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.

Thesis (Ph. D.)--Lehigh University, 2000. / Includes vita. Includes bibliographical references (leaves 162-174).

Thesis (M.S.)--Florida State University, 2004. / Advisor: Dr. Jim P. Zheng, Florida State University, College of Engineering, Dept. of Electrical and Computer Engineering. Title and description from dissertation home page (viewed June 18, 2004). Includes bibliographical references.

Thesis (Ph. D.)--University of Hong Kong, 2009. / Includes bibliographical references. Also available in print.

Thesis--M. Phil., University of Hong Kong, 1983.

Thesis (Ph.D)--Mechanical Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Graham, Samuel; Committee Member: Garimella, Srinivas; Committee Member: Kippelen, Bernard; Committee Member: Melkote, Shreyes; Committee Member: Ready, Jud. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Graham, Samuel; Committee Member: Joshi, Yogendra; Committee Member: Kumar, Satish. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Thesis (M.S.)--Youngstown State University, 2009. / Includes bibliographical references (leaves 44-45). Also available via the World Wide Web in PDF format.

This thesis reports on the design, fabrication, and testing of state-of-the-art, high-performance inverted top-emitting organic light-emitting diodes (OLEDs). The vast majority of research reports focuses on a device architecture referred to as a conventional OLED which has its anode on the bottom of the device and its cathode on the top. Moreover, most conventional OLEDs are bottom-emitting such that light exits the structure through both a semitransparent bottom electrode of indium-tin oxide and a glass substrate. The particular device architecture developed in this thesis is one in which the devices are inverted (i.e. their cathode is on the bottom as opposed to on top) and top-emitting. Despite the advantages that inverted top-emitting OLEDs possess over conventional bottom-emitting OLEDs, their development has been relatively slow. This is because inverted OLEDs have traditionally been hampered by the difficulty of injecting electrons effectively into the device. In this work, a novel method of injecting electrons from bottom cathodes into inverted OLEDs is discovered. In several previous reports, bottom Al/LiF cathodes had been used with the electron-transport material Alq3 to produce inverted OLEDs, but the resulting inverted OLEDs exhibited inferior performance to conventional OLEDs with top cathodes of Al/LiF. A new route for the development of highly efficient inverted OLEDs is shown through the use of electron-transport materials with high electron mobility values and large electron affinities. After systematic device optimization, inverted top-emitting OLEDs are demonstrated that currently define the state-of-the-art in terms of device efficiency. Optimized green and blue inverted top-emitting OLEDs are demonstrated that have a current efficacies of 92.5 cd/A and 32.0 cd/A, respectively, at luminance values exceeding 1,000 cd/m2. Finally, this discovery has enabled the development of the first stacked inverted top-emitting OLEDs ever made, combining all of the advantages offered by an inverted architecture, a top-emissive design, and a stacked structure. These OLEDs have a current efficacy of 200 cd/A at a luminance of 1011 cd/m2, attaining a maximum current efficacy of 205 cd/A at luminance of 103 cd/m2.

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