M. V. Kuz’min

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.

Modification of the surface and bulk electronic properties of ytterbium nanofilms under the action of adsorbed CO molecules

The modification of the surface and bulk electronic properties of ytterbium nanofilms under the action of adsorbed CO molecules has been investigated. It has been shown that adsorption of these molecules is accompanied by a fundamental rearrangement of the electronic structure of the films. This rearrangement is initiated by the formation of a donor-acceptor adsorption bond by a lone electron pair localized at the carbon atom of the CO molecule. The formation of this bond is accompanied by the lowering of the Yb 5d level below the Fermi level. After this lowering reaches a considerable value, the 4f electrons start to transfer to the 5d level; i.e., the 4f14 → 4f13 transition occurs. The transition opens a way to the N45N67N67 Koster-Kronig supertransition in the Auger spectra. As a result of these processes, the peaks observed in the spectra before adsorption of the CO molecules disappear and new peaks appear. Therefore, the adsorption of the CO molecules brings about qualitative changes in the Auger spectra of ytterbium. It has been found that the rearrangement of the ytterbium electronic structure due to the action of CO molecules involves both the surface of the ytterbium films and their near-surface layers. The thickness of these layers exceeds the Auger electron escape depth and can reach more than 11 monolayers.

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.

Effect of adsorbed oxygen on the properties of ytterbium nanofilms

The effect of O2 molecules adsorbed on the surface of ytterbium nanofilms on the properties of the volume and surface of these films has been studied. It has been shown that the dependence of the work function of the films on the concentration of O2 adsorbed molecules exhibits a nonmonotonic behavior: originally, the work function decreases, to start increasing again on passing through a minimum. At high oxygen doses, this increase stops. Adsorption of oxygen brings about a fundamental rearrangement of the Auger spectra of ytterbium; indeed, the Auger peaks observed before oxygen adsorption disappear completely after its deposition on the surface, to become replaced by other ones. The results obtained qualitatively agree with similar observations amassed by the present authors in studies of adsorption of CO molecules on the surface of ytterbium films. These results should be ascribed to a manifestation of complex processes of electron exchange between these films and adsorbed O2 molecules. These processes end up in a qualitative rearrangement of the electronic structure of the part of film volume that borders the surface, where ytterbium transforms into the d metal.

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 4d94f14 → 4d104f12 + 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.

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.

Influence of the metal nanofilm-semiconductor interface on surface properties of the nanofilm: The CO-Yb-Si(111) system

The influence of the “ytterbium nanofilm-single-crystal silicon substrate” interface on properties of the films has been investigated. It has been shown that, if the film thickness is less than 10 monolayers, the Friedel oscillations (standing waves of electron density) generated by the interface affect the work function of the films and the rate of adsorption of CO molecules on their surface. In turn, the CO molecules modify the electronic structure of ytterbium during adsorption on the surface of nanofilms by transforming ytterbium from the divalent to trivalent state. The completely filled layer of adsorbed CO molecules consists of two phases. The first phase is a two-dimensional gas whose molecules weakly interact with each other, but their lone electron pairs form a donor-acceptor bond with the Yb 5d level; as a result, this level is located below the Fermi level and the metal transforms into the trivalent state. After filling the two-dimensional phase, the second (island) phase, in which the CO molecule are bound by horizontal π-bonds, begins to grow. The formation of these bonds becomes possible due to the filling of 2π states in the molecules upon compaction of the adsorbed layer. The considered the adsorbed two-phase layer is responsible for the complete transition of ytterbium into the trivalent state.

Effect of chemisorbed oxygen and carbon monoxide molecules on the properties of Yb-Si(111) nanofilm structures

A study is reported of the effect of chemisorbed O2 and CO molecules on the properties of structures of the type “nanoscale-ytterbium-film-Si(111) silicon substrate” and, in particular, on the reactions between a metal and silicon. It has been shown that chemisorption of the above molecules on Yb-Si(111) structures initiates partial or total suppression of growth of ytterbium silicide and that the effect of adsorbates on reactions at the metal-silicon interface depends on the thickness of the ytterbium nanolayer. Indeed, at a film thickness of 8 monolayers and higher oxygen doses (360 Langmuir), chemisorbed molecules suppress completely the silicide formation process. With ytterbium films of larger thicknesses (16 monolayers) and higher doses of oxygen (360 Langmuir) or carbon monoxide (480 Langmuir), while the silicide does grow, this apparently takes place only in ytterbium nanofilm layers closest to the substrate. The effect of adsorbed molecules on the silicide formation becomes less pronounced also with decreasing number of these molecules on the surface, i.e., as their dose decreases.