Analysis of GaN crystal growth mechanism in liquid-phase epitaxial Na-flux method

The liquid-phase epitaxial growth (LPE) of GaN crystals on the MOCVD-GaN substrate using the Na-flux method was investigated under various growth times, 30, 60, 100, and 150 h. The morphology properties of as-grown GaN crystals were characterized by the scanning electron microscope (SEM). The variation of nitrogen concentration and supersaturation of GaN in the Ga-Na melt as a function of time is given computationally during the growth time of 150 h, as well as the distribution of nitrogen concentration for 30, 60, 100, and 150 h. In the early term of LPE growth (Stage I), the growth proceeds in a near-equilibrium laminar pattern at lower growth driving forces, corresponding to a screw dislocation growth mechanism. As the growth time extends, the GaN supersaturation in the Ga-Na melt increases with mass transfer continuing, leading to the mechanism of LPE growth changing from screw dislocation growth to continuous hexagonal pyramidal growth with deeper and deeper pit depths, and also thereby leading to the emergence of Stage II. A high level of GaN supersaturation in the solution could promote the growth along the vertical direction of Stage II but is detrimental to the layer growth of Stage I. Broadening the growth of Stage I could realize near-equilibrium growth of GaN crystals and improve the crystalline quality of epitaxial GaN crystals. The specific mechanisms of the different stages of GaN crystal growth were not clarified in the previous work. Through a combination of experimental and simulation calculations, the GaN crystal growth mechanism was discussed. Based on the growth mechanism acquired, the growth process may be effectively regulated to obtain high-quality GaN single crystals.