M. V. Loginov

Initial stages of formation of a Yb-Si(111) interface

The initial stages of formation of an Yb-Si(111) interface are investigated by several methods: thermal desorption spectroscopy, atomic beam modulation, and low-energy electron diffraction. The structure of the adsorbed films and ytterbium silicide films is analyzed over a wide range of surface coverage ratios, along with the desorption kinetics of Yb atoms. The desorption activation energies of Yb atoms are measured for 3×2, 5×1, and 2×1 submonolayer structures. The temperature interval in which ytterbium silicide decomposes and the activation energy of this process are determined. It is shown that the Yb-Si(111) phase interface evolves by a mechanism similar to the Stransky-Krastanov mechanism.

Friedel oscillations in ytterbium films deposited on the Si(111) 7 × 7 surface

The nonmonotonic dependence of the work function of ytterbium nanofilms deposited on singlecrystal Si(111) on their thickness is experimentally revealed and studied. This dependence is shown to be caused by electron density oscillations in the films (Friedel oscillations), which are generated by the ytterbium-silicon interface. These oscillations originate, in turn, from appreciable charge transfer from the ytterbium film having a low work function to silicon.

Size dependences in the adsorptive properties of the surface of ytterbium nanofilms deposited on silicon: The CO-Yb-Si(111)7 × 7 system

This paper reports on a study of the adsorption of CO molecules on the surface of ytterbium nanofilms of different thicknesses, which were sublimed on Si(111)7 × 7 at room temperature. Dependences of two types were investigated: the surface concentration of adsorbed molecules vs. CO dose expressed in langmuirs and the work function of films vs. CO dose. It was shown that the behavior of these dependences is mediated by size effects and Friedel oscillations generated by the ytterbium-silicon interface. Both effects exert an influence on the binding of CO molecules to the surface. At low molecule concentrations, this binding is effected through lone electron pairs localized at the carbon ends of the molecules. These electrons form a donor-acceptor bond to the vacant 5d level of the metal, with the level dropping below the Fermi level. At high CO molecule concentrations, the pattern becomes more complex; indeed, the enhanced Coulomb interaction gives rise to a partial transfer of electrons from the 5d level to the vacant 2π* orbital of CO molecules.

Specific features of the formation of ytterbium films on the Si(111) surface at room temperature

The processes accompanying the formation of ytterbium films on the Si(111) surface at room temperature are investigated by the contact potential difference method, Auger electron spectroscopy, low-energy electron diffraction, and thermal desorption spectroscopy. It is shown that the grown metal films are uniform in thickness and that Si atoms virtually do not dissolve in the films. The atoms of the silicon substrate can diffuse in limited amounts into the Yb metal film only when the surface is bombarded by high-energy primary electron beams employed in Auger electron spectroscopy. The results obtained permit the conclusion that the previously observed oscillations of the work function in Yb-Si(111) thin-film structures cannot originate from dissolution of silicon atoms in the ytterbium film.

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.

Valence transition 2+ → 3+ induced in ytterbium nanofilms by CO and O2 molecules chemisorbed on their surface

Transformations of the surface and bulk of nanoscale ytterbium films during the surface interactions of these films with different ligand molecules have been studied. It has been shown that a combination of two factors, i.e., the existence of a lone electron pair in CO and O2 molecules and the unoccupied 5d level lying near the Fermi level in metallic divalent ytterbium, causes the formation of a stable chemisorption state of the molecule-nanofilm surface layer in which donor-acceptor bonds between gas molecules and surface metal ions are formed. As a result, ytterbium on the surface and in the bulk of the nanofilm is oxidized to a non-autonomous trivalent electronic state. The depth to which this transition propagates in the nanofilm has been determined; its anomalously large value (from 9 to 22 layers according to different estimates) has been explained. It has also been shown that the ligand molecule layer on the ytterbium surface is filled in two stages. The two-stage mechanism of this process is reflected by the nonmonotonic behavior of concentration dependences of the work function of CO-Yb and O2-Yb structures.

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.

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.

Mechanism of the Yb2+ → Yb3+ valence transition in ytterbium nanofilms upon chemisorption of CO and O2 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.

Thermally activated reconstruction in Yb-Si(111) thin-film structures

The electronic properties and mechanisms of formation of Yb-Si(111) thin-film structures, produced by room-temperature deposition of Yb atoms on the Si(111)7×7 surface are studied by Auger electron and LEED spectroscopy and the contact potential difference method. A study is also made of the effect of heating to 800 K on the properties of these structures. The interface is shown to form by the Stransky-Krastanov mechanism. Heating the Yb-Si(111) system is found to result in a very high (up to 1 eV) increase of the work function for all Yb atom concentrations on the silicon surface. In the adsorption stage, this increase is due to the growth of 2D ytterbium domains, which is accompanied by the formation of polarized domains from the silicon surface atoms with dangling valence bonds. The dipoles are oriented in such a way that their formation reduces the total energy of the Yb-Si(111) system and increases the work function. In the stage of silicide formation, the increase of the work function under heating is ultimately due to the appearance of a layer of silicon atoms on the Yb-Si(111) surface.