transient hole formation during the growth of thin metal oxide layers

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Computational Materials Science 39 (2007) 794–802

Transient hole formation during the growth of thin metal oxide layers

X.W. Zhou a,*, H.N.G. Wadley b, D.X. Wang c

a Department of Materials Mechanics, 7011 East Avenue, Sandia National Laboratories, Livermore, California 94551-0969, United Statesb 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 phe-nomenon during oxidation of thin aluminum layers on Ni65Co20Fe15 substrates. Holes were found to first develop and expand at theearliest stage of the oxidation. These holes then shrank and finally disappeared as oxidation further proceeded. Thermodynamic analysisof the hole healing indicated that it is accompanied by a significant decrease in system potential energy. This suggests that the effect islargely driven by thermodynamics and is less related to the flux shadowing or kinetically introduced island coalescence. The simulationsprovide 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 withnanometer layer thicknesses. One typical example is themagnetic tunnel junction (MTJ) multilayers [1–3] that usethin metal oxide layers as the tunnel barriers to separatea pair of ferromagnetic metal alloy layers [4,5]. TheseMTJ multilayer structures can be used for non-volatilemagnetic random access memory (MRAM) [1,6] and mag-netic field sensing [7–9]. Similar tunneling barriers are alsobeing explored for spin injection [10,11]. Because the high-est tunneling conductance is usually obtained with the thin-nest barrier layer [5], there is a great interest in the growthof uniform metal oxide layers with reduced layer thickness.

The most commonly used tunnel barrier layers in MTJstructures has been the amorphous AlOx created by oxidiz-ing a pre-deposited aluminum layer. The current MTJstructures use relatively thick AlOx barrier layers madeby oxidizing aluminum layers that were at least 10 A thickprior to oxidation [12,13]. Experimental studies of the

0927-0256/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.commatsci.2006.10.006

* Corresponding author. Tel.: +1 434 982 5672; fax: +1 434 982 5677.E-mail address: [email protected] (X.W. Zhou).

growth of the AlOx barrier layer indicated that thereexisted a critical aluminum layer thickness between 6 and8 A below which oxide layers containing holes were likelyto form [14,15]. Increasing the aluminum layer thicknesssignificantly improves the uniformity of the oxide layer,resulting in hole free films [16]. High resolution transmis-sion electron microscopy (HRTEM) and three dimensionalatom probe (3DAP) analysis of the oxidation of �6 A (orgreater) thick aluminum layers indicated that holes devel-oped in the oxide layers while the aluminum layer wasunder-oxidized. Further oxidation eventually resulted incontinuous oxide layers [17]. Both the hole containingand the continuous oxide layers appeared to be relativelystable structures since they were present in samples thatwere subject to annealing prior to examination.

The processes responsible for the formation of holes andtheir disappearance in fully oxidized films are not wellunderstood. To explore the fundamental origin determin-ing the formation of the continuous metal oxide layer, wefurther apply the newest available molecular dynamics(MD) approach to simulate the atomic assembly processesduring oxidation of ultra-thin (�6 A) aluminum layer pre-deposited on an Ni65Co20Fe15 underlayer.

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X.W. Zhou et al. / Computational Materials Science 39 (2007) 794–802 795

2. Modeling methods

The MD method uses Newton’s equation of motion totrace the positions of all atoms in a simulated computa-tional cell. Accurate results about atomic scale structuresof any material can be gained provided a high fidelity inter-atomic potential is used to calculate the forces betweenatoms. In a material system composed of a metal oxide layeron a metal alloy layer, the atomic interaction changes dra-matically from the significantly ionic bonding in the oxidelayer to the predominantly metallic bonding in the metallayer through the oxide/metal interface. A fixed chargepotential [18], therefore, cannot be applied. The modifiedcharge transfer ionic embedded atom method potentialdeveloped recently [19] has begun to enable such a materialsystem to be simulated directly using the MD method.

2.1. Interatomic potential

The embedded atom method (EAM) potential devel-oped by Daw and Baskes [20] reasonably addresses theinteratomic forces between metal atoms [21,22]. When ametal is oxidized, metal atoms become positive charges(cations) and oxygen atoms become negative charges(anions). Depending on the local oxygen fraction and thelocal oxidation state, the magnitude of the charges onatoms can vary from zero in a metallic region (either ametal element or an alloy) to a high value in a fully oxi-dized region. The occurrence of such a dynamically varyingcharge distribution introduces a variable electrostaticenergy contribution to the interatomic potential.

A charge transfer ionic potential (CTIP) was proposedby Rappe and Goddard [23] to address the electrostaticenergy 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 theCTIP, the electrostatic energy is expressed as a sum of self-ionization energy and Coulomb energy. The self-ionizationenergy usually increases and the Coulomb energy betweencations and anions always decreases when the magnitudesof the charges are increased. As a result, there exists a setof equilibrium charges for all the atoms that minimizethe total electrostatic energy. It can be proven that for abinary (oxygen–single metal) system, the magnitude ofthe equilibrium charge on atom equals zero in a local metalor oxygen region, and it reaches the maximum value in afully oxidized (metal and oxygen mixed) region. Theseequilibrium charges can be dynamically solved from atompositions and the minimum energy condition during anatomistic simulation. They can be used to naturally definean electrostatic energy contribution to the interatomicpotential that is transferable from a metal region to a fullyoxidized metal oxide region. Integration of such a CTIPapproach with EAM has resulted in successful MD simula-tions of oxidation of aluminum [25,26].

In the earlier CTIP methods [23,24], the physical rangeof charges was not explicitly considered. This imposes a

restriction in the range of the model parameters in orderto ensure a reasonable range of charges [19]. In addition,the earlier CTIP models would fail to predict a zero chargefor a metal alloy system [19]. As a result, they have onlybeen used in (single metal–oxygen) binary systems and can-not yet be applied in material systems involving more thanone metallic element [19].

Without resorting to a more complicated potential for-mat, we recently proposed a simple empirical approachto additionally incorporate the range of charge in the exist-ing CTIP model [19]. The modified CTIP ensures stablenumerical calculations for any model parameters, andpredicts zero charge for any metal alloy systems. It has alsobeen coupled with a metal alloy EAM potential [21,27]to create a charge transfer potential for the quinternaryO–Al–Ni–Co–Fe system [28]. This potential now enablesdirect 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 com-plete atomic assembly process used to create the AlOx

barrier layer. This includes the deposition of the bottomNi65Co20Fe15 layer, the growth of an aluminum layer onthe Ni65Co20Fe15 layer, and the oxidation of the top alumi-num layer. An initial Ni65Co20Fe15 substrate crystalcontaining 54 ð22�4Þ planes in the x-direction, 3 (111)planes in the y-(growth) direction, and 32 ð2�20Þ planes inthe z-direction was created using the equilibrium (bulk)lattice parameter of a = 3.604 A. This crystal was approx-imated as an infinitely large layer in the x–z plane by usingperiodic boundary conditions in the x- and z-directions anda free boundary condition in the y-direction. The computa-tional cell measures about 40 · 40 A2 on the x–z growthplane. In our earlier work [16] on the same structure, anx–z cell dimension of about 88 · 20 A2 was used. Becauseone dimension of the plane was much smaller than theother, the previous work could not reveal the structuralevolution in the x–z plane. The computational cell usedin the present work was designed to overcome thisproblem.

Growth was achieved by continuously adding atoms tothe top y-surface of the layer and the evolution of the struc-ture was simulated by solving positions of all atoms inthe system using Newton’s equation of motion. To preventthe shift of the system during adatom impacts, atoms in thebottom 1–2 atomic layer of the substrate were fixed duringsimulation. The isothermal conditions that are usually usedin experiments were achieved by adding the Nose–Hooverdragging forces [29] to atoms in a thermostated regionbelow the surface. For a correct description of variousimpact mechanisms on the free surface, the upper bound-ary of the thermostated region was at least two atomic lay-ers below the surface and the lower boundary extended tothe 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 car-ried out and similar results were obtained.

During growth of the metal (either the Ni65Co20Fe15 layergrown further from the original Ni65Co20Fe15 substrate orthe aluminum layer), the corresponding metal atoms wereinjected to perpendicularly impact the surface from randomlocations far above. The species of the injected adatoms werestatistically assigned so that the deposited layer had approx-imately the desired composition. The adatom injection fre-quency 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 temper-ature and vapor pressure. The vapor temperature corre-

Deposited Ni65Co20Fe15

20 ps after oxidation

De

60 ps after oxidation 8

10Ao[111]y

[110

]z

[112]x

c d

e f

Fig. 1. Atomic structures at each stage of AlOx/Ni65Co20Fe15 bilayer growth.4 eV, a substrate temperature of 300 K, and a deposition rate of 10 nm/ns, (b) asubstrate temperature of 300 K, and a deposition rate of 1.5 nm/ns, (c)–(f) 20, 4atomic oxygen vapor pressure of 12 atmospheres, and a vapor temperature of

sponds to vapor oxygen atom kinetic energy. The vaporpressure corresponds to vapor density. A new oxygen atomwas added into the vapor region once an oxygen vapor atomwas found to condense into the film so that a near constantoxygen pressure was maintained in the simulations.

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 anincident atom energy of 4 eV, a growth temperature of300 K, and a deposition rate of 10 nm/ns. The as-depositedNi65Co20Fe15 structure is shown in Fig. 1(a). It can be seen

posited Al-on-Ni65Co20Fe15



O

A1

Co

Ni

Fe

(a) After deposition of the Ni65Co20Fe15 layer using an adatom energy offter deposition of the aluminum layer using an adatom energy of 0.2 eV, a0, 60 and 80 ps after oxidation using a substrate temperature of 300 K, an8000 K.


that a relatively flat Ni65Co20Fe15 surface was obtained dueto the impact flattening activated by the relatively high inci-dent atom energy [16].

Between two and three atomic layers (corresponding toa thickness of �6 A) of aluminum were then deposited onthe Ni65Co20Fe15 surface using an incident atom energy of0.2 eV, a growth temperature of 300 K, and a growth rateof about 1.5 nm/ns. The deposited Al-on-Ni65Co20Fe15

structure is shown in Fig. 1(b). Here, lower incident energyof 0.2 eV was used because flat aluminum layer can beeasily grown on a Ni65Co20Fe15 layer [16]. It can be seenfrom Fig. 1(b) that a relatively uniform aluminum layercompletely covered the underlying Ni65Co20Fe15 substrate.

MD simulations can be efficiently carried out for short(on the scale of ns) processes. To induce a sufficient struc-ture change within such a short time, an accelerated oxida-tion of the Al-on-Ni65Co20Fe15 surface was simulated. Thiswas done by exposing the surface that was kept at a sub-strate temperature of 300 K to a high pressure (12 atmo-spheres) atomic oxygen vapor held at a high vaportemperature of 8000 K. Although these vapor conditionsdiffer from the ones used in experiments, they acceleratedthe process kinetics and therefore may enable experimentalphenomena to occur within the simulated time scale. This,along with the analysis of thermodynamics, can result inthe correct insights about the atomic assembly mechanismsduring the realistic growth of the barrier layers.

Time-resolved atom position images obtained duringMD simulation of oxidation are shown in Fig. 1(c)–(f) toenable the detailed mechanisms of the oxide layer forma-tion to be examined. It can be seen that after 20 ps ofoxidation, oxygen vapor atoms had reacted with the alumi-num surface to form aluminum oxide. The initial oxidationwas not uniform and an aluminum depleted region devel-oped near the center of the simulated region, Fig. 1(c). Pre-vious studies indicated that this arises because the cohesiveenergy (eV/atom) in oxides (such as Al2O3) is much higher(more negative) than that of either pure aluminum or pureoxygen, and the first nucleated oxide regions then grow bydrawing the nearby aluminum atoms [16]. This results inthe depletion of aluminum in the nearby surface, leadingto the exposure of the underlying Ni65Co20Fe15.

Fig. 1(d) shows that after 40 ps of oxidation, the oxygenfraction in the oxidizing regions had increased, resulting in amuch denser oxide layer. However, the aluminum depletedzone 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 depletedzone had completely disappeared and the nickel alloy sub-strate was more or less covered by the aluminum oxide.

Additional oxidation conditions were explored. Thechanges of structures of the same Al-on-Ni65Co20Fe15 sur-face during oxidation using a different oxygen vapor pres-sure, a different oxygen vapor temperature, and a differentsubstrate temperature, are shown respectively in Fig. 2(a)–(c). It can be seen that the phenomenon observed in Fig. 1is rather general. The initial creation of the aluminum

depleted zone and its subsequent shrinkage occurred inall of the oxidation conditions we explored.

The effects of the system size have not been explored asthe work requires massive parallel calculations. While thephenomena associated with bigger systems remain to beseen, it is apparent that multiple aluminum depleted zoneswould be created if the planar size of the layer were signif-icantly increased. One additional mechanism for the evolu-tion of multiple aluminum depleted zones in a big planecan be the coalescence of the neighboring zones.

3.2. Magnetic layer coverage parameter

To quantify the observations, we have divided thesurface oxide layer of the system shown in Fig. 1 into a14 · 14 grid. If a grid element contained neither an oxygennor an aluminum atom, the substrate area beneath the ele-ment was considered to be uncovered by either (the un-reacted) aluminum or AlOx. Otherwise, the area was con-sidered as covered. The fraction of the grid elements thatcontained either aluminum or oxygen atoms was thendetermined and is referred to as the coverage parameter.

An oxygen fraction for a grid element was defined as theratio of the number of oxygen atoms to the total number ofoxygen and aluminum atoms in the grid. Let the oxygenfraction for grid element i be XO,i (i = 1, 2, . . . ,ng, whereng is the total number of grids containing either aluminumor oxygen atoms). The overall state of oxidation can berepresented by the average oxygen fraction, X O ¼1ng

Png

i¼1X O;i. The uniformity of the oxide can then be repre-sented by the standard deviation of the oxygen fraction,

r ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1ng

Png

i¼1X O;i�X O

X O

� �2r

.

The average oxygen fraction, X O, and the standard devi-ation of the oxygen fraction, r, are shown in Fig. 3(a) as afunction of oxidation time. The corresponding surface cov-erage parameter is shown in Fig. 3(b). Fig. 3(a) indicatesthat during oxidation, the oxygen fraction in the oxidelayer gradually increased, eventually approaching a valueof 0.7 (corresponding to AlO2.33). Formation of this highlyoxidized state is consistent with highly reactive oxygenvapor conditions used for the simulation. Fig. 3(a) alsoshows that the deviation in oxygen fraction was great earlyin the oxidation and decreased with elapsed oxidation time.It indicates that the oxide formed during the earliest stagesof oxidation was the least uniform. These are also thestages when holes have the highest nucleation probabilities.

Fig. 3(b) indicates that the initially fully aluminum cov-ered Ni65Co20Fe15 surface gradually became locally uncov-ered once the oxidation began. This was associated with theformation of holes in the oxide layer such as that in Fig. 1.However, the surface coverage parameter began to recoverafter an oxidation time of about 50 ps. The surface wasonce again fully covered (now by AlOx) after about 90 psof oxidation, consistent with the healing of the holes.

The elimination a hole requires atom diffusion fromsurrounding areas to the hole region. The transient hole

Fig. 2. Atomic structure evolution during oxidation of the Al-on-Ni65Co20Fe15 surface under different oxidation conditions. (a) Substrate temperature300 K, vapor pressure 4 atmospheres, and vapor temperature 8000 K, (b) substrate temperature 300 K, vapor pressure 12 atmospheres, and vaportemperature 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 oroxidation. To explore if diffusion alone can lead to thetransient hole phenomenon, the structure shown inFig. 1(d) was annealed at a temperature of 600 K for about100 ps, and the resulting atomic structure is shown inFig. 4. It can be seen that at least within the simulated timescale, the hole formed in Fig. 1(d) was quite stable andchanged little during the annealing. Clearly, the rapid elim-ination of holes such as that seen in Figs. 1–3 was causedby continuous oxidation rather than atom diffusion alone.

Unlike annealing, continued oxidation caused increases inboth thickness and oxygen composition of the oxide layer.

The hole elimination shown in Fig. 1 was observedunder an accelerated oxidation condition. It is not clear ifthe highly constrained kinetics promoted this phenomenon.However, this can be identified by exploring the stability ofthe structure shown in Fig. 1(f). During MD simulation, wecould ‘‘mechanically’’ introduce a hole in the structure ofFig. 1(f) by applying radial forces, f, to aluminum and oxy-gen atoms that fell within the pre-designated hole region,

Oxygen fraction

Al or AlOx coverage

0.0

0.2

0.4

1.0

0.6

0.8

0.1

0.3

0.9

0.5

0.7

Ave

rage

oxy

gen

frac

tion,

XO

0.2

0.4

1.0

0.6

0.8

0.1

0.3

0.9

0.5

0.7

Dev

iatio

n of

oxy

gen

frac

tion,

σ

Time (ps)

0.00 100908070605040302010

0.00

0.92

0.94

1.00

0.96

0.98

0.91

0.93

0.99

0.95

0.97

Al o

r A

lOx

cove

rage

Time (ps)0 100908070605040302010

transient hole formation transient hole elimination

average oxygen fraction

deviation ofoxygen fraction

Fig. 3. Oxygen fraction, deviation of relative oxygen fraction, andaluminum or AlOx coverage parameter as a function of oxidation timeduring a simulation using a substrate temperature of 300 K, an atomicoxygen vapor pressure of 12 atmospheres, and a vapor temperature of8000 K. (a) Oxygen fraction and deviation of relative oxygen fraction, and(b) aluminum or AlOx coverage.

Fig. 4. Effects of the annealing on the atomic structure of the surfaceshown in Fig. 1(d). Annealing temperature 600 K and annealing time100 ps.

Fig. 5. Time evolution of a mechanically introduced hole. (a) A hole witha radius of approximately 3 A introduced mechanically in the surfaceshown in Fig. 1(f) (by applying the radial force f to the oxygen andaluminum atoms that were within the designated hole region), and (b) theevolution of (a) after 10 ps at a temperature of 300 K.


Fig. 5(a). The magnitude of the force could be set to pro-portional to the distance between the atom and the periph-ery of the hole, Dr, Fig. 5. This corresponds to a quadraticincrease in energy (/Dr2) similar to the spring force. Oncethe hole was created, we then removed the radial forces andannealed the structure at 300 K for 10 ps. The structureafter the annealing is shown in Fig. 5(b). It can be seen thatthe hole was healed during the annealing process alone,


indicating that the hole free structure shown in Fig. 1(f) isstable and that the hole elimination is not an artifact of theaccelerated oxidation.

4. Discussion

4.1. Thermodynamic analysis

Once oxidation starts, local regions with rich oxygenconcentration randomly formed on the aluminum surfaceas oxygen atoms arrive at the surface at random locations.As has been discussed in the above, these oxygen-richregions can attract nearby aluminum atoms to formexpanding oxide nuclei because aluminum in the oxidehas a much lower energy than aluminum alone. Whenthe aluminum layer is very thin, this process can easilycause aluminum depleted zones around the oxygen-richregions. As a result, holes are most likely to form at theearliest stage of the oxidation.

The observation that holes begin to shrink when they arefully developed in the fully oxidized layer is a rather generalphenomenon. Suppose a round hole with a radius of r formsin the AlOx layer with a thickness h, Fig. 6. The formationof the hole results in (i) the creation of additional oxide sur-face area (on the interior of the hole), (ii) the elimination ofan oxide area (at the top of the hole), and (iii) the creation ofa Ni alloy surface area and the elimination of the Ni alloy/oxide interface area (at the bottom of the hole). The result-ing energy change, DE, can be written as

DE ¼ 2prhcAlOxþ pr2ðcNi65Co20Fe15

� cAlOx=Ni65Co20Fe15� cAlOx

Þ;ð1Þ

where cAlOxand cNi65Co20Fe15

are surface energies of AlOx andNi65Co20Fe15, respectively, and cAlOx=Ni65Co20Fe15

is the inter-

2r

h

h

Fig. 6. A continuum model of a hole in an oxide layer.

face energy between the AlOx layer and the Ni65Co20Fe15

substrate. Using the surface and interface energies deter-mined previously [16], the formation energy of a pinhole,DE, can be calculated as a function of radius, r, for variousoxide layer thicknesses, h, Fig. 7. It can be seen that thenucleation of pinholes always increases energy. However,there exists a critical pinhole size above which a further in-crease in the pinhole size reduces energy. This critical sizeincreases with increasing oxide layer thickness.

This analysis indicates that during the early stages ofoxidation of thin Al layers where very thin oxide layersare formed, the critical pinhole sizes are very small. Foran oxide layer thickness of 6 A, the critical size is about4 A. Pinholes above this size are easily formed at the startof oxidation due to the non-uniform nucleation of theoxide. These pinholes are then to the right of the peak inFig. 7. They therefore further expand. As pinholes expand,the material is transferred from holes to the nearby surfacearea, resulting in thickening of the surrounding oxide. Thisthickening increases the critical pinhole size. When theactual pinhole size approximately equals the critical pin-hole size, the pinhole expansion stops. Stable pinholes thendevelop when no new adatoms are added to the surface.This accounts for the stable pinhole seen during theannealing.

During continued oxidation where O atoms are addedto the surface, the oxide layer thickness is continuouslyincreased. The critical pinhole size then also increases andcan exceed the actual pinhole sizes, whereupon the pinholeswill shrink. This transient pinhole formation phenomenonis therefore driven by thermodynamics.

Eq. (1) can only qualitatively account for the observa-tions as it does not capture the effects of chemical compo-sition and atomic scale dimensions. An estimate of the holeformation energy that is most relevant to the simulationscan be obtained by calculating the change of total systempotential energy as a function of time during a realMD simulation of a hole shrinkage/expansion process.This was done for the 10 ps annealing process shown in

-100

0

100

300

200

-50

50

150

250

Hol

e fo

rmat

ion

ener

gy, Δ

E, (

eV)

Oxide hole radius, r, (Å)0 30252015105

h = 18 Å

h = 14 Å

h = 10 Å

h = 6 Å

shrink

holesize

expand

Fig. 7. Formation energy of oxide pinhole as a function of pinhole radiusat various oxide layer thicknesses.


Fig. 5, where a mechanically introduced hole of a radius ofr = 3 A was eliminated. The relative total potential energywith respect to the initial structure shown in Fig. 5(a) isshown as a function of annealing time in Fig. 8. It can beseen that upon the removal of the mechanical force, thesystem potential energy abruptly dropped by about850 eV. This is because atoms that were subject to themechanical force were no longer in balance and theyquickly move to new positions to reduce the system poten-tial energy. A further annealing of the system caused agradual decrease in the system potential energy by another350 eV until the hole was completely eliminated at anannealing time of about 10 ps. Clearly, for the two filmconfigurations shown in Fig. 5, the hole free structure ismore stable and the hole healing process is driven by thethermodynamics.

Under the accelerated oxidation conditions required forthe short time scale simulations, the transient hole phenom-enon was found to occur within a very short time (80 ps),Fig. 1. This means that during experiments where the timescale is significantly longer, the phenomenon is not likely tobe constrained by kinetics. As a result, holes are expectedto always form during the early stage of oxidation of verythin (say, 6 A thick) aluminum layer under conditions com-monly used in experiments. The healing of the holes isexpected to occur when the aluminum layer is relativelyfully oxidized. This suggests that the transient holes neverform during oxidation of thick aluminum layers becausethe surface aluminum is always fully oxidized before anyaluminum region is completely depleted to initiate a hole.The simulations also suggest that holes cannot be healedduring prolonged oxidation of very thin aluminum layersbecause when the aluminum is quickly fully oxidized, thedriving force for the hole healing is saturated and thereforethe hole ceases to further shrink. The transient hole forma-tion process identified above is consistent with the earlyresults of the MD simulations that rough (pinhole contain-ing) AlOx layers are obtained at aluminum layer thick-

-1500

-1200

-900

0

-600

-300

Ene

rgy

chan

ge d

urin

g an

neal

ing

(eV

)

0 10987654321

hole size r = 3 Å

hole size r = 0 Å

hole formation

Time (ps)

Fig. 8. Change of total system potential energy as a function of timeduring the annealing process shown in Fig. 5.

nesses around 2.5 A whereas continuous AlOx layers areobtained at aluminum layer thicknesses above � 6 A [16].

It can be seen that the thinnest continuous metal oxidelayer must be grown in the transient hole formation regime.Simulations then give important guidelines for reducing theoxide layer thickness by using full oxidation and the equi-librium-promoting annealing processes. It should be notedthat the transient hole formation was discovered in theAlOx-on-Ni65Co20Fe15 system. However, the phenomenonis general and likely to occur in other metal oxide on othermetal systems since metal oxides usually have much highercohesive energies than the reduced metals. It is thereforeenergetically favorable for metal oxides to form clusterson metals rather than a thin layer with atomic scalethickness.

4.2. Comparisons with experimental observations

A direct experimental observation of the atomic scalestructure of the AlOx-on-Ni65Co20Fe15 system has not beenfound in literature. However, Petford-Long et al., have car-ried out extensive HTEM and 3DAP experiments to exam-ine atomic scale structure of the AlOx oxide formed fromoxidation of 6 A thick aluminum layer on Co90Ni10 [17].Their experiments indicated that under the under-oxidationconditions, the AlOx layer exhibited discontinuous islandswith significant areas of the Co90Ni10 surface uncovered.This phenomenon closely corresponds to the formationof big holes in the oxide layer in the under-oxidized sam-ples. Annealing of these under-oxidized samples was foundto cause the AlOx islands to spread to form a networkalong the grain boundaries. Note that the experimentalannealing involved further oxidation. These experimentalobservations appear similar to the results of the simu-lations.

The experiments further indicated that more fully oxida-tion of the aluminum surface produced a more continuousAlOx layer. However, these as-grown films still containedholes on the scale of roughly 10 nm. The oxygen composi-tion in the AlOx layer, on the other hand, was found to bestill far below the one defined by the fully oxidized AlO1.5

(Al2O3). These holes were eliminated and a continuousAlO1.5 layer finally formed when the samples wereannealed. This means that oxygen diffused from furtheraway to complete the oxidation and that the layer laterallyspread to fill the holes. These observations are all consis-tent with the simulations.

5. Conclusions

A quinternary variable charge molecular dynamics sim-ulation method has been applied to simulate the growth ofa thin (�6 A thick aluminum prior to oxidation) AlOx spintunnel barrier layer on a Ni65Co20Fe15 surface. The resultsindicate that holes always form in the AlOx layer duringthe initial oxidation of the aluminum surface. Such holesare quite stable and are not seen to be eliminated by


annealing. However, these holes become unstable duringcontinued oxidation and are seen to be eliminated whenthe surface is fully oxidized. Thermodynamic analysis indi-cates that there exists a critical oxidation state above whichthe shrinkage of the holes reduces the total system potentialenergy. As a result, the hole healing process is driven bythermodynamics. This transient hole formation mechanismin many ways accounts for the experimental observation ofthe AlOx islands in under-oxidized samples and the smoothAlOx layer in fully oxidized, annealed samples.

Acknowledgements

This work was supported by NSF under Grant DMI-0214719, DARPA/USAAMC under contract W31P4Q-05-C-R141, and DARPA/ONR under Grant N00014-03-C-0288.

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transient hole formation during the growth of thin metal oxide layers

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