High pressure growth of bulk GaN from solutions in gallium

In this paper, the growth of GaN single crystals from solutions of atomic nitrogen in liquid gallium under high N-2 pressure is described. GaN single crystals obtained by the high nitrogen pressure solution method, without an intentional seeding, show strong growth anisotropy, which results in their platelet shape. The attempts to enhance the growth into (0001) directions by the increase of the integral supercooling in the solution often lead to growth instabilities on both N-polar and Ga-polar (0001) surfaces. This can be avoided only by the precise control of the growth conditions at the crystallization front on the particular surface. The results of the seeded growth in directions parallel and perpendicular to the c-axis of GaN are discussed. In particular, it is shown that dominating mechanisms of the unstable growth on (0001) polar surfaces such as the cellular growth or edge nucleation can be suppressed and the crystal can be grown in a much more stable way. Physical properties most relevant for understanding the growth of GaN crystals are reviewed. The most important feature of GaN crystals grown by the HNPS method is that they are almost free of dislocations and therefore used as substrates give a possibility to grow perfect epitaxial structures.