在這個研究中,呈現了三個以分子束磊晶法(MBE)生長在Si基底上的Si/Ge超晶格的拉曼光譜。超晶格樣本的週期數N=5,超晶格的生長條件﹕矽層的厚度均為50 nm;鍺層的厚度分別為2.2 nm、3.8 nm及5.4 nm。首先以樣品的TEM圖按比例測量,矽層的厚度為50.18 nm、48.65 nm及48.89 nm;鍺層的厚度為2.46 nm、3.78 nm及4.44 nm。 光譜中Si-Si Mode的峰線非常明顯;Ge-Ge Mode的峰線相對較弱;Si-Ge Mode的峰線則因週期數太少的關係,常溫下非常不明顯。三個超晶格的拉曼光譜在低頻(0~100 cm-1)區域,均有大量而顯著的折疊聲學聲子訊號,且其分裂的雙重線非常明顯。如以Rytov理論擬合其折疊聲子的頻率,可得矽層厚度為49.65 nm、45.98 nm及45.95 nm;鍺層的厚度為2.12 nm、3.78 nm及5.01 nm。誤差均在6%以內,可見Rytov理論是研究超晶格拉曼光譜的良好基礎。 如以彈性光學模式擬合折疊聲子的光譜強度,在遠離共振能量的波長(如476 nm),可以得到不錯的結果。同時得到矽層對厚度週期的比值分別為0.96(632 nm)、0.93(476 nm)及0.93(476 nm),與Rytov理論擬合折疊聲子的結果:0.96、0.92及0.90相較,除了樣品N107的差異3%較大外,大致上吻合良好。 另以線性鏈模式(LCM)檢驗光譜中的Ge-Ge Mode,可以檢驗鍺層的粗糙程度,並估算其厚度為2.22 nm、3.83 nm及5.37 nm,與聲子的擬合結果相較,差異在7%以內。 由變溫下的拉曼光譜(10 K~300 K),可以看到在200 cm-1附近有連續性散射的訊號,而且發現同一樣品溫度愈低,或不同樣品鍺層厚度愈薄,其連續性散射愈明顯。經由低頻的折疊聲子及Ge-Ge Mode共振及高頻的螢光光譜,可以看出鍺層的E1能階約在2.3 eV附近,而且分佈甚廣(2.29 eV~2.35 eV)。
This research presents three Raman Spectrums of Si/Ge superlattices growing on the substrate of Si by MBE. The period of superlattice samples N=5, the growing conditions of superlattice are: the Si-layer is 50nm;the Ge-layers are 2.2nm、3.8nm and 5.4nm. Measuring by TEM of samples firstly, the Si-layers are 50.18nm、48.65 nm and 48.89 nm;the Ge-layers are 2.46 nm、3.78 nm and 4.44 nm。 In the Spectrum, the peak of Si-Si mode is quite obvious;The peak of Ge-Ge mode is comparatively weak. In the room temperature, the peak of Si-Ge mode is quite unobvious due to the fewer periods. Raman Spectrum of three superlattices shows many and obvious folded phonon signals in the low frequency (0 ~100 cm-1) area , so does the doublets of phonon are quite obvious. Fittting the frequency of folded phonons by Rytov’s theory finds that the Si-layers are 49.65nm、45.98 nm and 45.95 nm, the Ge-layers are 2.12 nm、3.78 nm and 5.01 nm. All the errors are within 6%, so Rytov’s theory is a good foundation to study Raman Spectrum of superlattice. Fitting the intensity of folded spectrum of phonon with photoelastic mode , in the wavelength far away the energy of resonance ( As 476 nm) can get good result. Also getting the ratio of Si-layer thickness and the period of superlattice are 0.96(632 nm)、0.93(476 nm)and 0.93(476 nm), compared with the result of fitting frequency of folded phonon by Rytov’s theory:0.96、0.92及0.90, matchs very well, except the 3% difference of sample-N107. Besides, examining Ge-Ge mode in the spectrum with LCM, the roughness of Ge-layer can be examed, and the calculating layers are 2.22nm、3.83 nm and 5.37 nm, the difference is within 7% comparing with the result of fitting phonons. In the various temperature (10 K~300 K) Raman Sprctrum, there are signals of continuous scattering of phonons around 200 cm-1 , and the lower the temperature, or the thinner the Ge-layer, the more obvious the continuous scattering of phonon is. We can find the phenomenon that E1 energy of Ge-layer is in the vicinity of 2.3 eV and distributed widely ,when we observe the folded phonon in low frequency, the resonance of Ge-Ge Mode, and the fluorescence in high frequency.