The reduction of the dislocation density in a high-temperature (HT)-Al 0:17 Ga 0:83 N epitaxial layer was achieved by a middletemperature (MT)-intermediate layer technique, in which the HT and the MT were 1050 and 950 C, respectively. For one set of the MT-intermediate layer, the structure was 4.5-mm-thick HT-Al 0:17 Ga 0:83 N/1-mm-thick MT-intermediate layer/100-nmthick HT-Al 0:17 Ga 0:83 N layer/low-temperature (LT)-GaNP buffer/trenched (0001) sapphire. The full-width at half maximum values of ( 0002) and ( 101 12) diffraction peaks of the X-ray diffraction for the Al 0:17 Ga 0:83 N epitaxial layer using one set of the MT-intermediate layer were improved to 359 and 486 arcsec compared with 401 and 977 arcsec for the film without the MTintermediate layer technique, respectively. Transmission electron microscopy result showed that the dislocation density in the Al 0:17 Ga 0:83 N film using one set of MT-intermediate layer was reduced from ð1{ 4Þ Â 10 10 to 1:7 Â 10 9 cm À2 . The Al 0:17 Ga 0:83 N epitaxial film including two sets of MT-intermediate layers improved the most, showing a dislocation density of 9:3 Â 10 8 cm À2 .
Al x Ga 1Àx N (x 0:1) epilayers grown on (0001) sapphire substrates by low-pressure metalorganic chemical vapor deposition with low-temperature (LT)-GaNP buffer have been characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). It is found that the full width at half maximum (FWHM) values of the GaNP-buffer-based Al x Ga 1Àx N (x 0:1) epilayers decrease are lower than that of the conventional GaN-buffer-based Al x Ga 1Àx N (x 0:1) epilayers, but the difference becomes smaller as the Al composition increases. These results could be mainly attributed to a longer diffusion length with the GaNP buffer layer than with the GaN buffer layer.
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