Takeshi Eri

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 .

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.