Ammonothermal synthesis of aluminum nitride crystals on group III-nitride templates

Polycrystalline aluminum nitride (AlN) crystals were synthesized using the ammonothermal technique at temperatures between 525°C and 550°C. The growth of AlN was conducted in alkaline conditions with potassium azide (KN^sub 3^) as the mineralizer. The growth mechanism was found to be reverse-gradient soluble, necessitating the placement of the GaN and AlN seeds at a higher temperature than the aluminum metal source. Growth on the GaN seeds varied from 100 to 1500 µm in thickness at a gestation period of 21 days. Additionally, scanning electron micrographs revealed varying microstructure ranging from pointed hexagonal rods, which are approximately 5 µm wide and 20 µm long, to highly densified and contiguous films. Formation of hexagonal AlN was verified using x-ray powder diffraction measurements. Oxygen was detected at 3.7 at.% by inert gas fusion analysis on AlN nucleated on the walls of the autoclave and a qualitative indication of unintentionally incorporated impurities in the AlN grown on the GaN seeds was obtained using energy-dispersive x-ray analysis. Photoluminescence spectroscopy conducted at 20 K revealed a deep-level emission at 3.755 eV due to unintentionally incorporated impurities. [PUBLICATION ABSTRACT]