Microdisks fabricated with III-nitride materials grown on GaN substrates are demonstrated, taking advantage of the high material quality of homoepitaxial films and advanced micro-fabrication processes. The epitaxial structure consists of InGaN/GaN multi-quantum wells (MQWs) sandwiched between AlGaN/GaN and InAlN/GaN superlattices as cladding layers for optical confinement. Due to lattice-matched growth with low dislocations, an internal quantum efficiency of ∼40% is attained, while the sidewalls of the etched 8 µm-diameter microdisks patterned by microsphere lithography are optically smooth to promote the formation of whispering-gallery modes (WGMs) within the circular optical cavities. Optically pumped lasing with low threshold of ∼5.2 mJ/cm2 and quality (Q) factor of ∼3000 at the dominant lasing wavelength of 436.8 nm has been observed. The microdisks also support electroluminescent operation, demonstrating WGMs consistent with the photoluminescence spectra and with finite-difference time-domain (FDTD) simulations.

Optically pumped whispering-gallery mode (WGM) lasing is observed from a thin-film GaN microdisk processed from GaN-on-Si InGaN/GaN multi-quantum well wafers by selective wet-etch removal of the substrate. Compared with thin-film microdisks processed from GaN-on-sapphire wafers through laser lift-off of the sapphire substrate, the exposed surface is significantly smoother as laser-induced damage is avoided, with a root-mean-square roughness of 1.3 nm compared with 5.8 nm of the latter wafer. The ∼8- μ m diameter microdisks, fabricated by pattern transfer from a silica microsphere and dry etching, benefit from the surface smoothness to offer superior optical confinement within the cavity. WGM lasing thresholds of ∼2.9 mJ/cm2 and ∼3.5 mJ/cm2 with quality (Q)-factors of ∼3100 and ∼1700 are observed at the peak lasing wavelengths of ∼453 nm and ∼532 nm, respectively, which are significantly better than thin-film microdisks processed from GaN-on-sapphire wafers despite lower internal quantum efficiency, highlighting the importance of surface smoothness in such optical cavities.

The lasing characteristics of optically-pumped GaN microdisks of different configurations, including microdisks with undercuts, microdisks with cladding layers and thin-film microdisks are investigated in this paper. The microdisks, fabricated from a range of epitaxial structures containing blue-light emitting InGaN/GaN multi-quantum wells grown on Si, sapphire or GaN substrates, undergo different processes to form 8µm whispering-gallery mode (WGM) microdisks with different degrees of optical confinement. The microdisks have lasing thresholds ranging from 2.1 to 8.3 mJ/cm2 and quality factors of 1400 to 4200. The lasing characteristics are correlated to the material qualities, optical confinement as well as the overlap of the mode with the MQWs in the microdisk structures. The undercut microdisks benefit from high optical confinement factors but poor overlap factor, while the thin film structures have high overlap factors but low confinement due to absorption by the metallic bonding layers. The findings provide useful insight on ways to optimize GaN microdisk for improving lasing performances.

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