Continuous-wave operation of nonpolar GaN-based vertical-cavity surface-emitting lasers

This is the first demonstration of continuous-wave (CW) operation of nonpolar GaN-based VCSELs. These devices had a dual-dielectric distributed Bragg reflector (DBR) design with ion implanted apertures and III-nitride tunnel junction (TJ) intracavity contacts. Unlike c-plane devices, nonpolar GaN-based VCSELs have anisotropic gain that leads to a 100% polarization ratio and polarization-locked VCSEL arrays. Previous nonpolar devices were unable to lase under CW operation, notably due to the thermally-insulating bottom dielectric DBR. Based on thermal modeling using COMSOL, the main thermal pathway was restricted to a thin p-side metal contact that goes around the bottom DBR to the submount. Heat flow was further impaired as the Au-Au thermocompression flip-chip bond created cracks and voids in the p-side metal. The thermal performance was improved in our latest VCSELs by increasing the cavity length to 23 lambda and utilizing Au-In solid liquid interdiffusion bonding to create a more robust pathway for heat transport. This led to stable CW VCSEL operation for over 20 minutes. The peak output powers for a 6 mu m aperture VCSEL under CW and pulsed operation were 150 mu W and 700 mu W, respectively. Lasing wavelengths were observed at 406 nm, 412 nm, and 419 nm. The fundamental transverse mode was observed without the presence of filamentary lasing.