Morphology Control by Substrate Temperature

One of the most important issues of nanomaterial growth is the control of the morphology. This can be done by varying different process parameters. The effect of the growth temperature on the structure of the Si nanowires has been studied systematically [32, 33]. 

The Si nanostructures produced in this temperature region can be divided to two groups (1) Region I (1200 1100 °C) forming nanostructures by the metal catalyst VLS process (indicated by the presence ofmetallic caps), (2) Region II (1100 850 °C) where the OAG process is dominant (the nanostructures do not have metallic caps). HRTEM analysis of samples prepared in region I (temperatures of 1190, 1160, and 1130 °C denoted II, 12 and 13) and region II (temperatures 1050, 950, and 900 °C denoted III, II2 and II3) revealed the structure evolution at different temperatures. 

In region II the diameter of the Si nanostructures is constant ( 20 nm), but their shape changes from tadpole-like through chain-like to wire-like as the temperature decreased from 1050, through 950, to 900 °C respectively. The wires have SiO,c caps rather than metallic caps, and are encapsulated in a Si02 sheath, all indicative of an OAG process. 

We will now explain these results as a combination of (1) the dominant growth process (metal-assisted VLS or OAG) and (2) extended annealing of Si nanowires at high temperatures. 

Region I is characterized by a metal catalyst VLS growth, as indicated by the metal caps on top of the nanostructures. The diameter of the nanowires in this growth process is determined by the diameter of the liquid alloy droplet at their tips. Metal silicide clusters of different sizes are present in the flowing gas above 

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