Zinc oxide nanophotonics : toward quantum photonic technologies
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Zinc oxide (ZnO) is a large bandgap (3.37 eV at room temperature) semiconductor and is a good candidate for short wavelength photonic devices such as laser diodes. A large exciton binding energy (60 meV) at room temperature in addition to the advantages of being able to grow various nanostructure forms have made ZnO suitable for a wide range of applications in optoelectronic devices. Driven by the rapid advance of nanophotonics, it is necessary to develop single photon sources (SPSs) and optical resonators in new class of materials. In particular, SPSs are required for a wide range of applications in quantum information science, quantum cryptography, and quantum communications. ZnO has been investigated for classical light emitting applications such as energy efficient light emitting diodes (LEDs) and ultraviolet (UV) lasers. Significantly ZnO has recently been identified as a promising candidate for quantum photonic technologies. Thus in this thesis the optical properties of ZnO micro- and nano-structures were investigated for ZnO nanophotonic technologies, specifically their applications in single photon emission and optical resonators. Firstly, the formation of radiative point defects in ZnO nanoparticles and their photophysical properties were investigated. In particular, using correlative photoluminescence (PL), cathodoluminescence (CL), electron paramagnetic resonance (EPR), and x-ray absorption near edge spectroscopy (XANES) it is shown that green luminescence (GL) at 2.48 eV and an EPR line at g = 2.00 belong to a surface oxygen vacancy (V⁺o,s) center, while a second green emission at 2.28 eV is associated with zinc vacancy (VZn) centers. It is established that these point defects exhibit nanosecond lifetimes when excited by above bandgap or sub-bandgap (405 nm and 532 nm excitation wavelength) excitation. These results demonstrate that point defects in ZnO nanostructures can be engineered for nanophotonic technologies. ZnO nanoparticles were consequently studied for the investigation of room temperature single photon emission from defect centers in ZnO nanoparticles. Under the optical excitation with 532 nm green laser, the emitters exhibit bright broadband fluorescence in the red spectral range centered at 640 nm. The red fluorescence from SPSs in ZnO defect center is almost fully linearly polarized with high signal-to-noise ratio. The studied emitters showed continuous blinking; however, it was confirmed that bleaching can be suppressed using a polymethyl methacrylate (PMMA) coating. Furthermore, passivation by hydrogen treatment increase the density of single photon emitters by a factor of three. ZnO/Si heterojunctions were fabricated and used to investigate electrically driven light emission from localized defects in ZnO nanostructures at room temperature. It is shown that excellent rectifying behaviors were observed with the threshold voltages at ~ 18 V and ~ 7 V for ZnO nanoparticles and thin film-based devices, respectively. Both devices exhibit electroluminescence (EL) in the red spectral region ranging from ~ 500 nm to 800 nm when 40 V and 15 V were applied to ZnO nanoparticles/Si and ZnO thin film/Si, respectively. The emission is bright and stable for more than 30 minutes, providing an important prerequisite for practical devices. Finally, ZnO optical resonators were fabricated and investigated to enhance the visible light emission. Hexagonal ZnO microdisks with diameter ranging from 3 μm to 15 μm were grown by a carbothermal reduction method. Optical characterization of ZnO microdisks was performed using low temperature (80 K) CL imaging and spectroscopy. The green emission is found to be locally distributed near the hexagonal boundary of the ZnO microdisks. High resolution CL spectra of the ZnO microdisks reveal whispering gallery modes (WGMs) emission. Two different sizes (5 μm and 9 μm) of the ZnO microdisks were simulated to analyze the nature of light confinement in terms of geometrical optics. Respective analysis of the mode spacing and the mode resonances are used to show that the ZnO microdisks support the propagation of WGMs. The results show that the experimentally observed WGMs are in excellent agreement with the predicted theoretical positions calculated using a plane wave model. This work could provide the means for ZnO microdisk devices operating in the green spectral range.
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