Regina Dittmann

Coexistence of filamentary and homogeneous resistive switching in Fe-doped SrTiO3 thin-film memristive devices.

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 scalable future non-volatile memory concept. [ 1 , 2 ] These memory cells that can also be described as so-called memristors are particularly 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 specifi c material combination. In the search for promising oxide materials for future non-volatile memories, special attention 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 fi laments. While in the former case the scaling limit will be connected to the minimum device area, that guarantees suffi cient switching currents for a reliable circuit operation, in the latter case, scaling might suffer from too large fi lament dimensions or their insuffi cient density and regularity within the material. Complex transition metal oxides, e.g. manganites, [ 6–9 ] titanates 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, fi lamentary as well as homogenous switching has been reported in the literature. For thermally reduced SrTiO 3 single crystals it has been clearly demonstrated by conductive-tip atomic force microscopy (conductive AFM) that resistive switching at free surfaces occurs along conducting fi laments which can be identifi ed with the exits of dislocations. [ 13 ] For crystalline SrTiO 3 thin fi lm samples as well as amorphous TiO 2 capped between macroscopic electrodes, the indispensable electroforming process results in the formation of a single μ m-size fi lament. [ 14–18 ]

Impact of Defect Distribution on Resistive Switching Characteristics of Sr2TiO4 Thin Films

Adv. Mater. 2010, 22, 411–414 2010 WILEY-VCH Verlag Gm T IO N Recent rapid progress of technology has been brought in today’s advanced information society. However, conventional memory scaling is expected to come up against technological and physical limits in the near future. In order to overcome this problem and achieve further progress in information technology, nonvolatile memories with high-density, high-speed, and low-power which can replace so-called Flash memory and dynamic random access memory (DRAM) are required. Reversible resistive switching materials are highly promising candidates for next-generation nonvolatile memories, known as resistance random access memory (ReRAM) because of the simple device structure comprised of a semiconductor (or insulator) sandwich with metallic electrodes. Liu et al. reported that Pa0.7Ca0.3MnO3/ YBa2Cu3O7-d heterojunctions with Ag electrodes exhibited reversible switching by application of short voltage pulses. Beck et al. showed switching in Au/Cr-doped SrZrO3/SrRuO3 junctions. Since then, number of perovskite-type transition-metal oxides, especially titanates, zirconates, and manganites, have been investigated as a switching device. In order to develop resistive materials to applicable memory cells and to advance to the ultimate scaling limits for future ReRAM, deep understanding of switching mechanisms is indispensable. Thanks to recent further intensive investigations, there exists some general agreements in oxides that the migration of oxygen vacancies (or oxygen ions) under an applied electric field plays an important role and that interface effects are more pronounced than bulk switching. Two basic models have been proposed: (i) local redox process based on oxygenvacancy migration (or oxygen diffusion) including an electrochemical interface reaction and (ii) change in Schottky-like barrier height and/or width by trapping/detrapping effects at interface defect states. Although no attention has been paid to it in the literature yet, these two mechanisms exhibit different switching polarities. In n-type materials, the former changes resistance from a low resistance state (LRS) to a high resistance state (HRS) in the positive voltage polarity of the active electrode. The latter converts a HRS into a LRS under the same bias polarity. The same polarity changes occur in the negative bias in the case of p-type layers. Whereas interface defects will dominantly influence the trapping/detrapping-based switching mechanism, for the oxygen-diffusion-related switching mechanism, extended defects within the active thin film matrix have been claimed to play a crucial role. This is associated with the fact that oxygen-vacancy migration (or oxygen diffusion) is significantly enhanced along extended defects. However, direct experimental demonstration of a correlation between defect distribution and switching properties has not been reported up to now. One of the reasons is the lack of materials with a well-defined distribution of defects. On the other hand, it is of pivotal importance to fabricate resistive switching samples with a tailored defect configuration in order to gain a deeper understanding of the role of specific defects and to tune the material properties by ‘‘defect engineering.’’ In this study, we will compare thin film samples with significant different defect structure and will thereby provide the experimental evidence of the correlation between defect density and switching properties. We will furthermore demonstrate the coexistence of two different switching characteristics with opposite polarity which can be reversible and controllable selected by the current load. We will clearly show that the two switching mechanisms are differently pronounced in different defect density and thickness regimes. Nominally nondoped epitaxial Sr2TiO4(001) thin films were grown on nondoped and 0.5wt% Nb-doped SrTiO3(001) single-crystal substrates by pulsed laser deposition (PLD). Current–voltage (I–V) characteristics were measured with two-probe configuration. A schematic drawing of the measurement configuration is depicted in Figure 1a.

In situ observation of filamentary conducting channels in an asymmetric Ta2O5−x/TaO2−x bilayer structure

Electrically induced resistive switching in metal insulator-metal structures is a subject of increasing scientific interest because it is one of the alternatives that satisfies current requirements for universal non-volatile memories. However, the origin of the switching mechanism is still controversial. Here we report the fabrication of a resistive switching device inside a transmission electron microscope, made from a Pt/SiO₂/a-Ta₂O5-x/a-TaO2-x/Pt structure, which clearly shows reversible bipolar resistive switching behaviour. The current-voltage measurements simultaneously confirm each of the resistance states (set, reset and breakdown). In situ scanning transmission electron microscope experiments verify, at the atomic scale, that the switching effects occur by the formation and annihilation of conducting channels between a top Pt electrode and a TaO2-x base layer, which consist of nanoscale TaO1-x filaments. Information on the structure and dimensions of conductive channels observed in situ offers great potential for designing resistive switching devices with the high endurance and large scalability.

Resistive switching and data reliability of epitaxial (Ba,Sr)TiO3 thin films

We report on resistive switching of capacitor-like SrRuO3∕Ba0.7Sr0.3TiO3∕Pt thin films epitaxially grown on SrTiO3 substrates. We observe a weak but stable hysteresis in the current-voltage curve. By applying short voltage pulses, a high or low resistive state as well as intermediate states can be addressed even at room temperature. We demonstrate a multiple-branch hysteresis curve corresponding to multilevel switching modus revealing different subloops for different write voltages. Furthermore reliability issues such as cycling endurance and data retention are presented. Read-write operations over 10000cycles show a fatigue-like drift of both resistance states. No data loss is found upon continuous readout.

Improved endurance behavior of resistive switching in (Ba,Sr)TiO3 thin films with W top electrode

We compared the resistive switching performance of barium strontium titanate (BST) thin films with tungsten (W) and platinum (Pt) top electrodes, respectively. The yield, endurance, and reliability were strongly improved for the samples with W top electrode. Whereas the samples with Pt top electrode show a fast drop in the resistance for both high and low resistance states, the devices with W top electrode can be switched for 104 times without any obvious degradation. We attribute the improved switching performance to a reversible oxidation and reduction in a WOx layer at the W-BST interface, which was detected by time-of-flight secondary-ion-mass spectroscopy measurements.

Separation of bulk and interface contributions to electroforming and resistive switching behavior of epitaxial Fe-doped SrTiO3

We succeeded in the separation of bulk and interface contributions to the electroforming and resistive switching behavior of Pt/STO(Fe)/Nb:STO devices by performing impedance spectroscopy. Two distinctive features observed in the impedance spectra could be assigned to the STO(Fe) bulk and to the depletion layer of the Pt/STO(Fe) Schottky contact. We attribute the resistance change during the dc forming process to a local bypassing of the depletion layer caused by oxygen effusion to the environment. By comparing the impedance spectra in the resistive “on” and “off” states, we propose that the resistance of the STO(Fe)/Nb:STO interface locally changes during the switching process.

Impact of the electroforming process on the device stability of epitaxial Fe-doped SrTiO3 resistive switching cells

In this work, the results of our detailed investigations on the electroforming procedure in Pt/SrTi0.99Fe0.01O3/SrTi0.99Nb0.01O3 [Pt/STO(Fe)/Nb:STO] metal-insulator-metal (MIM)-devices and its impact on the performance of resistive switching memory devices are presented. Questions about the exact location of the modifications triggered by the electroforming procedure within the investigated MIM-devices will be addressed. From a technological point of view, the thermal stability of formed devices becomes important. An increase in the device resistances during retention measurements has been observed indicating the presence of internal redistribution effects. These may result from an oxygen vacancy gradient induced by the forming process. However, these internal relaxation effects will not end up in the unformed state. Annealing experiments under defined atmospheric conditions allowed distinguishing between internal and external rediffusion effects. We found that SrTiO3 starts to interact with the surroundi...

Pulsed laser ablation of complex oxides: The role of congruent ablation and preferential scattering for the film stoichiometry

By combining structural and chemical thin film analysis with detailed plume diagnostics and modeling of the laser plume dynamics, we are able to elucidate the different physical mechanisms determining the stoichiometry of the complex oxides model material SrTiO3 during pulsed laser deposition. Deviations between thin film and target stoichiometry are basically a result of two effects, namely, incongruent ablation and preferential scattering of lighter ablated species during their motion towards the substrate in the O2 background gas. On the one hand, a progressive preferential ablation of the Ti species with increasing laser fluence leads to a regime of Ti-rich thin film growth at larger fluences. On the other hand, in the low laser fluence regime, a more effective scattering of the lighter Ti plume species results in Sr rich films.

Reversible alternation between bipolar and unipolar resistive switching in polycrystalline barium strontium titanate thin films

The alternation from bipolar to unipolar resistive switching was observed in perovskite Ba0.7Sr0.3TiO3 thin films. By controlling the switching voltage, either bipolar or unipolar switching was obtained. When the switching voltage is higher than a threshold voltage, the device exhibits unipolar switching while if the switching voltage is lower than a threshold voltage, the device shows bipolar switching behavior. The bipolar-to-unipolar alternation is dynamically repeatable and may be related to the local modification of broken filaments by oxygen vacancy movement.

Realization of regular arrays of nanoscale resistive switching blocks in thin films of Nb-doped SrTiO3

We report on the realization of short-range-ordered arrays of nanoscale resistive switching blocks in epitaxial Nb-doped SrTiO3 thin films. These blocks can be individually addressed by the tip of a conductive tip atomic force microscope and reversibly switched between a high and a low resistance state reaching an Roff to Ron ratio of up to 50. Scanning micrometer-scale areas with an appropriately biased tip, all blocks within the scanned area can be switched between the two resistive states. We suggest a connection between these nanoscale switching blocks and defect-rich nanoclusters which were detected with high resolution transmission electron microscopy.