Linear and planar molecules formed by coupled P donors in silicon

Using the effective mass theory and the multi-valley envelope function representation, we have developed a theoretical framework for computing the single-electron electronic structure of several phosphorus donors interacting in an arbitrary geometrical configuration in silicon taking into account the valley-orbit coupling. The methodology is applied to three coupled phosphorus donors, arranged in a linear chain and in a triangle, and to six donors arranged in a regular hexagon. The results of the simulations evidence that the valley composition of the single-electron states strongly depends on the geometry of the dopant molecule and its orientation relative to the crystallographic axes of silicon. The electron binding energy of the triatomic linear molecules is larger than that of the diatomic molecule oriented along the same crystallographic axis, but the energy gap between the ground state and the first excited state is not significantly different for internuclear distances from 1.5 to 6.6 nm. Three donor atoms arranged in a triangle geometry have larger binding energies than a triatomic linear chain of dopants with the same internuclear distances. The planar donor molecules are characterized by a strong polarization in favor of the valleys oriented perpendicular to the plane of the molecule. The polarization increases with number of atoms forming the planar molecule

Multiple transitions of the spin configuration in quantum dots

Single electron tunneling is studied in a many electron quantum dot in high magnetic fields. For such a system multiple transitions of the spin configuration are theoretically predicted. With a combination of spin blockade and Kondo effect we are able to detect five regions with different spin configurations. Transitions are induced with changing electron numbers.Comment: 4 pages, 5 figure

Correlation between molecular orbitals and doping dependence of the electrical conductivity in electron-doped Metal-Phthalocyanine compounds

We have performed a comparative study of the electronic properties of six different electron-doped metal phthalocyanine (MPc) compounds (ZnPc, CuPc, NiPc, CoPc, FePc, and MnPc), in which the electron density is controlled by means of potassium intercalation. In spite of the complexity of these systems, we find that the nature of the underlying molecular orbitals produce observable effects in the doping dependence of the electrical conductivity of the materials. For all the MPc's in which the added electrons are expected to occupy orbitals centered on the ligands (ZnPc, CuPc, and NiPc), the doping dependence of the conductivity has an essentially identical shape. This shape is different from that observed in MPc materials in which electrons are also added to orbitals centered on the metal atom (CoPc, FePc, and MnPc). The observed relation between the macroscopic electronic properties of the MPc compounds and the properties of the molecular orbitals of the constituent molecules, clearly indicates the richness of the alkali-doped metal-phthalocyanines as a model class of compounds for the investigation of the electronic properties of molecular systems

Enhanced tunneling across nanometer-scale metal-semiconductor interfaces

We have measured electrical transport across epitaxial, nanometer-sized metal-semiconductor interfaces by contacting CoSi2-islands grown on Si(111) with an STM-tip. The conductance per unit area was found to increase with decreasing diode area. Indeed, the zero-bias conductance was found to be about 10^4 times larger than expected from downscaling a conventional diode. These observations are explained by a model, which predicts a narrower barrier for small diodes and therefore a greatly increased contribution of tunneling to the electrical transport.Comment: 3 pages, 2 EPS-figures; accepted for publication in Appl. Phys. Let

Interaction-Induced Spin Polarization in Quantum Dots

The electronic states of lateral many electron quantum dots in high magnetic fields are analyzed in terms of energy and spin. In a regime with two Landau levels in the dot, several Coulomb blockade peaks are measured. A zig-zag pattern is found as it is known from the Fock-Darwin spectrum. However, only data from Landau level 0 show the typical spin-induced bimodality, whereas features from Landau level 1 cannot be explained with the Fock-Darwin picture. Instead, by including the interaction effects within spin-density-functional theory a good agreement between experiment and theory is obtained. The absence of bimodality on Landau level 1 is found to be due to strong spin polarization.Comment: 4 pages, 5 figure

Non-invasive detection of molecular bonds in quantum dots

We performed charge detection on a lateral triple quantum dot with star-like geometry. The setup allows us to interpret the results in terms of two double dots with one common dot. One double dot features weak tunnel coupling and can be understood with atom-like electronic states, the other one is strongly coupled forming molecule-like states. In nonlinear measurements we identified patterns that can be analyzed in terms of the symmetry of tunneling rates. Those patterns strongly depend on the strength of interdot tunnel coupling and are completely different for atomic- or molecule-like coupled quantum dots allowing the non-invasive detection of molecular bonds.Comment: 4 pages, 4 figure

Scaling of nano-Schottky-diodes

A generally applicable model is presented to describe the potential barrier shape in ultra small Schottky diodes. It is shown that for diodes smaller than a characteristic length $l_c$ (associated with the semiconductor doping level) the conventional description no longer holds. For such small diodes the Schottky barrier thickness decreases with decreasing diode size. As a consequence, the resistance of the diode is strongly reduced, due to enhanced tunneling. Without the necessity of assuming a reduced (non-bulk) Schottky barrier height, this effect provides an explanation for several experimental observations of enhanced conduction in small Schottky diodes.Comment: 4 pages, 4 figures, accepted for publication in Appl. Phys. Lett., some minor additions and correction

Probing a Kondo correlated quantum dot with spin spectroscopy

We investigate Kondo effect and spin blockade observed on a many-electron quantum dot and study the magnetic field dependence. At lower fields a pronounced Kondo effect is found which is replaced by spin blockade at higher fields. In an intermediate regime both effects are visible. We make use of this combined effect to gain information about the internal spin configuration of our quantum dot. We find that the data cannot be explained assuming regular filling of electronic orbitals. Instead spin polarized filling seems to be probable.Comment: 4 pages, 5 figure