In situ observation of laser induced crystallisation in group IV semiconductors.

Nikolova, Liliya (2014). In situ observation of laser induced crystallisation in group IV semiconductors. Thèse. Québec, Université du Québec, Institut national de la recherche scientifique, Doctorat en sciences de l'énergie et des matériaux, 200 p.

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Résumé

The main objectives of this thesis were to study in situ the structure formation during laser induced crystallisation in group IV semiconductors. We were interested to reveal the sequence of change in the morphology during the growth of the different microstructures, to unveil the shape of the growth front during its propagation, to estimate the velocities of crystallisation and to shed light on the possible mechanisms of growth through estimation of the temperature within the films. For that, we used Dynamic Transmission Electron Microscopy (DTEM) because the technique offers the required combination of spatial and temporal resolutions for in situ visualisation of the phase transformation. We used freestanding and membrane-supported films with the goal of elucidating the influence of the supporting layer on the kinetics and thermodynamics of the phase transformation. In the first part of this work, we studied the crystallisation dynamics in membrane- Supported and self-sustained a-Ge films. We acquired time resolved images of the formation of the central nanocrystalline structure, the evolution of the size and the roughness of the crystallisation front during the two other structures growth i.e. dendritic and layered structure. This study allowed us to estimate the velocities of crystallisation and the evolution of the temperature within the film. We found that in the central region, nucleation from the bulk occurs at temperatures below the melting temperature of the crystalline material. We showed that the dendritic growth occurs by propagation of thin metastable liquid layer in front of the growing crystalline phase and is a result of high thermal instabilities at the growth front. We demonstrated that the growth of the layered structure takes place at temperature below the melting temperature of the crystalline material and proceeds in azimuthal direction, i.e. orthogonal to the direction of the net heat flow. However, the growth mechanisms remains similar to the dendritic growth because the heat diffusion can allow formation of very thin pocket-like layer to be formed and propagate III the azimuthal direction. These findings allowed us to overturn earlier hypothesis based solely on the post-mortem equilibrium investigation of the structure. In the second part of the thesis we studied the crystallisation dynamics in amorphous silicon. We showed that the formed structure is dependent on the fluence of the pump-Iaser. We demonstrated that at the fluences used in this study the process is melt-mediated favoring super-lateral growth. Nanocrystallization occurs at the periphery of the melted zone but with very limited extent. The crystallisation proceeds inward i.e. toward the center of the heated region resulting in the observed long-grained microstructure. Our observations are consistent with the earlier developed theory on super-lateral growth where the nucleation and nanocrystallisation occurs at the periphery of the heated region and the elongated grains are growing along the heat gradient i.e. toward the central part of the heated region. Our study contributed for better understanding of growth of these structures and permitted estimation of the evolution of the temperature within the film.

Type de document:	Thèse Thèse
Directeur de mémoire/thèse:	Rosei, Federico
Mots-clés libres:	laser, cristallisation; morphologie;
Centre:	Centre Énergie Matériaux Télécommunications
Date de dépôt:	04 déc. 2014 21:42
Dernière modification:	16 mars 2016 14:55
URI:	https://espace.inrs.ca/id/eprint/2428

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