Studies on Characteristics and Reliability Issues of High Power InGaN-based Light-Emitting Diodes

• Main Authors: Yang, Shih-Chun, 楊適存
• Other Authors: Lin, Pang
• Format: Others
• Language: en_US
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/42838918397481826100

博士 === 國立交通大學 === 材料科學與工程學系 === 99 === In recent years, InGaN-based light-emitting diodes (LEDs) have attracted much attention due to their application as indication lights and for illumination, such as the use of white light LEDs and their utilization as light sources for flat panel displays. LED reliability is a matter that has gathered great interest because high-power LEDs are the most favorable candidate to replace conventional incandescent bulbs and fluorescent lamps. Typically, to investigate the reliability of LEDs, aging tests are administered to estimate their lifetime and performance. During these tests, the devices are exposed to high junction temperature, high current density and electrostatic discharge (ESD) simultaneously to examine their gradual erosion. Therefore, LED degradation mechanisms are complex and difficult to be analyzed individually. The purpose of this study is to investigate the failure mechanisms of high power InGaN-based LEDs induced by heat, currents, and electrostatic discharge. This dissertation consists of three major portions. First, the impact factors of high-power 1W white LEDs with the commercial package under well-designed aging conditions were exposed to identify the degradation phenomena between LED chip and package materials. The most important index of LEDs, luminous flux, noticeably decreased with different electrical and thermal stresses. At various aging stresses, junction temperatures were compared to determine how thermal stresses factor into luminous degradation. Under short-term high electric stress, defects in LED chips were induced to increase the leakage current. As for the LED package, gradual yellowing and cracking of the encapsulating lens during the long-term aging process caused degradation of light extraction efficiency and the degradation levels of the LED package are strongly correlated with junction temperature. Secondly, we focused on the failure characteristics of InGaN-based LED chips and provided a degradation mechanism to realize one of the failure modes. An effective circuit model was proposed to analyze the behavior of premature turn-on diodes in the active layer. One of the key indicators to assess the reliability of InGaN-based LED chips is the uniformity of current spreading. According to the investigation of failure analyses, the simulation results matched the experimental data. Furthermore, V-shaped defects and the associated threading dislocations induced the inhomogeneous distribution of forward currents in LED chips because the threading dislocations associated with V-shaped defects act as very small shunt resistors connected across p-n junctions. Lastly, the ESD endurance of InGaN-based LED chips is also examined because ESD endurance is the important index for horizontal structure of the sapphire substrate. The experimental data shows that LED chips with a smaller reverse leakage current endures ESD better. Subsequently, in keeping with results from the previous stage of experimentation, it became necessary to research the correlations between electrical properties and the surface morphology of LED chips. Findings establish that local defects, such as V-shaped pits, are a major factor in the creation of high electrical fields and they augment ESD damage during ESD stress. Consequently, V-shaped defects and surface morphology are strongly correlated to the ESD endurance of InGaN-based LED chips. To conclude, well-designed thermal management, uniform current spreading and defect-free surface morphology can enhance the reliability of InGaN-based high power LEDs. LED packaging should be done at lower temperatures, which can minimize the LED degradation progress. It is also suggested that reducing the density of the threading dislocation and V-shaped defects during the processing of InGaN-based LED chips should effectively improve the reliability of LED chips.