Shaobo Mi

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.

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.

Sr2TiO4 layered perovskite thin films grown by pulsed laser deposition

We have fabricated epitaxial Sr2TiO4 thin films on SrTiO3 (100) single crystal substrates by pulsed laser deposition. We demonstrate that growth parameters including substrate temperature, oxygen pressure, as well as the laser fluence have to be chosen precisely to obtain stoichiometric well-ordered films of this complex layered structure. Films grown at low temperature showed three-dimensional random distribution of SrO double layers, causing a new extinction rule in x-ray diffraction. Stoichiometric Sr2TiO4 films with well-ordered SrO double layers were fabricated at higher temperature and under low oxygen pressures, where thermal energy was sufficient to compensate local composition fluctuation and Sr deficiency was very small.

Nonlinear Electrical Properties of Grain Boundaries in Oxygen Ion Conductors: Acceptor-Doped Ceria

Owing to the positively charged grain boundary cores in acceptor-doped ZrO 2 and CeO 2 , oxygen vacancies are depleted in the space charge layers. The validity of this space charge concept was checked for Y 2 O 3 -doped CeO 2 ceramic of high purity. Electrical fields up to 2 × 10 5 V cm - 1 were applied to the grain boundaries of 1.0 mol % Y 2 O 3 -doped CeO 2 at 400°C in air, and the grain boundary properties were separated by means of impedance spectroscopy. It was discovered that the current-voltage relation for individual grain boundary was nonlinear, and that the effective grain boundary thickness increased with increasing bias, which supports the space charge concept.

Growth of ternary PbTiOx films in a combination of binary oxide atomic layer depositions

Ternary PbTiOx films were deposited at 240°C on Pt-covered Si substrates using a combination of liquid injection atomic layer depositions of binary TiOx and PbO films. Ti(OC3H7)2(C11H19O2)2 [Ti(Oi−Pr)2(DPM)2] and Pb(C11H19O2)2 [Pb(DPM)2] dissolved in ethylcyclohexane and H2O were used as source materials. The deposition rates of Pb and Ti were enhanced in the ternary process compared to their binary processes under comparable deposition conditions. The Pb∕Ti ratio of PbTiOx films saturated with an increase in Ti precursor input, while it continued to increase with an increasing Pb precursor input. The self-regulated growth nature of the Pb–O layer in the binary film growth was lost in the mixed PbTiOx process as a result of interaction with the predeposited Ti–O layer. It was confirmed that for the PbTiOx film to grow on Pt substrates, an initial incubation period is required. Both Pb–O and Ti–O layers shared a common incubation period of up to ten sequences. The incubation period was shortened by increas...

Liquid Injection ALD of Pb ( Zr , Ti ) O x Thin Films by a Combination of Self-Regulating Component Oxide Processes

Quaternary Pb(Zr,Ti)O x [PZT] films were deposited at 240°C by a combination of liquid injection atomic layer depositions (ALD) of binary PbO, TiO x , and ZrO x thin films. We used water as the oxidant and two sets of precursors: Pb(C 11 H 19 O 2 ) 2 [Pb(DPM) 2 ], Zr(C 11 H 19 O 2)4 [Zr(DPM) 4 ], and either Ti(OC 3 H 7 ) 2 (C 11 H 19 O 2 ) 2 [Ti(Oi-Pr) 2 (DPM) 2 ] or (TiOC 3 H 7 ) 4 [Ti(Oi-Pr) 4 ]. These precursors were dissolved in ethylcyclohexane and separately injected into a vaporizer. The deposition rates of the metal elements were investigated as a function of the input of the solutions. We started the ALD-PZT process with Ti(Oi-Pr) 2 (DPM) 2 . When the input of one solution was increased, the deposition rates of the metal elements continued to increase or fluctuate, showing a complex interdependence. A PZT film deposited on a three-dimensional (3D) structure had an inhomogeneous cation composition. The film uniformity on the 3D structure was significantly improved by substituting Ti(Oi-Pr) 2 (DPM) 2 with Ti(Oi-Pr) 4 . In this ALD-PZT process, self-regulated growths were confirmed for Pb and Zr. Although the deposition rate of Ti did not saturate due to a catalytic decomposition, this study suggests that the multilayer stacking ALD process is an effective method for building up homogeneous layers of multicomponent materials on desired 3D structures.

Liquid Injection Atomic Layer Deposition of Crystalline TiO2 Thin Films with a Smooth Morphology from Ti ( O-i-Pr ) 2 ( DPM ) 2

TiO 2 thin films were grown at susceptor temperatures from 340 to 470°C by liquid injection atomic layer deposition (ALD) using Ti(O-i-Pr) 2 (DPM) 2 [Ti(OC 3 H 7 ) 2 (C 11 H 19 O 2 ) 2 , titanium dipivaloylmethanato di-isopropoxide] dissolved in ethylcyclohexane as a Ti source and H 2 O as an oxidant. The self-saturation growth behavior of the TiO 2 films was confirmed up to 390°C. Within the ALD window, the growth rate of the films increased from 0.022 nm/cycle at 340°C to 0.046 nm/cycle at 390°C, and the films exhibited a carbon content below 3 atom %. Due to the thermal decomposition of the precursor at 470°C, the growth rate of the films largely increased and the film density decreased by the increase in the carbon content of the film. The films showed quite a smooth surface morphology over the whole range of growth temperatures. The increase in the film thickness did not significantly change the surface morphology of the films due to the formation of the crystalline phase even at an initial growth stage. The relative permittivity of the TiO 2 films, which were crystallized into an anatase structure, was approximately 35-40. The films grown within the ALD window showed reasonable leakage current properties.

Pavlovian conditioning demonstrated with neuromorphic memristive devices

Pavlovian conditioning, a classical case of associative learning in a biological brain, is demonstrated using the Ni/Nb-SrTiO3/Ti memristive device with intrinsic forgetting properties in the framework of the asymmetric spike-timing-dependent plasticity of synapses. Three basic features of the Pavlovian conditioning, namely, acquisition, extinction and recovery, are implemented in detail. The effects of the temporal relation between conditioned and unconditioned stimuli as well as the time interval between individual training trials on the Pavlovian conditioning are investigated. The resulting change of the response strength, the number of training trials necessary for acquisition and the number of extinction trials are illustrated. This work clearly demonstrates the hardware implementation of the brain function of the associative learning.