以化學浴法在圖案化基板上側向成長非極性氧化鋅

Abstract

氧化鋅(ZnO)是重要纖鋅礦結構(wurtzite)的半導體，具有優越的光電性質。為了得到高效率的ZnO，需要成長出非極性面ZnO。 本論文研究主要是以化學浴法在85C 沉積ZnO於具有溝槽狀之圖案化基板上。圖案化基板以黃光及蝕刻製程技術製作，主要有兩種結構: 一種是SiO2/poly-ZnO/Si，鍍有多晶ZnO晶種層之(100)矽基板；另一種則是SiO2/GaN/Al2O3，使用a面氮化鎵(GaN)/r面藍寶石基板。300~400 nm 厚度之ZnO晶種層以濺鍍方式成長於矽基板；而GaN磊晶層則是以金屬有機化學氣相沉積法成長於r面藍寶石基板；兩者基板上皆覆蓋二氧化矽(SiO2)，可做為抑制ZnO微米柱c軸成長的阻擋層。SiO2以電漿輔助化學氣相沉積法鍍於Si上，再以黃光製程定義出圖形後，使用二次蝕刻的方式呈現出圖案；然後以化學浴法成長出ZnO微米柱。化學浴法(CBD, chemical bath deposition)是一種簡易而有效的成長方式；在低溫環境下就能製作出ZnO，特別是奈米晶柱或微米晶柱。CBD所使用的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晶柱之高寬比。非極性ZnO可從溝槽之ZnO和GaN側壁以側向成長方式沿著c軸方向形成奈米柱與微米柱之結構，進而經由合併過程形成連續膜。 利用SEM做表面形貌觀察，以X光繞射(XRD)做結晶性分析，TEM則針對局部區域做微結構分析。在SiO2/poly-ZnO/Si 圖案化基板，，以二階段式成長ZnO晶柱，由SEM觀察得知，晶柱從溝槽側壁以側向方式成長出，長度隨成長時間而增加，但無固定排列；在第二階段添加檸檬酸鈉，ZnO晶柱直徑也隨之變粗，促使晶柱間聚合。 在SiO2/GaN/Al2O3圖案化基板，由於GaN與ZnO同屬纖鋅礦結構，對於成長非極性ZnO而言，GaN是良好的晶種層。同樣的在二階段式成長條件所沉積之ZnO，經由SEM觀察得知，第一階段ZnO晶柱於溝槽側壁做側向成長，其晶柱為六角柱，柱面是m面，如屋脊狀形貌，晶向為a方向，且晶柱具有良好之準直性與方向性，呈規則排列。第二階段同樣添加檸檬酸鈉與溶液中，以填滿溝槽，經過成長時間後，晶柱合併成連續膜。由XRD可知ZnO晶柱為a面，溝槽中之ZnO晶柱與GaN之關係亦進行TEM觀察與分析。

Zinc oxide (ZnO) is an important wurtzite semiconductor for optoelectronic applications. To obtain high efficiency performance, nonpolar ZnO is required. In this thesis, chemical bath deposition (CBD) is applied to growth of ZnO on patterned substrate at 85C. Two kinds of patterned substrates with trenches/stripes structures fabricated by lithography and etching processes were used: one was SiO2/poly-ZnO/Si consisted of a 300~400 nm polycrystalline ZnO seeding layer covered with SiO2, and the other was SiO2/GaN/sapphire in which a-plane GaN was grown on r-plane sapphire. The stripe patterns allow the lateral growth of ZnO nanorods and microrods on the sidewalls of ZnO and GaN layers in c-axis. CBD is a simple and effective way for low temperature synthesis of ZnO with nanorod and microrod morphologies. The ZnO rods were grown using hexahydrate zinc nitrate as the zinc ion source, hexamethylenetetramine (HMT, C6H12N4) as pH buffer and sodium citrate (Na3C6H5O7) as capping agent to vary the density, growth rate, and aspect ratio of the Zn rods. The deposited ZnO rods were characterized with scanning electron microscopy (SEM), x-ray diffraction (XRD), and transmission electron microscopy (TEM) for the morphology, orientation, and microstructure with their distribution. On the SiO2/poly-ZnO/Si patterned substrate, it is observed that each ZnO nanorods grow laterally along c-axis from the sidewalls of the trenches and increases their size with the growth time. However, the rods exhibit without specific arrangements among them. After adding sodium citrate into the solution in the following CBD, ZnO nanorods significantly increase the lateral size into micrometers, which allows rods contact with each other and coalescence. As for the SiO2/GaN/ Al2O3 patterned substrate, GaN serves as an excellent seeding layer for ZnO growth as GaN and ZnO have the same wurtzite structure. Also with two-step CBD process, most of a-oriented ZnO nanorods in good alignments were laterally grown from inclined side walls of the trenches with m-facets and roof-like morphology. Further growth resulted in the coalescence of the ZnO microrods, and finally a continuous thin film formed by lateral overgrowth on the pattern. Also the microstructure of the ZnO rods and their relationships with GaN were analyzed with TEM.