Jaeyeong Heo

Photobias Instability of High Performance Solution Processed Amorphous Zinc Tin Oxide Transistors

The effects of the annealing temperature on the structural and chemical properties of soluble-processed zinc-tin-oxide (ZTO) films were examined by transmission electron microscopy, atomic force microscopy, high resolution X-ray reflectivity, and X-ray photoelectron spectroscopy. The density and purity of the resulting ZTO channel layer increased with increasing annealing temperature, whereas the oxygen vacancy defect density decreased. As a result, the device performance of soluble ZTO thin film transistors (TFTs) was improved at higher annealing temperature. Although the 300 °C-annealed ZTO TFT showed a marginal field-effect mobility (μFE) and high threshold voltage (Vth) of 0.1 cm(2)/(V s) and 7.3 V, respectively, the 500 °C-annealed device exhibited a reasonably high μFE, low subthreshold gate swing (SS), Vth, and Ion/off of 6.0 cm(2)/(V s), 0.28 V/decade, 0.58 V, and 4.0 × 10(7), respectively. The effects of dark negative bias stress (NBS) and negative bias illumination stress (NBIS) on the degradation of transfer characteristics of ZTO TFTs were also investigated. The instability of Vth values of the ZTO TFTs under NBS and NBIS conditions was suppressed with increasing annealing temperature. To better understand the charge trapping mechanism, the dynamics of Vth shift with NBS and NBIS time for all ZTO TFTs was analyzed on the basis of the stretched exponential relaxation. The negative Vth shift for each transistor was accelerated under NBIS conditions compared to NBS, which resulted in a higher dispersion parameter and smaller relaxation time for NBIS degradation. The relaxation time for NBS and NBIS instability increased with increasing annealing temperature, which is discussed on the basis of the transition mechanism of oxygen vacancy defects.

Joint Effects of Photoactive TiO2 and Fluoride-Doping on SnO2 Inverse Opal Nanoarchitecture for Solar Water Splitting.

Inverse opal (IO) films of tin dioxide (SnO2) were fabricated on polystyrene (PS) beads (diameter=350 nm (±20 nm) with a spin coating method. To compensate for the large band gap (Eg=3.8 eV), a thin TiO2 shell was deposited on the SnO2-IO films with atomic layer deposition (ALD), which produced shells with thicknesses of 10-40 nm. The morphological changes and crystalline properties of the SnO2 and TiO2-coated SnO2 (herein after referred to as TiO2/SnO2) IO films were investigated with field-emission scanning electron microscopy and X-ray diffraction, respectively. The photoelectrochemical (PEC) behavior of the samples was tested in a 0.1 M KOH solution under 1 sun illumination (100 mW/cm2 with an AM 1.5 filter). The highest PEC performance was obtained with the TiO2(10 nm)/SnO2 IO films, which produced a photocurrent density (Jsc) of 4.67 mA/cm2 at 0.5 V (vs NHE) and was sequentially followed by the TiO2(20 nm)/SnO2-IO, TiO2(30 nm)/SnO2-IO, TiO2 (40 nm)/SnO2-IO and SnO2 IO films. Overall, the thin TiO2 shell covered on the SnO2-IO core enhanced Jsc by 3 orders of magnitude, which in turn the PEC activity. This is mainly ascribed to the extremely low charge-transfer resistance (Rct) in the photoelectrode/electrolyte and at the TiO2/SnO2 interface, as well as the contribution of the photoactive TiO2 layer, which has an Eg of 3.2 eV. Moreover, to improve the electrical conductivity of the core SnO2 IO film, the films were doped with 10 mol % of F. The F- doped films were labeled as the FTO IO film. The Rct of the FTO-IO films decreased because of the improved electronic conductivity, enhancing the PEC performance of the TiO2(10 nm)/FTO-IO films by approximately 20%. The core-shell nanowire mesh nanoarchitecture is therefore suggested to provide an insight for designing the peculiar structure based on the materials properties and the engineering of their band gap energy for highly efficient PEC performance.

A novel route to the synthesis of silica nanowires without a metal catalyst at room temperature by chemical vapor deposition.

Silica nanowires were synthesized by employing inherent directionality of chemical vapor reaction between bis(ethylmethylamino)silane (H(2)Si[N(C(2)H(5))(CH(3))](2)) precursor and water without a metal catalyst at room temperature. The difference in the oxidation reactivity between Si-H and Si-N bonds with water leads to the formation of silica nanowires. The mean diameter and length of the silica nanowires grown for 10 min were 60-80 nm and 1.9 μm, respectively. Transmission electron microscopy revealed that the obtained nanowires had the concave tip, differing from other silica nanowires produced by a conventional vapor-liquid-solid method, and were amorphous. Energy dispersive X-ray spectroscopy, Fourier transform infrared, and X-ray photoelectron spectroscopy results also proved that the nanowires have a close composition to stoichiometric SiO(2). Silica nanowires were successfully synthesized on a poly(ethylene terephthalate) film. The nanowires can emit strong blue light and ultraviolet light under excitation at 266 nm.

Improving the Morphological and Optical Properties of Sputtered Indium Tin Oxide Thin Films by Adopting Ultralow-Pressure Sputtering

cm were also achieved using a continuous two-step depositionprocess, in which the initial layer was deposited using ULPS and then the final layer was deposited with a SP of 6.7 −110 Pa,without the use of any other additional steps. Both the ULPS and continuous two-step deposition methods were found to beeffective for producing ITO thin films with enhanced morphologies that make them suitable for use in display devices.© 2008 The Electrochemical Society. DOI: 10.1149/1.3005562 All rights reserved.Manuscript submitted June 26, 2008; revised manuscript received September 3, 2008. Published November 5, 2008.

Enhanced Nucleation Behavior of Atomic-Layer-Deposited Ru Film on Low-k Dielectrics Afforded by UV-O3 Treatment

The effect of UV-O 3 treatment on the nucleation behavior of ruthenium (Ru) film on low-k dielectrics was investigated. A continuous Ru film was not formed on the as-deposited or N 2 -annealed low-k layer, but after the UV-O3 treatment, full coverage with a continuous Ru layer was obtained. The microstructure and Ru/Row-k interface were studied using transmission electron microscopy, specular X-ray reflectivity, and Auger electron spectroscopy. The enhanced nucleation behavior of Ru may be due to the increased chemisorption probability of the Ru precursor on the low-k film surface, which comes from a modified oxygen-based dense layer (SiO x ).

The Role of the Methyl and Hydroxyl Groups of Low-k Dielectric Films on the Nucleation of Ruthenium by ALD

Nucleation of the ruthenium (Ru) precursor on low-k film surfaces during atomic layer deposition (ALD) of Ru was investigated. The adsorption tendency of the precursor decreased with increasing concentration of methyl groups on the low-k film surface, resulting in poor nucleation of Ru. The electron-deficient hydroxyl groups act as preferential adsorption sites for the electron-rich ligands of the aromatic Ru precursor through the formation of π-bonds. This leads to the enhanced nucleation of Ru. The roles of the functional groups were corroborated by silylation experiments.

Investigation into the Structural and Electrical Properties of a-SiCO:H as a Diffusion Barrier to Copper

In this study, we attempted to deposit low dielectric constant a-SiCO:H as a copper diffusion barrier by the plasma-enhanced chemical vapor deposition method using an organosilicon precursor, bis(trimethylsilylmethane) (BTMSM, C 7 H 20 Si 2 ). The dielectric constant of the a-SiCO:H films increased from 2.54 to 3.25 as the deposition temperature was increased from room temperature to 280°C. The refractive index increased gradually with increasing deposition temperature, indicating that the film has a denser structure as well as a higher dielectric constant at higher deposition temperatures. The Fourier transform infrared and X-ray photoelectron spectroscopy analyses indicated that the chemical structure of the Si atoms in the films changed from the D moiety to the T moiety with increasing deposition temperature. The electrical conduction mechanism of the a-SiCO:H films was determined to be a Schottky emission current in the high field region (E > 0.4 MV/cm).

Short-wavelength infrared photodetector on Si employing strain-induced growth of very tall InAs nanowire arrays

One-dimensional crystal growth enables the epitaxial integration of III-V compound semiconductors onto a silicon (Si) substrate despite significant lattice mismatch. Here, we report a short-wavelength infrared (SWIR, 1.4–3 μm) photodetector that employs InAs nanowires (NWs) grown on Si. The wafer-scale epitaxial InAs NWs form on the Si substrate without a metal catalyst or pattern assistance; thus, the growth is free of metal-atom-induced contaminations, and is also cost-effective. InAs NW arrays with an average height of 50 μm provide excellent anti-reflective and light trapping properties over a wide wavelength range. The photodetector exhibits a peak detectivity of 1.9 × 108  cm·Hz1/2/W for the SWIR band at 77 K and operates at temperatures as high as 220 K. The SWIR photodetector on the Si platform demonstrated in this study is promising for future low-cost optical sensors and Si photonics.

A facile and green synthesis of colloidal Cu2ZnSnS4 nanocrystals and their application in highly efficient solar water splitting

The development of solution-processable routes as well as compounds consisting of earth abundant elements is highly desirable to reduce the fabrication cost. Recently, kesterite Cu2ZnSnS4 (CZTS) nanocrystals (NCs) have attracted great attention for photoelectrochemical (PEC) water splitting owing to their suitable low-cost, earth-abundancy and suitable band gap energy. However, the environmentally benign synthesis of high-quality CZTS NCs without toxic solvents remains elusive. Here, a green chemistry approach employing vegetable oil as a non-toxic solvent for the synthesis of monodisperse and size-tunable CZTS NCs is introduced for the first time. Additionally, the relationship between the abnormal size behavior of the CZTS NCs and the degree of decomposition in the vegetable oil using electrospray ionization mass spectrometry (ESI-MS) measurements is elucidated for the first time. As a conceptual strategy, a ternary abundant compound based heterojunction nanostructure for efficient solar water splitting by introducing CZTS NCs onto 5 nm Zn(O,S) passivated layer/hydrothermally grown TiO2 nanorod arrays (TNRs) is designed and developed. Remarkably, this ternary CZTS NCs/Zn(O,S)/TNR photoelectrode shows a photocurrent density as high as 15.05 mA cm−2 at 1.23 V (vs. the NHE), which is the highest ever for previously reported CZTS NC-based photoelectrodes. The reasons for the enhanced PEC performance are discussed in detail based on different PEC characterizations. More importantly, this work reflects the sophistication of eco-friendly solution phase synthesized CZTS NCs without using any toxic chemicals as an earth abundant sensitizer and constitute a new paradigm towards the enhanced PEC performance with quantum dot based hetero-nanostructures.

Unipolar resistive switching characteristics of pnictogen oxide films: Case study of Sb2O5

Resistive switching (RS) and electrical endurance characteristics of amorphous Sb2O5 films in Pt/Sb2O5/Pt and Sb/Sb2O5/Pt structures were examined. Although both structures showed unipolar RS characteristics with power consumption smaller than those of other various transition metal oxides by ∼ two orders of magnitude, enhanced electrical endurance was obtained when Sb was employed as the top electrode than when Pt was employed as the top electrode. This improvement was explained by the creation of Sb clusters in the Sb2O5 films through the diffusion of oxygen from Sb2O5 to the Sb layer and the suppression of excessive oxygen loss during conducting filament formation process by the Sb top electrode. Metallic Sb clusters in pristine Sb2O5 films were confirmed by X-ray photoelectron spectroscopy depth profiling measurements and Auger electron spectroscopy. From the results of current density variation as a function of sample area, it was confirmed that the RS properties of Sb2O5 were controlled by the local...