氮化鋁奈米線成長條件和拉曼散射之特性研究

Abstract

近年來，氮化鋁和其它相關的半導體奈米線吸引了眾人目光在它獨特的特性上。許多報告指出，奈米材料的光學特性和它的結晶品質、成長軸向、結構缺陷、表面缺陷有相當大的關連性。在我們作的研究中，我們著重於單根奈米線在光學特性上的分析，像是偏振方向相關和奈米線尺寸相關的拉曼頻譜研究。為了成長出高品質的氮化鋁奈米線樣品，我們使用熱化學氣相沈積法來成長出低缺陷和筆直外形的奈米線。由掃描電子顯微鏡(SEM)，X-ray頻譜儀(XRD)，高解析穿透電子顯微鏡(HRTEM)量測儀器和散射能量頻譜儀，分析單晶結構的氮化鋁奈米線。 在將成長後的奈米線轉移在另一塊鍍有金方格的基板後，我們使用微米拉曼系統來研究在單根奈米線上的拉曼散射。在這一部份，偏極方向相關的單根拉曼散射研究詳盡地被呈現。在理論模擬方面，不同拉曼模態的強度是個別對應於其拉曼張量的。對於纖鋅礦(wurtzite)結構，光源偏極方向(電場向量)和晶軸方向(拉曼張量)之空間對應關係，是由兩種變數(θ & β)組成的方程式。其中，θ 是光源偏極方向對奈米線長軸的夾角，β是光入射方向對短軸的夾角。另一方面，從偏極拉曼頻譜獲得的實驗值可以用來和模擬結果作比較。 根據奈米線的外觀和其它特性，例如長寬比大約30:1的奈米線，與塊材結構的比較下，光學特性的表現有些差異。因此，我們使用不同半徑的奈米線來量測拉曼散射頻譜。我們發現到，當奈米線的直徑小於158奈米，將表現出特殊的光學現象。由於尺寸的因素，拉曼散射強度被明顯地增強。這是只有在幾百奈米尺度以下才能觀察到的特殊現象，可做為將來在開發奈米元件，像是奈米波導或是奈米雷射時的依據。

The present thesis has focused on the optical property, especially the polarization-dependent and size-dependent Raman spectroscopic studies on single aluminum nitride nanowire. For the growth of high quality aluminum nitride nanowires, thermal chemical vapor deposition (CVD) was employed. Scanning electron microscopy, x-ray diffraction, high resolution transmission electron microscope measurements and energy dispersive X-ray spectroscopy, revealed aluminum nitride nanowires to be single crystal of low defect. After transferring the CVD-grown nanowires on another patterned substrate, a micro Raman system was used to investigate well-isolated nanowire. Polarization-dependent Raman scattering of single aluminum nitride nanowire has been clearly observed. In theoretical simulation, intensity of different Raman modes is related to Raman tensor. For wurtzite structure, the relationship between polarization of light (electric field vector) and crystal axis (Raman tensor) of nanowire is a function of two variations (θ & β). The angle θ is the angle between the polarization of light to the long axis of wire, whereas the angle β is the angle between the incident light to the short axis of wire. Our experimental polarized Raman spectra obtained from nanowires with different crystal orientations, m-, a-, c- axis are closed to the simulated results. The nanowires, with inherently large length to width ratio (~30:1) make their optical property quite different to that of the bulk structure. We have performed a series of Raman studies, using different diameter of nanowires, ranging from 4000 to 30 nm. Strong size-dependent Raman scattering is observed. Enhancement of Raman scattering rapidly increases by orders of magnitude when the diameter of nanowires is reduced to below 158 nm.