GaN-based free-standing microdisks and nanostructures

Abstract

In this thesis, various micro- and nano-structures, fabricated by micro- and nanosphere lithography (NSL), were applied onto gallium nitride (GaN) based direct-bandgap semiconductors to develop optical micro-cavities supporting whispering gallery mode (WGM) lasing action. This work includes three major sections. The first section introduces the novel nanostructures patterned by NSL and discusses their characterization. The second section elaborates the free-standing microdisk (FSD) cavities supporting WGM lasing. Last section provides an effective approach for optimizing FSD system by replacing the interface layer. Novel nanostructures for various applications are fabricated by NSL. First we developed and demonstrated a drop-on-demand method for fabricating twodimensional (2D) photonic crystal (PhC) arrays. Different geometries, such as nano-pillar and nano-cone, were obtained with variant etching parameters. The GaN PhCs comprising nano-pillar array with diameter of 200 nm were fabricated by different nano-patterning methods, NSL, focused ion beam lithography (FIB), and E-beam lithography (EBL). The surface morphologies and optical properties of fabricated structures were evaluated by various characterization techniques, including scanning electron microscopy (SEM), photoluminescence (PL) spectroscopy, and transmission measurement. The NSL technique is regarded as a preferable approach in fabricating nanostructures. The FSD systems with diverse diameters of 1, 2, and 7 μm were prepared by laser lift-off (LLO) and microsphere lithography (MSL). The FSDs showed vertical and smooth sidewalls as confirmed by SEM images and served as an outstanding microcavities supporting WGM lasing. Optically pumped WGM lasing was obtained at room temperature for FSD systems. The lasing characteristics were fully studies. The thresholds were determined from the plots of PL peak intensity versus pumping energy density and the values were 5.01, 8.54, and 9.06 mJ/cm2 for FSDs with diameter of 1, 2, and 7 μm respectively. The remarkable quality (Q) factor of 2576 was achieved from 1 μm FSD. Anlysis for different thresholds and Q-factors were also discussed. By decreasing the FSD size, the number of lasing peaks is reduced to achieve single mode lasing due to the shrinkage of microdisk providing relatively large mode spacing. An elegant approach was conducted to improve the optical confinement in FSD system in the last section. The beneath layer, originally ITO, was replaced by a reflecting metallic Ni/Ag layer. The SEM images revealed that a thin FSD standing on metallic supporting layer was successfully fabricated. The WGM lasing was also obtained through optical pumping. Lasing mode centered at 430.0 nm has a threshold of 8.82 mJ/cm2 and a Q-factor of 1673. The free space range (FSR) was determined to be 3 nm. A comparison between FSD system with ITO layer and the one with NiAg layer indicated that the metallic layer can server as a reflecting layer and improved the optical confinement of the system.