The Applications of Subwavelength Semiconductor Lens-array for Optoelectronic Devices
Date Issued
2009
Date
2009
Author(s)
Wu, Han-Min
Abstract
In this dissertation, the colloidal lithography for casting close-packed distribution of self-assembled polystyrene spheres is demonstrated. High-order diffraction pattern and rainbow lines were observed due to close-packed spheres, showing the ordered-structure characteristics.ingle mode stimulated emission from optical pumping of prismlike gallium nitride (GaN) with a side length of 0.75 um is also reported. Using the self-assembled spheres as the mask, we applied the photo-enhanced etching and wet chemical etching to form prism-like GaN submicron cavities with nonpolar facet sidewalls. They were characterized by an average quality factor of above 1000 and an equivalent facet reflectivity of exceeding 98%, which allowed field amplification by repeated internal reflections in the transverse plane and field polarization along the c axis. These properties allow the observation of single mode stimulated emission by optical pumping the GaN prism at threshold intensity of 2 MW/cm2. A slight spectral blueshift of 0.35 nm and narrowing in linewidth of 0.4 nm were observed with the increase of pump intensity. These observations manifested resonant coupling of the band edge emission to a single mode of the prismlike GaN cavity.oreover, it is reported the use of recessive size reduction in self-assembled sphere mask with anisotropic etching to form lenslike nipple arrays onto the surface of silicon (Si) and GaN. These devices are shown to exhibit a filling factor near to an ideal close-packed condition and paraboloidlike etch profile with a slope increased proportionally to the device aspect ratio. Specular reflectivity of less than 3% was observed over the visible spectral range for the 0.35-um-period nipple lens arrays. The Si photodetector fabricated with nipple lens arrays exhibited a 40% enhancement in the photo-current measurement compared to the device without the nipple array.n order to obtain a surface structure with fine-tuned refractive index matching effect and antireflection for wide incident angles over full visible spectrum, we further combined the nipple lens array structure with thin film coating of zinc oxide (ZnO) to seek the optimised surface structure showing the best antireflection effect. It was found that Si nipple array coated with 50-nm-thick ZnO could exhibit specular reflectivity less than 1% over the visible spectral range at vertical incident angle. In the oblique incidence measurements, the ZnO/lens array structure showed suppressed reflection below 2.5% for TE mode at incident angle of 30? and reflection below 1% for TM mode at incident angle of 45?, respectively, further confirming the improvement of antireflection effect on ZnO/lens array structure.
Subjects
Subwavelength
Lens array
Antireflection
Type
thesis
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