Recent advances in nanotechnology have offered the hope of extending Moore’s law of large scale integration of semiconductor device circuits to nanodevices. Nanostructures based on Si will play an important role in future semiconductor technology. In situ transmission electron microscopy is a powerful means to understand the kinetics of nanoscale reactions. In this talk, I will present in-situ transmission electron microscopy of solid-state chemical reactions in the nanowire-based nanostructures and the control of the formation of nano-heterostructures.

Transition metal silicide nanowires are strong candidates for circuit elements in one-dimensional nanodevices, with applications including ohmic contacts, Schottky barriers, gate electrodes and interconnects. Silicidation reactions take place at the point contact between adjacent nanowires or nanoparticles, and high resolution imaging allows the reaction to be followed in real time as each layer of Si is transformed. It is possible to control the formation of the metal silicide accurately enough to fabricate nano-gap heterostructures, where silicide regions are separated by just a few atomic planes of Si. The atomic-level details of the growth process can also be examined directly, and I will show axial epitaxial stepwise growth of CoSi2, NiSi, and NiSi2 in silicon nanowires. This growth process is unique, in that each atomic plane of silicide nucleates at the center of the nanowire cross-section in a homogeneous nucleation event that had been theoretically predicted but not observed previously. The oxidized surface structure of the nanowire is of key importance in this nucleation and growth process, and I will discuss the differences in growth mechanism and morphology when silicidation reactions are carried out in-situ on nanowires with atomically clean surfaces.

Recent experimental and theoretical studies show that, in metal-catalyzed nanowire growth, sharp interfaces within nanowires can be achieved by using solid catalysts (the VSS growth mode). Solid catalysts have low solubility for the growth species and hence do not act as a reservoir when switching between materials. We propose a method using solid silver-based catalysts to grow abrupt Ge layers embedded in Si nanowires, and demonstrate the results using in situ transmission electron microscopy. Ag is completely miscible with Au without intermetallic compounds, so homogeneous Ag-Au alloys can be formed and act as catalysts. Ag-Au alloys have two advantages compared to Au or solid alloy catalysts: The eutectic temperatures of Ag-Au alloys with Si are higher than pure Au with Si so that, compared to pure Au, solid alloy catalysts are stable at higher temperatures where growth rates are faster. Furthermore, the resistance of Ag to oxidation makes it easier to work with than say Al alloys and potentially allows scaling up to standard CVD growth systems. We will show that the temperatures at which liquid and solid Ag-Au catalysts are stable, and hence growth occurs by the VLS and VSS processes, are tuned by adjustment of the Ag/Au ratio. In situ movies show that VSS Si growth occurs by ledge propagation at the alloy/silicon interface, similarly to AlAu alloys. We will discuss the details of the growth process, in particular periodic changes in the catalyst shape and growth interface morphology as individual ledge flow events occur. We can switch growth mechanisms controllably between VLS and VSS, enabling optimization of growth rates and the formation of abrupt interfaces and thin layers, thus opening up a promising route towards control of nanowire and heterostructure growth and morphology.

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