使用奈米結構提升發光二極體之性能

Using Nanostructures for the Performance Improvements of Light Emitting Diodes

鄭允瑋(Yun-Wei Cheng)

指導教授 ： 黃建璋

國立臺灣大學/電機資訊學院/光電工程學研究所/博士(2012年)

https://doi.org/10.6342/NTU.2012.03028

摘要

在本論文中，我們使用了包含電流導向結構、側壁奈米柱反射器、以及光子晶體等奈米結構來改善氮化鎵材料製成發光二極體之特性。這些結構為發光二極體元件之低光萃取效率、高接面溫度、以及發光指向性之控制等瓶頸提供了解決方案，以期能夠將發光二極體元件應用在一般照明以及顯示科技等應用。
電流導向結構是利用離子佈植製程方式降低Ｐ型氮化鎵摻雜濃度以達到一個相對高阻抗的區域。由此高阻抗的區域造成之電流以及載子的重新分佈，電流導向結構區域會有較低的電能及光能經過，因此可以把此結構視為相對低溫區域。而相對低溫的區域則提供了附近高溫區域的散熱途徑，因此可以得到較低的發光二極體元件接面溫度。我們使用了順向電壓法以及紅外線熱影像系統等兩種方式來分析元件的接面溫度。具有此類應用離子佈植製作之低溫區域的元件展示了較低的接面溫度（在順向電壓法下，注入電流為100毫安培量測下具有離子佈植低溫區域的元件接面溫度為39°C，而傳統元件為60.6°C）。較低的接面溫度也導致較低的元件效能下降，因此具有離子佈植低溫區域的元件展示較傳統元件高35毫安培的飽和電流以及300毫安培下 86.8% 的出光效率提升。
側壁奈米柱反射器是利用奈米球微影術製作於發光二極體發光區域的周圍以跟側向行進的光進行交互作用。奈米球微影術的製程包含了旋塗以及乾蝕刻的步驟，都是非常容易可以整合進入發光二極體元件的製程。此外，奈米柱陣列的填充係數則可以透過調整奈米粒子的濃度來加以控制。光輸出功率以及發光的場型都與奈米柱陣列的填充係數相關。其中奈米柱陣列排列最為稀疏的元件展示了最高的光輸出功率（較傳統元件提升30.43%），因為其具有最高的光耦合效率。另一方面，奈米柱陣列排列最為密集的元件具有最集中的發光場型，因為其緊密的分佈可以更有效地將側向行進的光導向至垂直元件表面的方向以被收集。除此之外，我們也設計了一個特殊的元件結構，利用金屬電極來覆蓋整個發光區域的表面，以期得到較高的出光極化比例。由於側向光與奈米柱陣列的交互作用，在垂直元件表面方向光的p/s極化比例可以達到1.88。
光子晶體是利用電子束微影術所製作的週期性結構，作為衍生自前一部份準週期性奈米柱陣列的延伸研究。首先我們將蝕刻超過發光二極體主動層的奈米洞光子晶體結構製作於發光區域的周圍以期得到氮化鎵材料中低階模態的高耦合效率。發光二極體的光輸出功率以及光束塑形的能力都跟光子晶體的週期以及圖樣直徑等參數有關。接著，我們結合奈米洞光子晶體的側壁反射器結構以及針對高階模態萃取之發光區域表面蝕刻較淺的粗化結構。對於具有奈米洞光子晶體側壁反射器、表面粗化光子晶體、以及結合兩者結構的發光二極體元件，其相對於傳統元件的光輸出功率提升分別為31.4%、40.2%、以及56.4%。表面粗化主要貢獻給光萃取效率的提升，而指向性的改善則是由於奈米洞側壁反射器對側向行進光的重新導向以達到垂直元件表面方向的高收集效率。

關鍵字

發光二極體 ； 奈米結構 ； 電流導向結構 ； 側壁奈米柱反射器 ； 光子晶體 ； 奈米球微影術

並列摘要

Several nanostructures including current diverting structure, nanorod sidewall reflectors, and photonic crystals are utilized to improve the performances of the GaN-based light-emitting diodes (LEDs). Solutions to some bottlenecks such as low extraction efficiency, high junction temperature and emission directionality control are addressed for the application of LEDs in general lighting and display technology, etc.
Current diverting structure is a relatively high resistivity region fabricated by ion implantation to lower the effective p-type GaN doping concentration. This structure can be regarded as a cold zone since less electrical and optical energies flow through it due to current and carrier re-distribution. The corresponding lower temperature region provides a path for heat dissipation, resulting in lower junction temperature of the LED device. The junction temperature is analyzed using forward voltage method and infrared thermal imaging system. Devices with the ion-implanted cold zone demonstrate lower junction temperature as compared with the conventional one (39°C vs. 60.6°C at 100 mA in forward voltage method). Lower junction temperature leads to less performance degradation, which is demonstrated by 35-mA higher power saturation current and 86.8% output power enhancement factor at 300 mA.
Nanorod sidewall reflectors are fabricated using nanosphere lithography (NSL) at the periphery of light-emitting mesa in order to interact with laterally propagated light. NSL includes spin-coating and dry-etching processes, which can be easily integrated into LED fabrication. Various fill factors of the nanorod arrays can be achieved by adjusting the concentration of silica nanoparticle. Output power enhancement and emission pattern are fill factor dependent, where the device with the most sparse nanorod arrangement exhibits the highest output power enhancement (30.43%) because of higher coupling efficiency. On the other hand, the device with the densest nanorod arrangement shows the most concentrated radiation profile because of more compact distribution which can redirect the laterally propagated light to be collected in the surface normal direction. Moreover, a special design where the light-emitting mesa is covered with thick metal electrode is utilized to improve the polarization ratio. The p/s ratio as high as 1.88 in the surface normal direction can be achieved for the lateral emission interacted with nanorod arrays.
Photonic crystals (PhCs) are defined by using electron-beam lithography as further research of the quasi-periodic nanorod arrays structure. First the deeply etched nanohole PhC structure is fabricated surrounding the light-emitting mesa area for higher coupling efficiency of the lower order modes inside GaN material. The output power and beam shaping capability are both dependent on the parameters of PhC structure including the pitch and the diameter of the pattern. Furthermore, we combine the nanohole PhC sidewall structure with the shallow PhC structure on the surface of the light-emitting mesa as surface texturing for the extraction of higher order modes. Output power enhancement factors of the devices with nanohole PhC sidewall reflectors, shallow PhC surface texturing, and both structures are 31.4%, 40.2% and 56.4%, respectively. The surface texturing contributes to the extraction efficiency enhancement while the directionality improvement is related to nanohole sidewall reflectors, which redirect the lateral emission for the collection in the surface normal direction.

並列關鍵字

light-emitting diodes ； nanostructures ； current diverting structure ； sidewall nanorod reflectors ； photonic crystal ； nanosphere lithography

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