Resistive switching in rectifying interfaces of metal-semiconductor-metal
structures
R. Zazpe, P. Stoliar, F. Golmar, R. Llopis, F. Casanova et al.

Citation: Appl. Phys. Lett. 103, 073114 (2013); doi: 10.1063/1.4818730
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Resistive switching in rectifying interfaces of metal-semiconductor-metal
structures
R. Zazpe,1 P. Stoliar,1,2,3,a) F. Golmar,1,4 R. Llopis,1 F. Casanova,1,5 and L. E. Hueso1,5
1CIC nanoGUNE, 20018 Donostia-San Sebastian, Basque Country, Spain
2IMN, Universite de Nantes, CNRS, 2 rue de la Houssinie`re, BP 32229, 44322 Nantes, France
3ECyT, UNSAM, Martın de Irigoyen 3100, B1650JKA, San Martın, Bs As, Argentina
4I.N.T.I.–CONICET, Av. Gral. Paz 5445, Ed. 42, B1650JKA, San Martın, Bs As, Argentina
5IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Basque Country, Spain
(Received 8 July 2013; accepted 30 July 2013; published online 15 August 2013)
We study the electrical characteristics of metal-semiconductor-metal HfO2x-based devices where
both metal-semiconductor interfaces present bipolar resistive switching. The device exhibits an
unusual current-voltage hysteresis loop that arises from the non-trivial interplay of the switching
interfaces. We propose an experimental method to disentangle the individual characteristics of
each interface based on hysteresis switching loops. A mathematical framework based on simple
assumptions allows us to rationalize the whole behavior of the device and reproduce the
experimental current-voltage curves of devices with different metallic contacts. We show that each
interface complementarily switches between a nonlinear metal-semiconductor interface and an
ohmic contact. VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4818730]
Resistive random-access memories (ReRAM) are
emerging as one of the most promising alternatives for the
next generation of non-volatile electronic memories.1–3 They
are based on the resistive switching (RS) phenomena, which
is the change of the resistance of a dielectric media by means
of electric pulses.4 The standard ReRAM cell is a capacitor-
like structure in which the active material is placed in
between two metal electrodes. Depending on the specific
materials and its configuration, the resistive switching can be
an interfacial effect (i.e., occurring in the proximity of the
electrodes) or a bulk effect. In general, when the RS is inter-
facial, the cell is designed to have only one switching inter-
face. Nevertheless, configurations with two ReRAM cells in
series or two switching interfaces are arising because they
hold notable technological advantages when integrated in
large crossbar-arrays.5
In this work, we study metal-semiconductor-metal
HfO2x-based devices where both interfaces display bipolar
resistive switching, i.e., the resistance of each metal-HfO2x
interface can be switched between a high-resistance state
(HRS) and a low-resistance state (LRS) by applying electric
pulses. The bipolar character of the RS means that the SET
process (switch from HRS to LRS) and the RESET process
(switch back from LRS to HRS) occur at opposite bias. In the
literature, this configuration was reported as a “switchable
rectifier”,6 and in a more general context, it belongs to the
family of devices that presents complementary resistive
switching.5,7–9 Due to the non-linear character of the resistive
switching phenomena, the configuration in series of two
switching interfaces leads to nontrivial electrical characteris-
tics; for example, hysteretic I–V (current-voltage) curves that
differ from the typical self-crossing curves found in most of
the RS devices.10 In some systems, it is possible to include
extra electrodes in order to monitor the contribution of each
interface separately,8 but when this alternative is not possible,
the electrical characteristics are rationalized by comparing
the behavior of a connection in series of two individual devi-
ces containing one switching interface,5,9,11,12 or by mathe-
matically modeling the system.13,14 Here, we use an analysis
method based on hysteresis switching loops (HSLs) that
allows us to individuate the role of each interface and to
model the switching mechanism. The understanding of the
individual interfaces allows us to mathematically reproduce
the behavior of the whole device and to understand the origin
of the non-conventional I–V curves. Finally, we verify our
procedure by changing the resistive switching character of
only one of the interfaces.
We fabricated metal-semiconductor-metal HfO2x-based
devices on Si/SiO2 (150 nm) thermal oxide substrates on which
the Ti bottom electrode (20 nm) was sputtered. The subsequent
HfO2 layer (20 nm) was grown by means of atomic layer depo-
sition (ALD) technique at 300 C using H2O as oxidant and
Tetrakis(dimethylamino)hafnium (TDMAH) as hafnium pre-
cursor. Finally, an array (16 5) of 35 nm-thick Co/Pd top
electrodes (200 200 lm2) was produced by sputtering and
photolithography. Electrical characterization was carried out at
a probe station (room temperature) using a Keithley 2635A
sourcemeter controlled by custom computer software.
In Figs. 1(a) and 1(b), we present a scheme of the exper-
imental setup and a typical current (I)-voltage (V) curve
obtained for a Ti/HfO2x/Co device. This curve does not cor-
respond to the typical hysteresis loop expected in RS devi-
ces. Figs. 1(c) and 1(d) sketch the conceptual difference
between the I–V characteristic of our device and, as a typical
example, the I–V loop of a bipolar RS device (Ref. 10
contains a comprehensive classification of I–V curves in RS
devices). In our devices, the two branches of the hysteresis
loop do not intersect at zero voltage. Moreover, the resist-
ance state sensed turns from LRS to HRS when the voltage
polarity is reversed. This kind of non-intersecting curve is
generally observed in systems with complementary RS
interfaces.5a)E-mail: pablo.stoliar@cnrs-imn.fr
0003-6951/2013/103(7)/073114/5/$30.00 VC 2013 AIP Publishing LLC103, 073114-1
APPLIED PHYSICS LETTERS 103, 073114 (2013)
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In order to disentangle the contribution of each inter-
face, HSLs at opposite bias voltages were conducted. HSL
is a parametric plot that displays the evolution of the resist-
ance of a RS device during the application of a train of
electrical pulses.15–17 Throughout this work, HSL plots are
defined as the sequence of points (VPULSE(i), RREM(i,
VREAD)), being i the pulse index, VPULSE the voltage of the
ith pulse and RREM is the remnant resistance after the ith
pulse, which is sensed with the reading voltage VREAD.
Here, the polarity of this VREAD is crucial due to the rectify-
ing character of the interfaces. At positive VREAD, the cur-
rent flowing through the device is mainly limited by the
Ti/HfO2x interface and the HfO2x layer; the other inter-
face is in direct bias so that it presents a negligible resist-
ance. Assuming that the HfO2x bulk does not modify its
resistance, any effect observed in the HSL that is acquired
with positive reading voltage, is attributed to resistive
switching in the Ti/HfO2x interface. Complementary, the
HSL obtained with negative VREAD reveals RS in the
Co/HfO2x interface.
The HSL of Fig. 2(a) was obtained with positive
VREAD¼ 4 V applying 20ms-width pulses with a sequence
0 V ! 15 V ! 5 V ! 0 V in steps of 100 mV. The data
clearly corroborates the bipolar character of the resistive
switching. It also shows that for the Ti/HfO2x interface the
minimum SET voltage is 5 V. Even if the RESET proce-
dure starts at low voltages, pulses should overcome 3 V
to ensure proper RESET. For negative VREAD¼4 V,
Fig. 2(b) evidences that the SET and RESET voltages of the
Co/HfO2x interface are 10 V and 3 V, respectively.
From these HSLs, it is also evident that the interfaces are
complementary, for example, a positive voltage SETs the
Ti/HfO2x interface but RESETs the Co/HfO2x interface.
The conceptual picture of these two complementary
interfaces presenting bipolar resistive switching, confirmed
by the HSLs, is schematized in Fig. 3. The device can be di-
vided into (from left to right in Fig. 3(a)) a left contact (CL),
an interface region (IL) modeled as a Schottky barrier, a bulk
layer, and finally the interface region (IR) corresponding to
the right contact (CR). The energy band diagram of the sys-
tem in thermal equilibrium, assuming an n-type semiconduc-
tor, is presented in Fig. 3(b).18 The RS mechanism found in
our devices is attributed to local changes in the oxygen
vacancy density at the interfaces by means of electric field.
These changes result in a modification of the injection bar-
riers, which in turn produces a modulation of their effective
resistance. This mechanism has been widely reported for
several systems,11,19,20 as well as for HfO2x.21–24 In fact,
oxygen vacancies originate a sub-band at the HfO2x
bandgap,25,26 turning the Schottky barrier into an ohmic
junction.27 The SET process occurs when local electric fields
increase the concentration of oxygen vacancies at the interfa-
ces. The RESET process (at opposite polarity) removes the
oxygen vacancies from the interfaces, restoring the original
injection barrier. Typically, our device can display three dif-
ferent configurations, with only one of the interfaces in LRS
(as in Figs. 3(c) and 3(d)) or with both interfaces in LRS
(Fig. 3(e)). As we shall see later, a state with both interfaces
in HRS is not recovered in normal operation.
We articulate the physical scenario described above into
the following mathematical framework. The current flowing
through the device, I, is modulated by the two interfaces
(IL and IR, modeled as Schottky barriers) and the semicon-
ducting bulk. The bulk is modeled as a linear resistor with
FIG. 1. (a) Schematic diagram of the experimental setup and (b) typical I–V
curve of the Ti/Hf2x/Co devices. (c), (d) Schemas pointing out the radically
different behavior observed in our devices (c) with respect to what is
expected in standard bipolar RS devices (d). It implies a discontinuity in the
resistance state when the polarity of the applied voltage changes.
FIG. 2. Hysteresis switching loops obtained with opposite sensing bias on
the same device. (a) When the sensing bias is þ4 V, it reveals the behavior
of the Ti/HfO2x interface. (b) Instead, when the bias is 4 V, it evidences
the behavior of the Co/HfO2x interface.
073114-2 Zazpe et al. Appl. Phys. Lett. 103, 073114 (2013)
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constant resistivity, i.e., we consider non-linear effects only
at the interfaces. The applied bias V is the sum of the voltage
drop at each of the three components
V ¼ VLðI; nLÞ þ I RB þ VRðI; nRÞ; (1)
where VL,R are the potential drop at each interface, nL,R
are the concentration of oxygen vacancies at the interfaces,
and RB is the resistance of the bulk region. The functions
VR;LðI; nR;LÞ express the rectifying characteristics of the
metal-semiconductor interfaces. Here, we assume that
the transport through the interfaces can be captured by the
thermionic-emission-diffusion model for conduction in
metal-semiconductor interfaces18
VR;L ¼
kBT
q
ln½1 þ ðI=i0ðnR;LÞÞ; (2)
where kB is the Boltzmann constant, q is the carrier charge, T
is the temperature, and i0 is the saturation current. This
assumption is based on the hypothesis that it is not possible
to develop a stable large reverse voltage across the interfa-
ces. In fact, as soon as the electric field increases, it activates
the introduction of oxygen vacancies towards the interface,
increasing the saturation current (i0 / nR;L).18 The RS behav-
ior is simulated as a modulation of the energy barriers by
means of nR and nL
d
dt nR;L ¼ sgnðVR;LÞ expðE0 þ jVR;L=WR;LjÞ; (3)
where E0 is the anchoring energy of the oxygen vacancies to
the HfO2x matrix and WR and WL are the depletion layer
widths of the interfaces. The application of voltage pulses
generates electric fields VL/WL and VR/WR strong enough to
overcome the anchoring energy of the ions (see Ref. 13),
triggering a redistribution of vacancies between the bulk and
the interfaces. Whether the electric field introduces or
removes ions from the interfaces depends on the polarity.
By numerical integration of Eq. (3), we obtain the
temporal evolution of vacancy distributions at the interfaces,
nL and nR. The integration must be performed in self-
consistency with Eqs. (1) and (2), obtaining also the evolution
of the device current, I, and the voltage drop at each interface
for any applied voltage waveform V(t).28 In short, we can
simulate the I–V characteristics and study the distribution V
along the interfaces and the bulk. Fig. 4(a) shows the excel-
lent agreement between experimental and simulated I–V
curves for Ti/HfO2x/Co.
Here, we comment about the slight mismatch between
the experimental data and the model. In the first place, both
leak currents as well as the sensitivity of the SourceMeter
prevent to measure currents below 1 pA, creating a mismatch
between experimental data and simulation when the voltage
approaches zero. The difference in the LRS is due to nonli-
nearities (see steps (3) and (6) in Fig. 4). The model consid-
ers that in the LRS the current is mainly limited by the linear
bulk resistance. Instead, the experimental data presents a
nonlinear behavior. This effect might rise from nonlinearities
in the bulk I–V characteristics and/or from a non-negligibly
influence of the injection barriers. Finally, a key point is that
the model is not considering the profile of the vacancies at
the interface; instead the interface is represented with a con-
centrated parameter model. We think this might be the
reason behind the differences between the simulated and
experimental data during the SET processes (see steps (2)
and (5) in Fig. 4).
The evolution of these simulated parameters is crucial
for the interpretation of the I–V curve, which is described in
Figs. 4(b)–4(d). The current in the first quadrant of the I–V
loop (positive bias) is governed by the Ti/HfO2x interface
(IL) and the bulk, whereas it is dictated by the HfO2x/Co
interface (IR) and the bulk in the third quadrant (negative
bias). We can only infer the behavior of the “hidden” interfa-
ces with the aid of the numerical simulations. The different
states of the system are numbered in a sequence from (1) to
(6) in both Figs. 4(a) and 4(d). We start our analysis with the
Ti/HfO2x interface in HRS (almost free of oxygen vacan-
cies) and the HfO2x/Co interface in LRS (with a high con-
centration of vacancies), corresponding to (1). Initially,
almost all the electrical potential drops in the Ti/HfO2x
interface, which is the main limitation for the current. When
the voltage reaches the SET voltage of the Ti/HfO2x inter-
face (2), it switches to the LRS. This SET voltage can be
observed in the HSL presented in Fig. 2(a). Subsequently,
the applied voltage turns to drop mainly at the bulk, which is
now the limiting factor for the current. Some electric field
also develops at the HfO2x/Co interface that RESETs (3).
Nevertheless, the RESET of this interface has no effect in
the current as long as the device is biased with positive
voltage (4). The interface can be observed in the HSL in
FIG. 3. (a) Schematic illustration of the structure of our devices. (b) Energy
band diagram at thermal equilibrium of the system, where EFm,L and EFm,R
are the Fermi energies of the metal contacts (CL, CR), WL and WR are the
widths of the depletion layers, Eg is the energy band gap of the semiconduc-
tor, qwbi is the built-in potential, EV and EC are the valence and conduction
band, respectively, and qUBn,L and qUBn,R are the energy heights of the
Schottky barriers. (c–e) Different possible configuration of the system; with
high concentration of oxygen vacancies (spheres) at the left interface (c),
right (d), and in both interfaces (e). For the sake of simplicity, in this plot,
we represented the injection barriers alike and not changing after the intro-
duction of the oxygen vacancies. Actually, in our model, we assume that the
injection barriers are strongly modified by the oxygen vacancies, turning the
metal-semiconductor interfaces into ohmic contacts. We also account for
unlike junctions.
073114-3 Zazpe et al. Appl. Phys. Lett. 103, 073114 (2013)
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Fig. 2(b). When the bias polarity is reversed to negative
values, the I–V reveals the HRS of the HfO2x/Co interface.
In this quadrant, the SET of the HfO2x/Co interface is
clearly reflected (5), but not the subsequent RESET of the
Ti/HfO2x interface (6). Essentially, for negative bias, the
HfO2x/Co interface limits the current until it SETs and then
the current is dominated by the bulk. As mentioned, a state
with both interfaces in HRS is not present in normal opera-
tion conditions.29
We finally consider a non-trivial confirmation of the
whole model, testing it in devices with configuration
Ti/HfO2x/Au, i.e., we substituted the Co electrode by Au.
The rationale behind this substitution lies in the marked
difference in the enthalpy of these two metals to react with
an HfO2 matrix.30 The modification of the IR should mainly
impact on the third quadrant (negative bias) and indeed it
creates a marked asymmetry in the I–V characteristics
(Fig. 4(e)). As predicted, positive bias senses the state of
the Ti/HfO2x interface (IL) while negative bias senses the
HfO2x/Au interface (IR). Moreover, Fig. 4(e) presents
the perfect agreement when we introduce an asymmetry in
the widths of the depletion layers WR¼ 3 WL in Eq. (3). This
is consistent with a picture in which a “textbook” Schottky
barrier is formed at IR when the Au electrode is in contact
with the semiconductive layer. In contrast, the higher reac-
tivity of the Ti effectively reduces the extension of the
barrier.19
In conclusion, we fabricated and studied the resistive
switching characteristics of metal-semiconductor-metal
HfO2x-based devices presenting a particular non-crossing
hysteresis loop. We proposed and validated the use of hys-
teresis switching loop with opposite sensing bias in order to
disentangle the contribution of each interface. In that way,
we confirm that both interfaces display bipolar resistive
switching. In fact, they commute between a HRS, due to the
injection barrier of the metal-semiconductor interface, and a
LRS caused by an accumulation of oxygen vacancies that
turns the interface into an ohmic contact. Moreover, at rela-
tively high voltages (10 V) the interfaces eventually com-
plement each other, so that if one is at HRS the other one is
at LRS. Based on this description, we were able to mathe-
matically study the interplay between both switching inter-
faces, and more importantly, to rationalize the particular
behavior observed in the I–V curves. Finally, we confirmed
the validity of all the procedure by verifying the prediction
that each interface is univocally related to a specific quad-
rant on the I–V hysteresis loop.
The authors acknowledge P. Levy for useful discussions.
This work was supported by the project MAT2009-08494 and
MAT2012-37638 from the Spanish Ministry of Science, by
the Basque Government through the Project No. PI2011-1 and
by the Marie Curie Reintegration Grant (ITAMOSCINOM)
from the European Commission. F.G. is a member of
CONICET.
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073114-5 Zazpe et al. Appl. Phys. Lett. 103, 073114 (2013)
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