Growth of bulk single crystal ScAlMgO₄ boules and GaN films on ScAlMgO₄ substrates for GaN-based optical devices, high-power and high-frequency transistors

Single crystal growth of ScAlMgO₄ boules with 10 mm up to 4 in. (c.a. 101.6 mm) in diameter by Czochralski technique was demonstrated. Some high quality ScAlMgO₄ single crystal boules with 2-inches (c.a. 50.8 mm) in diameter were successfully grown without dislocation, which was revealed by X-ray rocking curve and X-ray topography measurements for the c-plane wafers. After a GaN template was formed on a substrate cut out from a grown ScAlMgO₄ boule by using metalorganic vapor phase epitaxy, a thick GaN film was epitaxially grown on the template using hydride vapor phase epitaxy (HVPE). At the cooling stage after the growth in the HVPE process, the thick grown GaN film was separated by itself from the ScAlMgO₄ substrate due to high cleavability of ScAlMgO₄, which makes it possible the reuse of the substrate. The free-standing GaN grown on the reused ScAlMgO₄ substrate showed the same quality as a GaN fabricated on a fresh ScAlMgO₄ substrate. The reuse of ScAlMgO₄ substrates is beneficial for saving the GaN films fabrication cost. The fabrication of a free-standing GaN substrate with 2 in. in diameter and 1 mm in thickness on ScAlMgO₄ substrates was demonstrated to be more high-yield than the fabrication on a sapphire substrate. The growth method shown in this study is very promising, and it opens an efficient way to obtain GaN free-standing wafers and to reduce the production cost of GaN wafers.