Porosity modulation in nanoporous GaN distributed bragg reflectors via tri-subregion si-doping profiles using COMSOL multiphysics

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Abstract A long-standing challenge in visible GaN photonics is the realization of high-reflectivity distributed Bragg reflectors (DBRs) with a few mirror pairs, low optical loss, and compatibility with monolithic integration. Nanoporous GaN DBRs are attractive due to their lattice-matched nature and epitaxy-free fabrication process. However, their performance hinges on silicon (Si) dopant concentration gradients, which govern the resulting porosity and refractive index contrast. In this study, a tri-subregion Si doping modulation strategy is introduced for nanoporous GaN DBRs, in which each porous layer consists of high-porosity, large-pore outer regions surrounding a low-porosity, small-pore central region. This architecture enables controlled modulation of the refractive index while maintaining a compact DBR. Using COMSOL Multiphysics, a three-pair nanoporous GaN/GaN DBR with tri-subregion porous layers is modeled, and the effects of pore-size distribution, edge doping variation, and center doping variation are systematically investigated and compared with a conventional single-doping nanoporous GaN DBR. The optimized configuration achieves a peak reflectivity of 93% at 500 nm and a stopband width of 205 nm, corresponding to an effective refractive index contrast of 0.8, while requiring fewer than five mirror pairs. The graded porosity profile suppresses interface scattering and improves tolerance to structural variations. These results highlight the potential of tri-subregion nanoporous GaN DBRs for green-wavelength resonant-cavity light-emitting devices and GaN-based laser cavities.

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