Single-layer graphene is grown directly on Ti-buffered SiO₂ at 100℃. As a result of the AFM measurement of the Ti buffer layer, the roughness of approximately 0.2 nm has been improved. Moreover, the Raman measurement of graphene grown on it shows that the D/G intensity ratio is extremely small, approximately 0.01, and there are no defects. In addition, the 2D/G intensity ratio had a value of approximately 2.1 for single-layer graphene. The sheet resistance is also 89 Ω/□, demonstrating excellent characteristics. The problem was solved by using graphene and a lift-off patterning method. Low-temperature direct-grown graphene does not deteriorate after the patterning process and can be used for device and micro-patterning research.

Methylammonium lead iodide (MAPbI₃) thin films were grown at low temperatures on glass substrates via 3-zone chemical vapor deposition. Lead iodide (PbI₂) and lead bis (dipivaloylmethanate) [Pb(dpm)₂] precursors were used as lead sources. Due to the high sublimation temperature (~400℃) of the PbI₂ precursor, a low substrate temperature could not be constantly maintained. Therefore, MAPbI₃ thin films degraded into the PbI₂ phase. In contrast, for the Pb(dpm)₂ precursor, a substrate temperature of ~120℃ was maintained because the sublimation temperature of Pb(dpm)₂ is as low as 130℃ at a high vapor pressure. As a result, high-quality MAPbI3 thin films were successfully grown on glass substrates using Pb(dpm)₂. The rms (root-mean-square) roughness of MAPbI3 thin films formed from Pb(dpm)₂ was as low as ~19.2 nm, while it was ~22.7 nm for those formed using PbI₂. The grain size of the films formed from Pb(dpm)₂ was as large as approximately 350 nm.

We have investigated the characteristics of amorphous silicon (a-Si) thin-film solar cell by inserting barrier layer. The conversion efficiency of a-Si thin-film solar cells on graphite substrate shows nearly zero because of the surface roughness of the graphite substrate. To enhance the performance of solar cells, the surface morphology of the back side were modified by changing the barrier layer on graphite. The surface roughness of graphite substrate with the barrier layer grown by plasma enhanced chemical vapor deposition (PECVD) reduced from ~2 um to ~75 nm. In this study, the combination of the barrier layer on graphite substrate is important to increase solar cell efficiency. We achieved ~ 7.8% cell efficiency for an a-Si thin-film solar cell on graphite substrate with SiNx/SiOx stack barrier layer.

ZnO thin films have wide application areas due to its versatile properties as transparent conductors, wide-bandgap n-type semiconductors, gas sensor materials, and etc. We have performed a systematic investigation on ultraviolet-assisted CVD (chemical vapor deposition) method. Ultraviolet irradiation during the deposition of ZnO causes chemical reduction on the growing surface; which results in the reduction of the deposition rate, increase in the surface roughness, and decrease of the electrical resistivity. These effects produce larger characteristic variation with various deposition conditions in terms of surface morphology and optical/electrical properties compared to normal CVD deposited ZnO thin films. This versatile controllability of ultraviolet-assisted CVD can provide a larger processing options in the fabrication of nano-structured materials and flexible device applications.

Graphite electrodes are used for secondary batteries, fuel cells, and super capacitors. Research is underway to increase the reaction area of graphite electrodes. In this study, we have investigated the growth properties of carbon nanowall (CNW) according to the ingredient of gas. Microwave plasma enhanced chemical vapor deposition (MPECVD) system was used to grow CNW on Si substrate with a variety of the reaction gas. The planar and vertical growth conditions of the grown CNWs according to the ingredient of the gas were characterized by a field emission scanning electron microscopy (FE-SEM) and energy dispersive spectroscopy (EDS). The electrical characteristics of CNWs were analyzed using a4-point probe.

The carbon nanowall (CNW) is a carbon-based nanomaterials and it was constructed with vertical structure graphenes and it has the highest surface density among carbon-based nanostructures. In this study, we have checked the growth properties of CNW according to the substrate angle. Microwave plasma enhanced chemical vapor deposition (PECVD) system was used to grow CNW on Si substrate with methane (CH4) and hydrogen (H2) gases. And, we have changed the substrate angle from 0° to 90°in steps of 30°. The planar and vertical conditions of the grown CNWs according to the substrate angle were characterized by a field emission scanning electron microscopy (FE-SEM) and energy dispersive spectroscopy (EDS). In case of the growth angle increases, our experimental results showed that the length of the CNW was shortened and the content of carbon component was decreased.

Thermoelectric bismuth telluride (Bi2Te3) films were deposited on 4° off oriented (001) GaAs substrates using a modified metal organic chemical vapor deposition (MOCVD) system. The effects of substrate temperature on surface morphologies, crystallinity, electrical properties and thermoelctric properties were investigated. Two dimensional growth mode (2D) was observed at substrate temperature lower than 400℃. However, three dimensional growth mode (3D) was observed at substrate temperature higher than 400℃. Change of growth mechanism from 2D to 3D was confirmed with environmental scanning electron microscope (E-SEM) and X-ray diffraction analysis. Seebeck coefficients of all samples have negative values. This result indicates that Bi2Te3 films grown by modified MOCVD are n-type. The maximum value of Seebeck coefficient was -225 μV/K and the power factor was 1.86×10-3 W/mK2 at the substrate temperature of 400℃. Bi2Te3 films deposited using modified MOCVD can be used to fabricate high-performance thermoelectric devices.