在本研究中已成功在氫氟酸及硝酸銀混合溶液中，利用銀離子的氧化還原反應在(001)矽晶基材上製備出大面積且具方向性之矽晶奈米線陣列。為探討以此氧化還原反應之反應動力學，本實驗設計於不同反應溫度、時間及反應物濃度下進行矽晶奈米線之生成反應。經一系列實驗後發現，當反應於0–50 °C下進行時，所生成之矽晶奈米線長度與反應時間呈一次方線性關係，且經測量矽晶奈米線於不同溫度下之生成反應速率，可得到生成矽晶奈米線之反應活化能為0.41 eV。而當改變反應溶液之硝酸銀及氫氟酸濃度時，可獲知反應物之反應級數，經計算後可求得以此方式製備矽晶奈米線之生成反應速率經驗式。進一步以TEM、SAED及HRTEM鑑定分析所生成之矽晶奈米線，可發現其線寬約在30-200 nm之間且長寬比可達900-6000。而成長方向經鑑定後為[001]，此與本實驗所使用之(001)矽晶基材軸向相同。在本研究中亦提出生成矽晶奈米線可能之反應機制，推測是因銀離子與矽晶基材在微區域中發生氧化還原反應，且因矽晶基材於不同晶面之氧化速率不同而造成矽晶基材被垂直地向下蝕刻，繼而生成矽晶奈米線陣列。 此外，將Ni薄膜鍍在分散之矽晶奈米線上，經不同溫度退火後，可發現磊晶之NiSi2相於300 ℃退火時即可生成，相較於Ni薄膜在大面積矽晶基材上反應生成NiSi2所需之溫度，NiSi轉為NiSi2之相轉換溫度大幅降低400-450 ℃。且隨著矽晶奈米線線寬變窄，促進磊晶NiSi2生成之效應更為顯著。而所生成之鎳矽化物奈米線經TEM及SAED分析後可得，NiSi2與Si之磊晶關係為[100]NiSi2//[100]Si，(002)NiSi2//(004)Si及[110]NiSi2//[110]Si ，(1-11)NiSi2//(1-11)Si。此在奈米尺度下所發現有別於Ni薄膜與大面積矽晶基材試片之不同矽化反應的結果，可歸因於Si與NiSi2在室溫下之晶格不匹配程度僅約-0.4 %，且反應進行時主要擴散的原子-Ni受到一維奈米線結構之表面積/體積比變大的影響，使Ni原子得經表面快速擴散至NiSi2/Si界面處反應，進而降低生成磊晶NiSi2之反應溫度。 In this study, large area well-aligned silicon nanowire (SiNW) arrays were successfully synthesized on (001)Si substrates by redox reaction of Ag+ and Si in the aqueous solution containing silver nitrate (AgNO3(aq)) and hydrofluoric acid (HF(aq)). The growth kinetics of the SiNWs formed by redox reaction has been investigated. The lengths of the SiNWs were found to increase linearly in samples synthesized at 0–50 °C. By measuring the growth rate at different reaction temperatures, the activation energy for linear growth of the SiNWs was found to be 0.41 eV. In addition, by varying the concentrations of AgNO3 and HF solutions, the orders of reaction with respect to the reactants and the rate equation for the growth of SiNWs have been obtained. From TEM, SAED, and HRTEM analysis, the SiNWs are about 30-200 nm in width with aspect ratios of about 900-6000. The axial orientation of the SiNWs was identified to be along the [001] direction, which is the same as that of the initial (001)Si wafer. The possible mechanisms for the growth of SiNW arrays are discussed in the context of the localized electrochemical redox reaction processes and the oxidation and etching rates of different Si crystal planes. For the evaporated Ni thin films on SiNWs samples after different heat treatments, the epitaxial NiSi2 phase was observed to form at an annealing temperature as low as 300 ℃. The formation temperature of NiSi2 phase is about 400-450 ℃ lower than what is usually needed for the NiSi to NiSi2 transformation. In addition, the effect on the enhanced formation of epitaxial NiSi2 is apparently more pronounced with a decrease in the width of SiNWs. Based on the SAED and TEM analysis, the epitaxial orientation relationships between the NiSi2 and SiNWs were identified to be [100]NiSi2//[100]Si, (002)NiSi2//(004)Si and [110]NiSi2//[110]Si, (1-11)NiSi2//(1-11)Si. The observed result could be attributed to the small lattice mismatch (－0.4 %) between NiSi2 and Si at room temperature, and the fast diffusion of Ni atoms via the surface of Ni films to the NiSi2/Si interface since the surface/volume ratio is higher for the one-dimensional (1D) nanowire structures.
