**Acknowledgements**

and a high‐quality and atomically smooth AlN epilayer was obtained [22]. The same is true in the work of Zhang et al.; however, they introduced the PALE approach to enhance the migration of Al adatoms during the HT‐AlN growth [15]. But Hu et al. modified this method by performing nitridation pretreatment under H2/NH3 ambient prior to LT‐AlN BL growth. However, they ascribed the elimination of rotation domain to the different strain relaxation mechanisms induced by lattice mismatch [21]. In addition, nitridation pretreatment may induce rough AlN surface. Therefore, LT‐AlN BL is a promising technique for eliminating the rotation domain. It is noteworthy that LT BL has been used as a standard technique for the growth of GaN, resulting in crack‐free and high‐quality epilayer [31, 32]. However, to the best of our knowledge, this is the maiden report that clarifies the role of LT‐AlN BL for obtaining

**Pre/post‐treatment technique Suppression of**

As‐received Thermal cleaning [16, 26] ‐ + + As‐received Annealing in air [26] + + ‐ Nitrided Annealing [20] ‐ ‐ + Nitrided Nitrogen radical treatment [19] ‐ ‐ +

**Table 1.** Comparison among several techniques reported for eliminating SAGB in AlN, where "+" sign shows the

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.

**SAGB**

+ ‐

‐ + **Smooth surface** 

+ + +

+ +

+/‐ +/‐ **In situ treatment**

+ +

+ +

high‐quality AlN without SAGB.

54 Study of Grain Boundary Character

As‐received TMA preflow [25]

As‐received LT AlN BL without thermal cleaning (present technique)

NH3 preflow [25]

As‐received LT AlN BL with thermal cleaning [21]

Nitridation and LT AlN BL [21]

satisfied property, and the "‐" sign shows the unsatisfied property.

**Sapphire substrate** 

**5. Summary**

The authors acknowledge Dr. M. Sumiya, Dr. K. Watanabe, and Dr. N. Ishida for their helpful discussions and assistance throughout this work.
