Seok Keun Koh

Low temperature deposition of ITO thin films by ion beam sputtering

Abstract An ion beam sputtering system was used for the deposition of indium-tin-oxide (ITO) films at low temperatures (below 200°C). The electrical and optical properties and the microstructure were highly dependent on the growth temperature, the oxygen partial pressure and the ion beam energy. A reasonable resistivity (3.5×10 −4 Ωcm) was measured in the films deposited by Ar ion sputtering at as low as 50°C. In the films by Ar ion sputtering, the lowest resistivity was 1.5×10 −4 Ωcm at 100°C. Oxygen addition to the sputtering gas increased the resistivity, especially at low substrate temperatures. The addition of oxygen to the sputtering gas changed the microstructure from ‘domain’ (sub-grain) structure at 100°C to ‘grain’ structure. The oxygen addition induced the change in O/In ratios. The film composition also depended on the ion beam energy. The optical transmittance higher than 80% in the visible range was measured in the films deposited at above 100°C. The optical band gap calculated from the transmittance spectra was approximately 4.2 eV.

Surface modification of polyimide and polysulfone membranes by ion beam for gas separation

The surface carbonization of polyimide (PI) and polysulfone (PSf) by ion beam has been performed to adapt the carbon molecular sieve properties on the skin of the polymeric membranes without the deformation of the membrane structure. In order to control the structure of membrane skin and to improve gas transport properties, the irradiation conditions, such as the dosage and the source of ion beams, have been varied. The ideal separation factor of CO2 over N2 through the surface-modified PI and PSf membranes increased threefold compared to those of the untreated, pristine membranes, whereas the permeability decreased with almost two orders of magnitude. This appears to be due to the fact that the structure of membrane skin has been changed to a barrier layer. The formation of barrier layer was confirmed by comparing the calculated values of a simple resistance model with the experimental results, and the estimated permeability of this barrier was 10−4 barrer. It was concluded that ion beam irradiation could provide a useful tool for improving selectivity for gas separation membranes.

Effects of substrate treatment on the initial growth mode of indium-tin-oxide films

The initial growth mode of indium tin oxide (ITO) on polycarbonate (PC) substrates was investigated. Some of the PC substrates were bombarded by 1-keV Ar ions in an oxygen environment to modify the substrate surface before ITO sputter deposition. The initial part of the film growth was transformed from a three-dimensional island growth to a two-dimensional like growth as a result of the surface treatment. The change of the growth mode was attributed to oxygen-bound functional groups newly formed on the PC surface. Models based on thermodynamic theory and on atomic kinetic approach are presented to explain the transition, respectively.

Reactive ion (N+2) beam pretreatment of sapphire for GaN growth

Abstract Efficiency and lifetime of GaN based light emitting diodes and laser diodes can be improved by proper choice of substrate or deliberate modification of the substrate surface before deposition. Buffer growth or nitridation is the usual choices of surface modifications for GaN deposition to improve crystal quality and optical properties. It was our intention to study a possibility that a reactive ion beam (RIB) pretreatment of the sapphire substrate at room temperature could substitute the nitridation process at high temperature above 1000°C. The optical property of GaN films deposited on the sapphire surfaces pretreated by the reactive ion beam has improved significantly. Current observations clearly demonstrates that the RIB pretreatment of the sapphire surface can be used to improve the GaN films grown by metal-organic chemical vapor deposition (MOCVD).

Influence of seed layers on microstructure and electrical properties of indium-tin oxide films

Films of indium-tin oxide (ITO) were deposited by ion-beam sputtering. Two types of seed layers of ITO were deposited prior to bulk-layer deposition. The types of seed layers were determined by ion species, namely, either pure Ar+ or a mixture of Ar+ and O2+. The microstructure and the preferred orientation of the bulk films mimicked those of the seed layer. Films with larger grains were obtained when the seed layer was used. The electron mobility did not depend on the type of microstructure. The ability to control the microstructure without sacrificing the electrical conductivity was demonstrated.

Surface modification of α-Al2O3(0001) by N2 + ion irradiation

The surface of sapphire most widely used as a substrate for III-V nitride growth was modified by N2+ ion irradiation with ion-beam energies from 100 to 1000 eV. As the ion-beam energy increased from 100 to 500 eV, the root-mean-square roughness of the surface morphology decreased. However no significant change in surface roughness at energy higher than 500 eV was observed. From the N 1s x-ray photoelectron spectroscopy core-level spectra, no peak related to a nitrogen bond could be found in the samples irradiated with ion energy below 500 eV. An AlON peak began to appear in the samples irradiated with 600–900 eV N2+ ions, and two peaks corresponding to AlON and AlN were distinctively observed in the sample irradiated at 1 keV.

Effect of Oxygen Ion Energy and Annealing in Formation of Tin Oxide Thin Films.

Tin oxide ( SnOx ) thin films were deposited by ion-assisted deposition (IAD) at various ion beam voltages (V I) onto amorphous SiO2/Si substrate at room temperature. Tin oxide thin films with nonstoichiometric/stoichiometric composition were fabricated. The as-deposited SnOx films were mostly amorphous, but they exhibited various degrees of crystallinity and fine grain size after annealing at 500° C for 1 h in air. The annealed film deposited using an ion beam energy (E I) of 300 eV showed a preferred orientation along the SnO2 (110) plane. The preferred orientation changed to SnO2 (002) for film 1000 (the annealed film deposited with E I=1000 eV) through an amorphous intermediate structure of film 500 (the annealed film deposited with E I=500 eV). X-ray photoelectron spectroscopy study showed that the main Sn3d peaks in all samples were similar to the binding energy of Sn4+ and the atomic ratios for all the films were higher than 1.51. For the film grown under an average energy of 123 eV/atom, the refractive index was 2.0 and the estimated porosity was 5.2% smaller than that of the other films.

Low Dielectric Constant of MeV Ion-Implanted Poly(vinylidene fluoride)

Poly (vinylidene fluoride)(PVDF) samples were implanted by using high energy (MeV)F2+ and Cl2+ ions. We observed that AC dielectric constant of the ion-implanted PVDF samples decreased from 10.5 to 2.5 at 1 kHz as the ion dosage increased from 1011 to 3 × 1014 ions/cm2. From differential scanning calorimetry experiments, we observed that PVDF samples become more disordered state through the ion implantation. The decrease of the number of bonding of C-H and C-F and the increase of unsaturated bonding were observed from X-ray photoelectron spectroscopy experiments. The emission of HF and H2 molecules during the ion implantation was detected by residual gas analyzer spectrum. Based upon the results, we analyzed that the low AC dielectric constant of the MeV ion-implanted PVDF samples originated from the reduction of polarization due to the structural change of the CF2 molecules in the MeV ion-implanted PVDF samples.

Prepreg Treatment with Ar+ Ion Irradiation to Improve GIcof Unidirectional Graphite/Epoxy Laminated Composites

Currently, many methods are used to improve the fracture toughness of graphite/epoxy composites. However, no work has been done on the prepreg treatment with Ar+ ion irradiation. In the present work, the effect of surface treatment of prepreg by Ar+ ion irradiation on the delamination resistance of unidirectional graphite/epoxy laminated compositeswas studied. The surface of the prepregwas treated with Ar+ ion irradiation under oxygen environment, and the change of contact angle with surface-treatment was measured. Two kinds of [06//06] DCB (double cantilever beam) specimens were made: (1) DCB specimens made of graphite/epoxy prepregs that had been surface-treated with Ar+ ion irradiation under oxygen environment (Laminate A) and (2) DCBspecimens made of regular, untreated graphite/epoxy prepregs (Laminate B). The delamination resistance curve of Laminate A was compared with that of Laminate B so that the effect of prepreg treatment on the resistance behavior could be determined.

Effect of Ar + Ion Irradiation of Polymeric Fiber on Interface and Mechanical Properties of Cementitious Composites

The values of fracture energy and mechanical flexural strength of Fiber Reinforced Cement (FRC) with polypropylene (PP) fiber modified by Ion Assisted Reaction (IAR), by which functional groups were grafted on the surface of PP fiber, was improved about 2 times as those of fracture energy and flexural strength of cement reinforced by untreated PP fiber. PP fiber was irradiated in O₂ environment by Ar + ion. The contact angle of PP treated by IAR decreased largely when compared with untreated PP. From this result, we expected that surface energy and interfacial adhesion force of treated PP fiber increased. The strain hardening occurred in the strain-stress curve of FRC including PP treated by IAR when compared with that of FRC with untreated PP. These enhanced mechanical properties might be due to strong interaction between hydrophilic group on modified PP fiber and hydroxyl group in cement matrix. This hydrophilic group on surface modified PP fiber was confirmed by XPS analysis. We clearly observed hydration products that were fixed at modified PP fiber due to the strong adhesion force of interface in cement reinforced modified PP by SEM (Scanning Electron Microscopy) study.