The current status of bulk pressure grown GaN substrates and the lasers on them is summarized. Gallium nitride crystals grown from high temperature (~1500 oC) solutions in gallium at high pressure (~10-15 kbar) of N2 gas are usually almost free of dislocations (10-100 dislocations per cm2). Therefore, the construction of blue-violet lasers based on the epitaxial structures grown by both MOCVD and MBE methods on the pressure-grown substrates is possible. The crystals grown routinely by this method are limited in both their lateral size (up to about 10 mm) and their thickness (about 100 μm). Relatively large (exceeding 1 cm) GaN single crystalline platelets can be grown on free gallium surfaces which are in contact with N2 gas. It is shown that for solution growth of GaN on GaN substrates, macroscopically flat and uniform crystallization fronts can be obtained, even for very long processes, by the creation of a convection free zone at the growing surface. Due to a very uniform crystallization front achieved in the high pressure solution growth of GaN on GaN/sapphire substrates, the epitaxial lateral overgrowth (ELOG) became possible. The dislocation densities in the material resulting from lateral growth were in the range of 105-106 cm−2, that is 2 to 3 orders of magnitude lower than in the initial GaN. The GaN platelets grown under pressure were used as substrates for hydride vapor phase epitaxy (HVPE) which allows very fast growth of GaN in the c-direction. In this way single crystals of GaN with a thickness of about 2 mm were grown. These crystals were sliced along non-polar crystallographic planes. Then both {1 1 2 0} (non-polar) and {0 0 0 1} (polar) oriented platelets have been used for the epitaxial growth of GaN/AlGaN multi quantum wells by molecular beam epitaxy (MBE). According to the expectations, much stronger UV emission was obtained from the "non-polar" multi quantum wells at room temperature. The character of the low temperature photoluminescence spectra confirmed high structural quality of these samples.

Keywords: GaN single crystals, high pressure solution growth, HVPE of GaN, GaN based lasers by MOCVD, GaN based lasers by MBE.