Impedance spectroscopy characterization of resistance switching NiO thin films prepared through atomic layer deposition

Article abstract of DOI:10.1063/1.2392991

To understand electrical/dielectric phenomena and the origins of bistable resistive switching, impedance spectroscopy was applied to NiO thin films prepared through atomic layer deposition. The dc current-voltage characteristics of the NiO thin films were

Full text of DOI:10.1063/1.2392991

Impedance spectroscopy characterization of resistance switching NiO thin films
prepared through atomic layer deposition
Yil-Hwan You, Byung-Soo So, Jin-Ha Hwang, Wontae Cho, Sun Sook Lee, Taek-Mo Chung, Chang Gyoun Kim,
and Ki-Seok An
Citation: Applied Physics Letters 89, 222105 (2006); doi: 10.1063/1.2392991
View online: http://dx.doi.org/10.1063/1.2392991
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APPLIED PHYSICS LETTERS 89, 222105 ͑2006͒
Impedance spectroscopy characterization of resistance switching NiO thin
films prepared through atomic layer deposition
Yil-Hwan You, Byung-Soo So, and Jin-Ha Hwanga͒,b͒
Department of Materials Science and Engineering, Hongik University, Seoul 121-791, South Korea
Wontae Cho, Sun Sook Lee, Taek-Mo Chung, Chang Gyoun Kim, and Ki-Seok Ana͒,c͒
Thin Film Materials Laboratory, Korea Research Institute of Chemical Technology,
Daejeon 305-600, South Korea
͑Received 16 May 2006; accepted 13 October 2006; published online 28 November 2006͒
To understand electrical/dielectric phenomena and the origins of bistable resistive switching,
impedance spectroscopy was applied to NiO thin films prepared through atomic layer deposition.
The dc current-voltage characteristics of the NiO thin films were also determined.
Frequency-dependent characterizations indicated that the switching and memory phenomena in NiO
thin films did not originate from the non-Ohmic effect at the electrode/NiO interfaces but from the
bulk-related responses, i.e., from an electrocomposite where highly conducting components are
distributed in the insulating NiO matrix. Low dielectric constants and bias-independent capacitance
appeared to corroborate the bulk-based responses in resistive switching in NiO thin films. © 2006
American Institute of Physics. ͓DOI: 10.1063/1.2392991͔
High speeds and large amounts of digital data in global
information and communication products have brought the
continuous scaling down of the current dynamic random ac-
cess memory, in which 1 bit is made up of one capacitor in
series with one switching transistor. This requires a refresh-
ing function that impedes high speed and low power con-
sumption. With nonvolatile characteristics and a simplified
device structure, resistive random access memories provide a
simultaneous solution to the need for high speed, low power
consumption, and high-density integration, as one of the
next-generation nonvolatile memory candidates. Switching
phenomena are found in transition metal oxides and perov-
skite oxides, e.g., in NiO,1,2 TiO2,3 Nb2O5,4 Cr-doped
SrZrO3,5 SrTiOx,6 and ͑PrCa͒MnO3.7 Various mechanisms
have been proposed to explain the reversible switching be-
tween high and low resistance states, but detailed origins
remain unsolved, i.e., Schottky barriers with trapped charges
in the interface states,8 space-charge-limited current,9
phonon-assisted tunneling,10 formation and rupture of
filaments,1,11 etc.
bulk-related effects or from electrode-related phenomena.
Papagianni et al. claimed two bulk contributions and one
interfacial effect, only from impedance spectra of
Pr0.7Ca0.3MnO3 with no detailed analysis on capacitance.7 In
our study, ac impedance spectroscopy was employed to de-
termine the electrical/dielectric properties in NiO thin films
and the possible origins of resistive switching, in addition to
the dc current-voltage characterizations.
NiO thin films were deposited using atomic layer depo-
sition ͑ALD͒ which used Ni͑dmamb͒ as a Ni precursor,
2
where dmamb is described as 1-dimethylamino-2-methyl-2-
butanolate ͑–OCMeEtCH2NMe2͒ and water as an oxygen
source.15 Two amorphous NiO thin films, 40 and 190 nm,
were deposited onto commercial Pt-coated wafers ͑Inostek
Inc., Ansan, Korea͒ at 140 °C. To perform ac impedance
spectroscopy and determine dc current-voltage characteris-
tics in a two-point configuration, top Pt electrodes of 100 nm
thickness were sputtered into disk-shaped pads using metal
shadow masks. Impedance characterizations were performed
using
a low-frequency impedance analyzer ͑Hewlett-
Packard, 4192A, Palo Alto, CA͒: the oscillating amplitude
was fixed at 25 mV and impedance spectra were collected
between 1 MHz and 100 Hz in the logarithmic manner, ten
points per decade. dc current-voltage measurements were
made using a source-meter unit ͑Keithley 236, Cleveland,
OH͒ in combination with the controlled compliance in the
“current” mode.
Figure 1 displays the current-voltage characteristics in
ALD-deposited NiO thin films. Despite the amorphous state
of NiO thin films, resistance switching was noticeable and
comparable to resistance switching in polycrystalline NiO
thin films.2 Under the compliance of 100 mA, the 190 nm
NiO thin film was converted to the on state. The forming
state was achieved within the NiO thin films at a voltage of
8.1 V, equivalent to 0.43 MV/cm, without causing cata-
strophic breakdown. In successive voltage sweeping modes,
the current decreased abruptly near 1.0 V, shifting to the off
state, leading to an abrupt increase of three orders of magni-
tude in resistance. Another voltage sweep caused the current
ac impedance spectroscopy is a highly powerful tech-
nique used to investigate electrical/dielectric properties in
electroceramic materials, such as ferroelectric materials,
ionic conductors, positive-temperature coefficient resistors,
and voltage-dependent resistors.12–14 Frequency-dependent
impedance provides valuable information on the physical
origins of electrical/dielectric properties, e.g., separation of
bulk-based responses from electrode-related contributions,
along with the simultaneous measurement of materials con-
stants such as resistivity and dielectric constants. The estima-
tion of arc depression is attributed to the spatial distribution
of the microstructural components governing the correspond-
ing electrical responses.
The resistive switching mechanism remains controver-
sial over whether the memory characteristics originate from
a͒
Authors to whom correspondence should be addressed.
Electronic mail: jhwang@wow.hongik.ac.kr
b͒
c͒
Electronic mail: ksan@krict.re.kr
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222105-2
You et al.
Appl. Phys. Lett. 89, 222105 ͑2006͒
TABLE I. Summary of the equivalent circuit parameters obtained using
impedance spectra of Figs. 2͑a͒ and 2͑b͒ at the on and off states.
Parameter Value
R͑lead͒
L͑lead͒
R͑on͒
R͑off͒
CPE͑off͒
4.43 ⍀
3.54ϫ10−6
H
3.20 ⍀
5.49ϫ104 ⍀
7.75ϫ10−10 F and n=0.8206
NiO-based responses. The effect of lead wires is described
by a serial connection of an inductor, i.e., L͑lead͒, and a
resistor, i.e., R͑lead͒, and the NiO-related response can be
explained by a parallel combination of three components
from the on and off states, specifically R͑on͒, R͑off͒, and
CPE͑off͒. Through the equivalent circuit modeling, the pres-
ence of a highly conducting path, R͑on͒, that formed across
NiO thin films is believed to produce the Cole-Cole plot of
Fig. 2͑b͒. Detailed circuit parameters are summarized in
Table I. Initial resistances with no bias voltage were in agree-
ment with those of the low-frequency impedance in Figs.
2͑a͒ and 2͑b͒.
Corrected for the contribution of lead wires to the mea-
suring apparatus, the apparent resistance was approximately
3.20 ⍀, equivalently 131.9 ⍀ cm ͓see Fig. 2͑b͔͒. However, it
should be noted that the arc depression is not negligible with
regard to the real impedance axis, as shown in Fig. 2͑a͒
where the center of a simulated impedance arc is located
below the real axis of a complex plane. Depression analysis
in Nyquist plots ͑−Zimaginary vs Zreal͒ provides information
concerning homogeneity in the electrical/dielectric origins
governing the corresponding impedance responses. The im-
pedance information of Fig. 2͑a͒ reflects an assemblage of a
variety of time constants and leads to the hypothesis that the
corresponding electrical components are distributed with re-
gard to the average value. This feature appears to be due to
the amorphous quality of the NiO thin films, i.e., possibly
due to the lack in long-range order.
FIG. 1. ͑Color online͒ Current-voltage characteristics in the NiO thin films
͑40 and 190 nm͒ prepared through atomic layer deposition.
level to increase rapidly at approximately 1.7 V. The current-
voltage responses and the corresponding “reset” and “set”
voltages were reproducible even in amorphous NiO thin
films. The results of Fig. 1 show that the derivation of volt-
age versus current, dV/dI, defined as dynamic resistance was
not constant with respect to the applied voltage, implying
that the current-voltage characteristics were not linearly cor-
related in the on or off states. However, the 40 nm NiO thin
film exhibited higher off-state current, lower on-state current,
and lower forming voltage than those of the 190 nm NiO
film. The reset and set voltages approached those of the
thicker NiO film after two consecutive sweeping.
Three electronic states, such as forming, off, and on
were monitored using impedance spectroscopy, based on Fig.
1. In Fig. 2͑a͒, the initial state before electrical forming and
the off state exhibit impedance spectra of highly resistive
materials, whose equivalent circuit can be described by the
parallel connection of a resistor and a capacitor, or more
precisely a constant phase element. The off state can simu-
late a parallel combination of a highly resistive component,
R͑off͒, and a constant phase element, CPE͑off͒, as shown in
Fig. 2͑a͒. The on state induced very low impedance, as can
be seen in Fig. 2͑b͒. The equivalent circuit is modeled by a
serial connection of two contributions from lead wires and
The impedance information can incorporate, if present,
the contribution of non-Ohmic contacts between electrodes
and NiO thin films. Seo et al. tested a variety of electrodes
onto NiO thin films prepared through sputtering and found
that platinum and gold form Ohmic contacts.16 The relevant
impedance data can eliminate spurious contributions origi-
nating from Schottky effects due to the high values in work
FIG. 2. ͑Color online͒ Impedance
spectra at each step in the current-
voltage characteristics ͑a͒ before form-
ing and after forming ͑off state͒ and
͑b͒ after forming ͑on state͒ along with
the lead contributions occurring in the
measuring apparatus. ͑a͒ and ͑b͒ incor-
porate simulated impedance spectra
and the corresponding equivalent cir-
cuit models for off and on states, re-
spectively. ͑Thickness of NiO thin
film: 190 nm͒.
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222105-3
You et al.
Appl. Phys. Lett. 89, 222105 ͑2006͒
FIG. 3. ͑Color online͒ Bode plots of
absolute impedance ͉Z͉ and capaci-
tance as a function of frequency: ͑a͒
thickness dependence ͑190 and
40 nm͒ and ͑b͒ bias dependence in
NiO thin films between 0 and 3 V.
͑Thickness of NiO thin film: 190 nm͒.
functions. Therefore, the impedance spectra reflect the con-
tributions of the bulk-related responses, but not from the in-
terfacial contributions between NiO and the Pt electrode.
Bode plots of absolute impedance and capacitance are shown
as a function of frequency in Fig. 3͑a͒ for two different thick-
nesses, 40 and 190 nm. The corresponding capacitances at
1 MHz were calculated to be 7.9ϫ10−11 and 4.4ϫ10−11 F.
Corrected for the geometric factors, the effective dielectric
constants for the 40 and 190 nm films were estimated to be
approximately 8.86 and 11.51, respectively. The dielectric
constant is quite close to the reported value of bulk NiO
materials, i.e., 15: the switching behavior cannot be ex-
plained in terms of the interfacial responses originating from
the Pt/NiO contacts, since the non-Ohmic effects lead to
much higher capacitance, e.g., in the range of nanofarads.
The impedance information obtained during the off state
was similar to that before forming. The two impedance spec-
tra show almost identical capacitance as a function of fre-
quency, indicating that both states have the same origin. The
impedance of Fig. 2͑b͒ suggests percolated interconnections
of the highly conducting components between the two elec-
trodes embedded into the pseudoinsulating NiO matrix. Dur-
ing the bias sweeping from 0 to 3 V, the impedance arc de-
creased in magnitude after a maximum at 1 V ͓see Fig. 3͑b͔͒.
The bias sweeping showed a gradual decrease in impedance
unlike the rapid changes in the current-voltage characteris-
tics. The low-frequency capacitance was independent of the
applied bias. In the retention test, impedance at 100 Hz con-
tinued to retain the initial values without any significant
changes up to 1000 s.
As shown in Fig. 1, the initial “electroforming” can gen-
erate highly defective components, such as vacancies, metal-
lic clusters, phase separation, dislocations, etc., in the insu-
lating NiO matrix, leading to the interconnected filaments
between electrodes. In the on state, the resistance increases
with the applied voltage, probably in combination with local
Joule heating. Since NiO thin films are highly sensitive to
applied voltage, more precisely to power as mentioned
above, the Joule heating at local domains drives the defective
states to the oxidized ones, isolated into NiO matrix. The
formation and rupture of a conducting filament were used to
explain the NiO thin films deposited by sputtering.2,17 Taking
into account the power dependence of resistive switching
systems in transition metal oxides,3 impedance spectroscopy
is preferred because the uncontrolled dc probing technique
involves unexpected prolonged power accumulation and can
unpredictably modify the electronic state in switching
devices.
In summary, the use of impedance spectroscopy allows
for the separation of the bulk responses from the interfacial
effects between the electrode and thin films. ac impedance
measurements for ALD NiO thin films indicated several
unique features: ͑i͒ low dielectric constant, ͑ii͒ bias-
independent capacitance, and ͑iii͒ the capability of imped-
ance to monitor data retention. The low dielectric constant
and bias-independent capacitance indicate that the switching
behavior in NiO thin films originates from the filament-
controlled NiO matrix.
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