Doo Seok Jeong

Emerging memories: resistive switching mechanisms and current status

The resistance switching behaviour of several materials has recently attracted considerable attention for its application in non-volatile memory (NVM) devices, popularly described as resistive random access memories (RRAMs). RRAM is a type of NVM that uses a material(s) that changes the resistance when a voltage is applied. Resistive switching phenomena have been observed in many oxides: (i) binary transition metal oxides (TMOs), e.g. TiO(2), Cr(2)O(3), FeO(x) and NiO; (ii) perovskite-type complex TMOs that are variously functional, paraelectric, ferroelectric, multiferroic and magnetic, e.g. (Ba,Sr)TiO(3), Pb(Zr(x) Ti(1-x))O(3), BiFeO(3) and Pr(x)Ca(1-x)MnO(3); (iii) large band gap high-k dielectrics, e.g. Al(2)O(3) and Gd(2)O(3); (iv) graphene oxides. In the non-oxide category, higher chalcogenides are front runners, e.g. In(2)Se(3) and In(2)Te(3). Hence, the number of materials showing this technologically interesting behaviour for information storage is enormous. Resistive switching in these materials can form the basis for the next generation of NVM, i.e. RRAM, when current semiconductor memory technology reaches its limit in terms of density. RRAMs may be the high-density and low-cost NVMs of the future. A review on this topic is of importance to focus concentration on the most promising materials to accelerate application into the semiconductor industry. This review is a small effort to realize the ambitious goal of RRAMs. Its basic focus is on resistive switching in various materials with particular emphasis on binary TMOs. It also addresses the current understanding of resistive switching behaviour. Moreover, a brief comparison between RRAMs and memristors is included. The review ends with the current status of RRAMs in terms of stability, scalability and switching speed, which are three important aspects of integration onto semiconductors.

Coexistence of Bipolar and Unipolar Resistive Switching Behaviors in a Pt ∕ TiO2 ∕ Pt Stack

Bipolar resistive switching (BRS) as well as unipolar resistive switching (URS) behaviors in Pt/27 nm thick TiO 2 /Pt stacks were investigated. Depending on the current compliance during the electroforming process, either BRS or URS was observed. With a lower current compliance (<0.1 mA) during electroforming, asymmetric current-voltage curves showing BRS were observed in the voltage range -1.6 to +1.1 V, while with a higher current compliance (1-10 mA) URS behavior was observed. Furthermore, the permanent transition from BRS to URS was investigated by applying a voltage with a higher current compliance (∼3 mA).

Characteristic electroforming behavior in Pt/TiO2/Pt resistive switching cells depending on atmosphere

Electroforming effects on the composition, structure, and electrical resistance of Pt/TiO2/Pt switching cells are investigated. The correlation between the electroforming procedure and the resulting bipolar switching behavior is discussed. The dependence of electroforming behavior on atmosphere is also identified, from which we define symmetric or asymmetric electroforming. The symmetry of electroforming is a key factor determining the resulting bipolar switching characteristics. From the experimental results we suggest a possible mechanism for electroforming in Pt/TiO2/Pt in terms of the formation of oxygen gas and vacancies in the vicinity of the anode.

Impedance spectroscopy of TiO2 thin films showing resistive switching

Impedance characteristics of 27nm thick anatase TiO2 films showing bistable resistive switching were investigated in the frequency domain (100Hz–10MHz) in various resistance states, a fresh state (before electroforming), a high resistive state (HRS), and a low resistive state (LRS). dc conductance in the film becomes dominent in HRS and LRS and the capacitances in the various states are almost identical. Numerical calculations using finite element analysis were performed for the localized filament and homogeneous model, whose results suggest that the filament model is consistent with the experimental results.

Towards artificial neurons and synapses: a materials point of view

We overview several efforts to emulate functionalities of basic building blocks, i.e. neurons and synapses, of a mammal’s brain by means of non-biological inorganic systems. These efforts have been put to realize ambitious goals such as the achievement of artificial inorganic brains on silicon wafers, i.e. neuromorphic systems, and neuroprosthetic systems taking part in real brain functionalities by interfacing with real brains. In terms of the keywords, ‘threshold’, ‘analogue’, ‘plasticity’, and ‘elasticity’, which describe the behaviour of neurons and synapses, various functional systems, with particular emphasis on nanoionic systems, exhibiting these key behaviours, are dealt with in this review.

Size effects of metal nanoparticles embedded in a buffer layer of organic photovoltaics on plasmonic absorption enhancement

The effects of Au or Ag nanoparticles on optical absorption enhancement of organic photovoltaics based on blended poly(3-hexylthiophene)?:?phyenyl-C61-butyric acid methyl ester (P3HT?:?PCBM) were investigated using a finite-difference-time-domain method. The spherical metal nanoparticles were embedded in a buffer layer of thickness 20?nm and their size was varied from 10 to 50?nm. The metal nanoparticles with diameter 10?20?nm offered negligible absorption enhancement in the active layer. Unlike those short metal nanoparticles, the incorporation of metal nanoparticles taller than the buffer layer led to a significant absorption enhancement by plasmonic resonance especially in the case of Ag nanoparticles. Ag nanoparticles gave broader and stronger absorption enhancement in the active layer than Au nanoparticles. An enhancement of 34% in the optical absorption of the active layer was observed with Ag nanoparticles of 50?nm diameter at 10% coverage. The electric field distributions around metal nanoparticles, their self-absorption and the active layer thickness dependence on the absorption enhancement were studied.

Study of the negative resistance phenomenon in transition metal oxide films from a statistical mechanics point of view

The mechanism of the differential negative resistance behavior observable in transition metal oxides such as TiO2, NiO, and Nb2O5 is theoretically developed in terms of the order to disorder transition of local conducting pathways, so called filaments, by applying statistical mechanics. Joule heat is taken as a heat source giving rise to an increase of temperature in the filaments. The free energy of the filaments is evaluated by taking the site percolation model as well as the nonhomogeneity of an order parameter into account. It is concluded that the transition of a filament from the conducting state, which is dominated by the internal bonding energy term, to the nonconducting state is due to an increased contribution of the entropy term to the system’s total energy at a higher temperature. The calculation results also show that the rupture of a filament occurs in the middle of the filament when the filaments are in contact with thermally conducting metal electrodes, such as Pt.

Abnormal bipolar-like resistance change behavior induced by symmetric electroforming in Pt/TiO2/Pt resistive switching cells.

Abnormal bipolar-like resistive changes are reported in TiO(2) thin films sandwiched between Pt top and bottom electrodes. The abnormal behavior is shown relying on the applied voltage range. That is, normal bipolar switching is also shown in the same sample with the optimized voltage range. In the abnormal mode, both set- and reset-like changes in resistance take place under the same polarity of the applied voltage. This abnormal behavior is considered to be due to symmetric electroforming which is assumed to activate electrochemical reactions involving oxygen vacancies at both Pt/TiO(2) interfaces. We analyze the abnormal behavior in terms of the interfacial resistive switching taking place on both interfaces nearly simultaneously.

Threshold resistive and capacitive switching behavior in binary amorphous GeSe

A threshold switching (TS) event in a binary amorphous GeSe film placed between Pt top and bottom electrodes was examined. This GeSe film exhibits fast (<40 ns) TS behavior. The observed TS of the resistance was found to be accompanied with the TS of the capacitance. A mechanism for the TS of the GeSe film was suggested by revisiting the previous controversy about the thermal versus non-thermal electronic mechanism. The non-thermal electronic mechanism envisaging the double-injection of electronic carriers can qualitatively account for the measured threshold resistive and capacitive switching, whereas the TS behavior simulated using the thermal mechanism is inconsistent with the experimental observation.

Short-term memory of TiO2-based electrochemical capacitors: empirical analysis with adoption of a sliding threshold

Chemical synapses are important components of the large-scaled neural network in the hippocampus of the mammalian brain, and a change in their weight is thought to be in charge of learning and memory. Thus, the realization of artificial chemical synapses is of crucial importance in achieving artificial neural networks emulating the brains functionalities to some extent. This kind of research is often referred to as neuromorphic engineering. In this study, we report short-term memory behaviours of electrochemical capacitors (ECs) utilizing TiO2 mixed ionic-electronic conductor and various reactive electrode materials e.g. Ti, Ni, and Cr. By experiments, it turned out that the potentiation behaviours did not represent unlimited growth of synaptic weight. Instead, the behaviours exhibited limited synaptic weight growth that can be understood by means of an empirical equation similar to the Bienenstock-Cooper-Munro rule, employing a sliding threshold. The observed potentiation behaviours were analysed using the empirical equation and the differences between the different ECs were parameterized.