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.