Jae Gwan Chung

Anion control as a strategy to achieve high-mobility and high-stability oxide thin-film transistors

Ultra-definition, large-area displays with three-dimensional visual effects represent megatrend in the current/future display industry. On the hardware level, such a “dream” display requires faster pixel switching and higher driving current, which in turn necessitate thin-film transistors (TFTs) with high mobility. Amorphous oxide semiconductors (AOS) such as In-Ga-Zn-O are poised to enable such TFTs, but the trade-off between device performance and stability under illumination critically limits their usability, which is related to the hampered electron-hole recombination caused by the oxygen vacancies. Here we have improved the illumination stability by substituting oxygen with nitrogen in ZnO, which may deactivate oxygen vacancies by raising valence bands above the defect levels. Indeed, the stability under illumination and electrical bias is superior to that of previous AOS-based TFTs. By achieving both mobility and stability, it is highly expected that the present ZnON TFTs will be extensively deployed in next-generation flat-panel displays.

Electronic and optical properties of Al2O3/SiO2 thin films grown on Si substrate

The electronic and optical properties of Al2O3/SiO2 dielectric thin films grown on Si(1 0 0) by the atomic layer deposition method were studied by means of x-ray photoelectron spectroscopy and reflection electron energy loss spectroscopy (REELS). The band gaps of the Al2O3/SiO2 thin films before annealing and after annealing were 6.5 eV and 7.5 eV, respectively, and those of the γ-Al2O3 and α-Al2O3 phases were 7.1 eV and 8.4 eV, respectively. All of these were estimated from the onset values of the REELS spectra. The dielectric functions were determined by comparing the effective cross-section determined from experimental REELS with a rigorous model calculation based on dielectric response theory, using available software packages. The determined energy loss function obtained from the Al2O3/SiO2 thin films before annealing showed a broad peak at 22.7 eV, which moved to the γ-Al2O3 position at 24.3 eV after annealing. The optical properties were determined from the dielectric function. The optical properties of the Al2O3/SiO2 thin films after annealing were in good agreement with those of γ-Al2O3. The changes in band gap, electronic and optical properties of the Al2O3/SiO2 thin films after annealing indicated a phase transition from an amorphous phase to the γ-Al2O3 phase after annealing.

Band alignment of atomic layer deposited (ZrO2)x(SiO2)1−x gate dielectrics on Si (100)

The band alignment of atomic layer deposited (HfZrO4)1−x(SiO2)x (x = 0, 0.10, 0.15, and 0.20) gate dielectric thin films grown on Si (100) was obtained by using X-ray photoelectron spectroscopy and reflection electron energy loss spectroscopy. The band gap, valence band offset, and conduction band offset values for HfZrO4 silicate increased from 5.4 eV to 5.8 eV, from 2.5 eV to 2.75 eV, and from 1.78 eV to 1.93 eV, respectively, as the mole fraction (x) of SiO2 increased from 0.1 to 0.2. This increase in the conduction band and valence band offsets, as a function of increasing SiO2 mole fraction, decreased the gate leakage current density. As a result, HfZrO4 silicate thin films were found to be better for advanced gate stack applications because they had adequate band gaps to ensure sufficient conduction band offsets and valence band offsets to Si.

Effect of oxygen partial pressure on the Fermi level of ZnO1−x films fabricated by pulsed laser deposition

We investigated the influence of oxygen deficiency on the Fermi level (EF) of ZnO thin film prepared by pulsed laser deposition (PLD). For this purpose, we adopted in situ x-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. The oxygen deficiency was effectively controlled by varying the oxygen partial pressure [P(O2)] during the PLD. The EF shifted by +0.6 eV as the P(O2) decreased from 10 to 3.3 Pa. This shift indicates a significant change in the energy balance in the oxygen-deficient ZnO films. This fact suggests that the very large change in the resistivity of ZnO thin films resulting from the oxygen deficiency could be attributed to the EF shift rather than grain boundary formation in the ZnO film.

Dielectric and optical properties of Zr silicate thin films grown on Si(100) by atomic layer deposition

Dielectric and optical properties of (ZrO2)x(SiO2)1−x dielectric thin films, grown on Si(100) by the atomic layer deposition method, were studied by means of reflection electron energy loss spectroscopy (REELS). The quantitative analysis of REELS spectra was carried out by using the quantitative analysis of electron energy loss spectra-e(k,ω)-REELS software to determine the dielectric function and optical properties by using an analysis of experimental REELS cross sections from the simulated energy loss function (ELF). For ZrO2, the ELF shows peaks in the vicinity of 10, 15, 21, 27, 35, 42, and 57 eV. For SiO2, a broad peak at 23 eV with a very weak shoulder at 15 eV and a shoulder at 34 eV were observed, while for Zr silicates (x=0.75 and 0.5), the peak position is similar to that of ZrO2. For Zr silicates with high SiO2 concentration (x=0.25), the peak positions are similar to that of SiO2, but the peak at 42 eV, which is due to excitation of Zr N2,3 shell electrons, still exist. This indicates that the...

A High-Density Array of Size-Controlled Silicon Nanodots in a Silicon Oxide Nanowire by Electron-Stimulated Oxygen Expulsion

Methods of producing Si nanodots embedded in films of silicon oxide and silicon nitride abound, but fabrication of Si nanodots in a nanowire of these materials is very rare despite the fact that nanowire architecture enhances the charge collection and transport efficiencies for solar cells and field-effect transistors. We report a novel fabrication method for a high-density array of size-controlled sillicon nanodots from a silicon oxide nanowire using electron-beam irradiation. Our results demonstrate that a highly dense phase of Si nanodots with a narrow size distribution can be made from a silicon oxide nanowire with a core-shell structure of crystalline silicon-rich oxide (c-SRO)/amorphous silicon oxide (a-SiO(2)). This new nanomaterial shows the carrier transport characteristics of a semiconductor. The initially produced amorphous Si nanodots can be readily turned into crystalline Si (c-Si) nanodots by thermal annealing. Key characteristics of c-Si nanodots such as their size, number density, and rate of nucleation and growth are easily controlled by varying the electron radiation dose and annealing temperature. Nanodot formation is mechanistically initiated by electron trapping at the c-SRO core as well as at the core-shell interface, which leads to out-diffusion of the negatively charged oxygen through Coulomb repulsion, fostering the aggregation of Si atoms.

Band alignment of atomic layer deposited (HfZrO{sub 4}){sub 1−x}(SiO{sub 2}){sub x} gate dielectrics on Si (100)

The band alignment of atomic layer deposited (HfZrO{sub 4}){sub 1−x}(SiO{sub 2}){sub x} (x = 0, 0.10, 0.15, and 0.20) gate dielectric thin films grown on Si (100) was obtained by using X-ray photoelectron spectroscopy and reflection electron energy loss spectroscopy. The band gap, valence band offset, and conduction band offset values for HfZrO{sub 4} silicate increased from 5.4 eV to 5.8 eV, from 2.5 eV to 2.75 eV, and from 1.78 eV to 1.93 eV, respectively, as the mole fraction (x) of SiO{sub 2} increased from 0.1 to 0.2. This increase in the conduction band and valence band offsets, as a function of increasing SiO{sub 2} mole fraction, decreased the gate leakage current density. As a result, HfZrO{sub 4} silicate thin films were found to be better for advanced gate stack applications because they had adequate band gaps to ensure sufficient conduction band offsets and valence band offsets to Si.

Fabrication of surface plasmon-coupled si nanodots in au-embedded silicon oxide nanowires.

Adv. Mater. 2010, 22, 2421–2425 2010 WILEY-VCH Verlag G The bottom-up assembly of nanostructures on a dielectric matrix holds great promise for use in optoelectronic devices and photovoltaic solar cells because of the unique optical and electrical properties manifested only at the nanoscale. Notable examples include gold nanoparticle chains embedded in silicon oxide nanowires and crystalline Si (c-Si) nanodots embedded in a silicon oxide film. Recently, the simultaneous incorporation of both silicon and noble metal nanoparticles on a SiO2 matrix has become a particularly interesting issue because of the dramatic increase in light absorption, and emission of silicon-based materials related to the surface plasmon effect of the metal nanoparticles. Significant progress has been made on the synthesis of metal nanoparticles with various shapes and composition to control surface plasmonic properties. In our previous study, we reported a new fabrication method for size-controlled c-Si nanodots from a core–shell silicon oxide nanowire by electron-beam irradiation that leads to electron trapping followed by oxygen expulsion. In this paper, we aimed to fabricate a new noble metal–silicon hybrid nanowire with possible extraordinary optical properties by using a similar strategy as the one above to produce a high density of Si nanodots in gold-nanoparticle-embedded silicon oxide nanowires by electron-beam irradiation. We found that the formation mechanism of Si nanodots in this study is thermally driven, which is entirely different from earlier cases which resulted from electron trapping. Our method produces a high density (ca. 5.1 10 cm ) of amorphous Si (a-Si) and c-Si nanodots with a narrow size distribution. The optical characteristics of Si nanodots in the vicinity of the gold nanoparticles were investigated by monochromated electron energy-loss spectroscopy (EELS), which revealed a shift in the surface plasmon band of gold and a marked enhancement in the electron-induced excitation of Si nanodots. Furthermore, simple heat treatment at high temperature (1123K) yielded an unexpected Au/Si core–shell nanostructure with a corresponding shift in the plasmon band. We synthesized gold-nanoparticle-embedded silicon oxide nanowires by simply heating an n-type (100) silicon wafer onto which a thin (ca. 20 nm) gold layer had been deposited to 1273 K in a microchamber. The resulting nanowires were predominantly observed along the inner part of the downstream edge of the Si wafer. To understand how these hybrid nanowires came to be formed, we studied their structural evolution through detailed transmission electron microscopy (TEM) analyses, which revealed that gold silicide droplets that grow from the gold-coated substrate evolve into a gold peapod structure encapsulated in the silicon oxide nanowires (see Supporting Information, Fig. S1). A typical TEM image of the goldpeapod-embedded silicon oxide nanowire is shown in Figure 1a. The lattice-resolved TEM image (Fig. 1b) and its Fourier-transform diffractogram (Fig. 1b, inset) taken at the gold nanoparticle site unambiguously reveal the face centered cubic (fcc) structure of gold. On the other hand, both the high-resolution TEM (HRTEM) image (Fig. 1c) and the selective-area electron diffraction (SAED) pattern (Fig. 1d, inset) taken at the square-framed region in Figure 1a, together with its radial distribution function (RDF) (Fig. 1d), indicate that the nanowire matrix is in an amorphous phase, consisting of a short-range atomic arrangement similar to the monoclinic SiO2 structure. Figure 1e shows the energy-dispersive X-ray spectroscopy (EDS) data taken at the site marked by the red circle (A1) in Figure 1a, which reveals an approximate stoichiometry of SiO1.3 when compared against the EDS spectrum of our reference material, SiO2. In Figure 1f, the core-excitation electron energy-loss spectrum taken at A1 reveals a Si-L2,3 energy-loss near-edge structure (ELNES) of silicon-rich oxide (SRO), which is in accord with the EDS results. Electron-beam irradiation of these gold-peapod-embedded SRO nanowires yielded a high density of Si nanodots. Energy-filtered transmission electronmicroscopy (EF-TEM) gives the plasmon-loss images of Si filtered at 17 eV (Fig. 2a–c), which show that the number of Si nanodots formed between two gold particles increases with irradiation time. The bright zone in Figure 2b indicates the formation of a Si network following the initial nucleation of isolated clusters. As the irradiation proceeds further in time, Si atoms aggregate into nanodots in the amorphous SRO (a-SRO) matrix until eventually a high density of Si nanodots with a size of 4.0–5.5 nm are formed (Fig. 2c). If

Effects of gas environment on electronic and optical properties of amorphous indium zinc tin oxide thin films

The electronic and optical properties of indium zinc tin oxide (IZTO) thin films grown under different gas environments were investigated by means of x-ray photoelectron spectroscopy and reflection electron energy loss spectroscopy (REELS). REELS spectra revealed that IZTO thin films under argon mixed with oxygen had band gaps of 3.07 eV before annealing and 3.46 eV after annealing at 350 °C in air. Meanwhile, the band gap for IZTO thin film grown under oxygen mixed with water and annealed at 350 °C in air was 3.26 eV. Band gaps obtained from REELS spectra are consistent with the optical band gaps obtained using UV-spectrometry. The REELS spectra were quantitatively analyzed based on comparison of the effective cross section for inelastic electron scattering in the REELS experiment to determine the dielectric function and transmittance of the IZTO thin films. It was found that amorphous IZTO films grown under argon mixed with oxygen followed by annealing at 350 °C exhibit higher optical transmittance in t...

Understanding the structural, electrical, and optical properties of monolayer h-phase RuO2 nanosheets: A combined experimental and computational study

The structural, electrical, and optical properties of monolayer ruthenium oxide (RuO2) nanosheets (NSs) fabricated by chemical exfoliation of a layered three-dimensional form of K-intercalated RuO2 are studied systematically via experimental and computational methods. Monolayer RuO2 NS is identified as having a distorted h-MX2 structure. This is the first observation of a RuO2 NS structure that is unlike the t-MX2 structure of the RuO2 layers in the parent material and does not have hexagonal symmetry. The distorted h-MX2 RuO2 NSs are shown to have optical transparency superior to that of graphene, thereby predicting the feasibility of applying RuO2 NSs to flexible transparent electrodes. In addition, it is demonstrated that the semiconducting band structures of RuO2 NSs can be manipulated to be semi-metallic by adjusting the crystal structure, which is related to band-gap engineering. This finding indicates that RuO2 NSs can be used in a variety of applications, such as flexible transparent electrodes, atomic-layer devices, and optoelectronic devices.Two-dimensional materials: A more transparent way to get in contactNanosheets of ruthenium oxide could make excellent transparent electrical contacts, show researchers from Korea. Graphene is the wonder material of the last decade owing to its amazing electrical, mechanical and thermal properties. Scientists are thus keen to fabricate single layers of atoms other than carbon. Now, Dong-Su Ko, Jung-Hwa Kim and Jong Wook Roh from the Samsung Advanced Institute of Technology and co-workers have combined experiments and theory to fully characterize this unusual two-dimensional material. They created their nanosheets by exfoliating a three-dimensional block of ruthenium oxide. Transmission electron microscopy, X-ray diffraction experiments and first-principles calculations showed that the two-dimensional material has a distorted atomic arrangement, which makes it a semiconductor rather than a metal like its parent material. Furthermore, ruthenium oxide nanosheets are more transparent than graphene, making them useful for flexible transparent electrodes.As a new two-dimensional (2D) material, monolayer ruthenium oxide (RuO2) nanosheets (NSs) have distorted h-MX2 type crystal structures that lead to semiconducting properties and good optical transmittance. This study suggests that monolayer RuO2 can be useful in applications of flexible optoelectronics.