高功率發光二極體特性探討與可靠性問題研究

Abstract

氮化銦鎵發光二極體（InGaN-based LEDs）近年來因其在指示燈與一般照明的廣泛應用而備受關注,特別是最近在平板顯示器光源與白光照明的應用，目前高功率 LED 是最有可能取代傳統白熾燈和螢光燈的候選人，因此使得 LED 的壽命與可靠性問題益發重要，老化測試則是傳統最常用來研究並評估 LED 壽命和性能的方法之ㄧ，對發光二極體而言,如 PN 介面溫度、電流密度、環境溫度和靜電放電（ESD）均會導致發光二極體的逐步老化失效，由此可知，LEDs 的衰退機制是複雜且難以獨立分開分析的。本研究目的就是試圖逐步釐清熱應力、電應力和靜電放電對高功率氮化銦鎵發光二極體造成的傷害現象,並進一步探討氮化銦鎵發光二極體的老化與失效機制。 首先，本研究利用 1W 大功率的白光發光二極體,設計不同的老化條件,並經過 6000 小時以上長時間的燒機(burn-in)測試,觀察發光二極體晶片與封裝材料的逐步老化現象與相互影響因素。實驗結果顯示發光二極體的光通量會隨著不同的熱應力和電流密度出現不同的衰減，並計算在各種燒機應力下的 PN 界面溫度， 以探討熱應力對發光二極體光通量退化的影響。較高電應力的老化條件在短時間內即可導致發光二極體晶片內的逆向漏電流增加。而在長時間燒機實驗中,則可發現發光二極體的封裝因 PN 界面溫度影響而逐步老化，晶片上方的光學透鏡表面產生裂縫並且漸漸黃化，此現象將使得發光二極體的光取出效率變差而造成光通量退化。 其次，我們將重點放在解讀氮化銦鎵發光二極體晶片的各失效特徵，並且提供一個發光二極體晶片的老化失效模型，利用此老化電路模型對發光二極體活性層(active layer)中出現過低啟始電壓(turn-on voltage)的現象進行探討，研究此現象在發光二極體老化過程中扮演的角色，並利用失效分析驗證模型推估結果與實驗數據相匹配，進一步發現 V 型缺陷相關的差排(threading dislocation)在活性層扮演一個電流分流路徑,造成發光二極體電流分佈不均勻的現象，驗證氮化銦鎵發光二極體的電流分佈均勻性可做為評估其可靠度的關鍵性指標之ㄧ。 最後，對水平結構且以絕緣藍寶石為基板的氮化銦鎵發光二極體而言，靜電(ESD)耐受性的能力更是其磊晶與製程可靠性的重要指標之ㄧ，實驗數據顯示具有較小的逆向漏電流的發光二極體晶片擁有較好的靜電耐受性，而根據我們上一個的實驗結果，發光二極體晶片的相關電特性與晶片的表面形貌有很強的關聯，因此針對發光二極體表面的 V 型缺陷進行探討，發現局部性的缺陷，如 V 型缺陷和其相關差排，會在承受靜電時形成極高的局部電場，並造成小區域嚴重的靜 電放電損傷，因此，V 形缺陷和晶片的表面形貌都與發光二極體晶片的靜電耐受性能力有很強的關聯性。 總而言之，我們認為一個擁有良好熱管理設計、電流分佈均勻、表面缺陷較少和低差排密度的氮化銦鎵發光二極體晶片將具有較高的可靠度與使用壽命。另一方面，對於發光二極體的封裝形式與材料而言，降低封裝材料的操作溫度與採用耐高溫的封裝材料是改善可靠度的兩個主要方向，可有效減緩發光二極體的光通量衰減速度。

In recent years, InGaN-based light-emitting diodes (LEDs) have attracted much attention due to their application as indication lights and for illumination, such as the use of white light LEDs and their utilization as light sources for flat panel displays. LED reliability is a matter that has gathered great interest because high-power LEDs are the most favorable candidate to replace conventional incandescent bulbs and fluorescent lamps. Typically, to investigate the reliability of LEDs, aging tests are administered to estimate their lifetime and performance. During these tests, the devices are exposed to high junction temperature, high current density and electrostatic discharge (ESD) simultaneously to examine their gradual erosion. Therefore, LED degradation mechanisms are complex and difficult to be analyzed individually. The purpose of this study is to investigate the failure mechanisms of high power InGaN-based LEDs induced by heat, currents, and electrostatic discharge. This dissertation consists of three major portions. First, the impact factors of high-power 1W white LEDs with the commercial package under well-designed aging conditions were exposed to identify the degradation phenomena between LED chip and package materials. The most important index of LEDs, luminous flux, noticeably decreased with different electrical and thermal stresses. At various aging stresses, junction temperatures were compared to determine how thermal stresses factor into luminous degradation. Under short-term high electric stress, defects in LED chips were induced to increase the leakage current. As for the LED package, gradual yellowing and cracking of the encapsulating lens during the long-term aging process caused degradation of light extraction efficiency and the degradation levels of the LED package are strongly correlated with junction temperature. Secondly, we focused on the failure characteristics of InGaN-based LED chips and provided a degradation mechanism to realize one of the failure modes. An effective circuit model was proposed to analyze the behavior of premature turn-on diodes in the active layer. One of the key indicators to assess the reliability of InGaN-based LED chips is the uniformity of current spreading. According to the investigation of failure analyses, the simulation results matched the experimental data. Furthermore, V-shaped defects and the associated threading dislocations induced the inhomogeneous distribution of forward currents in LED chips because the threading dislocations associated with V-shaped defects act as very small shunt resistors connected across p-n junctions. Lastly, the ESD endurance of InGaN-based LED chips is also examined because ESD endurance is the important index for horizontal structure of the sapphire substrate. The experimental data shows that LED chips with a smaller reverse leakage current endures ESD better. Subsequently, in keeping with results from the previous stage of experimentation, it became necessary to research the correlations between electrical properties and the surface morphology of LED chips. Findings establish that local defects, such as V-shaped pits, are a major factor in the creation of high electrical fields and they augment ESD damage during ESD stress. Consequently, V-shaped defects and surface morphology are strongly correlated to the ESD endurance of InGaN-based LED chips. To conclude, well-designed thermal management, uniform current spreading and defect-free surface morphology can enhance the reliability of InGaN-based high power LEDs. LED packaging should be done at lower temperatures, which can minimize the LED degradation progress. It is also suggested that reducing the density of the threading dislocation and V-shaped defects during the processing of InGaN-based LED chips should effectively improve the reliability of LED chips.