ENERGETICS AND KINETICS OF SURFACE-STATES AT N-TYPE SILICON SURFACES IN AQUEOUS FLUORIDE SOLUTIONS

Electrochemical impedance spectroscopy (EIS) was used to analyze the e nergetics and kinetics of processes occurring at n-type silicon (111) surfaces in 1 M NH4F solutions in the pH range 3-11 in the dark. An ad ditional impedance parallel to that of the space charge layer was obse rved 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 x 10(-16) cm(2). The rate constant f or thermal excitation of electrons from the surface states into the co nduction band is essentially independent of pH, indicating the surface states are physically the same on both hydrogen terminated and oxidiz ed silicon;surfaces, The density of surface states is 2 x 10(10) cm(-2 ) at pH 3, characteristic of the hydrogen terminated surface, and incr eases to 1 x 10(12) cm(-2) at pH 11, corresponding to an oxide passiva ted surface.