Christophe Krzeminski

Molecular Rectifying Diodes from Self-Assembly on Silicon

We demonstrate a molecular rectifying junction made from a sequential self-assembly on silicon. The device structure consists of only one conjugated (π) group and an alkyl spacer chain. We obtain rectification ratios up to 37 and threshold voltages for rectification between −0.3 and −0.9 V. We show that rectification occurs from resonance through the highest occupied molecular orbital of the π group in good agreement with our calculations and internal photoemission spectroscopy. This approach allows us to fabricate molecular rectifying diodes compatible with silicon nanotechnologies for future hybrid circuitries.

Theory of electrical rectification in a molecular monolayer

The current-voltage characteristics in Langmuir-Blodgett monolayers of \ensuremath{\gamma}-hexadecylquinolinium tricyanoquinodimethanide

Effects of strain on the carrier mobility in silicon nanowires.

({\mathrm{C}}_{16}{\mathrm{H}}_{33}\mathrm{Q}\ensuremath{-}3\mathrm{CNQ})

Molecular dynamics simulation of the recrystallization of amorphous Si layers : Comprehensive study of the dependence of the recrystallization velocity on the interatomic potential

sandwiched between Al or Au electrodes is calculated, combining ab initio and self-consistent tight-binding techniques. The rectification current depends not only on the position of the LUMO and HOMO relative to the Fermi levels of the electrodes as in the Aviram-Ratner mechanism, but also on the profile of the electrostatic potential which is extremely sensitive to where the electroactive part of the molecule lies in the monolayer. This second effect can produce rectification in the direction opposite to the Aviram-Ratner prediction.

Energy-Band Engineering for Improved Charge Retention in Fully Self-Aligned Double Floating-Gate Single-Electron Memories

We investigate electron and hole mobilities in strained silicon nanowires (Si NWs) within an atomistic tight-binding framework. We show that the carrier mobilities in Si NWs are very responsive to strain and can be enhanced or reduced by a factor >2 (up to 5×) for moderate strains in the ± 2% range. The effects of strain on the transport properties are, however, very dependent on the orientation of the nanowires. Stretched 100 Si NWs are found to be the best compromise for the transport of both electrons and holes in ≈10 nm diameter Si NWs. Our results demonstrate that strain engineering can be used as a very efficient booster for NW technologies and that due care must be given to process-induced strains in NW devices to achieve reproducible performances.

Graphene buffer layer on Si-terminated SiC studied with an empirical interatomic potential

The molecular dynamics method is applied to simulate the recrystallization of an amorphous/crystalline silicon interface. The atomic structure of the amorphous material is constructed with the method of Wooten, Winer, and Weaire. The amorphous on crystalline stack is annealed afterward on a wide range of temperature and time using five different interatomic potentials: Stillinger-Weber, Tersoff, EDIP, SW115, and Lenosky. The simulations are exploited to systematically extract the recrystallization velocity. A strong dependency of the results on the interatomic potential is evidenced and explained by the capability of some potentials (Tersoff and SW115) to correctly handle the amorphous structure, while other potentials (Stillinger-Weber, EDIP, and Lenosky) lead to the melting of the amorphous. Consequently, the interatomic potentials are classified according to their ability to simulate the solid or the liquid phase epitaxy.

Optical absorption of silicon nanowires

We present a new fully self-aligned single-electron memory with a single pair of nano floating gates, made of different materials (Si and Ge). The energy barrier that prevents stored charge leakage is induced not only by quantum effects but also by the conduction-band offset that arises between Ge and Si. The dimensions and position of each floating gate are well-defined and controlled. The devices exhibit a long retention time and single-electron injection at room temperature.

Understanding of the Retarded Oxidation Effects in Silicon Nanostructures

The atomistic structure of the graphenebuffer layer on Si-terminated SiC is investigated using a modified version of the environment-dependent interatomic potential. The determination of the equilibrium state by the conjuguate gradients method suffers from a complex multiple-minima energy surface. The initial configuration is therefore modified to set the system in specific valleys of the energy surface. The solution of minimal energy forms a hexagonal pattern composed of stuck regions separated by unbonded rods that release the misfit with the SiC surface. The structure presents the experimental symmetries and a global agreement with an ab initio calculation. It is therefore expected that the interatomic potential could be used in classical molecular dynamics calculations to study the graphene growth.

Theoretical characterization of the electronic properties of extended thienylenevinylene oligomers

We report on simulations and measurements of the optical absorption of silicon nanowires (NWs) versus their diameter. We first address the simulation of the optical absorption based on two different theoretical methods: the first one, based on the Green function formalism, is useful to calculate the scattering and absorption properties of a single or a finite set of NWs. The second one, based on the finite difference time domain (FDTD) method, is well-adapted to deal with a periodic set of NWs. In both cases, an increase of the onset energy for the absorption is found with increasing diameter. Such effect is experimentally illustrated, when photoconductivity measurements are performed on single tapered Si nanowires connected between a set of several electrodes. An increase of the nanowire diameter reveals a spectral shift of the photocurrent intensity peak towards lower photon energies that allow to tune the absorption onset from the ultraviolet radiations to the visible light spectrum.

Efficient photogeneration of charge carriers in silicon nanowires with a radial doping gradient

A new understanding of the retarded or self-limited oxidation phenomenon observed during the oxidation of silicon nanostructures is proposed. The wet thermal oxidation of various silicon nanostructures such as nanobeams, concave/convex nanorings and nanowires exhibits an extremely different and complex behaviour. Such effects have been investigated by the mode\-lling of the mechanical stress generated during the oxidation process explaining the retarded or quasi self-limited regime. The proposed model describes the oxidation kinetics of silicon nanowires down to a few nanometers while predicting reasonable and physical stress levels at the Si/SiO