Influence of ITO Annealing Temperature on the Optoelectronic Characteristics and Reliability of InGaN Light Emitting Diode Chips

• Main Authors: Huang,Chun-Hsiang, 黃俊翔
• Other Authors: Huang, Man-Fang
• Format: Others
• Language: zh-TW
• Published: 2019
• Online Access: http://ndltd.ncl.edu.tw/handle/s4a7v5

碩士 === 國立彰化師範大學 === 光電科技研究所 === 107 === This thesis investigated the effects of thermal annealing temperatures of indium tin oxide (ITO) on the optoelectronic characteristics and reliability of InGaN blue light-emitting diodes (LEDs). The basic structures, optoelectronic characteristics and thermal characteristics of the III-N LEDs, and the importance of LED reliability were introduced in chapter one. In chapter two, a historical review on reliability-related literatures was presented. From exploring the variation in optoelectronic characteristics of LEDs under stress, the mechanisms of LED failures could be investigated, which included defect generation, increase of leakage current, current crowding effect and so on. Among various stresses, the application of high current stress may induce the defect generation quickly inside LEDs, resulting in the increase of leakage current and the decrease of resistance. This could lead to electrode deterioration and optical output power decrease which caused the nonuniform current spreading. From the analysis of the failure mechanism, we could understand how to improve the reliability of LEDs. In addition, the methods to improve the LED light output efficiency were also briefly reviewed. For instance, the use of patterned sapphire substrate could reduce the defect density and enhance the light extraction efficiency of LEDs, and the optimization of ITO characteristics could properly improve the conductivity and transparency of transparent contacts. The former research results showed that ITO films with lower thermal annealing temperatures had higher transparency in the range of blue wavelength, while ITO films with higher thermal annealing temperatures had superior conductivity. Despite these two very distinct indicators to LEDs, there was no or very few studies on the influence of ITO thermal annealing temperature on the optoelectronic characteristics and reliability of LEDs. Therefore, in this thesis, two LED samples from the same wafer were annealed at two different thermal temperatures for ITO and were then subjected to the same accelerated degradation for failure analysis. In the third chapter, the experimental results were presented and discussed. First of all, the results showed that the light output power of both samples decreased due to the defect generation inside LEDs and the deterioration of the electrode, which were verified from Log I-V, optoelectronic characteristics and the near-filed luminescence distribution. It was found that the sample with high annealing temperature had better conductivity in the initial state. However, due to worse ITO transparency and less light extraction leading to higher junction temperature, the electrode contacts and the optoelectronic characteristics of LEDs were degraded by a large extent. The near-field luminescence uniformity, the light output power and the reliability of LED samples were at lower performance. On the other hand, although the LED sample with a lower annealing temperature had worse conductivity in the initial state, the optoelectronic characteristics showed less degradation after high current stress because of better light extraction and thus the lower junction temperature. Therefore, the LED sample with lower annealing temperature demonstrated better reliability. The fourth chapter is the conclusion of this thesis. It was found that the electrical properties could be improved for the LED sample with high ITO annealing temperature, but the LED degraded after stress due to worse light extraction. While in spite of compromising its electrical property, the LED sample with lower ITO annealing temperature had superior optical properties and better reliability after stress aging because of its high light extraction efficiency.