利用主動層優化降低氮化鎵發光二極體效率下降特性之研究

Abstract

本論文中，我們利用改善主動層品質及優化量子井結構等方式,試圖提高氮化鎵發光二極體於高電流時的發光效率.我們將低溫成長之氮化鎵薄膜置於n-GaN及主動層間,並將此樣品與一般所使用的InGaN/GaN prestrain layer及未做任何結構之樣品作比較。並更進一步探討在高載子濃度下的LED發光效率.並用漸變式量子井結構進一步改善效率隨載子濃度遞減之情況。我們利用光激發螢光(Photoluminescence, PL)、電激發螢光(Electroluminescence, EL)、以及Advanced Physical Models of Semiconductor Devices (APSYS)模擬軟體等進行樣品的光學與電特性分析。 在本論文中,藉由變強度之光激發螢光實驗我們可以得知有插入prestrain layer 之樣品,當載子濃度提高時,其波長藍移量較小,亦即quantum confined Stark effect (QCSE)較小,因此我們可以得知使用低溫成長之氮化鎵薄膜的確是放了主動層內部之應力,並藉由室低溫變強度光激發螢光去定義出三塊樣品之內部量子效率,且有應力釋放之樣品期內部量子效率有明顯提高,並且在高載子濃度時,使用低溫成長之氮化鎵薄膜樣品之效率遞減情況最為輕微,從半高寬與載子濃度的相關圖及CL得表面影像中可以得知低溫成長之氮化鎵薄膜有效降低了主動層內部銦含量分布不均勻之情況,故可以減少載子被非輻射復合中心捕捉之機率 根據以上的實驗結果,使用低溫成長之氮化鎵薄膜及漸變厚度量子井結構的確能有效提升發光二極體之特性.分別在電流密度為22 A/cm2 及244 A/cm2 時提升效率36%及71%,並且效率遞減的情況從 22 A/cm2 到244 A/cm2 之間效率僅下降17%,優於傳統結構(54%),

Direct wide-bandgap gallium nitride (GaN) and other III-nitride-based semiconductors have attracted much attention for potential applications such as blue, green, and ultraviolet (UV) light-emitting diodes (LEDs) and blue laser diodes (LDs). Although InGaN-based LEDs have many excellent properties, the efficiency droop will occur under high current injection, resulting in the application of lighting was limited. In order to reduce the efficiency droop behavior in InGaN/GaN light emitting diodes, several semiconductor technologies have been used, such as non-polar material, AlInGaN barrier layer, thick DH active region or thick QW, barrier doping etc.. In this study, we tried to reduce the efficiency droop behavior in InGaN-based by using low temperature GaN (LT-GaN) pre-strained layer and graded quantum well (GQW) in which the well-thickness increases along [0001] direction. To better understand the influence of LT-GaN pre-strained layer and GQW on efficiency droop, a lot of measurement techniques were performed to investigate the optical and electrical properties of the grown specimens, including photoluminescence (PL), cathodoluminescence (CL), electroluminescence (EL), L-I-V curve and Advanced Physical Models of Semiconductor Devices (APSYS). Form power-dependent PL spectrum, the emission peak wavelength of the specimen with LT-GaN pre-strained layer and GQW exhibited relatively slight blue shift, which could be related to smaller quantum confined Stark effect (QCSE). Besides, the CL images shown that the specimen has more uniform emission area and less dark spots, resulting in the enhancement in emission efficiency. APSYS simulation analyzed that specimen with LT-GaN pre-strained layer and GQW revealed superior hole distribution as well as radiative recombination distribution. Additionally, according to the analysis of electroluminescence spectrum, specimen with LT-GaN pre-strained layer and GQW reveals additional emission peak from the following narrower wells within GQWs. These results are in good agreement with the result obtained from APSYS simulation. Based on the results mentioned above, a high-efficiency InGaN-based LED with LT-GaN pre-strained layer and GQW has been fabricated, which demonstrated an improvement in output power of 36% at current density of 22 A/cm2 and 71% at current density of 244 A/cm2. Besides, the efficiency droop was alleviated to be about 17% from maximum at current density of 22 A/cm2 to 244 A/cm2, which is much smaller than 54% of conventional LED.