Hidetoshi Takeda

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.

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.

LEDs on HVPE grown GaN substrates: Influence of macroscopic surface features

We demonstrate the strong influence of GaN substrate surface morphology on optical properties and performance of light emitting devices grown on freestanding GaN. As-grown freestanding HVPE GaN substrates show excellent AFM RMS and XRD FWHM values over the whole area, but distinctive features were observed on the surface, such as macro-pits, hillocks and facets extending over several millimeters. Electroluminescence measurements reveal a strong correlation of the performance and peak emission wavelength of LEDs with each of these observed surface features. This results in multiple peaks and non-uniform optical output power for LEDs on as-grown freestanding GaN substrates. Removal of these surface features by chemical mechanical polishing results in highly uniform peak wavelength and improved output power over the whole wafer area.

Planarization of brittle materials by laser assisted machining

Hard and brittle materials, such as wide-gap semiconductor materials, are difficult to obtain smooth surface by machining. Micro laser assisted machining (micro-LAM) was developed to achieve both smooth and high-speed machining of the hard and brittle materials, which a thermal assisted machining method by a laser irradiation. A purpose of this study is to investigate basic thermal characteristics of the micro-LAM toward high-speed face turning of hard brittle materials. A scratch test with the micro-LAM is applied to a well-known silicon substrate in this study. A relationship between temperature and critical cutting depth (dc) reveals thermal softening effect by micro-LAM and reduction of the crystal orientation dependence of dc. These results suggest that micro-LAM can achieve high efficiency planarization of hard and brittle materials.

Effect of internally focused laser processing of sapphire substrate on bowing management for III-nitride epitaxy

We review a new approach to the bowing management of sapphire substrates for III-nitride epitaxy based on the internally focused laser processing. The laser process modifies the phase of the sapphire, inducing a volume expansion effect that enables the bow to be managed. Bowing control is required in two main areas: 1) control of the initial bow of the sapphire substrate, and 2) reduction in the bow after the epitaxy. The initial bow control was demonstrated for ~250 μm pre-bowed convex and concave sapphire substrates. The effect of the pre-bowed substrate on III-nitride epitaxy was also experimentally verified with an in situ curvature monitoring system during the III-nitride epitaxy; the possibility to accommodate substrate bowing to any target values by applying the initial offset to the substrate was also confirmed. When applied to the substrate after the epitaxy, the same technique was successful in flattening of the substrate for a subsequent chip-fabrication process. This new approach provides wide flexibility in the design engineering of epitaxial and device fabrication processes. Thus, it accelerates the realization of larger diameter device processes with III-nitride/sapphire.

Fabrication of precision microholes and structures with smooth surfaces by a high repetition rate femtosecond laser

Precision micro holes and structures with smooth internal surfaces are introduced. We adopted a high repetition rate femtosecond laser to form optically damaged structures inside a silica glass. The glass is dissolved by HF aqueous solution after the laser irradiation. Through this process, we could obtain micro structures with smooth internal surfaces. We investigated the effects of laser repetition rate on surface roughness. It is suggested that one of the causes increased hole surface roughness is the increase in the defects by the laser irradiation.