K. H. Yoo

Effect of annealing temperature on the conduction mechanism for a sol–gel driven ZnO Schottky diode

The conduction mechanism of a sol?gel driven ZnO Schottky diode has been investigated with various annealing temperatures. A dense and pore-free ZnO film has been fabricated for a metal?semiconductor?metal structure via the sol?gel method with Zn acetate as a precursor, ethanol as a solvent and ethanol amine as a chelating agent. ZnO films were analysed by means of field emission scanning electron microscopy, ultraviolet visible absorption analysis and Fourier transform infrared (FTIR) analysis. The absorption shoulder appeared and shifted to a longer wavelength as the heating temperature and duration increased. The intensities of FTIR absorption peaks decreased as the heating temperature and duration increased. In particular, the strong and broad absorption peak of ?OH and ?NH at approximately 3400?cm?1 almost disappeared after the heating process. The intensity of the x-ray diffraction peak at 2? = 34? (2?0?0) was strongly dependent on the ZnO film thickness and annealing temperature. Schottky diodes comprising Al/ZnO/Au were fabricated with three different annealing temperatures, i.e. 100 (AZA100), 200 (AZA200) and 300??C (AZA300). The forward current of AZA300 has been increased more than twice with respect to that of AZA100. The plots of Ln(I) versus Ln(V), Ln(I) versus V1/2 and Ln(I/V) versus V1/2 based on the equations of the space charge limited conduction model, the Schottky conduction model and the Poole?Frenkel conduction model were employed to identify the conduction mechanism. All three plots showed a linear relationship. However, the slope and beta values do not match compared with the theoretical values. This result indicates that the conduction process is not a single conduction mechanism. Therefore, the forward current enhancement may be due to the reduction of ?OH and ?NH groups (acting as deep traps) instead of a fundamental conduction mechanism change.

The TiO2 nanoparticle effect on the performance of a conducting polymer Schottky diode

Among the conjugate polymers, poly(3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) has been paid a great deal of attention for various application fields. The absorption intensity of the whole UV-visible range increases linearly, as the concentration of PEDOT:PSS increases. When a small amount of TiO(2) nanoparticles are dispersed in the PEDOT:PSS solution, the absorption in the visible range normally increases, but the UV range absorption (TiO(2) absorption area) is greatly depressed as the concentration of PEDOT:PSS increases. Various weight ratios of TiO(2) nanoparticles in PEDOT:PSS were prepared. The TiO(2)/PEDOT:PSS solution was spin-coated onto the Al electrode and thermally treated to remove water molecules and densify the film. These thermal processes generated nanocracks and nanoholes on the surface of the TiO(2)/PEDOT:PSS film. As the heating temperature increased, wider and longer nanocracks were generated. These nanocracks and nanoholes can be removed by subsequent coating and heating processes. Schottky diodes were fabricated using four different concentrations of TiO(2)-PEDOT:PSS solution. The forward current increased nearly two orders of magnitude by doping approximately 1% of TiO(2) nanoparticles in PEDOT:PSS. Increasing the TiO(2) nanoparticles in the PEDOT:PSS matrix, the forward current was continuously enhanced. The enhancement of forward current is nearly four orders of magnitude with respect to the pristine PEDOT:PSS Schottky diode. The possible conduction mechanisms were examined by using various plotting and curve-fitting methods including a space-charge-limited conduction mechanism [Ln(J) versus Ln(V)], Schottky emission mechanism [Ln(J) versus E(1/2)], and Poole-Frenkel emission mechanism [Ln(J/V) versus E(1/2)]. The plot of Ln(J) versus Ln(V) shows a linear relationship, implying that the major conduction mechanism is SCLC. As the concentration of TiO(2) increased, the conduction mechanism slightly detracted from the ideal SCLC mechanism.

The effect of TiO2 nanoparticle concentration on conduction mechanism for TiO2-polymer diode

Polymer Schottky diodes using p-type poly(3,4-ethylenedioxythiophene:poly (styrenesulfonate) (PEDOT:PSS) doped with various concentrations of n-type TiO2 nanoparticles have been fabricated. Although Al/PEDOT:PSS/Au Schottky diode does not show a clear diode characteristics, the Al∕TiO2-PEDOT:PSS/Au Schottky diode exhibites excellent rectification characteristics. A Schottky diode with highly doped TiO2 nanoparticles (20wt%) shows high forward current having more than three orders of magnitude with respect to pristine PEDOT:PSS Schottky diode. The conduction mechanism of the TiO2 doped Schottky diodes shows best fit of space charge limited conduction process compared to the other mechanisms including Schottky emission and Poole–Frenkel emission.

Direct and indirect contact effect between Al and TiO2 on the conduction mechanism for polymer-TiO2 Schottky diodes

Two kinds of Schottky diodes comprising an of Al/a mixture of TiO2 nanoparticles and poly(3,4-ethylenedioxythiophene) : poly(4-styrenesulfonate) (PEDOT : PSS)/Au and Al/TiO2/PEDOT : PSS/Au were fabricated to investigate the effect of direct and indirect contacts of Al/TiO2 on the conduction mechanism. A mixture of TiO2 nanoparticles and PEDOT : PSS was coated to fabricate an Al/TiO2-PEDOT : PSS/Au Schottky diode (IATPA). TiO2 nanoparticles were directly coated on the Al electrode prior to PEDOT : PSS coating to fabricate an Al/TiO2/PEDOT : PSS/Au Schottky diode (DATPA). The forward current of DATPA increased by more than three-fold with respect to that of IATPA. Three conduction models including space charge limited conduction (SCLC), Schottky conduction and Poole–Frenkel conduction (PFC) models were employed to analyse the conduction mechanism. The major conduction mechanisms for both IATPA and DATPA closely follow the SCLC and PFC models. Higher slope and bigger beta value than those of the theoretical SCLC and PFC models for the DATPA indicate more involvement of the PFC mechanism in SCLC or vice versa. The reason for following the SCLC mechanism might be the low conductive property of PEDOT : PSS and the reason for following the PFC mechanism may be the partial positive and negative charges of the PEDOT : PSS.

Effects of annealing temperature on the performance of the Schottky diode fabricated with TiO2 sol-gel

Various conbinations of TiO2 sol-gel were prepared to fabricate Schottky diodes. Pure TiO2 sol-gel was spin-coated to the various substrates such as glass, silicon wafer, and cellulose. The sol-gel driven TiO2 films were generated cracks all over the surface during the annealing process. To prevent cracks, polyethylene glycol (PEG) was added to TiO2 sol-gel solution. TiO2-PEG sol-gel was spin-coated to the substrate and heat treated at 100, 200, and 300°C for 1 h. The film thicknesses were 230, 190, and 129 nm for the sample heated at 100, 200, and 300°C, respectively, and no cracks were observed. The FTIR pesk at 3380 cm-1 corresponds to -OH stretching mode and disappeared as the heating temperature increased. The characteristic peaks of PEG at 2875 and 1120 cm-1 also disappeared as the heating temperature increased. The Schottky diodes comprised of Al/PEG-TiO2/Au with various heat treatment were fabricated. The forward current was drastically increased as the annealing temperature increased. The plots of parabolic conduction curves based on Schottky conduction model, Poole-Frenkel conduction model, and space charge limitted conduction model show nonlinear relationship. These nonlinear relationship indicates that the conduction mechanism is not purely single conduction mechanism.

Transport mechanisms in polymer and TiO2 Schottky diode

The surface of directly coated TiO2 nanoparticles on the Al-electrode has many nanoscale holes and cracks. The defects of the TiO2 layers can be removed via coating PEDOT:PSS on top of the TiO2 layer. Schottky diodes having various thicknesses of the TiO2 layers and PEDOT:PSS layers were fabricated. The normalized forward current densities were almost the same for the Schottky diodes fabricated with various thicknesses of the TiO2 layers. The current densities of the Schottky diodes with double coatings have large deviation. The log(J) vs. log(V) and shows nonlinear J-V characteristics representing the multiple electron emission mechanisms such as space-charge limited conduction, Poole- Frenkel emission, or thermoionic emission. The plots of log(J) vs. E1/2 and log(J/E) vs. E1/2 show nonlinear behavior and with two distinctive slopes. Based on these results, the electron emission mechanism may have two distinctive mechanisms including Schottky emission mechanism and Poole-Frenkel emission mechanism.

Schottky diode made on cellulose paper with PEDOT:PSS and pentacene

Significant amount of pentacene can be dissolved in N-methylpyrrolidone (NMP) solvent. The solution color changed from deep purple to intense yellow. As the dissolution time increased, UV-visible absorption increased and several new absorption peaks were appeared. The solution was mixed with poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS). PEDOT:PSS or PEDOT:PSS doped with pentacene was spin-coated to the Al coated substrate. Au-electrode was fabricated on top of the semiconductor. Three-layered Schottkys diode comprised of Al, PEDOT:PSS or PEDOT:PSS-pentacene, and Au with thickness of 150nm, 420nm, and 1200nm, respectively were fabricated. The current densities of 4.8μA/cm2 at 2.5MV/m and 440μA/cm2 at 1.9MV/m were obtained for the Au/PEDOT:PSS/Al and Au/PEDOT:PSS-pentacene (3.2 mg)/Al Schottky diodes, respectively. The current density of Schottky diode enhanced about two order of magnitude by doping pentacene to PEDOT:PSS.

Feasibility of e-paper made with cellulose

Cellulose is a beneficial material that has low cost, light weight, high compatibility, and biodegradability. Recently electro-active paper (EAPap) composed with cellulose was discovered as a smart material for application to variety industrial fields such as smart wall-paper, actuator, and magic carpet. It also exhibited actuator property through ion migration and piezoelectric effect. Since cellulose acetate (CA) film has optically transparent property, we focused on optical field application, such as electronic paper, prismsheet, and polarized film. Since CA can be easily dissolved in variety of organic solvent, various weight % (from 1 to 25 wt. %) of CA solution in acetone was prepared. Polydimethylsilane (PDMS) master pattern was fabricated on the silicone wafer. CA solution was poured to the master mold and dried using spin-coating or tape casting method. Various shape and height patterns, such as circle, honeycomb, and rectangular patterns were fabricated using 12 wt. % CA solution. The resulting pattern showed uniform size in the large area without defect. These patterns can be utilized as a substrate and cell pattern for the electronic paper. To investigate saponification (SA) effect to convert CA to regenerated cellulose, CA film was immersed into the sodium methoxide solution in methanol for various times. The fabricated CA films were stretched and immersed into the sodium methoxide solution in methanol to desubstitute the acetate group. These regenerated cellulose films have larger mechanical strength than CA films. Although the UV-visible transmittance was decreased as increasing SA time, the transmittance of the further SA process and stretched film backed up near untreated CA film. Although the cross-sectional image of the saponified and unstretched CA film did not have specific directional structure, the cross-sectional FESEM image of the saponified and stretched CA film had one directional fiber structure. The fiber was aligned to the stretched direction. Most of the compositions were one directional ordered nanofibers having diameter of approximately 30nm.

Micro-contact printing method for metal micro-patterning with PUA

A cellulose solution was prepared using N,N-dimethylacetamide (DMAc), LiCl, and natural pulp. Transparent and smooth surface of the cellulose films were obtained after spin-coating and drying process. The cellulose films can be utilized as a biodegradable and flexible microelectromechanical system (MEMS) due to its electro-active and actuation properties. However, it is difficult to apply conventional lithography process to fabricate MEMS device because of its hydrophilic and flexible nature. Therefore, we applied unconventional lithography process to overcome those problems. Since polydimethylsiloxane (PDMS) has a modulus less than 10MPa, it is not suitable to fabricate high aspect ratio mold. Polyurethaneacrylate (PUA) having a modulus in the range of several hundred was utilized as a mold for micro-contact printing (MCP) process. Although high modulus PUA mold having more than 300MPa had edge defects during the mold-releasing process from the photoresist, the PUA mold having a modulus between 100MPa and 300MPa did not have the edge defect problem. Therefore, PUA mold with a modulus of 200MPa was used in this investigation. Gold was deposited onto the PUA mold, and mercaptopropyltrimethoxysilane (MPTMS) self-assembly monolayer (SAM) was fabricated to the gold surface. The gold was transferred to the cellulose film. The characteristics of the transferred gold electrode on cellulose film were investigated using field emission scanning electron microscope (FESEM).