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Article

Electronic band structure of various crystal orientations of relaxed and
strained bulk, 1D and 2D confined semiconductors are investigated using
nonlocal empirical pseudopotential method with spin-orbit interaction.
For the bulk semiconductors, local and nonlocal pseudopotential
parameters are obtained by fitting transport-relevant quantities, such
as band gap, effective masses and deformation potentials, to available
experimental data. A cubic-spline interpolation is used to extend local
form factors to arbitrary q and the resulting transferable local
pseudopotential V(q) with correct work function is used to investigate
the 1D and 2D confined systems with supercell method. Quantum
confinement, uniaxial and biaxial strain and crystal orientation effects
of the band structure are investigated. Regarding the transport relavant
quantities, we have found that the largest ballistic electron
conductance occurs for compressively-strained large-diameter [001] wires
while the smallest transport electron effective mass is found for
larger-diameter [110] wires under tensile stress.

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... Теперь рассмотрим кристаллическую решетку полупроводникового кремния. Для силы притяжения между соседними атомами можно выбрать известное выражение [4]: ...

In this work, the experimentally discovered effect of flexophotovoltaics (FFV) in silicon p-n structures under the influence of local mechanical stress on the front surface is theoretically justified for the first time. The regularities of the manifestation of the FFV effect are determined depending on the value of the local pressure force and the photoexcitation intensity. The experimental data were processed statistically by the least squares method and a new empirical formula was obtained for the experimentally determined dependence of the short-circuit photocurrent of a silicon structure on local mechanical stress.

... Теперь рассмотрим кристаллическую решетку полупроводникового кремния. Для силы притяжения между соседними атомами можно выбрать известное выражение [4]: ...

In this work, the experimentally discovered effect of flexophotovoltaics (FFV) in silicon p-n structures under the
influence of local mechanical stress on the front surface is theoretically justified for the first time. The regularities of the
manifestation of the FFV effect are determined depending on the value of the local pressure force and the photoexcitation intensity. The experimental data were processed statistically by the least squares method and a new empirical formula was obtained for the experimentally determined dependence of the short-circuit photocurrent of a silicon structure on local mechanical stress.

... Now consider the crystal lattice of semiconductor silicon. For the force of attraction between neighboring atoms, one can choose the well-known expression [4]: there Aproportionality coefficient depending on the type of interatomic bond, rinteratomic distance. If the supplied external force Fk causes a change in the interatomic distance and the conditions: r ~ L, r = γ L, dr = γdL , (7) there γaspect ratio. ...

In this work, the first experimentally discovered effect of flexophotovoltaics (FPV) in silicon p-n-structures under the influence of local mechanical stress on the frontal surface is theoretically substantiated.. The regularities of the manifestation of the FPV effect are determined depending on the magnitude of the local pressure force and the intensity of photoexcitation.. Statistical processing of the experimental data by the least squares method was carried out and a new empirical formula was obtained for the experimentally determined dependence of the short circuit photocurrent of a silicon structure on the local mechanical stress.

... Computing the electronic structure of extended molecules and materials can reveal important information such as the band gap, reactivity, and bulk properties like conductivity or polarizability. As such, accurate methods for computing the electronic structure of materials is of interest to many fields, including the study of inorganic polymers, organic electronics, semiconductors, and superconductors [1][2][3][4][5][6][7][8][9][10][11][12]. Electronic structure calculations on extended materials are only computationally tractable in cases where periodic boundary conditions can be imposed, which separate the electrons and orbitals into smaller, periodically repeating, unit cells. ...

Molecular periodic systems have the challenge of not only representing an infinite, albeit periodically repeating, molecule but also treating the strong electron correlation that emerges in such systems. Many post-Hartree-Fock methods that are efficient in moderately correlated molecules often fail to describe strongly correlated electrons and their properties in periodic systems. Here we develop and apply an electronic structure approach to periodic systems based on the variational calculation of the two-electron reduced density matrix (2-RDM). The variational 2-RDM method calculates a lower bound to the ground-state energy of a molecular periodic system without computation or storage of its many-electron wave function. The 2-RDM is constrained by $N$-representability conditions that are necessary for it to represent at least one $N$-electron density matrix---conditions that allow it to treat strongly correlated systems. Two periodic systems are treated to demonstrate the methodology: (i) a metallic hydrogen chain and (ii) a strongly correlated acene chain. In the hydrogen chains the 2-RDM theory describes the strongly correlated Mott metal-to-insulator transition while in acene chains it describes the emergence of polyradical character with increasing chain length. The methodology is applicable to treating strong electron correlation in more general periodic molecules and materials.

... C 11 , C 12 , and C 44 are the elastic constants of the GePb alloy. The details of the derivations can be found in Ref. [26]. ...

Correlation-driven phenomena in molecular periodic systems are challenging to predict computationally not only because such systems are periodically infinite but also because they are typically strongly correlated. Here, we generalize the variational two-electron reduced density matrix (2-RDM) theory to compute the energies and properties of strongly correlated periodic systems. The 2-RDM of the unit cell is directly computed subject to necessary N-representability conditions such that the unit-cell 2-RDM represents at least one N-electron density matrix. Two canonical but non-trivial systems, periodic metallic hydrogen chains and periodic acenes, are treated to demonstrate the methodology. We show that while single-reference correlation theories do not capture the strong (static) correlation effects in either of these molecular systems, the periodic variational 2-RDM theory predicts the Mott metal-to-insulator transition in the hydrogen chains and the length-dependent polyradical formation in acenes. For both hydrogen chains and acenes, the periodic calculations are compared with previous non-periodic calculations with the results showing a significant change in energies and increase in the electron correlation from the periodic boundary conditions. The 2-RDM theory, which allows for much larger active spaces than are traditionally possible, is applicable to studying correlation-driven phenomena in general periodic molecular solids and materials.

L’explosion de la demande en données a imposé de nouvelles exigences en terme de débit de transmission qui sont de plus en difficiles à satisfaire sans accroître considérablement les consommations énergétiques dans les centres de données, points névralgiques des réseaux de télécommunications. Dans ce contexte, la photonique silicium est considérée comme la solution la plus adaptée pour répondre de ces problématiques en remplaçant les interconnexions métalliques par des liaisons optiques à base de silicium. Le modulateur électro-optique constitue l’un des composants clés de ces liaisons optiques. Cependant, la centrosymétrie du silicium empêche l’exploitation de l’effet Pockels, un phénomène d’optique non linéaire très efficace dans la conception de modulateurs à très grande bande passante et à faible consommation énergétique. Cette limitation peut être néanmoins contournée lorsque des contraintes mécaniques sont appliquées au silicium de façon à briser sa symétrie d’inversion. Plusieurs travaux théoriques et expérimentaux ont alors été entrepris récemment pour mettre en évidence et quantifier l’effet Pockels induit par contraintes dans le silicium. Mais la nature semi-conductrice du silicium rend l’analyse de l’effet Pockels profondément complexe et cela a soulevé une controverse quant à sa réelle existence dans le silicium contraint. En effet, l’influence des porteurs libres dans le silicium et aux interfaces engendrent un fort signal de modulation, noyant la signature de l’effet Pockels. Pour enrayer les effets de porteurs, la solution apportée par le travail de thèse a été d’étudier le signal de modulation à hautes fréquences (> 5 GHz). Plusieurs études hyperfréquences de l’effet Pockels ont donc été menées dans des structures photoniques en silicium contraint et seront présentées dans ce manuscrit de thèse. Les premières études ont été réalisées sur une plate-forme SOI et les résultats expérimentaux ont permis de mettre en évidence la présence d’un signal de modulation électro-optique à hautes fréquences et dont l’intensité dépend clairement de l’orientation cristallographique du silicium et de l’amplitude de la contrainte appliquée sur celui-ci. Sur la base d’un modèle théorique décrivant le tenseur de susceptibilité électrique du second ordre χ(²), un modèle multiphysique a été développé et a permis de décrire de manière très précise à la fois les résultats expérimentaux et la distribution spatiale du χ(²) dans des guides d’onde silicium contraints. Ces travaux ont également permis de montrer que les faibles intensités des champs électriques appliqués dans les guides d’onde silicium, dues à la distribution des porteurs, sont en grande partie responsable de la faible efficacité de modulation par effet Pockels. Une seconde étude a donc été menée sur une plate-forme SOI modifiée et permettant la conception de circuits électriques plus performants avec des champs électriques générés plus intenses. Les résultats expérimentaux obtenus montrent une amélioration d’un facteur 20 de l’efficacité de modulation par effet Pockels en comparaison des premières études. De plus, le modèle multiphysique a de nouveau permis de décrire ces résultats, renforçant donc davantage sa validité. L’ensemble de ces travaux ouvrent notamment comme perspectives la possibilité d’obtenir un diagramme de l’œil électro-optique dans la mesure où une contrainte plus importante est appliquée aux guides d’onde silicium. De plus, le modèle décrivant le tenseur de susceptibilité électrique du second ordre χ(²) peut également être exploité pour décrire le phénomène de génération de seconde harmonique en optique guidée dont l’existence reste encore ambiguë à l’heure actuelle.

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