Postdocs et thèses

«Étude ab initio de la stabilité de fullerenes de SiC»

• Master and PhD thesis
• Supervisor: Pascal.Pochet  @  cea.fr

The study of low-dimensionality compounds made of silicon and carbon presents a number of interests, both from a theoretical and applied viewpoint.The exceptional physical and mechanical properties of silicon carbide make this material one of the most promising for the emerging fields of micro-electronics and of catalysis, among the others.The goal of this study is to calculate the stability of SiC cage-like structures by means of ab initio calculations using the BigFDT package.

«Simulation atomistique d'un surface plolaire de ZnO»

• Master and PhD thesis
• Supervisor: Pascal.Pochet  @  cea.fr

Ab initio simulation of systems in complex environments, like 2D slabs with a net charge dipole, is a challenging point which can efficiently be addressed only by flexible numerical treatments. BigFDT is a recently developed code, based on wavelets, whose features fit well with this task: the adaptivity of the description ease the treatment of particular environments like surface boundary conditions. ZnO is a frequently studied material in surface science for its wide range of technological applications in nanosciences (eg when doped with Co or Al). It crystallizes in the hexagonal wurtzite structure. One of the crystal termination of a ZnO surface exhibits a charge dipole. The mechanism of electronic stabilization of this polar surface is still theoretically unclear and object of debates. In this project, the trainee will use the BigFDT code for investigating some of the properties of the polar ZnO slab, like structural relaxation and the formation of defects . Besides the important studies of ZnO material preoperties, he will have the possibility to develop experience in ab initio simulations on massive parallel supercomputers, which is a powerful investigation tool widely used.

«Simulation ab initio de la génération des défauts ponctuels via une surface de silicium»

• Master and PhD thesis
• Supervisor: Pascal.Pochet  @  cea.fr

L_Sim laboratory has been developing an electronic structure code based on wavelets. This powerful formalism has the capabilities to handle versatile boundary conditions as surfaces. Furthermore, this tool is adapted to massively parallel and hybrid architecture based on GPU (graphics processor units). The group of Prof. Stefan Goedecker from the University of Basel is a main contributor of our code and has been developing an original method called minima hopping» which can explore minima of the atomic configuration space in order to find the global minimum. This class of methods is used to simulate protein folding. L_Sim laboratory in collaboration with CEA-Leti is studying from many years the formation and activation energies of point defects in semi-conductors such as Si, Ge and SiGe. Stresses and charged defects in silicon are the actual research topics. The goal of this academic training is to use both ab initio and «minima hopping» methods to study the point defect generation initiated at the silicon surface in particular with oxygen adatoms.

«Modélisation atomistique des semi-conducteurs magnétiques GeMn»

• Sujet de thèse
• Encadrant : Pascal.Pochet  @  cea.fr
• PDF français : download

Spintronic is a very active field in condensed matter research. A new magnetic semiconductor GeMn has been recently grown by means of molecular beam epitaxy in the laboratory [Nature Materials, 5 653 (2006)]. It consists of Mn-rich nanocolumns embedded in an almost purely Ge diamond matrix. The structure and the good magnetic properties of these Mn-rich nanocolumns are still puzzling. Our recent Extended X-Ray Absorption Fine Structure study [Applied Physics Letters, Vol. 92 242510 (2008)] reveals that these nano-columns show a complex local structure that does not correspond to any known stable GeMn compound . Electronic structure calculations give information about the magnetic and structural properties of a possible Ge1-xMnx compound which may form inside the nano-columns. The goal of this thesis is to study the kinetics of the nanocolumns growth as well as their transformation into stable compound upon annealing. This will be done by means of Monte Carlo simulations for the MBE growth and of Molecular Dynamics for the nanocolumns annealing. Both simulations will be based on an energy model that will be derived from ab initio calculations on stable compounds that should also reproduce magnetic ordering whitin theses compounds

Two postdoctoral positions in ab initio methods based on wavelets

• Post-doctorats
• Encadrant : Thierry.Deutsch  @  cea.fr

Two postdoctoral positions based at the atomistic Simulation group (L_Sim, CEA-Grenoble) are available to carry out research on ab initio methods based on wavelet.

The first position is sponsorised by the ANR ProHMPT to exploit hybrid architectures (graphics processor units, GPU). The goal is to optimize and does some applications with the BigDFT code on hybrid architectures. A first preliminary version has a speedup of 7 between GPU and a traditional core of the whole code. Target applications will be in the field of growth and kinetics in semi-conductors.

The second position is sponsorised by the ANR SAMSON to use adaptive methods with order N in the BigDFT code. The goal is to re-use previous ab initio calculations when a small displacement is applied to the system. Target applications are to study kinetics of materials or macro-molecules.

«Modélisation des propriétés de transport de nanofils de semiconducteurs»

• Sujet de thèse
• Encadrant : yniquet  @  cea.fr
• PDF français : download

Semiconductor nanowires are attracting much attention due to their promising properties and due to their possible applications in opto- and nano-electronics. The diameter of these nanowires ranges from a few to a few ten of nanometers, while their length can reach microns. These nanostructures provide many opportunities: For example, it is possible to vary the composition of the wires along their axis to introduce quantum dots or tunnel barriers whose width and position are well controlled. These nanowire heterostructures can then be connected to electrodes for charge transport measurements. Many such original experiments are being performed today to explore the potential of semiconductor nanowires for nano-electronics. The physics of semiconductor nanowires is complex and not yet fully understood. In this context, theory and modeling can bring valuable insights into the properties of these structures. The L_Sim laboratory is actively developing tools for the atomistic modeling of the electronic, optical and transport properties of semiconductor nanowires for a few years now (see web page below). We are now seeking a candidate for a PhD thesis on the modeling of the transport properties of semiconductor nanowires, using non-equilibrium Green function methods in an atomistic tight-binding framework. Various aspects such as the electron-phonon coupling, the treatment of contacts and open boundaries conditions, or charged defects might be explored during the thesis. These calculations will help achieving a better understanding of the physics of the nanowires and refining the interpretation of the experiments.