Published papers

In recent years, graphene growth technology has been greatly progressed, and it is possible to grow graphene with millimeter-size single crystals using metal-catalysis CVD. However, the transfer process is unavoidably which largely deteriorates the graphene. We have proposed direct precipitation method of graphene using a W capping layer. The method does not only require a transfer process, but also the introduction of wrinkles will be suppressed by optimizing the cooling rate at the graphene precipitation. The growth conditions such as thickness of amorphous carbon, annealing temperature, cooling rate and crystallinity of Ni catalyst were systematically changed to study the mechanism of the precipitation method using the W cap layer. By the optimization of the conditions, wrinkle-free multilayer graphene with D/G ratio of less than 0.1 were successfully obtained directly on a sapphire substrate.

Abstract In this study, the direct growth of multilayer graphene from amorphous carbon on a sapphire (0001) substrate by precipitation using a nickel catalyst layer and a tungsten capping layer was examined. The findings revealed that a tungsten carbide layer was formed on top of the catalyst, and this suppressed the diffusion of carbon atoms upwards towards the surface. This caused the graphene layer to precipitate below the catalyst layer rather than above it. Under optimized growth conditions, Raman spectroscopy indicated that a high-quality graphene layer was formed with a low D/G peak intensity ratio of 0.10.

ABSTRACT The mechanism for the precipitation of multilayer graphene was investigated with respect to the use of an Al2O3 barrier layer and Au capping layer. The Al2O3 barrier layer suppresses the dissolution of carbon into the catalyst, especially at low temperature, and assists a decrease in the density of graphene nuclei. On the other hand, the Au capping layer is beneficial to weaken the strong binding between the catalyst and the graphene carbon atoms, and enhances the surface migration of precipitated carbon adatoms. A combination of the Al2O3 barrier layer and Au capping layer is useful for the synthesis of high-quality graphene with large grains. On a sample with both layers annealed for 60 min, the area of 5-layer graphene islands is as large as 10 μm, and covers 60% of the entire surface. The Raman D/G band intensity ratio of 0.024 indicates the precipitated graphene is high quality.

The direct growth of graphene was investigated with precipitating graphene on a GaN template at various temperatures. In the method, a carbon source and catalyst were firstly deposited, and the sample was annealed to precipitate graphene. Tungsten capping layer was deposited on the surface to suppress the graphene precipitating to the sample surface. Consequently, the graphene was precipitated at the interface between the catalyst and the GaN template. After the removal of the catalyst, the graphene was successfully obtained on the GaN template. The Raman D/G ratio of the graphene decreased with increasing the annealing temperature. At 700 °C, fine graphene was obtained on the GaN template while maintaining the flatness and smoothness of the GaN surface. XRD and XPS measurements were also performed to investigate the precipitation of the graphene in detail.

In situ X-ray diffraction measurement was performed to study the precipitation mechanism of graphene from Ni catalyst with and without Ti capping layer using X-ray beam from a synchrotron radiation facility. The graphene precipitated on the surface of the catalyst in the case without the Ti capping layer while it did at the interface between the catalyst and the sapphire substrate in the case with the Ti capping layer. Each process, such as graphene nucleation and precipitation was successfully monitored even though graphene precipitated under the metal catalyst. The detailed mechanisms for both cases were discussed using the experimental results. Consequently, the crystallization of the catalyst is found to effectively suppress the precipitation at low temperature, which is useful to improve the quality of the precipitated graphene. The adjustment of the amount of the carbons is another important factor to precisely control the precipitation because graphene also precipitates after the carbons saturate the catalyst.

Abstract In this study, the growth behavior of Indium gallium nitride (InGaN)-based nanocolumn arrays was investigated, and red emission nanocolumn micro-light emitting diodes (μ-LEDs) were fabricated. The internal structure of the InGaN/GaN superlattice (SL) layer under the multiple-quantum-well (MQW) active layers was evaluated using scanning transmission electron microscopy (STEM) analysis. It was revealed that the InGaN crystal plane at the top of the nanocolumn changed from the c-plane, (1-102) plane, to the (10-11) plane as the number of SL pairs increased. A semipolar (10-11) plane was completely formed on top of the nanocolumn by growing InGaN/GaN SLs over 15–20 pairs, where the InGaN/GaN SL layers were uniformly piled up, maintaining the (10-11) plane. Therefore, when InGaN/AlGaN MQWs were grown on the (10-11) plane InGaN/GaN SL layer, the growth of the (10-11) plane semipolar InGaN active layers was observed in the high-angle annular dark field (HAADF)-STEM image. Moreover, the acute nanocolumn top of the (10-11) plane of the InGaN/GaN SL underlayer did not contribute to the formation of the c-plane InGaN core region. Red nanocolumn μ-LEDs with an φ12 μm emission window were fabricated using the (10-11) plane MQWs to obtain the external quantum efficiency of 1.01% at 51 A cm−2. The process of nanocolumn μ-LEDs suitable for the smaller emission windows was provided, where the flat p-GaN contact layer contributed to forming a fine emission window of φ5 μm.

Abstract A red InGaN-based nanocolumn micro μLED with an emission diameter of ϕ2.2 μm was demonstrated to achieve an on-wafer external quantum efficiency (EQE) of 2.1% at the peak wavelength of 615 nm. The LED was fabricated by repeating the electrode process on the same nanocolumn pattern area and reducing the emission diameter from ϕ80 to ϕ2.2 μm. The peak EQE, which was maximized at ∼25 A cm−2, increased by decreasing the emission diameter from 1.2% to 2.1%. This behavior, which differs from that of InGaN-film LEDs, is characterized as a unit of independent nano-LEDs with passivated sidewalls of nanocolumn LEDs.

InGaN-based light-emitting diodes (LEDs) exhibit excellent emission efficiency in the blue spectral region but experience severe degradation of external quantum efficiency in the red region, primarily due to lattice strain and increased defect density. To address this limitation, we introduced surface plasmon polaritons into InGaN/GaN nanocolumn (NC) arrays arranged in a honeycomb lattice to enhance red-light emission. Deposition of Ag films on the photonic crystal NCs generated plasmonic crystals that allowed precise control of photonic–plasmonic band interactions. Angle-resolved photoluminescence measurements combined with electromagnetic field analysis revealed that strong coupling occurs when photonic and plasmonic field modes are spectrally and spatially matched. Rabi splitting was observed due to the strong coupling between photonic and plasmonic band states, and a 9.6-fold enhancement in red emission was achieved through the combined effects of strong and weak coupling. This controllable strong coupling between photonic and plasmonic band states provides a promising approach for overcoming the intrinsic efficiency limitations of InGaN-based red LEDs while simultaneously enhancing their optical functionality.