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- 1. Phys. Rev. B 75, 033303 (2007) , “Excitons in silicon nanocrystallites: The nature of luminescence”, Eleonora Luppi, Federico Iori, Rita Magri, Olivia Pulci, Stefano Ossicini, Elena Degoli, and Valerio OlevanoThe absorption and emission spectra of silicon nanocrystals up to 1 nm diameter have been calculated within a first-principles framework. Our calculations include geometry optimization and the many-body effects induced by the creation of an electron-hole pair. Starting from hydrogenated silicon... (Read more)
- 2. Phys. Rev. B 75, 024205 (2007) , “Mechanical strength and coordination defects in compressed silica glass: Molecular dynamics simulations”, Yunfeng Liang, Caetano R. Miranda, and Sandro ScandoloContrary to ordinary solids, which are normally known to harden by compression, the compressibility of SiO2 (silica) glass has a maximum at about 2–4 GPa and its mechanical strength shows a minimum around 10 GPa. At this pressure, the compression of silica glass undergoes a change... (Read more)
- 3. Phys. Rev. B 75, 014102 (2007) , “First-principles study of vacancy formation in hydroxyapatite”, Katsuyuki Matsunaga and Akihide KuwabaraFirst-principles plane-wave calculations were performed for hydroxyapatite (HAp) in order to investigate the electronic structure and vacancy formation mechanisms. The HAp unit cell contains PO4 tetrahedra and OH groups formed by covalent P-O and H-O bonds. Ca ions play a role for... (Read more)
- 4. Appl. Phys. Lett. 88, 182903 (2006) , “Effects of Al addition on the native defects in hafnia”, Q. Li, K. M. Koo, W. M. Lau, P. F. Lee, J. Y. Dai, Z. F. Hou, X. G. GongTwo occupied native defect bands are experimentally detected in pure HfO2. The density of states of band one in the middle of the band gap reduces drastically with the Al addition, while that of band two slightly above the valence-band maximum remains rather unaffected. We attribute the... (Read more)
- 5. Appl. Phys. Lett. 88, 171912 (2006) , “Impact of nitridation on open volumes in HfSiOx studied using monoenergetic positron beams”, A. Uedono, K. Ikeuchi, T. Otsuka, K. Yamabe, K. Eguchi, M. Takayanagi, T. Ohdaira, M. Muramatsu, R. Suzuki, A. S. Hamid, T. ChikyowThe effects of nitridation on open volumes in thin HfSiOx films fabricated by metal-organic chemical vapor deposition were studied using monoenergetic positron beams. It was found that positrons were annihilated from the trapped state by open volumes that exist intrinsically in... (Read more)
- 6. Appl. Phys. Lett. 88, 012101 (2006) , “The structure of the SiO2/Si(100) interface from a restraint-free search using computer simulations”, Dominik Fischer, Alessandro Curioni, Salomon Billeter, and Wanda AndreoniThe structure of the interface between SiO2 and Si(100) is investigated using the replica-exchange method driven by classical molecular dynamics simulations based on ab initio-derived interatomic potentials. Abrupt interfaces are shown to be unstable, whereas a substoichiometric... (Read more)
- 7. J. Appl. Phys. 99, 054507 (2006) , “Characterization of HfSiON gate dielectrics using monoenergetic positron beams”, A. Uedono, K. Ikeuchi, T. Otsuka, K. Shiraishi, K. Yamabe, S. Miyazaki, N. Umezawa, A. Hamid, T. Chikyow, T. Ohdaira M. Muramatsu, R. Suzuki, S. Inumiya, S. Kamiyama, Y. Akasaka, Y. Nara, and K. YamadaThe impact of nitridation on open volumes in thin HfSiOx films fabricated on Si substrates by atomic layer deposition was studied using monoenergetic positron beams. For HfSiOx, positrons were found to annihilate from the trapped state due to open volumes which... (Read more)
- 8. Phys. Rev. B 74, 214105 (2006) , “Atomistic simulations of radiation-induced defect formation in spinels: MgAl2O4, MgGa2O4, and MgIn2O4”, D. Bacorisen, Roger Smith, B. P. Uberuaga, K. E. Sickafus, J. A. Ball, and R. W. GrimesMolecular dynamics simulations of collision cascades were performed in three spinel oxides with varying inversion, namely normal magnesium aluminate, MgAl2O4, half-inverse magnesium gallate, MgGa2O4, and inverse magnesium indate,... (Read more)
- 9. Phys. Rev. B 74, 205324 (2006) , “Surface smoothness of plasma-deposited amorphous silicon thin films: Surface diffusion of radical precursors and mechanism of Si incorporation”, Mayur S. Valipa, Tamas Bakos, and Dimitrios MaroudasWe present a detailed analysis of the fundamental atomic-scale processes that determine the surface smoothness of hydrogenated amorphous silicon (a-Si:H) thin films. The analysis is based on a synergistic combination of molecular-dynamics (MD) simulations of radical precursor migration on surfaces... (Read more)
- 10. Phys. Rev. B 74, 205202 (2006) , “Structural model of amorphous silicon annealed with tight binding”, N. Bernstein, J. L. Feldman,, and M. FornariWe present a model of amorphous silicon generated by extensive annealing of a continuous random network structure using a molecular dynamics simulation with forces computed by a tight-binding total energy method. We also produce a refined model by relaxing the annealed model using density functional... (Read more)
- 11. Phys. Rev. B 74, 174115 (2006) , “Modeling of damage generation mechanisms in silicon at energies below the displacement threshold”, Iván Santos, Luis A. Marqués, and Lourdes PelazWe have used molecular dynamics simulation techniques to study the generation of damage in Si within the low-energy deposition regime. We have demonstrated that energy transfers below the displacement threshold can produce a significant amount of damage, usually neglected in traditional radiation... (Read more)
- 12. Phys. Rev. B 74, 155208 (2006) , “Vibrational spectra of vitreous germania from first-principles”, Luigi Giacomazzi, P. Umari, and Alfredo PasquarelloWe report on a first-principles investigation of the structural and vibrational properties of vitreous germania (v-GeO2). Our work focuses on a periodic model structure of 168 atoms, but three smaller models are also studied for comparison. We first carry out a detailed structural... (Read more)
- 13. Phys. Rev. B 74, 125203 (2006) , “Density functional theory of structural transformations of oxygen-deficient centers in amorphous silica during hole trapping: Structure and formation mechanism of the Egamma[prime]" align="middle"> center”, T. Uchino and T. YokoWe investigate the hole trapping process of a neutral oxygen vacancy in amorphous silicon dioxide (a-SiO2) using cluster calculations based on the density functional theory (DFT) method. We show that trapping a hole at a neutral oxygen vacancy leads to the formation of several... (Read more)
- 14. Phys. Rev. B 73, 235211 (2006) , “Ab initio calculations for the interconversion of optically active defects in amorphous silica”, M. M. G. Alemany and James R. ChelikowskyUsing ab initio calculations on clusters, we have identified a new reaction path between the dicoordinated silicon atom defect and the paramagnetic Egamma[prime]" align="middle"> center in amorphous silica. Under ionizing irradiation, the dicoordinated silicon atom... (Read more)
- 15. Phys. Rev. Lett. 97, 155901 (2006) , “Proton Diffusion Mechanism in Amorphous SiO2”, Julien Godet and Alfredo PasquarelloWe study proton diffusion in amorphous SiO2 from the atomic scale to the long-range percolative regime. Ab initio molecular dynamics suggest that the dominant atomic process consists in cross-ring interoxygen hopping assisted by network vibrations. A statistical analysis accounting... (Read more)
- 16. Phys. Rev. Lett. 97, 066101 (2006) , “Structure and Interconversion of Oxygen-Vacancy-Related Defects on Amorphous Silica”, Chin-Lung Kuo and Gyeong S. HwangAtomic structure and structural stability of neutral oxygen vacancies on amorphous silica are investigated using combined Monte Carlo and density functional calculations. We find that, unlike their bulk counterparts, the Si-Si dimer configuration of surface oxygen vacancies is likely to be unstable... (Read more)
- 17. Phys. Rev. Lett. 97, 016102 (2006) , “Scaling and Universality of Roughening in Thermal Oxidation of Si(001)”, Hiroo Omi, Hiroyuki Kageshima, and Masashi UematsuBy analyzing atomic force microscopy images, we derive a continuum equation that quantitatively explains the roughening at the Si(001)-SiO2 interface during thermal oxidation at the temperature at 1200 °C in an Ar atmosphere containing a small fraction of O2. We also show... (Read more)
- 18. Phys. Rev. Lett. 93, 086102 (2004) , “Reaction of the Oxygen Molecule at the Si(100)–SiO2 Interface During Silicon Oxidation”, Angelo Bongiorno and Alfredo PasquarelloUsing constrained ab initio molecular dynamics, we investigate the reaction of the O2 molecule at the Si(100)SiO2 interface during Si oxidation. The reaction proceeds sequentially through the incorporation of the O2 molecule in a Si-Si bond and the... (Read more)
- 19. Phys. Rev. Lett. 88, 205502 (2002) , “Metastability of Amorphous Silicon from Silicon Network Rebonding”, R. Biswas, B. C. Pan, and Y. Y. YeWe propose a network rebonding model for light-induced metastability in amorphous silicon, involving bonding rearrangements of silicon and hydrogen atoms. Nonradiative recombination breaks weak silicon bonds and generates dangling bond–floating bond pairs, with very low activation energies. The... (Read more)
- 20. Phys. Rev. Lett. 86, 5522-5525 (2001) , “E' Centers in Amorphous SiO2 Revisited: A New Look at an Old Problem”, T. Uchino, M. Takahashi, T. YokoWe present theoretical evidence that the paramagnetic E? defect centers in amorphous silicon dioxide ( a-SiO2) do not have the same microscopic structures as those well-defined in the corresponding crystalline counterparts such as ?-quartz. We then present alternative models of... (Read more)
- 21. Phys. Rev. Lett. 86, 4560-4563 (2001) , “Structure and Generation Mechanism of the Peroxy-Radiacal Defect in Amorphous Silica”, T. Uchino, M. Takahashi, and T. YokoWe provide a new model of the peroxy-radical defect in amorphous silica on the basis of quantum-chemical calculations applied to clusters of atoms to model the defect. In this model, the 29Si hyperfine splittings of the peroxy radical arise from a single silicon, in agreement with the... (Read more)
- 22. Phys. Rev. Lett. 85, 2773-2776 (2000) , “Dangling Bond Defects at Si-SiO2 Interfaces: Atomic Structure of the Pb1 Center”, A. Stirling, A. Pasquarello, J.-C. Charlier, R. CarUsing a first-principles approach, we characterize dangling bond defects at Si-SiO2 interfaces by calculating hyperfine parameters for several relaxed structures. Interface models, in which defect Si atoms remain close to crystalline sites of the substrate upon relaxation, successfully... (Read more)
- 23. Phys. Rev. Lett. 85, 2324-2327 (2000) , “Fast Diffusion of H and Creation of Dangling Bonds in Hydrogenated Amorphous Silicon Studied by in situ ESR”, U. K. Das, T. Yasuda, and S. YamasakiThe interaction of atomic hydrogen with a-Si:H films was studied by means of in situ ESR during H plasma treatment. H diffuses into the a-Si:H film and creates additional Si dangling bonds (∼1013 cm -2). We observed a high diffusion coefficient (>10-10  cm... (Read more)
- 24. Nature 396, 58-60 (1998) , “Interface structure between silicon and its oxide by first-principles molecular dynamics”, A. Pasquarello, M. S. Hybertsen, R. CarThe requirement for increasingly thin (<50 Ã…) insulating oxide layers in silicon-based electronic devices highlights the importance of characterizing the Si–SiO2 interface structure at the atomic scale. Such a characterization relies to a large extent on an understanding of the atomic-scale mechanisms that govern the oxidation process. The widely used Deal–Grove model invokes a two-step process in which oxygen first diffuses through the amorphous oxide network before attacking the silicon substrate, resulting in the formation of new oxide at the buried interface1. But it remains unclear how such a process can yield the observed near-perfect interface2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12. Here we use first-principles molecular dynamics13, 14, 15 to generate a model interface structure by simulating the oxidation of three silicon layers. The resulting structure reveals an unexpected excess of silicon atoms at the interface, yet shows no bonding defects. Changes in the bonding network near the interface occur during the simulation via transient exchange events wherein oxygen atoms are momentarily bonded to three silicon atoms — this mechanism enables the interface to evolve without leaving dangling bonds. (Read more)
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