矽奈米線之合成與其場發特性研究

Abstract

隨著電子元件微型化，具奈米尺寸的一維之半導體奈米材料日漸受到重視，其中以矽(Si)奈米線具有整合與多變性優勢受到矚目。因此本論文主要以合成品質良好的單晶Si奈米線為主，並針對參數調變造成影響，與場發特性進行探討。 本論文利用兩種製程搭配氣-液-固成長(VLS)機制合成Si奈米線：熱蒸鍍法與化學氣相沉積法(CVD)，由實驗結果得知，腔體內含氧量為影響Si奈米線生長的重要因素。在合成Si奈米線時，會發現仍有二氧化矽(SiO2)奈米線的存在，且隨著氧含量之增加，於基板上形成SiO2奈米線機會增加。以熱蒸鍍法而言，調變氫氣(H2)於腔體內含量時，Si奈米線因H2含量增高而氧化趨勢下降，念珠結構減少。當壓力上升時，因腔體內部含氧量增加使得SiO2奈米線於試片比例上升，且Si奈米線核殼結構之SiO2層增厚。對於使用CVD法合成Si奈米線而言， 腔體內含氧量增加使得Au催化劑被SiO2層所包覆，抑制Si奈米線成核生長，Si奈米線生長速率受到抑制、密度降低且線徑分佈不均。H2量提升使得吸附於基板表面之SiCl2因SiCl2+H2□Si+2HCl可加速分解，並且增加腔體中HCl含量抑制SiO2層於基板表面生成機會。 由穿透式電子顯微鏡(TEM)與能量散射光譜儀(EDX)分析，熱蒸鍍法與CVD成長之Si奈米線，成長方向主要為[111]，奈米線之前端可發現Au訊號，且奈米線僅有Si與O訊號，可證實本實驗為利用VLS生長。相較於兩種製程方式，熱蒸鍍法合成之Si奈米線，於低壓與高壓時形貌差異極大，高壓時易觀察到念珠結構與鑲嵌Au顆粒之SiO2奈米線，且奈米線內部含大量疊差與雙晶缺陷，CVD製程生長Si奈米線外觀筆直，線徑均一且內部原子排列整齊無缺陷。 於場發特性方面，使用熱蒸鍍法合成Si奈米線之起始電場為12.4 V/μm，場發增強因子為574；應用CVD製程生長之Si奈米線其場發起始電壓為6 V/μm，場發增強因子為2245。造成兩者之間的差異為合成Si奈米線密度差異，當奈米線密度增加，可場發面積亦增加，場發特性因此提升。使用熱蒸鍍法製程容易合成Si與SiO2奈米線混合且所生長核殼Si奈米線SiO2殼層較厚情形，因SiO2功函數大於Si，故熱蒸鍍法生長Si奈米線的場發特性較差。此外，本實驗之CVD合成Si奈米線場發特性值2245相較於相同製程或摻雜後之Si奈米線量測場發特性之報導大可知本實驗CVD製程可生長單晶且場發特性良好的Si奈米線。 由本論文可知CVD製程為有效合成具較少缺陷Si奈米線之製程，並可藉由進一步控制反應物種於腔體內部含量達到改善Si奈米線生長狀況。

iii The Synthesis of Silicon Nanowires and their Field Emission Properties

Student：Yu-Jie Chiu Advisors：Dr. Wen-Wei Wu Department of Materials Science and Engineering National Chiao Tung University ABSTRACT With the miniaturization of electronic devices, one-dimensional semiconductor nanomaterials have drawn a large number of interests, especially Si nanowires. In this thesis, we focus on the synthesis of Si nanowires with good quality, and discussing the effect of modulating the synthesis parameters, and their field-emission properties. Two different processes were used to synthesize Si nanowires via vapor-liquid-solid (VLS) mechanism: thermal evaporation and chemical vapor deposition (CVD). Based on the experimental results, the content of O2 in the growth chamber is the key that affects the growth of Si nanowires. On synthesizing Si nanowires, there are some parts of SiO2 nanowires present on the substrates. The higher content of O2 is in the chamber, the more SiO2 nanowires are on the substrates. During the process of thermal evaporation, with the increasing of the content of H2, the portion of Si nanowires increases, and SiO2 nanowires and nanowires with oscillating diameters decrease. With the increasing of the pressure, the content of O2 increases so that the portion of SiO2 nanowires increases. So does the thickness of SiO2 shell. During the process of CVD, O2 would make Au catalysts covered by SiO2, suppressing the nucleation and growth of Si nanowires. The growth rate of Si nanowires was constrained, the density was lowered, and the diameter distribution broadened. The introduction of H2 would promote the adsorbed SiCl2 onto the surface of substrates to decompose according to the formula of SiCl2+H2 □Si+2HCl. The increasing content of byproduct HCl would prevent SiO2 from covering the surface of substrates. By means of TEM and EDX, the dominate growth direction of Si nanowires grown from thermal evaporation method and CVD is [111]. Au was detected on the top of nanowires, and only Si and O were detected in the nanowires. Those confirm the VLS growth mechanism. In the thermal evaporation method, the difference of Si nanowires synthesized from low pressure and high pressure is large. In high pressure, nanowires with oscillating diameters and Au-embedded SiO2 nanowires were often observed, and there were a lot of stacking faults and twin defects in the nanowires. From the CVD method, Si nanowires were straight and log, and exhibited narrower diameter distribution and few defects. In the investigation of field-emission properties, the turn-on electric field and the enhancement factor of Si nanowires synthesized from thermal evaporation are 12.4 V/μm and 574, respectively, while those of Si nanowires from CVD are 6 V/μm and 2245, respectively. The main difference between those two is the density of Si nanowires. While increasing the density of Si nanowires, the area contributing to the field emission increases, and therefore field-emission properties are intensified. By using thermal evaporation to synthesize Si nanowires, the SiO2 shell is thicker and the portion of SiO2 nanowires increases. Because the work function of SiO2 is higher than that of Si, the performance of field emission is lower. On the other hand, Si nanowires from the CVD process can serve as a far better field emitter due to the high quality of Si nanowires and thin SiO2 shell enclosing the Si nanowires. From this thesis, we can conclude that CVD process could be used to synthesize Si nanowires with few defects. By modulating species in the growth chamber, the diameter, density, and growth rate of Si nanowires can be controlled.