text: Si(100) is the way crystallographers have chosen to describe the crystal plane that is on the surface.  In a crystal, the positions of the atoms form a regular lattice structure, and the smallest unit of that structure that repeats itself is called a unit cell.  Si(100) tells you that the surface of the wafer is a plane that crosses the x-axis on the surface of a unit cell, and is parallel to the yz-plane.  This information can be used to determine the spacing between atoms on the surface, and their arrangement i.e. square, hexagonal, etc.  From the spacing and configuration, you can determine what materials will form a good interface with that surface-- things with a lattice mismatch of a few percent usually work well.