N AN O E X P R E S S Open Access
Coexistence of memory resistance and memory
capacitance in TiO
2
solid-statedevices
Iulia Salaoru
1
, Qingjiang Li
1,2
, Ali Khiat
1*
and Themistoklis Prodromakis
1
Abstract
This work exploits the coexis tence of both resistance and capacitance memory effects in TiO
2
-based two-terminal
cells. Our Pt/TiO
2
/TiO
x
/Pt devices exhibit an interesting combination of hysteresis and non-zero crossing in their
current-voltage (I-V) characteristic that indicates the presence of capacitive states. Our experimental results demonstrate
that both resistance and capacitance states can be simultaneously set via either voltage cycling and/or voltage pulses.
We argue that these state modulations occur due to bias-induced reduction of the TiO
x
active layer via the displacement
of ionic species.
Keywords: ReRAM; Memristor; Memcapacitor; TiO
2
;Nanoscale
Background
The memristor (short for mem ory-resistor) was theoret-
ically conceived in 1971 by Chua [1] through his famous
symmetry argument. Even though the first experimental
results on resistive memory have been reported as early
as in the 1960s [2-5], it did not attract at that time the
attention of the scientific community. Nonetheless, after
the announcement in 2008 [6] that the missing memristor
has been found by researchers at the Hewlett-Packard la-
boratory, the memristor is put again in the picture. Fur-
thermore, in 2009, Di Ventra et al. [7,8], advanced the
field by theoretically defining two other types of devices:
memory capacitors (memcapacitors) and meminductors
that can be considered as mem-devices [9-11]. To date,
concurrent resistive and capacitive switching effect have
been observed on practical devices, in perovskite oxide
[12], LaAlO
3
[13] and TiO
2
[14,15]. This further leads
to a re consideration of the existing memristor theory;
a plausible extension incorporates nano-battery effect.
In addition, recently, the research community has shown
great interest on exploiting the coexistence of distinct
memory modalities in developing adaptive circuits that
operate in radio frequencies (RFs) [16].
To date, a large number of materials have been exploited
including binary metal oxides, manganite, amorphous Si,
doped Si, perovskite oxides and even organic materials
[17-26]. Resistive switching has been observed in all these
materials and depending on the material employed, distinct
mechanisms have been propose d to be causing this re-
sistive change, including the formation and rupture of
conductive filaments [27-31] the modulation of Schottky
barriers [7,8], electrical trap-related processes [9] and
phase change [10] of the active mate rial. Moreover, it
was recently demonstrated that the memristor exhibit s
capacitive memory as well [ 11-15] that au gments the
interest of research and industrial communities by intro-
ducing novel functionalities and thus applications; beyond
what was previously proposed for memristors, i.e. recon-
figuration architectures [32], neuromorphic computing
[33] and artificial synapses [34].
In this paper, we provide experimental evidence of the
coexistence of both resistive and capacitive memory effects
in TiO
2
-based nanoscale devices. We present a complete
suite of electrical characterisation via quasi-static direct
current (DC) voltage sweep, sweeping potentials of static/
dynamic frequencies of alternating current (AC) and volt-
age pulsing. The results demonstrate the concurrent resist-
ive and capacitive switching behaviours in our solid-state
prototypes with the effective resistive and capacitive states
modulated simultaneously by appropriate voltage pulses.
We argue that this effect is related to a bias-induced reduc -
tion of the TiO
x
active layer via the displacement of ionic
species.
* Correspondence: A.Khiat@soton.ac.uk
1
Nano Research Group, School 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 Salaoru 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.
Salaoru et al. Nanoscale Research Letters 2014, 9:552
http://www.nanoscalereslett.com/content/9/1/552