Microscopic structure of the hydrogen-boron complex in crystalline silicon

The microscopic structure of hydrogen-boron complexes in silicon, which result from the passivation of boron-doped silicon by hydrogen, has been extensively debated in the literature. Most of the debate has focussed on the equilibrium site for the H atom. Here we study the microscopic structure of the complexes using parameter-free total-energy calculations and an exploration of the entire energy surface for H in Si:B. We conclusively show that the global energy minimum occurs for H at a site close to the center of a Si-B bond (BM site), but that there is a barrier of only 0.2 eV for movement of the H atom between four equivalent BM sites. This low energy barrier implies that at room temperature H is able to move around the B atom. Other sites for H proposed by others as the equilibrium sites are shown to be saddle points considerably higher in energy. The vibrational frequency of the H stretching mode at the BM site is calculated and found to be in agreement with experiment. Calculations of the dissociation energy of the complex are discussed.FWN – Publicaties zonder aanstelling Universiteit Leide

Microscopic structure of the hydrogen-phosphorus complex in crystalline silicon

The existing discrepancy between theoretical models and experimental results for hydrogen-donor complexes in crystalline silicon is resolved using first-principles pseudopotential-density-functional calculations for the hydrogen-phosphorus pair. In the configuration which is the global energy minimum, H is located on the extension of a P-Si bond on the Si side, with the Si-H pair relaxing away from P by 0.6 Å, leaving the P atom threefold coordinated. The calculated stretching and wagging vibrational frequencies associated with this configuration are in accord with experiment.FWN – Publicaties zonder aanstelling Universiteit Leide

Structure and properties of hydrogen-impurity pairs in elemental semiconductors

A variety of experiments have revealed several puzzling properties of hydrogen-impurity pairs. For example, H atoms passivate the electrical activity of some impurities, whereas they induce electrical activity in others; they appear to tunnel around some impurities but not around others. We report first-principles pseudopotential-density-functional calculations for several hydrogen-impurity complexes and unravel the origins and intricacies of the rich behavior of H bound to different substitutional impurities in Si and Ge.FWN – Publicaties zonder aanstelling Universiteit Leide

Electron Exchange Coupling for Single Donor Solid-State Qubits

Inter-valley interference between degenerate conduction band minima has been shown to lead to oscillations in the exchange energy between neighbouring phosphorus donor electron states in silicon \cite{Koiller02,Koiller02A}. These same effects lead to an extreme sensitivity of the exchange energy on the relative orientation of the donor atoms, an issue of crucial importance in the construction silicon-based spin quantum computers. In this article we calculate the donor electron exchange coupling as a function of donor position incorporating the full Bloch structure of the Kohn-Luttinger electron wavefunctions. It is found that due to the rapidly oscillating nature of the terms they produce, the periodic part of the Bloch functions can be safely ignored in the Heitler-London integrals as was done by Koiller et. al. [Phys. Rev. Lett. 88,027903(2002),Phys. Rev. B. 66,115201(2002)], significantly reducing the complexity of calculations. We address issues of fabrication and calculate the expected exchange coupling between neighbouring donors that have been implanted into the silicon substrate using an 15keV ion beam in the so-called 'top down' fabrication scheme for a Kane solid-state quantum computer. In addition we calculate the exchange coupling as a function of the voltage bias on control gates used to manipulate the electron wavefunctions and implement quantum logic operations in the Kane proposal, and find that these gate biases can be used to both increase and decrease the magnitude of the exchange coupling between neighbouring donor electrons. The zero-bias results reconfirm those previously obtained by Koiller.Comment: 10 Pages, 8 Figures. To appear in Physical Review

Theory of hydrogen diffusion and reactions in crystalline silicon

FWN – Publicaties zonder aanstelling Universiteit Leide

Fine Splitting of Electron States in Silicon Nanocrystal with a Hydrogen-like Shallow Donor

Electron structure of a silicon quantum dot doped with a shallow hydrogen-like donor has been calculated for the electron states above the optical gap. Within the framework of the envelope-function approach we have calculated the fine splitting of the ground sixfold degenerate electron state as a function of the donor position inside the quantum dot. Also, dependence of the wave functions and energies on the dot size was obtained

Modeling transport through single-molecule junctions

Non-equilibrium Green's functions (NEGF) formalism combined with extended Huckel (EHT) and charging model are used to study electrical conduction through single-molecule junctions. Analyzed molecular complex is composed of asymmetric 1,4-Bis((2'-para-mercaptophenyl)-ethinyl)-2-acetyl-amino-5-nitro-benzene molecule symmetrically coupled to two gold electrodes [Reichert et al., Phys. Rev. Lett. Vol.88 (2002), pp. 176804]. Owing to this model, the accurate values of the current flowing through such junction can be obtained by utilizing basic fundamentals and coherently deriving model parameters. Furthermore, the influence of the charging effect on the transport characteristics is emphasized. In particular, charging-induced reduction of conductance gap, charging-induced rectification effect and charging-generated negative value of the second derivative of the current with respect to voltage are observed and examined for molecular complex.Comment: 8 pages, 3 figure

Emerging Diluted Ferromagnetism in High-T-c Superconductors Driven by Point Defect Clusters

Defects in ceramic materials are generally seen as detrimental to their functionality and applicability. Yet, in some complex oxides, defects present an opportunity to enhance some of their properties or even lead to the discovery of exciting physics, particularly in the presence of strong correlations. A paradigmatic case is the high-temperature superconductor YBa2Cu3O7-delta(Y123), in which nanoscale defects play an important role as they can immobilize quantized magnetic flux vortices. Here previously unforeseen point defects buried in Y123 thin films that lead to the formation of ferromagnetic clusters embedded within the superconductor are unveiled. Aberration-corrected scanning transmission microscopy has been used for exploring, on a single unit-cell level, the structure and chemistry resulting from these complex point defects, along with density functional theory calculations, for providing new insights about their nature including an unexpected defect-driven ferromagnetism, and X-ray magnetic circular dichroism for bearing evidence of Cu magnetic moments that align ferromagnetically even below the superconducting critical temperature to form a dilute system of magnetic clusters associated with the point defects

Measuring the decoherence rate in a semiconductor charge qubit

We describe a method by which the decoherence time of a solid state qubit may be measured. The qubit is coded in the orbital degree of freedom of a single electron bound to a pair of donor impurities in a semiconductor host. The qubit is manipulated by adiabatically varying an external electric field. We show that, by measuring the total probability of a successful qubit rotation as a function of the control field parameters, the decoherence rate may be determined. We estimate various system parameters, including the decoherence rates due to electromagnetic fluctuations and acoustic phonons. We find that, for reasonable physical parameters, the experiment is possible with existing technology. In particular, the use of adiabatic control fields implies that the experiment can be performed with control electronics with a time resolution of tens of nanoseconds.Comment: 9 pages, 6 figures, revtex