使用掃描探針能譜術研究金屬薄膜中量子現象

Quantum Phenomena of Thin Metal Films by Scanning Tunneling Spectroscopy

呂欣明(Shin-Ming Lu)

指導教授 ： 鄭天佐

共同指導教授 ： 張嘉升 ； 蘇維彬

國立臺灣大學/理學院/物理研究所/博士(2006年)

https://doi.org/10.6342/NTU.2006.00600

摘要

當自由電子在金屬薄膜中運動時，其被散射而穿透薄膜的機率對應於不同能量的分布可以顯現出共振的現象。此穿透共振是一種能量高於真空能階的量子尺寸效應 (QSE)，而且它可以被掃描穿隧能譜術(STS)探測到。在一片銀薄膜長在Si(111) 7×7表面的樣品上,我們利用掃描穿隧能譜術觀察到穿透共振現象。在能譜中，不同偏壓下出現的穿透共振取決於薄膜的厚度。當穿透共振出現時，一般而言，其後的駐波態(standing wave states SWS)會被推上更高的能量。由於能譜的特徵取決於薄膜的厚度，如同指紋一般，因此他們可以用來鑑識薄膜的厚度。當薄膜覆蓋整個基材表面時,這個方法特別有用。此外，因為電子在介面上不同位置反射相位不同，在薄膜上不同位置取的能譜會顯現出輕微的差異。此差異可以用來探測埋藏在薄膜下的介面結構。我們也觀察到經由改變穿隧電流，亦即改變針和樣品間的電場，穿透共振的能階位置也隨之移動。隨著此電場增加，穿透共振的能階連續地上升，但剛超過臨界電場時，此能階會不連續地改變，突然下降到較低的位置。在量子化規則(quantization rule)中，加入由電場引進的相位變化，此電場相關行為可以得到定量化的解釋。
量子井態出現在薄膜中的真空能階以下，如同穿透共振，它是另一種量子尺寸效應。我們結合掃描穿隧能譜術和鎖相放大(lock-in)的技術，分別在兩種型式的三層鉛薄膜上觀察量子井態和圖案；觀察的能量範圍介於費米能階(Fermi Level)到 2伏特，而鉛薄膜是長在無理對應(incommensurate)的Pb/Si(111)表面上。我們的結果顯示這些鉛薄膜表面幾何結構(原子結構) ，由於鬆弛過程(relaxation)呈現週期性的扭曲，更源自介面上兩種表面的晶格不匹配(lattice mismatch)。此種週期性的扭曲會造成量子井態強度隨之明顯調變，甚至能顛倒圖案的對比。
當利用掃描穿隧能譜術探測金屬薄膜的電子結構時，能譜中可以同時顯現穿透共振和Gundlach 振盪(Gundlach oscillation) 。以往在金屬表面得到的Z-V能譜中，Gundlach 振盪代表觀察到的階梯特徵。因此，它是含有駐波態的次能帶(sub band)。此外，穿透共振的出現隱含能譜中應含有電子穿透率(transmissivity)的訊息，而此訊息可以分布在每個Gundlach 振盪的背景中。在Ag/Si(111) 7×7表面上和Pb/Si(111)帶有11×11的表面(無理對應的Pb/Si(111))上，Gundlach 振盪的能譜強度會隨著位置而起伏。這是由於穿透背景起伏造成的，而最終會與局部的穿透率有關。我們也觀察到一個現象，在單一Gundlach 振盪出現的能量範圍中，雖然能譜強度的分布與位置相關，其中穿透背景和駐波態的強度是互補的。

關鍵字

穿透共振 ； 量子井 (態) ； 駐波 (態) ； Gundlash 振盪 ； 量子尺寸效應 ； 量子化規則 ； 金屬薄膜 ； 掃描穿隧顯微儀 (能譜術) ； 穿隧 ； 能譜 ； 銀(111) ； 矽(111) ； 鉛(111)

並列摘要

The transmission probability of the free electron scattered by the quantum well in the metal film may reveals the phenomenon of resonance. This transmission resonance appearing above the vacuum level, which is a kind of quantum size effect (QSE), can be probed by the scanning tunneling spectroscopy (STS). We use the STS to observe the transmission resonance on Ag films grown on Si(111)7×7 surfaces. In the spectra, transmission resonance appears at different bias voltages depending on the film thickness. Its energy level moves toward the vacuum level with increasing film thickness. The existence of transmission resonance will affect the contemporary standing wave states (SWS) and, in general, push its following SWS to higher energies. The spectral lines and intensity vary with the film thickness thus can serve as finger prints for determining the film thickness. This is especially useful when the film covers the entire substrate. Besides, the spectra reveal a slight intensity variation with the location of the film due to a change in the electronic reflection phase at the buried interface. We also observed that the energy levels of transmission resonance can be shifted with the electric field in the tunneling gap, which can be adjusted by tuning the tunneling current. Our results demonstrate that the transmission resonance is shifted to higher energy the electric field is increased, but beyond a critical field it may drop to lower energy discontinuously. This field-dependent behavior can be qualitatively explained by a field-induced phase variation in the quantization rule.
Quantum-well state (QWS) is another kind of QSE as well as the transmission resonance, appearing below the vacuum level in metal film, can also be probed by STS. We combine the STS and locking-in technique to observe quantum-well states and patterns, between Fermi level to 2V, on three atomic layer lead films of type І and type П grown on the incommensurate Pb/Si(111) surface. Our result demonstrates that the film surface geometry (atomic structure) of both types are periodically distorted due to relaxation, which is originated from the lattice mismatch at interface. This distortion may cause the strength of QWS significantly modulated, thus rendering that the contrast of the pattern observed can be reversed.
When the electronic structure of a metal film beyond the vacuum level is probed by scanning tunneling spectroscopy (STS), both the transmission resonance and Gundlach oscillation may reveal in the tunneling spectrum. Gundlach oscillation, which represented the step like feature in Z-V spectrum acquired on metal surfaces, is a sub-band of SWS. The manifestation of the transmission resonance implies that the information of the electron transmissivity should involve in the spectrum and can be a background in the distribution of each Gundlach oscillation. The spectral intensity of Gundlach oscillation can be varied with observed location on Ag/Si(111)7x7 surface and Pb/Si(111) surface with 11x11 superstructure. It is due to the variation of the transmission background, and is ultimately related to local electron transmissivity. We also observe a phenomenon that the total intensity of the transmission background is complementary with the one of the standing-wave state although its intensity distribution is location-dependent.

並列關鍵字

transmission resonance ； quantum well (states) ； standing wave (states) ； Gundlash oscillation ； quantun size effect ； quantization rule ； thin metal film ； scanning tunneling microscopy (spectroscopy) ； tunneling ； spectroscopy ； Ag(111) ； Si(111) ； Pb(111)

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