High-temperature-annealed AlN (HTA-AlN) templates provide ideal substrates for high-quality AlGaN epitaxy. However, the significant compressive stress accumulated within the AlGaN layer makes it challenging to achieve a smooth surface free of hexagonal hillocks on these templates. To address this issue, we investigate the mechanism of compressive stress accumulation during the growth of AlGaN-based epilayers on HTA-AlN templates using in-situ curvature analysis in this study. To verify the mechanism, a low-Al-content AlGaN interlayer is introduced between the AlN epilayer and the subsequent AlGaN epilayer. The larger a-plane lattice constant of this interlayer relative to the AlGaN epilayer slows the accumulation rate of compressive stress. The hexagonal hillock can be effectively suppressed and the surface of AlGaN epilayer can be significantly regulated by adopting various low-Al-content AlGaN interlayers. This work provides a comprehension on the stress accumulation mechanism in AlGaN epilayers and a feasible method to obtain hillock-free surface of AlGaN epilayers on HTA-AlN templates, which will be beneficial for fabricating AlGaN based devices.

A high-speed single-mode vertical-cavity surface-emitting laser (VCSEL) is one of the most important light sources for optical interconnects in data centers. Single-mode VCSEL can improve the transmission distance. In this letter, we demonstrate a single-mode 850nm VCSEL with a bit rate of 60 Gb/s under NRZ modulation and 104 Gb/s under PAM4 modulation across a 100 m length of OM5 fiber, without the need for equalization or a filter. In addition, by using optical injection locking, the 3dB bandwidth is enhanced to 68.5 GHz.