分子束磊晶成長鉍薄膜的電特性與熱退火效應

Abstract

本論文利用分子束磊晶(Molecular beam epitaxy, MBE)的技術將不同厚度的鉍薄膜成長於輕摻雜P型矽(111)基板上並用高解析X 射線繞射儀(HRXRD)進行量測及分析。將鉍薄膜於150°C 的溫度在10-8 torr的高真空環境下以不同的加熱時間退火，並以電子背向散射繞射(EBSD)、室溫霍爾量測，以及傳輸線模型量測進行量測分析。 在厚度約80nm的鉍薄膜中，我們藉由EBSD量測結果發現經過熱退火後在側向沿著(10-10)及(01-10)晶面方向成長的兩種攣晶比例將重新分布，從約1：1轉變為約1：2，且薄膜結晶性也由於熱處理有所改善。以雙帶傳輸模型(two-band transport model)擬合室溫霍爾量測的實驗數據後，發現鉍薄膜在經過150°C、24小時的退火後由於非等向散射減少能讓載子遷移率增加12%，而材料缺陷密度下降使載子濃度減少30%。另外藉由文獻參考值以及我們模擬數值所計算出的載子平均自由徑能看見顯著的表面散射效應(Surface scattering effect)。 而對厚度約12nm的鉍薄膜進行熱處理後我們發現材料結構的轉變和80nm的樣品有相同的趨勢，除此之外12nm的樣品由於初始晶粒尺寸較小，在加熱後有晶粒成長的現象，經過150°C、60分鐘的退火後晶粒增大至兩倍以上。於室溫量測傳輸線模型後，發現鉍薄膜在熱處理後由於非等向散射、邊界散射的減少能讓鉍薄膜電阻率下降約40%。

In this thesis, the bismuth (Bi) thin film with various thickness is grown on lightly doped p-type Silicon (111) substrate by molecular beam epitaxy (MBE system), and high-resolution X-ray diffraction (HRXRD) is applied for measurement and analysis. We also investigate transformation of electrical properties and structure of bismuth thin film after annealing at 150°C under 10-8 torr by electron backscatter diffraction (EBSD), Hall measurement, and transmission line model in room temperature. Although there’re lots of twinning in 80-nm-thick bismuth film before thermal process, it will bring a (01-10)-preferred orientation structure and greater crystallinity after annealing the film at 150°C for 24 hr. By fitting the result of Hall measurement in room temperature with two-band transport model, we find that not only carrier mobility increases by 12% because of weaker anisotropic scattering, but carrier concentration also decreases by 30% due to diminishing lattice defect. In addition, by comparing film thickness with mean free path calculated by value of reference and our simulation, we find that there’s a significant classical size effect in 80-nm-thick bismuth film. There’s a same trend of structure transformation with 80-nm-thick bismuth film in 12-nm-thick film after annealing at 150oC for 60 min. However, as a result of its smaller initial grain size, we can clearly see grain growth of bismuth by more than 100% in 12-nm-thick film through thermal process. By analyzing the result of transmission line model in room temperature, we find that resistivity of bismuth film decreases by 40% because of weaker anisotropic scattering and diminishing boundary scattering.