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Current-driven insulator–conductor transition
and nonvolatile memory in chromium-doped
SrTiO_3 single crystals
Watanabe, Yukio
IBM Research, Zurich Research Laboratory
J. G. Bednorz
IBM Research, Zurich Research Laboratory
A. Bietsch
IBM Research, Zurich Research Laboratory
Ch. Gerber
IBM Research, Zurich Research Laboratory
他
http://hdl.handle.net/2324/4354929
出版情報：Applied physics letters. 78 (23), pp.3738-3740, 2001-06-04. American Institute of
Physics
バージョン：
権利関係：(C) 2001 American Institute of Physics.
Current-driven insulator–conductor transition and nonvolatile memory
in chromium-doped SrTiO
3
single crystals
Y. Watanabe,
a)
J. G. Bednorz,
b)
A. Bietsch, Ch. Gerber, D. Widmer, and A. Beck
c)
IBM Research, Zurich Research Laboratory, 8803 Ru
¨
schlikon, Switzerland
S. J. Wind
IBM Research, T. J. Watson Research Center, Route 134, Yorktown Heights, New York 10598
共Received 5 March 2001; accepted for publication 12 April 2001兲
Materials showing reversible resistive switching are attractive for today’s semiconductor technology
with its wide interest in nonvolatile random-access memories. In doped SrTiO
3
single crystals, we
found a dc-current-induced reversible insulator–conductor transition with resistance changes of up
to five orders of magnitude. This conducting state allows extremely reproducible switching between
different impedance states by current pulses with a performance required for nonvolatile memories.
The results indicate a type of charge-induced bulk electronic change as a prerequisite for the
memory effect, scaling down to nanometer-range electrode sizes in thin films. © 2001 American
Institute of Physics. 关DOI: 10.1063/1.1377617兴
Reversible resistive switching processes occurring in
thin films of amorphous semiconductors,
1
polymers,
2–4
and
ZnSe–Ge heterostructures
5
have engendered strong interest
in these materials for application in nonvolatile memories.
The memory behavior of oxides, based on current-induced
bistable resistance effects or voltage-controlled negative re-
sistance phenomena, as observed in compounds such as
Nb
2
O
5
,
6
TiO
2
,
7
Ta
2
O
5
,
8
and NiO,
9
has been studied in all-
oxide thin-film heterostructures involving ferroelectrics
10
and simple metal–insulator–metal 共MIM兲 structures.
11
The
latter showed memory retention times exceeding 18 months.
If bulk electronic mechanisms are considered to be of rel-
evance for the memory effect, they involve charge-transfer
processes including field and impact ionization of traps
8,12
and refilling by electron injection. To clarify whether the
switching effect and the current transport across the insulator
are mediated by microstructural defects in the thin films, we
examined Cr-doped SrTiO
3
crystals as a model system.
We used 共100兲-oriented crystals doped with 0.2% Cr
grown by flame fusion and thin films of SrZrO
3
doped with
0.2% Cr grown by pulsed-laser deposition.
11
Unless stated
otherwise, the results are obtained from a crystal with a
thickness of 10
m. The initial resistance of the crystal was
1G⍀ up to 100 V at 4.5 K 关Fig. 1共a兲兴. When sweeping the
voltage to 200 V, the resistance suddenly drops, and a hys-
teretic current–voltage (I–V) characteristic 共IVC兲 develops.
Figure 1共b兲 displays the effect at 296 K. Subsequent stressing
by pulsed or dc voltages of 50–90 V reduces the resistance
further by orders of magnitude and creates a conductive
state, which then enables memory switching between differ-
ent impedance levels. The resistance of the conductive state
is of the order of 500 ⍀ or5k⍀ at 4.5 K and 200 ⍀ or2k⍀
at 300 K for the low- 共on兲 and high-impedance 共off兲 state,
respectively, and an electrode diameter of 0.8 mm 共Au兲.
Typical IVCs of the conductive state after positive and
negative pulses applied in current-control mode are shown in
Fig. 1共c兲. The stability of the two levels was found to depend
on the current density and the total charge passing through
the crystal rather than on the amplitude of the voltage. A 1
ms pulse of 3.5 mA 共‘‘write’’兲 sets the system to the on state,
anda1mspulse of ⫺3.55 mA 共‘‘erase’’兲 sets it back to the
off state. The IVCs for two write and erase cycles were mea-
sured immediately and 5 h after application of the respective
pulse, Fig. 1共c兲. The perfect overlay demonstrates a superb
reproducibility. The resistance change is ‘‘read’’ by applying
a 1 ms pulse of ⫺0.5 V in voltage-control mode 关inset in Fig.
1共c兲兴. The data read immediately and 24 h after the write/
erase pulses show the perfect memory retention.
Between 4.5 and 10 K, the IVCs remain unchanged,
whereas at higher temperatures the resistance is increased
with the two states having different temperature coefficients,
Fig. 1共d兲. However, the temperature 共T兲 dependence of a cur-
rent 共I兲 in the off or on state as recorded during I –T scans
exhibits a more complex behavior. Nonlinear changes during
heating/cooling between 4.2 and 300 K originate from carrier
trapping or emission, as known from thermally stimulated
current spectroscopy.
13
Even at low voltages 共1mV兲 does the
on state tend to change into the off state during T cycles. At
extremely low scan speeds, the latter can even revert to the
insulating state. This transition, accompanied by carrier
emission, and the method of creating a conductive state by
exposing an insulating crystal to high voltages or high-
current densities suggest that this conductive state originates
from an excess of injected carriers. If so, creation of this
state should also be possible by a low voltage and small
currents. This was verified in thin-film experiments. The ap-
plication of ⫹0.4 V for a long period 共current density ⬃20
mA/cm
2
兲 increases the resistance of a SrZrO
3
:Cr thin film by
three orders of magnitude, Fig. 2共a兲, with the corresponding
IVCs shown in Fig. 2共b兲. Reversing I at ⫺0.4 V for a long
time brings the system back to the conductive state, a re-
quirement for memory switching between two or several
levels.
a兲
Present address: Department of Electrical Engineering, Kyushu Institute of
Technology, Sensui 1-1, Tobata, Kitakyushu, Fukuoka 804-8550, Japan.
b兲
Author to whom correspondence should be addressed; electronic mail:
bed@zurich.ibm.com
c兲
Present address: JDS Uniphase, Binzstrasse 17, 8045 Zurich, Switzerland.
APPLIED PHYSICS LETTERS VOLUME 78, NUMBER 23 4 JUNE 2001
37380003-6951/2001/78(23)/3738/3/$18.00 © 2001 American Institute of Physics
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Once the conductive state is established, crystals display
characteristics similar to the thin films,
11
underlining the po-
tential of these oxides as nonvolatile random-access memo-
ries 共RAMs兲. The record of the on and off states for hundreds
of write–read–erase cycles shows a remarkable stability
关Fig. 3共a兲兴 with no change of the readout signal after more
than 10
5
readouts 关Fig. 3共a兲, inset兴. A similar stability is con-
firmed in a SrTiO
3
:Cr crystal with a greater thickness, i.e.,
0.5 mm. As apparent in thin-film experiments,
11
write pulses
of different current amplitudes can create multiple on states.
At 4.5 K we could write and erase up to six states by 1 ms
pulses of 1.5, 2.2, 2.5, 2.7, 3, 3.5, and ⫺3.55 mA, Fig. 3共b兲.
To demonstrate the stability of the different levels, each write
or erase pulse was followed by 10
3
readout pulses at ⫺0.5 V.
Multiple states, not restricted to low T but also obtained at
300 K, are reproducibly controlled over 10
3
write/erase
cycles with a total of 10
4
read pulses.
The scalability of this remarkable performance will be of
great relevance for the technological realization as nonvola-
tile RAM. We have demonstrated well-defined switching and
memory behavior in SrZrO
3
:Cr films 共Fig. 4兲 with electrode
FIG. 1. IVCs recorded on a SrTiO
3
:Cr single crystal 共thickness, 10
m兲,
showing the change from an insulating 共dashed line兲 to a conducting state
共solid line兲 by current stress at 共a兲 4.5 K and 共b兲 296 K. IVCs of the on state
共solid line兲 and off state 共dashed line兲 at 共c兲 4.53 K and 共d兲 10 and 80 K. In
共c兲, the curves for two write and erase cycles 共four records per state兲 are
measured immediately and 5 h after the respective pulses. Inset shows ten
readouts of the corresponding states of the memory recorded immediately
after 共black circles兲 the write or erase pulses and 24 h later 共shaded circles兲.
FIG. 2. 共a兲 Two successively measured I –t characteristics of a SrZrO
3
:Cr
film 共100 nm兲 with a SrRuO
3
bottom electrode and a 0.5 mm
2
Au top
electrode. 共b兲 IVC before 共1兲 and after 共2兲 the I –t measurements 共at RT兲.
FIG. 3. 共a兲 Stability of the switching characteristics of a single crystal dur-
ing more than 500 write/erase cycles at 296 K. After each write 共⫹7.7 mA兲
or erase 共⫺8mA兲 pulse, five readout pulses 共⫺0.3 V兲 are applied. Inset
shows the stability of 10
5
readouts of an off state 共I兲 and on state 共II兲 at 296
K following a single write or erase pulse. 共b兲 Multiple on states obtained at
4.5 K after write pulses 共䉱兲 with different current amplitudes of 1.5, 2.2,
2.5, 2.7, 3.0, and 3.5 mA and erase pulses 共䊐兲 with a fixed amplitude of
⫺3.55 mA. The 䉱 and 䊐 pulses are shown only schematically on top. Each
pulse is followed by 1000 readouts 共⫺0.5 V兲. Inset shows readout currents
for 3.5 write–erase cycles with a total of 42 000 pulses.
3739Appl. Phys. Lett., Vol. 78, No. 23, 4 June 2001 Watanabe
et al.
Downloaded 08 Jun 2001 to 150.69.149.81. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp
共Ti/Pt兲 sizes down to 100⫻ 100 nm
2
. These contacts were
addressed via the conducting tip of an atomic-force micro-
scope 共AFM兲.
14
Currents scaled linearly with the electrode
area as estimated from experiments on mm-scale electrodes.
The results on crystals provide insight into the processes
leading to the complex behavior related to the memory ef-
fect. The existence of a high initial resistance up to 100 V
confirms that the bulk, and not the interface, determines the
current flow across the insulator and that the transition to the
conducting state originates from a change in the bulk prop-
erty. As this transition and well-defined memory switching
with long-time stability occur in single crystals having 0.01–
0.5 mm electrode separation, as well as in thin films with
submicron electrodes, a significant contribution to this phe-
nomenon by defects like grain boundaries can be excluded.
The stressing process required to create the conducting state,
the emission of trapped carriers, and giant conductance
changes during I–T scans indicate that this state is created
by carrier trapping in the bulk.
15,16
Likewise, the current con-
duction in SiO
2
could be explained by the injected-carrier-
induced change of the local electronic structure at
impurities,
17
suggesting a similar mechanism in perovskite
oxides. This is in contrast to thermally activated processes
for degradation and recovery of the bulk resistance
18
based
on oxygen–vacancy migration
19
because the conducting state
can even be obtained through stressing at 4 K. In addition,
various types of 共orbital兲 ordering occurring in perovskites,
such as ion ordering in ferro- and antiferroelectrics or the
spin-charge ordering in (La, Sr兲MnO
3
共Ref. 20兲 and high-T
C
superconductors,
21
suggest that changes of the electronic
structure, or a local variation of the oxygen polyhedra, or a
combination of both, can be induced by carrier injection.
Indeed, the destruction of charge order in (La, Sr兲MnO
3
by
current injection
22
resulted in an enhanced conductance.
When in SrTiO
3
crystals the bulk is sufficiently conduc-
tive, the interface, i.e., the Schottky barrier, starts to control
the current, as confirmed by the double Schottky-like IVCs
关Figs. 1共c兲 and 1共d兲兴 and the typical turn-on voltage of 1 V at
4.5 K. As apparent from the IVCs of the off states in Figs.
1共c兲 and 1共d兲, the stressing creates an asymmetry between
the two barriers of the otherwise symmetric MIM structure.
The switching can then be attributed to carrier-injection-
induced degradation and carrier-emission-induced recovery
of the Schottky barrier that is more resistive than the other.
The long-time memory retention probably needs an explana-
tion based on the specific valence properties of the perovs-
kites. This first demonstration of the memory effect in single
crystals allows the physics behind the effect to be studied by
various methods of solid-state physics. Electron-spin reso-
nance and optical absorption, used to determine the various
electronic levels of transition-metal impurities in
SrTiO
3
,
23–25
and photoconductivity experiments, which re-
vealed memory behavior at low T.
26
In conclusion, our results introduce a reversible
insulator–conductor transition, which implies that in the
analysis of insulating films along traditional lines, an inter-
pretation of the leakage–current phenomenon, where an ap-
parent breakdown can be electrically restored, is appropriate.
We gratefully acknowledge P. E. Blo
¨
chl for discussions,
J. W. M. Seo for the 10
m samples, and H. Rothuizen for
the lithography design. Y.W. is supported by Japanese Min-
istry of Education Grant-in-Aid No. 12134208.
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FIG. 4. 共a兲 AFM image of Ti/Pt electrodes (500⫻ 500 nm
2
兲 on a 3-nm-thick
SrZrO
3
:Cr film 共defined by electron-beam lithography兲. 共b兲 Switching prop-
erties 共at RT兲 on pads addressed by an AFM with write 共w兲, erase 共e兲, and
read 共r兲 by 1 ms pulses. 共I兲 labels the on state and 共II兲 the off state.
3740 Appl. Phys. Lett., Vol. 78, No. 23, 4 June 2001 Watanabe
et al.
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