In this paper, we investigated the failure mechanisms of blue InGaN LEDs grown on patterned sapphire substrates and demonstrated the influence of patterned sapphire substrates on the reliability of GaN LED by comparing with conventional LEDs grown on planar sapphire substrates. From experimental results, we found that InGaN LEDs grown on patterned substrates had a higher turn-on voltage but a smaller series resistance compared with conventional LEDs owing to rough inner patterns and small threading dislocation density. Both samples were then acceleratedly aged under a high DC current for two hours. Failure modes were studied with various measurements taken before and after aging. From the power evolution performance, we found that output power of LEDs with patterned substrates increased slightly due to fewer defects while output power of conventional LEDs decayed. This can be inferred from small reverse leakage currents and tunneling currents observed from Log I-V characteristics and EMMI measurement of P-LEDs. A slight redshift in emission wavelength was also found during aging because of possible leakage shunt paths caused by defect generation. Moreover, operation voltage increased slightly after aging which was caused by contact degradation induced by thermal annealing.
In recent years, with extensive use of InGaN LED, estimation of LED quality and improvement of LED reliability has become very important. In this report, the noise spectrum measurement techniques were used to estimate the reliability of InGaN LED devices and compare its reliability with its ESD tolerance test result. Experimental results show that the noise spectrum measurement more effectively distinguishes the LED device reliability than that of the current voltage curve measurement. EMMI, SEM and TEM images show that noise source and cause of failure of the LED device are attributed to poor quality of the SiO2 and ITO interface.
For energy-saving, high efficiency and low pollution, the lighting of LED systems is important for the future of green energy technology industry. The solid state lighting becomes the replacement of traditional lighting, such as, light bulbs and compact fluorescent lamps. Because of the semiconductor characteristics, the luminous efficiency of LEDs is sensitive to the operating temperature. Besides increasing the luminous efficiency, effective controlling electricity and thermal characteristics in the design of LED lighting products is the key point to achieve the best results. LED modules can be combined with multi-grain process or through a combination of multiple LED chips. Accurate analysis of this LED module for the electrical, thermal characteristics and high reliability is the critical knowledge of modular design. In this report, we studied the electrical and thermal coupling phenomenon in solid state lighting systems to analyze their reliability. By experiments and simulations, we obtained the apparent variation of temperature distribution of LED system due to differences of their forward voltages and thermal resistances. These events may reduce their reliability. Besides, the evaluation of optical and chromatic properties was based on the variation of temperature distribution and current of LED system. This is the key technology to predict the optical and chromatic properties of LED system in use.
The capability of high-power nitride-based light-emitting diodes (HPLED) to withstand electrostatic discharge
(ESD) is very important key index due to the horizontal structure of the insulating property of the sapphire substrate.
However, it is difficult to real-time monitor the damage caused by the ESD stress because it occurred in a very short
period. Current-voltage (I-V) curves and electroluminescence (EL) spectrum were applied to study the change during the
series ESD stress. Time-resolved optical beam induced current (TR-OBIC) was used to analyze the characteristics of the
delay time between normal region and the defect point caused by ESD stress. Transmission electron microscopy (TEM)
was used to compare to the difference in the distribution of damage region and investigate the failure modes. During the
series ESD stresses, V-shaped pits suffered from the high electrical field and the distance from multi-quantum well
(MQW). The bottom of V-shaped defect would be one of index to assess the ESD endurance of LED chips.
The die-attached quality and the thermal transient characteristics of high power flip chip light emitting diodes (LEDs)
are investigated using thermal transient tester. Various die-attached materials were utilized to compare the difference in
their thermal resistances and long term performance. By applying accelerated aging stress, the deterioration rates at the
die-attached layers were obtained. Numerical simulation provided further understanding of LED device temperature
distribution and also revealed that the thermal variance at the die-attached interface can be recognized within only few
milliseconds for the flip chip structure. The effects of bump arrangement and material were analyzed to achieve high
temperature uniformity and low thermal resistance for high power LEDs.
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