Ho Jung Chang

Characteristics of the ZnO thin film transistor by atomic layer deposition at various temperatures

ZnO thin films were deposited by atomic layer deposition (ALD) at various temperatures and the resulting electrical and chemical properties were examined. The fraction of O–H bonds in ZnO films decreased from 0.39 to 0.24 with increasing processing temperatures. The O/Zn ratio decreased from 0.90 at 70 °C to 0.78 at 130 °C. The carrier concentration and resistivity changed sharply with decreasing temperature. The ZnO thin film transistors (TFTs) were fabricated at processing temperatures of 70 to 130 °C and the electrical properties of the TFT were as follows: the field-effect mobility ranged from 8.82 × 10−3 to 6.11 × 10−3 cm2 V−1 s−1, the on/off current ratio ranged from 1.28 × 106 to 2.43 × 106, the threshold voltage ranged from −12.5 to 14.7 V and the subthreshold swing ranged from 1.21 to 24.1 V/decade. The electrical characteristics of the ZnO TFT were enhanced as the processing temperature decreased.

Structural and Electrical Properties of ZnO Thin Films Deposited by Atomic Layer Deposition at Low Temperatures

In this study, ZnO thin films were deposited by atomic layer deposition (ALD) at various process temperatures. The purpose of this paper was to investigate the controllability of the preferred orientations of ZnO thin films by varying the process temperature and to determine the effect of the preferred orientations on the electrical properties of the films. The process temperature was varied from 70 to 250°C at several increments while the other ALD process parameters were fixed. The deposition rates and uniformities, crystal structures, and electrical properties of these films were evaluated at the various process temperatures. At process temperatures of 70 and 90°C, ZnO thin films showed strong (002) preferred orientations with cylindrical, fine, columnar crystal structures, almost a 1:1 stoichiometric chemical ratio of Zn to O, and n-type carrier concentrations in the range of 10 16 cm -3 with resistivities of 0.1-1 Ω cm. ZnO thin films deposited at temperatures higher than 110°C had wedge-shaped crystal structures, high oxygen deficiencies, and higher n-type carrier concentrations up to 10 20 cm -3 than the films deposited at lower temperatures.

Improved performance of organic light-emitting diodes fabricated on Al-doped ZnO anodes incorporating a homogeneous Al-doped ZnO buffer layer grown by atomic layer deposition.

In this work, we investigated the use of a homogeneous Al-doped zinc oxide (AZO) buffer layer to improve the performance of an organic light-emitting diode (OLED) device fabricated on an AZO anode. For this, 10-nm-thick AZO buffer layers with Al doping concentrations of 3.1, 4.1, and 5.1 at % were grown on 140-nm-thick AZO anode films containing 2.1 at % Al by atomic layer deposition. The electrical resistivity of the AZO anode with a homogeneous AZO buffer layer decreased with an increase in Al doping concentration up to 4.1 at %; however, the resistivity increased at higher doping concentrations in the AZO buffer layer. On the other hand, the work functions of the AZO anode with the AZO buffer layer containing various Al doping concentrations gradually increased with an increase in Al doping concentration from 3.1 to 5.1 at %. Therefore, the best film properties were obtained for an AZO anode with an AZO buffer layer containing 4.1 at % Al, and the work function value for this film was 4.64 eV. The highest luminance and current efficiency values were optimized to be 20290 cd/m(2) and 13.4 cd/A, respectively, with the OLED device composed of a DNTPD/TAPC/Bebq2:10% doped RP-411/Bphen/LiF/Al structure on an AZO anode with an AZO buffer layer containing 4.1 at % Al.

Triplet host engineering for triplet exciton management in phosphorescent organic light-emitting diodes

The device performances of green phosphorescent organic light-emitting diodes with a triplet mixed host emitting layer were correlated with the energy levels and composition of the host materials. Two hole-transport-type host materials, (4,4′-N,N′-dicarbazole)biphenyl (CBP) and 4,4′,4″-tris(N-carbazolyl)triphenylamine (TCTA), were combined with two electron-transport-type host materials, 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBI) and PH1. The maximum quantum efficiency was obtained in the 5:5 mixed host in the case of TCTA:TPBI and TCTA:PH1, while CBP:PH1 showed the best performances in the 9:1 mixed host. The quantum efficiency of the green mixed host devices was improved by more than 50% compared with that of the corresponding single host devices.

Al2O3 buffer in a ZnO thin film transistor with poly-4-vinylphenol dielectric

We compared the characteristics of bottom-gate ZnO-thin film transistors using poly-4-vinylphenol (PVP) and PVP/Al2O3 dielectrics. The PVP dielectric is more hydrophobic than the PVP/Al2O3 dielectric and is not useful for TFT devices because of its high leakage current density, but this leakage current density can be significantly reduced by inserting Al2O3. We deposited ZnO and Al2O3 films by atomic layer deposition (ALD) because it is a low-temperature process. The ZnO-TFTs with either a PVP or a PVP/Al2O3 dielectric exhibit typical field-effect transistor characteristics with n-channel properties. The ZnO-TFT containing PVP/Al2O3 exhibits clear pinch-off and excellent saturation with an enhanced mode operation. The on/off ratio of 7.9 × 104 for the device containing the hybrid dielectric is about three orders of magnitude higher than the ratio of 47 for the device containing PVP. The subthreshold gate swings are 12 V/decade for the TFT containing PVP and 1.2 V/decade for the TFT containing PVP/Al2O3. The density of the interface trap state is significantly lower in the device containing PVP/Al2O3 than in the ZnO-TFT containing PVP. The saturation mobility was 0.05 and 0.8 cm2 V−1 s−1, respectively, in the TFTs containing PVP and PVP/Al2O3.

Synthesis of SrTiO3:Pr,Al by ultrasonic spray pyrolysis

Abstract SrTiO 3 :Pr,Al as an oxide compound phosphor shows potential application for a field emission display (FED). SrTiO 3 :Pr, Al phosphor was prepared directly by a continuous droplet to particle preparation using an ultrasonic spray pyrolysis. We have studied the processing mechanism of continuous droplet to particle. Powder surface and morphology were investigated as a function of reaction temperature and precursor solution. The powder surface was improved by adding H 2 O 2 because of controlling the decomposition rate. After post-heat treatment for short residence time, the cathodoluminescent measurement was carried out (operational pressure 10 −6 Torr) using an electron beam excitation at 100 μA/cm 2 and the 800 voltage, and we obtained a sharp peak at 615 nm. Our results obtained in this work suggest that SrTiO 3 :Pr,Al phosphors can be used as red phosphors in full-color FED.

Aluminum-doped zinc oxide formed by atomic layer deposition for use as anodes in organic light emitting diodes

Aluminum-doped zinc oxide films produced by atomic layer deposition were investigated for use as anodes in organic light emitting diode (OLED) devices. Al-doped ZnO (AZO) films (∼200 nm thick) were deposited at temperatures of 200, 230, and 260 °C and the AZO film deposited at 260 °C demonstrated carrier mobility, carrier concentration, resistivity, and transmittance values of 16.2 cm2 V−1 s−1, 5.18 × 1020 cm−3, 7.34 × 10−4 Ω cm, and 90%, respectively. OLED devices with a DNTPD/TAPC/Bebq2:10% doped RP-411/Bphen/LiF/Al structure on a glass substrate fabricated using an AZO anode formed at 260 °C showed turn-on voltage, maximum luminance, and current efficiency values of 5.3 V, 16680 cd/m2, and 4.8 cd/A, respectively.

Fabrication and Characterization of Direct-Patternable ZnO Films Containing Pt Nanoparticles

The optical and electrical properties of ZnO films incorporating 0.25 at. % Pt nanoparticles were studied. Pt nanoparticles were synthesized by a methanol reduction method and their size was controlled to 3 nm on average using poly(N-vinyl-2-pyrrolidon) as a protecting agent. The electrical conduction of the ZnO films was enhanced by the introduction of Pt nanoparticles without degradation of the optical transmittance. Well-defined 60-µm-wide direct-patterned ZnO films containing Pt nanoparticles were formed by using photosensitive ortho-nitrobenzaldehyde as a stabilizer to contribute the formation of a crosslinked network structure during a photochemical reaction.

Investigation of the surface chemical and electronic states of pyridine-capped CdSe nanocrystal films after plasma treatments using H2, O2, and Ar gases

Surface chemical bonding and the electronic states of pyridine-capped CdSe nanocrystal films were evaluated using x-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy before and after plasma treatments using H2, O2, and Ar gases from the viewpoint of studying the effects of surface capping organic molecules and surface oxidation. Surface capping organic molecules could be removed during the plasma treatment due to the chemical reactivity, ion energy transfer, and vacuum UV (VUV) of the plasma gases. With O2 plasma treatment, surface capping organic molecules were effectively removed but substantial oxidation of CdSe occurred during the plasma treatment. The valence band maximum energy (EVBM) of CdSe nanocrystal films mainly depends on the apparent size of pyridine-capped CdSe nanocrystals, which controls the interparticle distance, and also on the oxidation of CdSe nanocrystals. Cd-rich surface in O2 and H2 plasma treatments partially would compensate for the decrease in EVBM. After...

Effect of Plasma Treatment of ITO Electrode on the Characteristics of Green OLEDs with Alq3-C545T Emissive Layer

The influence of the plasma treatment of the ITO (Indium Tin Oxide)/glass substrate was investigated in the fabrication of green OLEDs (Organic Light Emitting Devices) using the Alq3-C545T fluorescent system. Various plasma powers of 100 W, 150 W, and 200 W were used under the fixed conditions of an Ar/O2 gas mixing ratio of 0.5 and pressure of 1 m Torr during the plasma treatment. The threshold times required for the inter-insulator width between the subpixel regions to be reduced to 80% of their original value were found to be 4, 3, and 2 minutes at plasma powers of 100 W, 150 W, and 200 W, respectively. The plasma treatment durations at each power were varied from 1 minute up to the threshold time with intervals of one minute. The basic structure of the fabricated devices was 2TNATA/NPB/Alq3-C545T/Alq3/LiF/Al and the best emission characteristics were obtained in the case of the plasma treatment at 150 W for 2 minutes. The luminance and power efficiency of the device treated with the optimum plasma condition were 20000 cd/m2 and 16 lm/W at 10 V, respectively. The peak wavelength and the CIE coordinates were found to be 522 nm and (0.32, 0.63), respectively.