High-temperature annealing (HTA) is a promising technique to improve the crystalline quality of AlN films. Combining HTA with RF sputtering, Miyake et al. have reported the fabrication of high-quality AlN templates on sapphire [J. Cryst. Growth 456, 155 (2016)], which paves the way to realizing low-cost AlGaN UV optical devices. In terms of AlN growth, DC sputtering is interesting because of its low cost and high throughput. We studied the effect of HTA on the morphology and crystalline quality of DC-sputtered AlN films. As with RF sputtering, HTA substantially improved the crystalline quality of DC-sputtered AlN films. In contrast, hillocks were formed on the surface after HTA, which plausibly stems from the rough surface with spiked structures of the DC-sputtered film. Additional growth of AlN on annealed samples improved the surface morphology and crystalline quality except for the sample containing large hillocks.

The influence of strain relaxation on the optical properties of AlGaN quantum wells (QWs) is investigated. The relaxation ratio is controlled by changing the thickness of an AlGaN buffer layer. The relaxation ratio increases from 20% to 100% with increasing the AlGaN layer thickness from 0.5 to 6.0 μm. Photoluminescence (PL) intensity reaches maximum at a relaxation ratio of 36%, which implies competing effects in radiative and nonradiative recombinations. From the viewpoint of radiative recombination, strain relaxation increases the polarization field in the QW, which decreases the electron–hole overlap. The observed gradual decrease in PL may come from this effect. In terms of nonradiative recombination, the dislocation density is nearly constant up to 4.5 μm from the AlN/AlGaN interface, and decreases at 6.0 μm. Therefore, it is difficult to explain the observed PL intensity by the dislocation‐related nonradiative pathways. Instead, relieved strain can decrease the point‐defect density. Strain relaxation can reduce the vacancy concentration as a result of increased formation energy with the partial relief of compressive strain. The effect of strain on the band structure change is also discussed, where the relieved compressive strain increases the crystal‐field split‐off component in the valence band maximum, resulting in a decreased PL intensity.

Strain plays a crucial role in the performance of ultraviolet (UV) optoelectronic devices. The strain states of AlGaN layers grown on different AlN templates are investigated. Two types of AlN templates are prepared: one is grown solely by metal‐organic chemical vapor deposition (MOCVD), and the other is fabricated with the combination of direct current (DC) sputtering, high‐temperature annealing, and MOCVD growth. The qualities of 4 μm‐thick MOCVD AlN and 1.2 μm‐thick sputter‐based AlN are almost equivalent in terms of dislocation density and surface morphology. However, the strain relaxation ratio of the AlGaN layer is higher for the 1.2 μm sputter‐based AlN than the 4 μm MOCVD AlN, indicating that the strain state depends on the total thickness of the epilayers on the sapphire substrate. In addition, it is found that the curvature of the sample at room temperature is governed mainly by the thickness of the AlGaN layer. As a result, sputter‐based AlN templates allow for the enhanced strain relaxation of the AlGaN layer with a small sample bowing.