Suitable thickness of the adhesive layer facilitates the release of thermal stress in AlN crystals†

AlN crystals have attracted wide attention because of their excellent optoelectronic properties. However, the huge growth difficulty limits their large-scale application; thus, the main challenge is how to reduce the thermal stress between the AlN seeds and the tungsten holder. The massive thermal stress will lead to the increase of dislocation density and cracks, which seriously affect the quality of AlN crystals. Therefore, this work investigates the relationship between the thickness of the adhesive layer and the thermal stress by combining theory and experiment. The thickness of the adhesive layer was inversely proportional to the thermal stress at the seed in the range of 0.1–0.8 mm, while positively proportional in the range of 0.8–1 mm. The distribution of the threading edge dislocations (TEDs) and threading spiral dislocations (TSDs) is closely correlated with the thermal stress distribution. As a result, when the thickness of the adhesive layer is 0.8 mm, it can release the thermal stress of AlN seeds most effectively to below 10⁶ Pa and reduce the probability of crystal cracking. This work provides a new direction for the quality optimization of AlN crystals and the expansion growth.