A freestanding GaN substrate over 2 inches in size was successfully prepared for the first time by hydride vapor phase epitaxy (HVPE) using GaAs as a starting substrate. In the experiment, a GaAs (111)A substrate with a SiO 2 mask pattern on its surface was used. A thick GaN layer was grown on the GaAs substrate at 1030 • C through the openings in the SiO 2 mask. By dissolving the GaAs substrate in aqua regia, a freestanding GaN substrate about 500 µm thick was obtained. The fullwidth at half maximum (FWHM) in the ω-mode X-ray diffraction (XRD) profile of GaN (0002) plane was 106 arcsec. The dislocation density of the GaN substrate obtained was determined to be as low as 2 × 10 5 cm −2 by plan-view transmission electron microscopy (TEM). Hall measurements revealed the n-type conductivity of the GaN substrate with typical carrier concentration and carrier mobility of 5 × 10 18 cm −3 and 170 cm 2 ·V −1 ·s −1 , respectively.

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AlN single crystal is one of the promising materials for substrates of GaN-based laser diodes. We prepared aluminum nitride single crystals by the sublimation method and characterized them. The crystals are transparent and slightly yellow. Some crystals are needle-shaped with a hexagonal cross section, diameter of 0.5 mm and length of 3 mm, grown parallel to |001|. Other crystals are plate-shaped with a maximum width of 3 mm, 5 mm length and 0.5 mm thickness, grown with a large (001) face. Also, other crystals are needle-shaped with a rectangular cross section, width of 1 mm, 7 mm length and 0.3 mm thickness, grown with a large (101) face. Their widths of X-ray rocking curves is about 39 arcsec, with a full width at half-maximum, 203 arcsec and 12 arcsec, respectively. The orientation of AlN single crystal axis is sufficient for use in substrates for GaN-based diodes.

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Electron spin resonance was used in the present study to lattice defects in the AlN lattice therefore is important for the development of high-thermal-conductivity AlN. detect lattice defects in an aluminum nitride lattice. TheThe aim of the present paper was to detect lattice defects in ESR spectra were obtained from polycrystalline AlN with polycrystalline AlN with various thermal conductivity values various thermal conductivities. Measurement of the AlN g by using electron spin resonance (ESR) and to investigate the values clearly indicated that the obtained ESR signals arose relationship between the detected defects and the thermal confrom electrons trapped by nitrogen vacancies. The ESR ductivity of the AlN. study revealed that thermal conductivity increases with an increase in the number of electrons trapped by nitrogen vacancies. The explanation for that phenomenon is that the II. Experimental Procedure thermal conductivity of AlN increases with a decreasing concentration of oxygen impurities incorporated into the AlN (1) Processing lattice, and the concentration of nitrogen vacancies changesTwo types of commercial AlN powder were prepared in the inversely with the concentration of oxygen impurities.present study. Powder 1 had a specific surface area of 3.5 m 2 /g and an impurity content of 0.92 wt% O, 300 ppm C, 9 ppm Si, I. Introduction and Ͻ10 ppm Fe. Powder 2 had a specific surface area of 4.0 m 2 /g and an impurity content of 0.90 wt% O, 0.05 wt% C,

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