Article

Crystallisation of amorphous germanium thin films.

Grup de Nanomaterials i Microsistemes, Departament de Física, UAB, 08193 Bellaterra, Spain.
Journal of Nanoscience and Nanotechnology (Impact Factor: 1.15). 06/2009; 9(5):3013-9. DOI: 10.1166/jnn.2009.225
Source: PubMed

ABSTRACT By combining cross-sectional transmission and scanning electron microscopy with Raman scattering we have investigated the mechanism of nanocrystal formation in ultrathin amorphous SiO2/Ge/SiO2 trilayers grown by e-beam evaporation as a function of annealing temperature and a-Ge layer thickness. We observe that with decreasing a-Ge thickness the amorphous-to-crystalline (a-to-c) transition occurs at considerably higher temperatures, even avoiding crystallisation for very thin films below 2 nm thickness. Furthermore, we demonstrate that the formation of Ge nanocrystals by annealing at around 900 degrees C takes place driven by a liquid-mediated mechanism. As indicated by the observed microstructure, the metallic liquid film dewets from the surface forming droplets that upon cooling and under the influence of the SiO2 capping layer, solidify into barrel-type nanocrystals.

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    ABSTRACT: Annealing of amorphous Si/SiO 2 or Ge/SiO 2 multilayers produces nanocrystals embedded between oxide interfaces. It is found that the crystallization temperature is strongly enhanced by the presence of the oxide interfaces and follows an exponential law. The crystallization temperature increases rapidly with decreasing Si layer thickness, and a nonstoichiometric interface decreases the crystallization temperature compared to a stoichiometric interface of the same thickness. A model is presented that takes into account the interface energies, the thickness of the layer, the melting point of the system, and the crystallization temperature of the thick amorphous layer. The evidence for a critical crystallization radius and the influence of deviations from a perfect stoichiometric interface are discussed.
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