R&D of Real-Time Non-Destructive Characterization Methods for Light Emitting Diodes
Date Issued
2016
Date
2016
Author(s)
Chou, Pei-Ting
Abstract
In recent years, gallium nitride (GaN) based light-emitting diodes, especially ultraviolet light-emitting diodes (UV LEDs) have been attracted much interest for several applications and purposes, with the rapid development and extensive use of UV LEDs, evaluating their quality and reliability is very important. For LED manufacturers, the optoelectronic characteristics of LED chips can be altered and controlled through adjusting epitaxial and process technology, and the quantity of luminous flux from LEDs is one of the most important parameters to lighting and luminaire manufacturers. This thesis studies four detection methods for improving the quality and reliability of LEDs, which are 1. monitoring the curvature of the LED wafer during the growth process, 2. analysis of the reliability of the Indium Gallium Nitride (InGaN) LED using the low-frequency noise spectrum, 3. photometric measurement of LED chips using a large-area silicon photodiode and 4. photometric measurement of the partial radiant flux of the LED using an assembled box of Si photodiodes. In the quality testing methods of LED chips, the measurements of LED wafer curvature and chip defect density are the critical issues. It is due to most commercial LED chips are grown on a substrate with a high lattice mismatch such as sapphire, silicon carbide and silicon, which may cause epitaxial substrate bending or crack, and largely increased dislocation density. Therefore, it is important to develop in-situ wafer curvature monitoring technology and further analyze different types of defect densities to enhance the luminous efficiency and reliability of LEDs. In this thesis, the reliability of InGaN LEDs was evaluated through monitoring the curvature of the LED wafer during the growth process and analyze low-frequency noise spectrum. Three measurement methods such as the distance between two circular laser spots, the length of a single line laser, and the vertical distance between the two parallel laser lines can be used to evaluate the curvature. The low-frequency noise spectra were measured and the defect density was compared with that calculated by XRD measurement. It is common using an integrating sphere to measure the total or partial luminous flux of LEDs, however, the small opening of an integrating sphere during partial flux may influence the measurement result. For UV LEDs, the BaSO4 coating layer of integrating sphere may be damaged by UV light and cannot stand prolonged exposure to the UV spectrum. In this thesis, a large-area Si photodiode was used to evaluate the output power of LED chips, and the assembling Si photodiodes box method are used to measure the output optical power of a UV LED. Optical simulations and experiments were performed through various measurement configurations to verify the measurement results.
Subjects
light-emitting diodes
in-situ curvature monitoring
low-frequency noise spectrum
silicon photodiode
photometric measurement
Type
thesis