圖案化藍寶石基板表面形貌對氮化鎵系發光二極體的影響

Abstract

高亮度的氮化鎵系發光二極體由於具有效率高、壽命長、小尺寸且環保等的優點而開始受到廣泛的應用。然而發光二極體必須更進一步提升其光電特性才能足以做為次世代固態照明應用。為達成這些目標，目前已發展出許多技術如表面粗化、加入金屬鏡面反射層、磊晶側向成長和圖案化藍寶石基板等。目前，由於圖案化藍寶石基板可以同時提升內部量子效率和光取出效率而最受到各界重視。 製備圖案化藍寶石基板的方式有兩種：(1)乾式蝕刻及(2)濕式蝕刻。其中乾式蝕刻在離子轟擊過程中會對圖案化藍寶石基板表面造成損傷而使得氮化鎵磊晶層內的貫穿差排密度增加。相反的，濕式蝕刻並不會對圖案化藍寶石基板產生損傷，同時濕式蝕刻亦是一種可以降低設備及製程成本的方式。 在進行濕式蝕刻時，通常是將覆以二氧化矽遮罩的藍寶石基板利用高溫的硫酸和磷酸混合溶液來進行蝕刻。而隨著濕式蝕刻時間增加，圖案化藍寶石基板底部c面面積的減小，其所製備的氮化鎵磊晶品質及發光二極體的性能也逐漸獲得提升。這是由於貫穿差排密度是會隨著底部c面面積減小而跟著減少。然而若當為了更減少c面面積而過度蝕刻反而會使得磊晶變得困難。在本論文我們發現磊晶會變得困難的原因在於過度濕式蝕刻後面指數為{4 -1 -3 18}的6C面會出現並取代底部c面使c面完全消失。此外我們也發現了大量的閃鋅礦結構氮化鎵從圖案的稜線處開始成長。 另一種提升氮化鎵磊晶品質與發光二極體光電特性的方法為將其上的圖案微縮至奈米尺寸。在本論文中，我們利用陽極氧化鋁的方法製備了具有奈米凹洞的奈米圖案化藍寶石基板。該基板在磊晶氮化鎵之後會在氮化鎵與藍寶石基板的界面處產生空氣孔隙。實驗中發現磊晶於此奈米圖案化藍寶石基板上的氮化鎵晶體品質優於磊晶於微米圖案化藍寶石基板及磊晶於傳統藍寶石基板。然而在光輸出功率卻是磊晶於微米圖案化藍寶石基板的發光二極體優於磊晶於奈米圖案化藍寶石基板的發光二極體，這乃是因為微米圖案化藍寶石基板的光取出效率大於奈米凹洞形貌的奈米圖案化藍寶石基板。 在附錄的部分，我們提出了一個新構想將鎳金屬誘發側向結晶多晶矽薄膜電晶體製備於氟矽玻璃基板上。由於鎳金屬誘發側向結晶的多晶矽薄膜電晶體其製程溫度低於600oC，因此相當適合用於玻璃基板上。然而在多晶矽和氧化層的界面以及材料內部的晶界都會有鎳與矽化鎳等鎳金屬相關雜質殘留，導致增加薄膜電晶體元件內的陷阱能態而損害電性。在利用氟矽玻璃做為基板所製備的鎳金屬誘發側向結晶的薄膜電晶體中，其電性、熱穩定性及可靠度的表現上都比起一般鎳金屬誘發側向結晶的薄膜電晶體來的優異。這是由於氟原子可以鈍化主動層內的懸鍵和應變鍵使得陷阱能態減少所致。

High-brightness GaN-based light-emitting diodes (LEDs) have been widely used in a variety of applications due to their advantages of high efficiency, long life, small size and environmental protection. For the purpose of next-generation application of solid-state lighting, LEDs with higher optoelectronic characteristics of LEDs are required. Many techniques such as surface roughing, metal mirror reflect layer, epitaxial lateral overgrowth (ELOG), and patterned sapphire substrate (PSS) have been developed to achieve these objectives. Currently, the PSS technique has attracted much attention because it can improve both internal quantum efficiency (IQE) and light extraction efficiency (LEE). There are two methods for fabricating PSSs: (1) dry etching and (2) wet etching. In dry etching, ion bombardment damages PSS surface and increases the threading dislocation density in GaN epitaxial layers. On the other hand, wet etching does not damage PSS surface. Moreover, wet etching is a less expensive method in terms of equipment and process costs. In wet etching, sapphire substrate covered with SiO2 hard mask is usually etched with a mixed solution of hot H2SO4 and H3PO4. When using wet-etched PSS, it was found that GaN quality and performance of LEDs improved with decrease in bottom c-plane areas of PSS. This was because the threading dislocation density decreased with bottom c-plane areas. However, further decrease in bottom c-plane areas made epitaxy of GaN film very difficult. In this dissertation, it was found that epitaxy difficulty was due to the appearance of 6C facets {4 -1 -3 18} and the disappearance of bottom c-plane. Besides, it was also found that most of the growth of zinc-blende GaN was initiated from the ridge of patterns. In addition, reducing the pattern size to nanoscale can further enhance the optoelectronic characteristics and GaN crystal quality of LEDs. In this dissertation, nano-sized PSS was fabricated by using anodic aluminum oxide (AAO). After growth of the GaN layer, air voids were formed at the GaN/sapphire interface. It was found that the crystal quality of GaN-based LEDs grown on nanoporous patterned sapphire substrate (NPSS) was better than that on micron-sized patterned sapphire substrate (MPSS) and on conventional sapphire substrate. However, the light output power of GaN-based LEDs grown on NPSS was smaller than that on MPSS because the light extraction efficiency (LEE) of MPSS was much larger than that of NPSS. In appendix of this dissertation, a new manufacturing method for Ni-metal-induced lateral crystallization polycrystalline silicon thin-film transistors (NILC poly-Si TFTs) using fluorinated silicate glass (FSG) was proposed. NILC is one of effect methods for manufacturing poly-Si TFTs because it could crystallize at a temperature below 600oC. Unfortunately, poly-Si/oxide interfaces and grain boundaries trap Ni and NiSi2 (Ni-related defects), which increases trap states density and degrades electrical performance of TFTs. It was found FSG-TFTs exhibited high electrical characteristics, good thermal stability and good electrical reliability compared with typical NILC-TFTs. This was because F atoms could passivate dangling bonds and strain bonds in active layer resulting in lower trap states.