以水熱法在異質基板上成長氧化鋅之研究

Abstract

水熱法(hydrothermal method)是一簡易而有效在低溫製作氧化鋅(ZnO)的方法，特別是奈米晶柱或微米晶柱。本研究主要利用X光繞射與掃描式電子顯微鏡觀察並探討水熱法在不同的基板上成長出的ZnO特徵；所使用的水熱法，溶液以硝酸鋅六結晶水(Zn(NO3)2∙6H2O)當作鋅離子源，六亞甲基四胺(HMT, C6H12N4)作為酸鹼緩衝劑；此外，在溶液加入檸檬酸鈉(Na3C6H5O7)改質劑，改變ZnO形貌。選用的基板分為三種: (1) 鍍有ZnO晶種層之矽基板及圖案化矽基板、(2)鍍有鉑(Pt)晶層之矽基板、(3)氮化鎵(GaN)單晶基板。 水熱法ZnO晶柱成長製程中，溶液溫度為85 C，探討改變其他溶液參數對ZnO的影響，其中改變前驅物濃度 (硝酸鋅和HMT)，則影響晶柱密度、方向性及成長速度等；檸檬酸鈉濃度則影響晶柱面的直徑大小；晶柱長度與水平方向聚合程度與製程時間有關。 觀察與分析不同基板上之ZnO晶柱成長，發現在Si(100)基板上直接成長之ZnO無特定方向而密度較低，而於Si基板上鍍上一層ZnO薄膜做為晶種層，則所成長的ZnO晶柱具有c軸方向性；另使用溝槽形貌的圖案化Si基板，以ZnO晶層輔助水熱法成長，因溝槽的水平及垂直面而得到不同方向的ZnO柱分佈，在平台上具極性面ZnO、在側壁則有非極性面ZnO，而轉角處呈現半極性面ZnO。 將室溫鍍膜的20 nm和54 nm Pt層退火成具Pt(111)方向的晶面可成長柱狀ZnO結構，Pt退火溫度對ZnO晶柱直徑與準直性有重大的影響，而在(111)面結晶性差的Pt層上，則促成ZnO形成片狀形貌。 GaN(0001)基板因晶格與ZnO匹配度高，晶柱在形貌上的準直性好且有磊晶關係，可成長出結晶性佳之ZnO磊晶連續膜，(0004) X光搖擺曲線(X-ray rocking curve)之半高寬約345 arcsec；此外，在GaN基板的N面及Ga面上成長之ZnO，晶柱密度因表面粗糙度等因素的不同而有所差異。

Hydrothermal method is a simple and effective way for low temperature synthesis of ZnO with nano- or micro-scale rod morphologies. This thesis studies the characteristics of ZnO rods grown on different substrates based on x-ray diffraction (XRD) analysis and scanning electron microscope (SEM) observations. The ZnO rods were grown by using hexahydrate zinc nitrate as the ionic-zinc source, hexamethylenetetramine (HMT, C6H12N4) as pH buffer and sodium citrate (Na3C6H5O7) as capping agent. The substrates used are of three types: (1) ZnO seeded Si(100) wafer and patterned Si(100) substrate, (2) annealed platinum (Pt) layer on Si(100) wafer, (3) c-plane gallium nitride (GaN) wafer. Hydrothermal growth parameters that affected ZnO morphologies were varied while maintaining the growth temperature at 85 ˚C. Precursor concentration (hexahydrate zinc nitrate and HMT) has an influence on rod density, alignment and growth rate. Adding sodium citrate in the solution causes changes in rod-diameter. It is also observed that the length and lateral-coalescence size of ZnO rods increase with growth time. The relations between substrate materials and rod growth morphology were also investigated. ZnO rods grown directly on the Si(100) substrate do not align in any specific directions, but hydrothermal growth after depositing a ZnO thin film on the substrate results in c-axis alignment of ZnO rods. ZnO seeded Si (100) substrates patterned with stripe shape are also applied for the hydrothermal growth of ZnO rods. The ZnO rods grown on the patterned substrate are commonly distributed perpendicular to the substrate surface, resulting in the formation of polar plane oriented ZnO on the mesa, non-polar plane oriented ZnO on the sidewalls, and semi-polar planes around the corners. After annealing of Pt sputtered on Si (100) substrate at room temperature, vertically aligned ZnO rods along c-axis can be grown on the Pt/Si substrate with Pt (111) preferred orientation. For growth on c-plane GaN, directly grown ZnO rods exhibit good alignment with each other and are in epitaxy with GaN due to the small lattice mismatch. Further growth of ZnO on GaN can form a continuous film after coalescence of the rods. Also, comparison of different ZnO growth on N-face and Ga-face GaN substrates has been made.