APPLIED PHYSICS LETTERS 94, 242902 共2009兲

Unipolar resistive switching characteristics of room temperature grown
SnO2 thin films
Kazuki Nagashima,1 Takeshi Yanagida,1,2,a兲 Keisuke Oka,1 and Tomoji Kawai1,b兲
1Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki,Osaka 567-0047, Japan
2
PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan

共Received 2 March 2009; accepted 28 May 2009; published online 18 June 2009兲
The resistive switching characteristics of room temperature grown SnO2 films were investigated byfabricating the metal-oxide-metal sandwich structures. The unipolar operation was found in alldevices. Experiments, including the size and material dependencies of the top electrodes and thethree terminal device structures, demonstrated the rupture and formation of conducting filaments
near the anode. The Ohmic behavior was observed in both on- and off-states when using Au and Ti
top electrodes, whereas the Schottky behavior was only found in the off-state for Pt. The analysis
on the transport properties indicates the presence of insulative crystalline SnO2 near the anode in theoff-state. © 2009 American Institute of Physics. 关DOI: 10.1063/1.3156863兴

Log Intensity (a.u.)

(a)

(b)

Si(004)Pt(111)

SnO2/Pt/Ti/Si (100)Pt(222)

Pt
SnO2

Pt/Ti/Si (100)Pt

Si (100)

20

30

40

50
60
70
2Theta (degree)

80900.335nm(110)

5nm

Intensity (a.u.)

Reversible resistive switching 共RS兲 behavior in a metaloxide-metal 共MOM兲 sandwich structure has attracted a great
deal of scientific and industrial interests due to the nonvolatile memory applications.1–8 There are various RS behaviors
when applying the electric field, including a bipolar,1unipolar,2–8 and threshold switching.9 Among them the unipolar RS mechanism has been intensively investigated due tothe many advantages in the device operations.3–8 A model
based on the local conducting filament has been proposed,and explained the many features of the unipolar RS characteristics in terms of the formation and rupture of such conducting filaments within an insulative matrix. Since the unipolar RS operation was reported to show the strong material
dependence,2–8 exploring the unipolar RS mechanisms forvarious oxide materials would be important to seek the universal nature. In previous investigations as to unipolar RS,binary oxides including NiO,2,3 TiO2,4,5 CuO,6 and others7,8
have been studied, and most of them have utilized the polycrystalline films. SnO2 共Eg ⬃ 3.5 eV兲 is an abundant materialand n-type semiconductor with oxygen vacancies.10 The
transport properties have been studied as transparent conducting oxides.11 On the other hand there are no reports as tothe RS characteristics of SnO2 so far. From the viewpoint ofthe practical applications, the use of low temperature grownfilms is preferred to integrate the oxide materials with existing technologies. Thus these backgrounds motivated us toinvestigate the RS characteristics of room temperature grown

CO01

Pt

SnO2 thin films by fabricating the MOM structures.
SnO2 thin films were grown on Pt/Ti/Si 共100兲 substrateby pulsed laser deposition method 共ArF eximer, ␭ = 193 nm兲.
The film thickness was controlled to be 40 nm. Prior to thedeposition of SnO2 thin films, Pt as the bottom electrode wasdeposited onto the substrate with Ti buffer layer. The thickness of Pt bottom electrode was 100 nm. Oxygen pressure
was controlled to be 10 Pa during the deposition of SnO2thin films. SnO2 pellet target was used, which was obtainedby sintering the milled SnO2 powder 共99.99% pure兲 at1000 ° C for 24 h. The laser power, the repetition rate, andthe distance between the target and the substrate were set to
be 40 mJ, 3 Hz, and 30 mm, respectively. The top electrodewas then deposited onto the SnO2 thin films by using a metalmask varying the area from 6.25⫻ 102共25⫻ 25兲 ␮m2 to1.0⫻ 104共100⫻ 100兲 ␮m2. Note that all depositions of SnO2and Pt electrodes were performed at room temperature.
X-ray diffraction 共XRD兲 and high resolution transmission
electron microscopy 共HRTEM兲 共JEOL JEM-3000F兲
equipped with energy dispersed spectroscopy 共EDS兲 wereused to evaluate the crystal structure of the fabricated films.
Current-voltage characteristics were evaluated using semiconductor parameter analyzer 共Keithley 4200-SCS兲 at room
temperature in the ambient atmosphere.
Figure 1共a兲 shows the XRD data of the SnO2 thin filmgrown on the substrate. 共111兲 oriented Pt bottom electrodecan be seen in the figure. In addition, there were no observ-Sn

2
3
4Energy (keV)

5FIG. 1. 共Color online兲 共a兲 XRD data of MOM structure.
共b兲 Cross-sectional HRTEM image of SnO2 thin film.
The insets show the magnified image of HRTEM, theFFT diffractogram, and EDS data.

100nma兲Electronic mail: yanagi32@sanken.osaka-u.ac.jp.
Electronic mail: kawai@sanken.osaka-u.ac.jp.

b兲0003-6951/2009/94共24兲/242902/3/$25.00

94, 242902-1© 2009 American Institute of Physics

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105

RESET

104

10-3

SET

10-4

10-1

10-5

10-5

10-6

10-9

0

10-7
0

0.5

103

Forming
2102

4

1
1.5
Voltage (V)2106(b)

10HRS
109
107

Resistance (Ω)(a)

Current (A)10-2

Resistance (Ω)

10-1Appl. Phys. Lett. 94, 242902 共2009兲

Nagashima et al.

t (s)242902-2

10years
300K

105

103
25 30 35 40
LRS
1/kT (eV-1)
1101041021102 103 104Time (s)(c)020
406080
Number of Cycles

for “set” and “reset” processes are closely related to the rupture and formation of local conducting filaments in previous“set” and “reset” process conditions. Note that as seen inFigs. 2 and 3, the voltages for “reset” processes for different
conditions were found consistently around 0.8–1.1 V range.
On the other hand, the voltages for “set” processes areranged from 1.2 to 2.5 V and dependent on the conditions.
The “set” voltage for 100⫻ 100 ␮m2 electrode was less thanthat for 25⫻ 25 ␮m2 electrode. This is possibly due to theprobability of the formation of local conducting filaments
near the anode, because the larger electrode might have more
conducting filaments and also higher probability to connect
the filaments with the electrode even under lower electricfields. Next, we examined the effect of top electrode material
by using Pt, Au and Ti, in order to investigate the effect of
the work functions of electrodes, i.e., Schottky barrier effecton the RS. In all top electrodes, the unipolar RS was consistently observed, although the Ohmic behavior was observedin both on- and off-states when using Au and Ti top electrodes, as shown in Figs. 4共a兲 and 4共b兲. Thus only theSchottky barrier based-model alone cannot rigorously explain the occurrence of the unipolar RS occurred within thepresent SnO2 thin films. Furthermore, we examined the presence of the conducting filaments and the location of the RS
by utilizing device structures comprised of two top electrodes and one bottom electrode as shown in the inset of Fig.
3共b兲. Figure 3共b兲 shows the I-V curves of the forming, set,and reset processes when applying the electric field between
the two top electrodes. Note that the distance between thetwo top electrodes 共50 ␮m兲 is long enough compared withthe distance between the top electrode and the bottom electrode, i.e., the film thickness 共40 nm兲. The forming voltage
was nearly double of that in Fig. 2共a兲, and then the reset
process was performed to obtain the off-state in the device
structure. Two I-V curves of Fig. 3共c兲 shows the data between each top electrode and the bottom electrode after
above reset process. The I-V curve for the top electrode,which was used as the cathode in the reset process in Fig.
3共b兲, showed the LRS, and the reset process occurred at thevoltage around 0.8 V. On the other hand, the I-V curve forthe top electrode, which was used as the anode in the reset
process in Fig. 3共b兲, showed the HRS, and the set process
occurred at the voltage around 1.1 V. Clearly these experi-

10-2
10-425µm××25µm
100µm××100µm

10-600.5

1 1.5 2Voltage (V)2.54 (b)3RESET

2+-Forming

1002468Voltage (V)Current (××10-2A)

Current (A)(a)

Current (××10-2A)

able peaks due to the presence of SnO2 thin film. The microstructure of SnO2 thin film was examined by HRTEM. The
cross-sectional HRTEM images are shown in Fig. 1共b兲. The
insets of Fig. 1共b兲 show the FFT diffractogram and the EDS
data, which indicate the presence of Sn within the film. Thefilm crystallinity seems to be amorphouslike phase, although
there are partially crystalline nanoclusters less than 10 nm,
which is fairly consistent with the XRD data. Figure 2共a兲shows the typical I-V data of the device structure. The inset
shows the data of forming process. The device showed theunipolar RS, which is in fact quite similar trend comparedwith existing devices using polycrystalline binary oxides.2–8
The forming, set and reset processes typically occurred
around 4–5, 1.5–2, and 0.5–1 V, respectively. Note that all
our devices consistently exhibited such unipolar RS. Figure
2共b兲 and 2共c兲 show the retention and the endurance data byplotting the resistance data at 0.1 V. The low resistance onstate 共LRS兲 and the high resistance off-state 共HRS兲 were almost maintained in both the retention data over at least 104 s
and the endurance data over 100 cycles. Note that the resistance of LRS state increased gradually with the time above
104 s due to the oxidization in atmospheric condition. Toestimate the lifetime of the device, we examined the retention limit by varying the temperature in vacuum. The data isshown in the inset of Fig. 2共b兲. The lifetime was found to beover 10 year at 300 K. Thus the lifetime should be over 10years in the presence of sealing layers. Thus observed RS
phenomenon within room temperature grown SnO2 thin film
is nonvolatile.
Next we question what causes the nonvolatile RS effecton room temperature grown SnO2 thin films. Previously, twomajor models including the Schottky barrier based-modeland the conducting filaments based-model have been
proposed.2–8 In many previous reports as to the unipolar RSof polycrystalline binary oxides, the conducting filaments
based-model has explained many features of unipolar RS
characteristics.2–8 The size dependence of the top electrodeon the RS has been frequently utilized to probe the presenceof local conducting filaments.3 Figure 3共a兲 shows the size
dependence of the top electrode on the I-V curves. There wasno significant size dependence of the top electrode on boththe on-state current and the reset current, inferring the presence of local conducting filaments. Assuming the presence oflocal conducting filaments, the distributions of the voltages
1104 +-+-

32(c)100100

FIG. 2. 共Color online兲 共a兲 Typical I-V data of the devicestructure. The inset shows the data of forming process.
共b兲 and 共c兲 show the retention and the endurance data ofboth LRS and HRS using the resistance data at 0.1 V.
The inset of 共b兲 shows the retention properties whenvarying the temperature. The lifetime was defined as thetime when the resistance of LRS reached at the resistance value of HRS.

0.5
11.5
Voltage (V)

2FIG. 3. 共Color online兲 共a兲 Size dependence of the Pt top
electrode on the I-V curves. 共b兲 I-V curves of the forming, and reset processes when applying the electric field
between the two Pt top electrodes. The inset shows theused device structure. 共c兲 I-V curves between each top
electrode and the bottom electrode after reset process.

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4I (A)

7××10-3

LRS0103 -7××10-3-0.1 0
V (V)

0.1

102

100
10-3(c) 0.51

AuTi

Pt

0.98

1.01

0.99

102-5××10-4-0.1

0101

0.1
0V (V)

slope

10-2
10-1
E (MV/cm)

10-1
10-2 -3
10100

6.5(d) 6Schottky emission

J ∝ exp E 1 / 2

slope=1Au

TiPt

1.02

1.00

0.87

10-2
10-1
E (MV/cm)

100

Poole-Frenkel emission

FIG. 4. 共Color online兲 共a兲 LRS and 共b兲 HRS data whenusing Pt, Au, and Ti top electrodes. 共c兲 and 共d兲 show thecomparisons with Schottky emission model and Poole–Frenkel emission model, respectively.

J ∝ E exp E 1 / 2

5.5

ln J/E

0-0.5-1-1.5-2-2.5-3-3.5

slope=1HRS

5××10-4103

100slope101

ln J(b)104
J (A/cm2)

J (A/cm2)(a) 10Appl. Phys. Lett. 94, 242902 共2009兲

Nagashima et al.

I (A)242902-3

54.5
4
3.5
3
2.5

00.1

0.2

0.3

1/2

√ E (MV/cm)

0.4

0.3

0.4

0.5

0.6

0.7

1/2

√ E (MV/cm)

mental results demonstrate the presence of the conducting
filaments and the rupture and formation of such conducting
filaments near the anode.
Next we analyzed the I-V curves of HRS and LRS whenusing Pt, Au, and Ti top electrodes to identify the transport
properties of HRS and LRS. Figures 4共a兲 and 4共b兲 show theLRS and HRS data when using Pt, Au and Ti top electrodes.
The Ohmic behavior of LRS was consistently observed for
all top electrodes. Although the Ohmic behavior of HRS wasobserved when using Au and Ti, the Schottky behavior wasobserved for HRS when using Pt. Considering the work
functions of the top electrodes 关Pt共5.65 eV兲, Au共5.10 eV兲,Ti共4.33 eV兲兴, the Ohmic behavior indicates the metallic
state of LRS, which might be related to the metallic nature ofSnO2−␦ 共␦ ⬎ 0.01兲 due to the oxygen vacancies.10 On theother hand, the Schottky behavior of HRS when onlyusing Pt infers the presence of the Schottky barriers between
Pt electrode and the insulating phase for HRS. The formulaof Schottky emission model: ln J = ln AⴱT2 − q共␾B− 冑qE / 4␲␧兲 / kBT, in which J is the current density, E is theelectric field, q is the electric charge, ␧ is the dielectric constant, kB is Boltzmann’s constant, T is the temperature, andAⴱ is the effective Richardson constant, can well describe theI-V data of HRS, which allows us to estimate the value of theSchottky barrier height to be 0.42 eV. Considering the work
functions of top electrodes,12 the bad gap, the electron affinity, and the activation energy of typical crystalline SnO2,13
the Schottky barrier height can be theoretically estimated to
be about 0.4 eV, which is clearly close to the experimentallyestimated barrier height of 0.42 eV. Thus these data highlightthat the off-state local structure near the anode might becomposed of insulative crystalline SnO2. Above the Schottky

barrier height of 0.42 eV, the I-V curves were well fitted by
the formula of Poole–Frenkel emission model ln共J / E兲
⬀ 共冑q3 / ␲␧ / kBT兲冑E.8 This indicates that the electron hopping
between the trap states under high electric field governs thetransport properties of insulative crystalline SnO2 near theanode.
This work is partially funded by SCOPE.
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