Michail Michailov

Non-steady state effects in MBE: oscillations of the step density at the crystal surface

Abstract An analysis of the microscopic processes (surface diffusion, nucleation) at the crystal surface during MBE provides a comparatively simple expression for the temperature dependence of the amplitude of the step density oscillations at a two-level and a three-level growth front. The time evolution of the terrace areas (in the case of complete condensation) is found to be temperature independent which leads to a temperature independent amplitude of the RHEED intensity oscillations when they originate from the interference of beams scattered by different terraces. The evolution of the growth front (the increase of its thickness) is found to depend only on the number of atoms in the critical nucleus. This provides the underlying physics for an understanding of the damping of the oscillations. When the RHEED intensity changes originate from diffuse scattering from the step edges, the oscillation amplitudes observed at different temperatures are expected to be essentially different, but the damped oscillations should coincide after appropriate scaling.

Two-dimensional overgrowth in the low submonolayer range: the case of c(2 × 2)-PbCu(110)

Abstract The vacuum deposition of lead on a copper (110) surface in the low coverage range has been followed by monitoring in a helium scattering experiment the intensity, position and full width at half maximum of selected diffraction peaks. Up to Pb coverages θ ⋍ 0.2 the scattered intensity of the (−1,0) peak, absent on the bare Cu surface, increases as θ2, thus indicating the “decoration” of the substrate by a lattice gas of adsorbed Pb atoms. Above this coverage to the completion of the condensed layer with c(2 × 2) structure, the existence of two levels is evidenced, one being the substrate covered by lattice gas, the other that of islands of the condensed phase. The difference in height between these levels favours the assumption that the Pb atoms of the lattice gas replace the Cu atoms of the outermost layer of the substrate. The thermal behavior of the scattered intensities of the two phases gives an indirect indication in the same sense.

Thermal Rupture of Monatomic Metal Nanowires

The present study deals with the problem of thermal stability and rupture of metal nanowires, considered here as a monatomic chain located on an atomically smooth crystalline substrate. Based on a recently developed Monte Carlo computational model, the simulation results reveal a scenario of nanowire breaking via formation of atomic vacancies. Depending on temperature, the fluctuations in the positions of the nanowire atoms generate wave-shaped nanowire configurations with specific active sites for breaking. This process is followed by formation of one-atom vacancies in the nanowire. Their overgrowth into vacancies of two, three and more atoms leads to permanent nanowire rupture and, hence, to loss of the nanowire electronic and other physical properties. The time evolution of the atomic-scale nanowire morphology and breaking mechanism are discussed in detail. The proposed model of nanowire rupture sheds light on the general problem of thermal stability of artificial atomic structures on crystal surfaces and their application to electronic and other devices with exotic physical features.

Surface Debye Temperatures and Specific Heat of Nanocrystals

The present study deals with the contribution of the surface atomic layer to the specific heat of nanocrystals. In the case of small phases with comparable number of bulk and surface atoms, the variation of heat capacity with respect to that of infinitely large phase is significant. To evaluate this variation, in the framework of the classical Debye model, we introduce surface excess heat capacity that accounts for the surface layer contribution. On that physical background, we draw a functional dependence of the specific heat of nano clusters on the number of their bulk and surface atoms and corresponding bulk, , and surface, , Debye temperatures. Since depends on the crystallographic orientation of the surface, the presented model also accounts for the symmetry and atomic density of the respective crystal faces of the nanocrystal.

Vacancy Mediated Diffusion at Surface-Confined Atomic Intermixing

The present study deals with diffusion behavior of adsorbed atoms on stepped crystal surfaces. In volume-immiscible systems, two-dimensional (2D) atomic intermixing at epitaxial interface could be completely blocked on step-free surface domains. This is a result of high diffusion barrier for direct atomic exchange between adsorbed layer and substrate. In that case, diffusion takes place exclusively across the steps of atomic terraces. In such systems, dynamic competition between energy gain by mixing and substrate strain energy results in diffusion scenario where adsorbed atoms form alloyed stripes in the vicinity of terrace edges. At high temperatures, the stripe width increases and finally completely destroys the terraces. This process leads to formation of alloyed 2D atomic islands on top of pure substrate layer. The atomistic Monte Carlo simulations reveal vacancy-mediated mechanism of diffusion inside atomic terraces as a result of spontaneous generation of vacancies at high temperature. Being in agreement with recent experimental findings, the observed surface-confined alloying opens up a way various surface pattern to be configured at different atomic levels on the crystal surface.