Dooyong Lee

Oxygen partial pressure dependent electrical conductivity type conversion of phosphorus-doped ZnO thin films

In this study, the oxygen partial pressure dependent physical properties of phosphorous-doped ZnO thin films were investigated. All thin films, grown on Al2O3(0 0 0 1) substrates using pulsed laser deposition, exhibited (0 0 2) orientation regardless of the oxygen partial pressure. However, as the oxygen partial pressure increased, the degree of crystallinity and the concentration of oxygen vacancies in the films decreased. All the thin-film samples showed n-type characteristics except for a sample grown at 100 mTorr, which exhibited p-type characteristics. The optical band gap energy also changed with the oxygen partial pressure. The feasible microscopic mechanism of conductivity conversion is explained in terms of the lattice constant, crystallinity, and the relative roles of the substituted phosphorous in the Zn-site and/or oxygen vacancies depending on the oxygen partial pressure.

Atomistic aspects of carrier concentration variation in post-annealed indium tin oxide films

Post-annealing environment-dependent optical and electrical properties of indium tin oxide films grown on glass were examined. X-ray diffraction measurements revealed that all of the films exhibited poly-crystallinity after annealing at 400 °C for 10 min O2, in-air and N2. The optical property measurements yielded >80% transmittances for all the films except for the as-grown and O2-annealed films, even though there were no significant optical band-gap energy differences. In the Hall measurements, all of the films exhibited n-type characteristics. However, the film annealed under the N2 environment showed the best electrical properties (highest carrier concentration and conductivity). The physical origin of electrical property variations due to annealing environment differences was explained by examining the core-level x-ray photoelectron spectra.

Ultrasonic-Assisted Spin-Coating: Improved Junction by Enhanced Permeation of a Coating Material within Nanostructures

Over the last decades, the spin-coating (SC) technique has been widely used to prepare thin films of various materials in the liquid phase on arbitrary substrates. The technique simply relies on the centrifugal force to spread a coating solution radially outward over the substrate. This mechanism works fairly well for solutions with low surface tension to form thin films of reasonable junctions on smooth substrates. Here, we present a modified SC technique, namely, ultrasonic-assisted spin-coating (UASC), to form thin films of coating solution having high surface tension on rough substrates with excellent junctions. The UASC technique couples SC with an external ultrasonic wave generator to provide external perturbation to locally break down big drops of the coating material into smaller droplets via Rayleigh instability. Because of their lower mass, these tiny droplets gain low momenta and move slowly both in radial and azimuthal directions, giving them an enough time to effectively permeate within pores, thereby yielding excellent junctions. Furthermore, we also investigated the effect of junction improvement on conventional and inverted bulk heterojunction organic solar cells. Intriguingly, the organic solar cells fabricated by the UASC method showed an improved efficiency compared to typical SC owing to efficient charge transfer across the junction. These results clearly imply that UASC is a simple and powerful technique which can significantly enhance the device performance by improving the junction. Moreover, we believe that UASC can be more effective for the preparation of devices composed of multilayers of different materials having complicated nanostructures.

The Optimization of Working Gas Composition for High-Efficiency PDP Discharges

Summary form only given. A plasma display panel (PDP) is one of the best candidates for a large-scale flat panel display using plasma technology for high definition TV. The gas mixture ratio of PDP discharge plays a very important role in several issues in AC PDP researches such as lowering operating voltage for cost reduction, increasing voltage margin for stable operation, and enhancing brightness and luminous efficiency for good image quality. In this study, experimental measurements are reported for firing and sustain voltages, voltage margin, luminance, and luminous efficiency with the change of the mixture ratio of Ne-Xe-He noble gases. In a Ne-Xe binary gas mixture, the increase of Xe contents results in the increases of luminance and luminous efficiency while it also results in the increase of the breakdown voltage and the discharge time lag. With the inclusion of He gas, the discharge time lag decreases, but the luminance and the luminance efficiency changes differently for high Xe and low Xe cases. With a low gas mixture ratio of Xe (less than 8%), the luminance efficiency increases with the amount of He gas at the same total pressure. With a high Xe partial pressure (more than 10%), however, there is an optimal value for He contents above which the luminance efficiency decreases as He partial pressure increases. From the experimental results, it was observed that a Ne:He mixture ratio of 9:1 yields the most efficient discharge characteristics for a standard AC PDP cell with Xe gas fraction of 10-30% at the total pressure of 400-500 torr. The experimental results are also compared well with those of a two-dimensional fluid simulation. A discharge road map is generated from the experimental and simulation results in order to explain the behaviors of firing and sustain voltages, discharge time lag, luminance, power loss, and luminance efficiency. Furthermore, experimental results are reported for the effect of other gases, such as Kr, Ar, and H2 , on luminance efficiency