A study on the growth process for liquid phase epitaxy of GaN crystal using Na–Li–Ca flux

The liquid epitaxial growth process on the HVPE-GaN seed of GaN crystal was investigated using the Na–Li–Ca flux method. The phase structure and morphology of as-grown GaN crystals were characterized by X-ray diffraction (XRD) and the scanning electron microscope (SEM), respectively. Under the growth time of 70h, a large area of dissolution morphology could be observed accompanied by a decrease in the thickness of the HVPE-GaN seed. At the bottom of the crucible, the thinning of the seed crystal thickness was the most obvious. For the growth time of 150h, there were two kinds of appearances along the height direction, presenting gathered ridges in the upper part and cellular organizations in the middle and bottom parts. For 420h, the as-grown crystal presented a typical hexagonal layered morphology and the side face developed outward forming regular hexagonal crystallographic planes. With prolonging time from 150h to 420h, the LPE growth shifted from non-equilibrium growth mode with an unstable cellular growth surface morphology to near-equilibrium growth mode with a typical hexagonal layered growth morphology. The thickness of as-grown crystal showed a decrease trend from the gas-liquid interface to the bottom of crucible. To get near-equilibrium growth mode for the whole crystal plate in Na-flux method, the promotion of N dissolving, GaN growth unit diffusion is the key issue.