• Proceedings of the Korean Vacuum Society Conference
• /
• 2000.02a
• /
• pp.138-138
• /
• 2000

Kinetic behaviors of homoepitaxial growth on Ni(110) surface was studied at the growth-temperature ranges 290~380 K with scanning tunneling microscopy. At low temperature (~290 K), deposited Ni grows layer-by-layer mode in the first several layers with one-dimensional islands but eventually (at > monolayers) forms three-dimensional islands througy the kinetic shortening of the average length of one-dimensional islands. At the intermediat temperature (~340 K), the three-dimensional islands were observed to be I) regular mesa-like structure with high aspect ratio (~1:10) at ~15 monolayer, ii) hut-like structure with low aspect ratio (~1:1.5) at ~35 monolayer, and iii) rounded mound structure at ~55 monolayers, due to the competition of kinetic and energetic terms. At the high temperature (~ 380 K), the flat surface with layer-by-layer mode was observed up to 50 monolayers. Microscopic orgins for the observations will be discussed on the basis of kinetic Monte Carlo simulations.

• Proceedings of the Korean Vacuum Society Conference
• /
• 2000.02a
• /
• pp.140-140
• /
• 2000

The initial growth mode of Nb on Ag(11) in sub-monolayer regime and the influence of subsequent 520K annealing are studied using UHV Scanning Tunneling Microscopy. E-beam evaporated Nb is deposited onto the substrate at RT, and STM measurements are carried out at RT and 78 K. With Nb being immiscible in bulk Ag, 3D islands formation begins at early stage and no particular ordered structure is found. After annealing to 520K, most of islands are disappeared from terrace. There exist 2 possibilities. : (1) Diffusion of Nb into the 2nd or 3rd layer of Ag substrate or (2) agglomeration of Nb on Ag at higher temperature. A model will be given to explain the evidence. In addition, we investigated the change of STM image according to bias voltage depending on island size. Possible physical mechanism responsible for such behavior together with interaction between Nb islands and reactive gases will be also discussed.

• Proceedings of the Korean Vacuum Society Conference
• /
• 2010.02a
• /
• pp.40-40
• /
• 2010

Employing synchrotron radiation based photoemission spectroscopy (PES) and scanning tunneling microscopy (STM), our group have investigated Si surfaces, various graphenes and molecular nanolayers. In this talk, I introduce recent results on the surface related systems. All experiments have been performed at the surface science beamlines, 3A2 and 7B1, in Pohang Accelerator Laboratory, where high resolution PES (HRPES) and angle resolved PES (ARPES) are available. Metals or molecules are adsorbed and sometimes extreme ultraviolet is irradiated onto surfaces to give them special functions. I show several examples for surface functionalzation and how to characterize solid surface using the analysis techniques. In particular, lots of ARPES and STM data are provided from graphenes, a strong candidate for replacing Si and conducting oxide currently used in many electronic and optical devices.

• Bulletin of the Korean Chemical Society
• /
• v.21 no.7
• /
• pp.705-708
• /
• 2000

Adsorption of CO and NO Moleculcs on a Pt(OO1)-hex R0.7° surface at 90 K is investigated by scanning tunneling microscopy (STM) in ultra-high vacuum environments. At an initial stage of adsorption, both molecules are preferentially adsorbed on th e Iess coordinated Pt atoms of the surface with hexagonal structure, which act as active sites. Domains of the adsorbates grow parallel to the stripe structure of the reconstructed surface because of Iower migration energy in this direction. The extra Pt atoms produced from adsorbate-induced restructuring give rise to anisotropic islands on the ( 1 x 1 ) surface. Each of the adsorbed NO molecules at low coveragcs is atomicalIy resolved during STM observation. However, the spots of the adsorbed CO are invisible.Such a behavior is probably explained in terms of different interactions between the adsorbates.

• Bulletin of the Korean Chemical Society
• /
• v.22 no.3
• /
• pp.267-270
• /
• 2001

The adsorption of chlorine on a Pt(111) surface at 90 K has been studied using scanning tunneling microscopy (STM) in ultra-high vacuum environments. The adsorbed chlorine atoms give rise to two different ordered structures, (3${\times}$3)-Cl and c(4${\times}$2)- Cl, depending on the coverage. It has been determined from the STM image that the absolute coverage of (3${\times}$3)-Cl structure is 0.44, which is not in agreement with coverage calibrated by a low energy electron diffraction technique together with an Auger electron spectrometer and a thermal desorption spectrometer. The Cl atoms bound to on-top sites at the Pt(111)(3${\times}$3)-Cl surface appeared to effectively perturb the density of states of Pt atoms, as compared with that bound to bridging sites. The other ordered structure, c(4${\times}$2)-Cl, with small domain sizes, consists of both on-top and bridge-bonded species with a saturation coverage of 0.5.

• Journal of Electrochemical Science and Technology
• /
• v.14 no.1
• /
• pp.21-30
• /
• 2023

This work presents a correlation between the behavior of formic acid oxidation (FAO) on various Bi-modified Pt(poly) disk electrodes and their morphologies observed on Bi-modified Pt(111) disk electrodes using electrochemical scanning tunneling microscopy (EC-STM) to understand the effects of Bi on Pt. To distinguish the FAO activities of Bi on Pt and plain Pt around Bi, additional Pt was intentionally deposited using two different routes: direct route and iodine route. In direct route, Pt was directly deposited on Bi islands and plain Pt sites around Bi islands, while in iodine route, Pt was exclusively deposited on Bi islands by protecting plain Pt sites with adsorbed iodine. Thus, a comparison of FAO performances on the two Bi-modified Pt electrodes with additional Pt (deposited in the different ways) disclosed a difference in FAO performances on plain Pt sites and Bi islands. When Bi coverage was ~0.04, the Bi deposits were scattered Bi islands enhancing FAO on Pt(poly). The additional Pt deposits using direct route increased FAO efficiency, while the ones using iodine route slightly decreased FAO current. The EC-STM observations indicated that Pt deposits around Bi islands, not on Bi islands, were responsible for the FAO current increase on Bi-modified Pt(poly). The FAO efficiency on Bi-modified Pt(poly) with a Bi coverage of ~0.25 increased by a factor of 2. However, the additional Pt deposits using the two Pt deposition routes notably decreased the FAO current. The dependency of FAO on Bi coverage was discussed in terms of electronic effect and ensemble effect.

• Proceedings of the Korean Vacuum Society Conference
• /
• 2011.02a
• /
• pp.77-77
• /
• 2011

These days, a growing demand for memory device is filled up with the flash memory and the dynamic random access memory (DRAM). Although DRAM is a reasonable solution for current demand, the universal novel memory with high density, high speed and nonvolatility, needs to be developed. Among various new memories, the magnetic random access memory (MRAM) device is considered as one of good candidate memories because of excellent features including high density, high speed, low operating power and nonvolatility. The etching of MTJ stack which is composed of magnetic materials and insulator such as MgO is one of the vital process for MRAM. Recently, MgO has attracted great interest in the MTJ stack as tunneling barrier layer for its high tunneling magnetoresistance values. For the successful realization of high density MRAM, the etching process of MgO thin films should be investigated. Until now, there were some works devoted to the investigations on etch characteristics of MgO thin films. Initially, ion milling was applied to the etching of MgO thin films. However, ion milling has many disadvantages such as sidewall redeposition and etching damage. High density plasma etching containing the magnetically enhanced reactive ion etching and high density reactive ion etching have been employed for the improvement of etching process. In this work, inductively coupled plasma reactive ion etching (ICPRIE) system was adopted for the improvement of etching process using MgO thin films and etching gas mixes of $CH_4$/Ar and $CH_4$/$O_2$/Ar have been employed. The etch rates are measured by a surface profilometer and etch profiles are observed using field emission scanning emission microscopy (FESEM). The effects of gas concentration and etch parameters such as coil rf power, dc-bias voltage to substrate, and gas pressure on etch characteristics will be systematically explored.

• Journal of Sensor Science and Technology
• /
• v.5 no.5
• /
• pp.1-10
• /
• 1996

Solar cells made of the edge-defined film-fed growth Si are characterized using current-voltage, surface photovoltage, electron beam induced current, electron microprobe, scanning electron microscopy, and electron backscattering. The weak temperature dependence of the I-V curves in the EFG solar cells is due to a voltage variable shunt resistance giving higher diode ideality factors than the ideal one. The voltage variable shunt resistance is modeled by a modified recombination mechanism which includes carrier tunneling to distributed impurity energy states in the band gap within the space-charge region. The junction integrity and the substrate quality are characterized simultaneously by combining I-V and surface photovoltage (SPV) measurements. The diode ideality factors and the surface photovoltages characterize the junction integrity while the SPV diffusion lengths characterizes the substrate quality. Most of the measured samples show the voltage variable shunt resistance although how serious it is depends on the solar cell efficiency. The voltage variable shunt resistance is understood as one of the most important factors of the degradation of EFG solar cells.

• Corrosion Science and Technology
• /
• v.6 no.4
• /
• pp.198-205
• /
• 2007

The initial stages of electrochemical oxidative CuI film formation on Cu(111), as studied by means of Cyclic Voltammetry (CV), in-situ Scanning Tunneling Microscopy (STM) and ex-situ Synchrotron X-ray Photoemission Spectroscopy (SXPS), indicate a significant acceleration of copper oxidation in the presence of iodide anions in the electrolyte. A surface confined supersaturation with mobile CuI monomers first leads to the formation of a 2D-CuI film via nucleation and growth of a Cu/I-bilayer on-top of a pre-adsorbed iodide monolayer. Structurally, this 2D-CuI film is closely related to the (111) plane of crystalline CuI (zinc blende type). Interestingly, this film causes no significant passivation of the copper surface. In an advanced stage of copper dissolution a transition from the 2D- to a 3D-CuI growth mode can be observed.

• Proceedings of the Korean Vacuum Society Conference
• /
• 2012.08a
• /
• pp.230-230
• /
• 2012

Graphene have showed promising performance as electrodes of organic devices such as organic transistors, light-emitting diodes, and photovoltaic solar cells. In particular, among various organic materials of graphene-based organic devices, pentacene has been regarded as one of the promising organic material because of its high mobility, chemical stability. In the bottom-contact device configuration generally used as graphene based pentacene devices, the morphology of the organic semiconductors at the interface between a channel and electrode is crucial to efficient charge transport from the electrode to the channel. For the high quality morphology, understanding of initial stages of pentacene growth is essential. In this study, we investigate self-assembly of pentacene molecules on graphene formed on a 6H-SiC (0001) substrate by scanning tunneling microscopy. At sub-monolayer coverage, adsorption of pentacene molecules on epitaxial graphene is affected by $6{\times}6$ pattern originates from the underlying buffer layer. And the orientation of pentacene in the ordered structure is aligned with the zigzag direction of the edge structure of single layer graphene. As coverage increased, intermolecular interactions become stronger than molecule-substrate interaction. As a result, herringbone structures the consequence of higher intermolecular interaction are observed.