In recent years, even though the light output of GaN-related LED continues to increase, the brightness is still low compared to conventional lighting systems and it is necessary to further improve the light extraction of LEDs. In this study, we utilize the ZnO nanotip with aqueous solution and flip-chip technique to increase the light extraction of GaN LEDs. Electroluminescence (EL) and angular optical distribution are used to measure the light output intensity of LED. In the results, ZnO nanotip after thermal annealing with N2O ambiance decrease the ZnO defects. Flip-chip LED has higher light intensity ( 1.25 times) than conventional one in vertical emitting area ( at 0 angles). The enhancement of light output is duo to the reduction of light absorption from the metal contact and Fresnel¡¦s transmission losses. Finally, we fabricate a high brightness LED with above light enhancement design. EL intensity of LED is increased about 1.38 times than conventional one. Therefore, we can manufacture a LEDs array with above designs to obtain high light output for future solid-state illumination.

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In recent years, even though the light output of GaN-based LED continues to increase, the brightness (~20 lm/W) is still low compared to conventional lighting systems and it is necessary to further improve the light extraction of LEDs. In this study, we utilize flip-chip technique, photoresist microlenses, reflectors and thermoelectric cooler to increase the light extraction of GaN MQW LED. Electroluminescence (EL) and power angular distribution are used to measure the light output intensity of LED. From temperature dependent current-voltage (I-V-T) characteristics, the charge carrier transport mechanisms at different biased regions are also investigated. In the results, back emission of LED with SiO2/Al reflector has maximum light intensity ( 3.28£gW ) , which is higher than front emission one ( 2.73£gW ) in vertical emitting area ( at 90 angles). LED with P.R. microlenses (refractive index, n=1.62) on backside could improve the light extraction of LED (about 1.2 times) as well. The enhancement of light output is duo to the reduction of light absorption from the metal contact and Fresnel¡¦s transmission losses at GaN (n=2.4)/air (n=1) interface. Finally, we fabricate a high brightness LED with above light enhancement design. EL intensity of LED is increased about 1.25 times than conventional one. Therefore, we can manufacture a LEDs array with above designs to obtain high light output for future solid-state illumination.

In recent years, even though the light output of GaN-related LED continues to increase, the brightness is still low compared to conventional lighting systems and it is necessary to further improve the light extraction of LEDs. In this study, the characteristics of LPD-SiO2 film and Al/SiO2/GaN MOS diode were investigated in advance of the formation of SiO2 micro structure for improving the oxide quality and controlling the deposition parameters. Temperature-difference method, post-annealing treatment, photochemical treatment, sulfurated treatment and etc. were used for the purposes of better properties of the MOS structure and the LED. To obtain higher light extraction efficiency of GaN LED, hemispherical SiO2 microlens was formed on the conventional and the flip-chip LEDs. The deposition mechanism had been developed to obtain the further improvements on the electrical and optical properties. The influences of epoxy encapsulation on the LEDs without and with microlens were also studied. Considering the refractive index of SiO2 is close to that of the epoxy, the enhancements of light extraction efficiency and angular optical distribution of GaN LED by using SiO2 microlens will be degraded after encapsulating. Therefore, we also tried to deposit ZnO film and rod on GaN LED by LPD method to maintain or further enhance the light extraction efficiency of GaN LEDs by the combining the micro structure and the epoxy encapsulation.

The objective of this thesis is to discuss the solutions investigated by AMDG (Advanced Materials and Devices Group) to reduce the "efficiency droop" effect that occurs in III-Nitrides Light Emitting Diodes (LEDs) when driven at high injection current densities. The efficiency droop refers to a decrease of the LED light emission efficiency when increases the current density from low values ~10 A/cm2 to higher values >100A/cm2. Many scientific papers have been written about the possible reasons for this phenomenon. Therefore, this thesis will discuss the different effects suspected to contribute to the droop, and discuss LED structure modifications studied by Dr. Dupuis' research group to reduce their impact. In addition to a description of a conventional LED structure, a discussion of the device fabrication process will be provided including the solutions investigated in our group to improve LED performance. Because measurement is critical to our studies, a description of the equipment used by the AMDG will be provided, e.g., the Electroluminescence (EL) and Photoluminescence (PL) test stations, Atomic Force Microscopy (AFM) for surface topology, TLM for metallic contact resistivity, X-Ray diffraction for crystal quality and epitaxial layer structure, and Hall-Effect measurement for doping concentration characterization and material resistivity. Because the IQE gives us a direct assessment of the active region's crystal quality, the setup and operation of a new Temperature-Dependent PL (TD-PL) system to measure the Internal Quantum Efficiency (IQE) was the main focus of this research. The External Quantum Efficiency (EQE) is measured using electroluminescence measurements. The EL measurements involve the acquisition of the emitted light spectrum along with different processed data such as the Full-Width at Half Maximum (FWHM) of the spectral intensity, the peak wavelength, output power, etc., which allows a comparison of the different LED structure performances. Within this work, a new LabVIEW© program (called QuickTest 2.0) has been developed in order to automate the instrumentation setup and improve both the speed and accuracy of EL acquisition. A brief description of the G language used by the LabVIEW© software will be provided along with the objective and motivation for upgrading the program, the general features of the program, and a comparison of spectrum acquisition and processed data results. The benefit for the research in the AMDG was to reduce measurement time, improve efficiency, supply a more user-friendly front-panel, and to enable transfer to other computers.

In this work, we propose a model to address the challenge of droop in internal quantum efficiency in InGaN/GaN Multiple Quantum Well LEDs. Efficiency droop limits the performance of high brightness LEDs as they operate at currents greater than 350mA. The efficiency droop is a multi-physics problem posed by various entities such as (1) dislocation recombination, (2) Auger recombination in active region, (3) non-radiative recombination, and (4) current overflow in the active region. This work aims at reducing the droop associated with non-radiative recombination by engineering the quantum well barrier thickness and materials. The goals are three-fold, namely: (1) To explore the role of barriers in determining the droop in internal quantum efficiency and to justify the use of multiple barriers to increase the carrier density and reduce the leakage current thereby increase the radiative recombination at higher current densities ; (2) Propose optimum barrier specifications such as number, material combination, and thickness for downscaling the efficiency droop, and thereby improving the device efficiency; and (3) Finally, obtain improved efficiency by engineering the barrier in a realistically-sized device by considering the effects of long-range strain fields in the device.