Yann Leroy

Semianalytical model of tunneling in nanocrystal-based memories

The purpose of this paper is to study single electron charging of a floating gate composed of nanocrystals in a metal-oxide-semiconductor transistor. We present a three-dimensional model of electron tunneling into quantum islands that are spherical in shape. This model can be numerically solved through a two-dimensional finite element approach. In this way, extensive and accurate numerical experimentations can be carried out due to the reduced computer time cost. The curves of tunneling time versus bias voltage exhibit complex serrated shapes, related to both the energy subbands of the channel and the energy states of the nanocrystal. The results are discussed for different channel doping densities.

Theoretical characterization of the topology of connected carbon nanotubes in random networks

In recent years, a lot of attention has been paid to carbon nanotube (CNT) networks and their applications to electronic devices. Many studies concentrate on the percolation threshold and the characterization of the conduction in such materials. Nevertheless, no theoretical study has yet attempted to characterize the CNT features inside finite size CNT networks. We present a theoretical approach based on geometrical and statistical considerations. We demonstrate the possibility of explicitly determining some relations existing between two neighbor CNTs and their contact efficiency in random networks of identical CNTs. We calculate the contact probability of rigid identical CNTs and we obtain a probability of 0.2027, which turns out to be independent of the CNT density. Based on this probability, we establish also the dependence of the number of contacts per CNT as a function of the CNT density. All the theoretical results are validated by very good agreement with Monte Carlo simulations.

Numerical model of a single nanocrystal devoted to the study of disordered nanocrystal floating gates of new flash memories

The improvement of our model concerning a single nanocrystal that belongs to a nanocrystal floating gate of a flash memory is presented. In order to extend the gate voltage range applicability of the model, the 3D continuum of states of either metallic or semiconducting electrodes is discretized into 2D subbands. Such an approach gives precise information about the mechanisms behind the charging or release processes of the nanocrystal. Then, the self-energy and screening effects of an electron within the nanocrystal are evaluated and introduced in the model. This enables a better determination of the operating point of the nanocrystal memory. The impact of those improvements on the charging or release time of the nanocrystal is discussed.

Organic solar cells: a rigorous model of the donor-acceptor interface for various bulk heterojunction morphologies

Theoretical studies of organic solar cells are mostly based on one dimensional models. Despite their accuracy to reproduce most of the experimental trends, they intrinsically cannot correctly integrate the effects of morphology in cells based on a bulk heterojunction structure. Therefore, accounting for these effects requires the development of two dimensional models, in which donor and acceptor domains are explicitly distinct. In this context, we propose an analytical approach, which focuses on the description of the interface between the two domains. Assuming pinned charge transfer states, we rigorously derive the corresponding boundary conditions and explore the differences between this model and other existing models in the literature for various morphologies of the active layer. On one hand, all tested models are equivalent for an ideal interdigitated bulk heterojunction solar cell with a planar donor-acceptor interface, but divergences between the models rise for small sizes of the donor domain. On ...

Organic Solar Cells: Extraction of Physical Parameters by Means of Markov Chain Monte Carlo Techniques

This paper presents an alternative approach to obtain, from experimental measurements, physical parameters of organic solar cells associated with a given model. In order to get rid of the limitations of common fitting methods, we use a specific Markov chain Monte Carlo technique. This method is applied to a two-dimensional model of an organic solar cell. Measurements carried out under dark and one sun conditions, from two complementary cells, allow access to more reliable values of the active layer parameters. The corresponding set of parameters generates JV -curves in excellent agreement with the measurements for a range of different illumination intensities. Similar extractions are applied on temperature-dependent parameters, from experimental data acquired at various temperatures. As the simulation results reproduce the measurement data rather well, we show that this approach can also be useful to test or determine the governing law associated with some of the temperature-dependent parameters. In addition, analyzing the simulated responses of the model allows the identification of model limitations. The approach discussed in this paper, not specific to organic solar cells, can be applied to a large range of condensed matter topics.

Modeling of the electrostatic coupling between nanocrystals of a disordered nanocrystal floating gate memory

This paper presents a realistic model that explicitly takes into account the electrostatic coupling between the nanocrystals of a disordered layer constituting the floating gate of a non-volatile memory. A statistical study of the neighborhood of a given nanocrystal is carried out, leading to the mean number of neighboring nanocrystals as a function of the radius of the central nanocrystal. We show that the empty neighborhood of every nanocrystal can be represented by an equivalent torus ring in the previous model of a single nanocrystal. Then the effects of charged nanocrystals are taken into account by an appropriate rigid shift of the energy levels of the central nanocrystal. The proposed model is validated by statistical comparisons with exact 3D computations, and the influence of the electrostatic coupling is analyzed and discussed.

Charging Model of a Si Nanocrystal-based Floating Gate in a Quantum Flash Memory

We propose a theoretical study for charging the floating gate composed of Si nanocrystals (NCs), in a non-volatile flash memory. Only a few electrons tunnel from the channel of a metal-oxide-semiconductor transistor into the two-dimensional array of nanocrystals. Our model is based on the geometrical and physical properties of the device, in order to take the dispersion of the relevant parameters into account: NC radii, inter-NC distances, tunnel oxide and gate oxide thicknesses. The energy subbands of the channel are explicitly included, together with the doping density. This three-dimensional model of electron tunneling into a NC is numerically solved through a two-dimensional finite element approach, which allows extensive numerical experimentations. The tunneling times to charge a single NC or the whole NC floating gate are evaluated in a finer detail, and the influence of the dispersion of the relevant parameters is discussed. Such a study may help the experimentalists to build efficient quantum flash memories.