Effect of Y, Gd, Dy, and Ce doping on the microstructural and electrical properties of sol-gel-deposited ZrO2 film

Article abstract of DOI:10.1149/1.3562971

ZrO₂ films doped with Y, Gd, Dy, and Ce at a concentration of ～13 cation atom were deposited by a sol-gel technique, and the early-stage dopant effect on the stabilization of the high temperature phase and dielectric properties was systematically compared after a low-temperature annealing process (400C). The high temperature phase formation of the ZrO₂ films was hindered by the Y, Gd, and Dy doping, thereby reducing the dielectric constant. In addition, the hysteresis was significantly increased via electron trapping, which was attributed to the increase in the number of oxygen vacancies possibly due to the difference in the valence number of the dopants with that of the substituted Zr atoms. However, the dielectric constant of the ZrO₂ film doped with tetravalent Ce atoms increased without retarding the stabilization of the high temperature phase and degrading the hysteresis characteristics.

Full text of DOI:10.1149/1.3562971

Journal of The Electrochemical Society, 158 (6) G133-G136 (2011)
0
G133
013-4651/2011/158(6)/G133/4/$28.00 VC The Electrochemical Society
Effect of Y, Gd, Dy, and Ce Doping on the Microstructural and
Electrical Properties of Sol-Gel-Deposited ZrO Film
2
z
Myung Soo Lee, Chee-Hong An, Kyung Park, Ju-Yun Choi, and Hyoungsub Kim
School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea
ZrO
early-stage dopant effect on the stabilization of the high temperature phase and dielectric properties was systematically compared
2
films doped with Y, Gd, Dy, and Ce at a concentration of ꢀ13 cation atom % were deposited by a sol-gel technique, and the
ꢁ
after a low-temperature annealing process (400 C). The high temperature phase formation of the ZrO films was hindered by the
Y, Gd, and Dy doping, thereby reducing the dielectric constant. In addition, the hysteresis was significantly increased via electron
trapping, which was attributed to the increase in the number of oxygen vacancies possibly due to the difference in the valence number
2
2
of the dopants with that of the substituted Zr atoms. However, the dielectric constant of the ZrO film doped with tetravalent Ce
atoms increased without retarding the stabilization of the high temperature phase and degrading the hysteresis characteristics.
VC 2011 The Electrochemical Society. [DOI: 10.1149/1.3562971] All rights reserved.
Manuscript submitted December 7, 2010; revised manuscript received February 9, 2011. Published March 30, 2011.
For many decades, metal-oxide systems have been widely inves-
tigated as a major component in various application areas, such as a
gate dielectric in complementary metal-oxide-semiconductor
Experimental
In preparing the precursor solutions for the sol-gel deposition,
acetic acid (CH CO H) and 2-methoxyethanol (CH OCH CH OH)
were used as solvents, and the following metal-based nitrate
hydrates were used as metallic components: zirconyl nitrate hydrate
1
,2
3
2
3
2
2
(
access memory devices. In particular, ZrO
MOS) devices and a capacitor dielectric in dynamic random
2
2 2
and HfO have
attracted a lot of attention due to their high dielectric constant, wide
band gap, and excellent electrical and chemical stability. Accord-
[
ZrO(NO
3
)
2
ÁxH
2
O], yttrium nitrate hexahydrate [Y(NO
3
)
3
Á6H
2
O],
3
gadolinium nitrate hexahydrate [Gd(NO ) Á6H O], dysprosium ni-
3
3
2
ing to their bulk phase diagrams, they hold a range of crystalline
phases—monoclinic, tetragonal, and cubic phases—and the mono-
clinic is the equilibrium phase at typical device fabrication tempera-
trate hydrate [Dy(NO
3
)
3
ÁxH
2
O], and cerium nitrate hexahydrate
Ce(NO O]. All the chemical precursors were thoroughly
[
mixed to form pure or doped ZrO solutions having an identical
3
)
3
Á6H
2
4
2
tures. Because the dielectric constant—one of the most important
dielectric characteristics—strongly depends on the crystalline phase,
concentration of 0.5 M by magnetic stirring at room temperature,
and each solution was filtered using a 0.5 ꢀm hole-sized syringe fil-
ter for particle removal. In the case of the doped ZrO films, the
2
5
–7
8–12
many theoretical
the possible stabilization of these high-k dielectric films, especially
HfO , from monoclinic to tetragonal and/or cubic phase by the dop-
and experimental studies
have focused on
mixing ratios of the solutes were controlled to achieve a dopant con-
centration of 10 cation atom %. Before the sol-gel deposition,
p-type Si (100) substrates of dimensions 2 Â 2 cm were degreased
using trichloroethylene to remove the possible organic contaminants
without removing the native oxide layer. Each precursor solution
containing different dopant species was then spin-coated on the
cleaned Si substrates at a rotational speed of 3000 rpm for 30 s. The
2
ing of several elements, such as Si, Ge, Al, Sc, Y, and various rare
earth elements. However, these studies have focused on the micro-
structural/dielectric properties of films undergoing either a nearly-
equilibrium deposition environment or a post-deposition annealing
process at high temperature, without revealing the initial, low-tem-
perature kinetics. In addition, the trivalent doping atoms have been
postulated to generate oxygen vacancies which may deteriorate the
ꢁ
spin-coated films were dried at 100 C for 10 min to remove the sol-
ꢁ
vents and finally sintered at 400 C for 60 min in air ambient.
After the formation of the various doped-ZrO films, the final
1
3,14
electrical properties of the dielectric film.
However, experimen-
2
tal confirmation by a systematic and in-depth comparison using dop-
ing elements with different valence numbers is rare.
doping concentrations (cation atomic ratio) were measured to be
around 13 atom %, which is close to the initial concentration of the
precursor solution (10 atom %), according to the separate energy
dispersive X-ray spectroscopy measurements made during high-re-
solution transmission electron microscopy (HR-TEM) analyses. The
Although many studies on these high-k dielectric films have
been performed by various vacuum-based deposition methods, such
as sputtering, chemical vapor deposition, and atomic layer deposi-
tion, chemical solution deposition, e.g., sol-gel technique, is a useful
method for fast and systematic studies of high-k films doped with
various elements due to its precise and wide composition controll-
2
phases and crystalline orientations of all the sintered ZrO films
were characterized by X-ray diffraction (XRD) with a Cu Ka X-ray
source. Detailed microstructural analyses were performed using a
HR-TEM operating at an acceleration voltage of 200 kV after mak-
ing cross-sectional TEM samples. In order to investigate the doping
effect with different elements on the electrical properties of the
15
ability. In addition, although solution-based deposition might not
be a suitable choice for a current integrated circuit fabrication
process compared to vacuum-based deposition, it can provide the
following useful information: the effect of the dopant types on the
stabilization strength and the microstructural/electrical properties of
high-k films at a low temperature regime under an nearly-identical
experimental condition.
ZrO films, MOS capacitors were fabricated by depositing TaN (50
2
nm) capped with Al (10 nm) as the gate electrode using DC magne-
tron sputter and patterning via a lift-off process. The capacitance–
voltage (C–V) characteristics were measured by using an Agilent
E4980A LCR meter to extract various MOS capacitor parameters
and the leakage current characteristics were also measured by using
an HP 4145B semiconductor parameter analyzer under a gate injec-
tion condition.
2
In this study, moderately-thick ZrO films (24–25 nm) doped
with various elements— Y, Gd, Dy, and Ce—at an approximate cat-
ion concentration of 13 atom % to Zr were deposited by a sol-gel
technique and the dopant effects on the microstructural evolution,
especially stabilization process to tetragonal/cubic phase and the
dielectric properties were studied after annealing at a relatively low
ꢁ
temperature (400 C), with a particular focus on the effect of the va-
lence number of dopants.
Results and Discussion
Figure 1a shows the XRD patterns obtained from the sol-gel-de-
posited, pure and doped ZrO films after the densification process at
2
ꢁ
4
00 C. For the pure ZrO film used as a reference, two diffraction
2
peaks coming from the film were identified: one indexed as mono-
clinic phase and the other as tetragonal or cubic phase. The
z
E-mail: hsubkim@skku.edu
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G134
Journal of The Electrochemical Society, 158 (6) G133-G136 (2011)
However, these results were obtained after reaching close to the
equilibrium stabilized state due to the nearly-equilibrium deposition
conditions or higher post-deposition annealing temperatures than in
ꢁ
our experiment (400 C). Therefore, according to our low-temperature
annealing experiment, we assumed that although all the dopants used
in this experiment may eventually produce the tetragonal or cubic
phase ZrO
low temperature regime is strongly dependent on the dopant identi-
ties. Furthermore, all the doped ZrO samples were converted into a
2
film at a much higher temperature, their effectiveness at a
2
partially-stabilized state with tetragonal/cubic and monoclinic phases
ꢁ
after high temperature annealing (1000 C, 1 min in N ambient) as
2
shown in Fig. 1b and exhibited a high dielectric constant (ꢀ28 for
17
Y-doped ZrO film), which corroborated our assumption.
2
In order to measure the thicknesses and study the detailed micro-
structural evolution of the sol-gel-deposited ZrO films doped with
2
various elements, cross-sectional HR-TEM pictures were taken and
their images are shown in Fig. 2. For all the samples, the thicknesses
of the high-k dielectric layers were within a similar range of 24–25
2
nm. Because the sol-gel deposition of all the ZrO films was per-
formed on the Si wafers having a native oxide without HF cleaning,
the pure ZrO sample had approximately 1.5 nm-thick interfacial
2
oxide layer. Although an identical Si wafer was used, the interfacial
layer thickness was increased to 2.1–2.4 nm for the doped ZrO2
samples. The similar interfacial oxide thickening that was also
18
observed in our previous experiment —sol-gel deposition of 3.5–4
nm-thick ZrO films with different Ce contents on the HF-cleaned
2
Si substrate—was attributed to the catalytic effect of the dopant in
oxidizing the underlying Si substrate during the post-deposition
annealing process. For the pure and Ce-doped ZrO samples (see
2
Figs. 2a and 2e), a crystalline image with relatively well-aligned
grains was observed. However, Y, Gd, and Dy doping resulted in a
random distribution of the crystalline directions of the nano-sized
grains, with even some disordered structures appearing as shown in
Figs. 2b–2d. A consideration of these HR-TEM analyses in conjunc-
tion with the previously discussed XRD results suggested that the Y,
Gd, and Dy doping atoms having a valence number that differed
Figure 1. XRD spectra of the sol-gel-deposited pure ZrO
2
and doped-ZrO
2
ꢁ
films (Y, Gd, Dy, and Ce) after (a) a densification (400 C, 60 min in air) and
ꢁ
(
b) an additional high temperature anneal (1000 C, 1 min in N
2
). The dif-
fraction peaks from the monoclinic and tetragonal/cubic phase are labeled as
M and T/C, respectively. The strong diffraction peak observed at ꢀ33
degrees in all of the samples is the artifact coming from the substrate Si
(
200).
tetragonal and cubic phase could not be differentiated by XRD
because the peak positions were too close with each other. Because
the vacuum-deposited ZrO thin film was known to have a mixed
2
state of monoclinic and tetragonal phases, i.e., a partially stabilized
1
6
state, at this thickness range, it would be more reasonable to assign
the low angle peak to the tetragonal (011) plane in our 25 nm-thick
2 2
ZrO film deposited by a sol-gel method. For the ZrO sample
doped with Ce atoms having the same tetravalent state with Zr, a
similar mixed state was observed. The low angle diffraction peak
from the Ce-doped ZrO sample may have arisen from the tetrago-
2
nal, cubic or mixed state. On the contrary, for the ZrO films doped
2
with elements having a trivalent state, i.e., Y, Gd, and Dy, only a
monoclinic phase was observed, which implied the possible retarda-
tion of the high temperature phase formation by these doping ele-
1
0–12
ments. Several published reports
element doping with trivalent states to be effective in stabilizing the
HfO film, which is similar to ZrO in terms of the electrical and
have shown Y or rare earth
2
2
microstructural properties, to tetragonal or cubic phase, and
increased the dielectric constant at an optimum doping concentra-
tion of around 10 atom %, which is close to our doping condition.
Figure 2. Cross-sectional HR-TEM images of the sol-gel-deposited (a)
pure, (b) Y-doped, (c) Gd-doped, (d) Dy-doped, and (e) Ce-doped ZrO
films. I.L. stands for the interfacial layer.
2
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Journal of The Electrochemical Society, 158 (6) G133-G136 (2011)
G135
Figure 4. (Color online) Leakage current characteristics of the pure and
doped-ZrO films measured under a gate injection condition.
2
low trapping characteristics during the voltage sweep. However, the
apparent increase of the hysteresis over 300 mV was incurred with
the other doping elements and a typical electron trapping behavior
was observed: a positive shift of the C–V curve during a reversal
voltage sweep after a positive-to-negative initial sweep. Because the
doping elements used in this study were incorporated into the ZrO
film through a substitutional mechanism rather than an interstitial
2
13,19
one,
the observed increase of the bulk trapping characteristics in
the Y, Gd, and Dy-doped samples was believed to be strongly
related to the formation of the oxygen vacancies due to the differ-
ence of their number of valence electrons from that of ZrO .
2
Because the formed oxygen vacancies are positively charged, they
are eager to trap the injected electrons and become neutralized,
which coincides well with the observed positive shift of the C–V
curve during the voltage sweeping.
Figure 3. (Color online) (a) C–V curves of the sol-gel-deposited ZrO
as a function of the doping species measured at a frequency of 100 kHz. (b)
The extracted dielectric constant and the amount of hysteresis in the ZrO
films with different doping species.
2
films
2
Figure 4 shows the leakage current characteristics of the sol-gel-
deposited doped-ZrO2 films as a function of the dopant species
measured under a gate injection condition. Y, Gd, and Dy doping
with that of Zr, hindered the stabilization of the ZrO film to the tet-
2
ragonal or cubic phase and deteriorated the crystallinity of the film
during the annealing process at the relatively low temperature.
Electrical analyses, such as C–V and leakage current density–
voltage (J–V) measurements, were performed after fabricating MOS
capacitor structures using Al/TaN gate electrodes. Figure 3a shows
the C–V curves measured at a frequency of 100 kHz as a function of
the dopant species, and the extracted dielectric constant and the
amount of the hysteresis are plotted in Fig. 3b. The dielectric con-
stants of the high-k films were calculated from the accumulation ca-
pacitance values measured at a gate bias of À2 V. In order to decou-
ple the effects of the interfacial oxide and high-k layers, the
physical thicknesses of all the layers measured from the HR-TEM
images were used to extract the dielectric constants of the high-k
were expected to increase the leakage current of the ZrO film due
2
to their generation of oxygen vacancy defects. However, the leakage
current was significantly decreased compared to the pure ZrO film.
2
Because the physical thicknesses of all the doped ZrO films are
2
quite similar, this suppression of the overall leakage current was
attributed mainly to the doping-induced thickening of the interfacial
oxide, as observed in the aforementioned HR-TEM results (see Fig.
2). In addition, the Ce-doped ZrO film exhibited the lowest leakage
2
current in the high field region at similar high-k and interfacial oxide
thicknesses, compared to the other doped samples, possibly due to
an additional densification effect induced by the Ce doping.
Conclusion
films. Also, the interfacial oxide was assumed to be a pure SiO
layer with a dielectric constant of 3.9. For the sol-gel-deposited pure
2
The dopant effect on the phase stabilization and dielectric prop-
erties of ZrO films after a low temperature anneal was investigated
ZrO film, the extracted dielectric constant was close to 20. When
2
2
Y, Gd, and Dy atoms were added to the ZrO
2
film, the dielectric
by doping trivalent (Y, Gd, and Dy) or tetravalent (Ce) atoms at
ꢀ13 cation atom % into the sol-gel-deposited ZrO films. After a
constant of the high-k layer was slightly lowered to 16–18 possibly
due to the retardation of the tetragonal/cubic phase formation and/or
decreased crystallinity, which were evidenced in the previous XRD
and HR-TEM results. However, the Ce doping significantly
2
ꢁ
densification process conducted at a temperature as low as 400 C
for 60 min in air ambient, the trivalent dopants retarded the stabili-
zation of the tetragonal or cubic phase and decreased the dielectric
constant as a result. In addition, the C–V hysteresis was significantly
increased by the trivalent atom doping, which was attributed to the
formation of numerous oxygen vacancies due to the difference of
the valence number with that of the substituted Zr atoms. On the
contrary, doping with Ce atoms, having the same valence number as
that of Zr, yielded an enhanced densification effect without hinder-
ing the tetragonal or cubic phase formation and hysteresis
increased the dielectric constant of the ZrO
2
film up to ꢀ26, which
could be explained not only by the Ce-O bond formation, but also
by the densification and tetragonal/cubic phase formation effect.
Another interesting trend observed in the C–V measurement
results was the effect of doping on the hysteresis characteristics of
2 2
the doped ZrO films. For the pure and Ce-doped ZrO films, the
measured hysteresis was lower than 30 mV, exhibiting excellent
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G136
Journal of The Electrochemical Society, 158 (6) G133-G136 (2011)
characteristics, and increased the dielectric constant compared to
the pure ZrO film.
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Acknowledgments
This work was supported by National Research Foundation of
Korea Grant funded by the Korean Government (2010-0017004).
Sungkyunkwan University assisted in meeting the publication costs of
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