氧化鋅奈米線之製備及特性研究

Abstract

在積體電路製程的急劇微型化過程裡，現今的製程結構材料技術均面臨了極為嚴苛與不凡的挑戰，使得奈米結構材料獲致大量與廣泛的研究與探討；單就微型化的奈米光電元件應用而言，無論在微分析技術、資料儲存與光學計算等等範疇裡，奈米雷射光源的取得與高穩定性的需求特性均被視為是奈米光電技術的一大挑戰。在現今的光電材料中，氧化鋅具備有較大的能帶差異(3.37eV)與極大的激發能(Exciton binding energy,60 meV)，很適合用來製作短波長的光學或電學激發奈米元件；相對於鍺、銦等三五族元素而言，氧化鋅具備有其所沒有的奈米雷射潛力。論文中針對氧化鋅奈米線的研製與其材料特性鑑別以及其光學特性的研究，作一完整而詳細的分析與研究，期能尋求出一穩定的奈米光電元件應用材料製程。 論文中首要提出的為一兩階段固-液-氣(VLS)沉積模型，藉由不同溫度的調控中獲得最佳成長溫度為攝氏900度與最佳成長時間為30分鐘，並從以下幾個方向萃取出製程上的最佳參數。一為不同基版潔淨流程(丙酮與標準RCA)的影響，藉由標準RCA潔淨步驟去除原生氧化矽層(native oxide)可獲致較具垂直成長方向之氧化鋅奈米線(與水平夾角約為85度)；透過不同催化金屬(金、銅)的選擇，均可以在經過RCA清洗的矽基板成長出高度一致性近垂直方向且完美烏采(wurtzite)結構之氧化鋅奈米線。其二在改變不同氣氛並選用氬氣與氮氣作為控制參數的成長環境中，肇因於氣體黏滯係數的不同，可以獲致不同外觀之氧化鋅奈米線；一般而言，在成功控制的氮氣氣氛下，可成長出較小直徑(<50 nm)、較長長度(~5 μm)及密度較高的氧化鋅奈米線，此製程結果對於氧化鋅奈米線其光致發光能力有絕佳的提升與改進。 此外，不同基板所能提供的不同表面能亦是探究高方向性氧化鋅奈米線製程的主要影響條件之一，在所選用的基板中，因為氧化鋁基板具有與氧化鋅一致的結晶結構，故可以輕易的在其上獲致結晶良好且垂直於基板之氧化鋅奈米線；但在鑽石結構的矽基板上卻無法順利的成長出具垂直基板方向之氧化鋅奈米線，因此透過緩衝層的引入，先於矽基板上成長一單晶超薄氧化鋅薄膜(0.7 nm)，在於其上使用兩階段固-液-氣(VLS)沉積方式，成長完全垂直於矽基板且結晶完美之氧化鋅奈米線，同時引入氣氛控制中表現極佳的氮氣做為氣流載體，以求得最佳之光學激發特性；經由成功引入超薄氧化鋅緩衝層的技術，使得此氧化鋅奈米線製程技術極易整合進入現今的矽積體電路製程，並能在矽基板上維持其完美的結晶與成長方向，保證其優良的光學激發特性。 最後，透過擴散與化學氣相沉積理論的導入，可以成功的建構氧化鋅奈米線在兩階段固-液-氣(VLS)沉積的理論模型，並且成功分析影響氧化鋅奈米線之成長直徑及長度與氣流載體及觸媒金屬與反應物三者的關鍵因素，進而獲致在兩階段固-液-氣(VLS)沉積的完整實驗與理論參數。此外，也從選用不同的氣流載體的過程中，獲致一些關於金屬鋅奈米線(Zn nanowires)，氧化鋅奈米線(ZnO nanowires)，過氧化鋅奈米線(ZnO2 nanowires)與氫氣及不同成長溫度的製程參數與三種相異奈米線結構的光學激發特性。 今後在光學上的努力目標，可以設法加強氧化鋅奈米線於紫外光頻段的發光特性與抑制綠光頻帶的發生，進而使其光學的應用上可以日臻便利與完善，達到商業化的需求。

Nanostructured materials represent an intellectually challenging and rapidly expanding area of research. These individual nanolasers could serve as miniaturized light sources for microanalysis, information storage, and optical computing. ZnO nanowires are considered as one of the most promising materials for the application of laser on account of its wide bandgap and higher exciton binding energy. In this thesis, the syntheses of ZnO nanowires by a new vapor-liquid-solid (VLS) process and the effects on the optical properties of ZnO nanowires have been investigated. First, ZnO nanowires are prepared by the VLS process. When the pre-treatment of the substrate is general acetone clean, ZnO nanowires are formed laying on the substrate. When the RCA clean is applied, however, the ZnO nanowires are inserted on the Si substrate and the ZnO nanowires with wurtzite structure is synthesized after reaction at 900 oC for 30 minutes.Various metal catalysts and atmospheres were adopted in this VLS process. It is found that ZnO nanowires prepared by Au and Cu both have high cryStallinity and emit uhraviolet light at room temperature. Comparing with the ZnO nanowires prepared in various atmospheres, the ZnO nanowires obtained by the nitrogen atmosphere exhibits smaller diameters, longer lengths, higher number of density and better photoluminescence ability. Secondly, the effects of substrates on the distribution of the ZnO nanowires have been investigated for the utilization of laser material. ZnO nanowires were well-align on the Sapphire and Si substrate with ZnO buffer layer. It is found that the factor of distribution influences the optical ability. The higher aligning degree of the ZnO nanowires exhibit superior optical properties. In addition, with nitrogen atmosphere, the growth of ZnO nanowires is enhanced and exhibits better optical characteristic. Finally, the gas phase VLS growth mechanism of ZnO nanowires with smaller diameters has been investigated. The smaller diameter of ZnO nanowires also exhibit good optical characteristic. In addition, Zn, Zno and ZnOz nanowires also carry out in various temperature regions by the VLS process with hydrogen reduction reaction.