介電奈米粒子與氧化金之光學性質探討及其在光電元件上之應用

Abstract

近來有越來越多研究指向利用不同尺度的傳統材料或是研發新穎材料來增進或改善光電元件的表現及效率。各類光電元件如：太陽能電池、發光二極體以及光偵測器都已經廣泛運用在不同地方。因此如何能更進一步增進這些元件的效能便是一重要課題。 本論文中首先將不同材料的介電質奈米粒子建構成漸變折射率抗反射層以增加矽基太陽能電池的效率。我們發現將二氧化鈦、二氧化鋯、三氧化二鋁以及二氧化矽奈米粒子依序旋轉塗佈於矽基太陽能電池上可以建構一寬波段及廣角度的抗反射層。使光電流分別在垂直入射以及以60o入射時在全波段增益超過30%及45%。 接著我們延續這樣的概念，建構漸變折射率的薄膜於發光二極體上，並再利用尺寸與光波長相近的二氧化矽散射粒子破壞發光二極體的主要材料氮化鎵以及空氣全反射面，使超過臨界角的光可以離開晶片。我們利用二氧化鋯、三氧化二釔、三氧化二鋁以及二氧化矽構成漸變折射率奈米粒子薄膜以及460奈米的二氧化矽散射粒子使藍光發光二極體的光激螢光光譜在波長為460奈米時增強6倍。 此外，本論文也對氧化金這個材料進行研究。金一般來說被認為活性最低的元素，不易與其它元素化合。我們利用離子濺鍍的方式鍍出三氧化二金的薄膜。並對其光學常數、還原機制做深入的探討。我們證實氧化金為一介電質材料，有著低導電度以及高折射率的特性。本論文也利用光譜分析發現氧化金在還原成金的過程會在波長為670奈米到700奈米左右有侷部表面電漿波共振的訊號。我們也將氧化金與n型矽形成接面製造光偵測器，在紅光及近紅外光波段有將近50%的外部量子效率以及超過1012 的偵測力。

Recently, there have been various studies focusing on producing materials with different scales or developing new materials to pursue better performance of optoelectronic devices. Several optoelectronic devices like solar cells, light emitting diodes (LEDs) and photo detectors have been applied in different fields. Therefore, how to improve their efficiencies has become an essential issue. In this study, we first arranged nanoparticles (NPs) of various types and sizes to enhance the omnidirectional light harvesting of solar cells. The nanoparticles we used include titanium dioxide (TiO2), zirconium dioxide (ZrO2), aluminum oxide (Al2O3), and silicon dioxide (SiO2). In contrast to previous reports, here we focused on choosing and arranging suitable materials and varying their sizes, rather than narrowly concentrating on optimization of a single type of particle. A graded-refractive-index NP stack could minimize reflectance, not only over a broad range of wavelengths but also at different incident angles; the photocurrents of silicon-based solar cells could also be significantly improved omnidirectionally. In addition, the optical gradient of an NP stack could also enhance the light extraction efficiency of LEDs, due to both the graded refractive index and the moderate surface roughness. Large particles having sizes on the same order of the wavelength of incident light roughened the LED surfaces further and extracted light from beyond the critical angle, as supported by three-dimensional finite-difference time domain simulations. Using this approach, we could increase the photoluminescence intensity by up to six fold. Moreover, we also investigated the basic properties of gold oxide (Au2O3). Gold has been widely thought of the most inert metal. We deposited gold oxide films by sputtering process through the help of O2/Ar plasma. We discussed the optical properties and the mechanism of reduction of this special material. We conclude that gold oxide is a dielectric material with poor conductivity and high refractive index. We also discovered that when reducing, gold nanoparticles would form and can be analyzed by spectroscopy due to the LSPR (Localized surface plasmon resonance) peak around 670nm-700nm. Furthermore we also fabricated photodetectors by forming Au2O3/n-Si contact showing EQE(external quantum efficiency) near 50% in the red light and near infrared wavelength regimes with detectivity over 1012.