Jun Sik Cho

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.

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.

Hydrophilic surface formation on materials and its applications

Abstract A new surface modification technique, ion assisted reaction (IAR) has been developed at the Korea Institute of Science and Technology (KIST) ion beam laboratory for modifying the surfaces of polymers. IAR, in which a kiloelectron volt ion beam is irradiated on the surface of a polymer in a reactive gases environment, has been developed for improving wettability of materials and enhancing adhesion to other materials. The contact angles of water drops with modified polymers were reduced more by Ar + ion irradiation with a flowing oxygen gas environment than without flowing oxygen gas. The change in contact angles for the modified polymers was explained by a two-step chemical reaction between the polymer matrix, energetic ions and oxygen gas. X-Ray photoelectron spectrometry (XPS) showed that hydrophilic groups were formed on the surface of polymers by chemical reaction between the unstable chains induced by ion irradiation and the oxygen gas, and the hydrophilic groups were identified as –(CO)–, –(CO)– and –(CO)O– bonds. The enhanced adhesion between metal and modified polymers was explained by the formation of electron acceptor groups in polymer and electron donors in metal.

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.

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.

The role of p-type buffer layers between ZnO:Al and P a-SiC:H for improving fill factor and V oc of a-Si:H solar cells

This study addresses the role of p-type buffer layers between ZnO:Al (AZO) TCO and p a-SiC:H window layer. When AZO is used as a front TCO in conventional a-Si:H solar cells incorporating p a-SiC:H window, the fill factor (FF) significantly decreases in different with SnO2:F (FTO). Various buffer layers with different conductivity and crystalline properties are inserted between AZO and p a-SiC:H and solar cell performances are compared. The FF deterioration of AZO/p a-SiC:H cells is directly related to the interface potential barrier. This potential barrier can be controlled by tuning electron affinity and mobility gap as well as electrical conductivity of buffer layers. The p μc-Si:H buffer improves FF up to 0.72, which results from high conductivity of buffer layer. The p a-Si:H buffers also improves FF although they have similarly low conductivity with p a-SiC:H because of low electron affinity and/or mobility gap.

Study on defects related to local bonding of oxygen in hydrogenated silicon oxide films

In this study, intrinsic a-SiO:H films were prepared by a conventional radio frequency (13.56 MHz) plasma enhanced chemical vapor deposition (PECVD) using a gas mixture of SiH<inf>4</inf>, H<inf>2</inf> and CO<inf>2</inf>. Changes in the optical, electrical and structural properties of the a-SiO:H films were investigated systematically by controlling the deposition parameters, mainly the gas ratio of CO<inf>2</inf> to SiH<inf>4</inf>, and hydrogen dilution concentration. By introducing the CO<inf>2</inf> gas, three kinds of O-related IR features are found at 780, 980 and 2090 cm⁻¹. With increasing the CO<inf>2</inf>/SiH<inf>4</inf> ratio, the absorption at 780 cm⁻¹ strongly coupled to the Si-H and Si-O-Si motions increases noticeably, indicating that the defect density in the films increases because the mode at 780 cm⁻¹ is unique signature of a particularly local geometry related to defects. In order to diminish the defect density level in the films, hydrogen dilution was performed. As the H dilution concentration increases, the defect density is reduced significantly and the photo-conductivity is improved to 10⁻⁴ S/cm. Influences of oxygen incorporation into the Si network and H dilution on the performance of a-SiO:H solar cells are also examined in detail.

Effect of texture morphology on the surface passivation and a-Si/c-Si heterojunction solar cells

The passivation characteristic of c-Si wafer with thin a-Si:H layer plays key parameter in a-Si:H/c-Si hetero-junction (HJ) solar cells. The significance of passivation depends not only on the quality of a-Si layers but also condition of c-Si substrate surface. C-Si substrates are textured to pyramid shape for effective absorption of light using NaOH or KOH with IPA. Textured c-Si substrates are different morphology and height of pyramid according to etching time. In this study, a-Si layers were deposited on c-Si substrates having different height of pyramid by PE-CVD method. High silicon pyramid with long etching time resulted into lower reflectance to illuminated light. This surface reflectance characteristic was closely related with short-circuit current (JSC) of heterojunction solar cell while minority carrier life time (MCLT) decides open-circuit voltage (Voc) and Fill Factor (F.F). Surface texturing of c-SI surface showed effects on both optical and electrical characteristics of heterojunction solar cell.

Silicon quantum dots thin films and superlattice in SiC matrix by co-sputtering of silicon and carbon

Silicon quantum dots (QDs) thin films and superlattice embedded in SiC matrix are prepared by co-sputtering of pure Si and C targets for all silicon tandem solar cell application. The composition of as-deposited Si1−xCx precursor films can be controlled by changing rf power of Si and C targets, respectively. The composition x from 0 to 0.43 is investigated by changing C power from 0 to 400W under the fixed Si power of 200W. In Raman spectrum, Si crystalline volume fraction decreases with increasing the composition and there is no phase transition from x ∼ 0.38. The Si QDs with 5 – 10 nm size are clearly observed through high resolution transmission electron microscopy. From the in-situ annealing and XRD analysis, Si signal precipitates to single crystalline phase above 900°. Si-rich SiC/stoichiometric SiC superlattice shows vague interfaces owing to re-arrangement and inter-diffusion after annealing. The absorption coefficient a of the sample shifts a high energy level and appears at 1.2∼1.4eV conjectured to exist Si quantum dots after annealing.