Showing papers by "Jaehoon Park published in 2018"

Investigation of the Electrical Characteristics of Bilayer ZnO/In2O3 Thin-Film Transistors Fabricated by Solution Processing

Abstract: Metal-oxide thin-film transistors (TFTs) have been developed as promising candidates for use in various electronic and optoelectronic applications. In this study, we fabricated bilayer zinc oxide (ZnO)/indium oxide (In2O3) TFTs by using the sol-gel solution process, and investigated the structural and chemical properties of the bilayer ZnO/In2O3 semiconductor and the electrical properties of these transistors. The thermogravimetric analysis results showed that ZnO and In2O3 films can be produced by the thermal annealing process at 350 °C. The grazing incidence X-ray diffraction patterns and X-ray photoemission spectroscopy results revealed that the intensity and position of characteristic peaks related to In2O3 in the bilayer structure were not affected by the underlying ZnO film. On the other hand, the electrical properties, such as drain current, threshold voltage, and field-effect mobility of the bilayer ZnO/In2O3 TFTs obviously improved, compared with those of the single-layer In2O3 TFTs. Considering the energy bands of ZnO and In2O3, the enhancement in the TFT performance is explained through the electron transport between ZnO and In2O3 and the formation of an internal electric field in the bilayer structure. In the negative gate-bias stress experiments, it was found that the internal electric field contributes to the electrical stability of the bilayer ZnO/In2O3 TFT by reducing the negative gate-bias-induced field and suppressing the trapping of holes in the TFT channel. Consequently, we suggest that the bilayer structure of solution-processed metal-oxide semiconductors is a viable means of enhancing the TFT performance.

Operational stability of solution-processed indium-oxide thin-film transistors: Environmental condition and electrical stress

Abstract: We investigate the operational stability of bottom-gate/top-contact-structured indium-oxide (In2O3) thin-film transistors (TFTs) in atmospheric air and under vacuum. Based on the thermogravimetric analysis of the In2O3 precursor solution, we utilize a thermal annealing process at 400 °C for 40 min to prepare the In2O3 films. The results of X-ray photoemission spectroscopy and field-emission scanning electron microscopy show that the electron is the majority carrier in the In2O3 semiconductor film prepared by a spin-coating method and that the film has a polycrystalline morphology with grain boundaries. The fabricated In2O3 TFTs operate in an n-type enhancement mode. When constant drain and gate voltages are applied, these TFTs in atmospheric air exhibit a more acute decay in the drain currents with time compared to that observed under vacuum. In the positive gate-bias stress experiments, a decrease in the field-effect mobility and a positive shift in the threshold voltage are invariably observed both in atmospheric air and under vacuum, but such characteristic variations are also found to be more pronounced for the atmospheric-air case. These results are explained in terms of the electron-trapping phenomenon at the grain boundaries in the In2O3 semiconductor, as well as the electrostatic interactions between electrons and polar water molecules.

Electrical stability of solution-processed indium oxide thin-film transistors under illumination stress

Abstract: We investigated the variation in the electrical characteristics of solution-processed indium oxide (In2O3) thin-film transistors (TFTs) under monochromatic illumination conditions at different wave...

Reduction of Hysteresis in Solution-Processed InGaZnO Thin-Film Transistors through Uni-Directional Pre-Annealing

Abstract: The hysteresis of the solution-processed oxide thin-film transistors (TFTs) is fatal issue to interrupt stable operation. So, we came up with uni-directional pre-annealing to solve the problem. There are inevitable defects when solution-processed oxide TFTs are fabricated, due to the porosities by the solvent volatilization. Also oxygen vacancies needed for carrier generation in metal oxide semiconductor can be trap states inducing charge carrier trapping. Uni-directional pre-annealing improved the hysteresis, preventing randomly solvent evaporation and decreased the defects of the film. We can result in advanced stability of the solution-processed oxide TFTs, at the same time showing that the field effect mobility was enhanced from 3.35 cm2/Vs to 4.78 cm2/Vs simultaneously, and exhibiting better subthreshold swing from 0.89 V/dec to 0.23 V/dec.

Load-balanced Resource Directory Architecture for Large-scale Internet of Things Local Networks

Abstract: This paper presents a load-balanced resource directory (LB-RD) architecture based on the constrained application protocol (CoAP), and is aimed at addressing the scalability issue with the centralized resource discovery approach in large-scale Internet of Things (IoT) local networks. To balance the traffic load due to a large number of messages concentrated on the resource directory (RD), the LB-RD serves as an intermediary agent between the servers and the RD. Its architecture includes the following three essential functional blocks: server group management (SGM), message aggregation (MA), and message forwarding scheduling (MFS). The SGM manages the uniform resource identifier (URI) list for a group of servers associated with the LB-RD. The MA aggregates the registration/update messages received from the servers into a single aggregated message. Finally, the MFS determines the forwarding time for the LB-RD to send its aggregated message to the RD. To verify the effectiveness of the LBRD architecture, an experimental simulation is conducted under various scenarios. The results show that the LB-RD architecture achieves better performance than the existing RD approach in terms of the number of lost messages and the message loss rate.

Surface Modification of Solution-Processed ZrO 2 Films through Double Coating for Pentacene Thin-Film Transistors

Abstract: We report the modification of surface properties of solution-processed zirconium oxide (ZrO2) dielectric films achieved by using double-coating process. It is proven that the surface properties of the ZrO2 film are modified through the double-coating process; the surface roughness decreases and the surface energy increases. The present surface modification of the ZrO2 film contributes to an increase in grain size of the pentacene film, thereby increasing the field-effect mobility and decreasing the threshold voltage of the pentacene thin-film transistors (TFTs) having the ZrO2 gate dielectric. Herein, the molecular orientation of pentacene film is also studied based on the results of contact angle and X-ray diffraction measurements. Pentacene molecules on the double-coated ZrO2 film are found to be more tilted than those on the single-coated ZrO2 film, which is attributed to the surface modification of the ZrO2 film. However, no significant differences are observed in insulating properties between the single-and the double-coated ZrO2 dielectric films. Consequently, the characteristic improvements of the pentacene TFTs with the double-coated ZrO2 gate dielectric film can be understood through the increase in pentacene grain size and the reduction in grain boundary density.

Structural Modification of Organic Thin-Film Transistors for Photosensor Application

Abstract: We investigated the light response characteristics of bottom-gate/top-contact organic TFTs fabricated using pentacene and polystyrene as an organic semiconductor and a polymeric insulator, respectively. The pentacene TFT with overlaps (50 μm) between the source and gate electrodes as well as between the drain and gate electrodes exhibited negligible hysteresis in its transfer characteristics upon reversal of the gate voltage sweep direction. When the TFTs were structurally modified to produce an underlap structure between the source and gate electrodes, clockwise hysteresis and a drain-current decrease were observed, which were further augmented by increasing the gate underlap (from 30 μm to 50 μm and 70 μm). Herein, these results are explained in terms of space charge formation and accumulation capacitance reduction. Importantly, we found that space charges formed under the source electrode contributed to the drain currents via light irradiation through the underlap region. Under constant bias conditions, the TFTs with gate underlap structures thus exhibited on-state drain current changes in response to light signals. In our study, an optimal photosensitivity exceeding 11 was achieved by the TFT with a 30 μm gate underlap. Consequently, we suggest that gate underlap structure modification is a viable means of implementing light responsiveness in organic TFTs.

Distributed Sensor Scheduling Scheme Using Euclidean Distance for Large-scale Internet of Things Local Networks

Abstract: In this paper, we present a distributed sensor scheduling scheme based on Euclidean distance (DSS-ED) for Internet of Things (IoT) local networks, with the aim of extending the network lifetime while maximizing the utilization of the limited network capacity of the IoT local network to support various IoT applications. For this purpose, the DSS-ED adapts the state of individual sensor devices by comprehensively considering the characteristics of various variables. In the DSS-ED, each sensor device first calculates the Euclidian distance between the measured variables and their ideal values, then notifies its neighbors of the calculated results. Afterward, the sensor device adaptively determines its own state by comparing its Euclidian distance with those of its neighbors. An experimental simulation is conducted to evaluate the performance of the DSS-ED. The results show that the DSS-ED obtains better performance than the lightweight redundancy-aware topology control protocol (LRTCP) in terms of throughput and energy consumption.

Adaptive Resource Observation for Congestion Alleviation using Constrained Application Protocol

Abstract: This paper presents an adaptive resource observation (ARO) for congestion alleviation using constrained application protocol (CoAP), which prevents buffer overflow of the client by adjusting observing period of the associated servers. The operation of ARO consists of two main phases; 1) buffer overflow estimation, 2) observing period adaptation. In the former, the client estimates whether buffer overflow will occur by comparing its service rate with packet arrival rate, then it determines the new observing period that can prevent buffer overflow of the client. The latter is used to adjust the observing period of servers considering the predefined the minimum and maximum queue threshold. ARO can significantly reduce the number of dropped packets caused by buffer overflow. The simulation results show that ARO achieves a higher network performance than legacy CoAP.