新穎氮化鎵發光元件磊晶結構之研究

Abstract

近年來，隨著發光二極體固態照明的開發及需求，具有廣泛波段涵蓋範圍的三族氮化物成為相當熱門的研究材料；但在三族氮化物在磊晶成長的過程當中存在一些無可避免的的問題，如磊晶層與基板間晶格不匹配的現象，造成晶體缺陷的產生與非輻射復合效應增加，結構應力所衍生的內建極化場及其效應皆已被證實會影響到發光二極體的元件效能。因此，本論文主要致力於提升晶體品質、降低結構應力與改善極化場效應的產生，進而提升發光二極體的發光效率。 本研究中，將針對上述的情況分別提出解決的辦法。首先在提升晶體品質與降低結構應力方面，分為二個方向探討，一是藉由超晶格結構的缺陷降低機制：使用嵌入氮化銦鎵/氮化鎵超晶格結構；二是晶格匹配的磊晶方式：使用氮化鎵同質磊晶結構。嵌入超晶格結構與同質磊晶結構的使用，確實明顯的降低的磊晶結構的線缺陷密度，同時結構應力也有顯著的改善；針對發光效率進行探討，嵌入超晶格結構之氮化物發光二極體結構內部量子效率約有1.2倍的提升；而同質磊晶結構之氮化物發光二極體結構內部量子效率則具有1.6倍的顯著提高。而在改善內建極化場及其效應的影響研究上，為了改善因結構應力所誘發的極化場效應的影響，利用有機金屬氣相磊晶法，將纖鋅礦結構的氮化物相關奈米結構沿非極性[11-20]軸向方位成長。根據實驗結果，非極性(11-20)氮化物奈米結構與(0001)基面堆疊缺陷的交互作用形成類量子線結構被觀察並具有強烈的載子侷限行為。此載子侷限效應被發現有助於氮化物奈米結構發光效率的提升，隨著侷限能量的增加，發光效率有明顯的提升。而值得注意的另一個發現是石墨烯覆蓋層之氮化鎵混合型結構所呈現出在改善內建極化場及其效應，石墨烯覆蓋層之氮化鎵混合型結構既不需要複雜的磊晶結構也不需要昂貴的高品質基板，也能有效地造成內部量子效率2倍的提升。根據實驗結果，石墨烯覆蓋層之混合型結構呈現出極化場效應的下降、輻射復合效率提升與表面態減少的現象，此被視為與內部量子效率的提升有相關，在提升氮化物磊晶層之發光效率有顯著成果。 在本論文中，我們提出數種磊晶結構及方法以提升氮化鎵發光二極體元件之元件效率，期許相關研究的成果能有助於氮化鎵系列光電元件的發展與進步。

In the past, with the developments and requirements of the solid-state lighting, the study of the wide bandgap III-nitrides semiconductors recently become a popular investigated topic. However, unavoidable issues existed during the heteroepitaxial growth of III-nitrides semiconductors. Due to the lattice mismatch between the epilayers and substrate, the existence of high defect density resulted in the increase of non-radiactive recombination centers. Furthermore, the strain-induced polarization field and its effects conspicuously diminished the LED performance, leading to the poor light emitting efficiency. Therefore, the research attends to improving the light emitting efficiency by using the differnet epitaxial structures. First, the different epitaxial structure including the insertion of InGaN/GaN superlattices (SLS) layer and GaN homoepitaxial structures have been presented respectively, based on the defect reduction mechanism and the lattice matching epitaxy to improve the large lattice mismatch and high defect density. The inserted SLS layer and homoepitaxial growth structures indeed effectively reduce the threading dislocation density and the structural strain. Based on the internal quantum efficiency measurements, the inserted SLS-LED structure and the homoepitaxial growth LED structure reveal the 1.2-fold and 1.6-fold enhancement respectively. Subsequently, to eliminate the strain-induced polarization field effects, the wurtzite GaN-based quantum-confined structure are grown along the non-polar a-plane orientation direction by using metal-organic chemical vapor deposition. According to the experimental results, the interaction between the (11-20) quantum-confined structure and the (0001) basal stacking faults forms the quantum-wire-like structure to cause the strong carrier localization behavior. The light emitting efficiency exhibits the increase with the carrier localized energy, and thus it is inferred that that defect-induced carrier localization behavior could be helpful to enhance the light emitting efficiency in non-polar GaN-based nanostructure. Finally, we find that a distinctive hybrid structure which need neither the complex epitaxial growth nor the expensive substrate, can improve the light emitting efficiency. The hybrid strucutre consists of the InGaN/GaN multiple quantum wells (MQWs) and the graphene capping layer. In terms of the experimantal results, the polarization-free-like behavior, enhanced radiative recombination rate, reduced surface potential, and significant efficiency enhancement have been demonstrated. The internal quantum efficiency is effectively enhanced 2.0-fold, which is attributed with the large free carriers in the interface between GaN-based MQWs and graphene capping layer, leading to the screening of polarization field. In this thesis, the several effective approaches in order to enhance the light emitting efficiency have been proposed, and then expected the outcome of the research could contribute to the development and progress for GaN-based optoelectronic components.