利用奈米柱陣列空氣洞之氮化鎵發光二極體以改善光光萃取效率及化學蝕刻剝離技術

Abstract

本篇論文包含如何利用氮化鎵奈米柱模板成長氮化銦鎵/氮化鎵發光二極體並使用化學蝕刻技術實現剝離基板之方法。首先利用奈米壓印製作氮化鎵奈米柱模板並使用多孔洞二氧化矽溶液保護奈米柱側壁後進行發光二極體結構成長，使用「奈米柱模板側向磊晶成長方式」，可以得到高結晶品質的氮化鎵，成長具有藍光460 nm的發光波段氮化銦鎵/氮化鎵多重量子井結構。使用「溝奈米柱模板側向磊晶成長方式」所得到的氮化鎵，可以發現再成長界面的氮化鎵存在含有二氧化矽材質填滿的孔洞，因此本文中利用蝕刻矽緩衝溶液去除奈米柱側壁的二氧化矽形成空氣孔洞，再使用氫氧化鉀溶液蝕刻對奈米柱模板進行側向蝕刻及晶面選擇蝕刻以達成剝離基板之目的。蝕刻處理所選定的試片包含了完整磊晶的氮化銦鎵/氮化鎵多重量子井結構，氫氧化鉀溶液蝕刻條件溶劑選擇為乙二醇及水；溫度控制在70°C、120°C；濃度條件維持在2 wt. %、5 wt. %、10 wt. %、20 wt. % 及30 wt. %；蝕刻時間則選擇固定20min，以利蝕刻結果分析。藉由調整不同條件組合的蝕刻條件發現在相對低溫低濃度的狀況下，氫氧化鉀溶液對於氮化鎵奈米柱模板產生等向性蝕刻效果，側向蝕刻速率約為10 nm/min；高溫高濃度的狀況下則產生非等向性蝕刻效果，即極性選擇蝕刻效果顯著，蝕刻速率約為17 nm/min，速率雖快但蝕刻差異大。為避免垂直方向蝕刻速率過快而破壞氮化銦鎵/氮化鎵多重量子井發光層，所以在本實驗中選擇低溫低濃度的蝕刻條件。在5 wt. %、70℃的氫氧化鉀溶液中成功剝離藍寶石機板而得到表面粗糙呈六角金字塔狀的N型氮化鎵薄膜。在剝離的表面可以觀察到12重對稱排列的奈米柱圖形。 本論文成功整合利用氮化鎵奈米柱模板成長高結晶品質氮化鎵的磊晶製程以及利用奈米柱間形成的孔洞達到的剝離基板技術，同時針對內部量子效益(奈米級二次成長技術)及光萃取效益(嵌入式多孔洞結構)之半導體發光二極體發光效益的因素改善之目的，並實現簡單化學蝕刻剝離基板技術以利發展高亮度固態照明之領域。

This dissertation reports to acquire high-quality GaN epitaxial layer by using nano-epitaxial lateral overgrowth (NELOG) technique growing InGaN/GaN heterostructures on GaN-nanorods (GaN NRs) /c-sapphire by metal-organic chemical-vapor deposition (MOCVD) and to achieve free-standing GaN thin-film by using chemical lift-off (CLO) technique to achieve separating the sapphire substrates. After wafer fabrication process, a GaN NRs-SiO2-mixed structure was observed on the GaN/sapphire template and there were embedded voids filling with SiO2. The peak emission wavelength of the GaN NRs-based LED structure was on the 460 nm. A wet chemical etching process including removing silicon oxide (SiO2) and lateral etching (LE) GaN NRs is used to remove the sapphire substrates. Then, an embedded air-void on GaN NRs structure that provided an etching channel to increase the etching rate of the GaN NRs was observed after using the buffered hydrofluoric aqueous to remove SiO2. The lateral etching rate (LER) of the GaN NRs layer was calculated at 10μm/min at low temperature (LT) and low concentration for the 300-μm-width LED chip that was lifted off from the GaN/sapphire substrate. A hexagonal-shaped GaN structure on the top-of n-GaN was observed on the lift-off GaN thin film surface, the stable crystallographic etching planes were formed as the GaN {10-1-1} planes after a N-face wet etching process on a GaN layer. The12-fold symmetry pattern of the n-GaN surface was shown the same with GaN NR arrangement. We can explain the transferring patter from etching the weakness connection between the top-head of GaN NRs and bottom of NELOG GaN. Comparing to the LED on sapphire substrate, a peak wavelength blueshift phenomenon of the micro-photoluminescence spectra was observed on the lifted off LED chip on adhesion tape caused by the release of a compressive strain at the GaN/sapphire substrate interface, also reduction strain of InGaN/GaN active layer. We found out that lift-off LED chip had increased light extraction efficiency causing by the surface hexagonal roughness. The chemical lift-off (CLO) process was achieved by using a GaN NRs layer as a sacrificial layer in a hot potassium hydroxide (KOH) solution, which has the potential to replace the traditional laser lift-off process for vertical LED applications.