不同量子井摻雜結構之氮化銦鎵/氮化鎵多重量子井發光二極體其光電特性之研究

Abstract

本論文主要研究不同量子井阻擋層摻雜結構的氮化銦鎵/氮化鎵發光二極體(LED)，並探討不同的摻雜結構對其光電特性之影響。在文中藉由低溫及室溫之光激螢光譜(low temperature and room temperature photoluminescence)測量，來估算不同摻雜結構的相對內部量子效率，實驗結果發現摻雜層數越多的LED樣品，會有較高之相對內部量子效率。由電激發光譜(electroluminescence, EL)的量測中發現，摻雜層數越多的樣品，其電流增加所產生的藍移量較少，我們相信這是由於摻雜所產生之載子，對量子井內的極化電場有屏蔽的效應，降低由極化電場所產生的量子侷限史塔克效應(quantum-confined Stark effect, QCSE)，而使後來注入之載子所造成的藍位移量降低。而在光強度-電流曲線的量測中，原本預估有最高內部量子效率的樣品 (6層全摻雜)，所量得的總光強度並沒有最高，反而是樣品C(3層摻雜)在量測的電流範圍內，有最高的光強度。6層全摻雜的樣品其發光強度在較低的電流即呈現飽和的情況，此結果顯示了當摻雜的層數較多時，其產生的載子填入量子井內，使後來注入的載子產生溢流(overflow)的現象。 本文另外探討了接面溫度對LED在光電特性上的影響，此實驗利用了順向電壓法(forward voltage method)來量測接面溫度，發現了在相同的操作環境下，摻雜層數越多的樣品，其熱平衡時的接面溫度會較低。在變溫的光學特性量測上，由實驗結果發現，發光效率在較低電流時會衰減的較快，然而在注入電流達到量子井的飽和量後，其發光效率隨接面溫度升高衰減的現象則會趨近於固定的比例。

In this thesis, the InGaN/GaN MQW LEDs with various barrier doping structures are investigated. As-grown samples are characterized with low temperature and room temperature photoluminescence measurement for the comparison of the relative internal quantum efficiency of different doping structures. The result reveals that higher internal quantum efficiency is achieved for more doped barriers LED. From the result of the electroluminescence measurement, LED samples with more doped barriers shows little blue-shift due to the injected carriers. We explain this phenomenon by Coulomb screening of the piezoelectric-field-induced quantum-confined Stark effect (QCSE). With an increasing number of doped-barriers, the internal field is more strongly compensated so that the magnitude of blue-shift due to the injected current is reduced. From the total radiation flux measurement, the sample with six doped barriers and highest internal quantum efficiency doesn’t give highest radiation flux as expected. Sample with 3 doped and 3 undoped barriers gives the highest output radiation flux for all injection currents instead. It is also revealed that sample with all barriers doped displays an earlier saturation of the radiation flux. The saturation of the radiation flux is a strong indication that carriers overflow the active region as current densities increase. The junction temperature measurement is also performed to investigate the influence of the junction temperature to the electrical and optical properties. We measure the junction temperature for all samples using forward voltage method. The result reveals that the sample with all doped barriers gives lowest junction temperature under the same operation condition. From the junction temperature-varied optical properties measurement, we find that the luminous efficiency will decay more quickly at low current operation. However, when the current density increases to the saturation capacity of the quantum well, the decay ratio will reach a constant.