Studying the effect of temperature and pressure on GaN crystals via the Na-flux method†

Owing to its wide band gap, extreme mechanical hardness and high thermal conductivity, GaN has found widespread applications in optoelectronic devices and high-power/-frequency devices. However, the growth of high-quality and large-size GaN crystal substrates is still a great challenge, which hinders the development of power and radio frequency (RF) devices. The Na-flux method can emerge as an effective strategy to address these challenges. Nevertheless, the growth quality of GaN crystals is influenced by several factors during the growth process. This study focused on investigating the growth rate and quality of GaN in relation to the growth temperature and pressure. It also explains the difference in the solubility between N³⁻ and GaN as a function of temperature in liquid phase melts. The intricacy of the Na-flux method and the opacity of the growth process present significant obstacles to the growth of GaN crystals. In order to accurately determine and optimise the growth conditions, the temperature distribution and material transport during the growth process are predicted by simulation. A series of validation experiments were conducted to investigate the influence of temperature and pressure on GaN crystallisation. Under optimised growth conditions, high-quality GaN crystals with a full width at half maximum of 433 arcsec (002) were obtained. This work provides an effective strategy for the liquid-phase growth of high-quality GaN crystals, facilitating the development of high-performance blue-green lasers, RF and power devices.