Inelastic electron tunnelling spectroscopy of magnetic tunnel junctions with AlN and AlON barriers

Article abstract of DOI:10.1016/j.jmmm.2003.12.1195

We report on electrical transport properties of magnetic tunnel junctions (MTJ) with AlN and AlON barriers. A series of junctions was made with the structure MnFe/NiFe/barrier/NiFe, where the type of the barrier layer was systematically varied. These samples were compared with a reference sample of MTJs with an alumina (Al₂O₃) barrier. Particularly, the bias dependence of tunnelling magneto resistance was found to be less pronounced in nitride barriers than in alumina, which was attributed to barrier-specific phonon-assisted inelastic electron tunnelling.

Full text of DOI:10.1016/j.jmmm.2003.12.1195

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) 1952–1953
Inelastic electron tunnelling spectroscopyof magnetic tunnel
junctions with AlN and AlON barriers
a,
M. Sharma *, S.-Y. Bae , S.X. Wang
b
b
a
Hewlett-Packard Laboratories, 1501 Page Mill Road, Palo Alto, CA 94304, USA
Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA
b
Abstract
We report on electrical transport properties of magnetic tunnel junctions (MTJ) with AlN and AlON barriers. A
series of junctions was made with the structure MnFe/NiFe/barrier/NiFe, where the type of the barrier layer was
systematically varied. These samples were compared with a reference sample of MTJs with an alumina ðAl2O3Þ barrier.
Particularly, the bias dependence of tunnelling magneto resistance was found to be less pronounced in nitride barriers
than in alumina, which was attributed to barrier-specific phonon-assisted inelastic electron tunnelling.
r 2004 Elsevier B.V. All rights reserved.
PACS: 73.40.Gk; 75.70.ꢀi; 85.30.Mn; 85.70.Kh
Keywords: Inelastic electron tunnelling spectroscopy; Magnetic tunnel junctions; Alumina; Aluminum nitride
1
. Introduction
barrier. Particularly, the bias dependence of tunnelling
magneto resistance (TMR) was found to be less
pronounced in nitride barriers than in alumina, which
was attributed to barrier-specific phonon-assisted in-
elastic electron tunnelling.
Due to their potential applications as magnetic read
head sensors and non-volatile memories, there has been
renewed interest in magnetic tunnel junctions (MTJs).
Formation of high-qualityultra-thin tunnel barrier in
MTJ is probablythe most challenging step in achieving
high performance MTJs. While alumina ðAl2O3Þ is the
most popular material for the barriers in MTJ,
tremendous efforts have been made in search of
alternative barrier materials [1–3] which maypromise
to supplement the drawbacks of alumina, such as high
junction resistance and high pin-hole densities. We
report on electrical transport properties of MTJs with
AlN and AlON barriers. A series of junctions was made
with the structure MnFe/NiFe/barrier/NiFe, where the
type and thickness of the barrier layer was systematically
varied [1,3]. These samples were compared with a
reference sample of MTJs with an alumina ðAl2O3Þ
2
. Experimental procedure
MTJs with the structure of Ta 5 nm=NiFe 6 nm=
MnFe 10 nm=NiFe 12 nm=barrier=NiFe 8 nm=Ta
5 nm were fabricated byUHV sputter deposition
followed byphotolithographic microfabrication. Tunnel
barriers were formed from 1:25 nm thick Al, through in
situ plasma oxidation, oxy-nitridation or nitridation.
TMR values observed were up to 16% in nitride
barriers and 19% in alumina. Transport properties
of the MTJ were characterized byDC four-probe
method. Inelastic electron tunnelling spectra (IETS)
employing
a standard AC lock-in technique and
custom-built electronic circuit measured at cryo-
genic temperatures to understand the systematic varia-
tion of transport characteristics with the barrier
structure.
*
Corresponding author. Tel.: +1-650-236-8030; fax: +1-
6
0
50-857-5559.
E-mail address: manish.sharma@hp.com (M. Sharma).
304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2003.12.1195

ARTICLE IN PRESS
M. Sharma et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 1952–1953
1953
Fig. 2. IETS spectra of the junction A and B measured at 15 K:
The spectra shown are for the parallel magnetization.
Fig. 1. Bias dependence of TMR in MTJs with various barriers.
Each curve has been normalized with TMRmax as indicated in
the legend.
barriers. Even though the signal related to the
phonon excitation in the barrier is not clearlyidentified
in Fig. 2, the strong contrast in the binding energies of
Al–O and Al–N should give rise to different modes of
phonon-assisted tunnelling in these junctions. In this
regard, the phonon-related features in the tunnel barrier
in addition to the magnon features mayalso playan
important role in spin-dependent transport in these
junctions [4].
3
. Results and discussion
Fig. 1 displays the bias dependence of TMR in
the MTJ with various barriers measured at room
temperature ðA : O2 for 2:5 min; B : O22N2 for
2:5 min; C : N2 for 2:5 min; D : N2 for 5:0 min;
E : N2 for 7:5 minÞ: The bias dependence of TMR
becomes systematically weakened as the degree of
nitridation increases.
To understand the difference in the bias dependence
of TMR in various barriers, we took the IETS at various
temperatures. Fig. 2 shows the spectra of the two MTJs
4
. Conclusion
We prepared various MTJs with AlO, AlON and AlN
(
A and B) measured at 15 K: A typical MTJ manifests as
tunnel barrier. These samples showed the systematic
variation in the transport properties, which correlated
with the degree of nitridation of the barrier. Particularly,
the bias dependence of TMR was found to be less
pronounced in nitride barriers than in alumina, which
was attributed to barrier-specific phonon-assisted in-
elastic electron tunnelling
sharp features at low bias, which have been attributed to
magnon-assisted tunnelling. This type of inelastic
electron tunnelling has explained the degradation of
TMR at finite biases [2].
While the characteristic magnon features are common
in both barriers, the energyscales involved in the
magnon excitation clearlydiffer in the two junctions. In
MTJs with alumina barrier, we observed inelastic
tunnelling features occurring over a broad range of bias
References
(
FWHM B100 mV). In contrast, these features in MTJs
with a nitride barrier occur at lower bias and were very
sharp (FWHM B20 mV). This difference mayexplain
the initiallysmaller zero-bias TMR and the lesser
dependence of TMR at high bias in MTJs with nitride
[
[
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2] M. Sharma, et al., Phys. Rev. Lett. 82 (1999) 616.
[3] M. Sharma, et al., Appl. Phys. Lett. 77 (2000) 2219.
[4] S. Zhang, et al., Phys. Rev. Lett. 79 (1997) 3744.