Takuji Okahisa

Preparation of Large Freestanding GaN Substrates by Hydride Vapor Phase Epitaxy Using GaAs as a Starting Substrate

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 SiO2 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 SiO2 mask. By dissolving the GaAs substrate in aqua regia, a freestanding GaN substrate about 500 µm thick was obtained. The full-width 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 2×105 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 5×1018 cm-3 and 170 cm2V-1s-1, respectively.

Epitaxial growth of cubic and hexagonal GaN by gas source molecular beam epitaxy using a microwave plasma nitrogen source

Abstract Cubic and hexagonal GaN epilayers were successfully grown on GaAs substrates by gas source molecular beam epitaxy using a microwave plasma nitrogen source. These GaN epilayers showed quite strong cathodoluminescence emission. X-ray and electron diffraction analyses indicated the improved crystalline quality, compared with GaN epilayers grown using dimetylhydrazine. GaN epilayers were also grown, by the same method, on 3C-SiC substrates, which have a lattice constant much closer to GaN than GaAs.

Preparation of large GaN substrates

Abstract A freestanding GaN substrate of over 2-in. size with low dislocation density was prepared by hydride vapor phase epitaxy (HVPE) using GaAs (111)A as a starting substrate. A SiO 2 mask pattern with round openings was formed directly onto the GaAs (111)A substrate. Then, a thick GaN layer was grown with numerous large hexagonal inverse-pyramidal pits constructed mainly by {11–22} facets maintained on the surface. After removing the GaAs substrate and subsequent lapping and polishing, a freestanding GaN about 500 μm in thickness was obtained. Etch pit observation reveals that etch pit groups with etch pit density 2×10 8 cm −2 at the center exist in the matrix area with etch pit density as low as 5×10 5 cm −2 . This distribution is due to the effect of large hexagonal pits on collecting dislocations at the bottom of the hexagonal pit. Dislocations propagate into the bottom of the pit mainly in the 〈11–20〉 or 〈1–100〉 direction parallel to (0001).

Homoepitaxial Growth of Cubic GaN by Hydride Vapor Phase Epitaxy on Cubic GaN/GaAs Substrates Prepared with Gas Source Molecular Beam Epitaxy.

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 ? m/h, which was 4?10 times higher than that of GSMBE or metalorganic vapor phase epitaxy (MOVPE), and an about 5 ? 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.