Energetics and Kinetics of Surface States at n-Type Silicon Surfaces in Aqueous Fluoride Solutions

Electrochemical impedance spectroscopy (EIS) was used to analyze the energetics and kinetics of processes occurring at n-type silicon (111) surfaces in 1 M NH 4 F solutions in the pH range 3−11 in the dark. An additional impedance parallel to that of the space charge layer was observed due to electrically active surface states. The surface states are located energetically at about 0.38 eV below the conduction band and have a capture cross section of 1 × 10 -16  cm 2 . The rate constant for thermal excitation of electrons from the surface states into the conduction band is essentially independent of pH, indicating the surface states are physically the same on both hydrogen terminated and oxidized silicon surfaces. The density of surface states is 2 × 10 10  cm -2  at pH 3, characteristic of the hydrogen terminated surface, and increases to 1 × 10 12  cm -2  at pH 11, corresponding to an oxide passivated surface.