氮化銦鎵/氮化鎵多重量子井結構發光二極體在高電流密度下其發光效率衰減之研究

Abstract

本論文主要研究不同量子井寬度結構的氮化銦鎵/氮化鎵發光二極體(LED)，其光電特性的比較，並著重於探討發光效率在高電流密度下衰減問題(efficiency droop behavior)。在低溫至室溫光激發螢光譜(low temperature to room temperature photoluminescence)量測中，發現量子井寬度較窄的LED結構，其估算的相對內部量子效率最高，相信這是由於量子井寬度較窄的結構，其所受的內部電場較小，其電子電洞對波函數分離較小，故其發光效率最高。另外在室溫下電激發螢光譜(electroluminescence, EL)量測中，亦可以發現量子井寬度較窄的LED結構，其電流增加所產生的藍位移量較少，同樣也跟其所受量子侷限史塔克效應(Quantum-confined Stark effect, QCSE)小有關係。在發光效率對電流曲線的量測中，我們發現量子井寬度較窄的LED結構，在通以電流至250 mA下，其總發光強度是最高的，但是其發光效率從峰值至最低值，其下降量是最多的，而量子井寬度較寬的結構，其發光效率在高電流密度下僅衰減2.9 %。此發光效率在高電流密度下衰減歸因於量子井寬度較窄的結構，其載子溢流(carrier overflow)情況較多。 同時，透過模擬方法我們得到載子在量子井內的分布，並透過能帶圖分布驗證內部電場的影響為何。發光效率衰減原因方面，由參考資料得知有歐傑復合(Auger recombination)跟載子溢流的兩種因素影響。透過模擬結果，我們提出發光效率開始下降這一段是由歐傑復合效應所影響，而當量子井內的載子飽和後，歐傑復合效應也開始趨於飽和，故在高電流密度下，載子溢流便是造成發光效率持續下降的主要原因。

In recent thesis, InGaN/GaN-based light-emitting diodes (LEDs) with various well thicknesses are investigated. The so-called efficiency droop behavior is the reduction of LED’s emitting efficiencies at higher current densities. In this research, to understand the efficiency droop phenomenon both experiment and simulation are performed. Photoluminescence experimental results showed that thinner well structure has higher relative internal quantum efficiency. This is attributed to the smaller electron-hole charge separation induced by internal field with thinner wells. Electroluminescence experimental results showed that thicker well structure behaves less efficiency droop. On the other hand, thinner well structure has significant efficiency reduction at high current densities. It is assumed that Auger recombination as carrier concentration raised and carrier overflow at higher current densities are the dominant mechanism for the reduction of efficiency. However, according to simulation results, the efficiency droop behavior from low to high current density is contributed by Auger recombination first, then by carrier overflow at higher current density. The more optical characteristic in each structure will be further discussed.