Nanowires: Fourth NW sample

Faebian Bastiman

For our 4th NW sample we took a little logical leap of faith based on our observations of the 3rd NW sample and the trend established by the first 3 in general. The decision was made to increase the growth temperature by 40°C AND to double the As flux compared to the 3rd NW sample. The results can be seen in Figure 1.

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Figure 1: SEM of 4th NW sample

 

The NWs are now around 1000 nm in length but possess a slight small (30nm) diameter. They are only subtly reverse taper (ice cream cone shaped). The NW density has also increased accompanied by a reduction in the parasitic 2D growth compared to the first 3 samples. There are however many tilted NWs, and that is something we wish to eliminate.

On the whole we have taken a big step in the right direction. The result it getting close to our desired NW shape and density profile, whilst the parasitic growth is reducing. Where to go next? Well, it seems that the amount of parasitic growth is directly related to growth temperature, so in the first instance it would be good to increase the growth temperature by 20°C and see what happens. In doing so we will also need to increase the As flux, since the required As flux is strongly related to the growth temperature, especially at these elevated temperatures (>600°C). On the other hand, we have grown 4 samples and not hit an “edge” yet. An edge is a condition that marks the boundary of a NW variable map. Edges are as even more useful that NW data points at this moment, because they define the boundary conditions (max and min): Tgrow, As flux, growth rate, oxide anneal temperature. This concept is explored more in the article Nanowires: Edgework.

For out 5th sample, let’s increase the growth temperature by 20°C but keep the As flux constant. In so doing we should reach a condition where As is limited AND parasitic growth is reduced/eliminated. The result could be interest. You can see how the 5th sample turned by reading the following article: Nanowires: Fifth NW sample.

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