Optical characterization and device fabrication of semiconductor micro- and nanostructures and their bio-applications

Abstract

Semiconductor micro- and nanostructures are promising building blocks for high-performance and high-resolution photonic devices due to their novel properties such as high material-quality, small foot-print, and so on. Furthermore, since they have small size sufficient to probe mammalian cells with subcellular resolution, semiconductor micro- and nanostructures based probes have possibility to provide revolutionary tools to investigate biological materials and systems. To realize the promising possibility of micro- and nanostructures, they should be scalable, well-controllable, and mechanically flexible. However, fabrication of micro- and nanostructure arrays satisfying these condition have been difficult due to problems related with material growth. In this thesis, it is demonstrated that this issue can be overcome by using graphene films. Optical characterization and device fabrication of semiconductor micro- and nanostructures grown on chemically vapor deposited (CVD) graphene films are studied. Using graphene films as intermediate layers to grow II-VI and III-V semiconductor micro- and nanostructures, it can be possible to fabricate well-controlled nano-waveguides and high-quality micro- and nanolasers. Furthermore, semiconductor micro- and nanostructures grown on graphene can be readily lifted off exploiting weak bonding between graphene films and supporting substrates by van der Waals force and transferred to foreign substrates that can complement the lasing performance and expand the application of lasers. ZnO nanotube laser arrays with low lasing thresholds and clear laser oscillations by Fabry-Perot (FP) resonance were monolithically fabricated on arbitrary substrates by using graphene films as intermediate layers. Moreover, exploiting the benefit of easy transfer, nanotubes were combined with silver films after the lift-off, which significantly enhanced lasing characteristics of nanotubes. In addition to ZnO one-dimensional laser arrays, lasing characteristics of GaN microdisk and microrods grown on CVD graphene films were also studied. Highly crystalline GaN microdisks having hexagonal facets were grown on graphene dots, and whispering-gallery-mode (WGM) lasing emissions from the GaN microdisks were observed. For GaN microrods, detection of FP and WGM lasing emissions from a single GaN microrod are reported. Additionally, demonstration the ability to switch between the two lasing mechanisms by translating the excitation beam along the microrod are described. Finally, using the advantageous properties of semiconductor nanostructures on graphene films, metal-deposited ZnO nanotube waveguide arrays were used for local optical excitation on biological cells. Mammalian cells were cultured on nanotube arrays and fluorescent dyes in cells were locally excited by light transmitted through nanotube waveguides.