JaeGwan Chung

Amorphous gallium indium zinc oxide thin film transistors: Sensitive to oxygen molecules

In this study, the authors report characteristic of indium gallium zinc oxides (GIZOs) which is strongly associated with the film surface. In ambient air, turn-on voltage of GIZO thin film transistors is approximately −7V. However, at the pressure of 8×10−6Torr, the turn-on voltage dramatically shifts to nearly −47V of the negative gate bias direction. When the oxygen is introduced in the chamber, the turn-on voltage returns to the normal value, that of air. It is believed that the adsorbed oxygen forms depletion layer below the surface, resulting in Von shifts. The carrier concentration of the channel varies from 1×1019to1×1020cm−3 due to oxygen adsorption.

Hole-injecting conducting-polymer compositions for highly efficient and stable organic light-emitting diodes

This letter introduces conducting polymer compositions which can be used for hole-injection layer in organic light-emitting diodes. The compositions are composed of poly (3,4-ethylenedioxythiophene) (PEDOT), polystyrene sulfonic acid (PSS) and a perfluorinated ionomer. The films based on these compositions showed much higher workfunction (∼5.3–5.7eV) than conventional PEDOT/PSS (∼5.0–5.2eV). When we fabricated blue polymer light-emitting diodes by using these compositions as a hole-injection layer, the luminescent efficiency was improved and the device lifetime was also enhanced relative to the device using the commercially available PEDOT/PSS. These compositions including perfluorinated ionomers can be one of the promising candidates for a hole-injection layer in organic light-emitting devices.

Observation of molecular nitrogen in N-doped Ge2Sb2Te5

Ge2Sb2Te5 (GST) film in the crystalline state was nitrogen doped using the reactive sputtering method in order to increase sheet resistance. High-resolution x-ray absorption spectroscopy revealed that molecular nitrogen (N2) existed in the N-doped GST film. This finding implies that both molecular nitrogen and atomic-state nitrogen should be taken into account in understanding the structures of N-doped GST film. The molecular nitrogen is believed to exist at interstitial and vacancy sites, and more likely at grain boundaries.

Bandgap measurement of thin dielectric films using monochromated STEM-EELS

High-resolution electron energy-loss spectroscopy (HR-EELS), achieved by attaching electron monochromators to transmission electron microscopes (TEM), has proved to be a powerful tool for measuring bandgaps. However, the method itself is still uncertain, due to Cerenkov loss and surface effects that can potentially influence the quality of EELS data. In the present study, we achieved an energy resolution of about 0.13 eV at 0.1s, with a spatial resolution of a few nanometers, using a monochromated STEM-EELS technique. We also assessed various methods of bandgap measurement for a-SiNx and SiO2 thin dielectric films. It was found that the linear fit method was more reliable than the onset reading method in avoiding the effects of Cerenkov loss and specimen thickness. The bandgap of the SiO2 was estimated to be 8.95 eV, and those of a-SiNx with N/Si ratios of 1.46, 1.20 and 0.92 were measured as 5.3, 4.1 and 2.9 eV, respectively. These bandgap-measurement results using monochromated STEM-EELS were compared with those using Auger electron spectroscopy (AES)-reflective EELS (REELS).

Large Work Function Modulation of Monolayer MoS2 by Ambient Gases

Although two-dimensional monolayer transition-metal dichalcogenides reveal numerous unique features that are inaccessible in bulk materials, their intrinsic properties are often obscured by environmental effects. Among them, work function, which is the energy required to extract an electron from a material to vacuum, is one critical parameter in electronic/optoelectronic devices. Here, we report a large work function modulation in MoS2 via ambient gases. The work function was measured by an in situ Kelvin probe technique and further confirmed by ultraviolet photoemission spectroscopy and theoretical calculations. A measured work function of 4.04 eV in vacuum was converted to 4.47 eV with O2 exposure, which is comparable with a large variation in graphene. The homojunction diode by partially passivating a transistor reveals an ideal junction with an ideality factor of almost one and perfect electrical reversibility. The estimated depletion width obtained from photocurrent mapping was ∼200 nm, which is much narrower than bulk semiconductors.

High-resolution soft x-ray spectroscopic study on amorphous gallium indium zinc oxide thin films

Amorphous gallium indium zinc oxide (a-GIZO) thin films of different compositions (Ga2O3:In2O3:ZnO=1:1:1,2:2:1,3:2:1,4:2:1) on Si substrates were investigated by high-resolution x-ray photoelectron spectroscopy and x-ray absorption spectroscopy (XAS) using synchrotron radiation. The O 1s, Ga 3d, In 4d, Zn 3d core, and shallow-core levels as well as the valence band maxima and O K-edge XAS were investigated. Each O 1s spectrum could be deconvoluted by a main component (O1 in the text) representing the Ga–In–Zn–O quaternary system along with two other higher-binding energy (BE) components (O2 and O3 in the text). The O2+O3 intensity increased as the Ga2O3 content increased. For the as-prepared samples, the spectral peak separations between the Ga 3d (∼20 eV) and Zn 3d (∼11 eV) orbitals and between the In 4d (∼18 eV) and Zn 3d orbitals became larger, respectively, as the Ga2O3 content increased. For the surface-cleaned samples, this trend was the same but with smaller increases in their separations. The sput...

Changes in Chemical and Structural Properties of Phase-Change Material GeTe with Nitrogen Doping and Annealing

In this study, changes in the chemical, structural, and electrical properties of undoped and 8.4 at. % nitrogen-doped GeTe films were investigated. The transition temperature of sheet resistance increased as a result of nitrogen doping, which corresponded well with phase transformations. The shift of chemical potential toward lower binding energies strongly depended on crystallization. The Ge/Te ratio showed a tendency to increase towards the surface only for undoped GeTe. Nitrogen doping may suppress the instability of GeTe.

Selective growth of carbon nanotube for via interconnects by oxidation and selective reduction of catalyst

We propose a selective growth approach of carbon nanotubes (CNTs) to prevent interface rupture in CNT via interconnects. Oxidation of the Ni catalyst layer is carried out before via hole patterning and the open hole area of the Ni layer is reduced by annealing in H2 ambient after via hole patterning. It is found that the present scheme provides robust selective growth of CNTs in the via hole and effectively prohibits the interface rupture due to the diffusion of carbon source into the Ni catalyst layer buried under the insulator.

An electronic structure reinterpretation of the organic semiconductor/electrode interface based on argon gas cluster ion beam sputtering investigations

The effects of the Ar gas cluster ion beam (GCIB) sputtering process on the structural and chemical properties of organic material and the energy-level alignment at the organic semiconductor/electrode interface are studied. The Ar GCIB sputtering process causes no damage to the molecular orientation and structure of the pentacene layer. The thin-film phase (001 at 5.74°, 15.4 A) in the X-ray diffraction patterns and the terrace-like structure in the atomic force microscope images are maintained even after the Ar GCIB sputtering process. Furthermore, there is no change in the chemical bonding state in the organic materials, including pentacene and poly(3,4-ethylenedioxythiophene) polymerized with poly(4-styrenesulfonate) (PEDOT:PSS). Finally, to investigate the preservation of the interface properties after the Ar GCIB sputtering process, the valence band structures of the pentacene/PEDOT:PSS and pentacene/Au structures are characterized using bottom-up (in situ ultraviolet photoemission spectroscopy (UPS) analysis with phased pentacene deposition) and top-down (in situ UPS analysis with Ar GCIB sputtering) methods, and the energy levels and chemical states are compared using the same sample. The Ar GCIB sputtering process causes no variation in the primary valence band structure, including the chemical state and configuration. Therefore, the energy-level alignment determined using the top-down method is comparable to that obtained using bottom-up method, since the Ar GCIB sputtering process is damage-free.The effects of the Ar gas cluster ion beam (GCIB) sputtering process on the structural and chemical properties of organic material and the energy-level alignment at the organic semiconductor/electrode interface are studied. The Ar GCIB sputtering process causes no damage to the molecular orientation and structure of the pentacene layer. The thin-film phase (001 at 5.74°, 15.4 A) in the X-ray diffraction patterns and the terrace-like structure in the atomic force microscope images are maintained even after the Ar GCIB sputtering process. Furthermore, there is no change in the chemical bonding state in the organic materials, including pentacene and poly(3,4-ethylenedioxythiophene) polymerized with poly(4-styrenesulfonate) (PEDOT:PSS). Finally, to investigate the preservation of the interface properties after the Ar GCIB sputtering process, the valence band structures of the pentacene/PEDOT:PSS and pentacene/Au structures are characterized using bottom-up (in situ ultraviolet photoemission spectroscopy (UPS)...

Nitrogen-Doping Effect on Ge2Sb2Te5 Chalcogenide Alloy Films during Annealing

The microstructural and electrical-property changes of undoped and 5.4% nitrogen-doped Ge2Sb2Te5 were investigated. The transition temperature of sheet resistance increased owing to nitrogen doping, which corresponded well with the observed phase-change states. The lattice parameters of the undoped and nitrogen-doped Ge2Sb2Te5 exhibited the same tendency of decrease with increasing annealing temperature. Considering the increase in the Ge2Sb2Te5 energy state owing to the presence of interstitial nitrogen, the increase in the crystallization temperature is contrary to the thermodynamic viewpoint. Nitrogen atoms and N2 gas can be located at the interstitial site without distorting the crystal structure.