Effect of Air Layer Thickness on AlN Crystal Growth by the PVT Method

The temperature distribution in the crucible plays a decisive role in increasing the growth rate and improving the quality of AlN crystals. Among the many variables affecting the temperature distribution, the air layer is often neglected. In this article, the influence of temperature and flow fields in crucibles with different air layer thicknesses on AlN crystal growth is investigated by numerical simulation and experimental results for the first time. The results show that the air layer thickness is positively related to the temperature gradient in the crucible. The greater the thickness of the air layer, the slower is the growth rate and gradual the decrease of the convexity of the crystal surface. In addition, it was also found that the vertical shear stress gradually increased in the first 7 h and gradually decreased from 7 to 25 h by the stress analysis. The dislocation density is in line with the distribution of the stresses, and the area of the stresses elevated contacts the tungsten metal. Under the condition of the air layer thickness of 10 mm, the crystal interface is slightly convex, the mass transfer route is shorter, the growth rate is moderate, and the shear stress is small, which are suitable for the growth of small-sized AlN crystals. These results demonstrate the feasibility of the simulation results and are supported by the available experimental data, which will provide important directions for the growth of high-quality AlN crystals.