| Summary: | 博士 === 國立交通大學 === 光電工程學系 === 98 === The next-generation of light-emitting diodes (LEDs), for applications such as projector displays, backlight displays, and automobile headlights, requires further improved light source properties. Within a few years, solid-state LEDs will be in a position to replace conventional lighting sources. Optimizing LED performance for a specific system requires detailed knowledge of light extraction, far-field emission distribution, and polarized characteristics. This dissertation studies the sapphire-based and micro-cavity (MC) types of GaN LEDs, using photonic crystal (PhC) nanostructures to improve light extraction efficiency, directional far-field emission, and polarized light emission.
First, anisotropic light extraction distribution in the azimuthal plane has been investigated through the optical images taken from GaN sapphire-based LEDs with a special annular PhC region in which the guided electroluminescence (EL) light generated at the center. With increasing PhC lattice constant, symmetric patterns with varying number of petals according to PhCs were observed and analyzed. In addition, a map of the anisotropic for various PhC lattice constants and numerical apertures was constructed. Then, polarized characteristics of the GaN sapphire-based PhC LEDs have been experimentally and theoretically studied. Strong polarization dependence on the lattice constant and lattice orientation of the PhC was observed. It is found that the PhC can be as a polarizer to improve the P/S ratio of the extracted EL emission. The results of the P/S ratio for light propagating in different lattice orientation were found to be consistent with the results obtained using the PhC Bloch mode coupling theory. This polarization behavior suggests an efficient means to design and control the GaN sapphire-based PhC LEDs for polarized light emission. Next, directional far-field emission measurements show a divergent angle distribution on GaN sapphire-based PhC LEDs. This is due to the PhC interacting with higher-order guided modes.
These results show that a surface PhC will not closely interact with the lower-order guided modes of GaN sapphire-based PhC LEDs because the GaN thickness is too thick. The solution to this problem is to make a GaN PhC MCLED to allow the lower-order guided modes to interact with the PhCs. This study examines the light extraction efficiency, directional far-field patterns, and polarization properties of GaN PhC MCLEDs. Angular-spectra-resolved electroluminescence (EL) measurements revealed directional far-field emission distribution from the GaN PhC MCLEDs based on the lower-order guided modes extraction. In addition, the light enhancement in GaN PhC MCLEDs compared to GaN non-PhC MCLEDs within a ±20° collection cone angle was enhanced by a factor of ~2.4. Additionally, angular-spectra-resolved EL measurements were also made in the polarized-resolved manner that has been used to analyze reveal guided modes polarization in the GaN PhC MCLEDs. The measurement results revealed the evolution of the collinearly and non-collinearly coupled modes. These results good agreement with the two-dimensional free-photon band structure is obtained. The present results indicate that the high directionality light extraction enhancement could contribute to many applications, especially for etendue-limited application such as pico-projector.
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