Mechanisms of crystallization of bulk GaN from the solution under high N2 pressure

The great advantage of the growth of GaN from high temperature (∼1500°C) gallium solutions under high nitrogen pressure (∼10–15 kbar) is that the crystals grown by this method are almost free of dislocations (density of 10–100 cm−2). Typically, the crystals grown under pressure during 150–200 h are relatively thin (about 0.1 mm) hexagonal platelets of 10–14 mm in their lateral size, which indicates strong anisotropy of such growth. The evolution of the crystal morphological forms in time and as a function of temperature distribution allows the discovery of qualitative relations between supersaturation and the nucleation rates on particular growth surfaces. It is of special importance for growth in directions parallel to the c-axis, which is very sensitive to the changes at the crystallization front at the higher supersaturations needed to increase the thickness of the final crystal. This often leads to the growth instabilities which can be avoided by special control of the conditions at the particular growing surface that is achieved by the application of directional methods of crystallization. The possible mechanisms of hindering the lateral growth of the platelets after 150–200 h can be related to changes of the mass transport mechanisms in the solution due to the presence of the crystals. Modeling of the mass transport phenomena in liquid gallium, based on the experimental temperature distribution data, shows that the convectional flows are again quite sensitive to small changes in the temperature gradients in the system. Such changes can be induced by a set of GaN crystals appearing in the solution. Therefore, to increase the lateral size of the crystals, an additional modification of temperature distribution during the crystallization process is necessary.