氮化鎵成長在不同形狀圖案畫藍寶石基板之成長力學及應力探討

Growth mechanism and strain evolution of GaN on various patterned sapphire substrates

王玫丹(Mei-Tan Wang)

指導教授 ： 曾雪峰 ； 李允立

國立臺灣大學/電機資訊學院/光電工程學研究所/博士(2013年)

https://doi.org/10.6342/NTU.2013.02862

摘要

在本篇論文中，針對於氮化鎵（GaN）材料成長在各種不同形式的圖案化藍寶石基板之生長機制及應力演變進行了研究探討，通過調整氮化鎵材料成長在平面藍寶石基板（FSS）和錐形圖案化藍寶石基板（CPSS）之成長參數（溫度，壓力，和V / III比），控制在生長過程中的固有應力，利用即時定量的方法分析晶圓的曲度，可用於追踪整個磊晶過程中每一個層間的內在應力。垂直溫度梯度、晶格不匹配和熱膨脹係數對應力的貢獻是被分開討論的，儘管氮化鎵成長在藍寶石基板上存在著很大的壓縮性晶格不匹配(16％)，氮化鎵在恆定的高溫生長下仍處於拉伸應力狀態，在這項研究中，當氮化鎵生長在錐形圖案化藍寶石基板的側向聚合時間被延遲時，一個明顯的壓縮應力被觀察到，在生長過程中的壓縮應力特性是來自於不匹配應力，毛細作用應力和圖案側壁擠壓應力。
利用成長非極性氮化鎵磊晶層取代極性c面氮化鎵磊晶層，來消除大應力造成之量子限制斯塔克效應（quantum-confined Stark effect, QCSE）的想法被提出，考慮到提升高光取出效率，引進錐形圖案化藍寶石基板是必不可少的，然而，它增加了取得氮化鎵平滑表面的困難度。在這項研究中，透過成長時的氮化鎵相控制，控制半極性面（112‾2）與非極性面a面（112‾0）之競爭成長，而達到光滑、單相的氮化鎵磊晶層成長在r平面錐形圖案化藍寶石基板；低溫成核層有利於半極性面（112‾2）成長，然而高溫成核層有利於非極性a面（112‾0）成長。儘管接續在成核層的低壓成長條件打破了單相的選擇性（兩相同時存在），但低壓成長對於促進側向聚合是不可或缺的，有效控制半極性（112‾2）和非極性a面（112‾0）之競爭關係是達到光滑氮化鎵磊晶表面的重要關鍵。
圖案化藍寶石基板的設計在橫向的聚合能力及應力鬆弛扮演著舉足輕重的角色，使用光微影技術和乾式蝕刻法，藍寶石基板被設計成梯形形狀的TPSS和錐形形狀的CPSS，有長周期6微米和短周期3微米的圖案。梯形圖案化藍寶石基板在頂面被設計有類c-面的平頂露台，相較於梯形圖案化藍寶石基板，錐形圖案化藍寶石基板具有較大範圍的傾斜側壁面積比率，這有助於利用橫向生長的方式來鬆弛殘餘應力及減少錯位缺陷的垂直傳遞；短周期的圖案化藍寶石基板較長周期的圖案化藍寶石基板有較大的傾斜側壁覆蓋面積，在這項研究中顯示在一個較短周期的的錐形圖案化藍寶石基板更能有效地抑制氮化鎵層內殘餘應力。
氮化銦鎵(InGaN)多量子井的發光二極管（LED）結構成長在3微米週期的梯形圖案化藍寶石基板和錐形圖案化藍寶石基板上，被用來探討殘餘應力對光學和電學性質的影響。通過分析電流相關的電致發光光譜和電壓相關的陰極發光光譜，顯示較低殘餘應力的氮化鎵成長錐形圖案化藍寶石基板上，有較均勻的銦含量(從量子井到量子井之間)，這也導致了一個較低的QCSE存在於錐形圖案化藍寶石基板的發光二極體內。

關鍵字

發光二極體 ； 應力 ； 成長

並列摘要

In this dissertation, the growth mechanisms and strain evolution of gallium nitride (GaN) layers grown on various sapphire substrates are studied. The intrinsic strain during GaN growth is controlled by adjusting the growth parameters (temperature, pressure, and V/III ratio) on flat sapphire substrates (FSS) and cone-shape patterned sapphire substrates (CPSS). A quantitative real-time analysis of wafer bowing is used to trace the intrinsic strain in every single layer throughout the full epitaxial process. The contributions of vertical temperature gradients, lattice mismatch, and thermal expansion coefficients are separated. Despite the large 16% compressive lattice mismatch of GaN grown on sapphire, GaN is typically under tensile strain at constant high-temperature growth. In this study, a clear compressive stress during GaN growth on CPSS is observed when the lateral coalescence time is delayed. The intrinsic compressive strain during growth is caused by misfit stress, capillarity stress, and extrusion from pattern sidewalls.
The idea of non-polar GaN epitaxy is proposed to replace the polar c-plane GaN to eliminate the quantum-confined Stark effect (QCSE) from the large strain. Considering the high light extraction, the introduction of CPSS is essential. However, it increases the difficulty of smoothing the GaN surface. In this study, the smooth, single-phased GaN layer on r-plane CPSS is obtained perfectly by controlling the growth phase. The semi-polar plane (112 ‾2) favors a low-temperature nucleation layer, whereas the non-polar a-plane (112 ‾0) favors a high-temperature nucleation layer. Although the low growth pressure following the nucleation layer breaks the single-phase selectivity (two phases existing simultaneously), it is necessary in facilitating lateral coalescence. For non-polar GaN on r-plane CPSS, the phase competition between the semi-polar (112 ‾2) and non-polar a-plane (112 ‾0) GaN phases is key to forming a smooth surface.
The design of the pattern plays an important role in lateral coalescence and strain relaxation. Using photolithography and dry etching process, sapphire substrates are designed as trapezoid-shape PSS (TPSS) and CPSS with 6 μm and 3 μm periods. TPSS are fabricated with a flat top terrace, which is a c-plane-like surface. In contrast with TPSS, CPSS has a larger area ratio of titled sidewalls, which contributes to the relaxation of the residual strain and the decrement of threading dislocations through lateral overgrowth. The patterning period is designed to be short to increase the covering region of the titled sidewalls. The residual strain of the GaN layer on a shorter CPSS period is suppressed effectively in this study.
The structure of InGaN multiple quantum-wells (MQWs) light-emitting diode (LED) is fabricated on a 3 μm period TPSS and CPSS to investigate the effect of the residual strain on the optical and electrical properties. The lower residual strain of the GaN layer on the CPSS results in a more uniform In content from well to well than that on the TPSS by voltage-dependent cathodoluminescence measurements. It also results in a lower QCSE for the CPSS-LED than that for the TPSS-LED, which are investigated by analyzing the current-dependent electroluminescence spectra.

並列關鍵字

light-emitting diodes ； strain ； growth

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