the saturation is estimated to
˚
be 8.8 A and exceeds the thickness of the amor-
mode arising from the perpendicular vibration of
Al atoms at the oxide/substrate interface. A similar
loss was already observed on NiAl(110) [13] and
NiAl(111) [13,14] surfaces. After the exposure of
phous Al O film grown on CoAl(100) at room
2
3
temperature [6].
The EEL spectra for different oxygen exposures
at 800 K are shown in Fig. 3. After the exposure of
0.8 L of O this loss vanishes because the oxide
2
0
.2 L of O the spectrum shows four phonons at
2
coverage becomes sufficient to screen the Al vibra-
tion. The remaining losses gain intensity, and the
mode initial 880 cm shifts slightly to 890 cm .
ꢀ1
235, 430, 635 and 880 cm . The mode at 235
cm can be assigned to a quasi-transverse optical
ꢀ1
ꢀ1
ꢀ1
Table 1
The characteristic frequencies of the losses in the EEL spectra and the band gap for well-ordered Al
intermetallic alloys
2 3
O films on surfaces of different
ꢀ1
Well-ordered Al
2
O
3
on
FK modes (cm
)
Oxidation temperature (K)
Structure
Bandgap (eV)
References
0
NiAl(100)
400, 600, 900
20, 603, 718, 896
700–1200
1200
c -Al
2
O
3
[17]
[17]
4
h-Al
2
O
3
0
NiAl(110)
NiAl(111)
410, 620, 850
427, 637, 887
1200
c -Al
2
2
O
3
3
[18]
[19]
0
900–1100
>1100
1150
c -Al
O
629, 911
415, 640, 875
a-Al
2
O
3
[19]
[20]
0
Ni
Ni
3
Al(100)
Al(111)
c -Al
2
O
O
3
4.3
4.5
0
3
440, 648, 910
ꢂ435, ꢂ665, 887
430, 630, 780, 920
1000
500
c -Al
2
3
[21]
[22]
Al(111)
CoAl(100)
800
h-Al
2
O
3
This work

V. Rose et al. / Surface Science 577 (2005) 139–150
143
With further increasing exposure, both the shift of
this mode and the gain of intensity of all modes
continue. The EEL spectrum of 400 L O exhibits
case of h-alumina on NiAl(100) in addition a
1
fourth loss at 718 cm was reported [17].
ꢀ
The comparison between measured and calcu-
lated EEL spectra based on dielectric theory
can give important indications about structure
and thickness of oxide films. We employed a pro-
gram published by Lambin et al. [23] to simulate
EEL spectra for electrons specularly reflected from
the surface. Because of missing data for CoAl,
parameters taken from NiAl [24] alloy were used.
Table 2 lists the IR parameters [25] we used to
simulate a c-Al O film on NiAl. A calculated
2
ꢀ1
modes at 430, 630 and 920 cm and a weak pho-
ꢀ
1
non at 780 cm . The loss structure is attributed to
the Fuchs–Kliewer (FK) modes [15,16] of a well-
ordered alumina layer. The rather large losses are
indicative of an oxide overlayer and caused by
the strong metal-oxide dipoles. Our results are in
good agreement with the investigations of the
growth of different phases of well-ordered alumina
films on NiAl(100) [17], NiAl(110) [18], NiAl
2
3
(
111) [19], Ni Al(100) [20], Ni Al(111) [21], or
3
spectrum is presented in Fig. 3 (marked by plus
signs). Calculations show that variation of the
oxide thickness leads to small energy shifts of
the FK modes (<50 cm ) while the number and
the relative intensities of the peaks remain
constant. Best agreement between measurement
and calculation is found for oxide thickness of
3
Al(111) [22] (see Table 1). Generally, all alumi-
nium oxides consist of a close-packed oxygen
sub-lattice with Al cations assuming the positions
of the tetrahedral and/or octahedral vacancies.
Different occupation probabilities of the vacancies
determine the crystal structure. According to
group theory four IR active modes are expected
for c-like or h-like phases (Al ions in tetrahedral
and octahedral vacancies) whereas in hexagonal
a-Al O the octahedral sites are solely occupied
ꢀ
1
˚
10 A at which FK modes at 400, 655, 775 and
ꢀ
1
900 cm
ness obtained from AES analysis. The loss at
occur, in agreement with the thick-
ꢀ
1
1300 cm can be assigned to a multiple excitation.
Although EEL spectra can ensure that the alumina
film is well-ordered, further determination of the
oxide structure can only be carried out with LEED
experiments.
2
3
and only two FK modes can be obtained [19].
The EEL spectra of the c-like or h-like phases like
here commonly show three distinct phonon fea-
ꢀ
1
tures in the frequency region of 400–440 cm
ꢀ
,
00–648 cm and 850—910 cm whereof a forth
1
ꢀ1
6
Fig. 4 shows LEED patterns of the CoAl(100)
surface after oxidation at 800 K with 0.7 and
100 L O . After the exposition with 0.7 L oxygen
loss remains normally unresolved due to the
broadness of the loss structure. However, in the
2
Table 2
˚
film with a thickness of 10 A (for
Oscillator parameters that were used for the calculated EEL spectrum in Fig. 3 of a h-Al
2
O
3
elucidations of the variables see [23])
c-Al
Dielectric constant
2 3
O
˚
Thickness of the layer = 10 A
e
0
2.9
Thick
.10D + 02
Resonant frequency
ꢀ
Strength factor
Resonant
frequency (cm )
Damping constant
1
)
ꢀ1
(
cm
x
x
x
x
TO1
TO2
TO3
TO4
357.0
536.0
744.0
807.0
S
S
S
S
k1
k2
k3
k4
2.52
2.38
0.34
0.17
x
x
x
x
LO1
LO2
LO3
LO4
403.0
669.0
783.0
917.0
c
c
c
c
k
/xTO1
k
/xTO2
k
/xTO3
k
/xTO4
0.202
0.189
0.076
0.054
Substrate (NiAl)
Bulk plasmon frequency
eV)
Thickness is assumed
infinity
Damping frequency
(
x
p
7.4
Thick
.100D + 75
c
0.03

144
V. Rose et al. / Surface Science 577 (2005) 139–150
of a-Al O . This suggests a complicated interface
2
3
structure favouring the formation of Al O stripes.
2
3
In a surface X-ray diffraction study [27] on the oxi-
dation of the NiAl(100) it was found that the
islands of h-Al O are embedded in a matrix of
2
3
alumina layers showing a strong deviation from
the pure h-phase at the metal-oxide interface and
at the oxide surface. Thus, the growth of a per-
fectly ordered, pure h-Al O layer on NiAl(100)
2
3
producing a streak-free (2 · 1) structure is some-
what complicated. We believe the scenario de-
scribed above for NiAl(100) may be also valid
for the oxidation of CoAl(100).
Fig. 5a shows a STM image of CoAl(100)
taken after the oxidation with 10 L of oxygen.
Rectangular domains rotated 90ꢁ with respect to
each other can be observed containing a unit cell
˚
with the lengths of the basis vectors a ꢂ 2.9 A
˚
and b ꢂ 5.7 A. Such unit cell corresponds to the
h-Al O layer and explains the (2 · 1) structure
2
3
observed by LEED. A top view structure model
of the oxide is depicted in Fig. 5b. In each oxygen
layer of h-Al O the Al ions form chains with a dis-
Fig. 4. The LEED images of (a) 0.7 and (b) 100 L of O
CoAl(100) exhibit
(2 · 1) structure with weak streaks
= 75 eV). Some reflexes are marked for better
understanding.
2
on
a
2
3
˚
(E
p
tance of b = 5.64 A. Thus the stripes in the STM
image can be either connected to aluminium chains
or Al missing chains. In case of the b-Ga O , that
2
3
is isomorphic to h-Al O , comparable stripes were
2
3
(
Fig. 4a) LEED shows (1 · 1) pattern of the clean
explained by Ga-missing rows with a depletion of
charge of Ga due to electron transfer to O during
oxidation [28]. Because we have used a positive
bias voltage at the tip a charge transfer from Al
to O in h-Al O favours the interpretation of Al
surface [6], a two-domain (2 · 1) structure, as well
as weak streaks along the [100], and the [010]
directions. With increasing oxygen dosage
the spots and streaks gain intensity. For the sake
of clarity the spots of one unit cell have been
marked in Fig. 4b. Based on the LEED measure-
ments, e.g. appearance of a (2 · 1) pattern, we con-
clude that a well-ordered h-Al O film grows on
2
3
missing chains.
Fig. 5c shows a qualitative 3D structural model
of a h-Al O thin film on CoAl(100). The h-Al O
3
2
3
2
˚
phase exhibits a monoclinic unit cell (a = 2.92 A,
2
3
˚
˚
CoAl(100).
b = 5.64 A, c = 11.78 A, b = 104ꢁ). Depending on
the domain orientation, the (110) plane of the
fcc oxygen sublattice is parallel to the [100] or
[010] direction of CoAl. Further, the oxygen
(110) plane is perpendicular to the CoAl(100) sur-
face. This is the classical Bain orientation between
bcc CoAl and the fcc oxygen sublattice structure.
According to this, the lattice mismatch between
CoAl(100) and h-Al O only amounts to ꢂ1%
The presence of streaks in LEED patterns is
usually assigned to a loss of order in the corre-
sponding direction. Such streak structures are also
reported for the high temperature oxidation of
FeAl [4]. For Al O on NiAl(100) high-resolution
spot profile analysis of low energy electron diffrac-
tion (SPALEED) exhibits a satellite structure in
the pattern corresponding to a (9 · 1) superstruc-
ture [26]. It was found that the crystalline Al O
2
3
2
3
˚
2
3
in a-axis direction (a
h-alumina
= 5.64 A, 2 ·
strips are embedded in re-grown NiAl terraces
aCoAl = 5.72) and ꢂ2% in the b-axis direction
˚ ˚
CoAl
pffiffi
pffiffi
(cð 2 ꢃ 3 2ÞR45ꢁ) and areas of ultra-thin layers
(b
= 2.91 A,
b
= 2.86 A). Therefore,
h-alumina

V. Rose et al. / Surface Science 577 (2005) 139–150
145
the h-Al O is compressed in one direction and ex-
2
3
panded in the perpendicular direction.
Fig. 6a–d show wide area STM images of
CoAl(100) at different stages of oxidation. Due
to experimental reasons we had to remove the
STM from the sample during the oxidation steps.
However, while every image corresponds to a dif-
ferent area of the sample surface it demonstrates
a surface morphology typical to the particular oxi-
dation step. The clean surface (Fig. 6a) exhibits 12
˚
large terraces with a width of around 400–1000 A
˚
separated by 2.8 A steps with assigned to double
atomic steps of the CsCl structure of CoAl [6]. Al-
ready after the exposure to 0.5 L of oxygen the
number of terraces decreases to 8 and a rearrange-
ment of the steps can be recognized. Only rectan-
gular angles are found on the steps between two
˚
terraces. The step heights amount to ꢂ3 A which
shows that the oxidation takes place on top of
all terraces in the same way. In addition, on top
of the terraces elongated Al (missing) chains can
develop, starting from the step edges in directions
perpendicular to each other (see arrows). After the
exposure of 1 L O (Fig. 6c) the oxide islands pref-
2
erentially start from the step edges (white arrows).
In addition, two types of isolated islands (Is and
a
Is ) with rectangular shape appear growing paral-
b
lel to the step edges. Isolated islands like marked
as Is appear as trenches on the CoAl surface (such
a
as the oxide located at the steps) and the Is islands
b
show up as protrusions on the same terraces. Thus,
the lateral growth of the oxide is accompanied by a
height growth with the formation of oxygen is-
lands of a distinct height. After the sample expo-
sure to 100 L of O2 (Fig. 6d) the surface is
entirely covered with oxide. The oxide domains
and the chain structure are very well visible over
the whole surface. The nanostructured Al O
2
3
exhibits stripes indicated by arrows that are ori-
ented in the domain directions starting at the step
˚
edges featuring length up to several hundred A.
Fig. 5. (a) STM image of 10 L O
= 2.00 V, I = 0.2 nA). The two domains of the oxide film
are denoted by the white stripes. For each domain a unit cell is
outlined. (b) Top view model of the h-Al surface plane. (c)
The structure model of h-Al grown on CoAl(100). In
addition a monoclinic h-Al unit cell and the close-packed
oxygen fcc lattice is marked. For the sake of clarity, only the
2
on CoAl(100) at 800 K
The discrete distribution of the stripe width (Fig.
˚
(
U
t
t
7) exhibits a pronounced maximum at 68 A and
a secondary maximum around 135 A. These values
˚
2
O
3
are multiples (12 times and 24 times respectively)
˚
2
O
3
of the lattice constant b = 5.64 A of h-Al O . The
2
O
3
2
3
latter observation may result from a periodical
change of the accumulated energy perpendicular
3+
octahedral occupation of Al ions is shown.

146
V. Rose et al. / Surface Science 577 (2005) 139–150
Fig. 6. Set of wide range STM images of CoAl(100) at different stages of oxidation. (a) Clean CoAl(100) surface (U
t
= 1.00 V,
I
t
= 0.2 nA); (b) 0.5 L O (U = 2.04 V, I = 0.2 nA); (c) 1 L O (U = 2.04 V, I = 0.2 nA); (d) 100 L O (U = 0.154 V, I = 0.2 nA).
t
2
t
t
2
t
t
2
t
to the oxide film, caused by the lattice mismatch
between h-Al O and CoAl(100), that prevents
the lateral growth perpendicular to the streaks.
2
3
3.3. Temperature dependence of the oxidation of
CoAl(100)
Now we focus on the temperature dependence
of the growth of alumina on CoAl(100). We de-
scribe both the case of partially oxygen-covered
CoAl(100) after the 1 L O exposure and the case
2
of an oxygen saturated surface (500 L O2).
Fig. 8 shows the AES peak-to-peak ratios
I(OKLL (503 eV))/I(CoLMM (775 eV)) as a function
Fig. 7. The analysis of the stripes width distribution of Fig. 6d
shows two pronounced maxima.

V. Rose et al. / Surface Science 577 (2005) 139–150
147
tion (Fig. 8 top). This indicates that the surface
has lost long-range order and an a-Al O film with
2
3
˚
a thickness of around 7 A is grown [6]. After
annealing at 700 K weak LEED spots at the
(±1,0) and (0,±1) positions can be found. Further
annealing leads to an increase of the intensity of
this spots. After 1000K, again, a (2 · 1) LEED pat-
tern can be obtained. The spot intensity increases
with rising temperature. Finally, after 1650 K the
(
1 · 1) LEED pattern corresponding to the clean
CoAl(100) substrate appears (not shown).
The vibrational properties during the anneal-
ing have been investigated with EELS. Generally,
we found that three regions exist with char-
acteristic temperatures (300–500, 600–800 and
900–1300 K) and special vibrational properties.
Fig. 9 presents typical EEL spectra from each
Fig. 8. Peak-to-peak intensity ratio of OKLL (503 eV)/CoLMM
775 eV) as a function of annealing temperature for 1 and 500 L
of O on CoAl(100). LEED pictures (E = 75 eV) of 500 L
top) and 1 L (bottom) oxygen on CoAl(100) show structural
changes during annealing.
(
2
p
(
range. After the 1 L O exposure at 300 K the
2
EEL spectrum exhibits losses at 370, 620 and
ꢀ
1
15 cm (Fig. 9 left). It has been already pre-
8
sented in Ref. [6] that more than two modes can
of annealing temperature for the 1 L O and 500 L
2
O exposures. After the exposure of 1 L O the
2
2
ratio amounts to 0.3 and remains constant up to
annealing at 1300 K. After annealing at 1400 K
the oxygen is fully desorbed. Although the con-
stant ratio demonstrates that the amount of oxy-
gen is constant LEED investigations reveal
structural changes. After oxidation with 1 L of
O2 at 300 K the (1 · 1) patterns of the clean
CoAl(100) sample (Fig. 8 bottom) are still visible.
While after the annealing at 700 K the LEED pat-
tern do not change, additional weak spots and
weak streaks along the [100] and [010] direction
appear after annealing at 1100 K. The latter
LEED patterns suggest formation of (2 · 1) struc-
ture with two perpendicularly aligned domains.
The peak-to-peak ratio of O and Co is 1.1 after
the 500 L O exposure at room temperature, show-
2
ing that the oxidation is complete [6]. The ratio is
constant up to 1300 K. Due to the oxide decompo-
sition the ratio decreases for annealing tempera-
tures above 1300 K and drops to zero at 1650 K.
It should be mentioned that lower annealing tem-
peratures are sufficient to remove the oxide, as well,
at longer annealing time. For that reason, 1 L O2
on CoAl(100) was already desorbed after anneal-
ing at 1400 K. After oxidation at saturation level,
the LEED screen exhibits only a diffuse illumina-
Fig. 9. EEL spectra of 1 L of O
2 2
(left) and 500 L of O (right)
for different annealing temperatures.

148
V. Rose et al. / Surface Science 577 (2005) 139–150
be expected for small oxygen exposures. A reason-
able explanation is the existence of several differ-
ent sites in which oxygen atoms are chemisorbed
at low coverages. The ratio between the intense
3.4. Band gap of h-Al O /CoAl(100)
2
3
Wide energy range EEL spectra were recorded
to determine the band gap of well ordered alumina
films on CoAl(100). In order to gain intensity low
energy resolution and long integration times were
chosen. Fig. 10 shows spectra that cover a range
of energy losses up to 10 eV. After the oxidation
at 800 K the spectrum (Fig. 10a) exhibits intense
FK modes in the low frequency region that were
already observed in Fig. 3. In addition, a very
broad loss structure around 4.5 eV appears, due
to the excitations of electrons from the valence
band into the conduction band of the oxide. Disre-
garding the matrix element effects, the loss proba-
bility is proportional to the joint density of states.
ꢀ
1
modes at 620 and 815 cm amounts to 1.2. The
ratio moves to lower values with increasing tem-
perature. Annealing at 700 K inverts the ratio of
the intensities. A different spectrum occurs with
ꢀ
1
modes at 400, 645 and 830 cm . This indicates
a transition from a-Al O to a different oxide
2
3
phase. In the thermal dehydration sequence of
boehmite and of trihydroxides a phase transition
to c-Al O occurs at about 700 K in vacuum
2
3
[
29]. The process involves an alteration of the
occupation probability of tetrahedral and octahe-
dral vacancies in the oxygen sub-lattice and may
account for the change in the EEL spectrum. On
the other hand, it is possible that in the tempera-
ture range between 600 and 800 K a-Al O trans-
2
3
forms into disordered clusters of h-Al O . The
2
3
absence of a (2 · 1) structure in LEED can be ex-
plained by both processes mentioned above. After
annealing at 1100 K the EEL spectrum exhibits in-
ꢀ
1
tense FK modes at 420, 625 and 900 cm , charac-
teristic for the temperature range between 800 and
1
LEED, this spectrum can be assigned to well-
400 K. In agreement with the (2 · 1) spots in
ꢀ1
ordered h-Al O . The weak shoulder at 250 cm
2
3
is the Al vibrational mode of the clean surfaces
slightly shifted in energy. Generally, with increas-
ing annealing temperature the intensity of the
phonons in the EEL spectra gradually increase.
Furthermore, the FWHM of the losses shrinks
indicating a gain of ordering as a function of
annealing temperature.
After a 500 L O exposure at room tempera-
2
ture the EEL spectrum shows FK modes at 640
ꢀ
1
and 890 cm (Fig. 9 right). These are the charac-
teristic FK phonons of a-Al O [6]. After anneal-
ing at 700 K an additional loss appears at
2
3
ꢀ1
4
well-ordered oxide, whereupon the formation of
00 cm . The spectrum is characteristic for a
c-Al O is most likely. The EEL spectrum of the
2
3
annealing at 1100 K exhibits modes at 430, 645
ꢀ
1
and 920 cm and corresponds to the spectrum
Fig. 10. Wide energy range EEL spectra of h-Al O on
CoAl(100). The band gap of the film is indicated by the broad
loss structure starting around (a) 4.5 eV after the growth at
2
3
of h-Al O . A forth mode, expected at around
2
ꢀ
3
1
780 cm , is unresolved due to the broadness of
the loss structure.
800 K and (b) 4.3 eV at 1125 K accompanied by an isolated gap
state at 3.4 eV.

V. Rose et al. / Surface Science 577 (2005) 139–150
149
Using a rough approximation with a parabolic
shape of the maximum and minimum of the band
edges the loss probability is proportional to
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4
3