應變矽鍺合金之拉曼光譜與EXAFS光譜研究

Abstract

本論文使用了拉曼散射光譜和延伸X光吸收精細結構光譜這兩種光學方法，來分析微觀狀態下鑽石結構(Diamond Structure)的矽鍺合金的結構性質及受應力變化的情形。主要討論三種不同成長結構的矽鍺合金：成長矽鍺合金在低溫矽(LT-Si) 、成長矽鍺合金於摻雜硼的低溫矽上(Boron-doped in LT-Si)、成長應變矽於矽鍺合金超晶格之上(Strained silicon on Si/SiGe Superlattice )。 成長於低溫成長矽(LT-Si)上的矽鍺合金層的特性，虛擬矽晶層的成長溫度從350oC到600oC，其鍺摻雜濃度x=0.3。在拉曼散射光譜中，除了LT500oc(＃323)樣品多了513cm-1的聲子訊號外，主要都可觀測到Ge-Ge(287.7cm-1)、Si-Ge(405cm-1)、Si-Si(503cm-1)、Si(520cm-1)等振動模的聲子訊號。由聲子的位置我們可以分析得知，除了＃323樣品的矽鍺合金層受有局部不均勻的應力外，其餘樣品中矽鍺合金聲子模皆在503cm-1附近，因此統稱無應力作用的矽鍺合金，鬆弛程度約在90%。並由X-ray吸收譜中可得到矽鍺間的原子距離變化量不大，約在±0.001 Å之間。 成長矽鍺合金於摻雜硼低溫矽上(Boron-doped in LT-Si)，摻雜濃度約x=0.2，成長溫度皆為600oC。由拉曼光譜中得知，當硼摻雜的濃度由0.49 x 1017 增加到123 x 1017 cm-3 時，Si-Si聲子模有非常明顯的紅移變化(RedShift)，並越接近完全鬆弛的狀態(508 cm-1)，鬆弛程度可高達70% 左右，原因是由於摻雜越多雜質時，附向取生的矽鍺合金會產生較嚴重的缺陷與錯位情形，並利用EXAFS光譜中得到相似的結果。當低溫矽層摻雜硼原子越多，上層的矽鍺合金中原子距離將被拉伸。 成長應變矽及矽鍺合金之超晶格，其鍺摻雜濃度約x=0.15。我們成長10個週期的矽及矽鍺合金超晶格，去分析應變矽受力的情形。利用可見光拉曼光譜分析超晶格成長的情形，可發現到樣品中來自於SiGe合金的Si-Si聲子模分布皆約在515 cm-1左右，其鬆弛程度大約為隨著矽鍺合金層的厚度提高而增大，且變化程度非常明顯，而UV紫外光拉曼光譜可分析出最頂層的應變矽受到底下未完全鬆弛的矽鍺合金影響情形。锗原子的周圍矽的結構及距離我們可以從ｘ-ray吸收譜光譜中得知。 利用拉曼光學以及EXAFS分析的方法檢測此三種不同鍺濃度的矽鍺合金結構，可清楚地分析出異質結構中矽/矽鍺受應力情況，這對製程應用或是理論分析上都有極大的助益。The strained silicon grown on the silicon-germanium alloy (SiGe alloy) virtual substrate can promote the electric-mobility by changing growing condition, and the residue stress in SiGe alloy has strong influence on the electron mobility of the strained Si. Therefore, the examination of the SiGe structure is crucial for the development of Silicon technology. In this paper, we report the utilization of Raman spectroscopy and Extend X-ray absorption fine structure (EXAFS) to analysis stress in Si1-xGex alloy. The relaxed Si0.7Ge0.3 virtual substrate grown on low-temperature-growth Si buffer layer (LT-Si) and partially strained Si0.8Ge0.2 virtual substrate grown on Boron-doped LT-Si were discussed, and, finally the strained Si grown on 10-periodes Si/Si0.85Ge0.15 superlattices were examined. Si0.7Ge0.3 alloy virtual substrates on Grown LT-Si (temperature varied from 350 oC to 600 oC) were relaxed over 90%, as observed by Raman measurements. Raman spectrum probed by 514.5-nm laser clearly showed three dominate peaks related to the Si0.7Ge0.3 alloy, and they are Ge-Ge mode near 287.7cm-1, Si-Ge mode around 405cm-1, and Si-Si at 503cm-1. However, the bulk modes of Si near 520cm-1 were also observed. Relaxation examined by the Si-Si modes clearly are over 90 %, closed to fully relaxation. EXAFS results give the average of the first neighboring distance of Ge atom are around 2.375Å. Both Raman and EXAFS results indicates the structure is cubic diamond structure.The strain of Si0.8Ge0.2 virtual substrate grown on Boron-doped LT-Si was found. The degree of relaxation of Si0.8Ge0.2 virtual substrate, as revealed by Raman scattering is proportion to the Boron concentration in LT-Si. On the other hand, the Ge-Si bond distance in virtual substrate detected by EXAFS is also proportional to the Boron-doped concentration in LT-Si, but the lattice constant calculated from the EXAFS deduced bond distance is larger than the lattice constant deduced from Raman measurement. This result indicates the strained SiGe alloy is slightly elongated in c-axis, i.e. along the growth direction. Capped strained-Si grown on Si/Si0.85Ge0.15 superlattice virtual substrate was examined by UV and Visible Raman to distinguish the strain in strained-Si and in superlattice virtual substrate. The degree of relaxation of strained-silicon is modulated by the thickness Si and Si0.85Ge0.15 of superlattice. The strain information of Si0.85Ge0.15 layers obtained by visible-Raman and EXAFS are similar to the results of Si0.8Ge0.2 virtual substrate grown on the Boron-doped LT-Si, and also indicate the elongation of c-axis is proportional to the strain in the grown alloy.