Normand Mousseau
Professeur de physique et titulaire de la Chaire UdeM
Matériaux complexes, énergie et ressources naturelles

Propriétés structurales, cinétiques et thermodynamiques des matériaux amorphes et désordonnés

Les semiconducteurs amorphes sont un de mes principaux thèmes de recherche 25 ans. À l’aide de méthodes diverses, telles que l’algorithme de Wooten, Winer et Weaire, ART, ART nouveau, ART cinétique et dynamique moléculaire, et en collaboration avec de nombreux collègues, j’ai travaillé depuis 10 ans à la caractérisation de la structure de ces matériaux, ainsi que des mécanismes de diffusion et de relaxation (Kerrache et al, 2011 ; Joly et al. 2013). Je me suis également intéressé à l’évolution de la surface d’énergie de matériaux amorphes et désordonnés, démontrant un excellent accord avec les mesures de diffraction par rayon X, de calorimétrie et de nanocalorimétrie (voir, par exemple, Kallel et al., PRL 2010 ; Béland et Mousseau, PRB 2013).

À plusieurs reprises, nos résultats ont permis d’avancer sur des questions débattues depuis longtemps. Par exemple, nous avons pu démontrer que, contrairement à une interprétation tirée des mesures expérimentales, les lacunes ne diffusent pas en bloc dans le silicium amorphe, mais qu’elles demeurent piégées ou se dissocient rapidement au moment de sauter (Joly et al., PRB 2013). Ces résultats nous forcent à repenser la notion de défauts dans les matériaux désordonnés. Grâce des simulations d’ART cinétique couvrant plus d’une seconde, nous avons aussi pu caractériser directement les mécanismes atomiques responsables de la relaxation de systèmes désordonnés, proposant un mécanisme en deux temps pour ces systèmes (Béland et al., PRL 2013). Sortant des approches traditionnelles, nous avons également démontré, parmi mes diverses contributions à ce domaine, qu’une phase liquide à faible densité observée dans de nombreuses simulations numériques était très sensible aux détails des champs de force utilisés, remettant en question la validité de ces observations difficilement vérifiables expérimentalement (Beaucage et Mousseau, 2005).

Quelques-uns de mes travaux sur le sujet



  • J. - F. Joly, L. K. Béland, P. Brommer, N. Mousseau, Contribution of vacancies to relaxation in amorphous materials: A kinetic activation-relaxation technique study, Physical Review B 87, 144204 (2013).
    Résumé : The nature of structural relaxation in disordered systems such as amorphous silicon (a-Si) remains a fundamental issue in our attempts at understanding these materials. While a number of experiments suggest that mechanisms similar to those observed in crystals, such as vacancies, could dominate the relaxation, theoretical arguments point rather to the possibility of more diverse pathways. Using the kinetic activation-relaxation technique, an off-lattice kinetic Monte Carlo method with on-the-fly catalog construction, we resolve this question by following 1000 independent vacancies in a well-relaxed a-Si model at 300 K over a timescale of up to one second. Less than one percent of these survive over this period of time and none diffuse more than once, showing that relaxation and diffusion mechanisms in disordered systems are fundamentally different from those in the crystal.
    Mots-clés : Amorphe.


  • A. Kerrache, N. Mousseau, L. J. Lewis, Amorphous silicon under mechanical shear deformations: Shear velocity and temperature effects, Physical Review B 83, 134122 (2011).
    Résumé : Mechanical shear deformations lead, in some cases, to effects similar to those resulting from ion irradiation. Here we characterize the effects of shear velocity and temperature on amorphous silicon (a-Si) modeled using classical molecular-dynamics simulations based on the empirical environment-dependent interatomic potential (EDIP). With increasing shear velocity at low temperature, we find a systematic increase in the internal strain leading to the rapid appearance of structural defects (fivefold-coordinated atoms). The impacts of externally applied strain can be almost fully compensated by increasing the temperature, allowing the system to respond more rapidly to the deformation. In particular, we find opposite power-law relations between the temperature and the shear velocity and the deformation energy. The spatial distribution of defects is also found to depend strongly on temperature and strain velocity. For low temperature or high shear velocity, defects are concentrated in a few atomic layers near the center of the cell, while with increasing temperature or decreasing shear velocity, they spread slowly throughout the full simulation cell. This complex behavior can be related to the structure of the energy landscape and the existence of a continuous energy-barrier distribution.
    Mots-clés : Amorphe.


  • H. Kallel, N. Mousseau, F. Schiettekatte, Evolution of the Potential-Energy Surface of Amorphous Silicon, Physical Review Letters 105, 045503 (2010).
    Résumé : The link between the energy surface of bulk systems and their dynamical properties is generally difficult to establish. Using the activation-relaxation technique, we follow the change in the barrier distribution of a model of amorphous silicon as a function of the degree of global relaxation. We find that while the barrier-height distribution, calculated from the initial minimum, is a unique function that depends only on the level of relaxation, the reverse-barrier height distribution, calculated from the final state, is independent of global relaxation, following a different function. Moreover, the resulting gained or released energy distribution is a simple convolution of these two distributions indicating that the activation and relaxation parts of the elementary relaxation mechanism are completely independent. This characterized energy landscape can be used to explain nanocalorimetry measurements.
    Mots-clés : Amorphe.
    Pièce jointe Full Text PDF 302 ko (source)


  • G. T. Barkema, N. Mousseau, High-quality continuous random networks, Physical Review B 62, 4985-4990 (2000).
    Résumé : The continuous random network (CRN) model is an idealized model for perfectly coordinated amorphous semiconductors. The quality of a CRN can be assessed in terms of topological and configurational properties, including coordination, bond-angle distributions, and deformation energy. Using a variation on the sillium approach proposed 14 years ago by Wooten, Winer, and Weaire, we present 1000-atom and 4096-atom configurations with a degree of strain significantly less than the best CRN available at the moment and comparable to experimental results. The low strain is also reflected in the electronic properties. The electronic density of state obtained from ab initio calculation shows a perfect band gap, without any defect, in agreement with experimental data.
    Mots-clés : Amorphe.
mercredi 2 juillet 2014

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