以高溫氧化矽鍺薄膜驅動固-液-固機制析出矽鍺半導體量子點成長之研究

Abstract

矽/矽鍺合金異質接面結構在電子、光電以及能隙工程領域都有應用的潛力。了解矽鍺合金奈米線中異質接面的成長機制在元件應用上材料物理性質的設計方面很有幫助。然而，與直接以VLS機制成長的異質接面奈米線不同，以高溫氧化低鍺濃度矽鍺合金奈米線製造異質接面的方法較為複雜。本研究發現在氧化過程中，奈米線頂端金矽鍺共晶液珠對於矽鍺奈米線的熱侵蝕十分顯著，而矽被優先氧化使得共晶液珠內Ge/Si的比例上升，進而影響了鍺、矽原子的溶解度。因此我們推斷金-矽-鍺三元合金系統在氧化過程中濃度的變化是造成奈米線異質接面處濃度差異的主因。這種藉由高溫氧化進行濃度調整的製程亦可被應用在製造矽鍺量子點上。Si0.99Ge0.01薄膜在700 °C空氣中氧化6小時後，具有較高鍺濃度的矽鍺量子點就會產生，而TEM與STEM影像顯示這些量子點結構是在冷卻過程中從金矽鍺共晶液珠中磊晶析出並鑲嵌在低鍺濃度的矽鍺薄膜中。之後我們更進一步證明藉由調整矽鍺薄膜的初始鍺濃度、氧化時間、氧化溫度等實驗參數，可以控制矽鍺量子點的成分、形貌以及尺寸。最後，我們量測量子點的PL光譜以探討此結構在後續應用上的可能性。

Group IV semiconductor Si/SiGe alloy heterojunction structures are potentially useful in electronics, optoelectronics, and bandgap engineering. Understanding the growth mechanism of heterojunctions in Si-Ge alloy nanowires is helpful for designing adequate physical properties in the material for device applications. However, unlike direct growth of heterojunction nanowires using the vapor-liquid-solid method, forming heterojunctions in Si-Ge nanowires by thermal oxidation in low Ge-content Si-Ge nanowires involves more complicated reaction routes. In the oxidation process, thermal etching of the Si-Ge alloy nanowires by the AuGeSi eutectic liquid at nanowire tip is found to be significant. Selective oxidation of Si increases the Ge/Si ratio in the eutectic liquid, which further modulates the solubility of Ge and Si atoms. The compositional variation in the Au-Ge-Si ternary alloy system during the oxidation process accounts for the observed concentration profile in the heterojunction nanowire. Such compositional modulation by thermal oxidation is applied to form Si-Ge quantum dots in a low Ge-content Si-Ge thin film. After Si0.99Ge0.01 thin film is oxidized at 700 °C for 6 hours in air, quantum dots with higher Ge concentration are produced. Transmission electron microscopy and scanning transmission electron microscopy images show that the quantum dots are epitaxially precipitated from the eutectic AuGeSi liquid during the cooling process. The concentration, dimensional, and morphology of the SiGe quantum dots can be controlled by adjusting several parameters such as initial Ge concentration in the SiGe thin film, oxidation time, and oxidation temperature. Photoluminescence spectra of the quantum dots are also measured.