Yong-June Choi

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.

A simple approach to the fabrication of fluorine-doped zinc oxide thin films by atomic layer deposition at low temperatures and an investigation into the growth mode

A simple low-temperature fabrication of fluorine-doped ZnO (ZnO:F) thin films by atomic layer deposition (ALD) was investigated and the growth mode of the films was analyzed. A novel method for fluorine doping into a ZnO matrix was successfully developed that uses ALD with a home-made fluorine source at a low deposition temperature of 140 °C, which is low enough for films to be applied to a plastic substrate. The fluorine doping concentration was controlled from 0 at.% to 1.2 at.% by manipulating the pulse sequence ratio of the oxygen source (deionized water) and the home-made fluorine source. Structural and morphological properties were investigated by X-ray diffraction and field emission scanning electron microscopy. The growth mode of the films was studied by grazing-incidence wide-angle X-ray diffraction. The grain growth orientation was found to change significantly as the fluorine concentration was increased due to the characteristics of fluorine doping in the oxygen sites of ZnO. This phenomenon could be explained by a perturbation and passivation effect resulting from the fluorine doping mechanism, with the fluorine anions filling oxygen-related defect sites in the ZnO lattice. A photoluminescence study confirmed the diminishment of defect sites and showed that, due to the Burstein–Moss effect, the optical transmittance in the visible region increased significantly with an increase in the fluorine doping concentration from 80.87% to 83.02%. The lowest resistivity was 1.876 × 10−3 Ω cm for ZnO thin films doped with 1.0 at.% fluorine when the carrier concentration and mobility were 1.375 × 1020 cm−3 and 24.20 cm2 V−1 s−1, respectively.

Structural characteristics of Y2O3 films grown on oxidized Si(111) surface

We investigated the characteristics of Y2O3 films grown on an oxidized Si(111) surface, using x-ray diffraction, Rutherford backscattering spectroscopy, and high-resolution transmission electron microscopy. The films grown on the oxidized Si show drastically improved crystallinity, compared with the film grown on clean Si surfaces: channeling minimum yield (Xmin) of 2.5% and full width at half maximum of rocking curve lower than 0.03°. The improvement of the crystallinity was due to the difference of the crystalline structure at the interface between the films grown on the oxidized and clean Si surfaces. Crystalline orientation of Y2O3 islands at the interfacial region was misaligned from the normal substrate direction. The misalignment decreased with increasing the substrate temperature. In particular, the ordering of the oxygen atom in the film grown on oxidized Si was improved compared to that of the Y atom, indicating that the crystallinity of the film is dominantly determined by the arrangement of th...

Effective atomic layer deposition procedure for Al-dopant distribution in ZnO thin films

A zinc-metal dopant-oxygen precursor exposure cycle is demonstrated as a modified deposition procedure for better distribution of Al-dopants in ZnO films by atomic layer deposition with the aim to reduce the formation of nanolaminate thin films that might form with the typically used alternating ZnO and metal oxide deposition procedure. The distribution and chemical bonding states of Al-dopants were studied with various dopant deposition intervals of Zn–Al–O precursor and Zn–O cycles at 1::4, 1::9, 1::14, and 1::19 ratios. The smallest resistivity of Al-doped ZnO film without degradation of transparency was obtained at 250 °C with 5.37×10−4 Ω cm.

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.

The CO gas sensing properties of direct-patternable SnO2 films containing graphene or Ag nanoparticles

The gas sensing properties of direct-patternable SnO2 thin films prepared by photochemical solution deposition were improved by an incorporation of graphene or Ag nanoparticles. The CO gas sensitivity of the SnO2 thin film was 3.65, but increased to 6.84 and 18.06 by the incorporation of graphene and Ag nanoparticles, respectively. Direct-patterning of graphene or Ag nanoparticles incorporated SnO2 thin films can be performed at the 40 μm scale pattern without a photoresist or an etching process.

Non-laminated growth of chlorine-doped zinc oxide films by atomic layer deposition at low temperatures

Chlorine doping in a ZnO matrix to a concentration of 0.65 ± 0.05 at% was accomplished via atomic layer deposition using a home-made chlorine source at a low deposition temperature of 140 °C. Structural and morphological properties were investigated using X-ray diffraction, field emission scanning electron microscopy, and grazing incidence wide-angle X-ray diffraction. The introduction of chlorine into the ZnO matrix resulted in significant grain growth reorientation due to chlorine doping in the oxygen sites of ZnO. Cl− ions preferentially occupied the substitutional O− ion site and O vacancies, and the preferential growth in the {100} planes changed to growth in the {002} planes along the longitudinal direction of the hexagonal wurtzite structure as a function of the Cl doping levels. This important phenomenon was explained by a passivation effect, resulting from the chlorine doping mechanism; this was elucidated using transmission electron microscopy. The optical transmittances of the undoped ZnO and ZnO:Cl films were approximately the same (88%), but the optical band gap was increased by the introduction of a Cl dopant in ZnO due to the Burstein–Moss effect. The lowest resistivity of ZnO:Cl was 1.215 × 10−2 Ω cm, and the corresponding carrier concentration and mobility were 5.715 × 1019 cm−3 and 31.81 cm2 V−1 s−1, respectively. Finally, the calculated doping efficiency of chlorine in ZnO was 10.8%, which was higher than that of aluminum-doped ZnO, even though the deposition temperature was very low when applied to plastic substrates due to the non-laminated growth of ZnO:Cl films.

Investigation of Ag-poly(3,4-ethylenedioxythiophene):polystyrene sulfonate nanocomposite films prepared by a one-step aqueous method

Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) hybrid films containing Ag nanoparticles were synthesized by a one-step aqueous method. The embedded Ag nanoparticles were produced using PEDOT as a reducing agent and PSS as a stabilizer. We have also demonstrated nanoparticle growth with increasing reaction time and silver nitrate content. Finally, as a nanocomposite-containing nanoparticle without surface-capping molecules and embedded in a conducting polymer matrix, the Ag NPs-PEDOT:PSS nanocomposite is interesting because it represents a type of material in which a metallic nano-island is embedded in the semiconducting matrix with a pure interface. One-step synthesized silver nanoparticles were found to induce a lowering of the work function and an increase of the highest occupied molecular orbital level in pristine PEDOT:PSS films by electron exchange with PEDOT, and also induced an inhibition of the electrostatic bond between PEDOT and PSS.

Enhanced hole injection into indium-free organic red light-emitting diodes by fluorine-doping-induced texturing of a zinc oxide surface

We investigated the effect of fluorine (F)-doping-induced texturing of a zinc oxide (ZnO) surface to enhance the hole injection properties of an organic light emitting diode (OLED) device. In this work, 250 nm thick ZnO anodes doped with 0, 0.2, 0.5, and 0.7 at.% F were grown on liquid crystal display (LCD) glass substrates by atomic layer deposition (ALD) at a low temperature (140 °C). The sheet resistance of undoped ZnO and F-doped ZnO (ZnO:F) anodes decreased with increased F-doping in the ZnO matrix. Conversely, the work functions of the doped ZnO anodes gradually increased with the increase in surface texturing caused by an increase in the amount of exposed (100) planes. Finally, the best OLED performance was obtained for a ZnO anode containing 0.5 at.% F; the work function value of this film was 4.74 eV. The highest luminance and current density values (at a bias of 20 V) were optimized to be 13 509.55 cd m−2 and 247.98 mA cm−2, respectively. These superior properties were obtained in an OLED device composed of a DNTPD/TAPC/Bebq2:10%-doped RP-411/Bphen/LiF/Al structure on a ZnO:F anode containing 0.5 at.% F.

Phase selective synthesis of gadolinium silicide films on Si(111) using an interfacial SiO2 layer

We synthesized a single phase GdSi2 film on a Si(111) substrate with an interfacial SiO2 layer. In order to take account of the role of the interfacial SiO2 layer, systematic investigations on clean and oxidized Si substrates were done by using in situ reflection of high energy electron diffraction, x-ray diffraction, and atomic force microscopy of the silicides formed with post annealing. Our result showed that the interfacial SiO2 layer enhanced the structural transformation of the initial GdSi1.7 hexagonal phase into the GdSi2 orthorhombic phase above the decomposition temperature of SiO2(∼800 °C). We proposed a reaction mechanism for the GdSi2 film formation with the help of the interfacial SiO2 layer. The measured electrical resistivity of the Gd-silicide film strongly depends on the silicide phase.