新型態高效率氮化鎵發光二極體之研究

Study of New Type High Performance GaN-Based Light-Emitting Diodes

陳碩偉(Shuo-Wei Chen)

指導教授 ： 盧廷昌

國立交通大學/電機學院/光電工程系所/博士(2020年)

摘要

本研究主旨為提升氮化鎵發光二極體之光電特性表現。論文主要分為三個部分探討。
第一部分為利用外部物理氣相沉積法濺鍍氮化鋁成核層(AlN nucleation layer)以提升後續磊晶層之結晶品質。相較於傳統內部成長低溫氮化鎵成核層(low temperature GaN nucleation layer)，此方式能有效降低缺陷密度並提升X光結晶品質分析結果。使用氮化鋁成核層所成長的發光二極體相較傳統使用低溫氮化鎵成核層，其發光強度能提升3.7%，且逆向漏電偏壓可提升將近兩倍。實驗結果清楚地呈現了使用外部物理氣相沉積法濺鍍氮化鋁成核層對於高品質發光二極體製造的潛力以及可能帶來的副作用。
第二部分接續探討應力在發光二極體中存在的狀況。利用拉曼光譜地圖式掃描分析(Raman spectral mapping)可解析出應力在發光二極體結構中的分布狀況與圖案化藍寶石基板(Patterned Sapphire Substrate, PSS)的圖案排列有強烈的相對應關係。此圖案排列會影響後續成長於其上的磊晶層應力直至表面。此外，更透過二維光激發光光譜儀(two-dimensional micro-PL)地圖式掃描分析氮化鎵發光二極體，也進一步瞭解到光激發光的強度分布與表面應力分布以及圖案化基板排列的密切關係。然而在實驗當中，我們發現了透過適當的應力釋放層(prestrained layer)能夠有效將圖案化基板排列所造成的應力分布差異與光激發光強度差異減小，對於未來設計發光二極體結構增進發光均勻度的考量有實質的貢獻。
最後，有鑑於V型凹陷(V-shaped pit)的生成與應力釋放層的設計有密不可分的關係，且近年來發現V型凹陷在氮化鎵發光二極體的發光效率上扮演極重要的角色。我們使用陰極發光光譜儀(CL)，光激發光光譜儀(PL)，以及縱深分析拉曼光譜儀(depth-resolved confocal Raman spectroscopy)針對成長於圖案化基板的氮化鎵發光二極體作分析。利用CL分析結果中應力釋放層與多重量子井(multiple quantum well, MQW)的強度比例關係，能夠有效辨識應力釋放的程度以及V型凹陷生成的位置，更能夠用來檢視應力釋放層的最佳成長條件。實驗結果表示配合適當的應力釋放層成長能夠生成最佳的V型凹陷大小並且有效將結構中應力釋放。最終我們得到高亮度發光二極體於晶粒尺寸530 × 1090 μm2並操作在60 A/cm2電流密度下可達到704 mW。

關鍵字

氮化鎵 ； 氮化鋁 ； 發光二極體 ； V型凹陷 ； 結構應力

並列摘要

This thesis focuses on methods aimed at improving GaN-based light-emitting diodes (LEDs) performance, and which is divided into three parts.
We first examine efforts to improve crystal quality via the ex-situ sputtered physical vapor deposition (PVD) of aluminum nitride (AlN) nucleation layers. The proposed scheme significantly outperformed the conventional use of an in-situ low temperature GaN nucleation layer, in terms of defect density and X-ray analysis. Compared to the conventional approach, the light output power of the proposed scheme 3.7% higher, and the reverse bias voltage of current leakage was double. Experiment results clearly demonstrated the potential and constraints of using ex-situ PVD AlN nucleation layers in the fabrication of high-quality GaN crystals for LED applications.
We then examined strain within the LED structure. Raman spectral mapping revealed that strain distribution within LED structures is strongly correlated to the patterned array on the patterned sapphire substrate (PSS). In fact, the surface array of the PSS even has a direct effect on strain formation during crystal growth in subsequent layers. Two-dimensional micro-PL mapping of GaN-based LED samples on PSS revealed a close relationship between PL intensity mapping and strain distribution on the sample surface. Nonetheless, we determined that internal strain and variations in PL intensity can both be mitigated through the use of prestrained layers.
Finally, V-shaped pits play a key role in the efficiency of GaN-based LEDs. Using cathodoluminescence (CL), micro-photoluminescence (PL), and depth-resolved confocal Raman spectroscopy, we systematically analyzed GaN LED structures on PSSs fabricated using two types of prestrained layer. We demonstrated the effectiveness of CL in the analysis of V-pits formation. The emission peak intensity ratio in the prestrained layers and multiple quantum wells (MQWs) provided crucial information related to strain relaxation between prestrained layers and MQWs. We also determined that the growth conditions of the prestrained layers played a dominant role in LED performance. The highest performance (704mW @ 530 × 1090 μm2, operating at 60 A/cm2) was achieved using an LED structure that included prestrained layers grown at suitable conditions, which resulted in adequate V-pits sizes and more pronounced strain relaxation.

並列關鍵字

GaN ； light emitting diodes (LEDs) ； V-shaped pits ； strain ； AlN

參考文獻

3. Wang, C.H.; Ke, C.C.; Lee, C.Y.; Chang, S.P.; Chang, W.T.; Li, J.C.; Li, Z.Y.; Yang, H.C.; Kuo, H.C.; Lu, T.C., et al. ”Hole injection and efficiency droop improvement in InGaN/GaN light-emitting diodes by band-engineered electron blocking layer”, Applied Physics Letters, 97, 261103, 2010.

4. Ling, S.-C.; Lu, T.-C.; Chang, S.-P.; Chen, J.-R.; Kuo, H.-C.; Wang, S.-C. ”Low efficiency droop in blue-green m-plane InGaN/GaN light emitting diodes”, Applied Physics Letters, 96, 231101, 2010.

5. Mukai, T.; Takekawa, K.; Nakamura, S. ”InGaN-based blue light-emitting diodes grown on epitaxially laterally overgrown GaN substrates”, Japanese journal of applied physics, 37, L839, 1998.

6. Pimputkar, S.; Speck, J.S.; DenBaars, S.P.; Nakamura, S. ”Prospects for LED lighting”, Nature Photonics, 3, 180-182, 2009.

7. Pankove, J.; Miller, E.; Berkeyheiser, J. ”GaN blue light-emitting diodes”, Journal of Luminescence, 5, 84-86, 1972.