The Si (100) surface: A theoretical study of the unreconstructed surface

The potential, charge density, and energy spectrum of an ideal unreconstructed Si (100) surface is obtained self-consistently. Two bands of surface states that arise from broken surface bonds are found in the energy gap between the valence and conduction bands. One band ($\lesssim 1$ eV wide) has its electron density localized about a line through and normal to the surface atoms, with a node on the atoms; the other band (∼ 2.5 eV wide) has its charge located around a line through a row of surface atoms, again having nodes on the surface atoms. The role these surface states play in surface reconstruction as well as in understanding the surface-state spectrum of stepped Si (111)-surface bands is discussed. Additional surface states within the valence band are found and their significance discussed.