st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Hybrid Bistable Memory Elements Processed by Plasma Polymerization
Y. Busby1, J.- J. Pireaux1, S. Nau2, S. Sax2, E. J. W. List2,3.
1
Centre de Recherche en Physique de la Matière et du Rayonnement (PMR), Laboratoire Interdisciplinaire de Spectroscopie Electronique (LISE),University of Namur, B-5000 Namur, Belgium.
2
NanoTecCenter Weiz, Forschungsgesellschaft mbH, Franz-Pichler Straße 32, A-8160 Weiz, (Austria).
3
Institute of Solid State Physics Graz University of Technology A-8010 Graz (Austria)
Abstract: We report the structural and electrical characterization of polymer resistive memory
devices where the polymer layer was deposited by plasma polymerization. I-V characteristics
showing resistive switching and memory performances are shown, the advantages of plasma
processing are also discussed.
Keywords: hybrid organic memory devices, plasma polymerization, resistive switching.
1. Introduction
Organic memory devices are considered the most
promising candidates for large scale, high density, and
cost efficient non-volatile memories production [1, 2].
The device architecture is very simple, and it is based on
an organic layer (which may be evaporated small semiconducting molecules, or a polymer film), sandwiched
between two metal electrodes. The device structure may
include some metal nanoparticles (NPs) which can be
randomly dispersed inside the organic layer or disposed to
form an intermediate metal layer (IML) in a Organic/Metal/Organic architecture. Recently, polymer memory
devices (PMDs), with high performances, have been
demonstrated based on a polystyrene (PS) layer solution
processed together with some added conducting cluster
(Au, or C60) or with metal nanowires (Ag) [3, 4, 5].
The deposition of the active layer through solution processing has two main limitations, one is that the colloidal
solution containing the NPs has to be compatible with the
polymer solvent, and the second is that the deposition of
hybrid (polymer/ metal) heterostructures is generally not
easy to control, since the deposition of the overlayers
will tend to dissolve the underlying ones.
In this work, we have studied the possibility to deposit
the polymer layer by plasma polymerization. Resistive
switching in plasma polymers has been poorly investigated since the pioneering exploratory works made in the
70s [6, 7].
2. Experimental and Results
We have studied a series of PMDs based on a PS thin
film deposited by plasma polymerization (P-PS) on a
Glass/ ITO or PET/ITO substrates, with Ag top electrode.
Silver nanoparticles where evaporated inside the organic
layer to form a discontinuous intermediate metal layer
(IML). The I-V characteristics of the PMDs have been
studied for different device architectures, with/without
IML, symmetric/asymmetric hybrid structures, and with
the polymer layer deposited at different plasma conditions
(from low to high monomer fragmentation regimes).
The typical behavior observed in the first I-V scans of our
memory devices in shown in Fig.1. It is interesting that
no forming step was needed in order to obtain the first
switching between the high to the low resistance state.
The I-V curves where measured with a Keithley 2400
source meter in ambient conditions.
The plasma polymerized PS devices have shown
ambipolar switching, with high On-Off ratio, typically
between 103 and 104, a retention time of >104s making
them suitable for non-volatile memory devices applications. The full cycling scheme was optimized, allowing
the memories to be written, erased, and rewritten, by applying short voltage pulses over the respective voltage
thresholds. The writing voltage was typically between 1
and 5 V depending on the device structure. The P-PS
memory elements were successfully cycled over hundreds
of cycles without losing their functionality (see Fig.2).
The On/Off ratio at the reading voltage of 1V was higher
than 100 testing, and the memory addressing was successful with rates up to 75%.
Fig.1: Addressing the P-PS memory devices by a voltage scan.
The grey curves show the switching from the high resistance
state to the low resistance state for the first two cycles, the black
curves show the memory erasing by polarizing the device
through the negative differential resistance region.
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Fig.2: Cycling of the P-PS memory devices by a voltage pulses (see inset for the full addressing scheme). The grey dots are
the on state currents measured at 1V, the black full dots are the
current values obtained after the memory erase step by a -15V
pulse. Addressing failures (35%) are not shown.
3. Discussion
The results has been compared to similar devices based
on solution processed PS. The switching mechanisms are
perfectly similar and can been fully interpreted in terms of
the formation and rupture of localized metallic filamentary paths inside the polymer layer [8].
We found that the switching voltage is weakly affected by
the polymer deposition parameters; moreover, switching
occurs even without any IML, most probably because of
the Ag NPs field assisted diffusion from the top electrode
into the organic layer. Our preliminary experiments confirmed that On/Off ratio is mainly affected by the metal
NPs density. Memory cycling studies has evidenced a
progressive On/Off decrease due to the increase of the
metal NPs density inside the organic layer; this can lead
to a permanent On-state when the particle density exceeds
the percolation threshold. Preliminary studies have shown
that by changing the polymer deposition parameters, the
metal clusters diffusion into the polymer matrix can be
reduced, which allows to increase the memory cycling
endurance.
The overall results show how plasma polymerization can
successfully be applied to PMDs processing and present
some technologically relevant advantages respect to conventional deposition methods. These are in particular, the
possibility to finely tune the polymer structure, and to
deposit complex hybrid heterostructures for tridimensional stacking.
Finally, plasma polymerization can be a very promising
technique for low cost, eco-friendly memory production.
The use of other monomer precursors may also give rise
to interesting results to reach a higher control on the
switching performances, reliability and endurance of organic memory devices.
5. References
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