Analysis of residual thermal stress for AlN crystal growth by PVT

The residual thermal stress of AlN grown by the PVT method during the cooling process is investigated. Firstly, the stress at the AlN interface is evaluated by measuring Raman frequency shifts and through finite element analysis. The reliability of the calculation model is verified by comparing the results of simulations and experiments. Next, the effect of the geometrical size of AlN and W on the thermal stress is discussed, and the evolution of thermal stress originating from the cooling process is studied. To further reduce the residual thermal stress of AlN, the thermal expansion coefficient of the W holder is adjusted by the addition of elements with high thermal expansion. The results show that the thermal stress concentration at the edges of AlN can be reduced by using a W holder with the same radius as the crystal. When a thinner W holder is used, the thermal stress formed at the AlN interface is lower. The evolution of thermal stress changes from compressive to tensile stress during the whole cooling process. Moreover, the thermal stress of AlN decreases with the increasing concentration of Re and Ta when the W alloy is used as a crystal holder.