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Two routes for the fabrication of bulk GaN microcavities embedded between two dielectric mirrors are described, and the optical properties of the microcavities thus obtained are compared. In both cases, the GaN active layer is grown by molecular beam epitaxy on (111) Si, allowing use of selective etching to remove the substrate. In the first case, a three period Al0.2Ga0.8N/AlN Bragg mirror followed by a λ/2 GaN cavity are grown directly on the Si. In the second case, a crack-free 2 μm thick GaN layer is grown, and progressively thinned to a final thickness of λ. Both devices work in the strong coupling regime at low temperature, as evidenced by angle-dependent reflectivity or transmission experiments. However, strong light-matter coupling in emission at room temperature is observed only for the second one. This is related to the poor optoelectronic quality of the active layer of the first device, due to its growth only 250 nm above the Si substrate and its related high defect density. The reflectivity spectra of the microcavities are well accounted for by using transfer matrix calculations. [ABSTRACT FROM AUTHOR]

The effects of lattice-matched (Al,In)N insertion layers on the optical and structural properties of GaN films grown epitaxially on freestanding GaN substrates are described. The intensities and energetic positions of the GaN excitonic transitions in the photoluminescence from material grown above (Al,In)N show that high material quality and low strain have been preserved. The free exciton energies of the overgrown GaN, measured by reflectivity and photoluminescence, are 3.477, 3.482, and 3.500 eV for A, B, and C excitons, respectively, corresponding to strain-free GaN. The spectra are compared with those of a similar heterostructure grown on sapphire and of GaN-on-sapphire templates. [ABSTRACT FROM AUTHOR]

Abstract: A novel epitaxial lift-off process for III-nitrides, involving selective removal of a sacrificial (Al, In)N layer in a hot nitric acid etchant, is reported. This was applied to the fabrication of GaN planar microcavities bounded by two dielectric DBRs, starting from epitaxial GaN–(Al, In)N–GaN trilayers grown on free-standing GaN or high-quality GaN template material. An optically smooth surface was retained on the GaN surface exposed to the nitric acid etch, with root mean square roughness values as low as 2 nm over 8 μm×8 μm areas. Photoluminescence and reflectivity spectra were recorded from completed microcavities, and the latter showed clear dips in the region of 3.5 eV. [Copyright &y& Elsevier]

Abstract: We describe the fabrication and optical properties of a InGaN/GaN-based microcavity using “upper” and “lower” silica/zirconia mirrors. The fabrication of this structure involved selective removal of an AlInN layer following multistep thinning of a free-standing GaN substrate. Photoluminescence spectra show a narrowing of the excitonic emission from InGaN/GaN quantum wells in the microcavity, giving a cavity quality factor exceeding 400. [Copyright &y& Elsevier]