| Summary: | 博士 === 國立臺灣大學 === 物理研究所 === 94 === 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.
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