N AN O E X P R E S S Open Access
Stochastic switching of TiO
2
-based memristive
devices with identical initial memory states
Qingjiang Li
1,2
, Ali Khiat
2
, Iulia Salaoru
2
, Hui Xu
1
and Themistoklis Prodromakis
2*
Abstract
In this work, we show that identical TiO
2
-based memristive devices that possess the same initial resistive states are
only phenomenologically similar as their internal structures may vary significantly, which could render quite
dissimilar switching dynamics. We experimentally demonstrated that the resistive switching of practical devices with
similar initial states could occur at different programming stimuli cycles. We argue that similar memory states can
be transcribed via numerous distinct active core states through the dissimilar reduced TiO
2-x
filamentary
distributions. Our hypothesis was finally verified via simulated results of the memory state evolution, by taking into
account dissimilar initial filamentary distribution.
Keywords: Resistive switching; Initial state; Filamentary distribution
Background
Among numerous candidates for the non-volatil e memories,
resistive random access memory (ReRAM) is highly consid-
ered for its advantageous attributes [1-3]. Nonetheless, the
operation mechanism of ReRAM devices remains a bone of
contention [4,5] with the formation and rupture of conduct-
ing filaments being ascertained as the functional switching
mechanism [6]. Understanding their switching dynamics is
thus of critical importance for the future implementation of
ReRAM. Surprisingly, there exist numerous studies that
highlight the stochastic switching in ReRAM [7-10]. In [8],
the experimental results show that both the distributions of
I
RESET
and V
RESET
arestronglyinfluencedbythedistribution
of initial resistance. In addition, Shibuya et al. [11] have
demonstrated the impact of pristine defect distribution on
current-voltage (I-V) characteristics of Sr
2
TiO
4
thin films,
demonstrating that the density of distinct initial defects
would result in two opposite I-V switching polarities.
One might expect that identical ReRAM devices that
possess the same initial effective resistance would attain
the same resistive state evolution when provided the
same programming stimulus. Nevertheless, this does not
always hold for practical devices. In practical devices,
randomly distributed local imperfections could act as
conductive percolation branches within the devices' active
cores. Such conditions employ the devices with a high
probabilistic nature, which could provide very dissimilar
switching characteristics. In this study, we experimentally
demonstrated stochastic resistive switching in TiO
2
-based
ReRAM devices that possess identical initial resistive
states. We further explore the origin of this phenomenon
by employing a random circuit breaker (RCB) network
model [9,12]. We show that ReRAM devices that have the
same initial resistance would attain distinct initial filament
distributions, which would finally result in very dissimilar
resistive switching dynamics even when programmed with
the same pulse schemes.
Methods
Fabrication of TiO
2
-based active cells
In this study, we employed the following fabrication
process flow. Firstly, 200-nm-thick SiO
2
was thermally
grown on a 4-in. silicon wafer. Then, e-gun evaporation
was employed to deposit 5-nm Ti and 30-nm Pt that serve
as adhesion and bottom electrode (BE) layers, respectively.
The stoichiometric TiO
2
-based layer with a total thickness
of 31 nm was then deposited by RF magnetron sputter-
ing at 300 W and with an Argon gas flow of 30 sccm.
Subsequently, a 30-nm-thick Pt top electrode ( TE) film
was deposited by e-gun evaporation. Optical lithography
and lift-off process were adopted to define the patterns of
each layer. The design allows having Pt/TiO
2
/Pt ReRAM
* Correspondence: t.prodromakis@soton.ac.uk
2
Southampton Nanofabrication Centre, Nano Group, Department of
Electronics and Computer Science, University of Southampton, Southampton
SO17 1BJ, UK
Full list of author information is available at the end of the article
© 2014 Li et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction
in any medium, provided the original work is properly credited.
Li et al. Nanoscale Research Letters 2014, 9:293
http://www.nanoscalereslett.com/content/9/1/293