Resistive switching characteristics of solution-deposited Gd, Dy, and Ce-doped ZrO2 films

Article abstract of DOI:10.1063/1.3700728

Several rare earth elements (Gd, Dy, and Ce) having different valence numbers were doped into a solution-synthesized ZrO₂ film, and the corresponding resistive memory characteristics were discussed in relation to the oxygen vacancies and film microstructure. Pure and trivalent ion-doped ZrO ₂ films showed forming-free behavior, probably because of the large amount of inherent and additional dopant-incurred oxygen vacancies, respectively. In contrast, tetravalent Ce ion doping caused the forming process to be required and afforded stable long-term switching characteristics with a relatively large memory window, which is attributed to the dopant-enhanced crystallization/densification effect without excessive oxygen vacancy generation.

Full text of DOI:10.1063/1.3700728

Resistive switching characteristics of solution-deposited Gd, Dy, and Ce-doped ZrO2
films
Myung Soo Lee, Sungho Choi, Chee-Hong An, and Hyoungsub Kim
Citation: Applied Physics Letters 100, 143504 (2012); doi: 10.1063/1.3700728
View online: http://dx.doi.org/10.1063/1.3700728
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APPLIED PHYSICS LETTERS 100, 143504 (2012)
Resistive switching characteristics of solution-deposited Gd, Dy,
and Ce-doped ZrO films
a)
2
Myung Soo Lee, Sungho Choi, Chee-Hong An, and Hyoungsub Kim
School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746,
Republic of Korea
(Received 9 January 2012; accepted 16 March 2012; published online 3 April 2012)
Several rare earth elements (Gd, Dy, and Ce) having different valence numbers were doped into a
solution-synthesized ZrO2 film, and the corresponding resistive memory characteristics were
discussed in relation to the oxygen vacancies and film microstructure. Pure and trivalent ion-doped
ZrO films showed forming-free behavior, probably because of the large amount of inherent and
2
additional dopant-incurred oxygen vacancies, respectively. In contrast, tetravalent Ce ion doping
caused the forming process to be required and afforded stable long-term switching characteristics
with a relatively large memory window, which is attributed to the dopant-enhanced
crystallization/densification effect without excessive oxygen vacancy generation. VC 2012 American
Institute of Physics. [http://dx.doi.org/10.1063/1.3700728]
Resistance-change random access memory (RRAM) has
been continuously recognized as a potential replacement for
contemporary flash memory due to its facile scalability and
future extensibility to a three-dimensionally stacked integra-
switching characteristics, especially emphasizing the effects
of dopant-instigated oxygen vacancy generation and crystal-
linity change in the film.
In the preparation of ZrO precursor solution for a sol-
2
1
tion scheme. A typical RRAM unit device consists of a sim-
ple metal-insulator-metal (MIM) structure with various
insulating layers, in particular, binary metal-oxide systems,
gel deposition, zirconyl nitrate hydrate (99%) was used with-
out further purification by slowly dissolving it in a mixture
of 2-methoxyethanol and acetic acid to reach a solute con-
centration of 0.3M. As dopants, gadolinium nitrate hexahy-
drate (99.99%), dysprosium nitrate hydrate (99.9%), and
cerium nitrate hexahydrate (99.99%), were added to give a
doping concentration of 10 cation at. %, maintaining the
same total solute concentration of 0.3M. The undoped and
doped ZrO films were spin-coated on Pt/Ti/SiO /Si sub-
2
3
4
5
6
such as NiO, TiO , Ta O , HfO , and ZrO , and rare
2
2
5
2
2
7
earth oxides. The operation principle of the RRAM device
is based on a reversible change between two definite resist-
ance states, viz., a high resistance state (HRS) and a low re-
sistance state (LRS), by a cyclic electrical biasing. Although
there is still a dispute surrounding the underlying working
mechanism, it is generally described by a reversible forma-
tion and rupturing process of local conducting filaments, in
2
2
ꢀ
strates and dried at 100 C for 10 min on a hot plate. The
final film thickness was measured to be around 13 nm, inde-
pendent of the dopant species, after a sintering process was
8
which generation and recovery of oxygen vacancies occur.
ꢀ
Accordingly, controlling the amount of oxygen vacan-
cies in the oxide film has become an important research topic
in order to improve both the switching characteristics and
uniformity of the RRAM devices, and several approaches
have been addressed, e.g., the insertion of an oxygen-
performed at 400 C for 60 min in air ambient.
The crystallinity of the sintered films was characterized
by using a grazing incident x-ray diffraction (GIXRD) sys-
tem with a CuKa x-ray source. The film microstructure was
inspected in further detail by using high-resolution transmis-
sion electron microscopy (HRTEM). In order to evaluate the
resistive switching characteristics, circularly patterned MIM
structures with a diameter of 100 lm were formed by a lift-
off process involving the sputter-deposition of a top elec-
trode [Al (10 nm)-capped TaN (50 nm)]. The resistive
switching characteristics, i.e., the repetitive cycling of the
current-voltage (I-V) measurement, were measured by using
an Agilent B1500A semiconductor device analyzer. A cur-
rent compliance of 1 mA was imposed during the SET pro-
cess to prevent irreversible dielectric damage (hard
breakdown).
5,9
gettering Ti interlayer under the metal electrode and the
introduction of foreign ions equipped with an oxygen va-
1
0,11
cancy generation capability.
method, recently Zhang et al.
In terms of the doping
experimentally demon-
1
0,11
strated a notable improvement of the RRAM characteristics
by incorporating a small amount of trivalent ions (Gd and
Al) into the ZrO2 or HfO2 films. They theoretically
accounted for this effect by using a first principles calcula-
tion involving a lowering of the oxygen vacancy formation
energy and the ensuing efficacious oxygen vacancy genera-
tion. However, an experimental comparison with tetravalent
ion-doped samples was not given, and the possible effects of
dopant-dependent microstructural change have not been
considered.
Figure 1 shows the bipolar resistive switching behaviors
of the pure and various rare earth element-doped ZrO sam-
2
ples. The devices were switched from the HRS to the LRS
by a positive bias sweep and switched back by a negative
bias sweep. It is well known that some pristine dielectrics
require an additional “electro-forming” process before the
repetitive dc sweep, which involves the initial application of
a high electric field to locally generate conductive current
In this study, we synthesized various ZrO films doped
2
with trivalent (Gd and Dy) or tetravalent (Ce) rare earth ele-
ments via a solution-based route and compared their resistive
a)
Email address: hsubkim@skku.edu. Tel.: 82-31-290-7363.
0003-6951/2012/100(14)/143504/4/$30.00
100, 143504-1
VC 2012 American Institute of Physics
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1

143504-2
Lee et al.
Appl. Phys. Lett. 100, 143504 (2012)
FIG. 1. Bipolar resistive switching I-V characteris-
tics of the MIM devices with various insulating
layers: (a) pure, (b) Gd-doped, (c) Dy-doped, and
(
2
d) Ce-doped ZrO films.
1
2
paths (filaments) across the dielectric via a soft breakdown.
mentally supported in an indirect way by our previous work,
which showed an increase in the hysteresis voltage during
capacitance-voltage measurements of the Gd-, Dy-, and
In our samples, the forming process was only required in the
Ce-doped ZrO sample, whereas the pure ZrO and other
doped samples (Gd- and Dy-doped ZrO ) could be made to
2
2
Y-doped ZrO films produced on Si substrates under similar
2
2
13
inhabit a SET state without it, as shown in Fig. 1. Among
many possible contributing factors, the initial amount of oxy-
gen vacancies existing in the dielectric film is acknowledged
to be closely related to the necessity of the forming pro-
deposition conditions. Therefore, the forming-free charac-
teristics observed in the Gd- and Dy-doped ZrO samples are
2
believed to be primarily caused by a further increase in the
oxygen vacancy concentration through the incorporation of
trivalent ions, which instigates the facile formation of con-
ductive filaments during the low field SET process. In the
12
cess. In this experiment, because the pure ZrO sample
2
was synthesized via a solution-based route and sintered at a
relatively low temperature producing an amorphous state as
revealed in the following XRD and HRTEM analyses, it has
a strong possibility of existing in a somewhat oxygen-
deficient defective state with a lower film density than the
crystalline films deposited by a vacuum-based process.
Therefore, it is most likely that the abundance of oxygen
vacancy-related and other film defects produced the
case of the tetravalent Ce-doped ZrO film, unlike the triva-
2
lent ion-doped film, the dopant-induced further oxygen va-
cancy generation may be greatly prevented. In addition, it is
known that the CeO -ZrO mixed system can form a com-
2
2
plete solid solution across the wide composition range and
that the Ce incorporation into the ZrO film can lower its
2
crystallization temperature due to the lower crystallization
Therefore,
14,15
observed forming-free characteristics of the pure ZrO sam-
2
temperature of CeO2 than that of ZrO2.
ple, in accordance with the results of a recent report using a
6
similar deposition technique.
crystallization-induced densification of the film and conse-
quent reduction of the number of film defects causing the ini-
tial high leakage paths are expected according to the
following XRD/HRTEM results and the results of our previ-
ous electrical measurement (reduction of electrical trap den-
Provided that oxygen vacancies are one of crucial agents
in removing the need for a forming process, the variation
among the doped-ZrO films could be accounted for by the
2
15
differences between the valence numbers of the doping ele-
ments in comparison with that of the substituted Zr atom.
The oxygen vacancy formation energy of the trivalent ions
sity), respectively. Considering these aspects, the necessity
of the forming process in the Ce-doped ZrO film is conjec-
2
tured to be closely related to the crystallization-promoted
densification effect as well as the absence of the dopant-
induced further oxygen vacancy generation.
in the hosting ZrO matrix was reported to be lower than that
2
of the tetravalent ions, according to the first principles calcu-
10
lation. Furthermore, the suggested enhancement in oxygen
vacancy generation induced by the difference between the
valence numbers of the doping and hosting ions was experi-
The crystallinity and phase of ZrO films doped with
2
various rare earth elements were investigated by using XRD
and HRTEM in order to obtain information on the
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143504-3
Lee et al.
Appl. Phys. Lett. 100, 143504 (2012)
FIG. 3. Endurance characteristics of the MIM devices with various insulat-
ing layers as a function of the number of dc sweep cycles, up to 5000 cycles:
(a) pure, (b) Gd-doped, (c) Dy-doped, and (d) Ce-doped ZrO films. The
2
HRS and LRS currents were measured at an applied voltage of 0.2 V.
fore, it is probable that the filament-forming oxygen
vacancies are randomly located in the amorphous phase,
which might have caused the unstable fluctuation of the HRS
FIG. 2. (a) GIXRD patterns of the pure and various rare earth element-
doped ZrO films. T and C refer to tetragonal and cubic phases, respectively.
Cross-sectional HRTEM images of (b) pure, (c) Gd-doped, (d) Dy-doped,
and (e) Ce-doped ZrO films sandwiched between TaN and Pt electrodes.
2
current, as observed in Figs. 3(a) and 4. For the ZrO films
2
2
doped with trivalent ions, a more uniform distribution of the
endurance characteristics was achieved; however, a some-
what increased HRS current level with a smaller memory
window (ON/OFF ratio) was identified [Figs. 3(b) and 3(c)].
Recently, a similar improvement of the device uniformity
was also reported for trivalent ion (Gd and Al) doped ZrO2
microstructural change of the ZrO film that is related to the
2
degree of densification. Figure 2(a) shows the measured
XRD patterns of the pure and doped ZrO films. There are a
couple of minor discrepancies with our previously reported
2
13
XRD results, which were obtained from systems of similar
materials on Si. These are due to differences in the film
thickness and substrate conditions. However, overall, we
find that the valance number of the introducing dopant has a
similar effect to that previously discovered: enhancement of
and HfO films synthesized by a metal deposition followed
2
10,11
by an oxidation process.
This was attributed to enhanced
oxygen vacancy generation by the trivalent ions and the sub-
sequent suppression of the randomness of the oxygen va-
cancy filaments’ formation, which can also be applied to our
the ZrO crystallization was only achieved by tetravalent ion
2
Gd- and Dy-doped ZrO samples. However, in contrast to
2
(Ce) doping. Coinciding with the XRD result, the cross-
sectional HRTEM images shown in Figs. 2(b)–2(e) also
their result, we observed a reduced ON/OFF ratio in the Gd-
and Dy-doped samples as a side disadvantage, which can be
explained by the difference in the doping concentrations; a
relatively higher doping concentration (ꢁ10 cation at. %)
was used in this experiment. In addition to the intrinsic
reveal that significant crystallization of the ZrO film only
2
happened after Ce doping, whereas the pure ZrO and the
2
other doped samples (Gd- and Dy-doped films) mostly
remained as an amorphous phase. In consequence, the
observed microstructural changes of the ZrO film as the
2
dopants with different valance numbers are introduced well
support the aforementioned necessity of the forming process
only in the Ce-doped ZrO film.
2
The switching endurance characteristics of undoped and
doped ZrO samples are displayed in Fig. 3. Additionally,
2
Fig. 3 is replotted in Fig. 4, which shows the statistical distri-
bution of the HRS and LRS currents measured up to a sweep
cycle number of 5000. The pure ZrO film shows unstable
2
endurance characteristics with random fluctuations, espe-
cially of the HRS current, resulting in a wide distribution, as
the number of sweep cycles is increased. As discussed
above, the solution-based deposition of the pure ZrO film
2
produced an amorphous phase, probably with lots of defects
(mainly oxygen vacancies) due to the solution-based deposi-
tion, which led to forming-free switching behavior. There-
FIG. 4. Statistical distribution of the HRS and LRS currents measured dur-
ing the sweep cycles up to 5000 cycles for the undoped and various rare
2
earth element-doped ZrO films.
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143504-4
Lee et al.
Appl. Phys. Lett. 100, 143504 (2012)
concentration of oxygen vacancies formed during the deposi-
tion process, a much greater generation of oxygen vacancies
by a large number of trivalent dopants may hinder the ruptur-
ing process of the conductive filament during the RESET
process, which can be attributed to an increased HRS current
level with a smaller ON/OFF ratio as observed in the Gd-
and Dy-doped samples.
increasing the amount of oxygen vacancies, which resulted
in forming-required, stable endurance characteristics with a
relatively large ON/OFF ratio. These results emphasize the
importance of simultaneous control of the oxygen vacancies
and film crystallinity by rare earth element doping to achieve
optimization of binary oxide-based RRAM devices.
This work was supported by National Research Founda-
tion of Korea funded by the Ministry of Education, Science
and Technology [Grant Nos. R32-2009-000-10124-0 (World
Class University program) and 2010-0017004].
For the Ce-doped ZrO sample, in contrast to the other
2
samples, very stable endurance characteristics with a reason-
ably large ON/OFF ratio were acquired, as shown in Figs.
3(d) and 4. As discussed above, only the Ce doping made the
ZrO film require a forming process by preventing further
2
1
C. K u¨ geler, M. Meier, R. Rosezin, S. Gilles, and R. Waser, Solid State
Electron. 53, 1287 (2009).
S. Seo, M. J. Lee, D. H. Seo, E. J. Jeoung, D.-S. Suh, Y. S. Joung, I. K.
Yoo, I. R. Hwang, S. H. Kim, I. S. Byun, J.-S. Kim, J. S. Choi, and B. H.
oxygen vacancy generation and by augmenting the crystalli-
zation. Similar to this result, the Ce doping is considered to
effectively densify the ZrO2 film without excessively
increasing the dopant-induced oxygen vacancies responsible
for the increased HRS current level, which resulted in a large
ON/OFF ratio. Furthermore, in a NiO-based RRAM device,
2
Park, Appl. Phys. Lett. 85, 5655 (2004).
B. J. Choi, D. S. Jeong, S. K. Kim, C. Rohde, S. Choi, J. H. Oh, H. J. Kim,
C. S. Hwang, K. Szot, R. Waser, B. Reichenberg, and S. Tiedke, J. Appl.
3
Phys. 98, 033715 (2005).
M.-J. Lee, C. B. Lee, D. Lee, S. R. Lee, M. Chang, J. H. Hur, Y.-B. Kim,
16
Kim et al. suggested that enhancing the film crystallinity is
an effective way in obtaining stable memory switching char-
acteristics. Therefore, it can be postulated that the grain
4
C.-J. Kim, D. H. Seo, U.-I. Chung, I.-K. Yoo, and K. Kim, Nat. Mater. 10,
6
25 (2011).
5
6
H. Y. Lee, Y. S. Chen, P. S. Chen, T. Y. Wu, F. Chen, C. C. Wang, P. J.
Tzeng, M.-J. Tsai, and C. Lien, IEEE Electron Device Lett. 31, 44 (2010).
B. Sun, Y. X. Liu, L. F. Liu, N. Xu, Y. Wang, X. Y. Liu, R. Q. Han, and J.
F. Kang, J. Appl. Phys. 105, 061630 (2009).
T.-M. Pan and C.-H. Lu, Appl. Phys. Lett. 99, 113509 (2011).
R. Waser and M. Aono, Nat. Mater. 6, 833 (2007).
boundaries formed by the Ce-induced ZrO crystallization
2
act as seeds for a uniformly distributed conductive filament
during both the forming process and the following switching
cycles, resulting in a uniform distribution of the HRS
currents.
7
8
9
C.-Y. Lin, C.-Y. Wu, C.-Y. Wu, T.-Y. Tseng, and C. Hu, J. Appl. Phys.
102, 094101 (2007).
In summary, the resistive switching characteristics of
sol-gel-deposited ZrO films doped with Gd, Dy, and Ce
1
1
1
1
0
1
2
3
H. Zhang, B. Gao, B. Sun, G. Chen, L. Zeng, L. Liu, X. Liu, J. Lu, R. Han,
J. Kang, and B. Yu, Appl. Phys. Lett. 96, 123502 (2010).
H. Zhang, L. Liu, B. Gao, Y. Qiu, X. Liu, J. Lu, R. Han, J. Kang, and B.
Yu, Appl. Phys. Lett. 98, 042105 (2011).
2
ions were discussed in terms of dopant-induced oxygen va-
cancy generation and microstructural change. The pure ZrO2
film exhibited a forming-free behavior with large HRS fluc-
tuation as the dc sweep cycle increased, which is attributed
to the large number of film defects (including oxygen vacan-
cies) that are formed by a non-equilibrium deposition pro-
cess. The trivalent ion (Gd and Dy) doping into the ZrO2
film increased the HRS current level by causing further ex-
cessive oxygen vacancy generation. However, doping with
K. M. Kim, D. S. Jeong, and C. S. Hwang, Nanotechnology 22, 254002
(
2011).
M. S. Lee, C.-H. An, K. Park, J.-Y. Choi, and H. Kim, J. Electrochem.
Soc. 158, G133 (2011).
14
T. Brezesinski, M. Antonietti, M. Groenewolt, N. Pinna, and B. Smarsly,
New J. Chem. 29, 237 (2005).
5
1
M. S. Lee, C.-H. An, J. H. Lim, J.-H. Joo, H.-J. Lee, and H. Kim, J. Elec-
trochem. Soc. 157, G142 (2010).
D. C. Kim, M. J. Lee, S. E. Ahn, S. Seo, J. C. Park, I. K. Yoo, I. G. Baek,
H. J. Kim, E. K. Yim, J. E. Lee, S. O. Park, H. S. Kim, U.-I. Chung, J. T.
Moon, and B. I. Ryu, Appl. Phys. Lett. 88, 232106 (2006).
16
tetravalent ions (Ce) enhanced the crystallization of the ZrO2
film and probably reduced film defects without further
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1