Si (111) Substrates By Low-temperature Growth Of Pb Films, The Electronic Structure And Quantum Effects Research

Electrons confined in a thin film with its thickness comparable to the Fermi electronwavelength, are quantized into discrete energy levels, known as quantum well states (QWS).In the vertical direction, this has been proven to greatly modulate the electronic distributionnear the Fermi level, thus significantly affects physical and chemical properties of the thinfilm. Metal thin films on a semiconductor or insulator substrate, where motion of theelectrons is confined in a potential well formed by the substrate energy gap and the vacuumlevel, establish an ideal one dimensional quantum well structure and can be used to study theQWSs and to explore some novel quantum phenomena. Due to the short Fermi electronwavelength of metals, for a successful research of this problem, atomically flat films must beprepared, and their electronic structure, which is atomic-layer sensitive, must be accuratelycharacterized. In this thesis, using reflection high energy electron diffraction (RHEED),angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy(STM), we have investigated the growth of the Pb thin films on the Si(111) surface, andsucceeded in preparing atomically flat Pb films with exactly known thicknesses in terms ofatomic layers, thus studied the electronic structure and the physical properties of the thin filmsassociated with the quantum size effects (QSE) in remarkable details. The main results andconclusions are summarized as follows:(1) By low temperature (145K) deposition method, we prepared such flat Pb films on theSi (111) substrate over a wide thickness range (from 10ML to 32ML). The unique growthbehaviors were investigated by RHEED, STM and ARPES. A critical thickness of 10ML isestablished, at which the atomically flat Pb films become stable at room temperature.Between 10ML and 20ML, the growth exhibits a quasi bi-layer growth mode. Above 21ML, alayer-by-layer growth mode was observed. We also found a beating pattern with 9ML period,at which the relative stabilities of the films alter from even (odd) layers to odd (even) layers.The transition points take place at 13ML, 22ML and 31ML, respectively.(2) Using our high resolution ARPES, we studied the electronic structure (the QWS) ofthe Pb films with the thicknesses from 10ML to 32ML, and observed intriguing oscillations ofthe QWS relative to the Fermi level with a period of 2ML. We employed the phaseaccumulation model to fit the experimental data within a free-electron approximation, anddeduced experimentally the accurate energy band of Pb. Our relative surface energycalculation of the Pb films manifests an energy landscape of a period of 9ML beating patternriding on a fast oscillation of a period of 2ML, which explains well the observed uniquegrowth mode of the Pb films.(3) Based on the well-characterized film morphologies and electronic structures, wefurther investigated the superconductivity of the Pb films with atomic-level precision, andobserved oscillations of the superconducting transition temperature of the Pb films as afunction of thickness, which we demonstrate a distinct reflection of the QSE. QSE inducedoscillatory superconductivity in thin films, a subject of fundamental importance to solid statephysics, was predicted early in the 1960`s, here we give the first deterministic andquantitative demonstration. The atomically uniform films obtained in our experiments providean ideal system to investigate and understand the superconducting mechanism at reduceddimensionality.(4) We also studied the temperature-dependence of the QWS of the Pb films, and foundthat the binding energies and line widths of the QWS exhibit a linear dependence ontemperature, with which the thermal expansion coefficient and the electron phonon couplingconstant of the Pb films were deduced. Strong quantum oscillation of these two quantitieswith a period of 2ML is identified. The in situ measurement of the electron phonon couplingrenders very important information for understanding the quantum oscillation insuperconductivity, which was measured ex situ with the Pb films covered by 4ML Au.In summary, we have shown clearly that the formation of QWS can dramaticallyinfluence the growth, the electron density of states near the Fermi level, and the physicalproperties of thin films, such as superconductivity, thermal expansion and electron-phononcoupling. These results are not only of fundamental importance to condensed matter physics,but also demonstrate that we have thus elevated this type of measurement to a new level ofprecision and sophistication.