氮化鎵發光二極體的發光效率改善

Emission Efficiency Improvement of GaN-based Light-emitting Diodes

呂志鋒(Chih-Feng Lu)

指導教授 ： 楊志忠

國立臺灣大學/電機資訊學院/光電工程學研究所/博士(2010年)

https://doi.org/10.6342/NTU.2010.03519

摘要

本論文中，一開始我們介紹預施應力之化學氣相沈積技術並描述其應用。我們證明了在預應變生長的氮化銦鎵/氮化鎵LED中隨著注入電流的增加，頻譜整體的紅移和頻譜藍移對於不同預應變位障層厚度的關連。由於較薄的預應變位障層具有較強烈的預應變效果，當注入電流增加時較薄的預應變位障層導致較大量的頻譜紅移及較少量的藍移。此外，可發現藉由元件電阻與飽和電流，預應變樣品製作的LED相較於傳統的LED有較佳的效能。這些現象我们歸因於具有較強烈預應變效果的樣品會有較高的平均銦含量,並且富銦聚集的行為也會較為明顯.
此外，相較於傳統長晶法的綠光樣品，我們利用預施應力生長法提高綠光樣品的長晶溫度，使得同樣發光波長的綠光發光二極體有較好發光效率。我們也利用此技術發展高效率的黃光量子井，在不需使用螢光粉下，混合上層量子井所發出的藍光而成功製作白光發光二極體，其電激螢光頻譜可接近理想白光的色座標位置 (1/3, 1/3)。
我們研發出以表面電槳和量子井之間的能量耦合機制來降低發光二極體外部量子效率的低垂。從最大外部量子效率所對應到的注入電流密度及低垂斜率的降低趨勢之結果顯示，較小的電流擴散網格可以使銀薄膜之下的區域達到較有效率的表面電槳和量子井之間的能量耦合，從而達到降低外部量子效率的低垂的目的。並且藉由改變p型氮化鎵的厚度所得到不同的低垂結果來釐清表面電槳和量子井之間的能量耦合與其他機制對元件的影響。
最後，經由改變發光二極體的接觸阻抗，我們使得量子井的接合溫度提高，我們觀察到此效應會讓載子因為晶格的震盪而有更大的機會進入更深的量子井複合，而此所對應量子井發出的藍光強度會提升到與最上層量子井發出的綠光強度相近。

關鍵字

氮化鎵 ； 發光二極體

並列摘要

In this dissertation, we have demonstrated the dependencies of output spectral overall red shift and spectral blue shift in increasing injection current on the prestrained barrier thickness in an InGaN/GaN QW LED of prestrained growth. It was found that a thinner prestrained barrier led to a larger general spectral red shift and a smaller blue shift in increasing injection current because of the stronger prestrain effect. Also, it was found that in terms of device resistance and saturation current, the LED performances of prestrained samples were better than that of a conventional LED. An LED of a thinner prestrained barrier had a better performance. These observations were attributed to the higher average indium content and stronger indium-rich clustering behavior in a sample of stronger prestrain. These attributions were supported by the SSA results in the TEM measurements.
Furthermore, a green LED was fabricated based on the prestrained growth technique to compare with an LED of the same emission wavelength based on the conventional growth method. Then, the prestrained underlying growth technique was used to grow three yellow-emitting QWs of high efficiency. The yellow photons were mixed with blue light from an overgrown blue-emitting QW to produce white light. The improved properties of the phosphor-free monolithic white-light LED have been discussed.
The reduction of the EQE droop of an LED through the SP-QW coupling mechanism has been demonstrated. With a current spreading grid pattern on the mesa surface, it was found that a smaller grid period led to more effective carrier transport into the QW regions under Ag deposition for stronger SP-QW coupling such that the droop effect was more significantly reduced, including the increase of injection current density of maximum EQE and the decrease of drooping slope. The observation of the SP-QW coupling effect in the samples of thin p-GaN was also supported by the different droop behaviors of the LED samples fabricated with the epitaxial structure of thick p-GaN, in which the SP-QW coupling effect is weak.
Besides, we have demonstrated the dependence of the output spectrum on the junction temperature of a blue/green two-color InGaN/GaN QW LED. By decreasing the metal thickness of the p-type Ohmic contact on the device, the contact resistance was increased and hence the junction temperature was also increased. With the junction temperature increased, the probability for the deeper QWs to capture holes became higher such that blue emission could be enhanced to compete with the dominating green emission from the first QW. The conclusion of a higher junction temperature in a sample of a thinner p-type metal layer was consistent with the measurements of the output power versus the injection current, the current versus the applied voltage, and the direct measurement of contact resistance.

並列關鍵字

GaN ； Light-emitting Diodes

參考文獻

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