Ah Rahm Lee

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.

All oxide semiconductor-based bidirectional vertical p-n-p selectors for 3D stackable crossbar-array electronics

Three-dimensional (3D) stackable memory devices including nano-scaled crossbar array are central for the realization of high-density non-volatile memory electronics. However, an essential sneak path issue affecting device performance in crossbar array remains a bottleneck and a grand challenge. Therefore, a suitable bidirectional selector as a two-way switch is required to facilitate a major breakthrough in the 3D crossbar array memory devices. Here, we show the excellent selectivity of all oxide p-/n-type semiconductor-based p-n-p open-based bipolar junction transistors as selectors in crossbar memory array. We report that bidirectional nonlinear characteristics of oxide p-n-p junctions can be highly enhanced by manipulating p-/n-type oxide semiconductor characteristics. We also propose an associated Zener tunneling mechanism that explains the unique features of our p-n-p selector. Our experimental findings are further extended to confirm the profound functionality of oxide p-n-p selectors integrated with several bipolar resistive switching memory elements working as storage nodes.

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.

Correlative analysis of conducting filament distribution at interfaces and bias-dependent noise sources in TiN/TiOx/Pt and Pt/TiOx/TiOy/Pt bipolar resistive switching frames

We evaluated conducting filament distributions occurring at interfaces of TiN/TiOx/Pt and Pt/TiOx/TiOy/Pt bipolar resistive switching elements after electroforming by identifying bias-dependent low-frequency noise sources. The TiN/TiOx/Pt switching element showed higher noise features at low and high resistance states (LRS and HRS) than the Pt/TiOx/TiOy/Pt one. These behaviors are predominantly associated with the presence of different resistance distributions at LRS and HRS observed in both switching I-V curves. We propose a possible mechanism to explain the unique observed features by employing the role of the oxygen reservoir and conducting filament stability at interfaces of the two switching elements.

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.