Improve Epitaxy and Efficiency Droop of GaN-based LEDs on Nano-Patterned Substrates with Spacing Optimization
Date Issued
2015
Date
2015
Author(s)
Hung, Zheng-Hung
Abstract
GaN-based Light-Emitting Diodes (LEDs) are typically grown on c-plane sapphire substrates. Due to the large lattice mismatch between GaN and the substrate, GaN on sapphire exhibits lots of threading dislocations (TDs), and as to control them, one can change the surface morphology of substrate, i.e. patterned sapphire substrates (PSSs). For further understanding on GaN epitaxy on nano-scaled PSSs, we fabricated a series of square-array PSSs in this research, providing various period and structure size but the same in depth. We exploited E-beam lithography system accompanied with dry etching method for precise pattern design. After that, GaN-based LED structure was developed on the fabricated PSSs via MOCVD system. A non-destructive analysis over epitaxial revolution was conducted with optical microscopy, micro-PL, and micro-Raman system, whereas a GaN epitaxial model was then proposed. In this research we observed that the epitaxial alteration was associated with spacing between structures, especially below 600nm, and can produce free-standing-like GaN, which is characterized with low defect density and strain simultaneously. For application, devices are capsulated and under electroluminescence (EL) measurement. And from it, a detailed research on device performance containing efficiency droop was discussed. We identified TDs as the main cause.
Subjects
GaN-based Light-emitting diodes
Nano-Pattered Sapphire Substrates
Epitaxial Model
Electron-beam Lithography
Efficiency Droop
Type
thesis
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