Strain-controlled impurity-induced disordered apertures for high-power single-mode VCSELs

Impurity-induced disordering in vertical-cavity surface-emitting lasers (VCSELs) has demonstrated enhanced performance such as higher modulation speeds, reduced series resistance, and higher-order mode suppression for singlemode operation. Initiated by the diffusion of Zn, impurity-induced disordering intermixes discrete AlGaAs-based distributed Bragg reflectors (DBR) pairs which leads to lower mirror power reflectivity and increased optical loss. When formed into an aperture where the center is non-disordered, suppression of higher-order transverse modes for high-power single-mode operation can be achieved. For maximal mode suppression, deep disordering apertures are desirable. However, due to the isotropic nature of diffusion, these apertures are limited to the lateral diffusion encroaching onto the fundamental mode. By tailoring the film stress of the SiNx diffusion mask, the capability to modify the diffusion front of the disordering aperture is demonstrated. Defined by their lateral-to-vertical (L/V) diffusion ratios, an L/V ratio of 3.7 to 0.90 is measured for corresponding SiNx diffusion mask strains ranging from a compressive -797 MPa to a tensile +347 MPa. This demonstrates that tensile strained diffusion masks limit the amount of lateral diffusion. To further reduce the lateral encroachment, increasingly tensile diffusion masks are deposited by modifying the SiH4/NH3 flow ratios. This diffusion mask is employed to fabricate high-power single-mode VCSELs designed for 850 nm emission. Compared to VCSELs fabricated with non-optimized disordering apertures, enhanced transverse-mode control is achieved and singlemode output power in excess of 3.8 mW with a side mode suppression ratio greater than 30 dB is measured.