Negative threshold voltage shift in an a-IGZO thin film transistor under X-ray irradiation.

We investigated the effects of X-ray irradiation on the electrical characteristics of an amorphous In-Ga-Zn-O (a-IGZO) thin film transistor (TFT). The a-IGZO TFT showed a negative threshold voltage (V TH) shift of -6.2 V after 100 Gy X-ray irradiation. Based on spectroscopic ellipsometry (SE) and X-ray photoelectron spectroscopy (XPS) analysis, we found that the Fermi energy (E F) changes from 2.73 eV to 3.01 eV and that the sub-gap state of D1 and D2 changes near the conduction band minimum (CBM) of the a-IGZO film after X-ray irradiation. These results imply that the negative V TH shift after X-ray irradiation is related to the increase in electron concentration of the a-IGZO TFT active layer. We confirmed that the sources for electron generation during X-ray irradiation are hydrogen incorporation from the adjacent layer or from ambient air to the active layer in the TFT, and the oxygen vacancy dependent persistent photocurrent (PPC) effect. Since both causes are reversible processes involving an activation energy, we demonstrate the V TH shift recovery by thermal annealing.

Highly Bendable and Durable Transparent Electromagnetic Interference Shielding Film Prepared by Wet Sintering of Silver Nanowires.

Electromagnetic (EM) wave emissions from wearable or flexible smart display devices can cause product malfunction and have a detrimental effect on human health. Therefore, EM shielding strategies are becoming increasingly necessary. Consequently, herein, we prepared a transparent acrylic polymer-coated/reduced graphene oxide/silver nanowire (Ag NW) (A/RGO/SANW) EM interference (EMI) shielding film via liquid-to-vapor pressure-assisted wet sintering. The film exhibited enhanced Ag NW network formation and antireflection (AR) effects. The wet-sintered Ag NW shielding film had a threshold radius of curvature (ROC) of 0.31 mm at a film thickness of 100 µm, demonstrating its high flexibility, whereas the conventional indium tin oxide (ITO) shielding film had a threshold ROC of ∼5 mm. The EMI shielding effectiveness (SE) of the A/RGO/SANW multilayer film was approximately twice that of the ITO film at a similar relative transmittance (84-85%). The optical relative reflectance of the Ag NW layer was reduced due to the AR effect, and the visible-light transmittance was considerably improved owing to the different refractive indices in the multilayer shielding film. Because the acrylic coating layer had a high contact angle, the multilayer film exhibited high temperature and humidity durability with little change in the SE over 500 h at 85 °C and 85% relative humidity. The multilayer film comprising wet-sintered Ag NW exhibited high flexibility and humidity durability, high shielding performance (more than 24 dB at a relative transmittance of 85% or more), and high mass productivity, making it highly applicable for use as a transparent shielding material for future flexible devices.

Conoscopy as a Failure Analysis Method for Single Crystals.

Conoscopy is widely used to evaluate single crystals used as substrates on which epitaxial layers are grown in the LED industry, where the quality of the single crystal affects the reliability of the final product, the LED chip, and the package. However, the application of this method is currently restricted to characterizing birefringence. We performed conoscopy measurements on single crystals with failure modes (e.g., birefringence, lineages, dislocations, polycrystallinity, and amorphousness) and examined whether it was possible to inspect such failures using conoscopy. Sapphire (α-Al2O3) and silicon carbide (6H­SiC) single crystals containing failures were investigated. X-ray diffraction and transmission electron microscopy analyses were also performed; their results were compared with the conoscopy results. Conoscopy was shown to inspect birefringence as well as other failure modes. Comparison of the conoscopic patterns obtained via simulation and experiment shows that quantitative evaluation of the failure level is possible. These results show that conoscopy can be used to quickly and easily investigate various failure mechanisms in single crystals.

Alignment of One-Dimensional SnO2 Lines by Electrohydrodynamic Jet Printing.

One-dimensional (1-D) SnO2 line as a representative semiconducting oxide were formed by electro- hydrodynamic jet-printing (EHD) of tin chloride pentahydrate and polyvinylpyrrolidone (PVP, 1,200 k, Aldrich) solution ink. The 1-D polymer lines including Sn precursors were created by controlling the viscosity, that is, polymer/tin precursor ratio, and adjusting printing conditions such as tip to substrate distance, applying voltage, flow rate of ink and velocity. The printed lines were dried at 200 degrees C to get rid of solvent and finally heat-treated at 600 degrees C to burn out PVP and form tin oxide line. We found out that the linearity and shape of the aligned 1-D SnO2 could be controlled by adjusting various parameters such as the viscosity of a precursor solution, the ratio of Sn to PVP polymer in the solution, the shape of a cone, the size of a droplet, the applied voltages, the working distance, the flow rate on the glass slides and the Si wafers with a SiPO2 layer, respectively. It is found out that the heat-treatment for the removal of polymers should be tailored to produce continuous 1-D SnO2 lines due to the drastic volume reduction (>90%) of the aligned fibers in the annealing process. The electrical properties of the 1-D SnO2 aligned on the Si wafers with Au electrode patterns were evaluated.

Photochemical Hydrogen Doping Induced Embedded Two-Dimensional Metallic Channel Formation in InGaZnO at Room Temperature.

The photochemical tunability of the charge-transport mechanism in metal-oxide semiconductors is of great interest since it may offer a facile but effective semiconductor-to-metal transition, which results from photochemically modified electronic structures for various oxide-based device applications. This might provide a feasible hydrogen (H)-radical doping to realize the effectively H-doped metal oxides, which has not been achieved by thermal and ion-implantation technique in a reliable and controllable way. In this study, we report a photochemical conversion of InGaZnO (IGZO) semiconductor to a transparent conductor via hydrogen doping to the local nanocrystallites formed at the IGZO/glass interface at room temperature. In contrast to thermal or ionic hydrogen doping, ultraviolet exposure of the IGZO surface promotes a photochemical reaction with H radical incorporation to surface metal-OH layer formation and bulk H-doping which acts as a tunable and stable highly doped n-type doping channel and turns IGZO to a transparent conductor. This results in the total conversion of carrier conduction property to the level of metallic conduction with sheet resistance of ∼16 Ω/□, room temperature Hall mobility of 11.8 cm(2) V(-1) sec(-1), the carrier concentration at ∼10(20) cm(-3) without any loss of optical transparency. We demonstrated successful applications of photochemically highly n-doped metal oxide via optical dose control to transparent conductor with excellent chemical and optical doping stability.

Characterization of Mn-Based Self-Forming Barriers on Low-k Samples With or Without UV Curing Treatment.

In the present work, we report a Cu-Mn alloy as a material for the self-forming barrier process, and we investigated the diffusion barrier properties of the self-formed layer on low-k dielectrics with or without UV curing treatment. Cu alloy films with 3.8 at% Mn were directly deposited onto low-k dielectrics by co-sputtering followed by annealing at various temperatures. The self-formed layers were investigated by transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS). In order to compare barrier properties between the Mn-based self-formed layer on low-k dielectric with UV curing and the interlayer on low-k dielectric without UV curing, thermal stability was measured at various thermal stress temperatures. Our results indicated that the formation of the barrier at the interface of Cu-Mn alloy/low-k dielectric was enhanced by UV curing due to changes in the porosity and C concentration in the dielectric layer.

Failure Analysis of a Nickel-Plated Electronic Connector Due to Salt-Induced Corrosion (ENGE 2014).

When electronic connectors in mobile devices are miniaturized, the thickness of plating decreases. However, this thin plating is expected to decrease the life of the connector due to problems with corrosion. In this study, salt spray aging tests were performed on miniaturized nickel-plated stainless steel electronic connectors to observe failure mechanisms in realistic environments. The tests were performed three times using a 5% NaCl solution in an atmosphere of 45 °C; each test included several cycles where one cycle was one 24-h period consisting of 8 h of salt spray and 16 h without salt spray. The nickel-plating layers were periodically observed by electron probe X-ray micro-analyzer, wavelength dispersive spectroscopy, and field-emission scanning electron microscopy to analyze and identify the corrosion mechanism. We found that the primary failure mode of the nickel plating is blistering and delamination. The corrosion mechanism is typically a chain reaction of several corrosion mechanisms: pitting corrosion --> stress corrosion cracking --> hydrogen-induced cracking --> blistering and delamination. Finally, we discuss countermeasures to prevent corrosion of the nickel layer based on the corrosion mechanisms identified in this study.

Dual active layer a-IGZO TFT via homogeneous conductive layer formation by photochemical H-doping.

In this study, InGaZnO (IGZO) thin film transistors (TFTs) with a dual active layer (DAL) structure are fabricated by inserting a homogeneous embedded conductive layer (HECL) in an amorphous IGZO (a-IGZO) channel with the aim of enhancing the electrical characteristics of conventional bottom-gate-structure TFTs. A highly conductive HECL (carrier concentration at 1.6 × 10(13) cm(-2), resistivity at 4.6 × 10(-3) Ω∙cm, and Hall mobility at 14.6 cm(2)/Vs at room temperature) is fabricated using photochemical H-doping by irradiating UV light on an a-IGZO film. The electrical properties of the fabricated DAL TFTs are evaluated by varying the HECL length. The results reveal that carrier mobility increased proportionally with the HECL length. Further, a DAL TFT with a 60-µm-long HECL embedded in an 80-µm-long channel exhibits comprehensive and outstanding improvements in its electrical properties: a saturation mobility of 60.2 cm(2)/Vs, threshold voltage of 2.7 V, and subthreshold slope of 0.25 V/decade against the initial values of 19.9 cm(2)/Vs, 4.7 V, and 0.45 V/decade, respectively, for a TFT without HECL. This result confirms that the photochemically H-doped HECL significantly improves the electrical properties of DAL IGZO TFTs.

Effect of Ag nanowire addition into nanoparticle paste on the conductivity of Ag patterns printed by gravure offset method.

This paper focuses on the effect of Ag nanowire addition into a commercial Ag nanopaste and the printability evaluation of the mixed paste by the gravure offset printing methodology. Ag nanowires were synthesized by a modified polyol method, and a small amount of them was added into a commercial metallic paste based on Ag nanoparticles of 50 nm in diameter. Two annealing temperatures were selected for comparison, and electrical conductivity was measured by four point probe method. As a result, the hybrid mixture could be printed by the gravure offset method for patterning fine lines up to 15 µm width with sharp edges and scarce spreading. The addition of the Ag nanowires was significantly efficient for enhancement of electrical conductivity of the printed lines annealed at a low temperature (150 degrees C), while the effect was somewhat diluted in case of high temperature annealing (200 degrees C). The experimental results were discussed with the conduction mechanism in the printed conductive circuits with a schematic description of the electron flows in the printed lines.

Gate insulator effects on the electrical performance of ZnO thin film transistor on a polyethersulphone substrate.

Low temperature processing for fabrication of transistor backplane is a cost effective solution while fabrication on a flexible substrate offers a new opportunity in display business. Combination of both merits is evaluated in this investigation. In this study, the ZnO thin film transistor on a flexible Polyethersulphone (PES) substrate is fabricated using RF magnetron sputtering. Since the selection and design of compatible gate insulator is another important issue to improve the electrical properties of ZnO TFT, we have evaluated three gate insulator candidates; SiO2, SiNx and SiO2/SiNx. The SiO2 passivation on both sides of PES substrate prior to the deposition of ZnO layer was effective to enhance the mechanical and thermal stability. Among the fabricated devices, ZnO TFT employing SiNx/SiO2 stacked gate exhibited the best performance. The device parameters of interest are extracted and the on/off current ratio, field effect mobility, threshold voltage and subthreshold swing are 10(7), 22 cm2/Vs, 1.7 V and 0.4 V/decade, respectively.

Fabrication of an a-IGZO thin film transistor using selective deposition of cobalt by the self-assembly monolayer (SAM) process.

Interest in transparent oxide thin film transistors utilizing ZnO material has been on the rise for many years. Recently, however, IGZO has begun to draw more attention due to its higher stability and superior electric field mobility when compared to ZnO. In this work, we address an improved method for patterning an a-IGZO film using the SAM process, which employs a cost-efficient micro-contact printing method instead of the conventional lithography process. After a-IGZO film deposition on the surface of a SiO2-layered Si wafer, the wafer was illuminated with UV light; sources and drains were then patterned using n-octadecyltrichlorosilane (OTS) molecules by a printing method. Due to the low surface energy of OTS, cobalt was selectively deposited on the OTS-free a-IGZO surface. The selective deposition of cobalt electrodes was successful, as confirmed by an optical microscope. The a-IZGO TFT fabricated using the SAM process exhibited good transistor performance: electric field mobility (micro(FE)), threshold voltage (V(th)), subthreshold slope (SS) and on/off ratio were 2.1 cm2/Vs, 2.4 V, 0.35 V/dec and 2.9 x 10(6), respectively.

Synthesis of titanate nanotube from different phases of TiO2 powders and its hydrogen absorption capacity.

Titanate nanotubes were synthesized by hydrothermal method with different NaOH concentration using various TiO2 powders (P-25, rutile, anatase, and Ni doped TiO2) at 120 degrees C for 24 hrs. At 10 M NaOH, Ni doped TiO2 powders formed the titanate nanotubes which consisted of layered structure such as A2Ti2O5.H2O, A2Ti3O7, H2TiO.H2O (A = Na and/or H) with outer and inner diameter of approximately 10 nm and 6 nm. Ni doped nanotubes absorbed a small amount of hydrogen at 6 and 10 atm, however, uptake of hydrogen was 1.2 wt% at 20 atm.

Structural characterization of titanate nanotubes for lithium storage.

Titanate nanotubes were synthesized by hydrothermal method using various TiO2 precursors as starting materials. The electrochemical properties were investigated by cyclic voltammetric methods. The microstructure and morphology of the synthesized powders were characterized by XRD, TEM. Titanate nanotubes composed of H2Ti2O5 x H2O with outer and inner diameter of approximately 10 nm and 6 nm, and the interlayer spacing was about 0.65 approximately 0.74 nm. Also, the titanate nanotubes showed a discharge capacity of 303 mAh/g and the highest cycle stability because of the open-end and rolled layers with suitable spacing. The relationships between morphology and electrochemical properties have been also discussed.