非極性氮化鎵光電元件之磊晶成長

Abstract

本論文旨在探討以有機金屬氣相沈積系統來成長非極性a面氮化鎵為主之光電材料與元件，包含了優化成長、減少材料缺陷、主動層氮化銦鎵量子井結構設計，最後完成a平面氮化鎵發光二極體製作與元件特性分析。 在磊晶之優化成長方面，我們利用有機金屬氣相沉積系統於高溫低壓下的成長環境下獲得高品質非極性a平面氮化鎵薄膜，改變成核層氮化鋁厚度與成長氮化鎵時的五三比，亦會影響非極性氮化鎵薄膜之品質。對於非極性氮化鎵之成長機制，我們利用Wulff圖形的概念以及選區成長的方法來分析非極性氮化鎵成長於r面藍寶石基板上的成長行為，此方法可以解釋非極性表面條紋以及坑洞的成因，並可進而預測成長，提供非極性氮化鎵成長之準則。 由於非極性a面氮化鎵成長不易，並具有很高的缺陷密度，因此在本論文中我們提出溝槽式磊晶側向成長以及氮化銦鎵/氮化鎵超晶格層的運用，來改善材料品質，由穿遂電子顯微鏡顯的結果，溝槽式磊晶側向成長可使a平面氮化鎵薄膜於氮極性方向上之晶格品質有明顯的提升，差排密度由原先的1×1010 cm−2 減少至 3×107 cm−2，而在氮化銦鎵/氮化鎵超晶格層的部份，差排密度可有效地從3×1010 cm-2降低到9×109 cm-2。此部份實驗確認達成晶格品質提升之目的與商業化的可行性。 為優化後續的非極性藍光雷射二極體的元件結構，我們改變非極性氮化銦鎵量子井的厚度並分析其光學特性。時間解析光致激發光譜結果指出當厚度逐漸變厚時，量子井產生的激子生命期逐漸增加。而在變溫的光致激發光譜中，我們發現在量子井厚度較薄的樣品具有較大的侷域深度可以有效地補獲激子，而較厚的樣品由於受到成長溫度較長時間下的影響使得其侷域紊亂程度變得更為嚴重導致發光效率明顯下降。 在製作發光二極體方面，我們運用溝槽式磊晶側向成長之a面氮化鎵作為基板，成功地在上面成長了發光二極體結構，發現該結構由於具有兩種不同缺陷密度之區域，因此在改變電流下，發光波長會因為電流流經不同區域而有所不同，在注入電流為140 mA時功率為0.2 mW；另外，在氮化銦鎵與氮化鎵組成的超晶格層部份，電致激發光譜的結果證實此樣品在強度上亦有3.42倍的增強，此元件並具有56.3%的偏振率。 因此，於本論文中我們已完成了非極性氮化鎵材料的成長以及元件製作相關的研究，成果包含了成長條件優化、成長行為以及量子井結構與光學分析，另外，我們亦提出數種可改善材料品質的方法，將在本論文中逐一討論並證明其可行性。我們冀望這一系列的實驗對未來在非極性氮化鎵之光電元件發展而言，提供有用的資訊與助益。

In this dissertation, the epitaxial growth of nonpolar a-plane GaN based optoelectronic materials grown using metal organic chemical vapor deposition (MOCVD) have been investigated. Main works include optimum growth, InGaN multiple quantum wells (MQWs) design, reduction of defects and the fabrication of a-plane GaN based optoelectronic devices and analysis of device characteristics. For optimum growth of a-plane GaN, we confirmed variation of thickness of AlN nucleation layer and V/III ratio of a-plane GaN growth influence crystal quality of a-plane GaN thin film. We also tried to figure out the mechanism of a-plane GaN by using Wulff plot and selective area growth to analyze the growth behavior of a-plane GaN grown on r-plane sapphire, which could be useful to explain the reasons account for stripes and pits exist on a-plane GaN surface and give us a guidance to predict growth of a-plane GaN. In this dissertation, we used trench epitaxial lateral over growth (TELOG) and InGaN/GaN supperlattices (SLs) to improve crystal quality of a-plane GaN. The threading dislocation (TD) density can be reduced largely from 1×1010 cm−2 to 3×107 cm−2 for the N-face GaN wing. As for SLs part, The TD density in the sample with SLs was reduced from 3×1010 cm-2 down to ~9×109 cm-2. For active layer structural design, a-plane InGaN/GaN MQWs of different width ranging from 3 nm to 12 nm have been grown. The peak emission intensity of the photoluminescence (PL) reveals a decreasing trend as the well width increases from 3 nm to 12 nm. Low temperature (9 K) time-resolved PL (TRPL) study shows that the sample with 3 nm-thick wells has the best optical property with a fastest exciton decay time of 0.57 ns. More effective capturing of excitons due to larger localization energy Eloc and shorter radiative lifetime of localized excitons are observed in thinner well width samples were observed in the temperature dependent PL and TRPL. In development of nonpolar light-emitting diodes (LEDs), we successfully fabricated a-plane LEDs structure by using TELOG GaN substrate. Due to there are two areas with different defect density in this kind sample, the emission wavelength will be changed when we increased injection current. The power was 0.2 mW at 140 mA injection current. On the other hand, we also fabricated nonpolar LEDs by using InGaN/GaN SLs layer. Electroluminescence intensity of the sample with InGaN/GaN SLs was enhanced by a factor of 3.42 times to that of the conventional sample without InGaN/GaN SLs. In this dissertation, we have achieved the studies on the growth of a-plane GaN and the fabrication of devices. Whole achievements include optimum growth, MQWs structural design, crystal improvement of material and fabrication of a-plane LEDs. We hope this series of experiments to provide a useful information and support for development of nonpolar optoelectronic devices in future.