Eungtaek Kim

Metal-containing thin-film encapsulation with flexibility and heat transfer

The thin-film encapsulation (TFE) technology is a salient technique for the realization of flexible organic light-emitting diodes. To reliably fabricate bendable and lightweight displays, ultra-thin and flexible encapsulation is required. Reported herein is a moisture-resistant, flexible, and thermally conductive TFE technology created by inserting a metal thin film with an inorganic–organic multibarrier structure to resolve the reliability and heat dissipation issues. Silica-nanoparticle-embedded sol-gel organic/inorganic hybrid nanocomposite (S-H) and Al2O3 were used as organic and inorganic materials, respectively. A silver (Ag) thin film used as a metal was deposited through thermal evaporation, and it had slight barrier properties, outstanding ductility, and high thermal conductivity. The proposed structure, which consists of three materials, resulted in a low water vapor transmission rate of 10−5 g/m2/day for a 240-nm-thick thin film, and showed improvement of the resistance to bending stress compared with the previous structure formed without an Ag thin film in terms of flexibility. A comparative analysis of the heat transfer properties of encapsulation structures was also performed through the investigation of the thermal conductivity of the materials, and thermal imaging measurement. The heat dissipation performance was confirmed to have been improved by the insertion of Ag thin films into the inorganic/organic multibarrier.

Influence of the charge trap density distribution in a gate insulator on the positive-bias stress instability of amorphous indium-gallium-zinc oxide thin-film transistors

We investigated the positive-bias stress (PBS) instability of thin film transistors (TFTs) composed of different types of first-gate insulators, which serve as a protection layer of the active surface. Two different deposition methods, i.e., the thermal atomic layer deposition (THALD) and plasma-enhanced ALD (PEALD) of Al2O3, were applied for the deposition of the first GI. When THALD was used to deposit the GI, amorphous indium-gallium-zinc oxide (a-IGZO) TFTs showed superior stability characteristics under PBS. For example, the threshold voltage shift (ΔVth) was 0 V even after a PBS time (tstress) of 3000 s under a gate voltage (VG) condition of 5 V (with an electrical field of 1.25 MV/cm). On the other hand, when the first GI was deposited by PEALD, the ΔVth value of a-IGZO TFTs was 0.82 V after undergoing an identical amount of PBS. In order to interpret the disparate ΔVth values resulting from PBS quantitatively, the average oxide charge trap density (NT) in the GI and its spatial distribution were i...

Suppressed Instability of a-IGZO Thin-Film Transistors Under Negative Bias Illumination Stress Using the Distributed Bragg Reflectors

We suggest functional passivation layers in the form of a distributed Bragg reflector (DBR) composed of ZnS and LiF for transparent thin-film transistors (TFTs) to improve the stability under negative bias illumination stress (NBIS). The luminous transmittance of the DBR was 82.0% when the number of dyads was 3.5, and the thicknesses of ZnS and LiF were 42 and 85 nm, respectively. We applied the DBR to TFTs based on amorphous indium-gallium-zinc oxide without the degradation of electrical performance, such as the mobility, ON-OFF ratio, subthreshold swing, and VON. The luminous transmittance of the TFT with the DBR was measured as 71.6%. ΔVON of the TFT with the DBR was reduced to -1.119 V compared with that of the reference TFT, which is -3.261 V when a 1.25 MV/cm electric field was applied, and white light was illuminated during 3000 s. This confirms that the functional passivation layers suggested, in this paper, provide a solution to suppress the instability of TFTs in the NBIS and enhance the optical transmittance of transparent displays.

Electrothermal Annealing (ETA) Method to Enhance the Electrical Performance of Amorphous-Oxide-Semiconductor (AOS) Thin-Film Transistors (TFTs)

An electro-thermal annealing (ETA) method, which uses an electrical pulse of less than 100 ns, was developed to improve the electrical performance of array-level amorphous-oxide-semiconductor (AOS) thin-film transistors (TFTs). The practicality of the ETA method was experimentally demonstrated with transparent amorphous In-Ga-Zn-O (a-IGZO) TFTs. The overall electrical performance metrics were boosted by the proposed method: up to 205% for the trans-conductance (gm), 158% for the linear current (Ilinear), and 206% for the subthreshold swing (SS). The performance enhancement were interpreted by X-ray photoelectron microscopy (XPS), showing a reduction of oxygen vacancies in a-IGZO after the ETA. Furthermore, by virtue of the extremely short operation time (80 ns) of ETA, which neither provokes a delay of the mandatory TFTs operation such as addressing operation for the display refresh nor demands extra physical treatment, the semipermanent use of displays can be realized.

Low-Resistive High-Work-Function Gate Electrode for Transparent a-IGZO TFTs

Highly transparent and low-resistive multilayered gate electrodes, MoO3/indium-tin oxide (ITO)/Ag/ ZnS (MIAZ) playing as the high-work-function layer, the nonreactive interface layer, the lateral conduction layer, and the index-matching layer, respectively, have been investigated for the application to the transparent oxide thin-film transistors (TFTs). The transmittance of the optimized MIAZ electrode is 92.46% and the sheet resistance is 7.77 Ω/□. The top gate InGaZnO TFT with this gate electrode shows the mobility of 11.57 cm2/(V · s) and positive Vth of 0.210 V compared with that with single ITO gate electrode of which Vth is -0.086 V.

Electro-Thermal Annealing Method for Recovery of Cyclic Bending Stress in Flexible a-IGZO TFTs

Amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) fabricated by low-temperature processes on a flexible substrate can easily be degraded by mechanical deformation. Furthermore, lower performance in terms of the initial characteristics and reliability levels compared to those fabricated on glass substrates with relatively high heat treatments is inevitable. To solve these problems, a local electro-thermal annealing (ETA) method was applied to flexible a-IGZO TFTs processed at low temperature to enhance the inferior initial characteristics and reliability under a bending state. The enhancement of the characteristics and reliability by ETA can be attributed to the reduction of defects related to the oxygen through a localized Joule heat treatment with an extremely short duration (~1 ms). In addition, the effectiveness of ETA to recovery from bending stress even under harsh cyclic bending operation (strain condition of 0.833%) is verified.