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\begin{document}

\title{A large memory window and low power consumption self-rectifying
memristor for electronic synapse}



\author[1]{Qingjiang Li}%
\author[1]{Shihao Yu}%
\author[1]{Peng Yang}%
\author[1]{Qin Wang}%
\author[1]{Sen Liu}%
\affil[1]{National University of Defense Technology}%


\vspace{-1em}



  
  \date{June 17, 2024}


\begingroup
\let\center\flushleft
\let\endcenter\endflushleft
\maketitle
\endgroup





\selectlanguage{english}
\begin{abstract}
Self-rectifying memristor has no need for selector devices, but possess
the one-way transmission behavior and multi-level non-volatile memory
characteristics, which makes it promising candidate for electronic
synapse. In this letter, we propose a novel self-rectifying memristor
based on Pt/Hf0.5Zr0.5O2/TiN structure. The devices show large memory
window (104) and high rectifying ratio (104), which can block the sneak
current in passive crossbar array without any additional hardware
overhead. Moreover, the devices demonstrate excellent multi-level states
modulation capability, low power consumption, high endurance and long
retention. The final benchmark demonstrates that the proposed
Pt/Hf0.5Zr0.5O2/TiN self-rectifying memristor is a promising candidate
for electronic synapse application.%
\end{abstract}%



\sloppy


A large memory window and low power consumption self-rectifying
memristor for electronic synapse

Qingjiang Li\textsuperscript{1}, Shihao Yu\textsuperscript{1}, Peng
Yang\textsuperscript{1}, Qin Wang\textsuperscript{1}, and Sen
Liu\textsuperscript{1}

\textsuperscript{1} \emph{College of Electronic Science and Technology,
National University of Defense Technology, Changsha, 410073, China}

Email: liusen@nudt.edu.cn

Self-rectifying memristor has no need for selector devices, but possess
the one-way transmission behavior and multi-level non-volatile memory
characteristics, which makes it promising candidate for electronic
synapse. In this letter, we propose a novel self-rectifying memristor
based on
Pt/Hf\textsubscript{0.5}Zr\textsubscript{0.5}O\textsubscript{2}/TiN
structure. The devices show large memory window (10\textsuperscript{4})
and high rectifying ratio (10\textsuperscript{4}), which can block the
sneak current in passive crossbar array without any additional hardware
overhead. Moreover, the devices demonstrate excellent multi-level states
modulation capability, low power consumption, high endurance and long
retention. The final benchmark demonstrates that the proposed
Pt/Hf\textsubscript{0.5}Zr\textsubscript{0.5}O\textsubscript{2}/TiN
self-rectifying memristor is a promising candidate for electronic
synapse application.

\emph{Introduction:} Due to the simple structure, multi-level operation
and non-volatile memory capacity, memristor has been considered as a
potential synapse device for constructing brain-inspired computing
systems, which is expected to break the von Neumann bottleneck in
traditional computing system {[}1,2{]}. Nonetheless, the sneak current,
flowing through unselected cells in passive crossbar array, will lead to
read-out errors and failure of program operations. Though additional
selectors (i.e. transistors or diodes) in serial can solve the sneak
path issues, they will lower the integration density due to their larger
size. The self-rectifying memristor cell, whose nonlinear and asymmetric
current-voltage characteristics can greatly suppress the sneak current
flowing through the unselected cells, is attracting significant
attention because of their simple two-terminal structure and 3D
integration ability {[}3,4{]}. However, most current type
self-rectifying memristors suffer from relatively small memory window
and high-power consumption. Thus, new self-rectifying memristor is
needed for being a promising candidate in electronic synapse
applications.

In this letter, by using the amorphous high-\emph{k} Zr-doped hafnium
oxide as resistive switching material, we propose a novel
self-rectifying memristor based on
Pt/Hf\textsubscript{0.5}Zr\textsubscript{0.5}O\textsubscript{2}/TiN
structure. Experimental results demonstrate the as-fabricated device has
high rectifying ration (10\textsuperscript{4}) and large memory window
(10\textsuperscript{4}), which is sufficient to suppress sneak path
issue in Mb-level passive crossbar array. Moreover, the devices possess
excellent multi-level states, endurance (10\textsuperscript{6}) and
non-volatility (10\textsuperscript{3} s) capability. The excellent
resistive switching performance can be ascribed to the trap-controlled
space charge limited current (SCLC) effect in
Hf\textsubscript{0.5}Zr\textsubscript{0.5}O\textsubscript{2} films. The
final benchmark demonstrates the outstanding self-rectifying performance
with previous reported devices.

\emph{Devices and experiments:} The proposed self-rectifying memristors
were fabricated with a 10-nm-thick
Hf\textsubscript{0.5}Zr\textsubscript{0.5}O\textsubscript{2} (HZO) layer
sandwiched between TiN top electrode and Pt bottom electrode, as shown
in Fig. 1\emph{a} . The vertical lines of Pt/Ti (25/5 nm) were deposited
on the SiO\textsubscript{2}/Si substrate as bottom electrode by e-beam
evaporation after the first lithography process. Then a 10-nm-thick HZO
layer was deposited on the Pt bottom electrode at a stage temperature of
280 \selectlanguage{ngerman}°C. The
Hf{[}N(C\textsubscript{2}H\textsubscript{5})CH\textsubscript{3}{]}\textsubscript{4},
Zr{[}N(C\textsubscript{2}H\textsubscript{5})CH\textsubscript{3}{]}\textsubscript{4}and
H\textsubscript{2}O were used as hafnium precursor, Zr precursor and
oxygen source, respectively. The hafnium/zirconium ratio was controlled
by alternate deposition of one HfO\textsubscript{2} cycle and one
ZrO\textsubscript{2} cycles. Finally, the horizontal lines of TiN (30
nm) were deposited by magnetron sputtering. The cell areas of the
devices range from 9 \selectlanguage{greek}μ\selectlanguage{english}m\textsuperscript{2} (3 \selectlanguage{greek}µ\selectlanguage{english}m\selectlanguage{ngerman}×3 \selectlanguage{greek}µ\selectlanguage{english}m) to 2500
\selectlanguage{greek}µ\selectlanguage{english}m\textsuperscript{2} (50 \selectlanguage{greek}µ\selectlanguage{english}m\selectlanguage{ngerman}×50 \selectlanguage{greek}µ\selectlanguage{english}m). During the electrical measurement,
the voltage was applied to the Pt bottom electrode and the TiN top
electrodes were grounded.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

Fig. 1 \selectlanguage{ngerman}\emph{(a) Device structure of self-rectifying Pt/HZO/TiN device.
(b) Typical I-V characteristics in DC-sweeping mode. (c) The memory
window with applied positive voltages. (d) The read currents
distribution under ±3V.}

\emph{Results and discussion:} Fig. 1\emph{b} shows 20
\emph{I-V}hysteresis loops of Pt/HZO/TiN device in DC sweeping mode.
Notably, no electroforming and no compliance current is needed to
trigger the reproducible switching behaviors. The memory window of
\textasciitilde{} 10\textsuperscript{4} and rectify ratio of
\textasciitilde{} 10\textsuperscript{4} can be obtained with 3 V read
voltage. The read currents of high resistance state (HRS) and low
resistance state (LRS) are \textasciitilde{} 10 pA and \textasciitilde{}
100 nA, indicating the potential for low-power applications. Fig.
1\emph{c} and 1\emph{d} show the memory window with applied positive
voltages and the read currents distribution under ±3 V, respectively.
These results further confirm the self-rectifying resistive switching
behaviors in the proposed Pt/HZO/TiN device. According to a SPICE-based
circuit analysis, where the parasitic wire resistance between adjacent
cells is estimated as R\textsubscript{wire} = 10 \selectlanguage{greek}Ω/\selectlanguage{english}cell, the rectify
ration of 10\textsuperscript{4} and memory window of
10\textsuperscript{4} are able to support passive crossbar array
integration scale up to Mb-level {[}5,6{]}.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

Fig. 2 \emph{(a) The I-t and V-t curves under triangle pulses (5.8 V, 10
ms). (b) Potentiation and depression process by positive pulses (5.8 V,
5 ms) and negative pulses (-4 V, 5 ms).}

The multi-level states modulation capability of self-rectifying
Pt/HZO/TiN device is shown in Fig. 2. The \emph{I-t} and
\emph{V-t}curves of Pt/HZO/TiN device under triangle pulses (5.8 V, 10
ms) are shown in Fig. 2\emph{a} . The gradual increase of response
current demonstrates the potential for multi-level modulation. Then, the
identical rectangle pulses (5.8 V, 5 ms) for potentiation and (-4 V, 5
ms) for depression are employed to further investigate the device
performance. It should be noted that all the current was readout at the
same read pulse (3 V, 5 ms). As seen from Fig. 2\emph{b} , the read
current was gradually tuned up by 50 potentiation pulses and down by 50
depression pulses. The above results confirm the good gradual switching
properties of self-rectifying Pt/HZO/TiN device under both potentiation
and depression pulses, which can be used to mimic the synaptic behaviors
in brain-inspired computing systems.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

Fig. 3 \emph{(a) PPF function under a pair of positive pulses (5.5 V, 10
ms). (b) short-term plasticity after removing the operating pulse.}

Pair-pulse facilitation (PPF) is an important short-term synaptic
function involved in several neuronal tasks, such as reinforcement
learning and information filtering {[}7{]}. Fig. 3\emph{a} shows the
experimental demonstration of PPF function in the proposed Pt/HZO/TiN
device. When a pair of pulses (5.5 V, 10 ms) was applied to the device,
the response current gradually increased during the positive bias, and
the maximum response current of the second pulse was clearly larger than
the first one. But the current at the beginning of the second pulse is
obviously smaller than that at the ending of the first pulse, which
means a decay during the pulse interval. Fig. 3\emph{b} further shows
the short-term plasticity of the self-rectifying device. After removing
the operation pulse (5.5 V, 10 ms), the device state can be maintained
for about 7 ms, monitoring by the 3 V bias voltage. Thus, the short-term
plasticity can also also be realized in Pt/HZO/TiN device.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

Fig. 4 \emph{(a) Endurance property under operation pulses (6 V, 20 ms)
and (-4.5 V, 20 ms). (b) Retention property of the representative three
conductance states.}

The endurance and long-term retention property are also investigated, as
shown in Fig. 4. The fatigue could be observed at
10\textsuperscript{2}\textasciitilde{} 10\textsuperscript{3} endurance
cycles, but no dielectric breakdown was observed even after
10\textsuperscript{6}pulses, as shown in Fig. 4\emph{a} . Three
representative conductance states were selected for retention tests, and
each state was monitored by a 3 V bias every 5 seconds. As seen from
Fig. 4\emph{b} , all the three states successfully lasted 1000 seconds
without noticeable degradation at room temperature.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

Fig. 5 \emph{(a) Area dependence of the read current. (b) Fitted I-V
curves with SCLC conduction mechanisms.}

Then, the underlying mechanism of the self-rectifying Pt/HZO/TiN device
is investigated. Fig. 5\emph{a} shows the area dependence of read
currents in LRS and HRS, respectively. The read current for each device
area were obtained from 20 switching cycles. As the device area increase
from 9 \selectlanguage{greek}µ\selectlanguage{english}m\textsuperscript{2} to 2500 \selectlanguage{greek}µ\selectlanguage{english}m\textsuperscript{2}, the read
currents of both LRS and HRS increase approximately linearly, indicating
an interface-type rather than localization-type resistive switching
behavior. Fig. 5\emph{b} shows the fitted \emph{I-V} curves with SCLC
conduction mechanism. A typical \emph{I-V} characteristic plotted in a
log-log curve for SCLC is bounded by the three limited curves, ohm's law
(\emph{I[?]V} ), traps filled limit current (\emph{I[?]V\textsuperscript{2}}
) and Child's law. The result in Fig. 5\emph{b} is consistent with the
features of SCLC conduction mechanism.

Finally, a benchmark especially on rectify ratio, memory window, read
power and endurance is provided in Table 1. Compared with previous
self-rectifying devices, the proposed Pt/HZO/TiN devices in our work
show competitive advantages, such as the largest memory window, lowest
read power in HRS state.

Table 1. Performance benchmarking between our self-rectifying device and
previous results.\selectlanguage{english}
\begin{longtable}[]{@{}llllll@{}}
\toprule
\begin{minipage}[b]{0.16\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[b]{0.16\columnwidth}\raggedright\strut
Device Structure\strut
\end{minipage} & \begin{minipage}[b]{0.16\columnwidth}\raggedright\strut
Rectifying Ratio\strut
\end{minipage} & \begin{minipage}[b]{0.16\columnwidth}\raggedright\strut
Memory Window\strut
\end{minipage} & \begin{minipage}[b]{0.16\columnwidth}\raggedright\strut
Read Power (HRS, nW)\strut
\end{minipage} & \begin{minipage}[b]{0.16\columnwidth}\raggedright\strut
Endurance\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1 {[}8{]}\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
TiN/Al-HfO\textsubscript{x}/SiO\textsubscript{2}/Si\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
300\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\textasciitilde{}7\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.2\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
10\textsuperscript{5}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
2 {[}3{]}\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Pt/C/NbO\textsubscript{x}/TiN\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
10\textsuperscript{6}\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\textasciitilde{}25\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
100\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\textasciitilde{}3\selectlanguage{ngerman}×10\textsuperscript{3}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.16\columnwidth}\raggedright\strut
3 {[}2{]}\strut
\end{minipage} & \begin{minipage}[t]{0.16\columnwidth}\raggedright\strut
Ru/HSO/Al\textsubscript{2}O\textsubscript{3}/ HSO/TiN\strut
\end{minipage} & \begin{minipage}[t]{0.16\columnwidth}\raggedright\strut
10\textsuperscript{4}\strut
\end{minipage} & \begin{minipage}[t]{0.16\columnwidth}\raggedright\strut
\textasciitilde{}5\strut
\end{minipage} & \begin{minipage}[t]{0.16\columnwidth}\raggedright\strut
0.8\strut
\end{minipage} & \begin{minipage}[t]{0.16\columnwidth}\raggedright\strut
10\textsuperscript{6}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\textbf{This work}\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\textbf{Pt/HZO/TiN}\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\textbf{10\textsuperscript{4}}\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\textbf{10\textsuperscript{4}}\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\textbf{0.03}\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\textbf{10\textsuperscript{6}}\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}

\emph{Conclusion:} In summary, we have proposed a novel self-rectifying
memristor based on Pt/HZO/TiN structure. The devices show large memory
window (10\textsuperscript{4}) and low power consumption (0.03 nW @
HRS). These properties make the devices especially suitable for
low-power artificial synapse and embedded non-volatile memory
applications.

\emph{Acknowledgments:} This research was funded by the National Natural
Science Foundation of China under Grant Nos. 61974164, 62074166,
61804181, 61704191, 62004219 and 62004220, as well as the National
University of Defense Technology Research Funding Program Grant No.
ZK19-05.

\emph{Conflict of interest:} There is no conflict of interest in this
paper.

\emph{Data availability statement:} Data available on request from the
authors (the data that support the findings of this study are available
from the corresponding author upon reasonable request).

? 2022 The Authors. \emph{Electronics Letters} published by John Wiley
\& Sons Ltd on behalf of The Institution of Engineering and Technology

This is an open access article under the terms of the Creative Commons
Attribution License, which permits use, distribution and reproduction in
any medium, provided the original work is properly cited.

\emph{Received:} xx January 2021 \emph{Accepted:} xx March 2021

doi: 10.1049/ell2.10001

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