霧化化學氣相沉積氧化鋅薄膜於藍寶石單晶

Abstract

霧化化學氣相沉積法(Mist Chemical Vapor Deposition, Mist-CVD)為一低成本、低危險性、低真空需求、製程簡單之綠色製程技術，此製程技術以超音波震盪將固態價廉的前趨物霧化，可在大氣壓力沉積氧化鋅(ZnO)薄膜。本研究利用X光繞射儀與掃描式電子顯微鏡分析在不同醋酸鋅濃度以及製程溫度以Mist-CVD沉積之氧化鋅結晶品質、磊晶關係與表面形貌。本研究使用之前趨物水溶液，係以去離子水(D.I. Water)以及冰醋酸(CH3COOH)做為溶劑，以醋酸鋅(Zn(OOCCH3)2˙2H2O)做為鋅元素來源，並以氮氣(N2)做為攜帶氣體、以氧氣(O2)做為反應氣體提供氧元素，使用基板為c面(0001)藍寶石(Sapphire)單晶。 Mist-CVD沉積氧化鋅製程中，腔體溫度維持500°C，探討改變醋酸鋅濃度對氧化鋅的影響，隨著濃度上升，氧化鋅之成核密度增加、沉積量提升，於0.05 M醋酸鋅濃度條件下沉積之氧化鋅開始出現奈米牆結構。 觀察並分析於500~800°C溫度條件下沉積之氧化鋅，發現氧化鋅厚度隨溫度上升而減少，於700°C條件下奈米牆結構不再出現，取而代之為平坦之晶粒，並有少部分未合併完全之區域；於800°C條件下沉積之試片伴隨著不規則形狀之晶粒散布表面。在600°C條件下，能得到較佳結晶品質之氧化鋅，其氧化鋅(0002) X光搖擺曲線 (X-ray rocking curve，XRC)之 半高寬 (Full width at half maximum，FWHM)為385 arcsec。 透過600°C 氧化鋅低溫晶種層沉積以及700°C高溫晶粒成長合併之兩階段成長，可有效地抑制奈米牆結構的成長，並能得到表面平整之氧化鋅薄膜，且(0002) XRC FWHM為450 arcsec之結晶品質。 以Mist-CVD成長之氧化鋅於PL量測結果顯示，主要發光能量為3.28 eV的紫外光發光(UV emission)，並伴隨著於發光能量2.5 eV強度微弱的綠光發光(Green emission)。

Mist chemical vapor deposition (Mist-CVD) is a low cost, safe, low vacuum system demand, and simple green process. By this process, inexpensive and solid precursors in solution can be vaporized by ultrasonic, which enables deposition of ZnO (zinc oxide) at atmospheric pressure. In this thesis , x-ray diffraction (XRD) and scanning electron microscopy (SEM) are mainly used to characterize crystallinities, epitaxial relationships and surface morphologies of ZnO deposited at various concentrations of zinc acetate (Zn(OOCCH3)2˙2H2O) and temperature. The precursor solution composed of deionized water, acetic acid as solvent, and zinc acetate as provision of zinc source was prepared. Nitrogen gas was used as carrier gas of mist, while oxygen gas which provided oxygen source was used as reaction gas. (0001) c-plane single crystal sapphire was used as substrate. For ZnO deposition at 500°C, both the nucleation density and the amount of deposition increased with the zinc acetate concentration. A nanowall structure emerged when the concentration increased to 0.05 M. Deposition of ZnO at temperatures between 500°C and 800°C was also investigated. It was found that the thickness of ZnO decreases with increasing temperature. When the temperature was raised to 700°C, the nanowall structure was not observed, instead a flat film surface appeared with incomplete coalescence of the grains in a few areas. For deposition at 800°C, the ZnO surface became rough due to the irregular grain shape morphologies. Good ZnO crystallinity can be obtained at 600°C with the FWHM (Full width at half maximum) of (0002) X-ray rocking curve (XRC) of 385 arcsec. To obtain a smooth and continuous ZnO film without the nanowall structure, a two-step growth was developed, in which the growth temperature in the first step was set at 600C, followed by the second step growth at 700C. The film quality with the (0002) XRC FWHM of 450 arcsec was obtained with a smooth surface morphology. Photoluminescence measurements at room temperature show that the deposited ZnO films exhibit strong near band edge emission at 3.28 eV with negligible green and yellow emissions.