A study of efficiency droop of green light emitting diodes grown by metalorganic chemical vapor deposition
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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.