Predictive design of heat management structures
Heat management is a paramount challenge in many cutting edge technologies, including new GaN electronic technology, turbine thermal coatings, resistive memories, or thermoelectrics. Further progress requires the help of accurate modeling tools that can predict the performance of new complex materials integrated in these increasingly demanding novel devices. However, there is currently no general predictive approach to tackle the complex multiscale modeling of heat flow through such nano and micro-structured systems. The state of the art, our predictive approach “ShengBTE.org”, currently covers the electronic and atomistic scales, going directly from them to predict the macroscopic thermal conductivity of homogeneous bulk materials, but it does not tackle a mesoscopic structure. This project will extend this predictive approach into the mesoscale, enabling it to fully describe thermal transport from the electronic ab initio level, through the atomistic one, all the way into the mesoscopic structure level, within a single model. The project is a 6 partner effort with complementary fields of expertise, 3 academic and 3 from industry. The widened approach will be validated against an extensive range of test case scenarios, including carefully designed experimental measurements taken during the project. The project will deliver a professional multiscale software permitting, for the first time, the prediction of heat flux through complex structured materials
of industrial interest. The performance of the modeling tool will be then demonstrated in an industrial setting, to design a new generation of substrates for power electronics based on innovating layered materials. This project is expected to have large impacts in a wide range of industrial applications, particularly in the rapidly evolving field of GaN based power electronics, and in all new technologies where thermal transport is a key issue.
Development of new processes for the fabrication of advanced interconnect structures of solar cells
The fabrication of solar cells with high performances at a reduced cost is a key challenge addressed by many research institutions and industrials worldwide. Many technological solutions are being investigated. Among them, a promising approach consists in forming narrower metal lines to limit shadowing of active areas of the cells. This work aims at replacing serigraphy by new fabrication processes able to reduce line width. For this purpose, the conducting substrate is coated by an insulating mask in which the lines are defined. The metal is then directly plated selectively onto the weakly conducting portions of the substrate, i.e. the lines, using electrolytic or electroless reactions. The process conditions will be adapted with regard to the nature of the initial conducting surfaces.
Modelling of interstitial cluster evolution in body-centered cubic metals after helium implantation
Under irradiation, structural materials inside nuclear reactors undergo changes in mechanical properties, which result from the formation of point defect clusters, such as cavities (clusters of vacancies) and interstitial dislocation loops (clusters of self-interstitial atoms). Understanding the formation processes of such clusters is thus of prime importance. Recently, three-dimensional interstitial clusters, known as C15 clusters, have been shown theoretically to be highly stable in iron. In order to detect such clusters experimentally, an idea is to make them grow, as shown for dislocation loops after helium implantation. This approach will be carried out experimentally in various bcc metals in the framework of the ANR project EPigRAPH, in collaboration with Chimie ParisTech, GEMaC and LPS.
In this project, the following modelling tasks will be performed by the postdoc:
- Electronic structure calculations will be done to obtain the energetic properties of point defects and point defect clusters in the bcc metals envisaged in the project.
- These data will then be used to parameterize a kinetic model based on cluster dynamics. This formalism is particularly well adapted to simulate the evolution of point defect clusters over long physical times.
Large-area processing and design of functional piezoelectric nanomaterials for flexible sensors and systems
CEA LETI develops innovative highly flexible strain sensors which exploit the piezoelectric properties of self-organized gallium nitride nanowires. The fabrication steps are basically: i) nanowire growth, ii) nanowire assembly, iii) encapsulation, iv) contacting. First demonstrators with small active area (1.5 cm²) have already been achieved using the Langmuir Blodgett (LB) technique for the assembly of nanowires. The present project is concerned with the scaling-up of the assembly process over large surface areas, as well as controlled patterning of nanowire assemblies in 1D and 2D by using an innovative CEA LITEN roll-to-roll technology called Boostream® which has the same functionalities as LB in its basic function.
The aim of the post doc is to develop a new building block for the Boostream® equipment enabling a controlled assembly of wires with a pre-defined design. The candidate will carry out studies to optimize the wire assembly, develop the process of film patterning and fabricate, integrate and characterize GaN nanowire piezoelectric transducers with dimensions of 15x15 cm².
More generally, this post doc will also provide the opportunity to develop a generic knowledge to manipulate micro or nano wires or fibers giving new solutions in various fields such as surface structuration, electronic skin, energy...
Minimizing modifications at III-V pattern sidewalls after plasma etching for heterointegrated optoelectronics and nonlinear photonics
This project will focus on understanding plasma-induced damage at the sidewalls of micro-nano-patterned III-V semiconductors to find relevant technological solutions capable to minimize this damage. There is a clear need of knowledge on by which mechanisms and to what extent the plasma etching process modifies the III-V pattern sidewalls and the consequences it has on the device optical performances. The selected III-V semiconductor will be aluminium gallium arsenide which exhibits excellent optoelectronic properties and strong nonlinear parametric gain.
The student will be mainly focused on understanding how the key plasma process parameters influence the structural and chemical changes at the III-V sidewalls, as well as changes of optical properties. This will require the development of a methodology for a 3D quantitative characterization of the sidewalls at the nanoscale, based on Auger microscopy and cathololuminescence. The main objective will be to correlate plasma-induced structural defects and modifications of the optoelectronics properties. The second step will consist in developing optimized plasma etching processes for III-V semiconductors, exploring alternative plasma technologies. You will also be involved in the development of processes for restoring and passivating the AlGaAs sidewalls.
Development of new processes for the fabrication of advanced interconnect structures of solar cells
The fabrication of solar cells with high performances at a reduced cost is a key challenge addressed by many research institutions and industrials worldwide. Many technological solutions are being investigated. Among them, a promising approach consists in forming narrower metal lines to limit shadowing of active areas of the cells. This work aims at replacing serigraphy by new fabrication processes able to reduce line width. For this purpose, the conducting substrate is coated by an insulating mask in which the lines are defined. The metal is then directly plated selectively onto the weakly conducting portions of the substrate, i.e. the lines, using electrolytic reactions. The process conditions will be adapted with regard to the nature of the initial conducting surfaces.
Multi-scale modelling of the structure and mobility of small defect clusters in metals
Recently, we have proposed a three dimensional periodic structure for self-interstitial clusters in body-centered-cubic metals, as opposed to the conventional two dimensional loop morphology [1]. The underlying crystal structure corresponds to the C15 Laves phase. Using Density Functional Theory and interatomic potential calculations, we have demonstrate that in a–iron these C15 aggregates are highly stable and immobile and that they exhibit large antiferromagnetic moments. They form directly in displacement cascades and they can grow by capturing self-interstitials. They thus constitute an important new element to account for when predicting the microstructural evolution of iron base materials under irradiation.
Despite their low concentration, these clusters are expected to play a crucial role in the behavior of iron and ferritic steels under irradiation and many questions remain to be elucidate: which clusters are the most stable in intermediate sizes, which are the reaction pathways which link the traditional clusters to new ones, how the new clusters interact with the dislocation loops, which are the effects of finite temperatures etc
CIGS solar cells optimized for energy harvesting applications in indoor environments
The goal of this post-doctoral fellowship is to develop solar cells based on CIGS thin films, for energy harvesting applications (powering of small electronic autonomous devices). This research project will aim at optimizing the solar cell performances in indoor environments, i.e., under low light intensity. The post-doctoral fellow will be involved in CIGS thin film elaboration by physical vapour deposition, film characterization, solar cell realization and test.