Improving thermal resistance in III-nitride blue and UV vertical-cavity surface-emitting lasers

Different mirror concepts are being explored in parallel for III-N vertical-cavity surface-emitting lasers (VCSELs), each with their own pros and cons. A general belief is that epitaxial distributed Bragg reflectors (DBRs) offer a VCSEL with superior thermal performance compared to all-dielectric DBRs. We here show that this is not the case for GaN-based VCSELs designed for 440 nm emission with cavity lengths ≥10λ due to a laterally dominated heat flow caused by the high thermal conductivity of GaN cavity material in contrast to the lower thermally conductive DBRs. If the same cavity design that is used for blue GaN VCSELs is applied to ultraviolet-C (UVC) AlGaN-VCSELs this will lead to detrimentally high internal temperatures (up to 370°C) due to the very low thermal conductivity of AlGaN which would prevent lasing. Increasing the cavity length to 30λ reduces the thermal resistance from 4400 K/W to 2600 K/W, but this is not enough. To drastically lower the internal temperature, we propose adding 300 nm AlN spacer layers to the AlGaN cavity, which reduces thermal resistance to 1100 K/W, which is similar to that of blue VCSELs. The low thermal resistance of this design shows promise for realizing electrically injected, continuous-wave AlGaN-based UVC VCSELs.