The Study of Photonic Crystal Arrays and AlN Nanorod Templates on GaN-based Light-emitting Diodes
Date Issued
2014
Date
2014
Author(s)
Tsai, Chen-Hung
Abstract
The thesis consist of two parts. First of all, we studied in the effects of photonic crystal nanohole arrays on light output properties of LEDs. Second, we used the high temperature tube furnace to grow AlN nanorod template. It acts as buffer layers between GaN epilayers and sapphire substrate and comes out with the epitaxial lateral overgrowth process.
In recent years, using photonic crystals (PhCs) in GaN LEDs as optical diffraction grating is a reliable way for improving light extraction. Generally, most researches utilized shallow PhCs in order not to damage the MQWs. Despite reducing the surface defects, the low-order modes could not couple with PhCs effectively and be extracted. We fabricated the LED, of which the entire mesa covered by PhC nanohole array with lattice constant of 400 nm, radius of 140 nm and etching through the MQWs layer. In this structure, the diffraction by PhCs in TM modes is effective than in TE modes, causing TM polarized light to congregate at 14° (another peak intensity is at 32.5°). Compared with conventional planar LED, the radiation profile of TE mode is much wider. In this chapter, we introduce two mechanisms, vertically guided mode extraction and laterally propagating light diffraction by PhCs, to explain the difference of radiation with polarization.
Besides, typically, GaN epilayers are grown on a planar sapphire substrate by metal-organic chemical vapor deposition (MOCVD). However, GaN epilayers suffer from a high threading dislocation density due to their large lattice mismatch (13%) and thermal expansion coefficient misfit (62%). The AlN nanorod templates were grown on sapphire substrate by VLS mechanism and have the effect of lateral epitaxial to improve the crystalline quality of GaN epilayers. The FWHM of (002) XRD rocking curve represent that the samples with the templates have higher crystal quality. It is found that the residual stress was reduced from 1.25 to 0.78 GPa in the GaN epitaxial layer by inserting the AlN nanorod templates. The TEM images reveal lower dislocation density in GaN epilayers on AlN nanorod template and the stacking fault on the templates can block the threading dislocation. Moreover, the internal quantum efficiency of MQWs is enhanced by 12.2% compare to reference sample.
Subjects
光子晶體
氮化鎵
奈米洞陣列
氮化鋁
奈米柱層
側向磊晶生長
發光二極體
Type
thesis
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