**5. Summary**

In summary, the surface structure of sapphire substrate is found to influence the formation of small‐angle grain boundary in subsequently grown AlN epilayer. The small‐angle grain boundary is formed when the surface of the sapphire (0001) substrate is terminated by *ABAB* oxygen stacking with monolayer steps, which is formed during high‐temperature thermal cleaning. To circumvent the small‐angle grain boundary, the LT‐AlN BL is introduced in order to circumvent the substrate structure from having monolayer steps. Rather, the substrate surface produced either rough structure at low BL *T*g or defined two‐ML‐step structure as LT‐ AlN BL *T*g is increased. CL measurement showed an increased emission from the AlN without SAGB. Thus, the LT‐AlN BL technique would be effective in eliminating the SAGB, thereby obtaining high‐quality AlN epilayer with improved optical and electrical properties.
