Suppression of substrate mode in GaN-based green laser diodes

Parasitic substrate mode readily appears in GaN-based laser diodes (LDs) because of insufficient optical confinement, especially for green LDs. Substrate modes affect the behavior of a LD severely, including the laser beam quality, the optical output power, the longitudinal mode stability, and the maximum modulation speed. In this article, systematic studies on the n-cladding layer (CL) design to suppress the substrate mode of GaN-based green LDs were carried out. We established a contour map to describe the relationship between the optical confinement (determined by the thickness and the refractive index) of n-CL and the substrate mode intensity by simulating the near-field pattern and the far-field pattern. We found that it was difficult to obtain the Gaussian-shape far-field pattern using AlGaN as a cladding layer due to the appearance of cracks induced by tensile strain. However, this can be realized by introducing quaternary AlInGaN as a cladding layer since refractive index and strain can be tuned separately for quaternary alloy.