Hideo Aida

Growth of β-Ga2O3 Single Crystals by the Edge-Defined, Film Fed Growth Method

The successful growth of 2-in. β-Ga2O3 crystals by the edge-defined, film fed growth (EFG) method was demonstrated. The optimization of growth conditions for larger single crystalline β-Ga2O3 is discussed in detail. The seeding conditions of temperature and neck width were found to be the most important factors to grow single crystals. X-ray rocking curve measurements of β-Ga2O3 crystals were conducted to estimate the dislocation densities of the grown crystals. Etch pit densities (EPDs) of the β-Ga2O3 crystals were also measured using KOH solution to measure the dislocation densities. The results were discussed combining with crystal growth parameters such as neck width to clarify the mechanisms of propagation and the origin of dislocations in crystals from phenomenological and crystallographic points of view.

Growth of Thick GaN Layers by Hydride Vapor Phase Epitaxy on Sapphire Substrate with Internally Focused Laser Processing

High-quality thick GaN layers were directly grown by hydride vapor phase epitaxy on sapphire substrates preliminary processed by a laser beam focused within the substrate. The critical thickness for crack generation was improved drastically from 10–20 µm for regular substrates to 220–250 µm by using the laser processed sapphire substrates. Layers of 200 µm thickness grown on the laser processed sapphire substrates exhibited a crack-free surface over the entire 2-in. area and a threading dislocation density as low as 1×107 cm-2, which is one order of magnitude lower than that achievable with a regular sapphire substrate.

III–Nitride Epitaxy on Atomically Controlled Surface of Sapphire Substrate with Slight Misorientation

The importance of the atomically well-controlled surface of sapphire substrate with slight misorientation and ideally minimized surface roughness for III–nitride epitaxy is discussed in detail. An atomically controlled surface of sapphire substrate with slight misorientation angle is modeled and an almost ideal level of atomic surface roughness of sapphire substrate is found to be obtained by a chemical mechanical polishing (CMP) with colloidal silica. Cathodoluminescence (CL) imaging indicated the complete absence of subsurface damage induced by mechanical polishing. GaN and AlN thin films are grown on misoriented sapphire substrate with an atomically controlled surface by the CMP to investigate the misorientation angle of both sapphire and grown GaN and AlN thin films. An interface model is proposed to explain the difference in misorientation angle between sapphire and III–nitride thin films, providing strong evidence of the necessity of atomically controlled surface of sapphire substrate for III–nitride epitaxy.

Reduction of Bowing in GaN-on-Sapphire and GaN-on-Silicon Substrates by Stress Implantation by Internally Focused Laser Processing

Reduction of the bowing of GaN-on-sapphire and GaN-on-silicon substrates was demonstrated with an internally focused laser processing. Stress implantation was successfully achieved inside the sapphire and silicon substrates by the internally focused laser process to compensate for the strain generated by the GaN/sapphire and GaN/Si systems which resulted in substrate bow reduction. This new approach gives us a larger flexibility in the design engineering of epitaxial and device fabrication processes and thus accelerates the realization of a larger diameter device process with GaN-on-sapphire and GaN-on silicon.

Fabrication of freestanding heteroepitaxial diamond substrate via micropatterns and microneedles

The fabrication of a high-quality freestanding diamond substrate was successfully demonstrated via heteroepitaxy by introducing diamond micropatterns and microneedles in the early stage of growth. Micropatterns contributed to a marked reduction in the number of dislocations induced by epitaxial lateral overgrowth, and microneedles relaxed heteroepitaxial strain. Raman spectroscopy indicated the absence of nondiamond carbon inclusions in the obtained freestanding substrate. The full width at half maximum of the X-ray rocking curve for diamond (004) reflections was 0.07°, the lowest value for heteroepitaxial diamond that has been reported so far. The results provide novel insights toward realizing large-diameter single-crystalline diamond substrates.

Effect of Initial Bow of Sapphire Substrate on Substrate Curvature during InGaN Growth Stage of Light Emitting Diode Epitaxy

A systematic study to understand the relationship between wavelength uniformity and substrate curvature during InGaN growth is described in relation to the initial bow of sapphire substrate. The initial bow of the substrate acted as the offset parameter for its curvature throughout the stages of the epitaxy process. Substrate flatness during InGaN growth was found to be important for achieving high wavelength homogeneity. The impact of n-GaN layer thickness and InGaN growth temperature on the substrate curvature shape was investigated to obtain a perfectly flat substrate shape at the InGaN growth stage. Temperature adjustment showed a strong impact on substrate curvature at the InGaN growth stage. The best initial bow was found as a function of InGaN growth temperature, and the importance of the initial bow of the sapphire substrate for obtaining high homogeneity in light emitting diode (LED) wavelength was experimentally verified.

Basic Study on Etching Selectivity of Plasma Chemical Vaporization Machining by Introducing Crystallographic Damage into Work Surface

Plasma chemical vaporization machining (PCVM) is a high-speed plasma etching method using atmospheric-pressure plasma. Although it does not leave an affected layer on the processed surface because of the small ion energy owing to the small mean free path of gas molecules, it is not suitable for planarization because of its isotropic etching. Thus, a combination of PCVM and a mechanical machining process is proposed. The convex parts of a substrate surface are considered to be affected by mechanical machining and are removed preferentially by PCVM. In this report, it is investigated whether etching rate of the affected layer becomes larger or not. As a result, it was found that the etching rate increased in the first 100 nm depth of the mechanically polished substrate, which corresponds to the thickness of the heavily damaged layer observed by cross-sectional transmission electron microscopy.

Effects of atmosphere and ultraviolet light irradiation on chemical mechanical polishing characteristics of SiC wafers

To establish a high-efficiency and high-quality polishing process by controlling the workpiece environment, a prototype chemical mechanical polishing (CMP) machine that can perform double-side CMP simultaneously in a sealed pressure chamber was developed. Using this new machine, polishing experiments on single crystalline silicon carbide (SiC) wafers were carried out. The results showed that applying a highly pressurized O2 gas and ultraviolet light irradiation were effective in SiC CMP.

Effect of Groove Pattern of Chemical Mechanical Polishing Pad on Slurry Flow Behavior

In chemical mechanical polishing (CMP), the slurry flow behavior on the polishing pad is very important both for improving polishing effectiveness and for reducing the slurry consumption. In this study, we aim to evaluate two types of CMP pad that have unique special groove patterns, slurry outflow and inflow pads, for controlling the slurry flow behavior. We describe the effect of the groove patterns on the slurry flow behavior observed using images recorded using a high-speed digital camera. The results of the study indicate several advantages of the proposed pads over the conventional pads from the viewpoint of slurry flow behavior.

Influence of Mis-Orientation of C-plane Sapphire Substrate on the Early Stages of MOCVD Growth of GaN Thin Films

We have studied the early stages of GaN growth to realize the growth mechanism of GaN thin films on mis-oriented sapphire substrates which affects the surface and crystal quality of GaN thin films. As the result, it was found that the larger mis-orientation angle helps the growth of the larger grain of GaN and leads to the earlier shift of growth mode from 3D to 2D. The AFM observation of closed-coalesced GaN thin films revealed the difference in the micro-step structures by the mis-orientation angle of sapphire substrate. The result of x-ray rocking curve as a function of mis-orientation angle well matched with the microstructure of GaN surface, indicating that the larger mis-orientation angle helps the column ordering of GaN crystals.