ISSN 10637834, Physics of the Solid State, 2010, Vol. 52, No. 12, pp. 2589–2595. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © S.E. Kulkova, S.V. Eremeev, S. Hocker, S. Schmauder, 2010, published in Fizika Tverdogo Tela, 2010, Vol. 52, No. 12, pp. 2421–2427.
LOWDIMENSIONAL SYSTEMS
AND SURFACE PHYSICS
Electronic Structure and Adhesion
on Metal–AluminumOxide Interfaces
S. E. Kulkovaa, b, *, S. V. Eremeeva, b, S. Hockerc, and S. Schmauderc
a
Institute of Strength Physics and Materials Science, Siberian Branch, Russian Academy of Sciences,
Akademicheskii pr. 2/4, Tomsk, 634055 Russia
b Tomsk State University, pr. Lenina 36, Tomsk, 634050 Russia
* email: kulkova@ispms.tsc.ru
c
Institute of Materials Testing, Materials Science and Strength of Materials, University of Stuttgart,
Pfaffenwaldring 32, Stuttgart, 70569 Germany
Received January 13, 2010; in final form, April 23, 2010
Abstract—This paper reports on the results of the systematic analysis of the atomic and electronic structure
of the Me/αAl2O3(0001) interfaces for two series of isoelectronic metals (Me = Cu, Ag, Au and Ni, Pd, Pt),
depending on the termination of the oxide substrate and the configuration of oxide films. The calculations
have been performed by the pseudopotential method in the planewave basis set. The adhesion energy of
metal films has been calculated depending on the cleavage plane. It has been shown that the adhesion energy
is maximum at the oxygen interface, which is caused by the ion component in chemical bonding at this inter
face. The aluminum and aluminumenriched interfaces are characterized by the metallic type of bonding.
The local densities of states and the charge distribution near the interface have been analyzed. It has been
demonstrated that oxygen vacancies at the interface substantially weaken the adhesion due to the partial
breaking of Me–O bonds.
DOI: 10.1134/S1063783410120243
1. INTRODUCTION
The structural and physicochemical properties of
metal oxides remain the subject of experimental and
theoretical studies due to their important technologi
cal applications in microelectronics, laser optics,
chemistry, highpressure physics, medicine, etc. [1].
Since the phenomena occurring at metal–oxide inter
faces are important for technical applications and pro
duction of materials, they have been intensively stud
ied in recent years in terms of the density functional
theory [2–13]. The majority of the works have been
devoted to studying the electronic structure of the
(0001) surface of aluminum oxide with the corundum
structure (αAl2O3), i.e., the material with a high bulk
modulus (320 GPa), and the Nb(111)/αAl2O3(0001)
interface with a high adhesion energy of ~9.8–10.6
J/m2 [2, 3, 5, 6, 13]. Despite the increasing number of
publications in the last decade, the mechanisms of
chemical bonding at metal–oxide interfaces remain
unclear. The available results of the study of the adhe
sion at the interfaces differ both in the values of ener
gies and in the conclusions regarding the stability of
different atomic configurations of films at the inter
faces and even in mechanisms of chemical bonding.
For example, the difference in the calculated energies
of adhesion of the copper film from the oxygen termi
nation of the surface is ~1.4 J/m2 [8, 12] and the values
on the aluminum termination of the surface [4, 8] sub
stantially differ from experimental data. Nearly equal
values (0.672 and 0.679 J/m2 [7]) were obtained for sil
ver at the interface with aluminum and aluminum
enriched terminations, whereas the difference of ~1.5
J/m2 was obtained in [11]. It should be emphasized
that the existing theoretical works give different values
of the relaxation of interface layers, splitting of the first
metal layer, and other structural and electronic char
acteristics. Since the mechanism of bonding of metal
adatoms and films with an oxide substrate is essentially
affected by the electronic structure of metals, system
atic and comparative theoretical studies of the atomic
and electronic structure of the interface between the d
metals and oxides in the framework of a unified model
of interface are needed.
The aim of the present work is to study the atomic
and electronic structure of the Me(111)/α
Al2O3(0001) interface with two series of isoelectronic
metals: Ni, Pd, and Pt (the first series) and Cu, Ag,
and Au (the second series), including the case with
interface defects, and to determine the electronic and
structural factors responsible for the metal–oxide
bonding.
2. CALCULATION TECHNIQUE
The atomic and electronic structures of three pos
sible terminations of an aluminum oxide surface with
2589
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KULKOVA et al.
(a)
(b)
(c)
Ag2
Ag1
Al0
Al1
O2
Al3
Al4
O5
Al6
Fig. 1. Atomic structure of the Me(111)/αAl2O3(0001) interface: (a) Me in the Altop configuration on the oxide surface termi
nated with Al (Me/(Al2O3)Al), (b) Me in the Altop configuration on the oxygen termination of the surface (Me/(Al2O3)O), and
(c) Me in the hollow Hposition on the surface terminated with a double aluminum layer (Me/(Al2O3)Al2).
the corundum structure αAl2O3(0001) and of
Me(111)/Al2O3(0001) interfaces were studied in the
framework of the pseudopotential approach imple
mented in the VASP computer code [14–17] with the
generalized gradient approximation [18] for the
exchange–correlation functional. The interface was
simulated by multilayer repeated oxidecontaining
films and two metal films on both sides of the oxide.
The metal films were separated by a vacuum gap of
~8 Å in order to exclude the interaction between metal
atoms on two surfaces. In contrast to the model used in
[13], where the metal layers fill the entire space of the
cell between oxide surfaces and which encounters dif
ficulties with correct preliminary estimation of the
interface volume, our approach is devoid of these
drawbacks. The calculation cell for describing alumi
num oxide with aluminum termination of the surface
contained six layers of oxygen atoms (three atoms in a
layer) and 12 aluminum atom layers (one atom in a
layer). For a surface terminated with a double alumi
num layer or oxygen, the number of aluminum layers
increased or decreased by two layers, respectively. The
thickness of metal films was limited by four atomic lay
ers on both sides of the oxide film. The structural
parameters of αAl2O3 are a = 4.763 Å and c =
13.003 Å. In consideration of metal films on an oxide
substrate, we took into account their tetragonal distor
tion. The parameters of metal lattices were expanded
in the case of Cu (6.34%) and Ni (9.48%) and com
pressed for Ag (–6.70%) and Au (–7.00%). For Pd
(0.05%) and Pt (0.91%), we have a good agreement
between the parameters of metal and oxide. For each
termination of oxide surface, we considered three
configurations of metal film: over aluminum (Altop)
or oxygen (Otop) atoms and the hollow (H) position.
The interface structure was optimized using a 4 × 4 × 1
grid of kvectors until the minimum forces on atoms,
not exceeding 0.01 eV/Å, were attained. The atomic
structure of several possible configurations of metal
films at the Me(111)/Al2O3 interface is presented in
Fig. 1.
The adhesion energy (or the ideal work of separa
tion) was calculated from the formula
W sep = ( E 1 – E 2 – E 12 )/2A,
(1)
where E12 is the total energy of a supercell containing
oxide and metal films, E1 and E2 are the total energies
of the same cell containing either oxide or metal films,
A is the interface area, and the divider 2 takes into
account the presence of two equivalent interfaces in
the supercell. In the modeling of oxygen vacancies at
the interface, their concentration is one vacancy per
considered interface area.
3. RESULTS AND DISCUSSION
First of all, we determined the most stable configu
rations of Me(111) metal films at different termina
tions of the oxide surface. As is evident from Table 1,
metals Ni, Pd, Pt, and Cu at the aluminum interface
prefer the Otop configuration over oxygen atoms,
which agrees with the results of [10] for Ni and Cu.
The energy gain in this configuration in the case of
copper as compared to the Altop configuration is
0.13 eV/atom (the value is given per surface atom),
whereas a greater difference (0.45 eV/atom) was found
in the case of nickel film. In [7], the Hconfiguration
for Ag(111) film was found to be more stable. How
ever, our calculations show that both for Ag film and
Au, the Altop configuration is more stable among the
three considered configurations. This conclusion is in
agreement with the results of [11]. It should be noted
that Pt in the monolayer cover also prefers the Altop
configuration. In this case, our results for a monolayer
of Pt and Ag agree with the data of [19]. A similar
result was obtained for an Ag monolayer in [9], where
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ELECTRONIC STRUCTURE
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Table 1. Stable configurations of the metal films at the Me(111)/Al2O3(0001) interfaces
Me(111)/Al2O3(0001)
Altermination
Ni/Al2O3
Otop
Otop [10]
H
Altop
Pd/Al2O3
Otop
H
Otop
Pt/Al2O3
Otop
H
Otop
Cu/Al2O3
Otop
Otop [10]
H
Altop
H [10]
Ag/Al2O3
Altop
H [7]
Altop [11]
H
H [11]
Altop
Altop [11]
Au/Al2O3
Altop
Altop [11]
H
H [11]
Altop
Altop [11]
the Hartree–Fock method with electron correlation
corrections was used.
All the studied metals at the aluminumenriched
interface (the oxide surface is terminated with a dou
ble layer of Al–Al2termination) prefer the hollow H
configuration, which agrees with the results for silver
and gold films [11]. However, the difference between
the energies of this and Otop configurations is small
and comprises 0.02 and 0.05 eV for silver and copper
films, respectively.
All metals isoelectronic with Cu prefer the Altop
configuration in the case of an interface terminated
with oxygen, whereas, among metals of the first series,
this configuration is most stable only for Ni(111). For
palladium and platinum, which are isoelectronic with
nickel, the Otop configuration is preferable. It should
be noted that the more stable configuration found in
[10] for Cu(Al2O3)O was the Hconfiguration; more
over, the authors emphasized the insignificant energy
difference between the H and Otop configurations.
In our calculation, the difference between the Otop
and Hconfigurations is substantial and comprises
~0.20 eV/atom. At the same time, the difference
between the H and Otop configurations for a mono
layer copper cover is insignificant, indeed
(~0.04 eV/atom). As is evident from Fig. 1, in the case
of the Altop configuration of films on aluminum or
oxygen terminations on relaxation of interfaces, the
first metal layer is split. In the case of an aluminum
interface, this splitting for silver and gold is 0.88 and
0.66 Å, respectively. As a rule, the second metal layer
from the interface is practically plane. As a whole,
metal atoms on the substrate with aluminum termina
tion tend to preserve the same interatomic distances as
in bulk metals. It is worth to note a substantial increase
in the distance between Al and O atoms, which, in the
case of a pure surface, lie practically in one layer. At
the oxygen termination of the interface, the first metal
layer for the Altop configuration is also split, because
PHYSICS OF THE SOLID STATE
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Al2termination
Otermination
one of the atoms of the film also tends to occupy the
position of the next aluminum atom. The calculated
splitting of 0.46 Å for Ni is comparable with the corre
sponding value of 0.57 Å for Cu but is substantially less
than the values obtained for Ag and Au (0.82 and
1.06 Å, respectively). Our calculation demonstrates
the presence of a plane first metal layer at the alumi
numenriched interface.
In order to analyze the interaction mechanisms at
different interfaces, we will consider the difference
charge density distribution Δρ(r) = ρ ox ( r ) + ρ Me ( r ) –
ρ Me/ox ( r ) at interfaces in the case of copper (Fig. 2).
For example, in the case of Cu/(Al2O3)Al, in addition
to the metal type of bonding between aluminum and
copper, a weak ion bonding of copper atoms with oxy
gen of the second layer from the interface is observed.
The stability of the Otop configuration can be caused
by increasing the ion component. The distance
(2.32 Å) between the metal and aluminum layers,
which is greater than the interatomic distances in
oxide (0.42 Å), correlates with the Cu–Al bond, which
is weaker than the Al–O bond in the oxide. The
increase in the distance between Ag(Au) in the split
layer and aluminum leads to weakening of the ion
component. The variation in the local density of states
of nonequivalent metal atoms in the interface layer
leads to their polarization, which, along with hybrid
ization of the s and d orbitals of Ag(Au) with the s and
p orbitals of Al, can provide the preference of the Al
top configuration in the given case. In the case of a
monolayer cover, the interaction of the metal layer
with the oxide substrate has a more pronounced ion
character [13] than in the case of thick films, in which
the metal bonding between atoms in films is strength
ened, which, in its turn, weakens the bonding of met
als with oxygen of the second layer from the interface.
As a whole, we can distinguish the following types of
bonding at the aluminum interface: the metal Me–Al
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2592
KULKOVA et al.
(a)
(b)
(c)
Fig. 2. Charge density difference Δρ(r) = ρ ox ( r ) + ρ Me ( r ) – ρ Me/ox ( r ) on three terminations of the Cu(111)/αAl2O3(0001)
interface in the plane perpendicular to the interface and passing through the interface atoms: (a) the Altop configuration of the
film at the Cu/(Al2O3)Al interface, (b) the Hconfiguration at the aluminumenriched Cu/(Al2O3)Al2 interface, and (c) the
Altop configuration at the oxygen Cu/(Al2O3)O interface.
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2010
ELECTRONIC STRUCTURE
(a)
4
Pd
Pdbulk
N(E), electrons/eV
2
0
~
~
(b)
EF
(c)
Pd1
EF
0
Al1 in Al2O3
~
~
0~
~
O2 in Al2O3
1
0
Al0 in Al2O3
Al1 in Al2O3
–8
E, eV
0
0
Pd1 EF
~
~
O2 in Al2O3
1
0~
~
O2 in Al2O3
1
–16
4
2
1
~
0
4
2
1
0~
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Al3 in Al2O3
1
–16
–8
E, eV
0
0
–16
–8
E, eV
0
Fig. 3. Local densities of electron states of the interface atoms at the Pd (111)/αAl2O3(0001) interface: (a) the Otop configura
tion at the Pd/(Al2O3)Al interface, (b) the Hconfiguration at the aluminumenriched Pd/(Al2O3)Al2 interface, and (c) Otop
configuration at the oxygen Pd/(Al2O3)O interface. The numeration of the interface atoms is given in accordance with Fig. 1.
bond, polarization of atoms in films, and the weak
Me–O bond. For an aluminumenriched interface,
the metal bonding is strengthened, which is clearly
seen in Fig. 2b. Finally, on the oxygen termination of
the interface, the ion type of bonding caused by the
charge transfer from the metal to the substrate prevails
(Fig. 2c).
The change in the type of bonding depending on
the atomic composition of the interface can be illus
trated by the curves of local densities of electron states
of interface atoms. For example, Fig. 3 presents the
calculated local densities of electron states for a
Pd/Al2O3(0001) interface. It is readily seen in Fig. 3a
that there are insignificant changes in the local densi
ties of electron states of palladium atoms near the
interface as compared to those in depth. In this case, a
relatively weak overlap of the states of Pd and Al is
observed. In Ag and Au films, where the Altop con
figuration is realized, the aluminum interface exhibits
a greater variation in the local densities of electron
states and a shift in the center of gravity of the valence
band of the metal atom nearest to the interface toward
the Fermi level. It is readily seen in Fig. 3b that the
metal bonding is strengthened for the aluminum
enriched interface, owing to a greater overlap of states
of Me and the interface Al layer. In this case, the oxy
gen band shifts from the Fermi level, which reduces
the overlap of Me–O orbitals. For the oxygen interface
(Fig. 3c), we observe a significant shift of local densi
ties of electron states of oxygen atoms to the Fermi
level and strong hybridization of oxygen orbitals and
palladium atoms. Such trends were obtained for all
metals that we considered. Moreover, the local densi
ties of electron states of metals of the second series are
situated closer to the Fermi level than the metals of the
PHYSICS OF THE SOLID STATE
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No. 12
first series, which causes their higher chemical activity
at interfaces.
The energies of separation of metal films from
oxide substrates are presented in Table 2. As expected
Table 2. Adhesion energies of the films (J/m2) at the stable
Me/Al2O3(0001) interfaces
Interface
Altermination
Al2termi
nation
Ni/Al2O3
Otermi
nation
1.24
3.91
1.30 [10]
3.78 [10]
Experiment 1.11 [20], 1.17 [21]
5.82
6.84 [10]
Pd/Al2O3
0.90
1.05 [22]
4.66
4.80
Pt/Al2O3
0.74
0.57 [19]
4.84
5.23
Cu/Al2O3
0.47
2.50
0.58 [10]
2.66 [10]
0.9 [4]
1.02 [8]
Experiment 0.44–0.50 [20, 21]
5.79
5.94 [10]
5.62 [8]
Ag/Al2O3
1.80
1.83 [11]
0.679 [7]
4.03
3.93 [11]
4.7 [9]
1.92
2.31 [11]
2.64
2.78 [11]
Au/Al2O3
0.41
0.33 [11]
0.672 [7]
0.27 [9]
0.36 [19]
0.17
0.29 [11]
Experiment 0.3–0.43 [23]
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KULKOVA et al.
Cu1 (a)
Cu3
N(E), electrons/eV
6
4
2
0
~
~
EF
4
2
0
O2 in Al2O3
1
~
~
(b)
EF
~
O2 in Al2O3
~
1
0~
~
Al3 in Al2O3
1
–16
–8
E, eV
0
0
~
~
(c)
EF
O2 in Al2O3
1
0~
Al3 in Al2O3
Cu2
Cu3
6
4
2
0
1
0~
0
Cu1
Cu3
6
Al3 in Al2O3
1
–16
–8
E, eV
0
0
–16
–8
E, eV
0
Fig. 4. Local densities of electron states of the interface atoms at (a) the perfect Cu/(Al2O3)O interface and (b, c) the Cu/(Al2O3)O
interface with (b) oxygen vacancies and (c) a vacancy in the first layer of the copper film. The numeration of the interface atoms
is given in accordance with Fig. 1. Symbols Cu1, Cu2, and Cu3 correspond to different atoms of the first metal layer.
from the analysis of electronic characteristics, the
maximum work of separation was obtained for
Me/(Al2O3)O interfaces, which points to the impor
tance of the ion component in the chemical bonding
for strong adhesion at metal oxide interfaces. It is
worth noting that the maximum values of Wsep were
obtained for interfaces with nickel or copper. Our
obtained value of the work of separation for nickel film
is less practically by 1 J/m2 than that obtained in [10],
although the results for other Ni/Al2O3 interfaces are
in a good agreement with [10]. It should be noted that
the work of separation is somewhat lower for films with
metals isoelectronic with nickel. A substantially
greater reduction in the work of separation is observed
for the second series, where the Altop configuration
at the oxygen interface is more stable, which causes a
sharp decrease in Wsep. The lowest work of separation
of metal films was obtained for interfaces with alumi
num termination. In this case, the results for the first
series of metals (Ni, Pd, and Pt) are practically twice
the values for the second series of metals (Cu, Ag, and
Au), which is caused by the presence of practically
totally filled d band for these metals. Our results are,
generally, in a good agreement with the experiment
[20, 21], except gold, for which the calculated values
are less than in the experiment [23]. It should be noted
that, as a whole, the adhesion energies in the case of
the aluminumenriched interface have intermediate
values but, for Pd, Pt, and Au, they are close to the
results obtained for the oxygen interface. Moreover,
the further filling of the d shell of metals from the sec
ond series favors weakening of the chemical bonding at
all studied interfaces as compared to the first series.
Since oxygen vacancies can substantially affect the
chemical bonding at interfaces, we calculated the work
of separation with allowance for the vacancies in the
interface layers at the interface with oxygen termina
tion. We found that oxygen vacancies reduce the value
of Wsep practically by a half. For example, the values of
the work of separation of 2.49 and 2.01 J/m2 were cal
culated for Cu/(Al2O3)O and Ag/(Al2O3)O. The nature
of reduction of adhesion in the presence of oxygen
vacancies can be understood from the analysis of den
sities of electron states. It is evident from comparison
of Figs. 4a and 4b that there is a substantial variation in
the local density of electron states of oxygen: namely,
a shift of the valence band of oxygen from the Fermi
level, which reduces hybridization of the p states of
oxygen with s and d orbitals of metal. At the same
time, vacancies in metal films near the oxygen inter
face influence the structure of the density of states of
oxygen and of copper atoms nearest to the interface to
a less extent. It is readily seen (Fig. 4c) that the local
density of states of metal atoms suffers a less change as
compared to the aboveconsidered case. It is worth
noting a decrease in the density of states at the Fermi
level and widening of the valence band of copper
atoms. Such insignificant changes are also observed in
the local density of states of oxygen in the given case.
Thus, a decrease in the number of Me–O bonds at the
interface at the expense of oxygen vacancies substan
tially reflects in the adhesion energy at the oxygen ter
mination of the interface, whereas the vacancies in the
interface layer of oxide films influence the adhesion at
interfaces to a less extent (the decrease in the work of
separation is 0.7–0.8 J/m2).
4. CONCLUSIONS
A comparative study of the atomic and electronic
structure of interfaces between two series of isoelec
PHYSICS OF THE SOLID STATE
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2010
ELECTRONIC STRUCTURE
tronic metals and aluminum oxide with the corundum
structure αAl2O3(0001) depending on the termina
tion of oxide film and configuration of metal films is
performed. It is shown that the stable configuration of
the aluminum interface in the case of metals isoelec
tronic with nickel is the Otop configuration. Stability
of this configuration is provided—in addition to metal
bonding—by the ion component due to the interac
tion of film atoms nearest to the interface with oxygen
of the second layer. For Ag and Au films, the Altop
configuration is more stable because of the weakening
of the interaction between metal atoms and oxygen
due to an increase in the distance between atoms of the
film and the oxygen layer. In this case, the main mech
anism of chemical bonding is the hybridization of
metal Me–Al orbitals and polarization of Me atoms. In
the case of an aluminumenriched interface, the metal
type of bonding prevails. The ion type of chemical
bonding is the main one at oxygen interfaces. Since
one of interface metal atoms tends to occupy the posi
tion of aluminum at the given interface, this results in
the splitting of the interface metal layer and leads to
increasing adhesion at the interface. The presence of
defects and, first of all, oxygen vacancies leads to a
substantial decrease in the adhesion energy, due to the
partial breaking of metal–oxygen bonds. The analysis
of electronic characteristics has shown that, in this
case, the overlap of orbitals of oxygen and metal
decreases due to a significant shift of the valence band
of oxygen from the Fermi level. As a whole, our study
made it possible to reveal the microscopic nature of
the interaction at metal–oxide interfaces, which is
important for understanding the ways of increasing the
adhesion of interfaces.
2595
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This study was supported by the Russian Founda
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Translated by E. Chernokozhin
2010