Available online at www.sciencedirect.com Organic Electronics 9 (2008) 119–128 www.elsevier.com/locate/orgel Switching in polymeric resistance random-access memories (RRAMS) H.L. Gomes a,*, A.R.V. Benvenho a, D.M. de Leeuw b,M.Co¨lle b, P. Stallinga a, F. Verbakel c, D.M. Taylor d a Universidade do Algarve, Centre of Electronic Optoelectronics and Telecommunications (CEOT), Campus de Gambelas, 8000 Faro, Portugal b Philips Research Laboratories, Professor Holstlaan 4, 5656 AA Eindhoven, The Netherlands c Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands d School of Electronic Engineering, University of Wales, Dean Street, Bangor, Gwynedd LL57 1UT, UK Received 17 July 2007; received in revised form 28 September 2007; accepted 11 October 2007 Available online 24 October 2007 Abstract Resistive switching in aluminum-polymer-based diodes has been investigated using small signal impedance measure- ments. It is shown that switching is a two-step process. In the first step, the device remains highly resistive but the low frequency capacitance increases by orders of magnitude. In the second step, resistive switching takes place. A tentative model is presented that can account for the observed behavior. The impedance analysis shows that the device does not behave homogenously over the entire electrode area and only a fraction of the device area gives rise to switching. Ó 2007 Elsevier B.V. All rights reserved. PACS: 73.40.Sx; 84.37.+q; 73.61.Ph; 85.65.+h Keywords: Organic memory; Switching; Impedance spectroscopy; Oxide; Traps 1. Introduction ing ON/OFF current ratios up to 106 and long retention times, thus opening interesting perspec- Electrically switchable solid state memories have tives for applications in the domain of resistance been demonstrated in diodes using a variety of random access memories (RRAMs). organic materials. The key feature of these memo- Several memory architectures have been ries is the ability to switch their resistance between reported. Bistable organic diodes made of a single two or more stable states simply by applying voltage organic layer between two metal electrodes [1–17], pulses. Many devices exhibit sufficiently fast switch- nanoparticles embedded into an organic matrix ing capability, low switching threshold voltages giv- [18–21], organic layers with granular metals [22– 25] and transistor type memories [26,27]. * Corresponding author. Tel.: +351 289 800 900. We also reported bistable resistance characteris- E-mail address: [email protected] (H.L. Gomes). tics in metal/polymer/metal diodes [28]. We showed 1566-1199/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2007.10.002 120 H.L. Gomes et al. / Organic Electronics 9 (2008) 119–128 that the current is transported through filaments measured. Switching to higher conducting levels and that the formation of a thin aluminum oxide can then be triggered by a subsequent voltage pulse. layer was crucial for the fabrication of reliable The frequency dependence of the admittance switching devices. Multi-level stages can be obtained reveals that the observed changes are located at depending on the number of filaments. Recently, the aluminum/polymer interface. The initial capaci- our findings were corroborated by Kartha¨user and tance change is caused by a trap filling mechanism co-workers [29] who elegantly showed that switch- occurring at the aluminium/polymer interface. ing characteristics can be obtained simply by depos- These traps were likely created in a previous pro- iting an aluminum film directly onto the substrate cess, the so-called ‘‘forming’’, a term first used by and without the need for an organic layer. Previ- Simmons and Verderber [38] in 1967. Since then, ously, Oyamada et al. [12] had also concluded that the term ‘‘forming’’ has been used by a number of the aluminum electrode is responsible for electrical authors to describe a permanent change in the switching. As long ago as 1990, Sato et al. [30] pro- oxide, induced by a once-only voltage or current posed that the contact between the aluminum and pulse in oxide-based memories [39–42]. Memory the semiconductor plays an essential role. This switching can only be initiated after this forming inference was supported by the experimental fact stage. The results described in this contribution that no observable switching effect could be found were on devices that had already been formed. when other electrode metals such as nickel or gold were employed instead of aluminum. Similar results 2. Experimental were reported by Beck et al. [31]. In practice, since the memory effect seems inti- The structure of the electrically-bistable device mately related to the presence of oxide, the switch- studied here and the schematic flat-band diagram ing in binary oxides and perovskite-type oxides is are presented in Fig. 1. The polymer used as the clearly relevant. In these materials switching is active layer was poly(spirofluorene) (PFO) for attributed to deep trapping processes. For instance, which the electron affinity, v = 2.2 eV, ionization trapped charges in nanometer-thin insulating layers potential, Ip = 5.1 eV and energy gap, Eg = 2.9 eV of Al2O3 were reported to exhibit a remarkable [16]. The aluminium (Al) and barium (Ba) elec- resistance to annihilation by opposite polarity carri- trodes have workfunctions /Al = 4.3 eV and ers passing through the oxide [32]. Retention times are estimated to be as long as 10 years [33]. Much knowledge about carrier trapping in inter- facial oxide layers in a variety of devices has been provided by small signal impedance spectroscopy. It is thus surprising that detailed studies of the impedance characteristics in switching devices have Barium not been conducted. The scarce reports [30,34,35] Polymer (80nm) do, however, show a behavior common to all mem- Aluminium ory device architectures: the switching is always accompanied by an increase in the device capaci- Glass substrate tance at low frequencies and explained as a trapping process. Simon et al. [36] have been shown that the trapped charge gives rise to inductive effects. Another study interpreted the impedance data in terms of different density of highly conducting mol- ecules [37]. Here, we report a systematic and detailed analysis of the impedance changes occur- ring in the devices as they are formed and subse- quently programmed into high conductance states. We show that prior to resistive switching, the device undergoes a dramatic increase in the capacitance. In Fig. 1. Schematic diagram showing: (a) the physical structure of spite of this change, the device still behaves as an the memory device (top electrode is Ba, Al) and (b) the energy insulator and no appreciable dc currents can be levels of the structure. H.L. Gomes et al. / Organic Electronics 9 (2008) 119–128 121 /Ba = 2.7 eV, respectively. The methods used for ously [28]. In the OFF state, the I–V curves can fabrication of these electron-only diodes were sometimes exhibit rectification properties as in described in Ref. [28]. The polymer thickness was Fig. 2. Current–voltage characteristics correspond- 80 nm and both metal electrodes were 30 nm thick. ing to the OFF state frequently show a shoulder The top electrode is 5 nm Ba with 100 nm Al. Effec- at low positive bias at about the built-in voltage tive device areas were either 0.01 or 0.09 cm2. The (see Fig. 2). This plateau/shoulder has often been aluminum electrode was treated with UV/O3 plasma observed in polymeric light emitting diodes treatment before polymer deposition. The current– (PLEDs) and will be discussed later when treating voltage (I–V) curves were obtained using a Keithley the capacitance–voltage (C–V) characteristics. 487 picoammeter/voltage source and capacitance– frequency and capacitance–voltage (C–V) curves 3.2. Small signal impedance characteristics were obtained using a Fluke PM 6306 RLC meter. In all the measurements, voltages are referenced The OFF state and the ON-state observed in the such that the Ba/Al electrode is held at local I–V characteristics are associated with correspond- ground, while the bias is applied to the Al electrode. ing changes in the impedance characteristics. How- ever, the changes do not occur simultaneously, 3. Results and discussion capacitance changes precede changes in the cur- rent–voltage characteristics. 3.1. Current–voltage characteristics The changes occurring in the frequency-depen- dence of the admittance of devices in the OFF and Fig. 2 shows typical current–voltage (I–V) char- ON states is shown in Fig. 3a. When the devices acteristics obtained for two programmed states. are OFF they behave as simple parallel plate capac- The lower I–V curve corresponds to the ‘‘OFF itors with capacitance (C) and dielectric loss (G/x) state’’ and the higher curve to what we designate almost independent of frequency (f), where G is the ‘‘fully-ON state’’. In between, multilevel stages the conductance and x =2pf, is the angular fre- can be observed, see Ref. [28]. The different conduc- quency. As no free carriers are able to follow the tive states can be programmed by application of a modulation of the external voltage at high frequen- voltage pulse, for example 3–5 V for 5 s. The process cies, the material system behaves like a passive is totally electrically reversible, as described previ- dielectric medium. If the devices are submitted to voltage ramps (0.1 V/s) within a small range (e.g. ±5 V) a dramatic increase in capacitance at low frequencies may be 10 -2 induced. The rise in capacitance is accompanied ON state by the appearance of a relaxation process with a cut-off frequency near 200 kHz, see Fig.