Research Article
www.acsami.org
Effect of Zr Content on the Wake-Up Effect in Hf1−xZrxO2 Films
Min Hyuk Park,†,‡ Han Joon Kim,† Yu Jin Kim,† Young Hwan Lee,† Taehwan Moon,† Keum Do Kim,†
Seung Dam Hyun,† Franz Fengler,‡ Uwe Schroeder,‡ and Cheol Seong Hwang*,†
†
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Department of Materials Science and Engineering, and Inter-University Semiconductor Research Center, Seoul National University,
Seoul 151-744, Republic of Korea
‡
NaMLab gGmbH, Noethnitzer Strasse 64, 01187 Dresden, Germany
ABSTRACT: In this study, the changes in the structural and electrical
properties of ferroelectric Hf1−xZrxO2 films with various Zr contents
(0.26−0.70) were systematically examined during electric field cycling,
resulting in a “wake-up” effect. To quantify the degree of wake-up effect, a
“variable” polarization as the difference between remanent and saturation
polarization was suggested as a new parameter, which could be calculated
by excluding the linear dielectric contribution from the total electric
displacement. Here, the variable polarization value could be minimized for
an optimized Zr content of 0.43, which was slightly lower than the value
for the largest remanent polarization. The polymorphism in Hf1−xZrxO2 thin films is known to be complicated due to the
relatively small energy differences between various phases, such as the monoclinic, tetragonal, and orthorhombic phases. The
variations in the polarization-electric field characteristics and dielectric constant values could be qualitatively and quantitatively
understood based on the competition of various polymorphs that are dependent on the Zr content. Furthermore, a schematic
model for the spatial distribution of mixed phases was suggested for Hf1−xZrxO2 films with various Zr contents based on the
experimental observations.
KEYWORDS: hafnium oxide, ferroelectrics, wake-up, endurance, nonvolatile memory
■
INTRODUCTION
The ferroelectricity and antiferroelectricity (more precisely,
field-induced ferroelectricity in this case) of doped HfO2 films
and Hf1−xZrxO2 (x = 0.0−1.0) films have been intensively
studied since they were first reported in the early 2010s.1−5 The
ferroelectricity of the Hf1−xZrxO2 thin film could be observed
with the Zr composition range near x = 0.5, which could be
ascribed to the emergence of the non-centrosymmetric
orthorhombic phase (o-phase; space group, Pca21), while
antiferroelectricity could be observed when x is larger than
∼0.7.2,3,6,7 Recently, an interesting two-step polarization
switching procedure involving an intermediate nonpolar
tetragonal phase (t-phase; space group, P42/nmc) was also
reported for the Hf0.40Zr0.60O2 film.8 In that report, the effect of
the composition on the polarization-electric field (P−E)
characteristics of the film could be understood based on the
classical first-order phase transition theory, and it was
elucidated that the Zr contents strongly affected the transition
temperature of the Hf1−xZrxO2 film.8 The Hf0.5Zr0.5O2 film
could have a remanent polarization (Pr) larger than ∼20 μC/
cm29,10 and could endure up to 109 switching cycles with an
electric pulse height of 2.5 MV/cm.11,12 The Hf0.5Zr0.5O2 films
are highly promising for ferroelectric memory applications due
to their Si compatibility, mature deposition processes including
atomic layer deposition (ALD), large electric band gap, and
strong resistivity to hydrogen-induced degradation.5,13 Cheng
and Chin reported that the ferroelectric field effect transistor
with Hf0.5Zr0.5O2 gate oxide can be utilized as a replacement of
© 2016 American Chemical Society
the current dynamic random access memory when it is
operated using a relatively low electric field.14 They also
reported that a subthreshold swing smaller than 60 mV/dec
could be achieved by utilizing a Hf0.65Zr0.35O2 film as a negative
capacitor,15 although the effective operation of negative
capacitance from the ferroelectric layer is currently being
debated.16−19 For ferroelectric HfO2 films doped with other
dopants, the application as a capacitive layer of the onetransistor-one-capacitor-type memory and as a gate oxide of the
one-transistor-type ferroelectric random access memory was
also reported.20,21 The antiferroelectric Zr-richer Hf1−xZrxO2 (x
> 0.7) films, on the other hand, are considered promising for
various applications, such as for electrostatic energy storage,
pyroelectric energy harvesting, and electrocaloric cooling.5,6,22
Recently, Hoffmann et al. also reported a similar application of
Si-doped HfO2 films.23
For the ferroelectric Hf0.5Zr0.5O2 films, pre-electric cycle
treatment is required to exploit their full Pr value due to the socalled “wake-up effect”.9−11,24−26 The wake-up effect can be
observed not only in the Hf1−xZrxO2 films but also in HfO2
films doped with other dopants.9−11,24−28 In the pristine state,
the shape of the P−E hysteresis is usually pinched, showing
relatively smaller Pr and coercive field (Ec) values.9−11,24−28
After a certain number of repetitive polarization switching
Received: March 24, 2016
Accepted: May 30, 2016
Published: May 30, 2016
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Figure 1. Topography images analyzed using an atomic force microscope for 9.2 nm thick (a) ZrO2, (b) Hf0.30Zr0.70O2, (c) Hf0.49Zr0.51O2, (d)
Hf0.74Zr0.26O2, and (e) HfO2 films, respectively, on a TiN/Ti/SiO2/Si substrate and (f) a TiN/Ti/SiO2/Si substrate.
cycles, however, the P−E loop is recovered from the pinching,
and the ferroelectric films have larger Pr values compared to
those in a pristine state.9−11,24−28 The possible origins of the
wake-up effect were reported to be the depinning of pinned
domains and/or nonferroelectric-to-ferroelectric phase transitions especially near the interface region with the electrodes.9−11,24−28 Zhou et al. first examined the wake-up effect in
Si-doped HfO2 films and suggested that the wake-up effect
might result from the depinning of the domains pinned by
defects in Si:HfO2 films.24 Martin et al. reported that the
monoclinic phase (m-phase; space group, P21/c) could be
transformed into o-phase during the wake-up process of Sidoped HfO2 films based on a transmission electron microscope
(TEM) study.27 Lomenzo et al. suggested that the t- to o-phase
transition might be the reason for the wake-up effect,28 and this
was recently reconfirmed by the authors using pulse switching
measurement.9 In the authors’ previous work, the thickness of
the t-phase interfacial layer was estimated to be ∼1.2 nm in the
pristine state, which became almost negligible after >105-time
wake-up field cycling.9
As both the requirement of pretreatment and the change in
the Pr value during the repetitive write/erase process are
unfavorable for the ferroelectric memories, an already wokenup state is necessary for pristine films. There have been only a
handful of reports on this issue, however, which may be
ascribed to the short history of the field. It has already been
reported that the polymorphism of Hf1−xZrxO2 films is highly
complicated, and that the problem of which phase will emerge
is affected by various factors. In addition, the direct structural
analysis of this complicated nanoscale behavior between the
various polymorphs is very difficult even with the use of the
state-of-the-art structural analysis method.29 As such, the
examination of the wake-up effect in a nanoscale thin film
should be highly challenging.9−11,24−28 In this study, the
evolutions of the P−E curves with ferroelectric switching cycles
from the various Hf1−xZrxO2 films with Zr contents (x value)
ranging from 0.26 to 0.70 were examined. Such evolutions were
used to elucidate the probable structural changes in the films
with cycling, which will contribute to the more detailed
understanding of the ferroelectric nature of this new class of
functional materials.
■
RESULTS AND DISCUSSION
Before the wake-up effect in the Hf1−xZrxO2 films are examined,
new parameters that could quantify the degree of wake-up
effect were defined. Up to now, the changes in the Pr and Ec
values have been considered parameters that can quantify the
degree of wake-up effect.9−11,24−28 This work, however,
adopted more refined parameters, as shown below, to more
quantitatively and accurately reveal the variation in the Pr value
with the wake-up cycles. From the careful review of the
previous reports on the wake-up effect of doped HfO2 or
Hf1−xZrxO2 films by the authors and other groups,9−11,24,30 an
interesting common observation was found. During the wakeup process, the changes in the maximum polarization (more
precisely, maximum electric displacement) value of Hf1−xZrxO2
or doped HfO2 films were lower than ∼5% while those in the Pr
value were higher than ∼30%. This appears reasonable because
the maximum polarization under the maximum electric field
condition includes the contributions from the Pr, which must be
largely influenced by the wake-up process, and the dielectric
displacement from the field-induced polarization of the lattice
(the background dielectric constant (εr) is responsible for this
component), which has little relevance to the wake-up process.
From the pulse switching measurement of Hf0.5Zr0.5O2 films in
the authors’ previous study, it was suggested that interfacial tphase materials are present near the top and bottom TiN
electrodes, and that the thicknesses of these interfacial layers
decreased during the wake-up field cycling.9−11 This nonferroelectric interfacial layer should form a large depolarization field,
and a part of the film could be depolarized when the electric
field is removed. If the interfacial t-phase is diminished by the
field cycling, the depolarization effect disappears, and the wakeup effect can be induced. On the other hand, the t-phase can be
transformed to a ferroelectric o-phase under sufficiently high
electric field, and changed back to a t-phase after removal of
electric field. Thus, this field-induced phase transition may also
vary during wake-up cycling.
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To accurately and quantitatively estimate such effects related
to the interfacial layers, the contribution of the εr of the
Hf1−xZrxO2 ferroelectric films to the estimated P−E curves
must be removed. Although the εr value of a ferroelectric
Hf1−xZrxO2 film (∼30) is smaller than those of perovskite
ferroelectrics such as Pb(Zr,Ti)O3 by an order of magnitude,
the electric field for P−E measurement is much higher than
those of perovskite ferroelectrics.5 As a result, the dielectric
contribution to electric displacement becomes significant while
the Pr value of Hf1−xZrxO2 films is relatively smaller than that of
Pb(Zr,Ti)O3, making the relative contribution of the dielectric
portion to the P−E curve rather significant. Therefore, the real
P−E curves, free from the influence of the linear dielectric
contribution, were estimated from the experimentally measured
P−E curves (D−E, in fact, where D is the total displacement)
using eq 1. By contrast, the depolarization and reverse phase
transition from o- to t-phase can be estimated using eq 2, which
is called variable polarization (Pv) in this work and plays a role
as a useful measure of the wake-up effect, when the wake-up
effect is assumed to originate from the removal of the
depolarization effect according to the t- to o-phase transition
of the interfacial layers.
2P = 2D − 2ε0εrE
(1)
2Pv = 2Dmax − 2ε0εrEmax − 2Pr = 2Ps − 2Pr
(2)
Figure 2. (a) Grazing angle incidence X-ray diffraction patterns and
(b) dielectric constant (εr)−electric field curves of the Hf1−xZrxO2 (x
= 0.26−0.70) films, respectively.
phase could be observed at 2θ ∼ 28.5° and 31.5°, respectively,
although its intensity was much lower than that of the (111)
diffraction peak from the o-phase at 2θ ∼ 30.5°. When the Zr
contents are further decreased to 0.26, the intensities of the
(111) and (−111) peaks from the m-phase significantly
increased. The peak near 2θ ∼ 30.5°, which was previously
assigned to the (111) peak of the o-phase, was then better
assigned as the mixture of (101) of the t-phase and (111) of the
o-phase.1−5 The Rietveld refinement of the pattern resulted in a
decrease of the m-phase with increasing ZrO2 content, but the
t- and o-phases were difficult to distinguish due to the
similarities in their lattice structures. The cross-section
transmission electron microscopy (TEM) previously confirmed
the uniform film deposition and the minimum intermixing
between the Hf1−xZrxO2 film and the TiN electrodes.6,7,9,11,31
Figure 2b shows the εr−E curves of the Hf1−xZrxO2 films with
various Zr contents (x = 0.26−0.70). To estimate the linear εr
of the Hf1−xZrxO2 films free from the domain wall motions and
field-induced phase transition, the εr values were extracted from
the maximum electric field region, and these values were used
in the analysis in the later parts of this work. The extracted εr
values were 34.2, 33.4, 32.5, 30.5, 26.6, and 20.3 for x = 0.70,
0.60, 0.51, 0.43, 0.35, and 0.26, respectively (see also Figure
5c). These εr values were consistent with those in the previous
reports.3,6,7
Figure 3a−f shows the D−E curves of the TiN/Hf1−xZrxO2/
TiN (x = 0.70, 0.60, 0.51, 0.43, 0.35, and 0.26, respectively.)
capacitors measured using a ferroelectric tester with a virtual
ground measurement circuit. As can be seen in Figure 3a, the
Hf0.30Zr0.70O2 film showed a double loop, meaning that fieldinduced phase transition occurred between the t- and ophases.6,7 With the increasing number of electric field cycles,
the electric field required for the field-induced phase transition
decreased. Its origin will be further discussed later. From the
D−E curve of the Hf0.40Zr0.60O2 film in Figure 3b, the
characteristic two-step polarization switching, which is a
different phenomenon from the double loop shown in Figure
3a, can be observed in the pristine sample.8 After 105-time field
cycling, however, the typical ferroelectric single hysteresis loop
could be observed. For the Hf0.49Zr0.51O2 film, ferroelectric
single hysteresis could be observed even in the pristine state,
but the Pr values and the slopes of the D−E curve near Ec
significantly increased after over 105 time electric field cycling.
For the Hf0.57Zr0.43O2 film, the P−E curve in the pristine state
was not very different from the P−E curve of the Hf0.49Zr0.51O2
film. There was a slight change, however, in the slope of the D−
E curve near Ec, and the changes in the Pr values were also
relatively smaller compared to those in the Hf0.49Zr0.51O2 film
with electric field cycling. This difference will be discussed in
greater detail later. With the further decreasing Zr contents, the
where ε0, Dmax, Ps, and Emax are the permittivity of vacuum,
maximum D, saturation polarization, and maximum electric
field, respectively. The Pv could result from the polarization
change both from the transition between ferroelectric and
nonferroelectric phase and depolarization of ferroelectric phase.
In fact, E must take into account the different εr values of the
ferroelectric bulk layer and nonferroelectric interfacial layers,
but as they are not much different, E was simply calculated by
dividing the applied voltage by the total film thickness.3 When
the depolarization effect disappears during the wake-up process,
the 2Pv value may become almost zero after the wake-up field
cycling. The relative ratio of 2Pv compared to 2Ps can be used
to quantify the degree of wake-up effect, and as such, by using
this normalized 2Pv value with respect to the 2Ps value (2Pv,nor
= 2Pv/2Ps), the degrees of wake-up effect of ferroelectric
Hf1−xZrxO2 films with different 2Ps values could be fairly
compared.
Figure 1a−e shows the atomic force microscopy (AFM)
topography images of the Hf1−xZrxO2 films with Zr contents of
1.00, 0.70, 0.51, 0.26, and 0.00, respectively, deposited on a TiN
bottom electrode, whereas Figure 1f shows the topography
image of the TiN bottom electrode before the deposition of
Hf1−xZrxO2 films. The root-mean-square (RMS) roughness of
the TiN bottom electrode and the Hf1−xZrxO2 films were 0.79
and 0.97−1.65 nm, respectively, confirming the uniform and
conformal growth of the Hf1−xZrxO2 films. The relatively high
RMS roughness of ZrO2 might have resulted from the partial
crystallization of the ZrO2 film due to its relatively low
crystallization temperature compared to that of the Hf-richer
films. Figure 2a shows the grazing incidence X-ray diffraction
(GIXRD) patterns of the ∼9.2 nm thick Hf1−xZrxO2 films with
various Zr contents (x = 0.26−0.70). For a Zr content of 0.51,
which is known to have the strongest ferroelectric properties
with the largest Pr value, the diffraction peaks from the m-phase
could be hardly observed while the diffraction peaks from the ophase had high intensities. For a Zr content of 0.43, the
diffraction peak from the (111) and (−111) planes of the m15468
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Figure 3. Total electrical displacement (D)−electric field curves of the Hf1−xZrxO2 films for (a) 0.70, (b) 0.60, (c) 0.51, (d) 0.43, (e) 0.35, and (f)
0.26 Zr contents (x), respectively.
Figure 4. Polarization−electric field curves of the Hf1−xZrxO2 films for (a) 0.70, (b) 0.60, (c) 0.51, (d) 0.43, (e) 0.35, and (f) 0.26 Zr contents (x),
respectively.
Pr value rapidly decreased, and the slope of the D−E curve in
the pristine state also decreased, as can be seen in Figure 3e,f.
The film eventually became a linear dielectric at x = 0 (data not
shown).3 These trends in the D−E characteristics with varying
x values were consistent with the previous study results.3
As previously mentioned, the D value contains the
contribution of the linear dielectric response of the Hf1−xZrxO2
films. Therefore, the real P−E curves were extracted by
eliminating the contribution of the linear dielectric response.
For this purpose, the εr values extracted from Figure 2b were
used, and the P−E curves were recalculated using eq 1. Figure
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Figure 5. Variations in (a) double remanent polarization (2Pr) and (b) double saturation polarization (2Ps) as a function of the number of electric
field cycles. (c) Variations in dielectric constant (εr) extracted from capacitance−voltage and polarization−voltage measurements with varying Zr
content.
the 2Pr values decreased almost linearly. For the 2Ps values, on
the other hand, the trend was quite different from that for the
2Pr values. The 2Ps value remained at quite a high level even for
x = 0.7, which could be ascribed to the field-induced formation
of a ferroelectric phase in the Zr-rich samples under the
presence of a high electric field. In the Zr-rich samples, there
should be more t-phase at the zero electric field, although this
could not be clearly noticed in the structural analysis. A quite
high portion of t-phase can be transformed to o-phase,
however, at a high electric field because the electric field
could lower the free energy of the polar phase by P·E.3,6−8 For
the Hf0.40Zr0.60O2 film, the 3.26 MV/cm electric field was high
enough to achieve a saturated P−E hysteresis curve, meaning
that the 2Ps value can be as large as that of the Hf0.49Zr0.51O2
film. For the Hf0.30Zr0.70O2 film, the free-energy difference
between the t- and o-phases should be larger than that in the
Hf0.40Zr0.60O2 film; as such, the 3.26 MV/cm electric field was
not sufficient to achieve the saturated P−E curve.6,7 As a result,
the 2Ps value of the Hf0.30Zr0.70O2 film was slightly smaller than
those of the Hf0.49Zr0.51O2 and Hf0.40Zr0.60O2 films. With the
decreasing Zr contents from 0.51 to 0.26, both the 2Pr and 2Ps
values decreased almost linearly. This should be ascribed to the
increasing portion of m-phase, which is consistent with the
changes in the GIXRD patterns shown in Figure 2a. Figure 5c
shows the changes in the εr values as a function of the Zr
contents, with the εr values extracted from the εr−E curves
shown in Figure 2b and from the D−E curves shown in Figure
3 in the high electric field regions. The εr values were taken
from the largest electric field region to minimize the nonlinear
portion of εr. The εr values from εr−E curves were taken at the
electric field of ∼2.2 MV/cm, while the εr values from D−E
hysteresis were taken at ∼3.0 MV/cm. The differences between
these εr values were always smaller than 5, which corresponds
to the errors of <1.3 μC/cm2 in Pv value at the largest. The
effect of the Zr contents on the εr well matches the results of
the previous experimental and theoretical works.3,6,7
Figure 6a,b shows the changes in the 2Pv and 2Pv,nor values of
the Hf1−xZrxO2 (x = 0.26−0.70) films as functions of the
number of wake-up cycles. In the pristine state, the 2Pv value of
the Hf0.30Zr0.70O2 film was the largest among the samples with
various Zr contents, and the 2Pv value was ∼33.4 μC/cm2,
which slightly decreased to 31.6 μC/cm2 after 106 field cycles,
meaning that ∼81% of 2Ps is depolarized after the removal of
the electric field even in the woken-up state. For the
Hf0.40Zr0.60O2 film, there was a very large decrease in the 2Pv
value after the wake-up field cycling. The 2Pv value was ∼28.7
4a−f shows the results for the same samples shown in Figure 3.
As the linear dielectric contribution was eliminated, plateau
regions could be found in the top and bottom portions of the
curves while the general features of the P−E curves for the
different samples were mostly retained. The variations in the
wake-up behaviors could be grouped into three according to
the x values: for x = 0.51 and 0.43, the films were ferroelectriclike; for x = 0.35 and 0.26, they were still ferroelectric-like, but
their Pr values were significantly decreased; and for x = 0.70 and
0.60, the films showed severely distorted P−E curves. As for the
Hf0.49Zr0.51O2 film (see Figure 4c), the slope of the P−E curve
near Ec significantly increased with the increasing number of
wake-up cycles while the change in the slope of the P−E curve
of the Hf0.57Zr0.43O2 film was negligible (Figure 4d). Moreover,
the relative changes in the 2Pr value during the wake-up field
cycling were much larger in the Hf0.49Zr0.51O2 film than in the
Hf0.57Zr0.43O2 film. These differences mean that there exist
significant differences between the wake-up processes of the
Hf0.49Zr0.51O2 and Hf0.57Zr0.43O2 films despite the relatively
small composition difference. When the Zr contents were
further decreased to 0.35 and 0.26, however, the changes in the
Pr and Ps values were very different from those in the
Hf0.49Zr0.51O2 and Hf0.57Zr0.43O2 films. For the Hf0.65Zr0.35O2
and Hf0.74Zr0.26O2 films, both the Pr and Ps values significantly
increased during the wake-up field cycling, although the
magnitude of Pr even in the woken-up state was still much
lower than that in the Hf0.49Zr0.51O2 film (Figure 4e,f). The
Hf0.40Zr0.60O2 film showed an almost ferroelectric-like P−E
curve after only 105 cycles while it showed a distorted P−E
curve in the pristine state. In contrast, the Hf0.30Zr0.70O2 film
still showed a distorted P−E curve even after 108 field cycles
while the degree of distortion slightly decreased with the
increasing number of wake-up cycles.
Figure 5a,b shows the changes in the 2Pr and 2Ps values of
the Hf1−xZrxO2 films as a function of the x-value for the pristine
and variously cycled cases. In the pristine state, the 2Pr value of
the Hf0.49Zr0.51O2 film was the largest (∼32.1 μC/cm2), which
further increased to 40.0, 41.7, 41.8, and 42.1 μC/cm2 after 105,
105.3, 105.7, and 106 field cycles, respectively. The 2Ps value of
the Hf0.49Zr0.51O2 film in the pristine state was 40.1 μC/cm2,
which is larger than the 2Pr value by 8.0 μC/cm2, meaning that
∼20% of the Ps at the high electric field was depolarized after
the removal of the electric field (2Pv ∼ 8.0 μC/cm2). With the
increasing number of wake-up cycles, however, the 2Pv value
significantly decreased, and it became almost zero after 106 time
field cycling. With the increasing Zr contents from 0.51 to 0.70,
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switching tests.9 Kim et al. reported that the thickness of the
nonferroelectric layer of the Hf0.5Zr0.5O2 film was estimated as
∼1.2 nm, which was 13% of the thickness of the whole film (9
nm) in the pristine state, but it decreased to an almost
negligible level after the wake-up cycling.9
Tagantsev et al. systematically examined the effect of the
nonferroelectric interfacial layer on the shape of the P−E
hysteresis, where they categorized the parameters extracted
from the P−E curves into two groups.33 In one group, the Pr
and slope of the P−E curve near Ec were strongly affected by
the presence of a thin nonferroelectric layer.33 In the other
group, the Ps and Ec were insensitive to the presence of a
nonferroelectric layer.33 Although this categorization was
originally suggested for the perovskite-based ferroelectrics, it
was also well applied to the case in this study. The 2Pv and
2Pv,nor values of the Hf0.57Zr0.43O2 film in the pristine state were
1.4 μC/cm2 and 5.0%, respectively, which are much lower than
those of the Hf0.49Zr0.51O2 film. These smaller variations from
the Hf0.57Zr0.43O2 film across all the switching cycles suggest
that the depolarization effect is much smaller. This finding in
turn suggests that the portion of the interfacial nonferroelectric
layer is also smaller despite the relatively minor composition
variation. The reasons for the formation of nonferroelectric
interfacial layers and for their different degrees depending on
the x values are discussed in detail below. The influences of the
bulk and interfacial regions on the estimated ferroelectric
performance should be considered simultaneously.
First, it should be noted that the m-phase is the most
thermodynamically stable phase for all the compositions if they
are in bulk material. The possible reasons for the formation of
the metastable phases other than m-phase, including the
nonferroelectric t-phase and the ferroelectric o-phase, are the
involvement of the surface (interface) energy, (asymmetric)
strain, and point defects. The relative free energy of the various
polymorphs in the Hf1−xZrxO2 films was calculated based on
the values reported by Materlik et al., and the results are
presented in Figure 7a,b.32 Here, the relative free energy of the
Figure 6. Variations in the (a) double variable polarization (2Pv) and
(b) normalized 2Pv(2Pv/2Ps) as a function of the electric field cycles.
μC/cm2 in the pristine state, which is ∼66% of 2Ps, but it
decreased to 14.0, 12.9, 11.8, and 10.8 μC/cm2 after 105, 105.3,
105.7, and 106 time electric field cycling, respectively. The
normalized 2Pv decreased from ∼66 to ∼25% after 106 time
electric field cycling. The difference between the changes in the
2Pv values of the Hf0.30Zr0.70O2 and Hf0.40Zr0.60O2 films could be
understood based on the phase transition during wake-up field
cycling. From the 2Pr values in the pristine state, it could be
estimated that a substantial portion of the films existed in tphase both for the Hf0.30Zr0.70O2 and Hf0.40Zr0.60O2 films under
zero external field. This estimation stems from the double-loopshaped P−E hysteresis curve. At a low E, the t-phase has the
lowest energy among the three phases, which shows an almost
nonferroelectric property where the c/a ratio of the crystal
lattice (1.015−1.029) is smaller than the 2a/(b + c) of the ophase (1.036−1.042). It should be noted that the longest axes
of the t- and o-phases are the c- and a-axes, respectively. Under
a high enough E, the electrostatic energy (P·E) overcomes the
energy difference between the t- and o-phases, and the film
transforms to o-phase, which induces large Ps values. When the
E is removed, the stable t-phase is recovered, and small Pr
values are generally achieved. From the difference in the
magnitude of the electric field required for field-induced phase
transition, it could be noticed that the energy difference
between the o- and t-phases in the Hf0.30Zr0.70O2 film was larger
than that in the Hf0.40Zr0.60O2 film, which is also consistent with
the results of the previous theoretical works.32 This energy
difference should affect the permanent phase transition during
the wake-up process as well as the reversible phase transition
between the t- and o-phases, which can be noted from the
double P−E hysteresis curve.32 The significant decrease in the
2Pv and 2Pv,nor values for the Hf0.40Zr0.60O2 film shown in the
figures suggest that the transition from t- to o-phase during the
wake-up process in the Hf0.40Zr0.60O2 film should be much
stronger than that in the Hf0.30Zr0.70O2 film.
The 2Pv value of the Hf0.49Zr0.51O2 film (∼8.0 μC/cm2) in
the pristine state was much smaller compared to the Zr-richer
films, but it was still ∼20% of the 2Ps value. After 105, 105.3,
105.7, and 106 time wake-up field cycling, the 2Pv (2Pv,nor) value
decreased to 2.9 μC/cm2 (6.7%), 1.2 μC/cm2 (2.8%), 0.6 μC/
cm2 (1.4%), and 0.2 μC/cm2 (0.5%), respectively. From the
quite large 2Pv value in the pristine state, the Hf0.49Zr0.51O2 film
contained a certain portion of t-phase, but its relative portion
should be much smaller than that in the Hf0.40Zr0.60O2 film. The
almost zero values of 2Pv (2Pv,nor) in the full wake-up state (106
cycles) suggest that almost the entire nonferroelectric phase
was transformed to the ferroelectric phase, which is consistent
with the results of the previous work based on the pulse
Figure 7. Calculated relative free energy of the monoclinic,
orthorhombic, and tetragonal phases for various Zr contents at (a)
room temperature and (b) annealing temperature (773 K).
m-, o-, and t-phase is shown for various Zr contents at room
temperature (Figure 7a) and at the annealing temperature of
773 K (Figure 7b), respectively. For this calculation, the film
thickness and grain diameter was assumed to be 9 nm for all the
films. Interestingly, at the 773 K annealing temperature, the
most stable phase was the t-phase for all the compositions.
Therefore, the Hf1−xZrxO2 films might be crystallized into the tphase at 773 K. At room temperature, however, the stable
phase for the samples with x = 0.26−0.51 was the o-phase while
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approximate calculations based on a previous study,32 however,
the grain diameters required for stabilization of t-phase for
Hf0.74Zr0.26O2 and Hf0.65Zr0.35O2 films are 3.3 and 4.4 nm,
respectively, when a columnar structure is assumed. Since the
required grain diameters are only ∼36 and 49% of the film
thickness, the relative volumetric portion of the grains with
such small diameters should be very low. Therefore, it is
believed that the contribution of t-phase in Hf0.74Zr0.26O2 and
Hf0.65Zr0.35O2 films might be negligible to the macroscopic
electrical properties. It should be noted that the mechanism of
wake-up effect in Hf-rich films are not clearly understood yet,
but the aforementioned explanation might be a reasonable
conclusion which could be drawn from the macroscopic change
in electrical characteristics.
The change in polymorphism in the Hf1−xZrxO2 film when x
increased from 0.51 is rather easy to understand. With
increasing Zr content, the relative free energy of the t-phase
compared to that of the o-phase decreases. As a result, the
portion of the t-phase under no external field increases with the
increasing Zr content. When the surface energy and grain
boundary energy effect are considered, small grains might tend
to exist in the t-phase. With a certain electric field magnitude,
however, the free energy of the o-phase can be lower than that
of the t-phase because the free energy of the polar phase can be
lowered. When a polarization of 20 μC/cm2 and a 3.26 MV/cm
electric field are considered, a free energy difference of 13
meV/f.u. can be overcome. This matches well the experimental
observation for the Hf0.40Zr0.60O2 film at the pristine state and
for the Hf0.30Zr0.70O2 film at both the pristine and woken-up
states. A possible migration of oxygen vacancies can also
produce changes in the P−E characteristics of Hf0.40Zr0.60O2
and Hf0.30Zr0.70O2 films. Field-induced phase transition may
occur in antiferroelectric films during the nucleation and
growth of the ferroelectric phase within the nonferroelectric
phase. This is similar to polarization switching through the
nucleation and growth of the oppositely polarized phase in
ferroelectrics. For this process, nucleation of the polar phase in
a nonpolar matrix should occur near the electrodes. Because of
the increased oxygen vacancy concentration near the TiN
electrode, the t-phase can be further stabilized compared to the
o-phase. This should increase the critical nuclei size of the polar
phase, and as such, field-induced phase transition from the t- to
ophase should occur at a relatively high electric field. With an
increasing number of field cycles, nonetheless, the oxygen
vacancies may diffuse though the film, and the initially high
oxygen vacancy concentration near the electrode region may
decrease.35 This mechanism for the wake-up effect was recently
suggested for Sr:HfO2 films, where the computational
simulation of the motion of the oxygen vacancies could explain
the experimental observations during wake-up field cycling.35
Starschich et al. also reported that the wake-up process in their
Y-doped HfO2 films should be strongly related to the motion of
the oxygen vacancies.36 As a result, the electric field required for
field-induced phase transition should decrease. On the other
hand, a portion of the Hf1−xZrxO2 film, which exists in the
metastable t-phase even after cooling due to sluggish kinetics,
may be permanently changed to a stable o-phase during
electrical field cycling. This mechanism can explain the
emergence of an almost ferroelectric-like P−E hysteresis loop
for the Hf0.40Zr0.60O2 film, which can also be ascribed to the
relatively small free-energy difference between the t- and ophase. Figure 8a−d shows the schematic film structure of the
that for the samples with x = 0.60 and 0.70 was the t-phase.
Therefore, the Hf1−xZrxO2 films might change to o-phase for
the samples with x = 0.26−0.51 when they are cooled down to
room temperature, while the Hf0.40Zr0.60O2 and Hf0.30Zr0.70O2
films still exist in t-phase. These expected phase changes after
crystallization annealing, however, were different from the
experimental observations for the Hf-rich samples. These
differences might have originated from the overestimated
surface energy effect due to the underestimated grain size of the
Hf1−xZrxO2 films. Park et al. and Kim et al. reported that the
average grain sizes of a ∼10 nm thick Hf0.5Zr0.5O2 film were
14.4 and 14.2 nm, respectively.4,31 These values are relatively
larger than 9 nm, which was used by Materlik et al.32 When the
larger grain size is considered, the relative free energy of the mphase can decrease by ∼9 meV/f.u. for the Hf0.74Zr0.26O2 film
when the hexagonal columnar grain is considered.32 Moreover,
there should be a certain grain size distribution, which strongly
affects the free energy of each phase. It should be noted that the
energy differences between the m- and o-phases for the
Hf0.74Zr0.26O2 and Hf0.65Zr0.35O2 films were smaller than 12
meV/f.u., and that those between the t- and o-phases in all the
samples were smaller than 7 meV/f.u.
The oxygen vacancies can also affect the polymorphism in
Hf1−xZrxO2 films.34 Hoffmann et al. reported that based on
their computational calculations, the presence of oxygen
vacancies further stabilizes the t-phase compared to the ophase.34 They also suggested that oxygen vacancies might be
formed in the HfO2-based dielectric layers due to a reduction
by reactive metal electrodes such as TiN and TaN.34 It was also
reported that the surface energy of the t-phase is lower than
that of the o-phase, meaning that the interfacial region should
favor the t-phase.32 Therefore, the interfacial region in
Hf1−xZrxO2 films tends to form a t-phase even when the bulk
region of the film forms an o-phase. The oxygen vacancy
formation energy of HfO2 is higher than that of ZrO2 due to
the stronger bond between Hf and O compared with that
between Zr and O. Therefore, as the Zr concentration
decreases (as the Hf concentration increases), the tendency
to have oxygen vacancies within the interfacial region
diminishes, which in turn results in decreased t-phase formation
at the interface. This can explain the decreased depolarization
effect when x = 0.43 compared with the case where x = 0.51.
Nevertheless, the further decreased Zr concentration induced
higher m-phase formation within the bulk region, decreasing
the Pr and Ps values. The variations in the properties for the
films with x = 0.35 and 0.26 can be understood from the same
line of reasoning. As the Zr content further decreased, the
depolarization effect could have also been further diminished
due to the even thinner interfacial t-phase formation. The 2Pv
values in Figure 6a, however, were in fact slightly higher than
those in the case with x = 0.43. This may indicate that the
interfacial region actually became slightly thicker, which may be
ascribed to the increased m-phase formation at such
composition. The m-phase content of the bulk portion must
have been much higher, and the overall Pr and Ps values must
have been the smallest among the six samples. The higher
2Pv,nor value of the sample shown in Figure 6b compared to that
of x = 0.43 was due to the largely decreased 2Ps value. Martin et
al. reported the m- to o-phase transition during the wake-up
process based on a TEM study.27 This can consistently explain
the wake-up effect in the aforementioned sample in this study.
It should be noted that the existence of t-phase in Hf0.74Zr0.26O2
and Hf0.65Zr0.35O2 films cannot be totally excluded. From the
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films with various compositions in the pristine state, which were
deduced from the discussion above.
6a. With further decreasing Zr content, an increase even in the
εr values could be observed. For the Hf0.65Zr0.35O2 film, the εr
value increased from 22.4 to 23.3 after 105.33 time field cycling,
and then it slightly decreased to 23.0 after 106-time field
cycling. The εr value of the Hf0.74Zr0.26O2 film slightly increased
from 21.2 to 21.5 after 105 time field cycling, and then it slightly
decreased to 21.3 after 106 cycles. The change of εr values with
increasing number of field cycling was estimated from the slope
of D−E curves under high electric field. The interfacial phase
transition can contribute to the change of εr with two different
aspects. First, the change of εr of interfacial region can affect the
average εr of the whole film. So, t- to o- and m- to o-phase
transition might result in an increase and decrease of the
average εr. Second, the decrease in thickness of interfacial
nonferroelectric region decreases the depolarization field, and
affects the slope of D−E hysteresis. Both t- to o- and m- to ophase transition may decrease the slope of D−E hysteresis, with
concurrent decrease in εr value. As a result, the magnitude of εr
change due to m- to o-phase transition is generally smaller than
that due to t- to o-phase transition.
Figure 9b shows the changes in the εr (Δεr) value from its
initial values as a function of the Zr content. For the
Hf0.30Zr0.70O2 film, the εr value slightly decreased after the
electric field cycling. This could be ascribed to the t- to o-phase
transition, but only a small portion of the film should be
transformed due to the relatively large energy difference
between the t- and o-phases. For the Hf0.40Zr0.60O2 and
Hf0.49Zr0.51O2 films, the changes in the εr values were much
larger than that in the Hf0.30Zr0.70O2 film. The small free-energy
difference between the t- and o-phases might be the reason for
this large change. The increase in the εr value in the initial stage
of the electric field cycling can be ascribed to the m- to o-phase
transition in the Hf0.65Zr0.35O2 and Hf0.74Zr0.26O2 films, while
the decrease in the εr value with the further increasing number
of electric field cycles might be related to the fatigue or leakage
current increase. For all the films, the breakdown mechanism
that was observed after the severe-fatigue test (electric field
cycling with E ∼ 3.0 MV/cm) was a hard breakdown, meaning
that there was an abrupt increase in the leakage current.
Therefore, the charges injected from the electrodes should
strongly affect the electrical properties of Hf1−xZrxO2 films,
especially with an increasing number of electric field cycles. The
m- to o-phase transition in the Hf-rich samples during the
wake-up field cycling could not be understood as having a
similar mechanism as in the Zr-rich samples because the
decrease in oxygen vacancy concentration in the interfacial
regions might further stabilize the m-phase interface. Therefore,
a slightly different mechanism is suggested for Hf-rich films.
When a high electric field is applied across Hf1−xZrxO2 films,
portions of the m-phase might be transformed to o-phase
because the free energy of the o-phase can be lowered by P·E
hysteresis measurements. At the same time, oxygen vacancies
diffuse from the interfacial to the bulk regions, although the
initial concentration of oxygen vacancies should be lower than
that in the Zr-rich films. As the oxygen vacancies tend to
stabilize the o-phase, a part of the m-phase may be transformed
to o-phase by the oxygen vacancies that diffused into the bulk
region under a high electric field. This phenomenon can occur
more preferentially along the grain boundary regions because
the diffusion of oxygen vacancies must be relatively higher in
the grain boundary region than in the film bulk region. Further
studies are required to verify the suggested mechanisms, which
is beyond the scope of the present study.
Figure 8. Schematic structure of the Hf1−xZrxO2 films with various Zr
contents (x): (a) 0.70, (b) 0.51, (c) 0.43, and (d) 0.35, respectively.
To further prove the suggested phase transition mechanism
during the wake-up process, the change in εr with an increasing
number of electric field cycles was examined. Figure 9a shows
Figure 9. Variations in the (a) dielectric constant (εr) as a function of
the electric field cycles. (b) Changes in εr (Δεr) as a function of the Zr
content vs the number of electric field cycles.
the change in εr with respect to the pristine values as a function
of the number of wake-up cycles. The εr values were extracted
from the highest electric field regions of P−E curves when the
electric field magnitude decreases. The εr values of the m-, o-,
and t-phases of most HfO2-based dielectric films are known to
be ∼17−20, 25−30, and 35−40, respectively.5 For the
Hf0.30Zr0.70O2 film in this study, the εr value in the pristine
state was 37.6, and it decreased to 37.5 and 36.3 after 105 and
106 wake-up cycles, respectively, meaning that the variation was
negligible. For the Hf0.40Zr0.60O2/Hf0.49Zr0.51O2 films, on the
other hand, the initial εr value of 34.6/34.5 decreased to 31.8/
31.5 and 30.3/28.7 after the occurrence of the same number of
wake-up cycles. This is consistent with the large changes in the
Pr and Pv values shown in Figures 5a and 6a. For the
Hf0.57Zr0.43O2 film, on the other hand, the variation in the εr
value was relatively small. The pristine εr value of 27.1
decreased to 27.0 and 26.7 after 105 and 106 time electric field
cycling, respectively. It was also consistent with the relatively
small change in the Pr and Pv values shown in Figures 5a and
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■
elsewhere.4−10 To electrically characterize the Hf1−xZrxO2 films, a
Pt(30 nm)/TiN(5 nm) top electrode was deposited via direct-current
sputtering through a shadow mask with a 300 μm hole diameter (the
TiN contacted the Hf1−xZrxO2 film). After the top-electrode
deposition, postmetallization annealing was performed for 30 s at
500 °C in an N2 atmosphere under 100 Torr pressure, using rapid
thermal annealing, for the crystallization of the films. The composition
and thickness of the Hf1−xZrxO2 films were examined using X-ray
fluorescence (Quant’X, Thermo SCIENTIFIC) and spectroscopic
ellipsometry (ESM-300, J.A. Woolam), respectively. The topography
of the samples was analyzed with an AFM (JSPM-5200, JEOL), using
the contact mode. The crystal structure of the Hf1−xZrxO2 films was
examined via GIXRD, using an X-ray diffractometer (X’pert Pro,
Panalytical). The incidence angle was fixed at 0.5°. The P−E
characteristics were measured using a ferroelectric tester (TF Analyzer
2000, Aixacct Systems). The capacitance−voltage characteristics were
measured using an impedance analyzer (4194A, HP) to achieve the
εr−E characteristics. For all the electric characterizations, the top
electrode was biased while the bottom electrode was grounded during
the measurements.
CONCLUSION
In this study, the effect of a different Zr content on the wake-up
process in Hf1−xZrxO2 films was systematically examined. For
the Hf0.30Zr0.70O2 and Hf0.40Zr0.60O2 films, an antiferroelectriclike double and broken P−E hysteresis behavior was shown in
the pristine state. With an increasing number of wake-up field
cycles, the electric field required for the nonferroelectric →
ferroelectric phase transition decreased. This phenomenon can
be ascribed to the decreasing oxygen vacancy concentration
near the electrodes, which may induce a decrease in the critical
nuclei size for field-induced phase transition. Moreover, it
appears that a portion of the metastable t-phase changed to ophase during the wake-up cycling, resulting in a decrease in the
εr value and an increase in the Pr value. For the Hf0.49Zr0.51O2
film, the deformed P−E hysteresis in the pristine state is
believed to have resulted from the existence of an interfacial
nonferroelectric t-phase, which induced the significant depolarization effect. This interfacial layer became negligible after the
wake-up field cycling. When the Zr content was slightly further
decreased to 0.43, the changes in the P−E characteristics with
wake-up cycling significantly decreased, suggesting a decrease in
the thickness of the interfacial t-phase. Therefore, adopting a
film with such Zr content can effectively suppress the wake-up
effect, although a slight decrease in the 2Pr value due to mphase formation is a trade-off. Therefore, this composition of
Hf0.57Zr0.43O2 would be beneficial for the memory application
with improved reliability with reasonable Pr value.
In this study, when the Zr content was further decreased to
0.35 and 0.26, it appeared that the m-phase portions in both the
interfacial layer and the bulk region increased, and the P−E
curve was largely degraded in the pristine state, which could
have been improved by a possible m- to o-phase transition
during wake-up cycling. This variation was accompanied by an
increase both in the εr and Pr values. Consequently, the wakeup effect of Hf1−xZrxO2 films may originate from the phase
transition from the nonferroelectric to the ferroelectric phase
both in the interface and in the bulk regions. The relatively
small differences in free energy of various phases with strong
surface and grain boundary energy effects and a large
magnitude of applicable electric field in HfO2-based ferroelectric materials result in quite strong wake-up effects during
electric field cycling. This has not been the case for the
conventional ferroelectric films, such as PZT. The high-enough
strength of the electric field to induce the diffusion of mobile
charged defects such as oxygen vacancies also affects the spatial
distribution and competition of various phases in the nanoscale
ferroelectric materials system.
Although the optimization of the composition of Hf1−xZrxO2
film could not totally suppress the wake-up effect, it
significantly weakened the wake-up effect. Moreover, there
might be several other control knobs that can further weaken
the wake-up effect besides composition control, such as varying
the process conditions, including the ALD temperature change,
annealing temperature change, precursor/oxygen source
replacements, and electrode materials change.
■
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: cheolsh@snu.ac.kr.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
The authors wish to thank Prof. A. Kersch and his group for the
valuable discussions the authors had with them. This work was
supported by the Global Research Laboratory Program
(2012K1A1A2040157) of the Ministry of Science, ICT, and
Future Planning of the Republic of Korea, and by a National
Research Foundation of Korea (NRF) grant funded by the
South Korean government (MSIP) (2014R1A2A1A10052979).
■
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