 Figure 1(b) shows a similar situation, but this time with the substrate rotated by 180° so that the incident beam is now in the "uphill" direction, and nearly parallel to the surface.In this case, the step faces provide the bonding sites and the deposited material grows along the steps, similar to the step-flow growth mechanism.In order to grow nanowires with a width of fifteen nanometers or less, the deposition temperature for both orientations should be chosen such that the mean free path of the adatoms on the surface is limited to a few nanometers.