Showing papers by "Young Jae Song published in 2020"

Color-Selective Schottky Barrier Modulation for Optoelectric Logic.

Abstract: The limitation on signal processes implementable using conventional semiconductor circuits based on electric signals necessitates a revolutionary change in device structures such that they can exploit photons or light. Herein, we introduce optoelectric logic circuits that convert optical signals with different wavelengths corresponding to different colors into binary electric signals. Such circuits are assembled using unit devices in which the electric current through the semiconductor channel is effectively gated by lights of different colors. Color-selective optical modulation of the device is cleverly achieved using graphene decorated with different organic dyes as the electrode of a Schottky diode structure. The drastic change in the electrode work function under illumination induces a change in the height of the Schottky barrier formed at the electrode/semiconductor junction and consequent modulation of the electric current; we term the developed device a photonic barristor. We construct logic circuits using an array of photonic barristors and demonstrate that they execute the functions of conventional NAND and NOR gates from optical input signals.

Quasiparticle interference and impurity resonances on WTe 2

Abstract: Using scanning tunneling microscopy/spectroscopy (STM/STS), we examine quasiparticle scattering and interference properties at the surface of WTe2 WTe2, layered transition metal dichalcogenide, is predicted to be a type-II Weyl semimetal The Weyl fermion states in WTe2 emerge as topologically protected touching points of electron and hole pockets, and Fermi arcs connecting them can be visible in the spectral function on the surface To probe the properties of surface states, we have conducted low-temperature STM/STS (at 27 K) on the surfaces of WTe2 single crystals We visualize the surface states of WTe2 with atomic scale resolution Clear surface states emerging from the bulk electron pocket have been identified and their connection with the bulk electronic states shows good agreement with calculations We show the interesting double resonance peaks in the local density of states appearing at localized impurities The low-energy resonant peak occurs near the Weyl point above the Fermi energy and it may be mixed with the surface state of Weyl points, which makes it difficult to observe the topological nature of the Weyl semimetal WTe2

Probing the Importance of Charge Balance and Noise Current in WSe2/WS2/MoS2 van der Waals Heterojunction Phototransistors by Selective Electrostatic Doping

Abstract: Heterojunction structures using 2D materials are promising building blocks for electronic and optoelectronic devices The limitations of conventional silicon photodetectors and energy devices are able to be overcome by exploiting quantum tunneling and adjusting charge balance in 2D p-n and n-n junctions Enhanced photoresponsivity in 2D heterojunction devices can be obtained with WSe2 and BP as p-type semiconductors and MoS2 and WS2 as n-type semiconductors In this study, the relationship between photocurrent and the charge balance of electrons and holes in van der Waals heterojunctions is investigated To observe this phenomenon, a p-WSe2/n-WS2/n-MoS2 heterojunction device with both p-n and n-n junctions is fabricated The device can modulate the charge carrier balance between heterojunction layers to generate photocurrent upon illumination by selectively applying electrostatic doping to a specific layer Using photocurrent mapping, the operating transition zones for the device is demonstrated, allowing to accurately identify the locations where photocurrent generates Finally, the origins of flicker and shot noise at the different semiconductor interfaces are analyzed to understand their effect on the photoresponsivity and detectivity of unit active area (25 µm2, λ = 405 nm) in the p-WSe2/n-WS2/n-MoS2 heterojunction device

Direct tuning of graphene work function via chemical vapor deposition control

Abstract: Besides its unprecedented physical and chemical characteristics, graphene is also well known for its formidable potential of being a next-generation device material. Work function (WF) of graphene is a crucial factor in the fabrication of graphene-based electronic devices because it determines the energy band alignment and whether the contact in the interface is Ohmic or Schottky. Tuning of graphene WF, therefore, is strongly demanded in many types of electronic and optoelectronic devices. Whereas study on work function tuning induced by doping or chemical functionalization has been widely conducted, attempt to tune the WF of graphene by controlling chemical vapor deposition (CVD) condition is not sufficient in spite of its simplicity. Here we report the successful WF tuning method for graphene grown on a Cu foil with a novel CVD growth recipe, in which the CH4/H2 gas ratio is changed. Kelvin probe force microscopy (KPFM) verifies that the WF-tuned regions, where the WF increases by the order of ~250 meV, coexist with the regions of intrinsic WF within a single graphene flake. By combining KPFM with lateral force microscopy (LFM), it is demonstrated that the WF-tuned area can be manipulated by pressing it with an atomic force microscopy (AFM) tip and the tuned WF returns to the intrinsic WF of graphene. A highly plausible mechanism for the WF tuning is suggested, in which the increased graphene-substrate distance by excess H2 gases may cause the WF increase within a single graphene flake. This novel WF tuning method via a simple CVD growth control provides a new direction to manipulate the WF of various 2-dimensional nanosheets as well as graphene.

Growth phase diagram of graphene grown through chemical vapor deposition on copper

Abstract: The phase diagram for graphene growth was obtained to understand the physics of the growth mechanism and control the layer number or coverage of graphene deposited on copper via low-pressure chemic...

Graphene and method for preparing same

Abstract: The present disclosure relates to a method for preparing graphene, including: forming a dielectric material; and applying heat treatment concurrently with a gaseous carbon source on the dielectric material to grow.