Optimization of GaN microdisks

Abstract

Optical microcavities based on III-V semiconductors have become the essential materials for a variety of optoelectronic devices. Among the laser diodes such as Fabry-Perot resonators, vertical-cavity surface-emitting lasers (VSCELs), whispering-gallery mode (WGM) microdisk occupies a place for their small mode volume, high optical gain and simple geometries. There are three main factors affecting light confinement within micro-cavity: (1) Internal quantum efficiency of gain; (2) surface and sidewall smoothness; (3) refractive index contrast between microdisk and the ambient. The promotion of light confinement within the cavity is required to optimize of GaN microdisks. GaN grown on non-optimized substrates generally results in higher defect densities, inducing sidewall roughness on microdisks which is detrimental for light confinement. Compared to GaN grown on Si or sapphire substrates, there is no lattice mismatch when grown on GaN substrates, offering materials with significantly reduced defect densities. In this thesis, GaN microdisks on GaN substrate are demonstrated, taking full advantage of the material qualities as well as the merits of the microsphere lithography process for patterning of microdisks. Laser performance with low threshold of around 5.2 mJ/cm2 and a quality factor Q ~3000 from the dominant mode at λ=436.8 nm is achieved. The whispering-gallery modes under continuous wave (CW) electrical pumping at room-temperature is also observed, identified with optical pumping lasing wavelengths, paving the way towards electroluminescent lasing. This is the first report of Whispering-Gallery mode lasing from InGaN/GaN microdisks on GaN substrate. Surface smoothness is also crucial to offer superior optical confinement within the micro-cavity. A new method of thin-film microdisks fabricated from GaN-on-Si wafers is developed. Compared with thin-film microdisks processed from GaN-on-sapphire wafers by laser lift-off, the exposed surface is significantly smoother with a root-mean-square roughness of 1.3 nm compared with 5.8 nm. The ~8 um dimension microdisks are fabricated on the blue-emitting light and green-light emitting light wafer, WGM lasing is observed with low threshold ~2.9 mJ/cm2, quality factor (Q) ~3100 at lasing wavelength ~453 nm and ~3.5 mJ/cm2 threshold with quality factor(Q) of ~1700 at lasing wavelength ~532 nm which imply the importance of surface smoothness in the optical cavity. Further study moving forward to the comprehensive comparison of the different InGaN/GaN microdisks developed from different structures including microdisks with undercuts, microdisks with cladding layers and thin-film microdisks. Whispering-gallery mode optically-pumped lasing is observed with the thresholds range from 2.1 to 8.3 mJ/cm2, quality factor from 1400 to 4200. Thresholds and quality factors are discussed combined with the material quality, process flow and morphology. The optical properties of the microdisks are also discussed with the aid of 3D-FDTD simulation. The promising results provide helpful insight on the optimization of GaN microdisk for enhancing lasing performances.