矽化鈷奈米線之合成與特性分析

Abstract

隨著電子元件日益微小化，具奈米尺度的結構及材料受到廣泛的重視，奈米材料的生長方式與其特性成了當前重要的課題，因此相關的研究也日新月異。鈷矽化物因為具有良好的電子特性，早在奈米線發展前，就常被利用在微電子元件上，做為歐姆接觸、內連線和閘極材料。本研究藉由熱蒸鍍方式，以氯化鈷為製備之前驅物，藉由控制爐管內矽和鈷元素的濃度，成功地合成出鈷矽化物奈米線。此實驗以鈷矽化物中之矽化鈷(CoSi)奈米線為主要研究材料，由結果得知，控制不同的反應溫度、壓力與時間，可以得到不同長寬比和密度的CoSi奈米線。此外，比較不同反應時間所生長之CoSi奈米線的形態，可推斷出本實驗中CoSi奈米線的生長為氣-固法(Vapor-Solid ,VS)的成長機制。 對CoSi奈米線而言，由於線直徑在奈米尺度下，有助其尖端放電，因此具有良好的場發性質，場發的增強因子和起始電壓大小則與奈米線的密度有很大的關連。在電性量測中，我們發現CoSi奈米線的電阻率會隨著直徑的增加而變大。除此之外，CoSi原為一反磁性材料，當合成為奈米線結構時，表面未成對的Co原子會誘發CoSi奈米線具有鐵磁性，在低溫下，由於較不受熱擾動影響，而具有較大的飽和磁化率。對不同直徑的CoSi奈米線來說，直徑越小的奈米線，磁化率越大且矯頑磁場越小，但總體而言，CoSi奈米線為一室溫鐵磁材料。

With the miniaturization of electron devices, the minuscule structures are important to state-of-the-art science and technology. Therefore, the growth methods and properties of nanomaterials have been extensively studied recently. Because of the excellent electronic properties, cobalt silicide has been used as the materials of ohmic contact, interconnect and gate of microelectronic devices. In this study, cobalt silicide nanowires were synthesized by chemical vapor transport process with cobalt chloride as the precursor under the appropriate concentration of silicon and cobalt. The fabrication process is economical as well as safe, and CoSi is the only thermodynamically stable phase that can be obtained on silicon wafer by solid phase reactions. By changing the reaction temperature and pressure, cobalt silicide nanowires with different size and density can be obtained. The CoSi nanowires were grown via a Vapor-Solid mechanism which can be inferred from different morphologies of CoSi nanowires with different reaction time, respectively. The as-prepared CoSi nanowires, owing to their sharp tips, display good field emission property. The field enhancement factor and the threshold voltage are highly related to the density of nanowires. Moreover, the electronic properties were studied. The resistivity is proportional to the diameter of the CoSi nanowires. The magnetic properties were also demonstrated in this work. The bulk CoSi is a diamagnetic semimetal. However, the CoSi nanowire ensemble exhibits magnetic property which induced by the reduced coordination of the surface Co atoms. At low temperature, the nanowires have relatively high saturated magnetization (Ms) due to less effect of thermal vibration. The CoSi nanowires with different diameters have diverse Ms and coercive force (Hc). In brief, the CoSi nanowires are room-temperature ferromagnetic materials.