Browsing by Author "Hakala, M."

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Calculation of valence electron momentum densities using the projector augmented-wave method
(2005-06) Makkonen, I.; Hakala, M.; Puska, M.J.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Correlation effects for electron-positron momentum density in solids
(1997) Barbiellini, B.; Hakala, M.; Puska, Martti J.; Nieminen, Risto M.; Manuel, A. A.
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
A method describing the calculation of the momentum density of annihilating electron-positron pairs in solids has been developed. One-electron states, which are not perturbed by the positron, are used. The effects due to the enhancement of the electron density near the positron are taken into account by a factor depending on the electron state in the annihilating pair. This enhancement is used both for the valence and core electron states. The calculated momentum densities are in good agreement with the two-dimensional angular correlation of the annihilation radiation measurements in defect-free Cu and GaAs.
Correlation effects for electron-positron momentum density in solids
(1997-09-15) Barbiellini, B.; Hakala, M.; Puska, Martti; Nieminen, R.M.; Manuel, A.A.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
A method describing the calculation of the momentum density of annihilating electron-positron pairs in solids has been developed. One-electron states, which are not perturbed by the positron, are used. The effects due to the enhancement of the electron density near the positron are taken into account by a factor depending on the electron state in the annihilating pair. This enhancement is used both for the valence and core electron states. The calculated momentum densities are in good agreement with the two-dimensional angular correlation of the annihilation radiation measurements in defect-free Cu and GaAs.
First-principles calculations of interstitial boron in silicon
(2000) Hakala, M.; Puska, Martti J.; Nieminen, Risto M.
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We perform first-principles total-energy calculations to identify the stable and metastable configurations of interstitial B in Si. We calculate formation energies and ionization levels for several equilibrium ionic configurations in different possible charge states. In all charge states the ground state consists of a B atom close to a substitutional site and a Si self-interstitial nearby. The binding energy of the self-interstitial to the substitutional B is, however, rather weak, of the order of 0.2–0.3 eV. The ground state has negative-U properties in accordance with experiments. We find several charge-state-dependent metastable configurations of interstitial B energetically close to the ground state. We discuss on the basis of formation energies the role of excess Si interstitials in the activation of B diffusion and the charge-assisted transport mechanism in the activation of B diffusion.
First-principles calculations of interstitial boron in silicon
(2000-03-15) Hakala, M.; Puska, M.J.; Nieminen, R.M.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We perform first-principles total-energy calculations to identify the stable and metastable configurations of interstitial B in Si. We calculate formation energies and ionization levels for several equilibrium ionic configurations in different possible charge states. In all charge states the ground state consists of a B atom close to a substitutional site and a Si self-interstitial nearby. The binding energy of the self-interstitial to the substitutional B is, however, rather weak, of the order of 0.2–0.3 eV. The ground state has negative-U properties in accordance with experiments. We find several charge-state-dependent metastable configurations of interstitial B energetically close to the ground state. We discuss on the basis of formation energies the role of excess Si interstitials in the activation of B diffusion and the charge-assisted transport mechanism in the activation of B diffusion.
Identification of vacancy-impurity complexes in highly n-type Si
(1999-03-01) Saarinen, K.; Nissilä, J.; Kauppinen, H.; Hakala, M.; Puska, M.J.; Hautojärvi, P.; Corbel, C.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We show that the detailed atomic structure of vacancy-impurity complexes in Si can be experimentally determined by combining positron lifetime and electron momentum distribution measurements. The vacancies complexed with a single impurity, V−P and V−As, are identified in electron irradiated Si. The formation of native vacancy defects is observed in highly As-doped Si at the doping level of 1020cm−3. The defects are identified as monovacancies surrounded by three As atoms. The formation of a V−As3 complex is consistent with the theoretical descriptions of As diffusion and electrical deactivation in highly As-doped Si.
Identification of Vacancy-Impurity Complexes in Highly n-Type Si
(1999) Saarinen, K.; Nissilä, J.; Kauppinen, H.; Hakala, M.; Puska, Martti J.; Hautojärvi, P.; Corbel, C.
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We show that the detailed atomic structure of vacancy-impurity complexes in Si can be experimentally determined by combining positron lifetime and electron momentum distribution measurements. The vacancies complexed with a single impurity, V−P and V−As, are identified in electron irradiated Si. The formation of native vacancy defects is observed in highly As-doped Si at the doping level of 1020cm−3. The defects are identified as monovacancies surrounded by three As atoms. The formation of a V−As3 complex is consistent with the theoretical descriptions of As diffusion and electrical deactivation in highly As-doped Si.
Microscopic identification of native donor Ga-vacancy complexes in Te-doped GaAs
(1999-07-15) Gebauer, J.; Lausmann, M.; Staab, T.E.M.; Krause-Rehberg, R.; Hakala, M.; Puska, M.J.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Native vacancies in Te-doped (5×1016–5×1018cm−3)GaAs were investigated by means of positron lifetime and Doppler-broadening coincidence spectroscopy. The experimental data were related to theoretical calculations of the positron lifetime and the annihilation momentum distribution. Monovacancies were observed in all Te-doped GaAs samples under study. It will be shown that they can directly be identified to be Ga-vacancy–TeAs-donor complexes. These complexes are the dominating type of vacancy defects in the doping range under observation.
Microscopic identification of native donor Ga-vacancy complexes in Te-doped GaAs
(1999) Gebauer, J.; Lausmann, M.; Staab, T. E. M.; Krause-Rehberg, R.; Hakala, M.; Puska, Martti J.
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Native vacancies in Te-doped (5×1016–5×1018cm−3) GaAs were investigated by means of positron lifetime and Doppler-broadening coincidence spectroscopy. The experimental data were related to theoretical calculations of the positron lifetime and the annihilation momentum distribution. Monovacancies were observed in all Te-doped GaAs samples under study. It will be shown that they can directly be identified to be Ga-vacancy–TeAs-donor complexes. These complexes are the dominating type of vacancy defects in the doping range under observation.
Modeling the momentum distributions of annihilating electron-positron pairs in solids
(2006) Makkonen, I.; Hakala, M.; Puska, Martti J.
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Measuring the Doppler broadening of the positron annihilation radiation or the angular correlation between the two annihilation gamma quanta reflects the momentum distribution of electrons seen by positrons in the material. Vacancy-type defects in solids localize positrons and the measured spectra are sensitive to the detailed chemical and geometric environments of the defects. However, the measured information is indirect and when using it in defect identification comparisons with theoretically predicted spectra is indispensable. In this article we present a computational scheme for calculating momentum distributions of electron-positron pairs annihilating in solids. Valence electron states and their interaction with ion cores are described using the all-electron projector augmented-wave method, and atomic orbitals are used to describe the core states. We apply our numerical scheme to selected systems and compare three different enhancement (electron-positron correlation) schemes previously used in the calculation of momentum distributions of annihilating electron-positron pairs within the density-functional theory. We show that the use of a state-dependent enhancement scheme leads to better results than a position-dependent enhancement factor in the case of ratios of Doppler spectra between different systems. Further, we demonstrate the applicability of our scheme for studying vacancy-type defects in metals and semiconductors. Especially we study the effect of forces due to a positron localized at a vacancy-type defect on the ionic relaxations.
Modeling the momentum distributions of annihilating electron-positron pairs in solids
(2006-01-06) Makkonen, I.; Hakala, M.; Puska, M.J.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Measuring the Doppler broadening of the positron annihilation radiation or the angular correlation between the two annihilation gamma quanta reflects the momentum distribution of electrons seen by positrons in the material. Vacancy-type defects in solids localize positrons and the measured spectra are sensitive to the detailed chemical and geometric environments of the defects. However, the measured information is indirect and when using it in defect identification comparisons with theoretically predicted spectra is indispensable. In this article we present a computational scheme for calculating momentum distributions of electron-positron pairs annihilating in solids. Valence electron states and their interaction with ion cores are described using the all-electron projector augmented-wave method, and atomic orbitals are used to describe the core states. We apply our numerical scheme to selected systems and compare three different enhancement (electron-positron correlation) schemes previously used in the calculation of momentum distributions of annihilating electron-positron pairs within the density-functional theory. We show that the use of a state-dependent enhancement scheme leads to better results than a position-dependent enhancement factor in the case of ratios of Doppler spectra between different systems. Further, we demonstrate the applicability of our scheme for studying vacancy-type defects in metals and semiconductors. Especially we study the effect of forces due to a positron localized at a vacancy-type defect on the ionic relaxations.
Momentum distributions of electron-positron pairs annihilating at vacancy clusters in Si
(1998-04-01) Hakala, M.; Puska, M.J.; Nieminen, R.M.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We report calculations of momentum densities of electron-positron pairs annihilating at various vacancy clusters in Si. The densities integrated along one direction, i.e., those corresponding to the spectra measured by the two-dimensional angular correlation of the annihilation radiation method, are shown to be isotropic if the positron is captured by a small vacancy cluster. The densities integrated along two directions correspond to the one-dimensional angular-correlation distribution or the Doppler-broadened annihilation radiation line shape. The characteristic narrowing of these one-dimensional distributions at vacancy clusters is demonstrated. The line-shape parameters S and W describing mainly the annihilations with valence and core electrons, respectively, are calculated and compared quantitatively with experimental data. We find a systematic increase in the S and a decrease in the W parameter as the size of the vacancy increases.
Momentum distributions of electron-positron pairs annihilating at vacancy clusters in Si
(1998) Hakala, M.; Puska, Martti J.; Nieminen, Risto M.
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We report calculations of momentum densities of electron-positron pairs annihilating at various vacancy clusters in Si. The densities integrated along one direction, i.e., those corresponding to the spectra measured by the two-dimensional angular correlation of the annihilation radiation method, are shown to be isotropic if the positron is captured by a small vacancy cluster. The densities integrated along two directions correspond to the one-dimensional angular-correlation distribution or the Doppler-broadened annihilation radiation line shape. The characteristic narrowing of these one-dimensional distributions at vacancy clusters is demonstrated. The line-shape parameters S and W describing mainly the annihilations with valence and core electrons, respectively, are calculated and compared quantitatively with experimental data. We find a systematic increase in the S and a decrease in the W parameter as the size of the vacancy increases.
Murtumissitkeyden määritys elastis-plastisissa murtumismekaniikkakokeissa
(1979) Saarelma, Heikki
Helsinki University of Technology | Master's thesis
Native defects and self-diffusion in GaSb
(2002) Hakala, M.; Puska, Martti J.; Nieminen, Risto M.
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The native defects in GaSb have been studied with first-principles total-energy calculations. We report the structures and the formation energies of the stable defects and estimate the defect concentrations under different growth conditions. The most important native defect is the GaSb antisite, which acts as an acceptor. The other important defects are the acceptor-type Ga vacancy and the donor-type Ga interstitial. The Sb vacancies and interstitials are found to have much higher formation energies. A metastable state is observed for the SbGa antisite. The significantly larger concentrations of the Ga vacancies and interstitials compared to the corresponding Sb defects is in accordance with the asymmetric self-diffusion behavior in GaSb. The data supports the next-nearest-neighbor model for the self-diffusion, in which the migration occurs independently in the different sublattices. Self-diffusion is dominated by moving Ga atoms.
Theoretical and experimental study of positron annihilation with core electrons in solids
(1996) Alatalo, M.; Barbiellini, B.; Hakala, M.; Kauppinen, H.; Korhonen, T.; Puska, Martti J.; Saarinen, K.; Hautojärvi, P.; Nieminen, Risto M.
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
A theory for calculating the momentum distribution of annihilating positron-electron pairs in solids is presented. To test the theory, momentum distributions are measured by the Doppler broadening of the annihilation radiation for several bulk metals and semiconductors, as well as for semiconductor alloys and for positrons trapped at vacancies in semiconductors. The theory is based on a two-particle description of the annihilating electron-positron pair. Then, the electron-positron correlation effects, i.e., the enhancement of the electron density at the positron, depend on the electronic state in question. The theory is suited for calculating the high-momentum part of the annihilation spectrum that arises from the core electrons and which can be measured by the Doppler broadening using coincidence techniques. The ideas of the theory are justified by a good agreement between theory and experiment in the case of positron annihilation in undefected bulk lattices. Moreover, the comparison of the theoretical and experimental spectra for alloys and vacancy defects tests the theoretical description for the positron distribution in delocalized and localized states, respectively.
Theoretical and experimental study of positron annihilation with core electrons in solids
(1996-07-15) Alatalo, M.; Barbiellini, B.; Hakala, M.; Kauppinen, H.; Korhonen, T.; Puska, M.J.; Saarinen, K.; Hautojärvi, P.; Nieminen, R.M.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
A theory for calculating the momentum distribution of annihilating positron-electron pairs in solids is presented. To test the theory, momentum distributions are measured by the Doppler broadening of the annihilation radiation for several bulk metals and semiconductors, as well as for semiconductor alloys and for positrons trapped at vacancies in semiconductors. The theory is based on a two-particle description of the annihilating electron-positron pair. Then, the electron-positron correlation effects, i.e., the enhancement of the electron density at the positron, depend on the electronic state in question. The theory is suited for calculating the high-momentum part of the annihilation spectrum that arises from the core electrons and which can be measured by the Doppler broadening using coincidence techniques. The ideas of the theory are justified by a good agreement between theory and experiment in the case of positron annihilation in undefected bulk lattices. Moreover, the comparison of the theoretical and experimental spectra for alloys and vacancy defects tests the theoretical description for the positron distribution in delocalized and localized states, respectively.