Analysis of the Switching Mechanism of Hafnium Oxide Layer with Nanoporous Structure
by RF Sputtering
LimJongho1
BaekMyung-Hyun1
KwonMin-Woo2,†
-
(Department of Electric Engineering, Gangneung-Wonju National University, Gangneung,
25457, South Korea)
-
(Department of Electronic Engineering, Seoul National University of Science and Technology,
Seoul 01811, South Korea)
Copyright © The Institute of Electronics and Information Engineers(IEIE)
Index Terms
RRAM, HfOx, rectifying properties, nanoporous structure, Schottky-like barrier modulation
I. INTRODUCTION
Metal-oxide-based RRAM is one of the most promising non-volatile memory devices due
to its high-speed switching, next-generation non-volatile memory technologies, low
power consumption, high scalability, and compatibility with complementary metal-oxide
semiconductor (CMOS) technology [2,3]. The resistive switching mechanism is typically attributed to the formation and rupture
of conductive filaments within the dielectric layer, which can be induced by applying
an external voltage. This mechanism makes RRAM a promising candidate for future memory
applications. Among high-k materials, hafnium oxide (HfO${}_{\rm x}$) is one of the
most widely used dielectric materials for the development of high-performance RRAM
devices [4]. Hafnium-based high-k dielectrics offer high dielectric constants, excellent thermal
stability, and compatibility with existing semiconductor fabrication processes [5]. In addition, HfO${}_{\rm x}$ exhibits a high affinity for oxygen, which significantly
facilitates the formation and migration of oxygen vacancies (V${}_{\rm o}$), an essential
aspect of the resistive switching mechanism. These properties make HfO${}_{\rm x}$
a highly desirable material for enhancing the reliability and performance of RRAM
devices. Consequently, we have selected hafnium oxide as the switching layer for our
RRAM device. The structure of the RRAM device is a metal-insulator-metal (MIM) structure
with an insulating layer sandwiched between two metal electrodes. Many studies have
explored high-density memory structures using the simple MIM configuration of RRAM.
In this crossbar array structure, the sneak path current that occurs between the memory
cells will induce read-out errors in the array [6]. Therefore, the sneak path current that occurs between the cells must be suppressed.
To solve the sneak-path problem, several research groups have used transistors or
diodes in each memory cell [7]. However, the use of selector device in memory cells leads to some limitations. First,
the selector device must match the operating range with the memory device [8]. Second, the addition of a selector device results in an increase in cell size, which
is a limiting factor for the integration density of memristor systems. Therefore,
the self-rectifying resistive switching memory becomes one of the most promising solutions
to overcome the sneak path current problem without requiring an additional diode.
We have derived these self-rectifying characteristics to a phenomenon that emerges
by RF sputtering the switching layer. Due to the nature of the sputtering process,
oxide films deposited by sputtering generally have nanoporous structures, and the
existence of these pores affects the performance of the device [9,10,11]. In this work, we have fabricated a RRAM device in which the HfO${}_{\rm x}$ layer
is sandwiched between titanium (Ti) and molybdenum disulfide (MoS${}_{2}$) layers
and studied the effect of the nanoporous structure on resistive switching behavior.
The resistive switching characteristics of the device have induced by RF sputtering
HfO${}_{\rm x}$ insulating layer to form pores. The device shows self-rectifying resistive
switching behavior with a nonlinear switching characteristics. This study will provide
valuable insight into the electrical characteristics of other similar devices.
II. DEVICE FABRICATION
The fabrication process of the Ti/HfO${}_{\rm x}$/MoS${}_{2}$ RRAM device is depicted
in Fig. 1(a). The Ti/HfO${}_{\rm x}$/MoS${}_{2}$ RRAM device was fabricated on a square SiO${}_{2}$/Si
wafer substrate. Fig. 1(b) shows the schematic of the RRAM device structure. The fabrication process of the
Ti/HfO${}_{\rm x}$/MoS${}_{2}$ RRAM device is described in the following. First, the
SiO${}_{2}$/Si substrate was rinsed with acetone, isopropyl alcohol, and deionized
water for 5 minutes for device fabrication and then dried with N${}_{2}$ gas. Then,
both the electrode and the switching layer were deposited by RF sputtering. The detailed
conditions of the sputtering method that used for the fabrication of the RRAM device
are summarized in Table 1. All the RF sputtering processes were carried out under the same conditions. The
MoS${}_{2}$ switching layer as the bottom electrode was deposited on SiO${}_{2}$/Si
substrate for 10 minutes. After that, to study the resistive switching characteristics
of the nanoporous structure, HfO${}_{\rm x}$ film was deposited as an insulator using
a shadow mask for 20 minutes. Finally, Ti top electrode with a diameter of 400 $\mu$m
was deposited through a shadow mask.
Table 1. RF sputtering deposition conditions.
Fig. 1. (a) The fabrication process of nanoporous HfO$_{\rm x}$ based on RRAM device;
(b) The proposed Ti/HfO${}_{\rm x}$/MoS${}_{2}$ RRAM structure.}
(a)
(b)
Fig. 2. (a) Representative I-V characteristics of the HfOx device; (b) Position of
Vmin when supply voltage is 6 V to 16 V; (c) I-V curve for $-5$ V to 5 V and 5 V to
$-5$ V voltage changes.
(a)
(b)
(c)
Fig. 3. Schematic cross-section of the Ti/HfO$_{\rm x}$/MoS${}_{2}$ device structure
with the negative oxygen ions in the pores driven by the external electric field.}
III. RESULTS AND DISCUSSION
The resistive switching characteristics of the Ti/HfO${}_{\rm x}$/ MoS${}_{2}$ RRAM
device were measured by a vacuum probe station and semiconductor parameter analyzer
(Agilent B1500). The voltage bias was applied to the top electrode, while the bottom
electrode was grounded. Fig. 2(a) shows the I-V characteristics of the two-terminal electrode nanoporous HfO${}_{\rm
x}$ memory device. A positive voltage sweep was followed by a negative voltage sweep.
The I-V curve exhibits two main characteristics. The first characteristic is self-rectifying
resistive switching behavior. During forward and reverse voltage sweep, the Ti/HfO${}_{\rm
x}$/MoS${}_{2}$ RRAM device exhibited a rectifying current ratio of approximately
$10 ^{2}$ at $\pm 2$ V. This self-rectifying property can be explained by barrier
modulation due to the migration of V${}_{\rm o}$ within the nanoporous HfO${}_{\rm
x}$ layer [12]. The self-rectifying switching operation of the device, which exhibits minimal current
flow at reverse voltage compared to positive voltage, can address the problem of sneak
path current, one of the major issues in crossbar arrays [13]. The second characteristic is that the voltage point of the minimum current during
the reverse sweep does not return to 0 V but exists at a specific negative voltage.
In the nanoporous switching layer, an internal electric field may be formed in accordance
with an external electric field in the pores [12]. Such a characteristic enables a wide range of switching operations. We conducted
two experiments to demonstrate these two features (rectification characteristics and
the specific voltage points of the minimum current) [14]. First, we set the applied voltage between 6 V and 16 V to observe the relationship
with V${}_{\rm min}$ (voltage position of minimum current). Fig. 2(b) shows the change of V${}_{min}$ according to the applied voltage. As the applied
voltage increased, a linear relationship emerged, with V${}_{\rm min}$ shifting in
the positive direction. This linear relationship supports the idea that the internal
electric field created by the negatively charged oxygen ions in the pores influences
the external electric field [14]. The external electric field provides the necessary energy for the migration of oxygen
ions within the insulating layer, causing them to move and become trapped in the defects
or pores. Trapped oxygen ions can lead to the formation of a local internal electric
field within the insulating layer. It indicates that more negatively charged oxygen
ions in the pores of the switching layer can move as the applied voltage increases.
The operation of the nanoporous RRAM device shows that the formation of the internal
electric field affects the resistance states of the device, allowing for the presence
of a minimum current at a specific voltage. The results of the first experiment show
that during the voltage sweep, the external electric field is offset by the internal
electric field generated by the nanoporous HfO${}_{\rm x}$ layer, counterbalancing
at a specific voltage point, so that the minimum current does not return to 0 V but
exists at a specific voltage point. In the second experiment, the rectification characteristics
resulting from Schottky barrier modulation due to the migration of V${}_{\rm o}$ within
the insulating layer were investigated. The impact of voltage polarity on current
and barrier modulation is explained by the I-V characteristics of the device, as illustrated
by two different sweeps. Each initial voltage polarity was applied in both positive
and negative directions, and the current flow was observed in the subsequent sweeps.
Fig. 2(c) shows the blue curve representing the voltage sweep from 5 V to $-5$ V, while the
red curve corresponds to the sweep from $-5$ V to 5 V. As shown in the I-V characteristics,
the current response depends on the direction of the initial voltage sweep. In the
5 V to $-5$ V sweep (blue curve), higher current is observed in the positive voltage
region compared to the negative voltage region. Conversely, in the $-5$ V to 5 V sweep
(red curve), higher current is observed in the negative voltage region. This non-linear
I-V characteristic can be attributed to the migration of V${}_{\rm o}$ within the
nanoporous insulating layer, modulating the Schottky barrier at the metal-insulator
interface; this corresponds to the schematics of Fig. 3 [14]. When voltage is applied, V${}_{\rm o}$ within the nanoporous insulating layer begin
to move according to the polarity of the electric field. In the positive sweep (5
V to $-5$ V), V${}_{\rm o}$ are pushed towards the bottom electrode by the initially
applied positive voltage at the top electrode. The accumulation of V${}_{\rm o}$ at
one end reduces the energy barrier, forming an Ohmic-like contact and enhancing current
flow in the positive voltage region. Conversely, at the opposite interface, the relative
lack of V${}_{\rm o}$ form a Schottky-like contact, suppressing current flow during
the subsequent negative voltage sweep (0 V to $-5$ V). In the reverse sweep ($-5$
V to 5 V), due to the polarity of the applied voltage, V${}_{\rm o}$ initially move
towards the top electrode. Such movement increases the barrier height at the interface
between the bottom electrode and the insulating layer, forming a Schottky-like barrier,
while an Ohmic-like contact is established at the opposite interface. Similarly, the
barrier modulation set by the initial negative voltage suppresses current flow during
the subsequent positive voltage sweep (0 V to 5 V). The results of the second experiment
demonstrate that the migration of V${}_{\rm o}$, influenced by voltage polarity, results
in the formation of Ohmic-like and Schottky-like contacts at the two interfaces of
the insulating layer. The barrier modulation exhibits Schottky diode behavior, showing
non-linear I-V switching operation as the device's self-rectifying characteristic.
Overall, this analysis explains the critical role of V${}_{\rm o}$ migration and internal
electric field generation within the nanoporous HfO${}_{\rm x}$ insulating layer.
The linear relationship between applied voltage and the voltage point of minimum current
highlights the dynamic nature of the internal electric field, driven by negatively
charged oxygen ions. Additionally, the observed Schottky-like and Ohmic-like contacts,
dependent on voltage polarity, underline the potential of this RRAM device in mitigating
sneak path currents in high-density crossbar arrays. These insights will not only
advance our understanding of the switching mechanisms in nanoporous HfO${}_{\rm x}$-based
RRAM but also pave the way for the optimization and development of next-generation
memory technologies.
IV. CONCLUSION
In summary, we analyzed the characteristics that emerge when the switching layer has
a nanoporous structure. The device can be fabricated at room temperature and exhibits
two distinct features that set it apart from conventional memory devices. Firstly,
the position of V${}_{min}$ varies with the applied voltage, which was demonstrated
to be due to the internal electric field created by the migration of negatively charged
oxygen ions within the pores. The linear relationship observed between the applied
voltage and V${}_{min}$ indicates a broad range of switching characteristics that
can be achieved across a range of operating voltages. Secondly, the self-rectifying
resistive switching behavior is shown to be due to Schottky-like barrier modulation
influenced by the initial polarity of the applied voltage. This self-rectifying property
has the potential to suppress sneak path currents from unselected cells in 3D vertical
crossbar array structures. This analysis demonstrates that nanoporous hafnium-based
memory devices offer significant advantages compared to non-porous oxide-based memory
devices. The results of this study not only enhance the understanding of the switching
mechanisms of these devices but also pave the way for the development of more efficient
and reliable memory technologies.
ACKNOWLEDGMENTS
This research was supported by the National R&D Program through the National Research
Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF-2022M3I7A1078936)
and also supported by "Regional Innovation Strategy (RIS)" through the National Research
Foundation of Korea(NRF) funded by the Ministry of Education (MOE)(2022RIS-005)
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Min-Woo Kwon received his B.S. and Ph.D. degrees in electrical and computer engineering
from Seoul National University (SNU) in 2012 and 2019, respectively. From 2019 to
2021, he worked at Samsung Semiconductor Laboratories, where he contributed to the
development of 1x nm DRAM cell transistors and their characterization. From 2021 to
2024, he worked at Gangneung-Wonju National University (GWNU) as an assistant professor
in the Department of Electric Engineering. Since 2024, he has been conducting research
in the Department of Electronic Engineering at Seoul National University of Science
and Technology, where he is currently a professor.
Jongho Lim entered the Department of Electronic Engineering at Gangneung-Wonju
National University in 2019, and is currently pursuing his B.S. degree. His main research
interests include RRAM (Resistive Random Access Memory), a type of next-generation
memory semiconductor device.
Myung-Hyun Baek was born in Seoul, Korea, in 1990. He received his B.S degree in
Electrical Engineering from Seoul National University (SNU), Seoul, Korea, in 2013,
and his Ph.D. in Electrical and Computer Engineering from SNU, Seoul, Korea, in 2020.
He worked at Samsung Electronics Co., Ltd. (Hwasung, Korea) as a Staff Engineer from
2020 to 2023. In 2023, he joined Gangneung-Wonju National University (GWNU, Korea)
as an assistant professor in the Department of Electronics and Semiconductor Engineering.
His main research interests are nonvolatile memory technologies and neuromorphic systems.