Sang Ho Jeon

Electrical manipulation of nanofilaments in transition-metal oxides for resistance-based memory.

The fabrication of controlled nanostructures such as quantum dots, nanotubes, nanowires, and nanopillars has progressed rapidly over the past 10 years. However, both bottom-up and top-down methods to integrate the nanostructures are met with several challenges. For practical applications with the high level of the integration, an approach that can fabricate the required structures locally is desirable. In addition, the electrical signal to construct and control the nanostructures can provide significant advantages toward the stability and ordering. Through experiments on the negative resistance switching phenomenon in Pt-NiO-Pt structures, we have fabricated nanofilament channels that can be electrically connected or disconnected. Various analyses indicate that the nanofilaments are made of nickel and are formed at the grain boundaries. The scaling behaviors of the nickel nanofilaments were closely examined, with respect to the switching time, power, and resistance. In particular, the 100 nm x 100 nm cell was switchable on the nanosecond scale, making them ideal for the basis for high-speed, high-density, nonvolatile memory applications.

First-principles modeling of resistance switching in perovskite oxide material

We report a first-principles study on SrRuO3∕SrTiO3 interface in the presence of the oxygen vacancy. While the oxygen vacancy on the side of SrTiO3 significantly lowers the Schottky barrier height, the oxygen vacancy close to the interface or inside the metallic electrode results in a Schottky barrier comparable to that of the clean interface. Based on these results, we propose a model for resistance-switching phenomena in perovskite oxide∕metal interfaces where electromigration of the oxygen vacancy plays a key role. Our model provides a consistent explanation of a recent experiment on resistance switching in SrRuO3∕Nb:SrTiO3 interface.

Anion control as a strategy to achieve high-mobility and high-stability oxide thin-film transistors

Ultra-definition, large-area displays with three-dimensional visual effects represent megatrend in the current/future display industry. On the hardware level, such a “dream” display requires faster pixel switching and higher driving current, which in turn necessitate thin-film transistors (TFTs) with high mobility. Amorphous oxide semiconductors (AOS) such as In-Ga-Zn-O are poised to enable such TFTs, but the trade-off between device performance and stability under illumination critically limits their usability, which is related to the hampered electron-hole recombination caused by the oxygen vacancies. Here we have improved the illumination stability by substituting oxygen with nitrogen in ZnO, which may deactivate oxygen vacancies by raising valence bands above the defect levels. Indeed, the stability under illumination and electrical bias is superior to that of previous AOS-based TFTs. By achieving both mobility and stability, it is highly expected that the present ZnON TFTs will be extensively deployed in next-generation flat-panel displays.

Role of structural defects in the unipolar resistive switching characteristics of Pt∕NiO∕Pt structures

We investigated the resistive switching characteristics of two types of Pt∕NiO∕Pt structures with epitaxial and polycrystalline NiO layers. Both of these Pt∕NiO∕Pt structures exhibited unipolar resistive switching. Pt/epitaxial-NiO∕Pt showed unstable switching or no resistance state change after several repeated runs. Pt/polycrystalline-NiO∕Pt showed very reproducible switching. The experimental data indicated that microstructural defects (e.g., grain boundaries) played crucial roles in the reliability of the unipolar resistive switching behavior. This was further supported by first-principles calculations.

Segregation of oxygen vacancy at metal-HfO2 interfaces

We perform first-principles calculations on metal-HfO2 interfaces in the presence of oxygen vacancies. Pt, Al, Ti, and Ag are considered as electrodes. It is found that oxygen vacancies are strongly attracted to the interface with binding energies of up to several eVs. In addition, the vacancy affinity of interfaces is proportional to the work function of metals, which is understood by the transition level of the vacancy and metal-Hf bonding. Interfacial segregation of vacancies significantly affects effective work functions of p metals. Our results are consistent with flatband shifts in p-type field effect transistors employing high-k dielectrics and metal gates.

High mobility zinc oxynitride-TFT with operation stability under light-illuminated bias-stress conditions for large area and high resolution display applications

We have investigated material and electrical properties of ZnON based on 1st principle calculations and TFT evaluations. Theoretically, ZnON has high mobility characteristics and band-structure for high stability. Fabricated TFTs exhibited high mobility (100 cm2/Vs), good uniformity, and stable operation performance such as -2.87 V of Vth-shift under light illuminated bias-stress condition. As a new approach to overcome the performance limit of oxide-semiconductors, ZnON technology is strongly promising to achieve high mobility and operation stability required for next generation displays.

Intrinsic nature of visible-light absorption in amorphous semiconducting oxides

To enlighten microscopic origin of visible-light absorption in transparent amorphous semiconducting oxides, the intrinsic optical property of amorphous InGaZnO4 is investigated by considering dipole transitions within the quasiparticle band structure. In comparison with the crystalline InGaZnO4 with the optical gap of 3.6 eV, the amorphous InGaZnO4 has two distinct features developed in the band structure that contribute to significant visible-light absorption. First, the conduction bands are down-shifted by 0.55 eV mainly due to the undercoordinated In atoms, reducing the optical gap between extended states to 2.8 eV. Second, tail states formed by localized oxygen p orbitals are distributed over ∼0.5 eV near the valence edge, which give rise to substantial subgap absorption. The fundamental understanding on the optical property of amorphous semiconducting oxides based on underlying electronic structure will pave the way for resolving instability issues in recent display devices incorporating the semiconducting oxides.

All-atom simulation of molecular orientation in vapor-deposited organic light-emitting diodes

Molecular orientation in vapor-deposited organic semiconductor films is known to improve the optical and electrical efficiencies of organic light-emitting diodes, but atomistic understanding is still incomplete. In this study, using all-atom simulation of vapor deposition, we theoretically investigate how the molecular orientation depends on various factors such as the substrate temperature, molecular shape, and material composition. The simulation results are in good agreement with experiment, indicating that the all-atom simulation can predict the molecular orientation reliably. From the detailed analysis of the dynamics of molecules, we suggest that the kinetics of molecules near the surface mainly determines the orientation of the deposited film. In addition, the oriented films have higher density and thermal stability than randomly oriented films. We also show that higher mobility of laterally oriented films can be explained in terms of the site-energy correlation.