氮化銦鎵高功率發光二極體之衰減機制

Abstract

近年來，高功率發光二極體被積極地使用於高亮度照明系統，肇因於其在高亮度應用之下的可靠度不如預期，發光二極體尚無法廣泛地取代傳統照明設備。在過去可靠度的研究當中，絕大部分的文獻是利用標準燒機測試來檢測發光二極體的壽命及衰減，本論文提出了有別於標準燒機測試之測試方式，藉由此方法可分開觀察熱、電流、以及近紫外光所引起的衰減，相較於燒機測試，本方法可進一步探究其衰減的機制。 本研究利用觀察數十顆市售高功率發光二極體光電特性的變化，並藉由下述之測試條件，來分析熱、電流、近紫外光引起的衰減。欲探究高溫對發光二極體的光電特性造成的衰減，將受試樣品置於攝氏 200 度以及 300 度的烤箱之中，此溫度是模擬高功率發光二極體在 350 mA 以及 500 mA 操作之下的接面溫度；欲探討電流以及近紫外光引起的衰減，將樣品操作在 350 mA 的電流之下，並利用風扇主動散熱系統來降低接面溫度至攝氏 64 度，在此條件之下的光電特性變化，可歸因於電流造成的晶片衰減以及近紫外光造成的封裝體衰減。除此之外，本研究也更進一步地探討矽膠、銀膠等數種封裝材料的衰減機制。 實驗結果指出：(1) 在高溫之下，發光二極體的塑膠封裝體、矽膠、銀膠、銀反射鏡以及螢光粉等封裝材料，其光特性以及熱特性都會明顯地衰減；(2) 銀金屬極容易與大氣中的分子反應，因此藉由可隔絕大氣之封裝，可以有效地降低銀膠、銀反射鏡的衰減；(3) 高溫使得 p 型氮化鎵之中的參雜不穩定，進而導致 p 型氮化鎵與氧化銦錫之間的歐姆接觸特性衰減；(4) 電流造成主動層區域的缺陷密度增加，導致反向、正向漏電流上升，也因此降低了其能量轉換效率；(5) 除了電特性之外，晶片的波峰波長以及半高寬等光特性也會有變化；(6) 發光二極體隨著測試時間的增加，接面溫度逐漸上升，除此之外，接面至周遭的熱阻也會上升。 不論是封裝還是晶片，高溫對其衰減扮演了很重要的角色，因此一個好的散熱設計，一方面可降低接面溫度，提高發光二極體的內部量子效率，另一方面亦可提高其可靠度。對於晶片來說，控制接面溫度低於攝氏 100 度即可有效地減緩晶片的衰減。但對封裝材料而言，除了溫度外，近紫外光也會造成其衰減，因此在選擇材料的時候，耐熱且抗紫外光的材料較適合使用在氮化銦鎵高功率發光二極體。

Reliability issues of light-emitting diodes (LEDs) have gathered great importance in recent years because the LED-based technologies are popular and have been widely used in daily life. For researches on reliability of LEDs, burn-in test is typically adopted to estimate the lifetime and performance of LEDs. Through a burn-in test, the devices are impacted by high junction temperature, high current density and high intensity of near-UV radiation simultaneously. As a result, the degradation mechanisms of LEDs are superimposed and hence are difficult to separately analyzed. By applying LEDs under well-designed aging conditions, the heat, current and near-UV radiation induced degradation are separately analyzed. For heat induced degradation, the analyzed LEDs are sent in temperature controlled ovens without current driving. For current and near-UV induced degradation, the LEDs are driven under 350 mA with various junction temperature. The mechanisms of chip level and package level degradation are further examined by considering the variation of optical and electrical properties for LEDs with various packaging styles. In addition, the degradation of silicone, silver paste, Ohmic contacts and current spreading characteristics are separately analyzed. Results of typical and accelerated burn-in tests show the optical output power is decreased over time. Also, the forward voltage, reverse leakage current and forward leakage current are increased. Further analysis show following: (1) High temperature stress can greatly impact the optical and thermal properties of package materials, including plastic leadframe, silicone, silver paste, silver reflectors and YAG phosphor; (2) A great encapsulant (e.g. silicone) is needed to minimize the chemical reaction of package material with atmosphere; (3) high temperature stress can significantly affect the Ohmic contact between p-type GaN and ITO and hence cause the modifications of I–V characteristics in forward region; (4) high current density can induce the increase of reverse and forward leakage current and is related to the variation of energy efficiency; (5) the optical properties of LEDs, including peak wavelength, FWHM and CCT, varied after burn-in tests; (6) the junction temperature and temperature resistance from junction to ambient of LEDs is increased after burn-in tests. To conclude, a well thermal management can enhance the reliability of InGaN-based high power LEDs. We suggest that the operation junction temperature which is lower than 100 ℃ is a reasonable value to minimize the degradation of LED processing and chip. To overcome the near-UV induced packaging degradation, the researches of high durability materials are believed to obviously enhance the reliability of devices.