掃描穿隧能譜術於金屬薄膜的局域功函數量測和電子共振態之研究

Scanning Tunneling Spectroscopy Studies on Local Work Function Measurement and Electronic Resonances of Metallic Films

黃旭昇(Hsu-Sheng Huang)

指導教授 ： 蘇維彬

共同指導教授 ： 張嘉升

國立臺灣大學/理學院/物理研究所/博士(2014年)

https://doi.org/10.6342/NTU.2014.00344

摘要

穿透共振是一種粒子的量子現象例如當電子被能量井散射時會發生。這個現象在銀薄膜成長於矽基板(111)的系統上可以被掃描穿隧能譜術觀測到。穿隧能譜指出穿透共振態的能量會隨著薄膜厚度的增加往低能量移動。運用從量子力學出發所推導出的公式指出這個穿透共振能量往低能量移動的過程正比於(w+1)^2/w^2，其中w是薄膜厚度的原子層數。這個關係可經由實驗結果證明但只適用於較薄的薄膜。此公式也預測了最低能階的穿透共振在銀薄膜達到某個臨界厚度時將會消失，而這個穿透共振消失的現象也已經在實驗所得到的穿隧能譜中獲得確認。而且從穿透共振的消失可以推論出銀薄膜中位能井的深度在真空/銀界面比在銀/矽界面來得大。
利用表面活性劑介導的外延生長鍺薄膜可成長於鉛單層膜和矽(111)基板之間。我們藉由掃描穿隧顯微儀來量測高階昆拉赫振盪的尖峰特徵，進而求得Pb/Si(111) 和 Pb/Ge/Si(111) 間的功函數差異。由於矽和鍺的介電常數並不相同，要在穿隧間隙中建立強度一樣的電場Pb/Si(111)上的穿隧電流必須比Pb/Ge/Si(111)的大上2-3倍。當這個條件下透過對昆拉赫振盪尖峰特徵的觀察，我們得出它們功函數的差異是200 mV。相信這裡所發展的技術也可以被擴大應用於導體塊材的功函數量測。
藉由掃描穿隧能譜術可觀察矽基板上鉛單層膜於非佔據態中所存在的兩個有震盪特徵的電子共振態。利用量測共振態的能量間距，可以發現能量間距會隨著鉛單層膜披覆量的增加而減少。造成能量間距改變的原因來自於高能量的共振態對於披覆量改變所引起的偏移量比低能量的共振態明顯。我們的結果表明這個能量間距會隨不同相位結構改變的關係是一個有用的物理量，它可以被用來證明室溫下1x1和低溫下 R7R3 下的相位結構有著相同的披覆量都是1.2 ML.

關鍵字

量子尺寸效應 ； 穿透共振 ； 昆拉赫振盪 ； 鉛單層膜 ； 功函數量測

並列摘要

Transmission resonance is a quantum phenomenon of particles such as electrons scattered by the potential well. This phenomenon can be observed on Ag films grown on Si(111)7x7 surface by using scanning tunneling spectroscopy. The tunneling spectra show that the energy of the transmission resonance moves toward lower energy with increasing film thickness. The formula used is derived from quantum mechanics to demonstrate that this lowering of the transmission resonance energy is proportional to (w+1)^2/w^2, where w is the number of the atomic layer of film thickness. This relation can be justified from experimental results, but only holds for thinner films. The formula also predicts that the lowest-order transmission resonance should disappear when the Ag film reaches a critical thickness. This disappearance of transmission resonance has also been experimentally confirmed in the tunneling spectrum. Moreover, it can be inferred from the disappearance of the transmission resonance that the depth of the inner potential of the Ag film at the vacuum/Ag interface is larger than that at the Ag/Si interface.

Ge films can be grown between the Pb overlayer and Si(111) substrate by the surfactant-mediated epitaxy. We detect the high-order Gundlach oscillation revealed in scanning tunneling microscopy (STM) to measure the work function difference between Pb/Si(111) and Pb/Ge/Si(111). Owing to different dielectric responses of Si and Ge, the tunneling current on Pb/Si has to be larger than that on Pb/Ge/Si by a factor of 2-3 to establish the same electric field in STM gap on both regions. This condition leads us to obtain a work function difference of 200 meV from observing Gundlach oscillation. It is believed that the method developed in this work can be extended to measure the surface work function difference of bulk conductors as well.

The unoccupied states of Pb dense overlayers on Si(111) reveal an oscillatory character with two electronic resonance peaks that can be observed by scanning tunneling spectroscopy. By measuring the energy spacing between resonance peaks, it is found that the energy spacing is reduced with increasing the coverage of dense overlayer. The change of energy spacing originates from that the movement of the high-energy resonance peak is more pronounced than that of the low-energy peak with varying coverage. We demonstrate that this phase-dependent energy spacing is a useful quantity to identify that the room-temperature 1x1 and the low-temperature R7R3 phases have an identical coverage of 1.2 ML.

並列關鍵字

Quantum size effect ； Transmission resonance ； Gundlach oscillation ； Pb overlayer ； Work function measurement.

參考文獻

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