氮化鎵奈米柱發光二極體之光萃取效率提升及偏極化光源達成

Abstract

自1990年後，p型氮化鎵發展完備，氮化鎵目前廣泛的被應用在發光二極體上。發光二極體具有反應時間短，可靠度高，元件壽命長，低耗電等優點。近幾年來，發光二極體廣泛地應用在液晶螢幕的背光模組。由於氮化鎵與空氣的折射係數差異，其介面處將會發生全反射，並使得光萃取效率降低。另一方面，現今業界將極化片放置於背光源以及液晶螢幕之間藉此達成極化光緣，但極化片將會使通過的光強度降低一半以上。因此，發光二極體仍有許多有趣的議題值得探討，例如，偏極化光緣的達成以及光萃取率的增加。 在本篇文章中，藉由塗佈自組排列的單層二氧化矽奈米小球做為蝕刻遮罩以製作自組性奈米柱陣列，並圍繞於發光二極體方型發光區，這種奈米小球自然微影技術不需增加額外光罩，所以製作成本低廉，為一種非常有發展潛力的奈米結構製作技術。我們證明此種自組排列的奈米柱陣列可以收集發光二極體內被量子井侷限之側向傳遞光。由光強度對電流作圖，可以得知，有20微米、密度為0.21的奈米柱陣列圍繞之氮化鎵發光二極體，其於15毫安培的電流注入下，將會有40%的光強度增加。由於隨機排列的奈米柱造成各向等性的光散射，各角度的光強度皆上升。 延續上個實驗，接下來我們提出一個新穎的低成本方法來提升發光二極體的光極化比例以及光萃取效率－將手指狀的p型發光區圍繞以自然微影製作的自組排列奈米柱陣列；由於奈米柱排列方向垂直於p型極化光的電場振動方向，所以奈米柱陣列對p型極化光的布拉格繞射效果比s型極化光大，故有奈米柱陣列圍繞的發光二極體，其極化比例高於沒有奈米柱陣列圍繞的元件。在發光二極體的正上方量測，有奈米柱陣列圍繞的元件，其p型極化光為s型極化光的1.96倍；若是將00到900的光強度積分起來，將得到p型極化光為s型極化光的1.52倍。

Since p-type GaN is well developed in 1990’s, GaN has been widely used in light emitting diodes. There are several advantages such as short response time, good reliability, long life time, and low power consumption for GaN light emitting diodes. In recent year, the demands of light emitting diodes such as backlight module increased dramatically. Since the refraction index is different between GaN and air, total reflection which decreases low external quantum efficiency causes at the interface. On the other hand, the commercial method to get polarized light is place polarizer between the backlight and liquid crystal. This method will leads light intensity decay at least 50% when light passes through the polarizer. Therefore, there are interesting topics of LEDs such as getting a polarized light source and improving the light extraction efficiency. In my thesis, by spin-coating a monolayer of self-aligned SiO2 nanosphere as the etching mask, nanorods array which encompassed with square shape light emitting mesa are fabricated. Since no additional lithography-mask needs by using the nanoparticle natural lithography, it has well potential to fabricate the low-cost nanostructure. We demonstrate a method of utilizing self-assembled nanorods array to collect the laterally propagating guided modes from a light emitting diode (LED). We measure a light intensity enhancement factor of 40% from GaN-based LEDs encompassed with 20μm thick nanorods array which filling factor is 0.21. Such power enhancement is found to be omni-directional due to a broken symmetry from a randomized distribution of the nanorods array placed along the periphery of the LED’s mesa. These observations indicate that the use of nanorods array can efficiently redirect the propagation of the laterally guided modes to the surface normal direction. Continuing from the previous experiment, we provide a noval and low-cost method by using nanorods to get polarized light source and enhance the output power of light emitting diodes－finger shape p-mase encompassed with self-aligned nanorods array by using natural lithography. Since the bragg diffraction of laterally propagated p-polarized mode by nanorods is more efficient than the s-polarized light, the p/s ratio of the device with nanorods is higher than that without rods. The polarization behavior of the light emitting diodes with and without nanorods surrounding the p-mesa is investigated. In my research, the p/s ratio of the LED with nanorods is 1.96 at 90˚, and is 1.52 when the integrating intensity between 0˚ and 90˚ is considered.