Minseok Jo

Three-Dimensional Integration of Organic Resistive Memory Devices

Since the discovery of conducting polymers [ 1 ] , organic-based electronics such as organic light-emitting diodes, transistors, photovoltaics, and memory devices have been spotlighted as potentially innovative devices given their easy and lowcost fabrication by spin-coating or ink-jet printing, and their fl exibility. [ 2–15 ] Among these, organic memories have been extensively investigated for data-storage application. [ 11 , 14 , 16–21 ]

Analog memory and spike-timing-dependent plasticity characteristics of a nanoscale titanium oxide bilayer resistive switching device.

We demonstrated analog memory, synaptic plasticity, and a spike-timing-dependent plasticity (STDP) function with a nanoscale titanium oxide bilayer resistive switching device with a simple fabrication process and good yield uniformity. We confirmed the multilevel conductance and analog memory characteristics as well as the uniformity and separated states for the accuracy of conductance change. Finally, STDP and a biological triple model were analyzed to demonstrate the potential of titanium oxide bilayer resistive switching device as synapses in neuromorphic devices. By developing a simple resistive switching device that can emulate a synaptic function, the unique characteristics of synapses in the brain, e.g. combined memory and computing in one synapse and adaptation to the outside environment, were successfully demonstrated in a solid state device.

Rewritable Switching of One Diode–One Resistor Nonvolatile Organic Memory Devices

[*] Prof. T. Lee, B. Cho, T.-W. Kim, S. Song, Y. Ji, M. Jo, Prof. H. Hwang, Prof. G.-Y. Jung Department of Materials Science and Engineering Gwangju Institute of Science and Technology 1 Oryong-Dong, Buk-Gu Gwangju 500-712 (Korea) E-mail: tlee@gist.ac.kr Prof. T. Lee, Prof. H. Hwang Department of Nanobio Materials and Electronics Gwangju Institute of Science and Technology 1 Oryong-Dong, Buk-Gu Gwangju 500-712 (Korea)

TiO2-based metal-insulator-metal selection device for bipolar resistive random access memory cross-point application

We report a simple metal-insulator-metal (MIM)-type selection device that can alleviate the sneak current path in cross-point arrays. By connecting a nanometer-scale Pt/TiO2/TiN selection device to a Pt/TiO2−x/TiO2/W resistive random access memory (RRAM), we could significantly reduce read disturbance from unselected memory cells. This selection device could be easily integrated into an RRAM device, in which it suppressed the sneak current and significantly improved the readout margin compared to that obtained for an RRAM not using a selection device. The introduction of this MIM device can fulfill the requirement for an appropriate selection device for bipolar-type RRAM cross-point applications.

Effect of ZrOx/HfOx bilayer structure on switching uniformity and reliability in nonvolatile memory applications

We have investigated the bilayer structure of binary oxides such as HfOx and ZrOx for applications to resistance memory. The ZrOx/HfOx bilayer structure shows a lower reset current and operating voltage than an HfOx monolayer under dc sweep voltage. Furthermore, the bilayer structure exhibits a tight distribution of switching parameters, good switching endurance up to 105 cycles, and good data retention at 85 °C. The resistive switching mechanism of memory devices incorporating the ZrOx/HfOx bilayer structure can be attributed to the control of multiple conducting filaments through the occurrence of redox reactions at the tip of the localized filament.

Diode-less nano-scale ZrO x /HfO x RRAM device with excellent switching uniformity and reliability for high-density cross-point memory applications

We report excellent switching uniformity and reliability of RRAM device with ZrOx/HfOx bi-layer films. Precise control of the oxygen vacancy concentration in HfO2 layer was achieved by depositing thin Zr metal (2–15nm) layer. Scaling down active device area (ϕ=50 nm) and film thickness (<2–5 nm) can significantly minimize the extrinsic defects-related non-uniform switching which was normally observed in large area (ϕ >um) device, with higher active layer thickness (>10 nm). Using back-to-back connection of two RRAM devices, we confirmed feasibility of a diode-free cross-point array with a wide readout margin and stable data reading. Considering excellent electrical and reliability characteristics of diode-free RRAM device, shows a great promise for future high density cross-point memory devices

HPHA Effect on Reversible Resistive Switching of Pt ∕ Nb -Doped SrTiO3 Schottky Junction for Nonvolatile Memory Application

We investigated the effect of high-pressure hydrogen annealing (HPHA) on a Pt/Nb-doped SrTiO 3 Schottky junction for nonvolatile memory applications. Hysteretic current-voltage (I-V) characteristic curves reveal that the HPHA-treated Schottky junction interface has a higher resistance ratio than the control sample in dc bias sweep. The HPHA sample also exhibits switching behavior by pulsed voltage stress with excellent electrical characteristics including voltage pulse duration as short as 1 μs, endurance cycles of up to 10 6 times, and retention time as long as 10 5 s. These indicate that HPHA improved resistance switching characteristics of the Schottky junction.

Excellent uniformity and reproducible resistance switching characteristics of doped binary metal oxides for non-volatile resistance memory applications

We have investigated various doped metal oxides such as copper doped molybdenum oxide, copper doped Al₂O₃, copper doped ZrO₂, aluminium doped ZnO, and Cu_xO for novel resistance memory applications. Compared with non-stoichiometric oxides (Nb₂O_5-x, ZrO_x, SrTiO_x), doped metal oxides show higher device yield. Moreover, Cu:MoO_x have demonstrated excellent memory characteristics such as reliability under programming cycles, potential multi-bit operation, good data retention, highly scalable property, and fast switching speed. The switching mechanism of the copper doped molybdenum oxide can be explained by the generation and rupture of multi-filaments under the electrical stress

Transient reverse current phenomenon in a p-n heterojunction comprised of poly(3,4-ethylene-dioxythiophene):poly(styrene-sulfonate) and ZnO nanowall

We report the characteristics of a p-n heterojunction diode comprised of a poly(3,4-ethylene-dioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) as the hole-conducting p-type polymer and n-type ZnO nanowall networks. ZnO nanowall networks were synthesized on a GaN/sapphire substrate without metal catalyst using hot-wall type metal organic chemical vapor deposition. The p-n heterojunction diodes of PEDOT:PSS/ZnO nanowall exhibited a space charge limited current phenomena at forward bias and a transient reverse current recovery when a sudden reverse bias was applied from the forward bias condition. The minority carrier lifetime was estimated to be ∼2.5 μs.

Improvement of reproducible hysteresis and resistive switching in metal-La0.7Ca0.3MnO3-metal heterostructures by oxygen annealing

Materials showing reversible resistance switching between high-resistance state and low-resistance state at room temperature are attractive for today’s semiconductor technology. In this letter, the improvement of reproducible hysteresis and resistive switching characteristics of metal-La0.7Ca0.3MnO3-metal (M-LCMO-M) heterostructures is demonstrated. The fabrication of the M-LCMO-M heterostructures is compatible with the standard complementary metal-oxide semiconductor process. The effect of oxygen annealing on the improvement of the hysteresis and resistive switching is discussed. The good retention characteristics are exhibited in the M-LCMO-M heterostructures by the accurate controlling of the preparation parameters.