Hyun Sik Im

Roles of interfacial TiOxN1−x layer and TiN electrode on bipolar resistive switching in TiN/TiO2/TiN frameworks

Reversible counter-clockwise and clockwise resistive switching in a TiN/TiO2/TiN structure was studied by different polarities of bias voltage. The nature of the bipolar switching phenomenon is related to the creation and annihilation of filament paths caused by redox reactions at locally confined interfaces between the TiO2 layer and TiN electrode. The analysis of electron energy loss spectroscopy (EELS) confirmed the formation of interfacial TiOxN1−x layer between the TiO2 and TiN bottom electrode. The TiOxN1−x layer reduces current levels of ON and OFF states by partially blocking oxygen ion drift to the TiN bottom electrode.

Multi-level resistive switching observations in asymmetric Pt/Ta2O5−x/TiOxNy/TiN/Ta2O5−x/Pt multilayer configurations

We examine multilevel (ML) resistance switching properties in a Pt/Ta2O5−x/TiOxNy/TiN/Ta2O5−x/Pt matrix, in which two bipolar resistive switching elements Pt/Ta2O5−x/TiOxNy and TiN/Ta2O5−x/Pt are anti-serially and electrically connected. The ML features for the three assigned, distinguishable resistance states are clearly identified by using an I–V device operation scheme, indicating that the middle TiN and TiOxNy electrodes are crucial for adjusting ML resistance states. Experimental observations suggest that the ML switching events rely on electrically induced oxygen ion drifts at interfaces between the top/bottom Ta2O5−x and middle TiN/TiOxNy layers.

Multifunctional resistive switching behaviors employing various electroforming steps

We examine the electroforming-dependent multifunctional resistive switching features by operating a merged Pt/Ta2O5−x/Ta–Ta/Ta2O5−x/Pt switching device under particular bias and polarity conditions. The basic Pt/Ta2O5−x/Ta resistive switching cell comprising the completely merged device shows two different bipolar switching behaviors with an initial forming process under different bias polarities. Therefore, two switching elements can be merged in various ways to produce diverse functionalities such as asymmetric complementary resistive switching (CRS) and typical CRS, achieved through control of the forming process. A possible mechanism to explain the unique features observed is discussed in terms of bias-driven oxygen ion drift and Joule-heating-based filamentary path models. This work suggests a suitable electroforming route for advancing symmetric CRS characteristics.

Revealing Al evaporation-assisted functions in solution-processed ZnO thin film transistors

Metal oxide semiconductors based on a solution process have facilitated major breakthroughs in the emerging field of flexible and transparent electronic devices. In particular, enhanced output performance of metal oxide semiconductors obtained by a solution process is desirable, because they are easy to fabricate and cost effective at low temperatures. To date, a carbon-free method involving an aqueous zinc amine complex has been employed to generate metal oxide active layers that have outstanding electrical features despite the formation of a very thin active layer. However, manipulation of trap states induced by chronic weak bonding structures initially present during the solution process remains a challenge. In addition, a thin active layer is highly susceptible to the initial surface and interface charge traps, resulting in the deterioration of electron transport by unclear mechanisms. Therefore, intentional control of intrinsic defects arising from porosity and pinholes is becoming one of the key issues in the development of highly stable solution-processed metal oxide semiconductors. Here, we describe a generic metal evaporation approach to enhance the electrical performance of solution-processed ZnO TFTs. In particular, we do not use passivation or post-annealing processes. Based on systematic structural and electrical analyses, we propose a mechanism based on Al metal evaporation-driven reduction of trap states that convincingly explains the unique features of the solution-processed ZnO TFTs obtained in this study. We anticipate that these findings will spur progress toward the realization of solution-processed electronic devices.

Memory window engineering of Ta2O5-x oxide-based resistive switches via incorporation of various insulating frames.

Three-dimensional (3D) stackable memory frames, including nano-scaled crossbar arrays, are one of the most reliable building blocks to meet the demand of high-density non-volatile memory electronics. However, their utilization has the disadvantage of introducing issues related to sneak paths, which can negatively impact device performance. We address the enhancement of complementary resistive switching (CRS) features via the incorporation of insulating frames as a generic approach to extend their use; here, a Pt/Ta2O5−x/Ta/Ta2O5−x/Pt frame is chosen as the basic CRS cell. The incorporation of Ta/Ta2O5−x/Ta or Pt/amorphous TaN/Pt insulting frames into the basic CRS cell ensures the appreciably advanced memory features of CRS cells including higher on/off ratios, improved read margins, and increased selectivity without reliability degradation. Experimental observations identified that a suitable insulating frame is crucial for adjusting the abrupt reset events of the switching element, thereby facilitating the enhanced electrical characteristics of CRS cells that are suitable for practical applications.

Oxide stoichiometry-controlled TaOx-based resistive switching behaviors

We examine the influence of variable oxygen concentration in TaOx active layers on the forming process and bipolar resistive switching (BRS) features of TaOx-based resistive switching cells. TaOx active layers prepared using various rf sputtering powers were systematically analyzed to identify the relation between initial compositions and BRS behavior. Proper control of oxygen vacancy concentration was clearly identified as a basic factor in ensuring typical BRS features without affecting the structural properties. We describe the possible origins of both conduction and switching based on the variation of oxygen concentrations initially provided by the growth conditions.

Observation of bias-dependent noise sources in a TiOx/TiOy bipolar resistive switching frame

We report the conduction features associated with the evolution of oxygen ions (or vacancies) under bias for a TiOx (oxygen ion-rich)/TiOy (oxygen ion-deficient) bi-layer cell by identifying low-frequency noise sources. It is believed that a low resistance state enhances the formation of conductive filaments exchanging electrons through a nearest-neighbor hopping process, while a high resistance state (HRS) emphasizes the rupture of conductive filaments inside the insulating TiOx layer and a reduction/oxidation reaction at the oxide interfaces. The high resolution transmission electron microscope images of as-grown and HRS cells are also discussed.

Oxygen ion drift-driven dual bipolar hysteresis curves in a single Pt/Ta2O5−x/TiOxNy framework

We describe abnormal dual bipolar resistive switching events in simple Pt/Ta2O5−x/TiOxNy and Pt/Ta2O5−x/TiN matrices in which the typical switching directions (SD) are initially clockwise (CW). The negative difference region in a high resistance state before reaching the typical “CW set” process enables the SD transition to a counterclockwise direction. It thereby emphasizes the occurrence of a highly stable secondary bipolar resistive switching curve. The origin of two different switching modes is described by adapting a bias-dependent oxygen ion accumulation and depletion process at TiOxNy and TiN electrode interfaces and by performing various structural analyses.

Nitride-Based Microlight-Emitting Diodes Using AlN Thin-Film Electrodes with Nanoscale Indium/Tin Conducting Filaments

Microlight-emitting diodes (µLEDs) are emerging solutions for both high-quality displays and lighting technologies. However, the overall light output power density of these devices is low, as the emission area is shielded by the p-electrodes required for current injection. In this study, instead of the more conventionally used indium tin oxide (ITO), an AlN thin film with nanoscale conducing filaments (CFs) is used, referred to as CF-AlN, as a transparent conducting electrode (TCE), to enhance the output power density from the same emission area. As a result of this modification, the electroluminescence intensity is enhanced by 10% at an injection current of 10 mA, and the current density is improved by 13% at a forward voltage of 4.9 V, in comparison to the parameters observed with ITO-based µLEDs. This improvement is attributed to the higher transmittance of CF-AlN TCEs, together with efficient hole injection from the p-electrode into the light-emitting layer, through the CFs formed in the AlN layer. In addition, using transmission electron microscopy analyses, the origin of the CFs is directly identified as the diffusion of In and Sn ions, which provides critical insight into the conduction mechanism of AlN-based TCEs.

Exploring oxygen-affinity-controlled TaN electrodes for thermally advanced TaO x bipolar resistive switching

Recent advances in oxide-based resistive switching devices have made these devices very promising candidates for future nonvolatile memory applications. However, several key issues remain that affect resistive switching. One is the need for generic alternative electrodes with thermally robust resistive switching characteristics in as-grown and high-temperature annealed states. Here, we studied the electrical characteristics of Ta2O5−x oxide-based bipolar resistive frames for various TaNx bottoms. Control of the nitrogen content of the TaNx electrode is a key factor that governs variations in its oxygen affinity and structural phase. We analyzed the composition and chemical bonding states of as-grown and annealed Ta2O5−x and TaNx layers and characterized the TaNx electrode-dependent switching behavior in terms of the electrode’s oxygen affinity. Our experimental findings can aid the development of advanced resistive switching devices with thermal stability up to 400 °C.