RESUMO
Silicon nanowires have been grown by a plasma-assisted vapour-liquid-solid method using tin as the catalyst. Transmission electron microscopy in the [12[combining macron]10] zone axis shows that the diamond hexagonal (P63/mmc) crystal structure is present in several nanowires. This is the first unambiguous proof of the natural occurrence of this metastable phase to our knowledge.
RESUMO
The crystallization enthalpy measured in a large series of amorphous silicon (a-Si) materials varies within a factor of 2 from sample to sample (Kail et al 2011 Phys. Status Solidi RRL 5 361). According to the classical theory of nucleation, this variation should produce large differences in the crystallization kinetics leading to crystallization temperatures and activation energies exceeding 550 °C and 1.7 eV, respectively, the 'standard' values measured for a-Si obtained by self-implantation. In contrast, the observed crystallization kinetics is very similar for all the samples studied and has no correlation with the crystallization enthalpy. This discrepancy has led us to propose that crystallization in a-Si begins in microscopic domains that are almost identical in all samples, independently of their crystallization enthalpy. Probably the existence of microscopic inhomogeneities also plays a crucial role in the crystallization kinetics of other amorphous materials and glasses.
Assuntos
Silício/química , Varredura Diferencial de Calorimetria , Cristalização , Cinética , Temperatura , TermodinâmicaRESUMO
Thermal crystallization experiments have been carried out on nanocrystalline silicon films. From the thermal activation of the process, it is concluded that the amorphous phase crystallizes by solid phase epitaxy around the preexisting crystallites. However, and in contrast with the usual epitaxial crystallization of wafers partially amorphized by ion implantation or ball-milled powders, the epitaxial growth is inhibited for most of the amorphous-crystalline interface. Our analysis indicates that a small fraction of the interface contributes to the epitaxial growth. Although the influence of oxidation and of Si-H groups located at the interface cannot be completely excluded, this behavior is explained in terms of a high density of microvoids located at the a-c interface. This result has implications for the models of electrical conduction of nc-Si:H.
RESUMO
Silicon nanowires have been grown with gallium as catalyst by plasma enhanced chemical vapor deposition. The morphology and crystalline structure has been studied by electron microscopy and Raman spectroscopy as a function of growth temperature and catalyst thickness. We observe that the crystalline quality of the wires increases with the temperature at which they have been synthesized. The crystalline growth direction has been found to vary between <111> and <112>, depending on both the growth temperature and catalyst thickness. Gallium has been found at the end of the nanowires, as expected from the vapor-liquid-solid growth mechanism. These results represent good progress towards finding alternative catalysts to gold for the synthesis of nanowires.
