The study of conductance in magnetic tunnel junctions with a thin MgObarrier: The effect of Ar pressure on tunnel magnetoresistance andresistance area productA. Zaleski, J. Wrona, M. Czapkiewicz, W. Skowroński, J. Kanak et al. Citation: J. Appl. Phys. 111, 033903 (2012); doi: 10.1063/1.3679543 View online: http://dx.doi.org/10.1063/1.3679543 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v111/i3 Published by the American Institute of Physics. Related ArticlesElectrical transport across metal/two-dimensional carbon junctions: Edge versus side contacts AIP Advances 2, 012132 (2012) Epitaxial Cr on n-SrTiO3(001)—An ideal Ohmic contact Appl. Phys. Lett. 100, 052106 (2012) Contact transport of focused ion beam-deposited Pt to Si nanowires: From measurement to understanding Appl. Phys. Lett. 100, 053503 (2012) Comment on “Simulation of Schottky and Ohmic contacts on CdTe” [J. Appl. Phys. 109, 014509 (2011)] J. Appl. Phys. 111, 026102 (2012) Preparation of Ohmic contacts to GaAs/AlGaAs-core/shell-nanowires Appl. Phys. Lett. 100, 042103 (2012) Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

The study of conductance in magnetic tunnel junctions with a thinMgO barrier: The effect of Ar pressure on tunnel magnetoresistanceand resistance area product

A. Zaleski,1,2,a) J. Wrona,1 M. Czapkiewicz,1 W. Skowronski,1 J. Kanak,1 and T. Stobiecki11Department of Electronics, AGH University of Science and Technology, Al. Mickiewicza 30,Krakow 30-059, Poland2Faculty of Physics and Applied Computer Science, AGH University of Science and Technology,al. Mickiewicza 30, Krakow 30-059, Poland

(Received 28 September 2011; accepted 23 December 2011; published online 3 February 2012)

The thickness dependence of tunnel magnetoresistance and resistance area product in Co40Fe40B20/

MgO wedge/Co40Fe40B20 magnetic tunnel junctions (MTJs) has been studied for multiple Ar

partial pressure (PAr) values during MgO sputtering. The extension of the simple equivalent circuit

model [B. Oliver et al., J. Appl. Phys. 91, 4348 (2002)] has been suggested in order to include

different transport mechanism contributions to the overall conductance of the MTJ as a function of

the MgO barrier thickness. Parameters of the model, used for quantitative description of the

conductivity of unpatterned MTJ stacks, have been analyzed as a function of PAr. VC 2012 AmericanInstitute of Physics. [doi:10.1063/1.3679543]

INTRODUCTION

Magnetic tunnel junctions (MTJs) based on CoFeB/

MgO/CoFeB trilayers are excellent candidates for future

spintronic devices, because they have a high tunnel magneto-

resistance (TMR) ratio and low resistance area (RA) prod-

uct.1 The controlled deposition of the MgO barrier in such

structures is one of the key steps to achieving optimal MTJ

parameters. The barrier smoothness, thickness (dMgO), and

(001) CoFeB/MgO/CoFeB texture play an important role,

particularly for the low RA product region with

dMgO< 1 nm, due to the complex problem of the MgO crys-

tallization on the amorphous CoFeB layer.2,3 It is well

known that optimal MgO deposition is the crucial factor in

obtaining high TMR and low RA product values.3–6

The tunneling type of carrier transport is characterized by

an exponential increase in resistance with increasing barrier

thickness that corresponds to the linear dependence of the RA

product logarithm as a function of dMgO. Fe/MgO/Fe MTJ

samples prepared using molecular beam epitaxy showed devi-

ation from a linear dependence of the RA product (in the log

scale) below dMgO¼ 1.5 nm, as reported by Yuasa et al.7

Later, it was demonstrated that such deviation may start below

dMgO¼ 1.0 nm by slightly changing the stack structure and

keeping the same growth conditions.8 For the sputtered

CoFeB/Mg/MgO/CoFeB MTJs, pure tunneling behavior was

reported for dMgO higher than 0.70 nm (see Fig. 1(b) in

Ref. 9). In our earlier studies, the deviation from a linear

dependence of the log RA product versus dMgO was similarly

observed at dMgO< 0.76 nm for sputtered CoFeB/MgO/

CoFeB wedge samples.10 Generally, it is known that, between

3 and 5 monolayers (MLs) of MgO (1 ML¼ 0.21 nm), there

is a transition between the amorphous and (001)-crystallized

MgO barrier in post-annealed MTJs.2

The production of several-MLs-thick barriers using RF

sputtering appears to be a big technological challenge.

Therefore, the characterization of inhomogeneities (e.g., hot-

spots) and defects (e.g., pinholes) in the barrier on the wafer

level is of importance.

Current-in-plane tunneling (CIPT) is a widely used tech-

nique for measuring the TMR and RA product on blanket

MTJ films without patterning.11 This technique, in combina-

tion with an X-ray diffraction and atomic force microscopy,

was used for studying the influence of PAr during MgO depo-

sition on the barrier thickness dependence of the TMR and

RA product.12 However, further analysis of the obtained data

was necessary in order to explain changes of the TMR and

RA product in the ultra-low dMgO range (i.e., below

0.76 nm). Several authors used simple equivalent circuit

models to describe transport mechanisms in the MTJ nano-

pillars with a thin tunnel barrier.13–17 This model assumes

that measured resistance of the MTJ can be expressed as a

combination of one tunneling resistance and one resistance

of the metallic nanobridge connected in parallel.

Nanobridges (or nanoclusters as they are called in

Ref. 18) are widely used in TMR read-heads to improve reli-

ability and to reduce the resistance at the cost of TMR signal

reduction.14,19 Komagaki et al.16 estimated their density in

[parts/lm2] by fitting a nanopillar breakdown voltage de-

pendence using Poisson distribution. They observed an expo-

nential decrease of nanobridge density with increasing dMgO.

In this paper, we suggest a simple approach that allows

one to describe quantitatively the effect of dMgO and PAr on

nanobridge contribution and, thereby, transport properties of

MTJ stacks, using an extended equivalent circuit model for

interpretation of CIPT results.

EXPERIMENTAL

Multilayer structures used in this study were deposited

onto thermally oxidized Si (100) wafers in a Timaris PVD

a)Author to whom correspondence should be addressed. Electronic mail:

zaleski@agh.edu.pl. Fax: þ48 (0)12 633 2398.

0021-8979/2012/111(3)/033903/5/$30.00 VC 2012 American Institute of Physics111, 033903-1

JOURNAL OF APPLIED PHYSICS 111, 033903 (2012)

cluster tool system from Singulus Technologies. All the

metallic layers were deposited by magnetron-dc-sputtering,

whereas the insulating MgO layer was rf-sputtered directly

from a sintered MgO target. The structure of the investigated

MTJs was the following: substrate/seed layers/PtMn(16)/

Co70Fe30(2.0)/Ru(0.9)/Co40Fe40B20(2.3)/MgO wedge/Co40Fe40

B20(2.3)/capping layers (thickness in nanometers). The MgO

wedge layer was sputtered using linear dynamic deposition

wedge technology. The applied working rf power density to the

MgO target during deposition was fixed at 6.6 W/cm2, while

the Ar partial pressure was varied from 1 mTorr to 15 mTorr.

After deposition, the MTJ structure was annealed in a high-

vacuum furnace at 360 �C for 2 h in a magnetic field of 10 kOe

(796 kA/m). Detailed information on sample preparation can be

found in Ref. 12.

Unpatterned MTJ wafers were characterized by CIPT

technique at room temperature. CIPT is an efficient tool,

which allows one to get information about electrical conduc-

tivity of the MTJ stack without a patterning procedure.11,20 It

uses the resistance versus magnetic field (R-H) minor loops

measured using probes with different spacing and special fit-

ting procedures20 in order to obtain values for RA product,

TMR, as well as minor loop coercivity and shift field. It is

noteworthy that information obtained using CIPT gives aver-

age values over an approximate length of 10 lm reciprocal

to wedge gradient direction. Further, we will focus on the

thickness dependence of TMR and RA product (in the low

resistance state) for samples with the MgO barrier sputtered

at PAr¼ 1, 2, 3.8, 5.6, and 15 mTorr.

Model description

An extension to an equivalent circuit model is proposed

for interpreting the relationship between the RA product and

dMgO. We have analyzed the MgO tunnel barrier thickness

dependence of the inverse of the RA product (further 1/RA

is called conductance) of the MTJ wafers measured using

CIPT. We have considered two conduction channels that cor-

respond to two resistors connected in parallel, characterized

by the tunneling and metallic transport mechanisms, respec-

tively. A peculiarity of our extended model is that we have

introduced the weight function as(dMgO), which has been

used to evaluate the contribution of tunneling and metallic

channel conductance for varied dMgO.

We have developed the following procedure in order to

evaluate as(dMgO) from the CIPT data.

Firstly, the RA product dependence on TMR has been

linearly extrapolated to the zero TMR level (TMR¼ 0%).

This gives us the value of the nanobridge resistance-area

product RA0. The procedure was first suggested by Oliver

et al. and applied for MTJs with an oxidized Al barrier.15

Similarly, the linear extrapolation of dMgO dependence on

TMR gives us the minimal nanobridge length c0. Figure 1

shows such extrapolations for all sets of CIPT data, corre-

sponding to PAr¼ 1, 2, 3.8, 5.6, and 15 mTorr. Extrapolated

data of RA0 and c0 have been shown in Table I.

Secondly, the relative contribution of nanobridges as has

been derived. Assuming that the sum of the nanobridges and

tunnel barrier (at) contributions is expressed as as þ at ¼ 1,

the total RA product is equal to21

RA ¼ RAt=at � RAs=as

RAt=at þ RAs=as¼ RAt � RAs

RAtas þ RAsat:

The nanobridge resistance-area product is given by

RAs¼RA0c0/dMgO. Values of RAt can be obtained by the lin-

ear extrapolation of log RA versus dMgO data to the low

thickness range.

Following that, the relative contribution of nanobridge

as can be expressed as

as ¼RAsðRA� RAtÞRAðRAs � RAtÞ

: (1)

FIG. 1. (Color online) (a) TMR vs RA product for all studied PAr. Linear extrapolation shows a way of determining nanobridge resistance RA0. (b) TMR vs

MgO thickness. Linear extrapolation shows a way of determining nanobridge length c0.

TABLE I. Nanobridge length c0 and nanobridge resistance RA0 as a function

of Ar partial pressure during MgO sputtering (PAr).

PAr [mTorr] c0 [nm] RA0 [Xlm2]

1.0 0.66 6 0.04 0.74 6 0.13

2.0 0.65 6 0.03 0.49 6 0.08

3.8 0.63 6 0.02 0.57 6 0.06

5.6 0.74 6 0.04 0.51 6 0.04

15.0 0.78 6 0.05 0.36 6 0.15

033903-2 Zaleski et al. J. Appl. Phys. 111, 033903 (2012)

The set of as values for different dMgO can be calculated with

Eq. (1) using the total RA product value measured by CIPT.

Thirdly, the nanobridge contributions as for a set of

measured dMgO points has been fitted with the exponential

decay function as(dMgO) in Eq. (2) in order to obtain decay

parameter t0 and exponential prefactor n.

asðdMgOÞ ¼ n � exp

�� dMgO � c0

t0

�: (2)

Finally, the conductance as a function of the barrier thick-

ness has been fitted using expression (3), that is the essence

of our extended equivalent circuit model

1

RAðdMgOÞ ¼ Gtð1� asðdMgOÞÞ � expð�2k0ðdMgO � c0ÞÞ

þ asðdMgOÞ � c0

RA0 � dMgO: (3)

The first term corresponds to the tunneling mechanism via

the MgO barrier (possibly affected by hot spots). Its contri-

bution changes continuously as a function of dMgO via pre-

factor (1 – as(dMgO)). For the high dMgO range, this prefactor

is close to one and its physical meaning is that tunneling is

the dominant mechanism of MTJ conductance.

The second term is responsible for the metallic transport

via nanobridges with different lengths for dMgO normalized

by the c0 parameter. Weight function as(dMgO) controls the

contribution of these mechanisms.

The schematic drawing of the MgO wedge and equiva-

lent circuit model have been presented in Fig. 2. The wavy

lines are a schematic representation of the interface character

as shown on a high resolution transmission electron micro-

scope (HRTEM) cross-section image (Fig. 2(b)). The

HRTEM data shows that, for the nominal thickness of the

tunnel barrier dMgO> 0.76 nm, the MgO layer is crystalline

and rather wavy, but smooth on an atomic scale (detailed

results to be presented elsewhere).

Three regions of the MgO wedged shape barrier can be

distinguished in our model. For the ultrathin dMgO below the

nanobridge length c0, the transport is purely metallic and no

tunnel contribution is present (the left side in Fig. 2(a)).

Within the medium range of dMgO, above c0, some part of

the electrons tunnel via the barrier and the rest flow via nano-

bridges, which are schematically shown in Fig. 2(a) as

shaded regions, where the wavy profiles overlap. The contri-

bution of the nanobridges decreases with increasing barrier

thickness. The extended equivalent circuit has been used to

describe conductance as a function of dMgO in this region.

Finally, for the high dMgO range (corresponding to the

HRTEM cross-section shown in Fig. 2(b)), conductance is

determined only by tunneling and there is no metallic contact

between the electrodes (the wavy profiles do not overlap).

Fitting the thickness dependence of the conductance

using expression (3), we obtain an intrinsic tunneling con-

ductance Gt and tunneling electron wave vector in the barrier

k0. We keep earlier input parameters n, c0, t0, and RA0 fixed.

Discussion of the results has been presented further on.

Free electron approximation (considering only the com-

ponents of the wave vectors that are perpendicular to the

CoFeB-MgO interface) allows one to derive the values for

the barrier height above Fermi energy Ub applying the fol-

lowing relationship, similarly to Yuasa et al.:7

Ub ¼ðk0�hÞ2

2meff: (4)

We assume effective electron mass in the barrier to be

meff¼ 0.4�m0, with m0 being the electron rest mass.22

FIG. 2. (Color online) (a) The schematic drawing of the MgO wedge cross-section of the MTJ and (b) a cross-sectional HRTEM image of a real MTJ stack

with the nominal thickness dMgO¼ 1.02 nm. The conductance of the mixed transport region above minimal nanobridge length c0 has been modeled using an

extended equivalent circuit model. The conductance in the tunneling region decreases exponentially with the dMgO thickness and exponential prefactor Gt

being the intrinsic tunneling conductance.

FIG. 3. (Color online) The tunnel barrier thickness dependence of the RA

product, PAr¼ 3.8 mTorr. The dashed line shows the tunneling fit (valid for

thick MgO above 0.76 nm); the solid line shows an extended equivalent

circuit fit (valid in the whole range).

033903-3 Zaleski et al. J. Appl. Phys. 111, 033903 (2012)

RESULTS AND DISCUSSION

Figure 3 shows an example of the tunnel barrier thickness

dependence of the RA product for PAr¼ 3.8 mTorr. The tun-

neling fit represents the known exponential dependence of the

RA product for thick enough barriers, so that the effective tun-

neling via the MgO barrier is the dominant transport mecha-

nism.1,12 This tunneling fit is analogous to the one presented

in Fig. 4 of our earlier study.12 A deviation from the linear de-

pendence of log RA was observed for dMgO< 0.76 nm for

sputtered CoFeB/MgO/CoFeB wedge samples.10

Linear extrapolations of TMR dependence of dMgO and

the RA product (see Fig. 1) allow one to obtain values for c0

and RA0, respectively. This procedure was performed with

CIPT data for all studied PAr. Obtained values for c0 and RA0

as a function of PAr have been summarized in Table I. High

errors in the case of the 15 mTorr sample have been caused

by the low quantity of experimental points. Two tendencies

are worth pointing out. For the nanobridge length analysis,

there are clearly two groups of values corresponding to the

low (PAr� 3.8 mTorr) and high pressures (PAr� 5.6 mTorr).

The lowest nanobridge length was obtained for PAr¼ 3.8 mTorr.

This stack was also considered as optimal in the earlier study.12

Nanobridge resistance shows a tendency to decrease with Ar

pressure.

Further, as has been calculated for those dMgO that have

a RA product lower than values obtained by linear extrapola-

tion of logRA(dMgO) to the low dMgO range (see Fig. 3 in

Ref. 12) and for different PAr using Eq. (1). Then, the set of

nanobridge contributions as has been fitted as a function of

dMgO using Eq. (2). The fits have been presented in Fig. 4(a).

Obtained values for decay parameter t0 for varied PAr

have been presented in Fig. 4(b). A clear distinction between

a low and high PAr range can be seen.

Further, the fits of the conductance (i.e., the inverse

resistance-area product) as a function of the barrier thickness

within an extended equivalent circuit model using Eq. (3)

and c0, RA0, n, and t0 as input parameters have been pre-

sented in Fig. 5.

Excellent agreement between fits and experimental data

was achieved. Two sets of output parameters (Gt and k0)

were obtained from those fits. Gt is the parameter describing

the intrinsic conductance in the tunneling regime (for

as¼ 0).

Figure 6 shows the influence of PAr on parameters

derived from fitting.

Parameter k0 allows us to calculate the energy barrier

height in the quasi-classical Wentzel–Kramers–Brillouin

(WKB) approximation using Eq. (4). Although it has been

shown that WKB-based models may give too low values for

the barrier height,22 we have focused on the influence of PAr

on the barrier parameters. The highest value of k0 (and Ub,

correspondingly) has been achieved for 3.8 mTorr. Inverse

intrinsic tunneling conductance (1/Gt) has a tendency to

decrease with increasing PAr in the low pressure range. For

both parameters, there is a pronounced difference between

high and low PAr ranges, which correlates with the x ray dif-

fraction data reported earlier.12 There, the MTJ samples

showed lower values of full width at high maximum rocking

curve MgO (002) peaks (higher interplanar distances) for

low pressure (up to 3.8 mTorr) contrary to the high pressure

range. Correlation between the effect of the MgO structure

properties on the wave vector in the barrier requires detailed

calculation of the influence of defect types on the band struc-

ture of CoFeB/MgO/CoFeB and is beyond the scope of this

paper.

FIG. 4. (Color online) (a) Nanobridge contribution as and (b) the decay parameter for nanobridge resistance contribution as a function of Ar partial pressure

during MgO sputtering. The dashed line is a guide for the eye separating low and high PAr ranges.

FIG. 5. (Color online) The inverse resistance-area product as a function of

MgO thickness for different Ar partial pressures during MgO sputtering

fitted using an extended equivalent circuit model.

033903-4 Zaleski et al. J. Appl. Phys. 111, 033903 (2012)

Nanobridge length c0 and nanobridge decay parameter

t0 are related to the barrier roughness averaged over the

CIPT probing area. Higher values for c0 and t0 for high pres-

sure range (Table I and Fig. 4(b)) are in concordance with

the higher roughness (RMS or Rq) parameters calculated

from the AFM scans of incomplete MTJ stacks sputtered at

PAr¼ 3.8 and 5.6 mTorr (the growth stopped at the MgO

layer) reported in Ref. 12. Similar results were obtained by

Shen et al.,5 where they observed more than double the

increase of Rq from 0.14 to 0.40 nm for 3.8 and 5.8 mTorr,

respectively.

CONCLUSIONS

In summary, we have introduced a simple procedure to an-

alyze CIPT data for MTJ wafers using an extended equivalent

circuit model, which is valid for a wide range of MgO barrier

thicknesses. Extrapolation of TMR¼ f(dMgO, RA) data to the

zero TMR level has led to values for the minimal nanobridge

length c0 and resistance RA0. Further analysis has led to the

thickness dependence of nanobridge contribution as for differ-

ent PAr. Conductance fits give us values for the tunneling elec-

tron wave vector (in the barrier) and intrinsic tunneling

conductance Gt. The suggested model, supported by structural

measurements, has allowed us to divide investigated samples

into two distinct groups deposited in low and high PAr.

ACKNOWLEDGMENTS

We would like to thank Singulus AG for multilayers

deposition, Professor S. van Dijken and Dr. L. Yao for pro-

viding us with HRTEM images. The study was co-financed

through the Swiss Contribution project NANOSPIN-PSPB-

045=2010. A.Z. was supported under SPINSWITCH Project

MRTN-CT-2006-035327 and AGH. W.S. and T.S would

like to thank the Foundation for Polish Science MPD. Pro-

gramme co-financed by the EU European Regional Develop-

ment Fund and the Polish Ministry of Science and Higher

Education grants (IP2010037970 and NN 515544538).

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FIG. 6. (Color online) (a) Electron wave vector in the MgO barrier k0, (b) barrier height Ub, and (c) inverse intrinsic tunneling conductivity 1/Gt as a function of Ar

partial pressure during MgO sputtering. Ub was calculated from the values of k0 obtained using an equivalent circuit model using quasi-classical approximation.

033903-5 Zaleski et al. J. Appl. Phys. 111, 033903 (2012)