表面微結構對氮化鎵發光二極體光電特性之影響

Abstract

本論文主旨為探討與增進氮化鎵發光二極體（GaN-Based Light-Emitting Diode，GaN-LED）表現相關的議題，包括了鏡面在元件中的位置對於發光功率的影響、n型氮化鎵碗狀結構與鏡面的結合在調變發光角度過程中所扮演的角色，以及利用自然微影方式對氧化銦錫透光層做表面粗化的結果探討。 在鏡面於元件中的位置對於發光功率影響的實驗中，我們發現DRSM-LED（LED結構包含雙粗化表面以及鍍附於藍寶石基板底面的鏡面結構）的發光強度為PR-LED（LED結構包含單一p型氮化鎵的粗化表面）的2.10倍。當改變鏡面位置將其從藍寶石基板底面移至無參雜氮化鎵與藍寶石基板的介面（即DRM-LED）時，發光強度得到了更大的提升（為PR-LED的3.05倍）。這是由於在DRSM-LED中，光子必須比在DRM-LED中多穿越額外的接合界面（環氧化物╱藍寶石基板）兩次。此外，當往元件底部行進的光子被DRSM-LED的鏡面改變方向回到環氧化物╱藍寶石基板的界面時，只有光徑在臨界角61.9°以內才能夠穿越，大於臨界角的部分會於藍寶石基板內形成全反射而損失。 在NBM-LED（LED結構包含p型氮化鎵粗化表面，以及n側碗型結構與鏡面結合的表面）的實驗中，我們利用三種不同尺寸的碗狀結構來調變發光角度與增進元件表現。我們發現發光強度與發光功率會隨著n側碗型結構尺寸的縮小而提升，其中以3微米者為最佳，其發光強度與功率為PR-LED的2.33與1.43倍。此外，NBM-LED的發光角度會隨著n側碗型結構尺寸的縮小而縮小，由PR-LED的130°縮小至3微米碗狀結構元件的118°，這是由於碗狀結構與鏡面的結合可視為凹面鏡，此凹面鏡不但可以將往元件底面行進的光子導至正面，且可將光徑在臨界錐形區外的光子修正回圓錐內。 在氧化銦錫透光層的粗化方面，我們提供了一種簡單而有效的自然微影法用以製作GaN-LED。此法是將不同塗佈厚度的光阻作為感應耦合電漿（ICP）乾蝕刻製程的結構轉印模具，於蝕刻過程中，光阻表面會因熱累積而產生縐折，此縐折並於其後轉印至氧化銦錫上。不同粗化程度的元件，其發光強度與功率皆高於傳統GaN-LED。以1.9微米光阻製作的元件，發光強度（71.6 mcd）為傳統LED的1.5倍，且為1.6微米的1.2倍。在發光功率方面，1.9微米光阻製作的元件（5.75 mW）約為傳統LED的1.27倍，且為1.6微米的1.07倍。這是因為粗化的氧化銦錫不但提供光子更高的機率穿越LED表面，而且具有改變光子路徑的優點。

The primary contents of this dissertation are discussing subjects which are relating to the performance enhancement of InGaN-GaN light-emitting diodes, including the influence of the mirror location to the light output, the role of the n-bowl/mirror structure played in managing the view angle, and the roughening of the ITO window layer by a natural lithography method. In the investigation of the effect of the mirror location on the performance of LEDs, we found that the light intensity of DRSM-LED (LED with double-roughened surfaces and a mirror system on the back side of sapphire substrate) was 2.10 times higher than that of PR-LED (LED with roughened p-GaN surface). By changing the mirror location from the backside of the sapphire to the u-GaN/sapphire interface (DRM-LED), the LED light intensity was further enhanced (3.05 times higher than the PR-LED). This is because, compared with DRM-LED, the photon path inside the DRSM-LED structure has to pass through an extra bonded interface (epoxy/sapphire) 2 times. Besides, when the mirror of DRSM-LED redirecting the downward-traveling light, light traveling from sapphire to the adhesive layer will only cross within a critical angle of 61.9°. The light reaching the adhesive layer beyond the critical angle will undergo total internal reflection. Three types of NBM-LEDs (LEDs composed of roughened p-GaN surface, n-bowl structure, and mirror on the n-bowl surface.) were introduced to adjust the view angle and improve the LED performance. We found that the light intensity and output power of NBM-LEDs increased with the decrease in n-bowl dimension. The luminance intensity and output power of the NBM-LED with 3 μm n-bowl were the highests, which were 2.33 and 1.43 times higher than PR-LED. Besides, the view angle decreased with the diameter of n-bowl. The viewing angle of PR-LED was 130°. As the diameter of n-bowl decreased to 3 μm, the view angle decreased to 118牵. This is because the n-bowl mirror structure acting as a concave mirror. It not only reflected the downward photons to the front side, but also redirected the photons which were originally emitted out of the escape cone, back into the escape cone. An investigation of the relationship between ICP etching conditions and LED performance has led to the development of a simple, effective natural lithography process for preparing ITO-textured surfaces useful for fabricating high-brightness GaN-based LEDs. In this lithography process, photoresist layers (AZ-1518) of different thicknesses (1.6 and 1.9 μm) were used as a mask for ICP dry etching. During etching, surface of the photoresist deformed because of the thermal accumulation, and this undulation was subsequently transferred to ITO surface. The light intensity and output power of IR-LEDs were better than those of the CV-LED. The luminance intensity of the IR1.9-LED was 71.6 mcd, which was 1.5 times higher than that of the CV-LED, and 1.2 times higher than that of the IR1.6-LED. The IR1.9-LED achieved an output power of 5.75 mW, which was 1.27 times higher than that of the CV-LED, and 1.07 times higher than that of the IR1.6-LED. This is because the roughened ITO surface provided the photons multiple opportunities to escape from the LED surface, and redirected the photons.