[en] Aims of inquiry: Development of computer simulation approach combining molecular dynamics and self-consistent tight-binding methods and simulation of charge state effects on structure and dynamics of clusters of semiconductors. Method of inquiry: Computer simulations; self-consistent tight-binding method; molecular dynamics method. The results achieved and their novelty: A computer simulation approach combining molecular dynamics and self-consistent tight-binding methods was developed and applied for studying clusters of Si and GaAs, as well as some electron-stimulated atomic processes in Si and GaAs. Stable geometries, binding energies, ionization potentials and electronic affinities of clusters were determined and their peculiarities versus number of atoms, charge states, and stoichiometry of clusters were revealed. The results are in good agreement with those of first-principles calculations. A new model of Si - H bond dissociation, consisting of Jahn-Teller destabilization of this bond due to charge trapping and followed migration of H around the neighborhood Si-Si bond, had been proposed and then was proved by the direct molecular - dynamics simulation of this dissociation. It was shown possibility of desorption of Ga and As from GaAs surface due to their four - fold ionization caused by Auger - effect. (author)

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[en] In the last years significant progress is achieved in development of neutron focusing methods (concentrating neutrons in a given direction and a small area). In this, main attention is given to focusing of neutron beams of reactor, particularly cold neutrons and their applications. [1,2]. However, isotope sources also let obtain intensive neutron beams and solve quite important (tasks) problems (e.g. neutron capture therapy for malignant tumors) [3], and an actual problems is focusing of neutrons. We developed a device on the basis of californium source of neutrons, allowing to obtain focused (preliminarily) beam of thermal neutrons with the aid of respective choice of moderators, reflectors and geometry of their disposition. Here, fast neutrons and gamma rays in the beam are minimized. With the aid of the model we developed on the basis of Monte-Carlo method, it is possible to modify aforementioned device and dynamics of output neutrons in wide energy range and analyze ways of optimization of neutron beams of isotope sources with different neutron outputs. Device of preliminary focusing of thermal neutrons can serve as a basis for further focus of neutrons using micro- and nano-capillary systems. It is known that, capillary systems performed with certain technology can form beam of thermal neutrons increasing its density by more than two orders of magnitude and effectively divert beams up to 20^o with length of system 15 cm. (author)

[en] A fragmentation of small stoichiometric gallium-arsenic clusters stimulated by Auger-ionization of atomic inner shells has been investigated by computer simulation method. Auger-ionization may be, for example, in laser field or at absorption of X-rays. The self-consistent tight-binding electronic and total energy calculation method and new algorithm for integrating the Newtonian equations are used for simulation of cluster fragmentation. It is shown that two-electron ionization of Ga₂As₂ cluster makes it unstable and cluster distorts on three parts - two Ga⁺ ions and As₂ molecule. (author)

[en] Full text: In the last years significant progress is achieved in development of neutron focusing methods (concentrating neutrons in a given direction and a small area). In this, main attention is given to focusing of neutron beams of reactor, particularly cold neutrons and their applications. [1,2]. However, isotope sources also let obtain intensive neutron beams and solve quite important (tasks) problems (e.g. neutron capture therapy for malignant tumors) [3], and an actual problems is focusing of neutrons. We developed a device on the basis of californium source of neutrons, allowing to obtain focused (preliminarily) beam of thermal neutrons with the aid of respective choice of moderators, reflectors and geometry of their disposition. Here, fast neutrons and gamma rays in the beam are minimized. With the aid of the model we developed on the basis of Monte-Carlo method, it is possible to modify aforementioned device and dynamics of output neutrons in wide energy range and analyze ways of optimization of neutron beams of isotope sources with different neutron outputs. Device of preliminary focusing of thermal neutrons can serve as a basis for further focus of neutrons using micro- and nano-capillar systems. It is known that, capillary systems performed with certain technology can form beam of thermal neutrons increasing its density by more than two orders of magnitude and effectively divert beams up to 20^o with length of system 15 cm

[en] Full text: The investigation of clusters of semiconductor materials has been the subject of great interest due to their unique properties that cardinally differs from those of bulk materials. Research on dependence of cluster properties on its size, charge states and stoichiometry is of obvious interest. In addition, processes of formation and fragmentation of clusters are rather complicated and have been insufficiently studied, particularly in respect of clarifying of the role of irradiation in these processes. Here we have simulated the structure and properties of Ga_mAs_n and Si_n clusters in different charge states (positive, neutral, and negative), as well as possible channels of their disintegration by the computational approach that combines molecular dynamics and self-consistent tight-binding methods. It has been revealed, for example, that for 4- and 5-atom clusters Ga_mAs_n there are four possible stable geometries depending on their charge state and stoichiometry. The anions Ga_mAs_n are found to be more stable with respect to disintegration. The probability of fragmentation is large for the positive and neutral clusters of GaAs₄, Ga₂As₃, Ga₃As₂, especially for Ga₂As₃, the neutral ones disintegrate into two opposite-charged fragments with more probability. Thus, the charged clusters may constitute considerable part of produced clusters due to their more stability than neutral ones in respect of disintegration, moreover, the additional charged clusters appear in result of disintegration of neutrals ones. The process of fragmentation of a number of clusters caused by the multiple ionization of an atom and then the whole cluster has been simulated. Such ionization can be triggered by irradiation with both high-energy X-rays and charged nuclear particles. The kinetic energy changes of the cluster with the time are obtained by molecular dynamic simulation of the time evolution of system It is shown that the multiple ionization of the cluster itself exerts a greater influence on cluster destruction than the single multicharged ion alone. For instance, the twofold-charged cluster [Ga₂As₂]⁺² disintegrates in three daughter fragments, exactly, two positive Ga⁺ ions and one As² molecule

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