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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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We have measured the photoluminescence and optical reflection spectra of cubic and hexagonal GaN films epitaxially grown on GaAs and 3C-SiC substrates by gas-source molecular beam epitaxy using microwave-activated NH3 source. The crystalline quality of the epilayers was improved by using an activated NH3 beam and 3C-SiC substrates, which enabled us to obtain intense optical signals. The measured spectra suggest that the band gap of cubic GaN is around 190 meV smaller than that of hexagonal GaN.

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Thick cubic GaN (c-GaN) layers were homoepitaxially grown on c-GaN/(100)GaAs by hydride vapor phase epitaxy (HVPE). The c-GaN crystals used as substrates in this work were prepared by gas source molecular beam epitaxy (GSMBE). When the growth temperature was too low (∼700° C) or too high (∼1000° C), hexagonal GaN (h-GaN) was included in the grown layer, but pure c-GaN was obtained at 900° C. The growth rate of c-GaN by HVPE in this work was about 1.6 µ m/h, which was 4–10 times higher than that of GSMBE or metalorganic vapor phase epitaxy (MOVPE), and an about 5 µ m thick c-GaN film was obtained by 3-h growth. The X-ray diffraction (XRD) patterns showed only the (200) and (400) c-GaN peaks but no h-GaN one. The cathodoluminescence (CL) spectra exhibited a strong peak at about 365 nm, which corresponds to the band edge emission. No emission due to deep levels was observed.

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