Abstract
By direct observations of transmission electron microscopy (TEM), irreversible morphological transformations of as-deposited amorphous Au/Si multilayer (a-Au/a-Si) were observed on heating. The well arrayed sequence of the multilayer changed to zigzag layered structure at 478 K (=Tzig). Finally, the zigzag structure transformed to Au nanoparticles at 508 K. The distribution of the Au nanoparticles was random within the thin film. In situ X-ray diffraction during heating can clarify partial crystallization Si (c-Si) in the multilayer at 450 K (= ), which corresponds to metal induced crystallization (MIC) from amorphous Si (a-Si) accompanying by Au diffusion. On further heating, a-Au started to crystallize at around 480 K (=Tc) and gradually grew up to 3.2 nm in radius, although the volume of c-Si was almost constant. Continuous heating caused crystal Au (c-Au) melting into liquid AuSi (l-AuSi) at 600 K (= ), which was lower than bulk eutectic temperature ( ). Due to the AuSi eutectic effect, reversible phase transition between liquid and solid occurred once temperature is larger than . Proportionally to the maximum temperatures at each cycles (673, 873 and 1073 K), both and Au crystallization temperature approaches to . Using a thermodynamic theory of the nanoparticle formation in the eutectic system, the relationship between and the nanoparticle size is explained.
Highlights
In metal silicides, metal-induced crystallization (MIC) [1]-[5] in thin films is well known to be a fundamental phenomenon relating to low eutectic point
Au nanoparticles existed in the thin film, where the nanoparticles randomly locate inside Si
Irreversible morphological transformation occurred accompanying by crystallization of the Au nanoparticles (c-Au)
Summary
Metal-induced crystallization (MIC) [1]-[5] in thin films is well known to be a fundamental phenomenon relating to low eutectic point. Rather than viewing this phenomenon from a solid-phase reaction perspective, studies from the diffusion process view point are necessary due to the fact that electric screening weakens the covalent bonds of the amorphous Si (a-Si). Surface sensitive diffusion is separated into Fickian-/non-Fickian-type, which is directly related to the surface stability The latter described in 2) is characterized by a variety of crystal structures of the AuSi particles, which are induced in hydrogenated amorphous Si (a-Si:H) [8] and c-Au/a-Si thin film [9]. At each thermal cycles, melting point of c-Au nanoparticle ( TmNP ) depends on the nanoparticle size of c-Au as predicted theoretically
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