Switching speed in Resistive Random Access Memories (RRAMS) based on plastic semiconductor
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Switching speed in Resistive Random Access Memories (RRAMS) based on plastic semiconductor Paulo F. Rocha1, Henrique L. Gomes 1, Asal Kiazadeh1, Qian Chen1, Dago M. de Leeuw2 and Stefan C. J. Meskers3 1 Center of Electronics Optoelectronics and Telecommunications (CEOT), Universidade do Algarve, Campus de Gambelas, 8005-139 Faro, Portugal 2 Philips Research Labs, High Tech. Campus, 5656 AE Eindhoven, The Netherlands 3 Molecular Materials and Nanosystems, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands ABSTRACT This work addresses non-volatile memories based on metal-oxide polymer diodes. We make a thorough investigation into the static and dynamic behavior. Current-voltage characteristics with varying voltage ramp speed demonstrate that the internal capacitive double-layer structure inhibits the switching at high ramp rates (typical 1000 V/s). This behavior is explained in terms of an equivalent circuit. It is also reported that there is not a particular threshold voltage to induce switching. Voltages below a particular threshold can still induce switching when applied for a long period of time. The time to switch is longer the lower is the applied voltage and follows an exponential behavior. This suggests that for a switching event to occur a certain amount of charge is required. INTRODUCTION Organic materials are becoming interesting candidates for electronic devices in new information technologies particularly on memories [1,2]. One type of memory offering excellent prospects is the resistive random access memory (RRAM), a simple diode structure whose resistance can be programmed reversibly to be high or low. In spite of intense effort, many details of the switching mechanism and charge transport have not clearly been identi¿ed. For example, there is no consensus on the write- and erase times. Reported switching times vary by orders of magnitude, the values range from nanoseconds to milliseconds. For the polymer/oxide memory diodes it was reported that short switching times (nanoseconds) are possible for individual switching events, but that repeated ON and OFF switching within a short time interval (< ms ) is not possible. This phenomenon has been refered to as ‘dead time‘ [3,4]. The origin of this effect is not known in detail. Yet it may put drastic limitation on the operation of these cells. In this contribution we characterize the switching speed of metal-oxide memory devices using dynamic and static measurements and equivalent circuit modeling. EXPERIMENTAL DETAILS The organic memory used consists on a diode structure (inset of figure 1) with an Al bottom electrode, a sputtered layer of Al2O3 (20 nm), a spirofluorene polymer (80 nm), and
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a Ba/Al (5 nm/100 nm) top electrode that forms an Ohmic contact with the polymer. The devices with an active area of 9 and 1 mm2 were encapsulated to exclude O2 and H2O. In all cases, positive bias voltage refers to the bottom aluminum electrode being positive with respect to the top electrode. The memories were formed; by applying
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