利用田口-灰關聯法探討以無電鍍沉積合成大面積矽奈米線陣列與其應用

Abstract

一維半導體奈米結構已被證實應用於光電子材料和高靈敏度的分子傳感器。近年來，矽奈米線因其特殊性質已被發展為各式元件，具有極大潛力並受到學術界注意。因此，本論文利用田口－灰關聯法探討以無電鍍沉積合成大面積矽奈米線陣列與其應用，以及將此大面積矽奈米線陣列應用於太陽能電池抗反射研究、無機－無機異質介面二極體與表面增強拉曼生物偵測等。 矽奈米線的應用關鍵是其幾何的控制，包括其長度、大小與成長方向。因此，本研究利用一種無電金屬沈積（Electroless metal deposition，EMD）的方法，以硝酸銀（silver nitrate，AgNO3）與氫氟酸（hydrogen fluoride，HF）混合溶液，在P(100) 矽晶片表面成長出高密度且垂直之單晶矽奈米線。在本論文中應用田口實驗設計法(Taguchi methods) L9(34)混合直交表，探討不同蝕刻參數(包括：蝕刻時間、溶液溫度（ AgNO3/HF ），硝酸銀（AgNO3 ）濃度與氟化氫（HF ）濃度)對矽奈米線陣列的長度、直徑、結構與表面形貌的影響。實驗中，配合灰關聯分析，求出四組系統之最佳因子水準組合，並驗證最佳因子水準組合之多重品質比現行的直交表要好，本實驗獲得多重品質特性最佳蝕刻矽奈米線之參數(A1B3C1D3)，藉由實驗驗証，及再現性測試，其矽奈米線陣列長度由15.80μm增至23.07μm，直徑由76.77nm減少至66.65nm；由此證實，導入灰關聯法不但能有效的解決田口方法只能針對單一品質最佳化的缺點，且擷取田口方法簡化實驗時間與成本的優點、灰關聯分析之計算簡單以及所需實驗數據少的優點，提供一套簡單易用的實驗分析法於多重品質最佳化上。硝酸銀（AgNO3）與氫氟酸（HF）混合溶液，可得到單晶的大面積矽奈米線陣列，合成的矽奈米線結晶方向與矽基板(100)為同方向 ，且相較於塊材矽基板，由於矽奈米線量子效應造成矽奈米線有藍位移之現象。而奈米線長度與蝕刻時間（15至45分鐘）或溫度（25至75°C ）呈線性之趨勢。此外，本研究將這些大面積之矽奈米線陣列發展相關的應用，如太陽能電池防反射， pn異質結二極體與表面增強拉曼（surface-enhanced Raman spectroscopy，SERS）生物檢測。 在抗反射方面，利用大面積矽奈米線陣列可以開發一個簡單、方便與有效的抗反射層來增加太陽光吸收，進而應用於太陽能電池。在本研究中，探討不同長度的奈米線之抗反射效果，發現奈米線在可見光的範圍中的抗反射效率隨矽奈米線長度增加而降低，其最小平均可達2%以下。該抗反射特性，應與蝕刻過程導致奈米線由空氣層到矽基板之空間平均填充率漸次變化，形成等效漸變折射率有關。 在pn異質介面二極體研究中，成功製備一罕見二氧化鈦與矽奈米線製作形成之無機-無機異質接面二極體。在n型二氧化鈦/p型矽奈米線異質介面方面，利用磁控濺鍍之二氧化鈦薄膜，由實驗結果分析得知，其二氧化鈦薄膜結構為多晶銳鈦礦，薄膜厚度約150 nm，且經由二氧化鈦的改質後，表面從超疏水性轉變為超親水性，另外，由電性量測得知，其n型二氧化鈦/p型矽奈米線異質介面有良好的二極體特性，其開路電壓約0.8V，且漏電流約10μA，未來可以進一步應用在太陽能電池上。另外，也探討不同長度的矽奈米線陣列之異質接面二極體之特性，結果顯示，矽奈米線陣列的電流隨著長度的增加而降低，這是因為在n型二氧化鈦/p型矽奈米線有缺陷存在，或者是當矽奈米線陣列長度的增加，其電阻值增加因而造成電流值下降。 在表面增強拉曼散射生物偵測應用上，一種新的表面增強拉曼散射已經成功被製造，此法利用射頻磁控濺射一層銀顆粒於高深寬比的矽奈米線作為表面增強拉曼散射測量基板。本研究發現，銀奈米顆粒均勻的沉積在矽奈米線陣列上，其直徑約30到75奈米。經過牛血清白蛋白（bovine serum albumin ，BSA) 固定於基板後，由分析結果證實，N - H 與 C＝O之吸收峰值增強，且牛血清白蛋白（bovine serum albumin ，BSA) 固定於基板後，表面呈現親水性，這些結果表示藉由物理吸附法成功的將牛血清蛋白固定於基板上；而鋅之偵測部分，牛血清白蛋白偵測硫酸鋅（zinc sulfate，ZnSO4）後，其表面呈現超親水性，且從其他儀器分析也偵測到鋅離子之訊號，由此可證實，牛血清蛋白藉由物理吸附成功固定於基材上並可偵測到鋅離子。另外，在表面增強拉曼上可知，牛血清蛋白成功固定於基材上並可偵測到鋅離子，未來可以發展無標記環境生物感測器以及檢測重金屬離子，如環境污染、醫學與食品行業。另外，此表面增強拉曼可用於相關的發展，如食品檢驗，敗血症檢測或其他等等。

One-dimensional semiconductor nanostructures have demonstrated to be good materials for novel nanoscale optoelectronics and high-sensitivity molecule sensors. Recent years have seen increased attention given to silicon nanowires (SiNWs), owing to their unusual quantum-confinement effects for developing various applied device, such as optoelectronics, biosensor, and other devices. Therefore, in the present thesis, the objective of this research work aims to fabricate SiNW arrays with electroless metal etching (EMD) by Grey-Taguchi method, as well as these large-area SiNW arrays were used in the development related applications such as solar cell anti-reflection, inorganic-inorganic heterojunction diode and surface-enhanced Raman spectroscopy (SERS) biodetection. The key application is the geometric control of fabricated SiNWs including their lengths, sizes, and orientations. Therefore, in this paper, simple and convenient approach to generate SiNWs of single-crystalline, well-aligned, and large area has been directly synthesized on p-type (100) silicon wafer via an EMD method. On the other hand, in order to control size (lengths and diameters) of well-aligned SiNW arrays in this study, the Taguchi-Grey method is used. The experimental results show that microstructures of SiNWs have been observed at AgNO3/HF solution with different parameters. This study present an effective method to optimize the etching process parameters for SiNW arrays with multiple performance characteristics based on Grey-Taguchi analysis. The effect of the etching process parameters (etching time, solution (AgNO3/HF) temperature, silver nitrate (AgNO3) concentration, and hydrogen fluoride (HF) concentration) on the length, diameter, structural, and morphological of SiNW arrays were studied. In the confirmation runs, by applying grey relational analysis, the SiNW arrays length was increased from15.80 μm to 23.07 μm, and diameter was decreased from 76.77 nm to 66.65 nm, respectively. Further, the linear relationship of SiNW arrays could be adjusted by increasing the etching time (from 15 to 45 min) and the solution temperature (from 25 to 75 °C). The axial orientation of the SiNWs was identified to be along the [100] direction, which is the same as that of the initial Si wafer. Moreover, these large-area SiNW arrays were used in the development related applications such as solar cell anti-reflection, p-n heterojunction diode and SERS biodetection. A simple, convenient and effective approach to generate SiNW arrays which have shown to absorb solar light very satisfactorily can be used as the solar cell absorber in this study. The antireflection properties of SiNWs with different lengths were studied. The reflectivity of SiNWs were decreases as length increases The result showed that SiNWs gave the lowest reflectance (<2% in the broad visible band). The effective graded index between air and silicon substrate could be responsible for the broad band anti-reflection of the SiNW arrays. A rare inorganic-inorganic heterojunction diode using n-itanium dioxide (n-TiO2) and p-silicon nanowires (p-SiNWs) was fabricated. The thickness of thin film is about 150 nm. The HRTEM image and corresponding SAED pattern of TiO2 layer also reveal its polycrystalline nature and the crystal structure of TiO2 layer reveals its anatase. Electrical properties of n-TiO2/p-SiNW heterojunction have been investigated through I-V measurements, which displayed a well-defined rectifying behaviour with a low turn-on voltage of about 0.8 V in forward bias. Additionally, the leakage current is 10μA at a reverse bias up to 5 V. Its good property proved its potential applications such as nanoelectronic and solar cell devices in the future. Besides, the p-n junctions with different length of the SiNW arrays are fabricated. The results displayed the current of SiNW arrays also decreases as length increases. It is because there exists the interface defects between n-type TiO2 thin films and p-SiNW arrays, and the resistance of SiNW arrays increases as length increases. A novel kind of SERS active substrates to detection of heavy metal ions (Zn2+) through decorating with AgNPs by RF magnetron sputter has been fabricated.A layer of AgNPs were deposited on SiNWs surface by RF magnetron sputter. High resolution transmission electron microscope (HRTEM) revealed that the AgNPs are directly deposited on SiNW arrays and the size of AgNPs is distributed from 30 to 75 nm. After bovine serum albumin (BSA) immobilization, the bulk surface induces hydrophilic functional groups. The Ag-SiNW arrays become more hydrophilic after zinc sulfate (ZnSO4) immobilization process than BSA immobilization. BSA and ZnSO4 conjugations through physical adsorption gave specific and good-quality SERS spectra with higher sensitivity, which shows great promises in developing label-free biosensors and detection of heavy metal ions such as Zn2+ is a vital issue in various fields ranging from environmental science to medicine and food industry. Besides, SERS spectra could be expected to have favorable applications such as food examination, detection septicemia or so on.