Nanostructured Isolation Layer for Enhanced III-Nitride Photonics Performance

Operation in the visible and ultraviolet frequencies are enabled by III-nitride based nanophotonics that are important for many applications like solid-state lighting, lasers, flat optics and quantum information science. A low refractive index interface cladding region between the high refractive index substrate GaN and the active layer enables the confinement of electromagnetic field in the active layer leading to superior performance. While there are some examples in III-nitrides of low threshold lasing in nanowire array geometry photonic crystals [Sci. Rep. <b>2013</b>, 3, 2982] achieving simple optical isolation for membrane PhCs, metasurfaces, and VCSEL structures has been a challenge. In silicon photonics, natural oxide (SiO₂) provides such isolation from the substrate and similarly in III-V semiconductors active GaAs region can be isolated from the substrate by oxidizing an Al_xGa_1-xAs buffer layer with high Al content. The oxidized Al_xGa_1-xAs has lower refractive index (~ 1.7) than GaAs (3.5). Since GaN and other III-nitrides do not possess naturally low refractive index oxide other approaches are required to address this challenge. A nanoporous layer with an effective low refractive index formed by electrochemical etching in a highly doped layer of GaN shows to be a promising approach [App. Phys. Lett. <b>2018</b>, 112, 041109]. Here we will discuss achieving optical substrate isolation in III-nitride nanowire array-based resonators using electrochemically etched nanoporous layer as well as creating an undercut layer below the active layer. Optical reflectance characterization of the nanowire resonators exhibits the emergence of distinct magnetic and electric dipole resonances depending upon the refractive index of the isolation layer which can be used to characterize the refractive index of the isolation layer.<br/>Sandia National Laboratories is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.