Improvements of Light-emitting Diode Performances with Novel Device Structures
Date Issued
2015
Date
2015
Author(s)
Lin, Chun-Han
Abstract
Several novel device structures of light-emitting diode (LED) for enhancing LED emission efficiency and modulation bandwidth are demonstrated in this dissertation. First, to understand the optimum grating period in forming a surface grating on a vertical light-emitting diode (VLED), we construct a Llyod’s interferometer within photoelectrochemical (PEC) electrolyte (KOH) to fabricate surface gratings of various periods on VLEDs for comparing their light extraction efficiencies. The comparisons of VLED characterizations show that among those grating VLEDs, the light extraction is more effective in a VLED of a smaller grating period. Compared with VLEDs of rough surfaces, the grating VLEDs of short grating periods (<2 m) have the higher light extraction efficiencies. Second, the enhancement of output intensity, the generation of polarized output, and the reduction of the efficiency droop effect in a surface plasmon (SP) coupled VLED with an Ag nano-grating structure located between the p-GaN layer and the wafer bonding metal for inducing SP coupling with the InGaN/GaN quantum wells (QWs) are demonstrated. Based on the reflection measurement from the metal grating structure and the numerical simulation result, it is found that the localized surface plasmon (LSP) resonance induced around the metal grating crest plays the major role in the SP-QW coupling process. By adding a surface grating structure to the SP-coupled vertical LED on the n-GaN side, the output intensity is further enhanced, the output polarization ratio is further increased, and the efficiency droop effect is further suppressed. Third, the enhanced SP coupling effects in a blue LED with regularly patterned (REG) and randomly distributed (RAN) surface Ag nanoparticles (NPs) on a dielectric interlayer (DI) of a lower refractive index overgrown on p-GaN are demonstrated. Without a DI, the surface Ag NPs induced SP coupling with the QWs in the LED can lead to the increases of internal quantum efficiency (IQE) and LED output intensity, the reduction of the efficiency droop effect, and the enhancement of modulation response. By adding a DI, the SP coupling effect is enhanced, resulting in the further improvements of all the aforementioned factors. Although the REG Ag NPs can produce stronger collective LSP resonance with a narrower spectral width, the SP coupling effect depends mainly on the LSP resonance strength at the QW emission wavelength. Fourth, the modulation bandwidths of the LEDs of different mesa sizes with and without SP coupling effect are compared. Due to the significant increase of carrier decay rate, within the size range of LED square-mesa from 60 through 300 micron and the injected current-density range from 139 through 1667 A/cm2, the SP coupling can lead to the enhancement of modulation bandwidth by 44-48 %, independent of the variations of LED mesa size or injected current level. The increases of the RC time constants in the samples with SP coupling are attributed to the increases of their device resistance levels when the Ag nanoparticles and GaZnO dielectric interlayer are added to the LED surface for effectively inducing SP coupling. Fifth, the emission behaviors of four LEDs of different substrate structures, including a lateral LED grown on sapphire, a vertical LED wafer-bonded onto Si (111), a bendable LED Ag-epoxied onto a flat metal, and another bendable LED Ag-epoxied onto a metal of a curved surface, under different duty cycles of current injection are compared. Their different variation trends of emission behavior with injection duty cycle are attributed to the different thermally-induced strain conditions in the epitaxial layers, which are controlled by their substrate structures, in increasing injection duty cycle or current level. The results of Raman scattering measurements during LED operation show that a stronger tensile strain is generated under heating for reducing the quantum-confined Stark effect and hence increasing emission efficiency when the epitaxial layer is not tightly bonded onto a hard substrate. Such a behavior is particularly stronger when the epitaxial layer is bent. Sixth, to identify the individually optimized thermal annealing conditions for reducing the contact resistivity between highly Ga-doped ZnO (GaZnO) and doped-GaN and for improving the electrical and optical properties of GaZnO, the effects of the junction-layer (JL) and whole-layer (WL) thermal annealing processes of GaZnO at various temperatures are compared. GaZnO is grown with plasma-assisted molecular beam epitaxy. The JL-annealing process always results in lower contact resistivity on either p-GaN or n-GaN at any annealing temperature. Lateral and VLEDs with GaZnO layers on the top are fabricated to show that the LED samples with the JL-annealing process have lower device resistance levels, higher emission efficiencies, and weaker efficiency droop effects, when compared to those with the WL-annealing process.
Subjects
light-emitting diode
Type
thesis
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