Please use this identifier to cite or link to this item: http://hdl.handle.net/11455/10504
標題: 以有機金屬化學氣相沉積法進行氧化鋅奈米結構之成長與特性分析
Growth and characterization of ZnO nanostructures by metalorganic chemical vapor deposition
作者: 吳嘉城
Wu, Chia-Cheng
關鍵字: ZnO nanostructures
氧化鋅奈米結構
metal organic chemical vapor deposition
transparent conducting layer
sacrificial layer
有機金屬化學氣相沉積
透明導電膜
犧牲層
出版社: 材料科學與工程學系所
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摘要: ZnO是一種寬能隙的II-VI族化合物半導體。在室溫下是直接能隙半導體,其能隙約為3.37 eV,結構屬於wurtzite結構,且c軸取向良好。ZnO材料光譜範圍符合紫外光放射,激子束縛能是60 meV,為GaN(約為25 meV)的兩倍多,常溫的熱擾動無法使激子分開成為自由電子與自由電洞,因此氧化鋅的激子可以在室溫下存在,故在室溫下將具有更多的激子放射(more efficient excitionemission),這有助於高效率光電元件的發展。因此本論文將探討不同氧化鋅結構之特性與成長,其中,我們所使用的磊晶設備為自行組裝之氧化鋅有機金屬化學氣相沉積(Metalorganic Chemical Vapor Deposition, MOCVD)系統,主要是由Emcore D-180氮化鎵磊晶系統所改裝並自行完成週邊(水、電、氣體等)設施,並與美國Structured Materials Industries公司合作開發此沉積系統腔體的內部設計。相較於其它成長方式,有機金屬化學氣相沉積法可成長高品質磊晶,且為一量產型的技術,因此,製程參數(製程溫度、腔體壓力與鋅氧比等)之建立及其對氧化鋅結構的影響,與結構特性之分析,將會是論文主要的方向。 藉由調變製程參數,可以在藍寶石基板上得到不同的氧化鋅結構,包含奈米結構與薄膜結構。其中,在沉積氧化鋅奈米柱、奈米管和奈米牆的過程中,並未使用任何催化劑,所以並不是金屬誘發的Vapor-Liquid-Solid (VLS)成長機制。研究發現,於基材上先行沉積不同厚度之緩衝層,可以調變氧化鋅奈米柱之垂直性及密度,藉由X光繞射(XRD)的分析可以得到氧化鋅奈米柱擇優成長是沿c軸(002)的面,並且由光激發光譜(PL)、掃描式電子顯微鏡(SEM)及高解析穿透式電子顯微鏡(TEM)的量測,證明其結構性及垂直性相當良好;我們亦發現,在奈米柱沉積的過程中,改變沉積溫度,結構會由奈米柱轉變為奈米管,亦即僅柱子周圍繼續向上成長,形成奈米管結構。綜合TEM、能量散佈光譜(EDS)及X射線光電子光譜(XPS)的量測結果,推測氧化鋅奈米結構其成因為一種Self-catalyst成長機制,並且結合氧化鋅不同面,具有不同成長速率所致,[0001] > [01-1-1] > [01-10] > [01-10] > [000-1]。此外,製程溫度的變化,將會大幅改變氧化鋅的結構,為了得到氧化鋅薄膜,我們提出一個新的製程方法-反覆磊晶與退火成長模式(Repeated Growing and Annealing, RGA),可以沉積出高品質氧化鋅薄膜結構,得到低阻值(~3.427×10-3 ohm-cm)和高遷移率(~85.2 cm2/V-s)的氧化鋅薄膜。 在應用方面,我們在氧化鋅中摻雜鋁,即氧化鋁鋅(Aluminum Zinc Oxide, AZO) 。比較不同鋁摻雜及不同厚度之氧化鋁鋅,發現當厚度增加時,會形成奈米柱狀結構,當氧化鋁鋅厚度為1.2 μm時,表面粗糙度為75 nm。並且由發光二極體(Light Emitting Diode, LED)之發光強度可以得知,使用1.2 μm之氧化鋁鋅的LED有較佳的亮度,其主要是利用氧化鋁鋅柱狀結構,來改變LED與出光介質之間的折射角度,使光有較大的比例可以小於臨界角而射出,突破原來全反射的限制,同時,1.2 μm的氧化鋁鋅具有更佳的電流分佈。本研究亦提出智慧型分離技術,主要是於氮化鎵磊晶膜與藍寶石基板間加入一層200 nm的氧化鋅犧牲層,利用濕式蝕刻技術移除氧化鋅,探討將2吋氮化鎵磊晶膜由熱傳導特性不佳藍寶石基板轉移至電鍍銅基板的可行性。經由X-射線繞射光譜儀分析分離前後之基板表面,藍寶石基板及銅基板上皆呈現氮化鎵波鋒,證實氮化鎵已成功轉移至另外電鍍的銅基板之上;成功轉移至銅基板之試片,經由能量散射光譜儀分析表面,重量百分比及原子百分比皆有Ga原子及N原子訊號,驗證了氮化鎵的存在。
ZnO is a wide bandgap (3.37 eV) semiconductor in wurtzite structure. ZnO exhibits a large excition binding energy of 60 meV at room temperature. The strong exciton binding energy, which is much larger than that of GaN (25 meV) and the thermal energy at room temperature (26 meV), can ensure an efficient exciton emission at room temperature under low excitation energy. Therefore, the growth of ZnO structures was carried out by metalorganic chemical vapor deposition (MOCVD) system, which was refitted from an Emcore D-180 GaN-MOCVD system. The building of ZnO-MOCVD system relies on the cooperation with Structured Materials Industries especially on the design of the chamber. It is known that MOCVD is a mainstay of semiconductor wafer processing because it can be readily controlled and scaled for large wafers. Therefore, it is necessary to investigate the effects of different growth parameters on the growth of ZnO, e.g., growth temperature, chamber pressure, and flow ratio of group VI source gas to group II source gas (DEZn/O2 ratio). The surface morphology of ZnO grown by MOCVD was systematically investigated particularly as a structure of nanorods, nanotubes and nanowall networks. For the growth of ZnO nanorods, the ZnO buffer layer was used as the nucleation template to control the growth direction and density of the ZnO nanorods. It was found that the ZnO nanorods were preferred oriented in the (002) c-axis direction of X-ray diffraction (XRD). Besides, photoluminescence, scanning electron microscopy, and transmission electron microscopy (TEM) results further confirm that ZnO structures have high crystal quality. Vertically well-aligned ZnO nanotube arrays were synthesized using a three-step (buffer→nanorod→nanotube) growth process by MOCVD without using any metal catalysts. When the growth temperature was switched from 650 to 450oC during the growth process, structure of ZnO changed from ZnO nanorods into ZnO nanotubes. The TEM, energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy analyses do not reveal any metal catalysts or any other type of additives upon the growth of ZnO nanostructures. Therefore, the formation of ZnO nanostructures is said to follow the self-catalyzed growth mechanism instead of metal-catalyzed Vapor-liquid-solid mechanism and the growth rates of the ZnO crystal in different directions have been reported to have the order of [0001]>[01-1-1]>[01-10] >[01-10]>[000-1]. Results also show that the growth of various ZnO nanostructures strongly depends on the growth temperature. Therefore, we proposed a new growth mode of repeated growing and annealing (RGA) as a reliable and reproducible way to grow high performance ZnO film. By using RGA growth mode, a low resistivity (3.43×10-3 Ω-cm) and high mobility (85.2 cm2/V-s) were achieved, respectively. For further improving the light output intensity of GaN-based light emitting diodes (LEDs), it is essential to obtain high light-extraction efficiency from the devices. Al-doped ZnO (AZO) microstructures were used as the transparent conducting layers on GaN-based LEDs to replace ITO thin films. First, effects of Al-doping on the optical property and morphology of AZO were discussed. Then, the performances of the GaN-based LED with various thickness of AZO on the top were compared. It was found that the output power of the GaN-based LED with a 1200 nm AZO exceeded that with a 375 nm AZO. Thick 1200 nm-AZO was used as surface texturing, uniform current spreading, and thick window layer for achieving highly efficient LEDs. Furthermore, a method for removing the original sapphire substrate by wet etching the ZnO sacrificial layer has been presented. It is known that laser lift-off technique has been established as an effective tool for the integration of GaN-based LEDs with a variety of dissimilar substrate. Nevertheless, it needs a laser beam to scan an epitaxial wafer more than one time, which decreases the throughput and increases the chance to damage the epitaxial layer of LEDs. In this work, GaN epitaxial layer was successfully separated from sapphire substrate by wet etching the ZnO sacrificial layer. The XRD results confirm that the GaN is transferred to the copper substrate. In EDS analysis, one can only observe the peaks corresponding to the elements of Ga and N which transformation of the as grown 2-inch GaN epitaxial layer to a heat sink substrate can be easily achieved and contributes to the reuse of the original sapphire substrate.
URI: http://hdl.handle.net/11455/10504
其他識別: U0005-2108200714430800
文章連結: http://www.airitilibrary.com/Publication/alDetailedMesh1?DocID=U0005-2108200714430800
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