Computational Materials Science 39 (2007) 794–802
www.elsevier.com/locate/commatsci
Transient hole formation during the growth of thin metal oxide layers
X.W. Zhou
a
a,*
, H.N.G. Wadley b, D.X. Wang
c
Department of Materials Mechanics, 7011 East Avenue, Sandia National Laboratories, Livermore, California 94551-0969, United States
b
Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22904, USA
c
Nonvolatile Electronics Corporation, Eden Prairie, MN 55344, USA
Received 21 April 2006; received in revised form 1 October 2006; accepted 4 October 2006
Abstract
Using a quinternary variable charge molecular dynamics simulation technique, we have discovered a transient hole formation phenomenon during oxidation of thin aluminum layers on Ni65Co20Fe15 substrates. Holes were found to first develop and expand at the
earliest stage of the oxidation. These holes then shrank and finally disappeared as oxidation further proceeded. Thermodynamic analysis
of the hole healing indicated that it is accompanied by a significant decrease in system potential energy. This suggests that the effect is
largely driven by thermodynamics and is less related to the flux shadowing or kinetically introduced island coalescence. The simulations
provide insights for the growth of dielectric tunnel barrier layers with reduced layer thicknesses.
2006 Elsevier B.V. All rights reserved.
Keywords: Molecular dynamics; Charge transfer potential; Embedded atom method; Magnetic tunnel junction; Aluminum oxide; Multilayer
1. Introduction
Many devices require the growth of multilayers with
nanometer layer thicknesses. One typical example is the
magnetic tunnel junction (MTJ) multilayers [1–3] that use
thin metal oxide layers as the tunnel barriers to separate
a pair of ferromagnetic metal alloy layers [4,5]. These
MTJ multilayer structures can be used for non-volatile
magnetic random access memory (MRAM) [1,6] and magnetic field sensing [7–9]. Similar tunneling barriers are also
being explored for spin injection [10,11]. Because the highest tunneling conductance is usually obtained with the thinnest barrier layer [5], there is a great interest in the growth
of uniform metal oxide layers with reduced layer thickness.
The most commonly used tunnel barrier layers in MTJ
structures has been the amorphous AlOx created by oxidizing a pre-deposited aluminum layer. The current MTJ
structures use relatively thick AlOx barrier layers made
by oxidizing aluminum layers that were at least 10 Å thick
prior to oxidation [12,13]. Experimental studies of the
*
Corresponding author. Tel.: +1 434 982 5672; fax: +1 434 982 5677.
E-mail address: [email protected] (X.W. Zhou).
0927-0256/$ - see front matter 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.commatsci.2006.10.006
growth of the AlOx barrier layer indicated that there
existed a critical aluminum layer thickness between 6 and
8 Å below which oxide layers containing holes were likely
to form [14,15]. Increasing the aluminum layer thickness
significantly improves the uniformity of the oxide layer,
resulting in hole free films [16]. High resolution transmission electron microscopy (HRTEM) and three dimensional
atom probe (3DAP) analysis of the oxidation of 6 Å (or
greater) thick aluminum layers indicated that holes developed in the oxide layers while the aluminum layer was
under-oxidized. Further oxidation eventually resulted in
continuous oxide layers [17]. Both the hole containing
and the continuous oxide layers appeared to be relatively
stable structures since they were present in samples that
were subject to annealing prior to examination.
The processes responsible for the formation of holes and
their disappearance in fully oxidized films are not well
understood. To explore the fundamental origin determining the formation of the continuous metal oxide layer, we
further apply the newest available molecular dynamics
(MD) approach to simulate the atomic assembly processes
during oxidation of ultra-thin (6 Å) aluminum layer predeposited on an Ni65Co20Fe15 underlayer.
X.W. Zhou et al. / Computational Materials Science 39 (2007) 794–802
2. Modeling methods
The MD method uses Newton’s equation of motion to
trace the positions of all atoms in a simulated computational cell. Accurate results about atomic scale structures
of any material can be gained provided a high fidelity interatomic potential is used to calculate the forces between
atoms. In a material system composed of a metal oxide layer
on a metal alloy layer, the atomic interaction changes dramatically from the significantly ionic bonding in the oxide
layer to the predominantly metallic bonding in the metal
layer through the oxide/metal interface. A fixed charge
potential [18], therefore, cannot be applied. The modified
charge transfer ionic embedded atom method potential
developed recently [19] has begun to enable such a material
system to be simulated directly using the MD method.
2.1. Interatomic potential
The embedded atom method (EAM) potential developed by Daw and Baskes [20] reasonably addresses the
interatomic forces between metal atoms [21,22]. When a
metal is oxidized, metal atoms become positive charges
(cations) and oxygen atoms become negative charges
(anions). Depending on the local oxygen fraction and the
local oxidation state, the magnitude of the charges on
atoms can vary from zero in a metallic region (either a
metal element or an alloy) to a high value in a fully oxidized region. The occurrence of such a dynamically varying
charge distribution introduces a variable electrostatic
energy contribution to the interatomic potential.
A charge transfer ionic potential (CTIP) was proposed
by Rappe and Goddard [23] to address the electrostatic
energy due to the dynamically induced charges on atoms.
It was later integrated with EAM by Streitz and Mintmire
[24] to study metal and metal oxide heterostructures. In the
CTIP, the electrostatic energy is expressed as a sum of selfionization energy and Coulomb energy. The self-ionization
energy usually increases and the Coulomb energy between
cations and anions always decreases when the magnitudes
of the charges are increased. As a result, there exists a set
of equilibrium charges for all the atoms that minimize
the total electrostatic energy. It can be proven that for a
binary (oxygen–single metal) system, the magnitude of
the equilibrium charge on atom equals zero in a local metal
or oxygen region, and it reaches the maximum value in a
fully oxidized (metal and oxygen mixed) region. These
equilibrium charges can be dynamically solved from atom
positions and the minimum energy condition during an
atomistic simulation. They can be used to naturally define
an electrostatic energy contribution to the interatomic
potential that is transferable from a metal region to a fully
oxidized metal oxide region. Integration of such a CTIP
approach with EAM has resulted in successful MD simulations of oxidation of aluminum [25,26].
In the earlier CTIP methods [23,24], the physical range
of charges was not explicitly considered. This imposes a
795
restriction in the range of the model parameters in order
to ensure a reasonable range of charges [19]. In addition,
the earlier CTIP models would fail to predict a zero charge
for a metal alloy system [19]. As a result, they have only
been used in (single metal–oxygen) binary systems and cannot yet be applied in material systems involving more than
one metallic element [19].
Without resorting to a more complicated potential format, we recently proposed a simple empirical approach
to additionally incorporate the range of charge in the existing CTIP model [19]. The modified CTIP ensures stable
numerical calculations for any model parameters, and
predicts zero charge for any metal alloy systems. It has also
been coupled with a metal alloy EAM potential [21,27]
to create a charge transfer potential for the quinternary
O–Al–Ni–Co–Fe system [28]. This potential now enables
direct MD simulations of the reactive growth of an AlOx
tunnel barrier layer like that used in the MTJ multilayers
[16], and is used in the present work.
2.2. Molecular dynamics model
The MD [16,21,22] model was used to simulate a complete atomic assembly process used to create the AlOx
barrier layer. This includes the deposition of the bottom
Ni65Co20Fe15 layer, the growth of an aluminum layer on
the Ni65Co20Fe15 layer, and the oxidation of the top aluminum layer. An initial Ni65Co20Fe15 substrate crystal
containing 54 ð2 2 4Þ planes in the x-direction, 3 (1 1 1)
planes in the y-(growth) direction, and 32 ð2 2 0Þ planes in
the z-direction was created using the equilibrium (bulk)
lattice parameter of a = 3.604 Å. This crystal was approximated as an infinitely large layer in the x–z plane by using
periodic boundary conditions in the x- and z-directions and
a free boundary condition in the y-direction. The computational cell measures about 40 · 40 Å2 on the x–z growth
plane. In our earlier work [16] on the same structure, an
x–z cell dimension of about 88 · 20 Å2 was used. Because
one dimension of the plane was much smaller than the
other, the previous work could not reveal the structural
evolution in the x–z plane. The computational cell used
in the present work was designed to overcome this
problem.
Growth was achieved by continuously adding atoms to
the top y-surface of the layer and the evolution of the structure was simulated by solving positions of all atoms in
the system using Newton’s equation of motion. To prevent
the shift of the system during adatom impacts, atoms in the
bottom 1–2 atomic layer of the substrate were fixed during
simulation. The isothermal conditions that are usually used
in experiments were achieved by adding the Nose–Hoover
dragging forces [29] to atoms in a thermostated region
below the surface. For a correct description of various
impact mechanisms on the free surface, the upper boundary of the thermostated region was at least two atomic layers below the surface and the lower boundary extended to
the fixed region. For the results reported in this work, the
796
X.W. Zhou et al. / Computational Materials Science 39 (2007) 794–802
size of the periodic cell was assumed to be fixed. However,
test simulations using flexible periodic length were also carried out and similar results were obtained.
During growth of the metal (either the Ni65Co20Fe15 layer
grown further from the original Ni65Co20Fe15 substrate or
the aluminum layer), the corresponding metal atoms were
injected to perpendicularly impact the surface from random
locations far above. The species of the injected adatoms were
statistically assigned so that the deposited layer had approximately the desired composition. The adatom injection frequency was determined from the desired deposition rate.
Each adatom was given a remote incident kinetic energy.
For the oxidation simulations, the Al-on-Ni65Co20Fe15
surface was simply exposed to an atomic oxygen vapor.
The main characteristics of the vapor are the vapor temperature and vapor pressure. The vapor temperature corre-
3. Results
3.1. Simulated atomic structures
To mimic the surface formed during vapor deposition,
about six additional atomic layers of the Ni65Co20Fe15
alloy were deposited on the initial substrate using an
incident atom energy of 4 eV, a growth temperature of
300 K, and a deposition rate of 10 nm/ns. The as-deposited
Ni65Co20Fe15 structure is shown in Fig. 1(a). It can be seen
Deposited Al-on-Ni65Co20Fe15
Deposited Ni65Co20Fe15
[112]
z
[110]
x
sponds to vapor oxygen atom kinetic energy. The vapor
pressure corresponds to vapor density. A new oxygen atom
was added into the vapor region once an oxygen vapor atom
was found to condense into the film so that a near constant
oxygen pressure was maintained in the simulations.
y
[111]
o
10A
c
20 ps after oxidation
d
e
60 ps after oxidation
f
40 ps after oxidation
80 ps after oxidation
O
A1
Co
Ni
Fe
Fig. 1. Atomic structures at each stage of AlOx/Ni65Co20Fe15 bilayer growth. (a) After deposition of the Ni65Co20Fe15 layer using an adatom energy of
4 eV, a substrate temperature of 300 K, and a deposition rate of 10 nm/ns, (b) after deposition of the aluminum layer using an adatom energy of 0.2 eV, a
substrate temperature of 300 K, and a deposition rate of 1.5 nm/ns, (c)–(f) 20, 40, 60 and 80 ps after oxidation using a substrate temperature of 300 K, an
atomic oxygen vapor pressure of 12 atmospheres, and a vapor temperature of 8000 K.
X.W. Zhou et al. / Computational Materials Science 39 (2007) 794–802
that a relatively flat Ni65Co20Fe15 surface was obtained due
to the impact flattening activated by the relatively high incident atom energy [16].
Between two and three atomic layers (corresponding to
a thickness of 6 Å) of aluminum were then deposited on
the Ni65Co20Fe15 surface using an incident atom energy of
0.2 eV, a growth temperature of 300 K, and a growth rate
of about 1.5 nm/ns. The deposited Al-on-Ni65Co20Fe15
structure is shown in Fig. 1(b). Here, lower incident energy
of 0.2 eV was used because flat aluminum layer can be
easily grown on a Ni65Co20Fe15 layer [16]. It can be seen
from Fig. 1(b) that a relatively uniform aluminum layer
completely covered the underlying Ni65Co20Fe15 substrate.
MD simulations can be efficiently carried out for short
(on the scale of ns) processes. To induce a sufficient structure change within such a short time, an accelerated oxidation of the Al-on-Ni65Co20Fe15 surface was simulated. This
was done by exposing the surface that was kept at a substrate temperature of 300 K to a high pressure (12 atmospheres) atomic oxygen vapor held at a high vapor
temperature of 8000 K. Although these vapor conditions
differ from the ones used in experiments, they accelerated
the process kinetics and therefore may enable experimental
phenomena to occur within the simulated time scale. This,
along with the analysis of thermodynamics, can result in
the correct insights about the atomic assembly mechanisms
during the realistic growth of the barrier layers.
Time-resolved atom position images obtained during
MD simulation of oxidation are shown in Fig. 1(c)–(f) to
enable the detailed mechanisms of the oxide layer formation to be examined. It can be seen that after 20 ps of
oxidation, oxygen vapor atoms had reacted with the aluminum surface to form aluminum oxide. The initial oxidation
was not uniform and an aluminum depleted region developed near the center of the simulated region, Fig. 1(c). Previous studies indicated that this arises because the cohesive
energy (eV/atom) in oxides (such as Al2O3) is much higher
(more negative) than that of either pure aluminum or pure
oxygen, and the first nucleated oxide regions then grow by
drawing the nearby aluminum atoms [16]. This results in
the depletion of aluminum in the nearby surface, leading
to the exposure of the underlying Ni65Co20Fe15.
Fig. 1(d) shows that after 40 ps of oxidation, the oxygen
fraction in the oxidizing regions had increased, resulting in a
much denser oxide layer. However, the aluminum depleted
zone was also further developed. As oxidation continued,
Fig. 1(e), the aluminum depleted zone began to shrink.
After 80 ps of oxidation, Fig. 1(f), the aluminum depleted
zone had completely disappeared and the nickel alloy substrate was more or less covered by the aluminum oxide.
Additional oxidation conditions were explored. The
changes of structures of the same Al-on-Ni65Co20Fe15 surface during oxidation using a different oxygen vapor pressure, a different oxygen vapor temperature, and a different
substrate temperature, are shown respectively in Fig. 2(a)–
(c). It can be seen that the phenomenon observed in Fig. 1
is rather general. The initial creation of the aluminum
797
depleted zone and its subsequent shrinkage occurred in
all of the oxidation conditions we explored.
The effects of the system size have not been explored as
the work requires massive parallel calculations. While the
phenomena associated with bigger systems remain to be
seen, it is apparent that multiple aluminum depleted zones
would be created if the planar size of the layer were significantly increased. One additional mechanism for the evolution of multiple aluminum depleted zones in a big plane
can be the coalescence of the neighboring zones.
3.2. Magnetic layer coverage parameter
To quantify the observations, we have divided the
surface oxide layer of the system shown in Fig. 1 into a
14 · 14 grid. If a grid element contained neither an oxygen
nor an aluminum atom, the substrate area beneath the element was considered to be uncovered by either (the unreacted) aluminum or AlOx. Otherwise, the area was considered as covered. The fraction of the grid elements that
contained either aluminum or oxygen atoms was then
determined and is referred to as the coverage parameter.
An oxygen fraction for a grid element was defined as the
ratio of the number of oxygen atoms to the total number of
oxygen and aluminum atoms in the grid. Let the oxygen
fraction for grid element i be XO,i (i = 1, 2, . . . ,ng, where
ng is the total number of grids containing either aluminum
or oxygen atoms). The overall state of oxidation can be
represented
by the average oxygen fraction, X O ¼
Png
1
X
.
The
uniformity of the oxide can then be repreO;i
i¼1
ng
sented by the standard deviation of the oxygen fraction,
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Png X O;i X O 2
r ¼ n1g i¼1
.
X
O
The average oxygen fraction, X O , and the standard deviation of the oxygen fraction, r, are shown in Fig. 3(a) as a
function of oxidation time. The corresponding surface coverage parameter is shown in Fig. 3(b). Fig. 3(a) indicates
that during oxidation, the oxygen fraction in the oxide
layer gradually increased, eventually approaching a value
of 0.7 (corresponding to AlO2.33). Formation of this highly
oxidized state is consistent with highly reactive oxygen
vapor conditions used for the simulation. Fig. 3(a) also
shows that the deviation in oxygen fraction was great early
in the oxidation and decreased with elapsed oxidation time.
It indicates that the oxide formed during the earliest stages
of oxidation was the least uniform. These are also the
stages when holes have the highest nucleation probabilities.
Fig. 3(b) indicates that the initially fully aluminum covered Ni65Co20Fe15 surface gradually became locally uncovered once the oxidation began. This was associated with the
formation of holes in the oxide layer such as that in Fig. 1.
However, the surface coverage parameter began to recover
after an oxidation time of about 50 ps. The surface was
once again fully covered (now by AlOx) after about 90 ps
of oxidation, consistent with the healing of the holes.
The elimination a hole requires atom diffusion from
surrounding areas to the hole region. The transient hole
798
X.W. Zhou et al. / Computational Materials Science 39 (2007) 794–802
Fig. 2. Atomic structure evolution during oxidation of the Al-on-Ni65Co20Fe15 surface under different oxidation conditions. (a) Substrate temperature
300 K, vapor pressure 4 atmospheres, and vapor temperature 8000 K, (b) substrate temperature 300 K, vapor pressure 12 atmospheres, and vapor
temperature 1000 K, and (c) substrate temperature 500 K, vapor pressure 12 atmospheres, and vapor temperature 8000 K.
phenomenon could then be caused by either diffusion or
oxidation. To explore if diffusion alone can lead to the
transient hole phenomenon, the structure shown in
Fig. 1(d) was annealed at a temperature of 600 K for about
100 ps, and the resulting atomic structure is shown in
Fig. 4. It can be seen that at least within the simulated time
scale, the hole formed in Fig. 1(d) was quite stable and
changed little during the annealing. Clearly, the rapid elimination of holes such as that seen in Figs. 1–3 was caused
by continuous oxidation rather than atom diffusion alone.
Unlike annealing, continued oxidation caused increases in
both thickness and oxygen composition of the oxide layer.
The hole elimination shown in Fig. 1 was observed
under an accelerated oxidation condition. It is not clear if
the highly constrained kinetics promoted this phenomenon.
However, this can be identified by exploring the stability of
the structure shown in Fig. 1(f). During MD simulation, we
could ‘‘mechanically’’ introduce a hole in the structure of
Fig. 1(f) by applying radial forces, f, to aluminum and oxygen atoms that fell within the pre-designated hole region,
X.W. Zhou et al. / Computational Materials Science 39 (2007) 794–802
799
1.0
0.9
0.9
0.8
0.8
0.7
0.7
average oxygen fraction
0.6
0.6
0.5
0.5
0.4
0.4
deviation of
oxygen fraction
0.3
0.3
0.2
0.2
0.1
0.1
0.0
Deviation of oxygen fraction, σ
Average oxygen fraction, XO
Oxygen fraction
1.0
0.0
0
10
20
30
40
50
60
70
80
90
100
Time (ps)
Al or AlOx coverage
1.00
Al or AlOx coverage
0.99
0.98
0.97
0.96
0.95
0.94
transient hole formation
0.93
transient hole elimination
0.92
0.91
0.00
0
10
20
30
40
50
60
70
80
90
100
Time (ps)
Fig. 3. Oxygen fraction, deviation of relative oxygen fraction, and
aluminum or AlOx coverage parameter as a function of oxidation time
during a simulation using a substrate temperature of 300 K, an atomic
oxygen vapor pressure of 12 atmospheres, and a vapor temperature of
8000 K. (a) Oxygen fraction and deviation of relative oxygen fraction, and
(b) aluminum or AlOx coverage.
Fig. 5. Time evolution of a mechanically introduced hole. (a) A hole with
a radius of approximately 3 Å introduced mechanically in the surface
shown in Fig. 1(f) (by applying the radial force f to the oxygen and
aluminum atoms that were within the designated hole region), and (b) the
evolution of (a) after 10 ps at a temperature of 300 K.
Fig. 4. Effects of the annealing on the atomic structure of the surface
shown in Fig. 1(d). Annealing temperature 600 K and annealing time
100 ps.
Fig. 5(a). The magnitude of the force could be set to proportional to the distance between the atom and the periphery of the hole, Dr, Fig. 5. This corresponds to a quadratic
increase in energy (/Dr2) similar to the spring force. Once
the hole was created, we then removed the radial forces and
annealed the structure at 300 K for 10 ps. The structure
after the annealing is shown in Fig. 5(b). It can be seen that
the hole was healed during the annealing process alone,
800
X.W. Zhou et al. / Computational Materials Science 39 (2007) 794–802
indicating that the hole free structure shown in Fig. 1(f) is
stable and that the hole elimination is not an artifact of the
accelerated oxidation.
4. Discussion
4.1. Thermodynamic analysis
Once oxidation starts, local regions with rich oxygen
concentration randomly formed on the aluminum surface
as oxygen atoms arrive at the surface at random locations.
As has been discussed in the above, these oxygen-rich
regions can attract nearby aluminum atoms to form
expanding oxide nuclei because aluminum in the oxide
has a much lower energy than aluminum alone. When
the aluminum layer is very thin, this process can easily
cause aluminum depleted zones around the oxygen-rich
regions. As a result, holes are most likely to form at the
earliest stage of the oxidation.
The observation that holes begin to shrink when they are
fully developed in the fully oxidized layer is a rather general
phenomenon. Suppose a round hole with a radius of r forms
in the AlOx layer with a thickness h, Fig. 6. The formation
of the hole results in (i) the creation of additional oxide surface area (on the interior of the hole), (ii) the elimination of
an oxide area (at the top of the hole), and (iii) the creation of
a Ni alloy surface area and the elimination of the Ni alloy/
oxide interface area (at the bottom of the hole). The resulting energy change, DE, can be written as
DE ¼ 2prhcAlOx þ pr2 ðcNi65 Co20 Fe15 cAlOx =Ni65 Co20 Fe15 cAlOx Þ;
ð1Þ
where cAlOx and cNi65 Co20 Fe15 are surface energies of AlOx and
Ni65Co20Fe15, respectively, and cAlOx =Ni65 Co20 Fe15 is the inter-
face energy between the AlOx layer and the Ni65Co20Fe15
substrate. Using the surface and interface energies determined previously [16], the formation energy of a pinhole,
DE, can be calculated as a function of radius, r, for various
oxide layer thicknesses, h, Fig. 7. It can be seen that the
nucleation of pinholes always increases energy. However,
there exists a critical pinhole size above which a further increase in the pinhole size reduces energy. This critical size
increases with increasing oxide layer thickness.
This analysis indicates that during the early stages of
oxidation of thin Al layers where very thin oxide layers
are formed, the critical pinhole sizes are very small. For
an oxide layer thickness of 6 Å, the critical size is about
4 Å. Pinholes above this size are easily formed at the start
of oxidation due to the non-uniform nucleation of the
oxide. These pinholes are then to the right of the peak in
Fig. 7. They therefore further expand. As pinholes expand,
the material is transferred from holes to the nearby surface
area, resulting in thickening of the surrounding oxide. This
thickening increases the critical pinhole size. When the
actual pinhole size approximately equals the critical pinhole size, the pinhole expansion stops. Stable pinholes then
develop when no new adatoms are added to the surface.
This accounts for the stable pinhole seen during the
annealing.
During continued oxidation where O atoms are added
to the surface, the oxide layer thickness is continuously
increased. The critical pinhole size then also increases and
can exceed the actual pinhole sizes, whereupon the pinholes
will shrink. This transient pinhole formation phenomenon
is therefore driven by thermodynamics.
Eq. (1) can only qualitatively account for the observations as it does not capture the effects of chemical composition and atomic scale dimensions. An estimate of the hole
formation energy that is most relevant to the simulations
can be obtained by calculating the change of total system
potential energy as a function of time during a real
MD simulation of a hole shrinkage/expansion process.
This was done for the 10 ps annealing process shown in
h
2r
Hole formation energy, ΔE, (eV)
300
250
h = 18 Å
hole
size
200
150
h = 14 Å
shrink
100
h = 10 Å
50
expand
0
-50
h=6 Å
-100
0
h
Fig. 6. A continuum model of a hole in an oxide layer.
5
10
15
20
Oxide hole radius, r, (Å)
25
30
Fig. 7. Formation energy of oxide pinhole as a function of pinhole radius
at various oxide layer thicknesses.
X.W. Zhou et al. / Computational Materials Science 39 (2007) 794–802
Fig. 5, where a mechanically introduced hole of a radius of
r = 3 Å was eliminated. The relative total potential energy
with respect to the initial structure shown in Fig. 5(a) is
shown as a function of annealing time in Fig. 8. It can be
seen that upon the removal of the mechanical force, the
system potential energy abruptly dropped by about
850 eV. This is because atoms that were subject to the
mechanical force were no longer in balance and they
quickly move to new positions to reduce the system potential energy. A further annealing of the system caused a
gradual decrease in the system potential energy by another
350 eV until the hole was completely eliminated at an
annealing time of about 10 ps. Clearly, for the two film
configurations shown in Fig. 5, the hole free structure is
more stable and the hole healing process is driven by the
thermodynamics.
Under the accelerated oxidation conditions required for
the short time scale simulations, the transient hole phenomenon was found to occur within a very short time (80 ps),
Fig. 1. This means that during experiments where the time
scale is significantly longer, the phenomenon is not likely to
be constrained by kinetics. As a result, holes are expected
to always form during the early stage of oxidation of very
thin (say, 6 Å thick) aluminum layer under conditions commonly used in experiments. The healing of the holes is
expected to occur when the aluminum layer is relatively
fully oxidized. This suggests that the transient holes never
form during oxidation of thick aluminum layers because
the surface aluminum is always fully oxidized before any
aluminum region is completely depleted to initiate a hole.
The simulations also suggest that holes cannot be healed
during prolonged oxidation of very thin aluminum layers
because when the aluminum is quickly fully oxidized, the
driving force for the hole healing is saturated and therefore
the hole ceases to further shrink. The transient hole formation process identified above is consistent with the early
results of the MD simulations that rough (pinhole containing) AlOx layers are obtained at aluminum layer thick-
Energy change during annealing (eV)
0
-300
-600
hole size r = 3 Å
-900
hole formation
1
2
3
4
5
6
Time (ps)
7
8
9
nesses around 2.5 Å whereas continuous AlOx layers are
obtained at aluminum layer thicknesses above 6 Å [16].
It can be seen that the thinnest continuous metal oxide
layer must be grown in the transient hole formation regime.
Simulations then give important guidelines for reducing the
oxide layer thickness by using full oxidation and the equilibrium-promoting annealing processes. It should be noted
that the transient hole formation was discovered in the
AlOx-on-Ni65Co20Fe15 system. However, the phenomenon
is general and likely to occur in other metal oxide on other
metal systems since metal oxides usually have much higher
cohesive energies than the reduced metals. It is therefore
energetically favorable for metal oxides to form clusters
on metals rather than a thin layer with atomic scale
thickness.
4.2. Comparisons with experimental observations
A direct experimental observation of the atomic scale
structure of the AlOx-on-Ni65Co20Fe15 system has not been
found in literature. However, Petford-Long et al., have carried out extensive HTEM and 3DAP experiments to examine atomic scale structure of the AlOx oxide formed from
oxidation of 6 Å thick aluminum layer on Co90Ni10 [17].
Their experiments indicated that under the under-oxidation
conditions, the AlOx layer exhibited discontinuous islands
with significant areas of the Co90Ni10 surface uncovered.
This phenomenon closely corresponds to the formation
of big holes in the oxide layer in the under-oxidized samples. Annealing of these under-oxidized samples was found
to cause the AlOx islands to spread to form a network
along the grain boundaries. Note that the experimental
annealing involved further oxidation. These experimental
observations appear similar to the results of the simulations.
The experiments further indicated that more fully oxidation of the aluminum surface produced a more continuous
AlOx layer. However, these as-grown films still contained
holes on the scale of roughly 10 nm. The oxygen composition in the AlOx layer, on the other hand, was found to be
still far below the one defined by the fully oxidized AlO1.5
(Al2O3). These holes were eliminated and a continuous
AlO1.5 layer finally formed when the samples were
annealed. This means that oxygen diffused from further
away to complete the oxidation and that the layer laterally
spread to fill the holes. These observations are all consistent with the simulations.
5. Conclusions
hole size r = 0 Å
-1200
-1500
0
801
10
Fig. 8. Change of total system potential energy as a function of time
during the annealing process shown in Fig. 5.
A quinternary variable charge molecular dynamics simulation method has been applied to simulate the growth of
a thin (6 Å thick aluminum prior to oxidation) AlOx spin
tunnel barrier layer on a Ni65Co20Fe15 surface. The results
indicate that holes always form in the AlOx layer during
the initial oxidation of the aluminum surface. Such holes
are quite stable and are not seen to be eliminated by
802
X.W. Zhou et al. / Computational Materials Science 39 (2007) 794–802
annealing. However, these holes become unstable during
continued oxidation and are seen to be eliminated when
the surface is fully oxidized. Thermodynamic analysis indicates that there exists a critical oxidation state above which
the shrinkage of the holes reduces the total system potential
energy. As a result, the hole healing process is driven by
thermodynamics. This transient hole formation mechanism
in many ways accounts for the experimental observation of
the AlOx islands in under-oxidized samples and the smooth
AlOx layer in fully oxidized, annealed samples.
Acknowledgements
This work was supported by NSF under Grant DMI0214719, DARPA/USAAMC under contract W31P4Q05-C-R141, and DARPA/ONR under Grant N00014-03C-0288.
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