Hu-Jong Lee

Bio: Hu-Jong Lee is an academic researcher from Pohang University of Science and Technology. The author has contributed to research in topics: Josephson effect & Superconductivity. The author has an hindex of 29, co-authored 147 publications receiving 2927 citations.

Fabrication and electrical characteristics of high-performance ZnO nanorod field-effect transistors

Abstract: We report on fabrication and electrical characteristics of high-mobility field-effect transistors (FETs) using ZnO nanorods. For FET fabrications, single-crystal ZnO nanorods were prepared using catalyst-free metalorganic vapor phase epitaxy. Although typical ZnO nanorod FETs exhibited good electrical characteristics, with a transconductance of ∼140nS and a mobility of 75cm2∕Vs, the device characteristics were significantly improved by coating a polyimide thin layer on the nanorod surface, exhibiting a large turn-ON/OFF ratio of 104–105, a high transconductance of 1.9μS, and high electron mobility above 1000cm2∕Vs. The role of the polymer coating in the enhancement of the devices is also discussed.

ZnO Nanorod Logic Circuits

Abstract: Logic devices, including OR, AND, NOT, and NOR gates, based on single-crystalline ZnO nanorods are demonstrated. In these devices, ZnO nanorods are employed as semiconducting channels. They control metal/oxide semiconductor junction characteristics, to yield either good ohmic or Schottky contacts, ensuring fabrication of high-performance Schottky diodes and metal-semiconductor field-effect transistors.

Observation of negative refraction of Dirac fermions in graphene

Abstract: Negative refraction has now been observed for Dirac fermions in graphene, and is used to create an electronic Veselago lens. Half a century ago, Veselago1 proposed ‘left-handed’ materials with negative permittivity and permeability, in which waves propagate with phase and group velocities in opposite directions. Significant work has been undertaken to attain this left-handed response, such as establishing a negative refractive index in so-called metamaterials, which consist of periodic sub-wavelength structures2,3,4. However, an electronic counterpart has not been demonstrated owing to difficulties in creating repeated structures smaller than the electronic Fermi wavelength of the order ∼10 nm. Here, without needing to engineer sub-wavelength structures, we demonstrate negative refractive behaviour of Dirac fermions in graphene, exploiting its unique relativistic band structure5. Analysis of both electron focusing through an n–p–n flat lens and negative refraction across n–p junctions confirms left-handed behaviour in the electronic system. This approach to electronic optics is of particular relevance to the on-going efforts to develop novel quantum devices with emerging6 layered materials.

Inelastic scattering in a monolayer graphene sheet: A weak-localization study

Abstract: Charge carriers in a graphene sheet, a single layer of graphite, exhibit distinct characteristics from those in other two-dimensional electronic systems because of their chiral nature In this paper, we focus on the observation of weak localization in a graphene sheet exfoliated from a piece of natural graphite and nanopatterned into a Hall-bar geometry Much stronger chiral-symmetry-breaking elastic intervalley scattering in our graphene sheet restores the conventional weak localization The resulting carrier density and temperature dependence of the phase coherence length reveal that the electron-electron interaction including a direct Coulomb interaction is the main inelastic-scattering factor while electron-hole puddles enhance the inelastic scattering near the Dirac point

Ultimately short ballistic vertical graphene Josephson junctions

Abstract: Much efforts have been made for the realization of hybrid Josephson junctions incorporating various materials for the fundamental studies of exotic physical phenomena as well as the applications to superconducting quantum devices. Nonetheless, the efforts have been hindered by the diffusive nature of the conducting channels and interfaces. To overcome the obstacles, we vertically sandwiched a cleaved graphene monoatomic layer as the normal-conducting spacer between superconducting electrodes. The atomically thin single-crystalline graphene layer serves as an ultimately short conducting channel, with highly transparent interfaces with superconductors. In particular, we show the strong Josephson coupling reaching the theoretical limit, the convex-shaped temperature dependence of the Josephson critical current and the exceptionally skewed phase dependence of the Josephson current; all demonstrate the bona fide short and ballistic Josephson nature. This vertical stacking scheme for extremely thin transparent spacers would open a new pathway for exploring the exotic coherence phenomena occurring on an atomic scale.

A comprehensive review of zno materials and devices

Abstract: The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. ...

Nitrogen-doped graphene for high-performance ultracapacitors and the importance of nitrogen-doped sites at basal planes.

Abstract: Although various carbon nanomaterials including activated carbon, carbon nanotubes, and graphene have been successfully demonstrated for high-performance ultracapacitors, their capacitances need to be improved further for wider and more challenging applications. Herein, using nitrogen-doped graphene produced by a simple plasma process, we developed ultracapacitors whose capacitances (∼280 F/gelectrode) are about 4 times larger than those of pristine graphene based counterparts without sacrificing other essential and useful properties for ultracapacitor operations including excellent cycle life (>200000), high power capability, and compatibility with flexible substrates. While we were trying to understand the improved capacitance using scanning photoemission microscopy with a capability of probing local nitrogen–carbon bonding configurations within a single sheet of graphene, we observed interesting microscopic features of N-configurations: N-doped sites even at basal planes, distinctive distributions of N...

Intrinsic ripples in graphene

Abstract: The stability of two-dimensional (2D) layers and membranes is subject of a long standing theoretical debate. According to the so called Mermin-Wagner theorem, long wavelength fluctuations destroy the long-range order for 2D crystals. Similarly, 2D membranes embedded in a 3D space have a tendency to be crumpled. These dangerous fluctuations can, however, be suppressed by anharmonic coupling between bending and stretching modes making that a two-dimensional membrane can exist but should present strong height fluctuations. The discovery of graphene, the first truly 2D crystal and the recent experimental observation of ripples in freely hanging graphene makes these issues especially important. Beside the academic interest, understanding the mechanisms of stability of graphene is crucial for understanding electronic transport in this material that is attracting so much interest for its unusual Dirac spectrum and electronic properties. Here we address the nature of these height fluctuations by means of straightforward atomistic Monte Carlo simulations based on a very accurate many-body interatomic potential for carbon. We find that ripples spontaneously appear due to thermal fluctuations with a size distribution peaked around 70 \AA which is compatible with experimental findings (50-100 \AA) but not with the current understanding of stability of flexible membranes. This unexpected result seems to be due to the multiplicity of chemical bonding in carbon.

Graphene Spintronics

Abstract: The isolation of graphene has triggered an avalanche of studies into the spin-dependent physical properties of this material, as well as graphene-based spintronic devices Here we review the experimental and theoretical state-of-art concerning spin injection and transport, defect-induced magnetic moments, spin-orbit coupling and spin relaxation in graphene Future research in graphene spintronics will need to address the development of applications such as spin transistors and spin logic devices, as well as exotic physical properties including topological states and proximity-induced phenomena in graphene and other 2D materials