A comparative study of optical confinement and strain in GaN microdisk lasers with semi-suspended and DBR-based structures

In this study, GaN-based microdisk lasers with two different underlying structures, namely, a bottom distributed Bragg reflector (DBR) based structure and a semi-suspended structure formed through partial undercutting of the former, are fabricated from the same epitaxial wafer and systematically compared. The distinct strain distributions, optical confinements, and their combined effects on the lasing characteristics are thoroughly analyzed. The bottom DBRs introduce additional tensile strain in the top GaN layer, effectively mitigating the quantum-confined Stark effect, but provide no significant advantage in whispering-gallery-mode confinement compared to the semi-suspended structure. Owing to the synergistic influence of strain and optical confinement, the semi-suspended devices exhibit lower thresholds, narrower linewidths, shorter lasing wavelengths, and faster stimulated-emission rates.