Research Article
www.acsami.org
A Complementary Metal Oxide Semiconductor Process-Compatible
Ferroelectric Tunnel Junction
Fabian Ambriz-Vargas,*,† Gitanjali Kolhatkar,† Maxime Broyer,† Azza Hadj-Youssef,† Rafik Nouar,‡
Andranik Sarkissian,‡ Reji Thomas,† Carlos Gomez-Yáñez,§ Marc A. Gauthier,† and Andreas Ruediger*,†
Centre Énergie, Matériaux et Télécommunications, INRS, Varennes, Québec J3X1S2, Canada
Plasmionique Inc., 9092 Rimouski, Brossard, Québec J4X2S3, Canada
§
Departamento de Ingeniería en Metalurgia y Materiales, Instituto Politécnico Nacional, Zacatenco 07738, México
†
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‡
S Supporting Information
*
ABSTRACT: In recent years, experimental demonstration of ferroelectric tunnel junctions (FTJ) based on perovskite tunnel barriers has
been reported. However, integrating these perovskite materials into
conventional silicon memory technology remains challenging due to
their lack of compatibility with the complementary metal oxide
semiconductor process (CMOS). This communication reports the
fabrication of an FTJ based on a CMOS-compatible tunnel barrier
Hf0.5Zr0.5O2 (6 unit cells thick) on an equally CMOS-compatible TiN
electrode. Analysis of the FTJ by grazing angle incidence X-ray
diffraction confirmed the formation of the noncentrosymmetric
orthorhombic phase (Pbc21, ferroelectric phase). The FTJ characterization is followed by the reconstruction of the electrostatic
potential profile in the as-grown TiN/Hf0.5Zr0.5O2/Pt heterostructure. A direct tunneling current model across a trapezoidal
barrier was used to correlate the electronic and electrical properties of our FTJ devices. The good agreement between the
experimental and theoretical model attests to the tunneling electroresistance effect (TER) in our FTJ device. A TER ratio of ∼15
was calculated for the present FTJ device at low read voltage (+0.2 V). This study suggests that Hf0.5Zr0.5O2 is a promising
candidate for integration into conventional Si memory technology.
KEYWORDS: ferroelectric tunnel junctions, CMOS process, nanoscale characterization, tunneling electroresistance effect,
electronic band alignment
endurance (106 cycles), nonvolatility, and a simple structure.2
The theoretical concept of an FTJ was proposed by Esaki in
1971.4 A conventional FTJ is composed of an ultrathin
ferroelectric layer (∼2.8 nm thick) sandwiched between two
metallic electrodes. By an electric field, the step barrier height at
both interfaces (electrode-ferroelectric) can be modulated due
to the polarization reversal of the tunnel barrier. This in turn
gives rise to two different electrical resistance states (tunneling
electroresistance effect (TER)) that can be codified as binary
information (“ON” and “OFF” states).5 Despite the advantages
of FTJs, this concept lost attention for more than 30 years due
to the difficulty in producing ferroelectric films with a thickness
compatible with coherent quantum mechanic tunneling.2 In
fact, for a long time, it was believed that the critical thickness
for ferroelectricity was in the 10−100 nm range. However,
recent strain engineering studies have shown that the
ferroelectricity of a given film can be markedly enhanced by
the strain caused by lattice mismatch between the film and the
substrate.6 In 2009, Garcia et al. reported the fabrication of an
1. INTRODUCTION
Nowadays, ferroelectric random access memories (Fe-RAM)
are commercially available products. This type of memory
combines the fast read/write access (∼10 ns) of dynamic RAM
(DRAM) with the nonvolatility (the ability to retain
information when the power supply is off) of FLASH memory.1
Fe-RAM is composed of a ferroelectric capacitor in which a
ferroelectric thick film (∼100 nm thick) is sandwiched between
two electrodes. This film’s remnant polarization is then
switched by an electric field between the top and bottom
electrodes, resulting in two stable polarization states that are
interpreted as “1’ and ‘0” in binary code.2 However, Fe-RAM
faces some challenges such as destructive read-out, which
destroys the stored information during the reading process and
makes it necessary to restore the data after the reading
operation. A second drawback is the scaling limit; Fe-RAM is
based on charge sensing, and the lateral size of the capacitor
cannot be reduced to nanometer scale.3
Among the different notable types of semiconductor
memories, ferroelectric tunnel junction (FTJ) devices are
excellent candidates for overcoming the current Fe-RAM
limitations, as they combine good scalability with low operating
energy, high operation speed (write time = 10 ns), high
© 2017 American Chemical Society
Received: December 16, 2016
Accepted: April 3, 2017
Published: April 3, 2017
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Figure 1. (a) AFM topography image (1×1 μm2) of a Hf0.5Zr0.5O2 film on a TiN substrate, (b) XRR spectra measured on Hf0.5Zr0.5O2 films, and (c)
grazing incidence angle XRD spectrum of a Hf0.5Zr0.5O2 film deposited on a TiN bottom electrode.
electrode was deposited at 400 °C in a N2 and Ar atmosphere (pN =
N2/(Ar + N2) = 70%) using a sputtering pressure of 10 mTorr. The
subsequent growth of Hf0.5Zr0.5O2 was performed at 425 °C. During
this step, the sputtering medium consisted of a mixture of Ar and O2
(pO = O2/(Ar + O2) = 50%) and the RF power on the one inch in
diameter polycrystalline Hf0.5Zr0.5O2 target was fixed to 20 W. In both
cases, the sputtering chamber was pumped prior to deposition to a
base pressure of ∼10−5 Torr using a dry pumping station. To eliminate
contaminations and to maintain the target composition homogeneous,
the target surface was cleaned for 15 min by presputtering prior to all
the depositions (see Supporting Information, section 1).
Microstructural and Structural Characterization. The surface
morphology of each film was analyzed by atomic force microscopy
(AFM, Smart SPM1000-AIST-NT Inc.), and the layer thickness of the
ferroelectric films was determined by X-ray reflectivity (XRR, Philips
X’Pert Materials Research Diffractometer) and the software Gen X.
For films thicker than 5 nm, the structural properties were studied by
high-resolution X-ray diffraction (XRD) in grazing incidence mode
using Cu Kα radiation.
Electronic Characterization. The bandgap of a 20 nm thick
Hf0.5Zr0.5O2 layer grown on Si quartz (0001) was calculated with the
Tauc plot method using the transmission spectra obtained from a
PerkinElmer UV/vis spectrometer. X-ray photoelectron spectroscopy
(XPS, VG Escalab 220i XL) was employed to determine the band
alignment of the Hf0.5Zr0.5O2 film with the TiN and Pt substrates,
which records the shallow core level, valence band spectra, and Fermi
energies. All high-resolution spectra were collected in constant pass
energy (20 eV) mode. The binding energy scale was calibrated using a
pure gold standard sample and setting Au 4f7/2 at a binding energy of
84 eV (see Supporting Information, section 2).
Nanoscale Characterization. The ferroelectric switching of the
films was studied by piezoresponse force microscopy (PFM) using
conductive Pt−Ir-coated Si cantilever tips (radius ≈ 30 nm).
Nanoscale electrical measurements were performed in conductive
AFM (C-AFM) mode. For the electrical characterization, a 30 nm
thick polycrystalline Pt top electrode was deposited using DC
sputtering through a shadow mask consisting of an array of 300 μm
diameter holes (see Supporting Information, section 3). Local
current−voltage (I−V) characteristics were measured by placing the
FTJ device based on a highly strained BaTiO3 ultrathin film (3
nm thick), where a giant tunneling electroresistance effect was
demonstrated.7 Since then, a large TER effect has been
reported in FTJ devices based on perovskite materials such as
BaTiO3,8 PbTiO3,9 and BiFeO3.10 However, integrating FTJs
with ferroelectric ultrathin films having a perovskite structure
into conventional Si-based memory technology remains
challenging for many reasons, such as poor interfacing with
silicon (Si), an elevated crystallization temperature, and an
electrical degradation caused by a forming gas treatment.11
Hafnium zirconium oxide (Hf0.5Zr0.5O2) is a very promising
material for FTJ devices and could present a solution for
overcoming the actual limitations of the semiconductor
memories. Just a few years ago, in 2011, its ferroelectric
behavior in the ultrathin film form (layer thickness below 10
nm) was first discovered.11 This was a surprise because hafnia
and zirconia had been studied for more than a century without
evidence of polar ordering. In contrast to traditional perovskite
materials, Hf0.5Zr0.5O2 presents advantages such as a high
compatibility with the CMOS process, a low crystallization
temperature (∼400 °C), and excellent compatibility with TiN
electrodes, a favored material for mass production.12−14
Therefore, because of the importance of Hf0.5Zr0.5O2 to the
semiconductor industry, this work reports the fabrication of an
FTJ comprising a TiN bottom electrode, a 6 unit cell thick
Hf0.5Zr0.5O2 tunnel barrier (∼2.8 nm thick) and a Pt top
electrode. We also present an investigation of the TER effect by
correlating the reconstructed electrostatic potential profile with
the electrical transport properties using a direct tunneling
current model as proposed by Brinkman.15
2. EXPERIMENTAL SECTION
Sample Preparation. Hf0.5Zr0.5O2/TiN bilayers were grown by
radio frequency magnetron sputtering (SPT310, Plasmionique Inc.)
on (100) p-type Si substrates (ρ = 1−10 Ω cm). The TiN bottom
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acquired on the Hf0.5Zr0.5O2 film integrated in the FTJ device
(Figure S4). These results confirm the ferroelectric behavior of
the ultrathin Hf0.5Zr0.5O2 film. Figure 2b shows a PFM phasecontrast image depicting intentionally written ferroelectric
patterns. Three intersecting squares located from the bottom
left to the top right were poled with voltages of −3, +3, and −3
V, demonstrating upward, downward, and upward polarizations, respectively. The remaining area corresponds to the
virgin state, which exhibits a preferential downward polarization. This spontaneous polarization was caused by the
presence of substrate-induced strain in the Hf0.5Zr0.5O2 film,18
and the intersecting squares show the capability of the material
to go through a full cycle of ferroelectric polarization switching.
The presented ferroelectric patterns are stable for over 60 h,
which suggests a stable and robust ferroelectric polarization of
the Hf0.5Zr0.5O2 layer (Figure S5). This PFM analysis is in
agreement with the XRD results, confirming the formation of
the noncentrosymmetric orthorhombic ferroelectric phase.
Optical constants such as the bandgap (Eg) can be estimated
from UV−visible transmission spectroscopy measurements,
whereas the carrier transport properties at the ferroelectric/
metal interface can be well understood by determining the
valence band offset as well as the potential step barrier.19,20
Thus, accurate knowledge of these optical and electronic
parameters is essential for identifying a suitable ferroelectric
layer for FTJ.21
conductive tip in contact with the platinum top electrode while the
TiN bottom electrode was grounded and performing a “DC” voltage
sweep.
3. RESULTS AND DISCUSSION
The surface morphology of the as-grown Hf0.5Zr0.5O2 sample is
shown in Figure 1a. It depicts a grainy layer uniformly covering
the TiN substrate surface. The layer is thin enough that some
atomic steps/terraces are still visible. The small grains
characteristic of Hf0.5Zr0.5O2 are also visible, and a root-meansquare (rms) roughness of ∼0.25 nm is obtained over a 1×1
μm2 area, attesting the atomic flatness of the Hf0.5Zr0.5O2/TiN
heterostructure’s surface.
As the TER effect is highly dependent on the barrier width,5
it is crucial to determine the layer thickness of our films. Using
XRR measurements (Figure 1b), the layer thickness was
estimated to be ∼2.8 nm (6 unit cells of Hf0.5Zr0.5O2) for the
films used in the fabrication of the FTJ and ∼5.2 nm for those
used in the structural analysis.
For insight to be gained into the structural properties of the
ultrathin Hf0·5Zr0.5O2 film on the TiN substrate, a phase
identification was performed using grazing incidence angle
XRD (Figure 1c). The diffraction peaks from the (111), (200),
and (220) planes belong to the noncentrosymmetric
orthorhombic phase, which has ferroelectric properties. The
ferroelectric phase is obtained during the initial stages of film
growth. It starts with the nucleation of small grains with a high
surface and volume ratio, which results in the formation of a
tetragonal phase (P42/nmc). However, the large tensile strain
along the c-axis induced by the coalescence of the nucleating
grains allows the formation of the ferroelectric orthorhombic
phase (Pbc21).16 The XRD presented in Figure 1c confirms the
film composition, as a change in the Hf/Zr ratio would result in
the appearance of other peaks in 2θ.17
The first requirement to realize a ferroelectric tunnel junction
device is for the nanometer-thick film to present ferroelectric
properties.5 For this reason, the ferroelectric properties of the 6
unit cell thick Hf0.5Zr0.5O2 film were investigated at room
temperature on the bare surface of the ultrathin film as well as
on the FTJ devices. Starting with the PFM response on the bare
surface of the Hf0.5Zr0.5O2 film, Figure 2a reveals a clear
hysteretic behavior in both phase, as shown by the ∼180°
difference, and the amplitude signals, acquired out of the
resonance frequency of the cantilever, as a function of voltage.
The ferroelectric switching voltages that create the downward
and upward polarization were found to be +1.9 and −1.9 V,
respectively. Similar local phase and amplitude signals were
Figure 3. Tauc plot of a Hf0.5Zr0.5O2 film.
Eg is determined from transmission measurements. The
corresponding Tauc plot is illustrated in Figure 3. The
absorption coefficient (∝) was calculated using the relation19
∝=
1 ⎛⎜ 100 ⎞⎟
ln
d ⎝ %T ⎠
(1)
where d is the layer thickness and T is the transmission. Eg is
estimated by extrapolating the linear portion of the absorption
curve to the x-axis, where the absorption coefficient becomes
zero, and a value of 5.06 eV was obtained for a Hf0.5Zr0.5O2 film
with a thickness of 20 nm.
To gain quantitative information on the electronic properties
at both interfaces of the FTJ system (TiN/Hf0.5Zr0.5O2 and
Hf0.5Zr0.5O2/Pt), we used the Kraut procedure.22,23 This
method consists of measuring the valence band offset (VBO)
by finding the core level signals provided by the film and the
substrate, the Fermi energy level (EF) of the metallic bulk
material, as well as the valence band maximum (VBM) of the
ferroelectric layer. Starting with the first interface (TiN/
Hf0.5Zr0.5O2), VBO was calculated using the equation
Figure 2. (a) Local PFM hysteresis loops of the phase (blue curve)
and the amplitude signal (green-red curve) and (b) 3 × 3 μm2 PFM
phase contrast image.
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Figure 4. XPS spectra taken on the (a) thick TiN film, (b) thick Hf0.5Zr0.5O2 layer, (c) TiN/Hf0.5Zr0.5O2 interface, (d) bulk Pt film, and (e)
Hf0.5Zr0.5O2/Pt interface.
VBO = (BE Ti2p − E F)TiN − (BE Hf4f − VBM)HZO
− (BE Ti2p − BE Hf4f )HZO/TiN
(2)
The energy separation between the Ti 2p centroid with
respect to the leading edge of the Fermi level for the bulk TiN
sample was found to be 455.4 ± 0.05 eV. The energy difference
between the Hf 4f centroid and the VBM for the bulk
Hf0.5Zr0.5O2 was estimated to be 14.1 ± 0.05 eV. The binding
energy difference between Ti 2p and Hf 4f core levels in the
ultrathin film was calculated to be 438.1 ± 0.05 eV. Substituting
these values in eq 2, we determined the VBO to be 3.2 ± 0.05
eV at the TiN/Hf0.5Zr0.5O2 interface. This analysis and the
obtained values are schematically described in Figure 4.
The same procedure was employed to measure the VBO at
the second interface (Hf0.5Zr0.5O2/Pt, Figure 4). For the
reference bulk metallic Pt sample, we obtain an energy
difference equal to 70.5 ± 0.05 eV, and the core level line
separation on the Pt/Hf0.5Zr0.5O2 ultrathin sample was found to
be 53.7 ± 0.05 eV. From these values, we found the second
VBO to be equal to 2.70 ± 0.05 eV (see Supporting
Information, section 5).
The conduction band offset (or potential step barrier,φ) is
given by
φ = Eg − VBO
Figure 5. Schematic representation of the electronic band diagram for
the TiN/Hf0.5Zr0.5O2/Pt heterostructure.
(3)
electrical resistance between the two states suggest a TER
effect.8,18,24
To confirm that the resistive switching observed in Figure 6a
is due to the TER effect rather than another resistive switching
mechanism, we used a method proposed by Zenkevich et al.24
It correlates the interfacial electronic properties with the
electrical transport properties of the TiN/Hf0.5Zr0.5O2/Pt
heterostructure by employing a direct tunneling current
model across a trapezoidal barrier.15 This model consists of
calculating the tunneling current density (J) as a function of
voltage (V) using the potential barrier steps at both
ferroelectric−electrode interfaces (φ1 and φ2), layer thickness
(d), electron charge (e), and effective electron mass (m) as
described by the Brinkman model8,15
Therefore, the potential step barriers at the TiN/Hf0.5Zr0.5O2
(φ1) and Hf0.5Zr0.5O2/Pt (φ2) interfaces were calculated to be
1.86 ± 0.07 and 2.36 ± 0.07 eV, respectively. The
reconstructed electronic band diagram for the TiN/
Hf0.5Zr0.5O2/Pt heterostructure with spontaneous downward
polarization is shown in Figure 5.
The TER effect in our system was investigated using
nanoscale electrical measurements. A representative I−V curve
measured on the TiN/Hf0.5Zr0.5O2/Pt devices is presented in
Figure 6a. It depicts an hysteretic I−V behavior characterized
by an initial high resistance state (“OFF” state, green line),
which switches to a low resistance state (“ON” state, blue line).
The hysteretic electrical behavior as well as the large change in
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Figure 6. (a) Typical I−V characteristics measured on a 300 μm diameter TiN/Hf0.5Zr0.5O2/Pt heterostructure displaying low- and high-resistance
states and (b) electrostatic potential barrier as a function of the polarization direction.
Figure 7. Memory performances of the FTJ devices: (a) Fatigue measurements on a TiN/Hf0.5Zr0.5O2/Pt (diameter of 300 μm) heterostructure
displaying the “OFF” (green) and “ON” (blue) resistance values over 1000 write/read cycles. (b) “ON” and “OFF” resistances state (upper panel)
and ON/OFF ratios (bottom panel) of 30 different FTJ memory cells. (c) Retention properties, where the “ON” resistance state was measured as a
function of time at a read voltage of 0.4 V.
obtained by XPS (φ1 = 1.86 eV and φ2 = 2.36 eV), and adding
only the scaling factor, which was assumed to be the same for
both states following the method described in the literature.8,24
Then, using the same scaling factor and eq 4, the I−V curve for
the “OFF” state was fitted to obtain the potential barriers for
that state. As shown in Figure 6a, a good agreement was
obtained during the fitting of the I−V curve with the theoretical
model described in eq 4 for both the “OFF” and “ON” states.
From this model, we obtained two potential barriers (φ1 = 2.75
eV and φ2 = 2.20 eV) that produce the “OFF” state in the I−V
curve (upward polarization). Hence, the polarization reversal
changes at the TiN/Hf0.5Zr0.5O2 and Hf0.5Zr0.5O2/Pt interfaces
⎛ ⎧
⎞
⎡
eV 3/2
eV 3/2 ⎤⎫
⎬⎟
− φ1 + 2
⎜ exp⎨α(V )⎢ φ2 − 2
⎥
⎣
⎦
⎛ 4em ⎞
⎩
⎭⎟
J = − ⎜ 2 3 ⎟⎜
2
⎟
⎝ 9π ℏ ⎠⎜
1/2
1/2
⎡
⎤
eV
eV
⎜ α 2(V )⎢ φ2 − 2
− φ1 + 2
⎥⎦ ⎟⎠
⎣
⎝
(
(
)
)
(
(
)
)
⎧
⎡⎛
⎪ 3
⎪
eV ⎟⎞1/2 ⎜⎛
eV ⎟⎞1/2 ⎤ eV ⎫
⎥ ⎬
× sinh⎨
− φ1 +
α(V )⎢⎜φ2 −
⎪
⎝
2 ⎠
2 ⎠ ⎦2 ⎪
⎣⎝
⎩2
⎭
(4)
where ∝(ν) = [4d(2m)1/2/[3ℏ(φ1 + eV − φ2)] and ℏ is the
reduced Planck constant (see Supporting Information, section
6). The theoretical I−V curve for the “ON” state (downward
polarization) was plotted using the potential barrier values
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■
are Δφ1 = 0.89 eV and Δφ1 = 0.16 eV, respectively. The
derived changes in the potential barrier profile are shown in
Figure 6b. Indeed, the electrostatic potential profile was
modulated by the polarization reversal of the ultrathin layer.
When the polarization vector of Hf0.5Zr0.5O2 points to the TiN
bottom electrode, the conduction band will bend downward
due to the buildup of negative screening charges in the TiN
electrode. This will be followed by the lowering of the potential
barrier height at the Hf0.5Zr0.5O2/TiN interface, resulting in a
low-resistance state (Figure 6b, downward state). On the other
hand, when the polarization vector of Hf0.5Zr0.5O2 points to the
Pt top electrode, positive screening charges will build up at the
Hf0.5Zr0.5O2/TiN interface, bending the conduction band
upward and resulting in a high-resistance state (Figure 6b,
upward state).The TER effect in our FTJ was quantified by the
TER ratio (J> − J</J<), resulting in a TER ratio of ∼15 at +0.2
V.
The I−V curve illustrated in Figure S11 showed a change
from the “ON” to “OFF” state at a read voltage of +2.2 V, and
the change from the “OFF” to “ON” state takes place at a read
voltage of −1.7 V. The endurance of the TiN/Hf0.5Zr0.5O2/Pt
heterostructures was evaluated by recording 1000 I−V cycles
under quasi-static conditions on the same FTJ memory cell.
Given that contact by the AFM was required, the acquisition
speed was limited by the AFM specifications and a 1000 cycle
quasi-static measurement already took a day. For each I−V
curve, we obtained the “ON” and “OFF” resistances at a read
voltage of +0.2 V (Figure 7a). They remain stable over 1000
cycles, indicating good endurance of the FTJs.25
The reproducibility of the TER effect in the FTJs under
study was investigated by recording I−V cycles from 30
different FTJ devices. For each memory cell, the resistance
values for both the “ON” and “OFF” states were recorded at 0.2
V. The 30 memories cells presented an ON/OFF ratio, or TER
ratio, of 15 ± 3 on average (Figure 7b). This result attests to
the high reproducibility of the TER effect in the TiN/
Hf0.5Zr0.5O2/Pt FTJ memory devices.25
The retention properties of the present FTJ were evaluated
on a single memory cell by recording the “ON” resistance state
value over time at a read voltage of +0.4 V (following the
method described by Abuwasib et al.26). As shown in Figure 7c,
the “ON” resistance state can still be read after 8 h,
demonstrating that our FTJ devices present a long time
retention.
Research Article
ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsami.6b16173.
Synthesis of the Hf0.5Zr0.5O2 and TiN films, complete
XPS binding energy calibration, structural properties of
the platinum top electrode, complete ferroelectric
studies, complete band alignment analysis, and additional
information related to the Brinkman model (PDF)
■
AUTHOR INFORMATION
Corresponding Authors
*E-mail: fabian.ambriz.vargas@emt.inrs.ca.
*E-mail: ruediger@emt.inrs.ca.
ORCID
Fabian Ambriz-Vargas: 0000-0002-9069-9710
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
F.A.-V. is thankful for an individual FRQNT MELS PBEEE 1M
scholarship and financial support from CONACYT-Mexico.
G.K. acknowledges a postdoctoral FRQNT research scholarship. A.R. and M.A.G. acknowledge generous support through
NSERC discovery grants. The collaboration with C.G.-Y. was
generously sponsored through an MRIFE grant.
■
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In summary, we reported the successful fabrication of an FTJ
device based on a CMOS-compatible tunnel barrier
(Hf0.5Zr0.5O2) and a TiN bottom electrode. The formation of
the metastable ferroelectric orthorhombic phase was confirmed
by correlating the results of grazing angle incidence XRD with
those acquired by PFM. By combining XPS and UV−visible
spectroscopies, the potential step barriers were determined to
be 1.86 and 2.32 eV at the bottom and top interfaces of the
TiN/Hf0.5Zr0.5O2/Pt heterostructure, respectively. Using the
Brinkman model, we confirmed the presence of a large
electroresistance effect with a TER ratio equal to ∼15 (at
+0.2 V). Our results hold promise for novel semiconductor
memory devices because FTJs have the potential to overcome
the limitation of conventional semiconductor memory devices
based on charge storage such as Fe-RAM and DRAM.
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DOI: 10.1021/acsami.6b16173
ACS Appl. Mater. Interfaces 2017, 9, 13262−13268
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DOI: 10.1021/acsami.6b16173
ACS Appl. Mater. Interfaces 2017, 9, 13262−13268