Transformation of auger electron spectra of ytterbium nanofilms under the action of adsorbed carbon monoxide and oxygen molecules

The transformation of the Auger electron spectra of ytterbium nanofilms as a result of chemisorption of CO and O₂ molecules on their surface has been studied. It has been shown that the adsorption of these molecules is accompanied by a radical transformation of the electronic structure of nanofilms, during which the 5d level of ytterbium drops below the Fermi level. As a consequence, one electron can transfer from the 4f level to the 5d level. In turn, this provides the conditions for a giant resonance 4d → 4f and a subsequent Coster-Kronig supertransition 4d ⁹4f ¹⁴ → 4d ¹⁰4f ¹² + Auger electron, which is accompanied by emission of one 4f electron to vacuum. The results obtained have demonstrated that molecules chemisorbed on the surface of nanofilms can cause qualitative changes in the properties of the surface and in the bulk of these films. It is obvious that this offers a means for designing nanoobjects with controlled properties.     

Study of reactive film heterostructures with several interfaces using the thermal desorption spectroscopy

The thermal desorption spectroscopy is used to study the interaction of the chemisorbed oxygen and carbon monoxide molecules with the nanometer-thick ytterbium films that are formed on the surfaces of silicon substrates at room temperature. In accordance with the results at a temperature of 300 K, the O₂ and CO molecules are chemisorbed on the surface of a metal film and do not exhibit dissociation to atoms under such conditions. The molecular dissociation is observed at higher temperatures. The liberated oxygen is involved in reactions with ytterbium and silicon that lead to the formation of complicated silicates, which dissociate at even higher temperatures.     

Adsorption stage in the formation of Eu-Si(111) thin-film structures

The adsorption stage in the formation of the Eu-Si(111) interface has been studied within a broad temperature range by thermal and isothermal desorption spectroscopy, low-energy-electron diffraction, Auger electron spectroscopy, and the contact potential difference method. It is shown that the ordering of an adsorbed europium film is accompanied by silicon surface reconstruction throughout the coverage range studied, 0<θ≤1.8. This self-organized process is also shown to be thermally activated. Ordered adsorbed europium layers have been found to be made up of 2D islands, whose structure depends on the amount of the metal deposited on the surface. The energy required to remove atoms from an island to vacuum has been determined. This energy decreases with decreasing 2D lattice constant of the islands. This pattern of its variation is accounted for, in the final count, by the decrease of the number of the Si surface atoms not bound directly to Eu atoms.     

Nonmonotonic dependence of the work function of ytterbium nanofilms deposited on the Si(111)7 × 7 surface at room temperature on the film thickness

The dependences of the work function of ytterbium nanofilms on their thickness are studied. The films are evaporated at room temperature on the Si(111)7 × 7 surface of silicon samples doped to different levels and having different types of conduction (n and p). It is shown that these dependences exhibit a pronounced nonmonotonic behavior, which does not depend on the type of silicon used. It is established that the amplitude of the nonmonotonic variations in the work function is governed by the surface microroughness of the deposited layers, so that larger amplitudes correspond to smoother films. The variations in the work function of the films due to the deposition of electrically negative Si atoms on their surface are investigated. It is revealed that the sign of the variation depends on the film thickness. This result strange at first glance is associated with the fact that the electron density distribution at the metal-film-vacuum interface depends nonmonotonically on the amount of deposited ytterbium. This nonmonotonic behavior is a manifestation of electron density standing waves (Friedel oscillations) generated in the films by the ytterbium-silicon interface.     

Transfilm passivation of a silicon–ytterbium nanofilms interface with chemisorbed CO and O2 molecules

Physics of the Solid State - Passivation of a silicon–ytterbium nanofilm interface with СО and О2 molecules chemisorbed on the opposite side of films is studied. The...     

Interaction of ytterbium nanofilms grown on tungsten substrates with oxygen

The interaction of ytterbium nanofilms evaporated on tungsten substrates with oxygen has been studied by Auger electron spectroscopy, thermal desorption spectroscopy, and contact potential difference measurements. It has been shown that at room temperature, no oxide is formed in the above interaction. In place of the oxide, a chemisorbed layer of nondissociated O₂ molecules is formed on the surface of the ytterbium nanofilms. This layer modifies the ytterbium. This modification transforms ytterbium from the divalent state into the trivalent state.     

Initial stages in the Sm-Si(111) interface formation

The initial stages in the formation of the Sm-Si(111) interface have been studied by thermal desorption, atomic beam modulation, and low-energy-electron-diffraction spectroscopy. The structure of adsorbed films and samarium silicide films, as well as the Sm atom desorption kinetics have been investigated within a broad range of surface coverages and temperatures. The activation energy of desorption from the thermally most stable 3×2 submonolayer structure, as well as the binding energy of a single samarium atom with the substrate, have been measured. The temperature of the onset of silicide decomposition and the activation energy of this process have been determined. It is shown that the Sm-Si(111) interface forms by a mechanism close to that of Stransky-Krastanov. Fiz. Tverd. Tela (St. Petersburg) 40, 371–378 (February 1998)     

Growth of an Eu-Si(111) thin film structure: The stage of silicide formation

Silicide formation in thin films produced by depositing Eu atoms on the Si(111) surface is studied using LEED, Auger electron spectroscopy, contact potential difference, and isothermal thermal-desorption spectroscopy. It is shown that if Eu is deposited on a substrate at room temperature, the growing film is disordered and consists of almost pure Eu. At high temperatures (T≥500 K), the Eu-Si(111) system forms through the Stranski-Krastanow mechanism; namely, first a two-dimensional transition layer (reconstruction) with the (2×1) structure forms and then three-dimensional silicide crystallites grow on it. A specific feature of this system is a low rate of diffusion of Si atoms in the europium silicides. This feature accounts for the concentration gradient of Si atoms across the silicide film thickness and, as a consequence, the multiphase film composition.     

Mechanism of the Yb<Superscript>2+</Superscript> → Yb<Superscript>3+</Superscript> valence transition in ytterbium nanofilms upon chemisorption of CO and O<Subscript>2</Subscript> molecules on their surface

The dependences of the work function of ytterbium nanofilms with a thickness ranging from 1 to 32 monolayers on the amount of CO or O₂ molecules chemisorbed on their surface have been investigated experimentally. It has been found that these dependences have a pronounced nonmonotonic character. The mechanism of the Yb²⁺ → Yb³⁺ valence transition, which occurs upon the chemisorption of CO or O₂ molecules on the surface of ytterbium nanofilms, has been developed using the results of this study together with the previously obtained data.