| Summary: | 博士 === 國立臺灣大學 === 光電工程學研究所 === 99 === In this dissertation, the fabrications of high-quality nitride-based light-emitting devices by using photoelectrochemical (PEC) etching technique are demonstrated. First, the implementation of a series of optically-pumped GaN photonic crystal (PhC) membrane laser at room temperature is demonstrated. The photonic crystal is composed of a scalene triangular arrangement of circular holes in GaN. Three defect structures are fabricated for comparing their lasing characteristics with those of perfect PhC. It is observed that all the lasing defect modes have the lasing wavelengths very close to the band-edge modes in the perfect PhC structure. Although those lasing modes, including band-edge and defect modes, have different optical pump thresholds, different lasing spectral widths, different quality factors (Q factors), and different polarization ratios, all their polarization distributions show the maxima in the directions around one of the hole arrangement axes. The similar lasing characteristics between the band-edge and defect modes are attributed to the existence of extremely narrow partial band gaps for forming the defect modes. Also, the oriented polarization properties are due to the scalene triangle PhC structure. In one of the defect lasing modes, the lasing threshold is as low as 0.82 mJ/cm2, the cavity Q factor is as large as 1743, and the polarization ratio is as large as 25.4. Such output parameters represent generally superior lasing behaviors when compared with previously reported implementations of similar laser structures.
Besides, the enhancement of light extraction by fabricating a surface grating structure around the mesa of a nitride-based light-emitting diode (LED) with an approach combining PEC wet etching and phase mask interferometry is demonstrated. The PEC etching rate is controlled by the intensity of illuminating UV light, which is spatially modulated by the fringe pattern of phase mask interferometry, for forming the grating structure. Without affecting the resistance characteristics of the device, the diffraction of such a grating structure leads to LED output enhancement by >43 % on either the top or bottom side.
In addition, the method of sapphire substrate liftoff for the fabrication of vertical nitride-based blue LED, based on the combination of the PEC etching and epitaxial lateral overgrowth (ELOG) techniques, is demonstrated. This method relies on the formation of connected voids during the ELOG process on a GaN template such that PEC electrolyte can approach the GaN portions above the SiO2 masks. Also, the GaN template must be thin enough for the illuminating ultraviolet light to reach the GaN portions above the SiO2 masks. It is shown that PEC etching starts from a very thin layer of GaN right above a SiO2 mask. It then extends into the window regions of ELOG to completely separate GaN from sapphire. The performances of a vertical LED are illustrated.
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