© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 4819 www.advmat.de www.MaterialsViews.com COMMUNICATION wileyonlinelibrary.com Adv. Mater. 2010, 22, 4819–4822 By Ruth Muenstermann,* Tobias Menke, Regina Dittmann, and Rainer Waser Coexistence of Filamentary and Homogeneous Resistive Switching in Fe-Doped SrTiO 3 Thin-Film Memristive Devices [∗] R. Muenstermann, Dr. T. Menke, Dr. R. Dittmann, Prof. R. Waser Institute of Solid State Research Research Center Juelich 52425 Juelich (Germany) E-mail: ru.muenstermann@fz-juelich.de DOI: 10.1002/adma.201001872 Resistance random access memory, short RRAM, which employs two or more resistive states of a material for data storage, has attracted considerable attention as a highly scal- able future non-volatile memory concept. [1,2] These memory cells that can also be described as so-called memristors are par- ticularly interesting when multilevel resistance values or even analogue values should be stored and processed. [3–5] A large variety of binary and ternary oxides exhibit resistive switching phenomena, however, the details of the complex microscopic mechanisms are rarely understood and depend strongly on the specific material combination. In the search for promising oxide materials for future non-volatile memories, special atten- tion has to be paid to their scaling capabilities. The issue of scaling is strongly linked to the question of, whether the switching current is distributed homogeneously across the device area or localized to one or a few conducting filaments. While in the former case the scaling limit will be con- nected to the minimum device area, that guarantees sufficient switching currents for a reliable circuit operation, in the latter case, scaling might suffer from too large filament dimensions or their insufficient density and regularity within the material. Complex transition metal oxides, e.g. manganites, [6–9] titan- ates and zirconates, [10,11] usually exhibit different resistance states at opposite polarities of electrical stimulation. It has become widely accepted that this so-called bipolar resistive switching is connected with a voltage-driven oxygen vacancy movement and a resulting redox process. [12] Both, filamentary as well as homogenous switching has been reported in the liter- ature. For thermally reduced SrTiO 3 single crystals it has been clearly demonstrated by conductive-tip atomic force micros- copy (conductive AFM) that resistive switching at free surfaces occurs along conducting filaments which can be identified with the exits of dislocations. [13] For crystalline SrTiO 3 thin film sam- ples as well as amorphous TiO 2 capped between macroscopic electrodes, the indispensable electroforming process results in the formation of a single μm-size filament. [14–18] Besides these reports about filamentary-type resistive switching, Sawa et al. reported a so-called homogeneous interface-type switching, which is observed at the interface between different complex oxides and the metal electrode, e.g. Pr 0.7 Ca 0.3 MnO 3 /Ti and Nb-doped SrTiO 3 /SrRuO 3 . [19] For this type of devices, On and Off resistance are reported to scale with the device area, [20] which hints on a homogeneous conduction mechanism, and the change of the resistance is attributed to the field-induced change of the Schottky-barrier at the interface. However, whether filamentary or interface-type switching exhibits superior scaling capabilities has to be clarified in the future. In addition to the investigation of area scaling of metal-insulator-metal (MIM) structures, conductive AFM is a powerful tool to elucidate the nanoscale current distribution and switching properties of thin film heterostructures. In this work we used conductive AFM combined with a delamination technique to remove the top electrode of Fe-doped SrTiO 3 MIM structures to gain insights into the active switching interface with a lateral resolution of a few 10 nm. This enabled us to prove the coexistence of a filamentary and an area-dependent switching process with opposite switching polarities in the same sample. Resistive switching MIM structures have been fabricated from 1 at% Fe-doped SrTiO 3 thin film grown on a metallic single crystal substrate (1at% Nb-doped SrTiO 3 ) by pulsed laser deposition (PLD). Pt top electrodes were deposited onto the film either by DC sputtering or by electron-beam-evaporation. The current-voltage ( I–V) characteristics of the resulting MIM struc- tures were measured in a two-probe configuration, applying the bias always to the Pt top electrode. The asymmetric sample structure results in an inherently ohmic bottom interface and a Schottky-like upper interface. To activate the resistive switching properties in MIM structures, typically a forming process has to be applied to the devices (see supplementary information). The resistive switching behaviour of a formed Pt/500 nm SrTiO 3 (Fe)/Nb:SrTiO 3 MIM-device is shown in Figure 1a in terms of a current-voltage ( I–V) characteristic. Starting at 0 V and sweeping to -2.2 V, the sample resides in branch “1”. If, in a subsequent sweep, the voltage is swept back from -2.2 V to 0 V (curved green arrow) the sample switches into a lower resistance state (branch “2”). Going up to 2.8 V and then back to 0 V again completes the switching cycle. A stable resistive switching state, shown in green, is reached (1-2-3-4-1). In the following we call the “sense of rotation” or switching polarity of this curve “counter eightwise” polarity. If, however, the sample resides in branch “1” and –upon reaching -2.2 V – the voltage is swept further down to -3.5 V (curved orange arrow), a higher resistance state is reached (branch “5”). Going back to positive voltages reverses this switching again (branch “6” to branch “4”). Keeping this higher negative voltage amplitude, a stable second type of switching can be achieved,