從奈米到微米結構設計以達成氮化銦鎵/氮化鎵元件之高效能光擷取/光萃取

Abstract

在本篇論文中，我們將先討論氮化鎵系的太陽能電池，接著為氮化鎵系的發光二極體，最後是我們的總結。 首先，在氮化銦鎵系的多重量子井太陽能電池上，利用自組裝的銀奈米小球當作蝕刻遮罩，去做反應式離子蝕刻，製做出二氧化矽奈米柱陣列。由於光捕捉效應及折射率的匹配(在空氣及元件間)，使此二氧化矽奈米柱陣列可有效地降低元件的表面反射率(從330至570奈米波段)。電池在模擬太陽光源(air mass 1.5G)的照射下，其短路電流明顯提升，而轉換效率可增加21 %。模擬軟體的分析也進一步証明此表面結構能改善電池的光伏特性。 第二，將太陽能電池的p型氮化鎵層製做成微米鐘的結構，也可以顯著的提升其轉化效率達102 %之多。此微米鐘結構能降低元件表面的反射率，增加電池的光吸收能力，並提升短路電流及填充因子。此經由磊晶直接成長出微米鐘的方法，可有效的改善元件的光伏特性。 第三，二氧化矽奈米柱陣列/p型氮化鎵微米鐘的分層結構被應用在氮化銦鎵的多層量子井太陽能電池上，以當作光擷取層。同樣以自組裝的銀球當作蝕刻遮罩來做反應式離子蝕刻，來將二氧化矽奈米柱陣列製作於p型氮化鎵微米鐘之上。由於此粗糙結構的光捕捉效應以及奈米柱具匹配的折射率，使得介面的菲涅耳反射(Fresnel reflection)能被更有效地降低。具此分層結構的電池表現出優異的光伏特性，能提升短路電流及填充因子，進而使轉換效率增加1.47倍。此外，元件光吸收能力的增加與以有限差分時域法(finite-difference time-domain, FDTD)分析的結果相吻合。 最後，我們將此分層結構應用在LED上，發現能增加LED的出光強度。與表面未經粗化的LED相比，在20mA注入電流下，微米鐘LED出光強度增強16.7 %，而奈米柱/微米鐘LED則增強了36.8 %之多。此結果歸因於粗化結構能使出射光散射並提供一等效折射率，來降低元件的內部全反射，進而提高光萃取率。此LED出光強度的增加也同樣可由有限差分時域法來分析得到。

In this thesis, we will firstly focus on InGaN/GaN solar cells, and secondly we move to GaN/InGaN light emitting diodes. The final is our conclusion. First, SiO2 nanorod arrays (NRAs) are fabricated on InGaN-based multiple quantum well (MQW) solar cells using self-assembled Ag nanoparticles as the etching mask and subsequent reactive ion etching. The SiO2 NRAs effectively suppress the undesired surface reflections over the wavelengths from 330 to 570 nm, which is attributed to the light-trapping effect and the improved mismatch of refractive index at the air/MQW device interface. Under the air mass 1.5 global illumination, the conversion efficiency of the solar cell is enhanced by ~21 % largely due to increased short-circuit current from 0.71 to 0.76 mA/cm2. The enhanced device performances by the optical absorption improvement are supported by the simulation analysis as well. Second, InGaN-based multiple quantum well (MQW) solar cells (SCs) employing the p-GaN microdome were demonstrated to significantly boost the conversion efficiency by 102 %. The improvements in short-circuit current density (Jsc, from 0.43 to 0.54 mA/cm2) and fill factor (from 44 % to 72 %) using the p-GaN microdome are attributed to enhanced light absorption due to surface reflection suppression. The concept of microdome directly grown during SC epitaxial growth preserving mechanical robustness and wafer-scale uniformity proves a promising way in promoting the photovoltaic performances of SCs without any additional process. Third, the hierarchical structure of SiO2 nanorod arrays/p-GaN microdomes was applied as a light harvesting scheme on InGaN-based multiple quantum well solar cells. Using self-assembled Ag nanoparticles as the etching mask and subsequent reactive ion etching, SiO2 NRAs were fabricated upon the p-GaN microdomes. Due to the light trapping effect of the roughness and the improved match of refractive index by SiO2 nanorod arrays, the undesired Fresnel reflections are effectively suppressed. Cells with the hierarchical surfaces exhibit excellent photovoltaic performances including enhanced short-circuit current densities and fill factor, and the measured conversion efficiency is enhanced by 1.47-fold. The improved light absorption in device is consistent with the finite-difference time-domain analysis. Finally, we report the enhanced light extraction efficiency of the hierarchical structure, SiO2 nanorods/p-GaN microdomes, fabricating on InGaN/GaN LEDs. Compared with conventional flat LEDs, the light output intensity of bare microdome LED presents an improvement of 16.7 % at 20 mA, yet it boosts to 36.8 % for SiO2 NRA/p-GaN microdome LED. The results are attributed to the scattering effect and the effective refraction indexes of the textured structures that reduce the total internal reflection, contributing to the most light extraction. The enhanced optical performances are supported by the improved light output power calculated by finite-difference time-domain analysis.