Sung Mook Chung

Transparent Al–Zn–Sn–O thin film transistors prepared at low temperature

We have fabricated transparent bottom gate thin film transistors (TFTs) using Al-doped zinc tin oxide (AZTO) as active layers. The AZTO active layer was deposited by rf magnetron sputtering at room temperature. The AZTO TFT showed good TFT performance without postannealing. The field effect mobility and the subthreshold swing were improved by postannealing below 180 °C. The AZTO TFT exhibited a field effect mobility (μFET) of 10.1 cm2/V s, a turn-on voltage (Von) of 0.4 V, a subthreshold swing (S/S) of 0.6 V/decade, and an on/off ratio (Ion/Ioff) of 109.

4.3: Transparent ZnO Thin Film Transistor Array for the Application of Transparent AM-OLED Display

We have fabricated 2.15 inch AM-OLED panel composed of 176 × 144 (106 dpi) transparent pixels driven by ZnO-TFT. Transparent ZnO thin film deposited by means of atomic layer deposition at 100°C was used for the active channel. The ZnO TFT had a mobility of 1.13cm2/V.s, a threshold voltage of 0.8V, and an on/off ratio of 106. The fabrication of the ZnO TFT array for AM-OLED display has been carried out with four-mask process. and the integration of the ZnO TFT array and the transparent OLED consisted of a transparent anode and cathode and organic layers led to really transparent display.

Effects of ZnO buffer layer on the luminous properties of thin-film phosphors deposited on ZnO/ITO/glass substrates

ZnGa2O4:Mn and CaTiO3:Pr thin-film phosphors which emit green and red light, respectively, were deposited on ZnO/ITO/glass and ITO/glass substrates by a rf reactive magnetron-sputtering method. To improve photo- and cathodoluminescent properties of the thin-film phosphors, ZnO buffer layers were deposited between the thin-film phosphor and the ITO-coated glass substrate. A transparent c-axis preferentially oriented ZnO thin-film buffer layer could be obtained on the ITO film, while ITO had an amorphous-like structure. ZnGa2O4:Mn and CaTiO3:Pr thin-film phosphors were used to evaluate the effects of the ZnO buffer layer. The crystal structures of thin-film phosphors deposited on ZnO/ITO/glass substrates were developed better than those on ITO/glass, consequently, the former showed photo- and cathodoluminescent intensity at least twice as high as the latter.

P‐18: Improvement of Stability in ZnO TFT Under Bias Stress

The stability of ZnO TFT under bias stress was investigated. Transparent ZnO thin films deposited by means of atomic layer deposition(ALD) and plasma enhanced atomic layer deposition(PEALD) at 100 °C were used as the active channel. The TFT with PEALD grown ZnO layer has better stability under bias stress than the TFT with ALD grown ZnO layer.

A protective layer on the active layer of Al‐Zn‐Sn‐O thin‐film transistors for transparent AMOLEDs

Abstract Transparent top‐gate Al‐Zn‐Sn‐O (AZTO) thin‐film transistors (TFTs) with an Al2O3 protective layer (PL) on an active layer were studied, and a transparent 2.5‐inch QCIF+AMOLED (active‐matrix organic light‐emitting diode) display panel was fabricated using an AZTO TFT backplane. The AZTO active layers were deposited via RF magnetron sputtering at room temperature, and the PL was deposited via two different atomic‐layer deposition (ALD) processes. The mobility and subthreshold slope were superior in the TFTs annealed in vacuum and with oxygen plasma PLs compared to the TFTs annealed in O2 and with water vapor PLs, but the bias stability of the TFTs annealed in O2 and with water vapor PLs was excellent.

P‐8: Effects of Active Thickness in Oxide Semiconductor TFTs

The effects of active thickness on the characteristics of oxide TFTs were investigated for the case of ZnO TFT and IGZO TFT. As the decrease of active thickness, the Von (Turn-on Voltage of TFT) was shifted to near zero Volt and negative shift of Von as decrease of channel length was also diminished. The overall performance (including subthreshold swing and leakage currents) was improved with thinner active layer. This behavior could be explained under the consideration of unintentional n-type doping in active layer. But, for the constant-current stress, thinner active layer TFT showed less stable behavior.

Effects of substrates and heat treatment on growing behavior and luminescent characteristics of ZnGa2O4:Mn thin film

ZnGa2O4:Mn phosphor thin films were deposited on ZnO:Ga/glass, ZnO:Al/glass, ZnO/ITO/glass, and ITO/glass by rf magnetron sputtering. Effects of substrates and heat treatment on the structural and luminescent properties were investigated. The surface morphology, growing behavior, and luminescent properties of thin films depend on the crystallinity of the substrates. Thin film phosphors were heat-treated in a N2+vacuum, vacuum (∼10−3 Torr), and air atmosphere, respectively. The annealing conditions did not affect the surface morphology and crystallinity of the films. However, the Ga/Zn atomic ratios and luminescent characteristics were dependent on these conditions.

Exciton Confinement and Triplet Harvesting for Efficient White Organic Light Emitting Diodes

A blue light emitting diode based on wide band gap host for a blue fluorescence light emitting layer has been developed to fabricate hybrid white organic light emitting diode comprising of a fluorescence blue and a phosphorescence red light emitting layers. We have tested various wide band gap host materials including CBP, BAlq, and TcTa, and among them the device with TcTa host has shown good performance which could be attributed to tight exciton confinement. A phosphorescence red dopant (RD) has been introduced into BAlq layer to harvest triplet states of the fluorescence blue emission layer and we have succeeded in obtaining the balanced emission of the blue and the red emissions with high efficiency. The device showed the maximum external quantum efficiency of 16% at 0.1 mA/cm2 and 13% of external quantum efficiency with (0.29, 0.23) of CIE coordinates at 10 mA/cm2.

66.3: Emission Characteristics of ZnO-incorporated CaTiO3:Pr3+ Phosphor and its Application for Solid-state Lightings

We have investigated ZnO-incorporated CaTiO3:Pr3+, a red phosphor, and its application for the solid-state lighting. Efficient red emission from the phosphor was obtained by the excitation of wavelength 400 nm. Potentially, ZnO-incorporated CaTiO3:Pr3+ can be used as a complementary phosphor in the red region for the white LEDs.

Harvest of Triplet Excitons in Fluorescence Emission Layer based on a wide band gap host of TcTa for Efficient White Organic Light Emitting Diodes

Selective energy transfer from triplet states of the fluorescent blue emission layer to a red phosphorescent dye in a neighbored triplet harvesting layer has been achieved, which has provided improved efficiency with emissions from fluorescent and phosphorescent dyes. First of all, it is crucial to find a wide band gap host for a fluorescent blue emission layer which has larger triplet state band gap than green or red phosphorescent dye. It was found that TcTa is a good wide band gap host for fluorescent blue dopant(BD) and a efficient blue device was obtained. A phosphorescent red dopant (RD) was introduced into a neighboring electron transporting layer to harvest triplet states in the fluorescent blue emission layer and by optimizing the distance between the blue emission layer and the red triplet harvesting layer, we have succeeded in obtaining the balanced emission of the blue and the red emissions with high efficiency from the device structure of NPB/TcTa:BD/BAlq/BAlq:RD/BAlq/LiF/Al. The device showed maximum external quantum efficiency of 16 % at 0.1 mA/cm2 and 13 % of external quantum efficiency, (0.29, 0.23) of CIE coordinates and 920 cd/m2 at 10 mA/cm2. To realize RGB WOLED, a fluorescent green dopant was introduced into the blue emission layer. The RGB WOLED was successfully obtained through optimization of doping concentration for green dopant and it showed 10 % of external quantum efficiency, (0.36, 0.36) of CIE coordinates and 1400 cd/m2 at 10 mA/cm2.