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Another aspect of the silicon deposition is the formation of steps. Steps form as the silicon is deposited on the lattice. There are two types of growth: island growth and step flow growth. For island growth, atoms find a place to bond on a terrace. Other atoms join because they can fill more of their bonds if they attach to three atoms instead of just two by attaching anywhere on the surface. Eventually an island forms in the middle of the terrace. As the island grows it develops steps and terraces of its own. Step flow growth is the growth of steps. Silicon atoms like to bond to steps because they are an energetically favorable position. An atom bonding at the base of a step bonds with three other atoms and only has one bond free. It is much more unlikely for an atom to stick in the middle of a terrace with only two bonds available. With each silicon atom that joins the step the step grows.
The dimer rows on the surface of the lattice also affect the formation of the steps. There are two types of steps: A and B. The A-steps are always smooth while the B-steps can be jagged. This difference is caused by the dimer rows (see Figure 2). By looking at the illustration we can see that the dimer rows form little channels along the surface of the silicon. It is easier for the silicon atoms to move along the dimer rows than to hop from one dimer row onto another. From the illustration you can see that the dimer rows of the A-step run perpendicular to the dimer rows of the B-step. This change in direction of the dimer rows is a product of the unit cell of silicon and the (001) surface orientation. The A-step is along the side of the dimer rows. It is more difficult to extend the A-step by another row because an entire row of atoms is required. In comparison, the B-step can be extended with the addition of only two atoms. This easy increase is a product of the dimer rows. Only two atoms are needed to extend the length of any dimer row. This also causes the jaggedness of the B-step. Because only two atoms are required, a small portion of the B-step may extend at a time. In Figure 3, notice how the dimer rows are aligned with the B-step.
Chris Siefert and Molly Moore 2002
