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The crystal quality evolution of AlN films via high-temperature (HT) annealing under nitrogen is investigated. It is found that the best crystal quality can be realized using an optimized combination of the annealing temperature at 1700 °C and an AlN thickness of 540 nm, respectively. Thus the X-ray diffraction ̉-scan full width at half maximum (FWHM) values of 59 and 284 arcsec for (0002) and (101̅2) diffractions were achieved, respectively. It is verified that the significant reduction in the threading dislocation density (TDD) via HT annealing starts from the interface zone between AlN and sapphire anterior to the zone far from the interface, which is caused by less energy being required to decrease the larger twist angle among the columns in the interface zone. Benefiting from the low-TDD annealed AlN template, the internal quantum efficiency of the 282-nm AlGaN-based multiple quantum wells reached 57% at 300 K. [ABSTRACT FROM AUTHOR]