| Summary: | 碩士 === 國立彰化師範大學 === 光電科技研究所 === 106 === In this thesis, the influence of defect density and buffer layer thickness on the reliability of InGaN light emitting diodes (LEDs) was investigated. First of all, basic electro-optical characteristics of LEDs and operation principle of measurement instruments are introduced.
In chapter two, some reliability-related literatures were reviewed. The failure mechanisms of LEDs can be understood by studying the change in electro-optical characteristics of LEDs after burn-in stress, including increase of leakage current shunt paths, increase of reverse-bias leakage current, wavelength shift and change in current spreading or light emission pattern. When LEDs were operated under a high current stress, defects originated from lattice mismatch might be further generated owing to heat generation caused by non-radiative recombination. Furthermore, defect-related leakage current shunt paths could also cause nonuniform current spreading which leads to electrode degradation and decrease output power. In order to reduce defect density, thick buffer layers and patterned sapphire substrates (PSS) have been employed to grow InGaN-based LEDs. Therefore, several LEDs with different buffer layer thicknesses grown on either conventional sapphire substrate (CSS) or PSS were investigated to study their reliability in this thesis.
In chapter three, threading dislocation densities and indium compositions of LED wafers were analyzed by XRD first. From experimental results, we found that InGaN LEDs grown on CSS with a 2.5-m buffer layer had a similar threading dislocation density with the LED grown on PSS with a 1.5-m buffer layer. Under a high current stress, both LEDs degraded due to defect generation, which leads to a low-resistance current path, and causes current crowding effect and electrode degradation. However, PSS LED with a 1.5-m buffer layer degraded even severer because of having a thinnest buffer layer. Therefore, with similar defect density, CSS LEDs with a thicker buffer layer could have a better reliability that PSS LEDs. Furthermore, by comparing the influence of buffer layer thickness, we also found that the PSS LED with a 2.5-m buffer layer has similar performance and defect density with the PSS LED having a 3.5-m buffer layer. However, the PSS LED with a 2.5-m buffer layer degraded also severer than the one with a 3.5-m buffer layer. Therefore, we conclude that InGaN-based LEDs even grown on PSS should have at least a buffer layer thickness of 3.5 m. We conclude that a thick buffer layer is a key to improve the reliability of InGaN-based LEDs.
Finally, chapter four is the conclusion of this thesis.
Keyword: light emitting diodes、defect density、buffer layer thickness
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