Maria Ganchenkova | Moscow Engineering and Physics Institute (original) (raw)

Papers by Maria Ganchenkova

The wide scatter in experimental results has not allowed drawing solid conclusions on self-diffus... more The wide scatter in experimental results has not allowed drawing solid conclusions on self-diffusion in the chalcopyrite CuInSe 2 (CIS). In this work, the defect-assisted mass transport mechanisms operating in CIS are clarified using first-principles calculations. We present how the stoichiometry of the material and temperature affect the dominant diffusion mechanisms. The most mobile species in CIS is shown to be copper, whose migration proceeds either via copper vacancies or interstitials. Both of these mass-mediating agents exist in the material abundantly and face rather low migration barriers (1.09 and 0.20 eV, respectively). Depending on chemical conditions, selenium mass transport relies either solely on selenium dumbbells, which diffuse with a barrier of 0.24 eV, or also on selenium vacancies whose diffusion is hindered by a migration barrier of 2.19 eV. Surprisingly, indium plays no role in long-range mass transport in CIS; instead, indium vacancies and interstitials participate in mechanisms that promote the formation of antisites on the cation sublattice. Our results help to understand how compositional inhomogeneities arise in CIS. V C 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4799064] JOURNAL OF APPLIED PHYSICS 113, 133510 (2013) 133510-2 Oikkonen et al.

Mechanical properties of silicon microstructures and basic structural properties of crystalline s... more Mechanical properties of silicon microstructures and basic structural properties of crystalline silicon are discussed here. At ordinary pressure silicon crystallizes in a diamond structure with a basis of two atoms. All the theoretical calculations using force-field methods correctly describe bulk silicon in its diamond structure ground state, giving a value for the lattice parameter that is close to the value that is experimentally observed. Effects of pressure are explained here in detail. At high-elastic strains the harmonic approximation becomes insufficient to correctly describe the elastic energy. An alternative way to treat the nonlinearity effects is to include the higher than second-order terms in the formal expansion of the elastic energy in strains. Extended defects could be classified with respect to their dimensionality. The partial dislocations are always associated with stacking faults. Two types of dislocation in silicon are of special interest. Silicon has been a favorite material for theoretical and experimental investigations of dislocation nature and mobility. The second approach is the so-called cluster method, where a finite cluster surrounding the defect is constructed. Some examples of the proposed core reconstruction are presented. The dislocation segment where the dislocation line jumps over the Peierls barrier is called kink. Silicon belongs to the class of intrinsically brittle solids. The convenience and success of silicon material and micromachining technology have made silicon a natural choice for many MEMS applications, such as actuator, power generator, etc. The reliability of these applications is extremely important to ensure their effective performance.

The effect of strengthening of steels by yttrium oxide nanoparticles on their interaction with hy... more The effect of strengthening of steels by yttrium oxide nanoparticles on their interaction with hydrogen is studied.Hydrogen uptake and effective activation energy of its diffusion and trapping are calculated from the TDS spectra.Embedding of oxide nanoparticles lead to significant increase of hydrogen uptake compared to the EUROFER 97 steel.High-temperature components of the TDS peak in the ODS steels are reflect the hydrogen de-trapping from the nanoparticles.In this work EUROFER 97, ODS-EUROFER, and PM2000 steels are studied in terms of their interaction with hydrogen at ambient temperature. Hydrogen uptake and effective activation energy of its diffusion and trapping are calculated from the thermal desorption spectra obtained for the studied steels in the temperature interval up to 1123 K after the electrochemical hydrogen charging. It was observed that hydrogen uptake in ODS-EUROFER and PM2000 is markedly higher than in EUROFER 97 steel. It is shown that hydrogen in ODS-EUROFER has the effective diffusion activation energy, which is 0.08 and 0.06 eV larger than that in EUROFER 97 and PM2000, respectively. The high-temperature components of the TDS peaks in the studied ODS steels are suggested to reflect hydrogen de-trapping from the oxide nanoparticles. The mean particle size, the size distribution and chemical composition are studied using SEM and TEM techniques.

Density-functional-theory calculations have often been used to interpret experimental observation... more Density-functional-theory calculations have often been used to interpret experimental observations of defects in CuInSe 2 (CIS). In this work, we bring back under scrutiny conclusions drawn from earlier calculations employing the (semi)local-density approximation. We present hybrid-functional results showing that copper-or indium-related defects such as V Cu or In Cu do not create charge transition levels within the band gap in CIS. Instead, deep levels in CIS can only arise from selenium-related defects, which act as recombination centers in this material.

The wide scatter in experimental results has not allowed drawing solid conclusions on self-diffus... more The wide scatter in experimental results has not allowed drawing solid conclusions on self-diffusion in the chalcopyrite CuInSe 2 (CIS). In this work, the defect-assisted mass transport mechanisms operating in CIS are clarified using first-principles calculations. We present how the stoichiometry of the material and temperature affect the dominant diffusion mechanisms. The most mobile species in CIS is shown to be copper, whose migration proceeds either via copper vacancies or interstitials. Both of these mass-mediating agents exist in the material abundantly and face rather low migration barriers (1.09 and 0.20 eV, respectively). Depending on chemical conditions, selenium mass transport relies either solely on selenium dumbbells, which diffuse with a barrier of 0.24 eV, or also on selenium vacancies whose diffusion is hindered by a migration barrier of 2.19 eV. Surprisingly, indium plays no role in long-range mass transport in CIS; instead, indium vacancies and interstitials participate in mechanisms that promote the formation of antisites on the cation sublattice. Our results help to understand how compositional inhomogeneities arise in CIS. V C 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4799064] JOURNAL OF APPLIED PHYSICS 113, 133510 (2013) 133510-2 Oikkonen et al.

The paper is aimed at getting deeper insight into the fundamental mechanisms that govern CoSi2 pr... more The paper is aimed at getting deeper insight into the fundamental mechanisms that govern CoSi2 precipitate nucleation and growth during Co ion implantation at high temperatures (500–650 °C). Information about nucleation and growth of metal silicides as a function of temperature and implantation flux is provided by experiments on cobalt implantation in silicon, performed directly by in situ transmission electron microscopy. The main attention is paid to the nucleation of B-type precipitates, which dominate under ion implantation conditions. The obtained quantitative behavior of precipitate number density and size and the scaling of these values with implantation flux are discussed and rationalized in terms of analytical and simulation approaches. An atomistic model of B-type precipitate nucleation based on the first-principles calculations of relative energetic efficiency of different Co clusters is proposed.

Inorganic Materials: Applied Research, 2011

The article discusses first-principles calculations of hydrogen solubility in various interstitia... more The article discusses first-principles calculations of hydrogen solubility in various interstitial positions in L10 ordered FePd alloy. It is demonstrated that the predominant localization of hydrogen in solid solution is in octahedral positions in the Fe and Pd planes, where the position in the Fe plane is somewhat more favorable (by 0.13 eV). It is also demonstrated that addition of a hydrogen atom to an FePd crystal results in modification of the magnetic moment of the system by approximately one Bohr magneton.

Journal of Applied Physics, 2008

The paper is aimed at getting deeper insight into the fundamental mechanisms that govern CoSi2 pr... more The paper is aimed at getting deeper insight into the fundamental mechanisms that govern CoSi2 precipitate nucleation and growth during Co ion implantation at high temperatures (500-650 °C). Information about nucleation and growth of metal silicides as a function of temperature and implantation flux is provided by experiments on cobalt implantation in silicon, performed directly by in situ transmission electron microscopy. The main attention is paid to the nucleation of B-type precipitates, which dominate under ion implantation conditions. The obtained quantitative behavior of precipitate number density and size and the scaling of these values with implantation flux are discussed and rationalized in terms of analytical and simulation approaches. An atomistic model of B-type precipitate nucleation based on the first-principles calculations of relative energetic efficiency of different Co clusters is proposed.

Physical Review B, 2011

The results of molecular dynamics (MD) simulations of atomic hydrogen kinetics on graphene are pr... more The results of molecular dynamics (MD) simulations of atomic hydrogen kinetics on graphene are presented. The simulations involve a combination of approaches based on Brenner carbon-hydrogen potential and first-principles force calculations. Both kinds of MD calculations predict very similar qualitative trends and reproduce equally well the features of hydrogen behavior, even such sophisticated modes as long correlated jump chains. Both approaches agree that chemisorbed hydrogen diffusion on graphene is strongly limited by thermal desorption. This limitation rules out long-range diffusion of hydrogen on graphene but does not exclude the short-range hydrogen diffusion contribution to hydrogen cluster nucleation and growth.

Materials Science and Engineering B-advanced Functional Solid-state Materials, 2009

In this paper, using first-principles calculations, we demonstrate that vacancies and E-centers (... more In this paper, using first-principles calculations, we demonstrate that vacancies and E-centers (AsV, SbV) in silicon can co-exist in several metastable configurations with notably different relaxation patterns, which have very similar formation energies. Thus these vacancy-type defects can be considered as multi-symmetry defects in the sense that, at elevated temperatures, the probabilities to find vacancies in different stable configurations are comparable. From an experimental point of view, the co-existence of various symmetries can complicate the identification of the defect.

Materials Science and Engineering B-advanced Functional Solid-state Materials, 2006

In this paper, we investigate the effect of uniaxial strain on the interaction of a pair of neutr... more In this paper, we investigate the effect of uniaxial strain on the interaction of a pair of neutral vacancies in pure silicon at distances up to the fifth-nearest-neighbors. The calculation of the total energies of vacancy pairs at different pair orientations was performed using the first-principles approach. It is demonstrated that the energy of a vacancy pair is sensitive to the pair orientation with respect to the direction of applied stress. The effect of such orientational dependence of vacancy interaction on the formation of vacancy clusters in a uniaxially strained monocrystalline Si is studied using kinetic Monte-Carlo simulation. It is shown that anisotropy in vacancy–vacancy interaction leads to the formation of planar vacancy clusters with preferred orientation with respect to the strain axis.

Journal of Physics-condensed Matter, 2011

We calculate the energetics of vacancies in CuInSe2 using a hybrid functional (HSE06, HSE standin... more We calculate the energetics of vacancies in CuInSe2 using a hybrid functional (HSE06, HSE standing for Heyd, Scuseria and Ernzerhof), which gives a better description of the band gap compared to (semi)local exchange-correlation functionals. We show that, contrary to present beliefs, copper and indium vacancies induce no defect levels within the band gap and therefore cannot account for any experimentally observed levels. The selenium vacancy is responsible for only one level, namely, a deep acceptor level ɛ(0/2-). We find strong preference for VCu and VSe over VIn under practically all chemical conditions.

Physical Review B, 2011

The paper presents a systematic study of the trends in the interaction of hydrogen with carbon fu... more The paper presents a systematic study of the trends in the interaction of hydrogen with carbon fullerenes versus their curvature, where graphene is taken as the limit of zero curvature. The efficiency of hydrogen incapsulation in fullerenes, penetration into them, and adsorption on their surface are analyzed and discussed. The effects on magnetism are also considered; in particular, it is shown that hydrogen adsorption to some fullerenes induces magnetism to initially nonmagnetic systems. In addition, highly hydrogen-saturated fullerenes are examined and the suitability of fullerenes for hydrogen storage is discussed.

Physical Review B, 2011

Formation energies of various intrinsic defects and defect complexes in ZnO have been calculated ... more Formation energies of various intrinsic defects and defect complexes in ZnO have been calculated using a density-functional-theory-based pseudopotential all-electron method. The various defects considered are oxygen vacancy (VO), zinc vacancy (VZn), oxygen at an interstitial site (Oi), Zn at an interstitial site (Zni), Zn at VO (ZnO), O at VZn(OZn), and an antisite pair (combination of the preceding two defects). In addition, defect complexes like (VO+Zni) and Zn-vacancy clusters are studied. The Schokkty pair (VO+VZn) and Frenkel pairs [(VO+Oi) and (VZn+Zni)] are considered theoretically for the first time. Upon comparing the formation energies of these defects, we find that VO would be the dominant intrinsic defect under both Zn-rich and O-rich conditions and it is a deep double donor. Both ZnO and Zni are found to be shallow donors. The low formation energy of donor-type intrinsic defects could lead to difficulty in achieving p-type conductivity in ZnO. Defect complexes have charge transitions deep inside the band gap. The red, yellow, and green photoluminescence peaks of undoped samples can be assigned to some of the defect complexes considered. It is believed that the red luminescence originates from an electronic transition in VO, but we find that it can originate from the antisite ZnO defect. Charge density and electron-localization function analyses have been used to understand the effect of these defects on the ZnO lattice. The electronic structure of ZnO with intrinsic defects has been studied using density-of-states and electronic band structure plots. The acceptor levels introduced by VZn are relatively localized, making it difficult to achieve p-type conductivity with sufficient hole mobility.

Defect and Diffusion Forum, 2001

Journal of Nuclear Materials, 2009

Here we present the results of first-principles quantum mechanical calculations of energetic para... more Here we present the results of first-principles quantum mechanical calculations of energetic parameters of vacancies, interstitials, gas atoms (H, He) and some small point defect complexes in beryllium. It is found that the most energetically favorable position for self-interstitial is the basal octahedral one, for hydrogen atoms is the basal tetrahedral, while He atoms can be located with equal probability in basal octahedral and basal tetrahedral interstitial positions. The formation of divacancies and tri-vacancies from individual vacancies is shown to be energetically unfavorable, which implies high stability of beryllium against vacancy clustering. The preliminary estimates of diffusion pathways for H and He interstitials indicate essential differences: while hydrogen diffusion is nearly isotropic, that of He occurs preferentially in basal planes with the migration energy of only 0.1 eV.

Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms, 2005

In this paper we discuss possible mechanisms of PV and V2 annealing in Si. Our approach includes ... more In this paper we discuss possible mechanisms of PV and V2 annealing in Si. Our approach includes a combination of density functional theory and lattice kinetic Monte-Carlo (LKMC) simulations. The density functional theory is used to find the binding energies and jump barriers for P–V and V–V pairs at different separations (from one to three interatomic bonds between complex constituents) and in different charge states. The mobility and interaction of vacancies and vacancy complexes is simulated by LKMC with event probabilities calculated based on the energies from ab initio calculations.

Physical Review B, 2008

The paper deals with multiscale modeling of the minor component ordering in wurtzite Ga 1−x In x ... more The paper deals with multiscale modeling of the minor component ordering in wurtzite Ga 1−x In x N ͑x Ͻ 0.5͒ alloys. The treatment combines the total-energy density-functional calculations of the metal atom interaction parameters and the atomistic description of the alloy decomposition using lattice kinetic Monte Carlo. It is demonstrated that the phase decomposition patterns in wurzite GaInN are very sensitive to the interplay of metal atom interactions at several interatomic distances ͑at least to the fourth nearest neighbors͒ on the cation sublattice. Variation of the metal interaction energies within reasonable limits resulted in pronouncedly different relaxation patterns ͑linear or wall ordering of In and Ga atoms along c-axis, planar ordering parallel to basal plane, spinodal decomposition͒. The high sensitivity of the GaInN decomposition to relatively small variations of the metal interaction energies could be the main reason for the experimentally observed versatility of the alloy decomposition patterns and their sensitivity to the particular experimental conditions.

Physical Review Letters, 2006

We present results of ab initio calculations for vacancies and divacancies in GaN. Particular att... more We present results of ab initio calculations for vacancies and divacancies in GaN. Particular attention is paid to nitrogen vacancies and mixed Ga-N divacancies in negatively charged states, which in n-type GaN are found to be energetically comparable with gallium vacancies. We also demonstrate that the activation energy for self-diffusion over the nitrogen sublattice is lower than over the gallium one for all Fermi-level positions, which implies the nitrogen vacancies are major defects in samples annealed at high temperatures. Possibilities for direct observations of nitrogen vacancies through positron annihilation experiments are discussed.

Physical Review B, 2006

A combined first-principles total energy Monte Carlo method is used to study the Curie temperatur... more A combined first-principles total energy Monte Carlo method is used to study the Curie temperature T C and its evolution in ͑Ga,Mn͒As. The inter-and intra-cluster magnetic interactions are treated separately in a modified Heisenberg Hamiltonian. We find for random Mn distributions ͑mimicking as-grown samples͒ a T C maximum at the Mn concentration of 6%. During annealing we show that a slow clustering process of substitutional Mn atoms takes place and reduces T C . Our calculated temporal evolution of T C during annealing agrees closely with experiment.

The wide scatter in experimental results has not allowed drawing solid conclusions on self-diffus... more The wide scatter in experimental results has not allowed drawing solid conclusions on self-diffusion in the chalcopyrite CuInSe 2 (CIS). In this work, the defect-assisted mass transport mechanisms operating in CIS are clarified using first-principles calculations. We present how the stoichiometry of the material and temperature affect the dominant diffusion mechanisms. The most mobile species in CIS is shown to be copper, whose migration proceeds either via copper vacancies or interstitials. Both of these mass-mediating agents exist in the material abundantly and face rather low migration barriers (1.09 and 0.20 eV, respectively). Depending on chemical conditions, selenium mass transport relies either solely on selenium dumbbells, which diffuse with a barrier of 0.24 eV, or also on selenium vacancies whose diffusion is hindered by a migration barrier of 2.19 eV. Surprisingly, indium plays no role in long-range mass transport in CIS; instead, indium vacancies and interstitials participate in mechanisms that promote the formation of antisites on the cation sublattice. Our results help to understand how compositional inhomogeneities arise in CIS. V C 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4799064] JOURNAL OF APPLIED PHYSICS 113, 133510 (2013) 133510-2 Oikkonen et al.

Mechanical properties of silicon microstructures and basic structural properties of crystalline s... more Mechanical properties of silicon microstructures and basic structural properties of crystalline silicon are discussed here. At ordinary pressure silicon crystallizes in a diamond structure with a basis of two atoms. All the theoretical calculations using force-field methods correctly describe bulk silicon in its diamond structure ground state, giving a value for the lattice parameter that is close to the value that is experimentally observed. Effects of pressure are explained here in detail. At high-elastic strains the harmonic approximation becomes insufficient to correctly describe the elastic energy. An alternative way to treat the nonlinearity effects is to include the higher than second-order terms in the formal expansion of the elastic energy in strains. Extended defects could be classified with respect to their dimensionality. The partial dislocations are always associated with stacking faults. Two types of dislocation in silicon are of special interest. Silicon has been a favorite material for theoretical and experimental investigations of dislocation nature and mobility. The second approach is the so-called cluster method, where a finite cluster surrounding the defect is constructed. Some examples of the proposed core reconstruction are presented. The dislocation segment where the dislocation line jumps over the Peierls barrier is called kink. Silicon belongs to the class of intrinsically brittle solids. The convenience and success of silicon material and micromachining technology have made silicon a natural choice for many MEMS applications, such as actuator, power generator, etc. The reliability of these applications is extremely important to ensure their effective performance.

The effect of strengthening of steels by yttrium oxide nanoparticles on their interaction with hy... more The effect of strengthening of steels by yttrium oxide nanoparticles on their interaction with hydrogen is studied.Hydrogen uptake and effective activation energy of its diffusion and trapping are calculated from the TDS spectra.Embedding of oxide nanoparticles lead to significant increase of hydrogen uptake compared to the EUROFER 97 steel.High-temperature components of the TDS peak in the ODS steels are reflect the hydrogen de-trapping from the nanoparticles.In this work EUROFER 97, ODS-EUROFER, and PM2000 steels are studied in terms of their interaction with hydrogen at ambient temperature. Hydrogen uptake and effective activation energy of its diffusion and trapping are calculated from the thermal desorption spectra obtained for the studied steels in the temperature interval up to 1123 K after the electrochemical hydrogen charging. It was observed that hydrogen uptake in ODS-EUROFER and PM2000 is markedly higher than in EUROFER 97 steel. It is shown that hydrogen in ODS-EUROFER has the effective diffusion activation energy, which is 0.08 and 0.06 eV larger than that in EUROFER 97 and PM2000, respectively. The high-temperature components of the TDS peaks in the studied ODS steels are suggested to reflect hydrogen de-trapping from the oxide nanoparticles. The mean particle size, the size distribution and chemical composition are studied using SEM and TEM techniques.

Density-functional-theory calculations have often been used to interpret experimental observation... more Density-functional-theory calculations have often been used to interpret experimental observations of defects in CuInSe 2 (CIS). In this work, we bring back under scrutiny conclusions drawn from earlier calculations employing the (semi)local-density approximation. We present hybrid-functional results showing that copper-or indium-related defects such as V Cu or In Cu do not create charge transition levels within the band gap in CIS. Instead, deep levels in CIS can only arise from selenium-related defects, which act as recombination centers in this material.

The wide scatter in experimental results has not allowed drawing solid conclusions on self-diffus... more The wide scatter in experimental results has not allowed drawing solid conclusions on self-diffusion in the chalcopyrite CuInSe 2 (CIS). In this work, the defect-assisted mass transport mechanisms operating in CIS are clarified using first-principles calculations. We present how the stoichiometry of the material and temperature affect the dominant diffusion mechanisms. The most mobile species in CIS is shown to be copper, whose migration proceeds either via copper vacancies or interstitials. Both of these mass-mediating agents exist in the material abundantly and face rather low migration barriers (1.09 and 0.20 eV, respectively). Depending on chemical conditions, selenium mass transport relies either solely on selenium dumbbells, which diffuse with a barrier of 0.24 eV, or also on selenium vacancies whose diffusion is hindered by a migration barrier of 2.19 eV. Surprisingly, indium plays no role in long-range mass transport in CIS; instead, indium vacancies and interstitials participate in mechanisms that promote the formation of antisites on the cation sublattice. Our results help to understand how compositional inhomogeneities arise in CIS. V C 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4799064] JOURNAL OF APPLIED PHYSICS 113, 133510 (2013) 133510-2 Oikkonen et al.

The paper is aimed at getting deeper insight into the fundamental mechanisms that govern CoSi2 pr... more The paper is aimed at getting deeper insight into the fundamental mechanisms that govern CoSi2 precipitate nucleation and growth during Co ion implantation at high temperatures (500–650 °C). Information about nucleation and growth of metal silicides as a function of temperature and implantation flux is provided by experiments on cobalt implantation in silicon, performed directly by in situ transmission electron microscopy. The main attention is paid to the nucleation of B-type precipitates, which dominate under ion implantation conditions. The obtained quantitative behavior of precipitate number density and size and the scaling of these values with implantation flux are discussed and rationalized in terms of analytical and simulation approaches. An atomistic model of B-type precipitate nucleation based on the first-principles calculations of relative energetic efficiency of different Co clusters is proposed.

Inorganic Materials: Applied Research, 2011

The article discusses first-principles calculations of hydrogen solubility in various interstitia... more The article discusses first-principles calculations of hydrogen solubility in various interstitial positions in L10 ordered FePd alloy. It is demonstrated that the predominant localization of hydrogen in solid solution is in octahedral positions in the Fe and Pd planes, where the position in the Fe plane is somewhat more favorable (by 0.13 eV). It is also demonstrated that addition of a hydrogen atom to an FePd crystal results in modification of the magnetic moment of the system by approximately one Bohr magneton.

Journal of Applied Physics, 2008

The paper is aimed at getting deeper insight into the fundamental mechanisms that govern CoSi2 pr... more The paper is aimed at getting deeper insight into the fundamental mechanisms that govern CoSi2 precipitate nucleation and growth during Co ion implantation at high temperatures (500-650 °C). Information about nucleation and growth of metal silicides as a function of temperature and implantation flux is provided by experiments on cobalt implantation in silicon, performed directly by in situ transmission electron microscopy. The main attention is paid to the nucleation of B-type precipitates, which dominate under ion implantation conditions. The obtained quantitative behavior of precipitate number density and size and the scaling of these values with implantation flux are discussed and rationalized in terms of analytical and simulation approaches. An atomistic model of B-type precipitate nucleation based on the first-principles calculations of relative energetic efficiency of different Co clusters is proposed.

Physical Review B, 2011

The results of molecular dynamics (MD) simulations of atomic hydrogen kinetics on graphene are pr... more The results of molecular dynamics (MD) simulations of atomic hydrogen kinetics on graphene are presented. The simulations involve a combination of approaches based on Brenner carbon-hydrogen potential and first-principles force calculations. Both kinds of MD calculations predict very similar qualitative trends and reproduce equally well the features of hydrogen behavior, even such sophisticated modes as long correlated jump chains. Both approaches agree that chemisorbed hydrogen diffusion on graphene is strongly limited by thermal desorption. This limitation rules out long-range diffusion of hydrogen on graphene but does not exclude the short-range hydrogen diffusion contribution to hydrogen cluster nucleation and growth.

Materials Science and Engineering B-advanced Functional Solid-state Materials, 2009

In this paper, using first-principles calculations, we demonstrate that vacancies and E-centers (... more In this paper, using first-principles calculations, we demonstrate that vacancies and E-centers (AsV, SbV) in silicon can co-exist in several metastable configurations with notably different relaxation patterns, which have very similar formation energies. Thus these vacancy-type defects can be considered as multi-symmetry defects in the sense that, at elevated temperatures, the probabilities to find vacancies in different stable configurations are comparable. From an experimental point of view, the co-existence of various symmetries can complicate the identification of the defect.

Materials Science and Engineering B-advanced Functional Solid-state Materials, 2006

In this paper, we investigate the effect of uniaxial strain on the interaction of a pair of neutr... more In this paper, we investigate the effect of uniaxial strain on the interaction of a pair of neutral vacancies in pure silicon at distances up to the fifth-nearest-neighbors. The calculation of the total energies of vacancy pairs at different pair orientations was performed using the first-principles approach. It is demonstrated that the energy of a vacancy pair is sensitive to the pair orientation with respect to the direction of applied stress. The effect of such orientational dependence of vacancy interaction on the formation of vacancy clusters in a uniaxially strained monocrystalline Si is studied using kinetic Monte-Carlo simulation. It is shown that anisotropy in vacancy–vacancy interaction leads to the formation of planar vacancy clusters with preferred orientation with respect to the strain axis.

Journal of Physics-condensed Matter, 2011

We calculate the energetics of vacancies in CuInSe2 using a hybrid functional (HSE06, HSE standin... more We calculate the energetics of vacancies in CuInSe2 using a hybrid functional (HSE06, HSE standing for Heyd, Scuseria and Ernzerhof), which gives a better description of the band gap compared to (semi)local exchange-correlation functionals. We show that, contrary to present beliefs, copper and indium vacancies induce no defect levels within the band gap and therefore cannot account for any experimentally observed levels. The selenium vacancy is responsible for only one level, namely, a deep acceptor level ɛ(0/2-). We find strong preference for VCu and VSe over VIn under practically all chemical conditions.

Physical Review B, 2011

The paper presents a systematic study of the trends in the interaction of hydrogen with carbon fu... more The paper presents a systematic study of the trends in the interaction of hydrogen with carbon fullerenes versus their curvature, where graphene is taken as the limit of zero curvature. The efficiency of hydrogen incapsulation in fullerenes, penetration into them, and adsorption on their surface are analyzed and discussed. The effects on magnetism are also considered; in particular, it is shown that hydrogen adsorption to some fullerenes induces magnetism to initially nonmagnetic systems. In addition, highly hydrogen-saturated fullerenes are examined and the suitability of fullerenes for hydrogen storage is discussed.

Physical Review B, 2011

Formation energies of various intrinsic defects and defect complexes in ZnO have been calculated ... more Formation energies of various intrinsic defects and defect complexes in ZnO have been calculated using a density-functional-theory-based pseudopotential all-electron method. The various defects considered are oxygen vacancy (VO), zinc vacancy (VZn), oxygen at an interstitial site (Oi), Zn at an interstitial site (Zni), Zn at VO (ZnO), O at VZn(OZn), and an antisite pair (combination of the preceding two defects). In addition, defect complexes like (VO+Zni) and Zn-vacancy clusters are studied. The Schokkty pair (VO+VZn) and Frenkel pairs [(VO+Oi) and (VZn+Zni)] are considered theoretically for the first time. Upon comparing the formation energies of these defects, we find that VO would be the dominant intrinsic defect under both Zn-rich and O-rich conditions and it is a deep double donor. Both ZnO and Zni are found to be shallow donors. The low formation energy of donor-type intrinsic defects could lead to difficulty in achieving p-type conductivity in ZnO. Defect complexes have charge transitions deep inside the band gap. The red, yellow, and green photoluminescence peaks of undoped samples can be assigned to some of the defect complexes considered. It is believed that the red luminescence originates from an electronic transition in VO, but we find that it can originate from the antisite ZnO defect. Charge density and electron-localization function analyses have been used to understand the effect of these defects on the ZnO lattice. The electronic structure of ZnO with intrinsic defects has been studied using density-of-states and electronic band structure plots. The acceptor levels introduced by VZn are relatively localized, making it difficult to achieve p-type conductivity with sufficient hole mobility.

Defect and Diffusion Forum, 2001

Journal of Nuclear Materials, 2009

Here we present the results of first-principles quantum mechanical calculations of energetic para... more Here we present the results of first-principles quantum mechanical calculations of energetic parameters of vacancies, interstitials, gas atoms (H, He) and some small point defect complexes in beryllium. It is found that the most energetically favorable position for self-interstitial is the basal octahedral one, for hydrogen atoms is the basal tetrahedral, while He atoms can be located with equal probability in basal octahedral and basal tetrahedral interstitial positions. The formation of divacancies and tri-vacancies from individual vacancies is shown to be energetically unfavorable, which implies high stability of beryllium against vacancy clustering. The preliminary estimates of diffusion pathways for H and He interstitials indicate essential differences: while hydrogen diffusion is nearly isotropic, that of He occurs preferentially in basal planes with the migration energy of only 0.1 eV.

Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms, 2005

In this paper we discuss possible mechanisms of PV and V2 annealing in Si. Our approach includes ... more In this paper we discuss possible mechanisms of PV and V2 annealing in Si. Our approach includes a combination of density functional theory and lattice kinetic Monte-Carlo (LKMC) simulations. The density functional theory is used to find the binding energies and jump barriers for P–V and V–V pairs at different separations (from one to three interatomic bonds between complex constituents) and in different charge states. The mobility and interaction of vacancies and vacancy complexes is simulated by LKMC with event probabilities calculated based on the energies from ab initio calculations.

Physical Review B, 2008

The paper deals with multiscale modeling of the minor component ordering in wurtzite Ga 1−x In x ... more The paper deals with multiscale modeling of the minor component ordering in wurtzite Ga 1−x In x N ͑x Ͻ 0.5͒ alloys. The treatment combines the total-energy density-functional calculations of the metal atom interaction parameters and the atomistic description of the alloy decomposition using lattice kinetic Monte Carlo. It is demonstrated that the phase decomposition patterns in wurzite GaInN are very sensitive to the interplay of metal atom interactions at several interatomic distances ͑at least to the fourth nearest neighbors͒ on the cation sublattice. Variation of the metal interaction energies within reasonable limits resulted in pronouncedly different relaxation patterns ͑linear or wall ordering of In and Ga atoms along c-axis, planar ordering parallel to basal plane, spinodal decomposition͒. The high sensitivity of the GaInN decomposition to relatively small variations of the metal interaction energies could be the main reason for the experimentally observed versatility of the alloy decomposition patterns and their sensitivity to the particular experimental conditions.

Physical Review Letters, 2006

We present results of ab initio calculations for vacancies and divacancies in GaN. Particular att... more We present results of ab initio calculations for vacancies and divacancies in GaN. Particular attention is paid to nitrogen vacancies and mixed Ga-N divacancies in negatively charged states, which in n-type GaN are found to be energetically comparable with gallium vacancies. We also demonstrate that the activation energy for self-diffusion over the nitrogen sublattice is lower than over the gallium one for all Fermi-level positions, which implies the nitrogen vacancies are major defects in samples annealed at high temperatures. Possibilities for direct observations of nitrogen vacancies through positron annihilation experiments are discussed.

Physical Review B, 2006

A combined first-principles total energy Monte Carlo method is used to study the Curie temperatur... more A combined first-principles total energy Monte Carlo method is used to study the Curie temperature T C and its evolution in ͑Ga,Mn͒As. The inter-and intra-cluster magnetic interactions are treated separately in a modified Heisenberg Hamiltonian. We find for random Mn distributions ͑mimicking as-grown samples͒ a T C maximum at the Mn concentration of 6%. During annealing we show that a slow clustering process of substitutional Mn atoms takes place and reduces T C . Our calculated temporal evolution of T C during annealing agrees closely with experiment.