| Summary: | 碩士 === 國立彰化師範大學 === 光電科技研究所 === 105 === In this thesis, the influence of growth-induced surface roughness on the optoelectronic characteristic and reliability of InGaN light emitting diode chips are investigated. First of all, the basic characteristics of light emitting diodes (LEDs) including the operation principle of luminescence, strucures and the importance of reliability were introduced in chapter one.
In chapter two, some literatures related to LED reliability and light extraction were reviewd. Failure mechanisms of degraded LEDs can be analyzed from the variation of electro-optical characteristics of LEDs, which include defect generation, increase of leakage current shunt paths, change of voltage and decrease of light output power. Some methods have been proposed to improve the output power of LEDs such as optimization of material quality based on epitaxial growth and roughen LED surface to enhance light extraction. However, with textured transparent contact surface, it was found that forward voltage could be increased due to reduction of cross sectional area or dry-etching induced damages. Manufacture cost is increased too. Therefore, we intend to use single epitaxy process and control the growth rate as well as the thickness of buffer layer to obtain LEDs with two different surface roughnesses. The optoelectronic characteristics and the reliability of these two LEDs are then studied and compared.
In chapter three, threading dislocation densities and active region structures of these two LEDs, with rough or smooth surface, were analyzed by XRD first. From experimental results, we found that these two samples have the same order of threading dislocation densities. While the the slope efficiency (S/E) of rough LED is better than that of smooth one because of enhancing ligh extraction effiency caused by rough LED surface. Under high current stress, the LED with smooth surface degraded severely because of worse and non-uniform light extraction caused by the smooth surface, which accumulated heat inside and caused more defects to generate. Besides, the current injection as well as non-radiation recombination also resulted in self-heating effect inside LED which futher caused the decay of electrodes. On the other hand, under high temperature stress only, the output power of two samples degraded slightly along with voltage decrease owing to the better current spreading which casued by thermal annealing of surface transparent contact and p dopant activation.
Finally, chapter four is the summary of this thesis. We concluded that LED with rough surface has well-distributed light extraction, and better reliability. LED with smooth surface has worse light extraction which accumulates more heat inside LED and shows larger power degradation. Therefore, the non-radiative recombination and tunneling of carriers causes the generation of defects and power degradation. The self-heating effect results in the degradation of electrode or ITO/semiconductor interface which causes power and voltage degradation.
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