以低壓成長之有機金屬氣相磊晶法改善氮化銦鎵/氮化鎵發光二極體元件特性之研究

Abstract

InGaN/GaN材料因為缺乏適當的基板可以作為材料磊晶成長之用，所以GaN薄膜之缺陷密度高達108-1010 cm-2，且因為GaN材料中原生的缺陷會使材料呈現n型半導體的特性，所以高電洞濃度之p型GaN材料製作不易，因此GaN材料所製作之發光二極體元件具有許多特性上之缺陷。 論文中將針對目前GaN發光二極體元件的缺點提出可行的方法，以改善目前InGaN/GaN LED元件所面臨的缺陷，並有效獲得下列成果，包括(1)利用調節InGaN/GaN之多層量子阱(multiple quantum well, MQW)厚度及In成分，以平衡MQW材料間之壓電效應，有效提昇LED元件之亮度及改善元件在不同操作電流下之波長位移。(2)外部量子效率改善方面，利用二次成長微米丘(micro-hillocks) p-GaN以及低溫成長六角形空穴表面之p-GaN以粗化LED表面元件，上述兩種方式對LED元件之出光效率提昇分別高達52%以及47%，有效提昇LED元件支出光效率。(3) 利用具應力之InGaN歐姆接觸層改善透明導電層ITO與p-GaN間之歐姆接觸，與不具SCL層之標準LED結構相比，此方法可以使元件在20mA驅動電流下之之操作電壓降低約0.35V，解決長期以來ITO與p-GaN之間接觸電阻偏高之問題。(4) 利用平坦化p-GaN磊晶條件改善發光二極體元件之抗ESD能力，使得LED元件之抗ESD能力由原先MM (machine mode) -300V測試通過率由0%提昇至82%，HBM (human body mode) -4000V測試通過率由0%提昇至92%，LED元件之抗ESD能力獲得大幅度之改善。

Due to the lack of native substrates for InGaN/GaN materials epitaxy growth, the densities of threading dislocation are as high as 108-1010 cm-2. The other challenge of GaN devices is the high holes concentration of p type GaN are very difficult to made, because the native defects in GaN material will reveal high electrons carrier concentrations. These drawbacks of GaN material lead to many poor characteristics in the GaN devices. In this study, we propose many suitable methods to improve the drawbacks of InGaN/GaN light-emitting diodes (LEDs) and greatly improvement on the poor characteristics of LED devices. There are four topics in this study list below: (1) Optimize the well thickness and In composition in InGaN/GaN multiple quantum wells (MQW). Due to the optimum the composition of MQW, the piezoelectric field in the MQW will be reduced. The internal quantum efficiency and the wavelength shift of the LED devices will be improved. (2) Due to the effect of total reflectance, only few part of light from MQW will escape from device to air. In this study we use two methods to improve the light extraction efficiency. One is using LPMOCVD to regrowth the micro-hillock p-GaN on the surface of the LED devices. The other is using LPMOCVD to grow the low temperature p-GaN on the surface of LED devices. These two methods will make a texture surface on the device to break the symmetric of the light path and then the total reflectance will be reduced. Finally, the output power will be increased 52% and 47%, respectively. (3) Due to poor Ohmic contact between ITO and p-GaN, we using strained InGaN as an interlayer between ITO and p-GaN. Compare with the standard LEDs without strained contact layer between ITO and p-GaN, the forward voltage at 20mA driven current is 0.35V lower. It can successfully solve the high contact resistance when the ITO contact with p-GaN. (4) Using the planarized growth conditions to fill the hexagonal pits on the LED surface, the LED device can endure higher ESD stress. In this study, we can improve the surviving rate from 0% to 82% under machine mode -300V stress and the surviving rate from 0% to 92% under human body mode -4000V stress. The ability to endure ESD stress will be greatly improved.