High-resolutioncore-level and Auger-electron spectroscopy, polarization-dependent near-edgex-ray absorption, and angle-resolved photoemission are used to study the electronic structure and the bonding at the CaF2/Si(111) interface. Si core-level shifts of + 0.4 and -0.8 eV show that both Ca and F bond to Si and that the interface is atomically sharp. Interface-derived Ca and F core-level and Auger-electron shifts are found indicating layer-by-layer growth. The interface Ca 2p, 3p, 3score-level shifts are about 2 eV and the Ca 2p Auger energy shift is =4.S eV. The F 1s,2s core levels show no interface shift but a shift of 1.7 -2 eV in the initial adsorption regime indicating a rearrangement of F after the completion of the first layer. The F 1s Auger electrons show an interface shift of 2.0 eV. Initial-state and relaxation contributions to the shifts are considered. In the Ca 2p and F 1s near-edge x-ray-absorption fine-structure (NEXAFS) spectra several unoccupied Caand F-derived interface states are found. The orientation of the corresponding orbitals is revealed by the polarization dependence. The oxidation state of the Ca atoms at the interface is found to be changed to 1 + . The CaFq valence bands start to form at 2 layers with an overall bandwidth of 3.3 eV. An occupied interface state is found at 1.2 eV. The Fermi level shifts by 0.6 -0.6S eV when 2 CaF2 layers are deposited and a new pinning position is established at the Si valence-band maximum. A bonding model for the interface is proposed.
Quantum-well states are observed for Fe films embedded in Au(100) using inverse photoemission.They can be analyzed with a simple interferometer model, using bulk states from the I &26&H» band that are modulated by an envelope function with wavelengths in the order of 7-10 atomic layers (10-14 A). These states can be followed down to a monolayer, where two states are seen at 0.6 and 1.9 eV. They are tentatively assigned to the 551 monolayer state and a A~quantum-well state. The resulting ferromagnetic exchange splitting of the h, q state is 2.7 eV, exceeding the splitting of 1.8-2. 1 eV for the analogous bulk band.The attempts to fabricate ever finer quantum structures, such as superlattices and quantum wells, are approaching the atomic limit. The study presented here makes the connection from a narrow but still bulklike quantum well to a monolayer quantum well. The latter is strongly inAuenced by interface phenomena since the atoms in the well are all interface atoms. The transition from conventional quantum-well states to monolayer states is observed with inverse photoemission. In contrast to previously studied quantum wells, such as semiconductors and noble metals, the system considered here consists of a ferromagnetic well, all the way down to a monolayer.In the monolayer limit a ferromagnetic exchange splitting of 2.7 eV is found for the 5, & state, which is significantly larger than the splitting of 1.8-2. 1 eV for the analogous bulk band. This confirms predictions of enhanced monolayer magnetism.The Fe/Au(100) system is suited particularly well for the purpose of creating narrow quantum wells. There is a good lattice match (better than 1%), with the lattice constant of fcc Au a factor of J2 larger than that of bcc Fe. This provides a one-to-one match of the (100) surface lattices, due to the extra face-centered atom in Au. The two band structures, however, are quite different from each other, thereby providing a large band offset and well depth of 9 eV. This is an order of magnitude larger than in any other quantum-well system studied to date. ' As a consequence, the exponential decay of the wave function is extremely fast in the barrier region, thereby confining the states to the well within a single layer. The growth of Fe on Au(100) has been found to proceed layer-by-layer, with one layer of Au staying on top of the growing Fe surface. This Au overlayer acts as a surfactant by lowering the surface energy of the growing film and thereby preventing island formation. The resulting uniformity of the Fe film makes it possible to see variations in the electronic structure on a layer-by-layer basis. Since Fe is capped on both sides by Au one obtains a nearly symmetric quantum-well structure.The Au(100) substrate was electropolished (using a cyanide-based method), sputtered (500 eV Ar+ at + 30' from grazing), and annealed (400'C). It exhibited a bright 5&&20 low-energy electron-diffraction (LEED) pattern, which converted to 1 x 1 at submonolayer Fe cover-age. Fe was evaporated from a miniature electron-beam evap...
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