A loss analysis of blue semipolar (20-2-1) vertical-cavity surface-emitting lasers with ion implanted apertures (IIA) reveals the presence of loss due to absorption in the implant and other absorbing regions. Devices using a buried tunnel junction (BTJ) scheme to confine the current are then analyzed to find the absence of any excess losses. The effect of changing the number of DBR periods on both device types is simulated to give a 70% and a 95% increase in output power for the IIA and BTJ devices, respectively, with the removal of one period from the top DBR at 10 kA/cm2. The mode structure of two different BTJ devices with different index confinements is compared to show that the 0.034 increase in refractive index difference significantly increased the prevalence of higher order modes.
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 23λ 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 μm aperture VCSEL under CW and pulsed operation were 150 μW and 700 μW, 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.
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