氮化鎵垂直結構發光二極體之高反射性歐姆接觸層; High Reflective Ohmic Contact Layer for GaN-based Vertical Light Emitting Diode

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Title:	氮化鎵垂直結構發光二極體之高反射性歐姆接觸層;High Reflective Ohmic Contact Layer for GaN-based Vertical Light Emitting Diode
Authors:	陳柏翰;Po-han Chen
Contributors:	化學工程與材料工程研究所
Keywords:	垂直結構發光二極體;反射層;薄膜轉移;光萃取;歐姆接觸層;ohmic contact layer;light extraction;vertical LED;reflectivity layer;thin-film transfer
Date:	2009-06-04
Issue Date:	2009-09-21 12:29:08 (UTC+8)
Publisher:	國立中央大學圖書館
Abstract:	過去10年來由於有機氣相化學磊晶的突破，半導體固態照明成為未來新世代光源的矚目技術。而氮化鎵為主的半導體藍綠發光材料，更被視為固態照明實現的關鍵議題。隨著磊晶技術突破，氮化鎵發光原件的製作工藝也日趨複雜。經由不同結構的調整可使光萃取的效率有更好地表現。其中垂直結構發光二極體由於不再受制於導熱性差且絕緣的藍寶石基板，被認為會有較佳的光萃取效率與較好的散熱效能。目前垂直結構發光二極體效率提升的關鍵是在氮化鎵薄膜的轉移技術、散熱效能的強調以及光萃取效率地提升三方面。本論文將針對這三方面分別探討，並提出新的概念與理論實際應用於垂直結構發光二極體效率的提升。在薄膜轉移的過程中主要包含新基板的鍵合與雷射剝離藍寶石基板。在晶圓鍵合方面，我們利用積體電路封裝製程上廣泛使用的金矽鍵合技術。應用此技術在大面積的薄膜轉移上時，我們發現會發生共晶反應不均勻、鍵合接面產生孔隙等問題。本論文提出使用一非晶形矽層於金矽鍵合的接面。由於非晶形矽結構較鬆散，因此相較於單晶矽晶圓可更快速的和金形成均勻的共晶液態接面。如此可有效的增加鍵合接面的反應均勻性與減少孔隙的生成。在雷射剝離藍寶石基板方面，我們探討了溫度對於雷射剝離的影響。藉由變化溫度來探討雷射剝離時起始雷射能量與溫度的關係，以及高溫雷射剝離所引發的現象。此研究對於改善過大雷射能量造成的傷害以及氮化鎵薄膜轉移過程中因應力變化造成的傷害有很大的幫助。垂直結構發光二極體的基板由導熱性較差的藍寶石轉移到導熱較佳的基板，大大的提升了發光二極體的散熱效能。但在電流灌注時的熱點依然會對發光二極體可靠度造成影響。因此我們導入熱分散層的概念，利用一具超高導熱係數之類鑽膜，可在第一時間將發光層所產生之熱往水平方向傳遞。如此可有效處理熱點問題，並提升新基板之整體散熱效能。經由實驗證實在高電流灌注下此熱分散層效果更為顯著。垂直結構發光二極體的光萃取分為上部與底部。上部為雷射剝離後所露出的N-Polar表面的粗化。此一部分已可利用鹼液蝕刻達成均勻的六角錐狀粗化。其光出量提升可達260%，已是一成熟之技術。但在底部光萃取的部分則著重在利用底部反射層將量子井向下發散的光反射而由上部出光面萃取之。不同於一般反射鏡，垂直結構發光二極體所用反射鏡必須要同時與p型氮化鎵具有低的接觸電阻值。由於p型氮化鎵高功函數的特性，一般常用的銀、鋁等高反射性金屬將會使金屬-半導體接面電阻上升而使元件電性變差。而鉑金等高功函數金屬卻又無法滿足高反射率的需求。因此，我們利用鉑金在氮化鎵表面自我成形的現象，製做了一奈米尺度的六角網狀電極，再鍍上高反射性的銀層。利用鉑金的高功函數以及與氮化鎵的界面反應來達到降低接觸電阻地目的。並利用網目的簍空部分使得光直接由銀鏡面層反射，而效率地成為被萃取的光源。這是首次有關鉑金在氮化鎵表面形成奈米尺度的六角網狀圖形的發表。我們認為這現象與鉑金與氮化鎵的界面因為異質晶格介面的原子重組有很大的關聯，而使鉑金在氮化鎵表面形成規則排列的圖案。經由實驗發現此一有序排列的網狀結構狀態為一暫穩態。鉑金在氮化鎵的表面上最終仍會轉換成獨立島狀以滿足最低表面能的要求。本研究中，我們利用底層氮化鎵的六方晶格排列，來說明六方網狀自我形成的機制，並透過實驗條件控制金屬網狀的覆蓋率(孔隙率)，以研究金屬覆蓋率(孔隙率)與整體Pt/Ag歐姆反射層的光電特性關係，又根據本研究實驗結果，我們提出散射因子的概念以討論因為網狀結構而造成的光散射損失的現象，並歸納出散射因子與孔隙率間的關係。網狀電極的概念可同時滿足新型結構發光二極體在電性與光性的需求。因此鉑金的自我聚集形成的六角網狀結構可被利用在氮化鎵垂直結構發光二極體的電極上，本研究結果顯示此一網狀電極有良好的效果。 Because of the breakthrough of the epitaxial growth technique in the past decade, the semiconductor solid-state lighting becomes the focus of new lighting source. GaN-based LED (light emitting diode) emitting in blue to green region is one of the topics that have been expected to realize the solid-state lighting. The fabrication technique of GaN-based LED device not only depends on GaN crystal quality but also the light extraction efficiency. The different LED structures have been developed to improve the light extraction efficiency. Vertical LED structures attract a lot of attention because it has the excellent light extraction efficiency and heat dispersion ability. The efficiency improvement of the vertical LED relies on three key process, which are thin-film transfer, heat dissipation and light extraction. We provide and apply some new idea and concept to improve the efficiency of vertical LED in this study. In the view point of thin-film transfer, it includes the wafer bonding of new substrate and stripping the sapphire substrate. We use the Au/Si wafer bonding, which is generally used in the package of IC (integrated circuit) chips. When the Au/Si bonding is applied in large area thin-film transfer, the bonding uniformity and voids formation at the bonding interface will be a serious problem. We add an amorphous Si layer at the bonding interface and the loose amorphous Si layer react with Au layer to form an uniform liquid alloy firstly. It has been proved that an uniform and void-free interface can be obtained. We also study the temperature effect on the laser lift-off process on sapphire substrate. The elevated temperature was employed to reduce the laser damage that from the abruptly stress change and adjust the excess laser energy in LLO process. The most important advantage of the vertical LED is the excellent heat dissipation ability that caused by the high conductive transferring substrate. But, the heat spot that caused by high current injection is still a big problem in the vertical LED. Therefore, we propose a concept of the heat spreading layer to promote the heat dissipation ability. A high thermal conductive diamond-like carbon layer was employed to spread the heat in the horizontal direction rapidly. The experiment verifies the heat spreading layer can solve the problem of heat spot and enhance the heat dissipation ability of new substrate and the enhancement effect will be more obvious in the high current injection condition. The light extraction of the vertical LED can be separated to the top side and the bottom side. The top side is the texturing of N-polar surface that appear after LLO process. It has been reported that the uniform hexagonal cone can be obtained by the wet etching process and the light output enhancement can reach 260%. It is a mature process and has been generally used technique. The reflector in the bottom side is used to reflect the light from MQW, which is the other key point to improve the light extraction of vertical LED. The reflector in LED device has to satisfy both requirements of the high reflectivity and low contact resistance between metal and GaN semiconductor. Yet, due to the low work functions of Ag and Al, they have been shown to have poor specific contact resistance to p-GaN. Among the high work function metals, Pt can achieve a good ohmic contact to p-GaN due to its high work function (5.36 eV). But, the low reflectivity of Pt layer in the visible region will limit Pt layer used in the high performance LED. In this study, we observed a hexagonal nano meshed Pt thin-film, which self-formed on the GaN surface. Combining with a metal reflector (Ag), meshed Pt/Ag can provide a low contact resistance to p-GaN and the mesh can provide a directly reflection path to Ag layer. This is the first report about the self-assembled Pt hexagonal nano-meshed Pt layer. We believe that the hexagonal mesh formation is related to the regular arrangement of Pt in the GaN surface. A meta-stable state between Pt thin film on GaN leads de-wetting of Pt layer on the GaN surface form a specific pattern. In meshed Pt/Ag metal contact scheme to p-GaN, the meshed Pt thin-film plays the role of the current paths and the meshes allow the light get reflected by the Ag layer directly. An energy favorite stacking configuration is proposed to explain the mechanism of the hexagonal structure formation. Also, the origin of meshes on the meshed Pt layer is explained by the defect theory. As for optical property of the meshed Pt layer, we conclude a relation between the reflectivity (transmittance) of the meshed Pt layer and the size of meshes on the meshed Pt layer.
Appears in Collections:	[National Central University Department of Chemical & Materials Engineering] Electronic Thesis & Dissertation

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