Sang Han Park

Improved Growth Behavior of Atomic-Layer-Deposited High-k Dielectrics on Multilayer MoS2 by Oxygen Plasma Pretreatment

We report on the effect of oxygen plasma treatment of two-dimensional multilayer MoS2 crystals on the subsequent growth of Al2O3 and HfO2 films, which were formed by atomic layer deposition (ALD) using trimethylaluminum and tetrakis-(ethylmethylamino)hafnium metal precursors, respectively, with water oxidant. Due to the formation of an ultrathin Mo-oxide layer on the MoS2 surface, the surface coverage of Al2O3 and HfO2 films was significantly improved compared to those on pristine MoS2, even at a high ALD temperature. These results indicate that the surface modification of MoS2 by oxygen plasma treatment can have a major impact on the subsequent deposition of high-k thin films, with important implications on their integration in thin film transistors.

High concentration of nitrogen doped into graphene using N2 plasma with an aluminum oxide buffer layer

We performed plasma doping of nitrogen into single-layer graphene on SiO2. Using aluminum oxide as a buffer layer to reduce the plasma damage, up to 19.7% nitrogen was substitutionally doped into graphene. The nitrogen doping of graphene was confirmed by Raman and X-ray photoemission spectroscopy analyses. The n-doping property of the N-doped graphene was measured by Raman spectroscopy. Raman mapping was carried out to statistically confirm the Dirac cone shift of graphene resulting from the N-doping. The Dirac cone shift was directly measured by ultraviolet photoemission spectroscopy (UPS). The UPS result was consistent with the value calculated from the Raman G peak shift.

The effect of ZnO surface conditions on the electronic structure of the ZnO/CuPc interface

The interfacial electronic structures of zinc oxide (ZnO)/copper-phthalocyanine (CuPc) were investigated by in situ x-ray and ultraviolet photoelectron spectroscopy (UPS) to determine the effects of air contamination on the ZnO substrate. UPS spectra showed that the 0.2 eV of the interface dipole is generated at the interface of the air exposed ZnO/CuPc while the interface of the annealed ZnO/CuPc generated −0.2 eV. In both cases, no band bending was observed. On the other hand, band bending at 0.3 eV and an interface dipole of 0.2 eV were observed at the interface of the sputter cleaned ZnO/CuPc. The energy offset between the conduction band maximum of ZnO and the highest occupied molecular orbital of CuPc was determined to be 0.6–0.7 eV for the contaminated ZnO interface while the offset was 1.0 eV for the cleaned ZnO interface. Contaminating moisture has little effect on the offset while the charge transfer was blocked and the offset was decreased in the presence of hydrocarbons.

Reversible Fermi Level Tuning of a Sb2Te3 Topological Insulator by Structural Deformation

For three-dimensional (3D) topological insulators that have a layered structure, strain was used to control critical physical properties. Here, we show that tensile strain decreases bulk carrier density while accentuating transport of topological surface state using temperature-dependent resistance and magneto-resistance measurements, terahertz-time domain spectroscopy and density functional theory calculations. The induced strain was confirmed by transmittance X-ray scattering measurements. The results show the possibility of reversible topological surface state device control using structural deformation.

The modulation of Si1−xGexnanowires by correlation of inlet gas ratio with H2 gas content

Si1−xGexnanowires (NWs) were prepared by a Vapor–Liquid–Solid (VLS) procedure using Au as the catalyst at a fixed growth temperature of 400 °C. The alloy composition was adjusted and the growth rate of the Si1−xGex NWs was achieved by varying the inlet gas ratio and the H2 flow rate. The growth of Si1−xGex NWs can be explained by two mechanisms that are related to growth kinetics; first, collisional activation is a dominant factor at flow rates of H2 100 sccm and second, in the case of a H2 flow rate of 200 sccm, the reaction is unimolecular. In addition, a Ge concentration (0.56 < x < 0.91) in Si1−xGex NWs is observed at a relatively high growth temperature of 400 °C as compared with data reported in the literature. The findings herein indicate that the high Ge concentration (x) can be attributed to the presence of interstitial Ge atoms in the Si1−xGex NWs, when they are grown under non-equilibrium conditions. This was confirmed by comparing the measured Ge concentration between EDX and XRD, Raman and strongly demonstrated by XPS results indicating the development of Ge interstitial states at lower binding energy, rather than bulk-like bonding.

Induction of the surface plasmon resonance from C-incorporated Au catalyst in Si1−xCx nanowires

Si1−xCx nanowires (NWs) were synthesized by varying the ratio of SiH4 and CH3SiH3 gases using a vapor–liquid–solid (VLS) procedure using Au as a catalyst. The growth rate of the Si1−xCx NWs and the change in the wire shape from straight to helical near the Au tip were found to be closely related to the ratio of the CH3SiH3 content. The large concentration of C in the Si1−xCx NWs was proportional to the CH3SiH3 content, overcoming the extremely low solubility of C in Si, resulting in an interstitial incorporation of C atoms in the wire. This incorporation can be attributed to the cleavage of Si–C bonds in the CH3SiH3 compound through the Au catalyst (an Au–Si liquid-state cluster of about 70–100 nm) during wire growth by the VLS method. Simultaneously supplying CH3SiH3 and SiH4 gases enhanced the diffusion of Au atoms from the tip to the sidewall of the wire, while also deforming the shape of the Au tip. When the CH3SiH3 gas was increased to 1.5 sccm, the number of Au nanoparticles (2–3 nm in size) at the lateral surface induced a surface plasmon resonance (SPR) and improved the optical conductivity (σ) of the Si1−xCx NWs. For 2 sccm of CH3SiH3, a remarkable increase in the number of C atoms incorporated in the Au nanoparticles along the sidewall red shifted the SPR peak, suggesting that the SPR can be modulated by the Au–C interactions in the nanoparticles.

Tuning the Fermi level with topological phase transition by internal strain in a topological insulator Bi2Se3 thin film

In a three-dimensional topological insulator Bi2Se3, a stress control for band gap manipulation was predicted but no systematic investigation has been performed yet due to the requirement of large external stress. We report herein on the strain-dependent results for Bi2Se3 films of various thicknesses that are grown via a self-organized ordering process. Using small angle X-ray scattering and Raman spectroscopy, the changes of d-spacings in the crystal structure and phonon vibration shifts resulted from stress are clearly observed when the film thickness is below ten quintuple layers. From the UV photoemission/inverse photoemission spectroscopy (UPS/IPES) results and ab initio calculations, significant changes of the Fermi level and band gap were observed. The deformed band structure also exhibits a Van Hove singularity at specific energies in the UV absorption experiment and ab initio calculations. Our results, including the synthesis of a strained ultrathin topological insulator, suggest a new direction for electronic and spintronic applications for the future.

Electronic structure of low work function electrodes modified by C{sub 16}H{sub 33}SH

Abstract Organic and printed electronics technologies require electrodes with low work functions to facilitate the transport of electrons in and out of various optoelectronic devices. We show that the surface modifier of 1-hexadecanethiol reduces the work function of conductors using in situ ultraviolet photoemission spectroscopy, and we combine experimental and theoretical methods to investigate the origin of the work function changes. The interfacial electronic structures of pentacene/1-hexadecanethiol/Au were investigated via in situ ultraviolet photoemission spectroscopy and X-ray photoemission spectroscopy in order to understand the change in the carrier injection barrier and chemical reactions upon surface modification. Theoretical calculations using density functional theory were also performed to understand the charge distribution of 1-hexadecanethiol, which affects the reduction of the work function. The 1-hexadecanethiol surface modifier is processed in air from solution, providing an appealing alternative to chemically-reactive low-work-function metals.

Performance Improvement of the Organic Light-Emitting Diodes by Using a LiF/Pyromellitic Dianhydride Stacked Cathode Interfacial Layer

A bilayered cathode interfacial structure consisting of lithium fluoride (LiF) and pyromellitic dianhydride (PMDA) was used between Al and tris-(8-hydroxyquinoline) aluminum (Alq 3 ) in the organic light-emitting diode (OLED), and a better performance enhancement compared to the OLED with a single LiF interfacial layer was achieved by using an optimal thickness combination of the bilayered interfacial structure (0.3 nm LiF/0.7 nm PMDA). The bilayered interfacial stucture with an optimum thickness combination decreased the parallel bulk resistance and lowered the electron injection barrier height between Al and Alq 3 according to the impedance and in situ ultraviolet photoelectron spectroscopy measurements, respectively.