Masatomo Shibata

Preparation of freestanding GaN wafers by hydride vapor phase epitaxy with void-assisted separation

We have developed a novel technique for preparing large-scale freestanding GaN wafers. Hydride vapor phase epitaxy (HVPE) growth of thick GaN layer was performed on a GaN template with a thin TiN film on the top. After the cooling process of the HVPE growth, the thick GaN layer was easily separated from the template by the assistance of many voids generated around the TiN film. As a result, a freestanding GaN wafer was obtained. The wafer obtained had a diameter of 45 mm, and a mirror-like surface. The-full-width-at-half-maximum (FWHM) of (0002) and (1010) peaks in the X-ray rocking curve profile were 60 and 92 arcsec, respectively. The dislocation density was evaluated at 5×106 cm-3 by etch pit density measurement.

Thermal and optical properties of bulk GaN crystals fabricated through hydride vapor phase epitaxy with void-assisted separation

The fundamental material parameters associated with GaN, which are important for the design of devices such as light-emitting diodes and laser diodes, were investigated using large high-quality GaN single crystals fabricated through hydride vapor phase epitaxy using the void-assisted separation method. The thermal-expansion coefficients (298–573K) along the C[0001], A[112¯0], and M[101¯0] axes (αC, αA, and αM) were measured. Thermal expansion in each direction, approximately proportional to the temperature, was observed throughout the measured temperature range. Although the thermal-expansion coefficients in the high-temperature range, i.e., αC(573K)=7.2±0.02×10−6∕K, αA(573K)=5.7±0.2×10−6∕K, and αM(573K)=5.8±0.2×10−6∕K,were relatively close to the reported values, the thermal-expansion coefficients along the C axis in the low-temperature range, i.e., αC(298K)=5.3±0.02×10−6∕K, was significantly larger than the reported values. Thermal conductivities parallel and perpendicular to the C axis were almost the ...

Fabrication of Freestanding GaN Wafers by Hydride Vapor-Phase Epitaxy with Void-Assisted Separation

A novel technique for fabricating large-diameter freestanding GaN wafers has been developed. This technique uses a porous GaN template with a TiN nano-net on top as the starting substrate for hydride vapor-phase epitaxy (HVPE) growth. A mechanically weak layer containing numerous small voids is formed between the template and the thick GaN layer during HVPE growth. The thermal stress due to the difference in thermal expansion coefficients between the GaN and sapphire results in self-separation during the cooling process after HVPE growth. As a result, freestanding GaN wafers with a diameter of 45 mm are obtained. These wafers are crack-free and have a mirror-like surface. The full widths at half maximum of GaN (0002) and (1010) peaks in the X-ray rocking curve profile are narrow, i.e. 60 and 92 arcsec, respectively. The dislocation density is low, i.e. 5 × 10 6 cm -2 .

Observation of edge-facets in 〈100〉 InP crystals grown by LEC method

Abstract The growth of edge-facets on 〈100〉 InP crystals has been investigated using microscopy. The solid/liquid interface in the region of edge-facets was found to have a knife-edged shape, while that of a region without facets had a smooth shape. The irregular patterns of edge-facets point to instability in the growth at the periphery of a crystal, and this instability is found to be related to the generation of twins.

Synthesis of gallium nitride by ammonia injection into gallium melt

Abstract Gallium nitride (GaN) was synthesized by injecting ammonia gas into molten gallium at 900–980°C under atmospheric pressure. A large amount of GaN powder was reproducibly obtained using a simple apparatus. The synthesized powder was characterized by scanning electron microscopy, X-ray diffraction, photoluminescence and energy dispersive X-ray spectroscopy, and was found to consist of fine crystals of hexagonal GaN of good quality. The total of GaN obtained was far more than the amount calculated from expected saturation solubility in the Ga melt at that temperature. We speculate that the GaN crystals were largely formed by direct reaction between Ga and the gaseous N source at the surface of the NH 3 bubbles in the melt. GaN synthesized by this method may be useful as a starting material for bulk growth.

LEC growth of large GaAs single crystals

Abstract As the origin of polycrystallization in LEC grown GaAs crystals, accumulation of dislocations has been studied by microscopic observation of polycrystallized crystals. It has been found that dislocations have a tendency to propagate perpendicular to the solid⌜quid interface during growth. The shape of the interface near periphery of the growing crystal is usually concave towards the melt, this concave interface can lead to an accumulation of dislocations. This allocation causes polycrystallization. By control of the growing interface shape, polycrystalline growth has successfully been suppressed. Using this technology, GaAs single crystals of 75 mm diameter and 500 mm long, of 100 mm diameter and 320 mm long, and of 150 mm diameter and 170 mm long have been obtained with good stability.

Role of TiN Film in the Fabrication of Freestanding GaN Wafers Using Hydride Vapor Phase Epitaxy with Void-Assisted Separation

We investigate the role of a TiN film on epitaxial growth and crystal quality in the void-assisted separation (VAS) method. Plan-view TEM images show the TiN film contains numerous nanometer-scale holes, resulting in a nano-net structure. X-ray rocking curve data show the crystal quality of GaN layers on the TiN nano-net is very high, having the tilt/twist angles of 60/90 arcsec. Numerous small GaN islands were generated on the TiN nano-net in the beginning of the growth. These islands having crystal facets may introduce the dislocation bending. We show that the TiN film plays a very important role in improving the crystal quality in the VAS method.

Thermal and Electrical Properties of High-Quality Freestanding GaN Wafers with High Carrier Concentration

The thermal and electrical properties of high-quality bulk GaN crystals fabricated by hydride vapor phase epitaxy were investigated in the high-carrier-concentration region between 1.0×1018 and 1.24×1019 cm-3. The carrier concentration was almost identical to the Si doping concentration. An electrical resistivity as low as 2.5 mΩ cm was obtained at the carrier concentration n=1.24×1019 cm-3. The electron mobilities from Hall measurements were 441 and 200 cm2 V-1 s-1 for n=1.0×1018 and 1.24×1019 cm-3, respectively, which are significantly higher than those reported in the literature. The thermal conductivity measured by the laser flash method was consistently high in the measured carrier concentration range, i.e., about 2.0 W cm-1 K-1 for n=1.0×1018 cm-3 and 1.87 W cm-1 K-1 for n=1.24×1019 cm-3.

Freestanding GaN Wafers by Hydride Vapor Phase Epitaxy Using Void-Assisted Separation Technology

An outline is presented of the fabrication technique of freestanding GaN wafers by hydride vapor phase epitaxy using the void-assisted separation method and the properties of resulting crystals. A thick GaN layer of large area can be separated with excellent reproducibility from a base substrate by the application of thermal stress. This process is assisted by numerous voids formed near the interface between the thick GaN layer and the base substrate. By using this method, high-quality GaN wafers of large area with diameters of over 3 in. have been prepared.