Growth and oxidation of a Ni3Al alloy on Ni(1 0 0)

Article abstract of DOI:10.1016/S0039-6028(03)00516-8

The growth and oxidation of a thin film of Ni₃Al grown on Ni(1 0 0) were studied using Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and high resolution electron energy loss spectroscopy (EELS). At 3 0 0 K, a 12 ? thick layer of aluminium was deposited on a Ni(1 0 0) surface and subsequently annealed to 1150 K resulting in a thin film of Ni₃Al which grows with the (1 0 0) plane parallel to the (1 0 0) surface of the substrate. Oxidation at 300 K of Ni₃Al/Ni(1 0 0) until saturation leads to the growth of an aluminium oxide layer consisting of different alumina phases. By annealing up to 1000 K, a well ordered film of the Al₂O₃ film is formed which exhibits in the EEL spectra Fuchs-Kliewer phonons at 420, 640 and 880 cm^-1. The LEED pattern of the oxide shows a twelvefold ring structure. This LEED pattern is explained by two domains with hexagonal structure which are rotated by 90° with respect to each other. The lattice constant of the hexagonal structure amounts to ～2.87 ?. The EELS data and the LEED pattern suggest that the γ′-Al₂O₃ phase is formed which grows with the (1 1 1) plane parallel to the Ni(1 0 0) surface.

Full text of DOI:10.1016/S0039-6028(03)00516-8

Surface Science 531 (2003) 287–294
www.elsevier.com/locate/susc
Growth and oxidation of a Ni3Al alloy on Ni(1 0 0)
*
A. Wehner , Y. Jeliazova, R. Franchy
Institut f€ur Schichten und Grenzfl€achen (ISG 3) Forschungszentrum J€ulich D-52425, Germany
Received 18 December 2002; accepted for publication 11 March 2003
Abstract
The growth and oxidation of a thin film of Ni3Al grown on Ni(1 0 0) were studied using Auger electron spectroscopy
(AES), low energy electron diffraction (LEED), and high resolution electron energy loss spectroscopy (EELS). At 300
ꢀ
K, a 12 A thick layer of aluminium was deposited on a Ni(1 0 0) surface and subsequently annealed to 1150 K resulting
in a thin film of Ni3Al which grows with the (1 0 0) plane parallel to the (1 0 0) surface of the substrate. Oxidation at 300
K of Ni3Al/Ni(1 0 0) until saturation leads to the growth of an aluminium oxide layer consisting of different alumina
phases. By annealing up to 1000 K, a well ordered film of the Al2O3 film is formed which exhibits in the EEL spectra
Fuchs–Kliewer phonons at 420, 640 and 880 cmꢀ1. The LEED pattern of the oxide shows a twelvefold ring structure.
This LEED pattern is explained by two domains with hexagonal structure which are rotated by 90ꢀ with respect to each
ꢀ
other. The lattice constant of the hexagonal structure amounts to ꢁ2.87 A. The EELS data and the LEED pattern
suggest that the c0-Al2O3 phase is formed which grows with the (1 1 1) plane parallel to the Ni(1 0 0) surface.
ꢁ 2003 Elsevier Science B.V. All rights reserved.
Keywords: Nickel; Aluminum oxide; Alloys; Oxidation; Growth; Low energy electron diffraction (LEED)
1. Introduction
films of Al2O3 are utilized in different fields of
applications as catalysis [2] and microelectronics
and are also used as insulating barrier in magnetic
tunnel junctions [3]. In general, the controlled
epitaxial growth of ultrathin oxide (Al2O3, Ga2O3)
layers on ferromagnetic substrates with a well de-
fined ferromagnet/insulator interface is of crucial
importance for the understanding of the spin-
dependent tunnelling. Therefore, many attempts
are done to find procedures to grow well defined
homogenous oxide layers with a thickness in
Intermetallic alloys have gained much interest
because of their physical and mechanical proper-
ties, like high melting point, low density, and good
corrosion resistance. Some possible applications of
intermetallics, e.g. in areas of energy storage and
furnace hardware, have been reviewed by Stoloff
et al. [1]. Also, thin films of metals and intermet-
allics are intensively studied, because, due to the
low dimensionality, they have some interesting
properties different from those of the bulk. Thin
ꢀ
the range of few tens of A. Recently, tunnel mag-
neto resistance (TMR) amplitudes of more than
40% at room temperature have been achieved [4].
The growth of thin films of aluminium oxides on
metals and metal alloys has recently been reviewed
by Franchy [5]. In general, upon oxidation of the
* Corresponding author. Tel.: +49-2461-614633; fax: +49-
2461-613907.
E-mail address: a.wehner@fz-juelich.de (A. Wehner).
0039-6028/03/$ - see front matter ꢁ 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0039-6028(03)00516-8

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A. Wehner et al. / Surface Science 531 (2003) 287–294
surface of a NiAl compound a thin film of Al2O3 is
grown [6–12], whereas in most studies in UHV
no evidence for the oxidation of Ni was found
[7,12,13]. Well ordered, ultrathin Al2O3 films have
been prepared at 700–1200 K on Ni3Al(1 1 1) [7,
14], Ni3Al(1 0 0) [9,15], NiAl(1 1 1) [10], NiAl(1 1 0)
[12], and NiAl(1 0 0) [11]. The preparation of
Al2O3 films was also studied on pure aluminum
[16], Ru(0 0 0 1) [17] and Mo(1 1 0) [18]. The ad-
vantage of preparing Al2O3 on surfaces of NiAl
alloys is that these substrates have a high melting
temperature which allows the formation of well
ordered (crystalline) ultrathin oxide layers.
Al2O3 exists in different phases, all of which are
based on a close-packed oxygen lattice [19] in
which the Al3þ cations occupy tetrahedral and/
or octahedral sites. Differences in occupation
probability of these sites determine the crystal
structure. For example, the amorphous alumina
can be described as clusters of close-packed oxy-
gen with Al ions sitting only in tetrahedral va-
cancies [20], while c- and c0-Al2O3 have a cubic
spinel structure with Al ions in tetrahedral and
octahedral positions.
By deposition and subsequent annealing at 820
K of one monolayer (ML) Al on a Ni(1 0 0) surface
Lu et al. [21] and OÕConnor et al. [22] have pre-
pared a thin layer of Ni3Al. The thin film of Ni3Al
grows with the (1 0 0) plane parallel to the (1 0 0)
surface of Ni(1 0 0). The surface structure of the
Ni3Al/Ni(1 0 0) was investigated by low energy
electron diffraction (LEED) [21,22] and ion scat-
tering [22]. Deposition of Al at 520 K on Ni(1 0 0)
also leads to the formation of Ni3Al [21].
In this paper we report on the preparation of a
thin film of Al2O3 which was grown by oxidation
of a Ni3Al(1 0 0) thin layer grown on Ni(1 0 0).
Oxidation at 300 K of Ni3Al/Ni(1 0 0) leads to the
formation of an oxide film consisting of a mixture
of different Al2O3 phases (amorphous and unor-
dered). By annealing up to 1200 K a well ordered
thin film of c0-Al2O3 is formed. The paper is or-
ganized as follows: Section 2 presents the experi-
mental conditions and the cleaning procedure of
the sample. Section 3.1 deals with the growth of a
thin Ni3Al layer on Ni(1 0 0). The oxidation of
Ni3Al is described in Section 3.2. Section 3.3 pre-
sents the formation of a well-ordered c0-Al2O3 film
on Ni(1 0 0). In section 4 some conclusions are
summarized.
2. Experimental
The experiments were performed in a two level
ultra-high vacuum (UHV) chamber with a base
pressure of about 4 Â 10ꢀ8 Pa. The upper level is
equipped with a cylindrical mirror analyzer (CMA)
for Auger electron spectroscopy (AES), a three grid
LEED optics, an ion gun, and a quadrupole mass
spectrometer (QMS). Auger spectra were recorded
in the N(E) mode at a primary energy of 4 keV and
the dN(E)/dE spectra are obtained by numerical
differentiation. The LEED images were recorded
with a digital camera (Kodak DC290) at an expo-
sure time of typically 16 s. The lower level of the
UHV chamber contains a computer controlled
high resolution electron energy loss spectrometer
(EELS) which is based on the 127ꢀ cylindrical de-
flector [23]. EEL spectra were taken in specular
direction at room temperature.
The Ni(1 0 0) single crystal has a diameter of 8
mm and a thickness of 2.5 mm. The surface was
polished mechanically, and the crystal orientation
has an accuracy of 0.5ꢀ. In UHV, the cleaning of
Ni(1 0 0) was performed by repeated cycles of
sputtering (Arþ ions, 1 keV/0.8 lA) and annealing
to 1200 K. Carbon was removed by oxidation at
1000 K and subsequent annealing at 1200 K. The
clean Ni(1 0 0) surface exhibits a sharp (1 Â 1)
LEED pattern.
Al was evaporated from a crucible filled with
pestled crystals of NiAl. The calibration of the Al
evaporator was performed with a quartz micro-
balance. In our experiments a deposition rate of
ꢀ
r ¼ 0:48 A/min was used. Oxidation was per-
formed by backfilling the chamber with O126 at
partial pressures varying between 1 Â 10ꢀ5 and
1 Â 10ꢀ4 Pa. Oxygen exposures are expressed in
Langmuirs (1 L ¼ 1.33  10ꢀ4 Pa  1 s). Annealing
of the sample was performed by electron impact,
and the temperature was measured via a W–Re
thermocouple which was mounted on the backside
of the sample.
Thin films of Ni3Al and finally Al2O3 on Ni-
(1 0 0) were prepared in the way indicated in Fig. 1:

A. Wehner et al. / Surface Science 531 (2003) 287–294
289
25
20
15
10
5
100
80
60
40
20
0
(a)
Step 1
Ni (848)
Al (1396)
r = 0.48 Å/min
0
0
300 600 900 1200 1500
Fig. 1. Schematic illustration of the preparation process. Step 1
represents the deposition of Al on Ni(1 0 0), step 2 the annealing
from room temperature to 800 K in steps of 100 K and from
800 to 1150 K in steps of 50 K. During annealing the Ni3Al
alloy is formed. Step 3 is the oxidation at 300 K of the Ni3Al
layer, and step 4 represents the annealing of the oxide from
room temperature to 1200 K in steps of 100 K.
Deposition time[s]
80
60
40
20
0
(b)
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Al/Ni
Ni
ꢀ
• Step 1: 12 A of Al are deposited at room tem-
perature on the Ni(1 0 0) surface.
• Step 2: The system Al/Ni(1 0 0) is annealed to
800 K in steps of 100 K, afterwards to 1150 K
in steps of 50 K. During annealing a Ni3Al layer
is formed. Each step lasted for 2 min.
• Step 3: The system Ni3Al/Ni(1 0 0) is oxidized at
room temperature with oxygen to a total expo-
sure of 2500 L. By oxidation a thin film of
Al2O3 is formed.
• Step 4: The system Al2O3/Ni3Al/Ni(1 0 0) is an-
nealed to 1200 K in steps of 100 K. By anneal-
ing up to 1200 K a well-ordered c0-Al2O3 phase
is formed. Each annealing step lasted for 2 min.
Step 2
400
Al
1000 1200
600
800
T [ K ]
Fig. 2. Part (a) shows the peak-to-peak (p-to-p) intensity in
arbitrary units (a.u.) of the AES transition of Al at 1396 eV (left
axis) and Ni at 848 eV (right axis) as a function of the depo-
sition time. Part (b) shows the p-to-p intensities of the Al
transition at 1396 eV (right axis) and the Ni transition at 848 eV
(right axis) as a function of the annealing temperature. Also
shown is the p-to-p ratio Al1396/Ni848 (left axis).
ꢀ
mean free path of about 24 A [24] in aluminium.
3. Results and discussion
Thus, at this thickness the intensity of the Auger
transition should show a saturation level. In our
ꢀ
case only 12 A were deposited and consequently
3.1. Formation of Ni3Al
no saturation is reached as can be seen in Fig. 2a.
After a deposition time of 1500 s and a deposition
ꢀ
rate of 0.48 A/min the thickness of the Al layer
Fig. 2a shows the p-to-p (peak-to-peak) ampli-
tude of the KLL AES transition of aluminium at
1396 eV as a function of the deposition time. The
exact shape of this curve depends of course on the
growth mode of aluminium on Ni(1 0 0). However,
independent of the growth mode, a saturation of
the Al signal is reached, when the thickness of the
layer exceeds the mean free path of the electrons at
the energy of the respective AES transition. Elec-
trons with an energy of 1396 eV (Al, KLL) have a
ꢀ
amounts to 12 A.
Fig. 2b shows the p-to-p intensities of the Ni
LMM transition at 848 eV, the Al KLL transition
at 1396 eV and the ratio Al/Ni as a function of the
annealing temperature. Up to 700 K, the p-to-p
intensity of the Ni transition does not change sig-
nificantly, while above 700 K the signal increases
continuously. The increase of the Al intensity and

290
A. Wehner et al. / Surface Science 531 (2003) 287–294
ꢀ
sample after annealing to 1150 K of a 12 A thick
the ratio Al/Ni in the region from 300 K to ꢁ600
K might be due to structural changes and ordering
of the Al layer. For T P 700 K, the ratio Al/Ni
decreases due to the increase of the Ni signal. This
suggests that the interdiffusion (alloying) of Ni and
Al atoms sets in. After annealing to 1150 K the
ratio of Al/Ni corresponds to the ratio of Al/Ni in
bulk Ni3Al. After annealing to 1200 K (not shown)
the intensity of the Al transition is close to zero. At
this stage the aluminium is most likely completely
diffused into the bulk.
Al film on Ni(1 0 0). After annealing, the surface
exhibits a c(2 Â 2) structure with respect to the
Ni(1 0 0) surface. The unit cell is quadratic with
ꢀ
a lattice constant of (3.52 Æ 0.1) A. This unit cell
corresponds to that which is expected for the
Ni3Al(1 0 0) surface. This strongly suggests, that
on Ni(1 0 0) a surface alloy of Ni3Al is formed. The
LEED pattern is in agreement with that found by
Lu et al. [21] and OÕConnor et al. [22] who have
grown a layer of Ni3Al on Ni(1 0 0) by depositing
approximately 1 ML of Al onto the Ni(1 0 0) sur-
face and annealing at 820 K for 5 min. Although in
our case higher annealing temperatures were nec-
essary to obtain an ordered LEED pattern, the
LEED pattern and the AES data are clear evi-
dences for the existence of Ni3Al.
Fig. 3 shows: (a) a LEED pattern of the clean
Ni(1 0 0) surface and (b) a LEED image of the
From our experiments we cannot determine the
thickness of the Ni3Al layer strictly. However, we
can give an upper limit. The upper limit results
ꢀ
from the fact that if the a 12 A thick Al layer is
completely used to form Ni3Al, then the total
number of layers can be calculated from the lattice
constant of Ni3Al and the thickness amounts to
ꢀ
about 35 A.
3.2. Oxidation of Ni3Al(1 0 0)/Ni(1 0 0)
In order to grow a thin aluminum oxide the
surface of Ni3Al(1 0 0)/Ni(1 0 0) was oxidized at
room temperature. Fig. 4 shows the AES p-to-p
ratios O/Ni
and O/Al
as a function of
ð848 eVÞ
ð1396 eVÞ
oxygen exposure (Fig. 1, step 3). The O/Al and O/
Ni ratios increase strongly up to 200 L and above
500 L both of the ratios clearly indicate that a
saturation is reached. The saturation value (ꢁ6.5)
of the O/Al ratio is approximately the same as for
bulk Al2O3 [25]. We now focus on the AES spectra
in Fig. 5, which trace the emergence of Al and Al3þ
transition in the energy region between 10 and 120
eV. Spectrum a shows the spectrum of the clean
Ni(1 0 0) surface and accordingly two characteris-
tic transitions at 61 and 102 eV occur. The situa-
ꢀ
tion after the deposition of a 12 A thick Al layer is
shown in spectrum b. The Ni-transitions at 61 eV
and at 102 eV are disappeared and a transition at
68 eV is emerged, which corresponds to the LMM
transition of Al. The disappearing of the Ni tran-
Fig. 3. (a) LEED pattern of the clean Ni(1 0 0) surface. (b)
LEED image of the Ni3Al layer. The primary energy of the
electrons was 122 eV for both of the images.

A. Wehner et al. / Surface Science 531 (2003) 287–294
291
MNN) and 68 eV (Al, LMM) are clearly resolved
(see spectrum c). These facts indicate the reap-
pearance of Ni in the first few layers. After oxi-
dation of the sample to saturation at 300 K only
the transitions at 51, 43 and 35 eV occur (spectrum
d), which are typical for the formation of Al3þ [25].
Summarized this is strong evidence for the oxida-
tion of Al, whereas Ni atoms seem to be not in-
volved. In UPS investigations of NiAl [12], Ni3Al
[7] and FeAl [26] it was found that only the Al
atoms are oxidized. In our investigation AES is
not sensitive enough to rule out completely the
Ni-O interactions. Maybe the Ni MNN transition
at 61 eV overlaps with the strong signal of the Al3þ
transition at 51 eV, which is observed by the very
broad line shape at around 51 eV. The oxidation
process can be described as follows: adsorption of
oxygen induces the segregation of Al atoms to the
surface where by reaction with the adsorbed oxy-
gen an aluminium oxide film is formed. This pro-
cess is the so called ‘‘preferential segregation
oxidation’’ of intermetallic alloys which have Al
(or Ga) as a component [5]. Generally, the oxida-
tion of Al is thermodynamically favored over the
oxidation of Ni, because the heat of formation of
Al2O3 is much larger (DHf ¼ ꢀ1676 kJ/mol) than
that of NiO (DHf ¼ ꢀ240 kJ/mol) [27]. Finally, we
give a rough estimate of the thickness of the oxide
layer. Assuming that a pure Al2O3 layer is grown
that contains no Ni, the thickness can be deter-
mined by the exponential decrease of the intensity
of the Ni AES transition at 848 eV. The inelastic
mean free path (IMFP) of electrons with energy
Fig. 4. Peak-to-peak ratios of the AES signal of O/Ni and O/Al
during oxidation at 300 K (step 3). Both of the ratios indicate a
saturation and the value of the O/Al ratio corresponds to that
of bulk Al2O3 [25].
ꢀ
848 eV in Al2O3 was calculated to be 18 A [28].
The thickness of the Al2O3 layer amounts to
Fig. 5. AES spectra in the energy region between 10 and 120
eV. (a) clean Ni(1 0 0) surface, (b) after deposition of Al (step 1),
(c) after annealing to 1200 K (step 2), (d) after oxidation of
Ni3Al at 300 K (step 3), (e) after annealing of the oxide layer at
1200 K (step 4).
ꢀ
d ¼ ð11 Æ 2Þ A. It was found that the thickness of
an Al layer is increased by oxidation. For example,
Chen et al. claimed that the thickness of Al-oxide
is nearly double of the Al film [29]. By assuming
that in our case a similar increase takes place, a
ꢀ
sitions is expected, because the mean free path of
pure Al film of ꢁ5.5 A would be needed to grow a
ꢀ
electrons in this energy region is only few A, which
is much smaller than the Al layer thickness of 12
ꢀ
A. Maybe some amount of Ni is also evaporated
from the pestled NiAl crystals, but the total
amount is below the detection limit of AES. After
annealing the sample to 1150 K, the transition at
102 eV reemerges and two transitions at 61 eV (Ni,
ꢀ
11 A thick layer of Al2O3. This is about one half of
the initially deposited amount of Al. However, we
have to mention that this consideration is valid
only for a pure Al layer and not for the oxidation
of an intermetallic alloy like Ni3Al. In principle, it
should be possible to trace the oxidation of Al by
the KLL AES-transition of metallic Al (1396 eV)

292
A. Wehner et al. / Surface Science 531 (2003) 287–294
and Al3þ (1378 eV) [25]. However, during oxida-
tion only a broadening of the transition is ob-
served, which indicates a coexistence of Al0 and
Al3þ. Since in spectrum d of Fig. 5 there is no hint
of Al0 it appears likely, that Al0 atoms are not
grown Al2O3 film does not only consist of an
amorphous phase, but rather of a mixture of dif-
ferent Al2O3 phases. This assumption is supported
by the LEED pattern, which is diffuse due to the
lack of a long-range order. Between room tem-
perature and 700 K the EEL spectra do not change
significantly. Upon annealing above 800 K, the
intensity of the losses (the magnification factor of
the losses decreases from 50 at 300 K to 10 after
annealing at 1200 K) and of the elastic beam in-
crease, indicating that an ordering of the oxide
layer takes place. The spectra after annealing to
1000 and 1200 K are shown in Fig. 6 and exhibit
losses at 420, 640 and 880 cmꢀ1. EEL spectra of
thin well-ordered alumina films (c-, c0- and h-
Al2O3) on metal and metal alloy surfaces show
commonly three (sometimes four) distinct phonon
features in the frequency regions of 380–430, 620–
660, and 850–990 cmꢀ1 with slightly different rel-
ative loss intensities between the modes (Ref. [5]
and the references therein). Therefore, we conclude
that in the case of Al2O3 on Ni(1 0 0) the Al oxide
belongs to an ordered Al2O3 phase. The increase
of ordering is in accordance with the emergence of
Bragg reflections in the LEED images at 900 K.
1200 K seems to be the temperature where the
oxide got the best ordering. However, we have to
mention that the unoxidized Ni3Al layer on
Ni(1 0 0) is destroyed at around 1200 K and may
be this is at least in part also the case after an-
nealing the oxide covered Ni3Al layer. Unfortu-
nately, no evidences on the hidden interfaces
Al2O3/Ni3Al and Ni3Al/Ni(1 0 0) are available in
our experiment.
ꢀ
within the topmost 5 A of the film. Whether
the unoxidized Al is in the film or in an interface
layer between the Ni(1 0 0) substrate and the Al2O3
film cannot be determined.
3.3. Annealing of the oxide
The vibrational properties of the Al2O3 film on
Ni3Al/Ni(1 0 0) were investigated by EELS. Fig. 6
shows EEL spectra of the sample during annealing
of the oxide. The spectrum at the bottom was re-
corded after oxidation at 300 K (saturation) and
exhibits three broad losses at about 420, 620, and
860 cmꢀ1. After annealing to 1200 K, the losses at
620 and 860 cmꢀ1 are shifted to 640 and 880 cmꢀ1
.
For an amorphous Al2O3 (a-Al2O3 ) only two IR
active modes are expected [5], because the Al3þ
ions occupy exclusively tetrahedral sites [20,30].
The fact that already at room temperature three
losses are observed indicates, that even at room
temperature both tetrahedral and octahedral sites
of the O2ꢀ lattice are occupied by Al3þ ions (see
e.g. [5] and references therein). Thus, at 300 K the
860
880
Spectrum e in Fig. 5 shows an AES spectrum of
the oxide layer in the energy range between 10 and
120 eV after annealing to 1200 K. This spectrum is
similar spectrum d, which suggests that during
annealing no change of the ionicity of the Al de-
velops.
620
640
420
1200K
*10
Fig. 7(a) shows a LEED pattern of the sample
taken after annealing the oxide to 1200 K and (b)
shows a schematic representation of the pattern. A
twelvefold ring structure is observed which results
from two domains with hexagonal structure which
are rotated with an angle of n ꢂ 30ꢀ (n odd) with
respect to each other. Because of the square lattice
(C4 symmetry) of the substrate it appears reason-
1000K
300 K
*15
*5
-200
0
200
400
600
800 1000 1200
-1
Energy loss [cm
]
Fig. 6. EEL spectra taken during annealing of the oxide layer.
After annealing to 1200 K, the frequency of the losses originally
at 620 and 860 are shifted to 640 and 880 cmꢀ1, respectively.

A. Wehner et al. / Surface Science 531 (2003) 287–294
293
the (1 0 0) plane of the Ni substrate. The lattice
constant of the hexagonal unit cell was determined
ꢀ
to be ahex ¼ ð2:87 Æ 0:2Þ A. This agrees with the
distance between two adjacent O2ꢀ ions in the
(1 1 1) plane of a number of cubic phases of Al2O3
[32]. From the agreement of the EELS [9] and
LEED [7,9] data obtained for the oxidation of
Ni3Al single crystal surfaces with those of this
study, the growth of the c0-Al2O3 phase is con-
cluded, which grows with the (1 1 1) surface par-
allel to the Ni(1 0 0) surface. The thickness of the
Al2O3 film after annealing to 1200 K amounts to
ꢀ
(8 Æ 2) A.
Fig. 8 shows the ratio of the peak to peak in-
tensities of O/Al as a function of the annealing
temperature of the oxide. Since the value of this
ratio does not change significantly it appears rea-
sonable to assume that the stoichiometry does not
change during annealing. From this ratio together
with the relative Auger sensitivities the ratio be-
tween the aluminium and oxygen ions in the Al2O3
layer can be estimated. The ratio is determined to
be 6.2, by using the relative sensitivity of 0.4 for
the oxygen transition and 0.09 for the aluminium
transition [33]. Therefore the ratio between the
amount of O and Al ions in the oxide layer is ꢁ1.4
which is close to the value of 1.5 of Al2O3. The
discrepancy can be assigned to the experimental
inaccuracy.
Fig. 7. (a) LEED image of the oxide after annealing at 1200 K.
The primary energy was 122 eV. (b) schematic representation:
substrate spots are indicated by dots, spots stemming from the
oxide are indicated by asterisks.
able to assume an angle of rotation of 90ꢀ. F rom
the half width of the spots the diameter of the
ꢀ
domains is estimated to be about 30 A. In order to
determine the structure of the Al2O3 film we have
to consider again the EELS data. The three losses
in the EEL spectra clearly rule out the existence of
a-Al2O3 [31], because in a-Al2O3 the Al3þ ions
occupy only octahedral sites which induce only
two IR active modes and not three. Therefore the
observed hexagonal structure has to be explained
by a (1 1 1) plane of a cubic Al2O3 phase which
grows with the (1 1 1) plane of the oxide parallel to
Fig. 8. The peak-to-peak ratio of O/Al during annealing of the
oxide. The horizontal line is a guide to the eye.

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A. Wehner et al. / Surface Science 531 (2003) 287–294
[9] V. Podgursky, I. Costina, R. Franchy, Appl. Surf. Sci. 206
(2003) 29.
4. Conclusion
[10] R. Franchy, J. Masuch, P. Gassmann, Appl. Surf. Sci. 93
(1996) 317.
In conclusion, upon deposition and annealing of
Al on a Ni(1 0 0) surface, a thin layer of Ni3Al is
formed. Exposing the surface of this alloy to oxy-
gen and subsequent annealing, leads to the for-
mation of Al2O3. At 300 K the thickness of the
[11] P. Gassmann, R. Franchy, H. Ibach, Surf. Sci. 319 (1994)
95–109.
[12] M. Jaeger, K. Kuhlenbeck, H.J. Freund, M. Wuttig, W.
Hoffmann, R. Franchy, H. Ibach, Surf. Sci. 259 (1991) 235.
[13] A. Venezia, C. Loxton, Surf. Sci. 194 (1988) 136.
[14] C. Becker, J. Kandler, H. Raaf, R. Linke, T. Pelster, M.
Draeger, M. Tanemura, K. Wandelt, J. Vac. Sci. Technol.
A 16 (1998) 1000.
ꢀ
Al2O3 layer is approximately (11 Æ 2) A, which
ꢀ
changes to (8 Æ 2) A after annealing to1200 K.
After annealing up to 1200 K, the aluminium
oxide shows a sharp LEED image with a twelve-
fold ring structure, indicating long range order and
a hexagonal structure of two domains which are
rotated by 90ꢀ with respect to each other. The
oxide grows with the (1 1 1) plane of the c0-Al2O3
lattice parallel to the (1 0 0) plane of the Ni sub-
strate. The lattice constant of the hexagonal unit
[15] I. Costina, PhD-Thesis. Duesseldorf: Heinrich-Heine-Uni-
versitaet, 2002.
[16] J.L. Erskine, R.L. Strong, Phys. Rev. B 25 (1982) 5547.
[17] M.B. Lee, J.H. Lee, B.G. Frederick, N.V. Richardson,
Surf. Sci. 448 (2000) L207.
[18] P.J. Chen, M.L. Colaianni, J.T. Yates Jr., Phys. Rev. B 41
(1990) 8025.
[19] H. Jagodzinski, Z. Krist, 109 (1957) 388.
ꢁ ꢁ
[20] R. Manaila, A. Devenyi, E. Candet, Thin Solid Films 122
ꢀ
cell was determined to be ꢁ2.87 A.
(1984) 131.
[21] S.H. Lu, D. Tian, Z.Q. Wang, Y.S. Li, F. Jona, Solid State
Comm. 67 (1988) 325.
Acknowledgements
[22] D.J. OÕConnor, M. Draeger, A.M. Molenbroek, Y.G.
Shen, Surf. Sci. 357–358 (1996) 202.
This work was supported by the HGFproject
‘‘Magnetoelectronics’’ and represents a part of the
PhD work of A.W. at the Heinrich Heine Uni-
[23] H. Ibach, Electron Energy Loss Spectroscopy––The Tech-
nology of High Performance, Springer, Berlin, 1991.
[24] S. Tanuma, C.J. Powell, D.R. Penn, Surf. Interface Anal.
17 (1991) 911.
€
€
versitat Dusseldorf, Germany.
[25] L.E. Davis, N.C. MacDonald, P.W. Palmberg, G.E. Riach,
R.E. Weber, Handbook of Auger Electron Spectroscopy,
Physical Electronics Industries, Eden Prairie, 1976.
[26] H. Graupner, L. Hammer, K. Heinz, D.M. Zehner, Surf.
Sci. 380 (1997) 335.
References
[27] D.R. Lide, Handbook of Chemistry and Physics, CRC
Press, 1994.
[1] N.S. Stoloff, C.T. Liu, S.C. Deevi, Intermetallics 8 (2000)
1313.
[28] S. Tanuma, C.J. Powell, D.R. Penn, Surf. Interface Anal.
17 (1991) 927.
[2] H.J. Freund, E. Umbach, Adsorption on ordered Surfaces
of Ionic Solids and Thin Films, Springer, Heidelberg,
1993.
[29] E.Y. Chen, R. Whig, J.M. Slaughter, D. Cronk, J. Goggin,
G. Steiner, S. Tehrani, J. Appl. Phys. 87 (2000) 6061.
[30] G. Gutierrez, B. Johansson, Phys. Rev. B 65 (2002) 104202.
[31] M. Liehr, P.A. Thiry, J.J. Pireaux, R. Caudano, J. Vac. Sci.
Technol. A 2 (1984) 1079.
[3] J.S. Moodera, L.R. Kinder, J. Appl. Phys. 79 (1996) 4724.
[4] S.S. Parkin et al., JAP 85 (1999) 5828.
[5] R. Franchy, Surf. Sci. Rep. 38 (2000) 195.
[6] Y.G. Shen, D.J. OÕConnor, R.J. MacDonald, Surf. Inter-
face Anal. 17 (1991) 903.
[32] H.D. Megaw, Crystal Structures: A Working Approach,
Sanders, Philadelphia, 1973.
[7] U. Bardi, A. Atrei, G. Rovida, Surf. Sci. 268 (1992) 87.
[8] G.F. Cotterill, H. Niehus, D.J. OÕConnor, Surf. Rev. Lett.
3 (1996) 1355.
€
[33] G. Ertl, J. Kuppers, Low Energy Electrons and Surface
Chemistry, 2nd ed, Weinheim, New York, 1985.